1. Definitions
- Codec
-
Refers generically to an instance of
AudioDecoder
,AudioEncoder
,VideoDecoder
, orVideoEncoder
. - Key Chunk
-
An encoded chunk that does not depend on any other frames for decoding. Also commonly referred to as a "key frame".
- Internal Pending Output
-
Codec outputs such as
VideoFrame
s that currently reside in the internal pipeline of the underlying codec implementation. The underlying codec implementation MAY emit new outputs only when new inputs are provided. The underlying codec implementation MUST emit all outputs in response to a flush. - Codec System Resources
-
Resources including CPU memory, GPU memory, and exclusive handles to specific decoding/encoding hardware that MAY be allocated by the User Agent as part of codec configuration or generation of
AudioData
andVideoFrame
objects. Such resources MAY be quickly exhausted and SHOULD be released immediately when no longer in use. - Temporal Layer
-
A grouping of
EncodedVideoChunk
s whose timestamp cadence produces a particular framerate. SeescalabilityMode
. - Progressive Image
-
An image that supports decoding to multiple levels of detail, with lower levels becoming available while the encoded data is not yet fully buffered.
- Progressive Image Frame Generation
-
A generational identifier for a given Progressive Image decoded output. Each successive generation adds additional detail to the decoded output. The mechanism for computing a frame’s generation is implementer defined.
- Primary Image Track
-
An image track that is marked by the given image file as being the default track. The mechanism for indicating a primary track is format defined.
- RGB Format
-
A
VideoPixelFormat
containing red, green, and blue color channels in any order or layout (interleaved or planar), and irrespective of whether an alpha channel is present. - sRGB Color Space
-
A
VideoColorSpaceInit
containing «[ "primaries" →bt709
, "transfer" →iec61966-2-1
, "matrix" →rgb
, "fullRange" →true
]». - REC709 Color Space
-
A
VideoColorSpaceInit
containing «[ "primaries" →bt709
, "transfer" →bt709
, "matrix" →bt709
, "fullRange" →false
]». - Codec Saturation
-
The state of an underlying codec implementation where the number of active decoding or encoding requests has reached an implementation specific maximum such that it is temporarily unable to accept more work. The maximum may be any value greater than 1, including infinity (no maximum). While saturated, additional calls to
decode()
orencode()
will be buffered in the control message queue, and will increment the respectivedecodeQueuSize
andencodeQueueSize
attributes. The codec implementation will become unsaturated after making sufficient progress on the current workload.
2. Codec Processing Model
2.1. Background
The codec interfaces defined by the specification are designed such that new
codec tasks can be scheduled while previous tasks are still pending. For
example, web authors can call decode()
without waiting for a previous decode()
to complete. This is achieved by offloading underlying codec tasks
to a separate parallel queue for parallel execution.
This section describes threading behaviors as they are visible from the perspective of web authors. Implementers can choose to use more threads, as long as the externally visible behaviors of blocking and sequencing are maintained as follows.
2.2. Control Messages
A control message defines a sequence of steps corresponding to a
method invocation on a codec instance (e.g. encode()
).
A control message queue is a queue of control messages. Each codec instance has a control message queue stored in an internal slot named [[control message queue]].
Queuing a control message means enqueuing the message to a codec’s [[control message queue]]. Invoking codec methods will generally queue a control message to schedule work.
Running a control message means performing a sequence of steps specified by the method that enqueued the message.
The steps of a given control message can block processing later messages in the
control message queue. Each codec instance has a boolean internal slot named [[message queue blocked]] that is set to true
when this occurs. A
blocking message will conclude by setting [[message queue blocked]] to false
and rerunning the Process the control message queue steps.
All control messages will return either "processed"
or "not processed"
.
Returning "processed"
indicates the message steps are being (or have been)
executed and the message may be removed from the control message queue. "not processed"
indicates the message must not be processed at this time
and should remain in the control message queue to be retried later.
To Process the control message queue, run these steps:
-
While [[message queue blocked]] is
false
and [[control message queue]] is not empty:-
Let front message be the first message in [[control message queue]].
-
Let outcome be the result of running the control message steps described by front message.
-
If outcome equals
"not processed"
, break. -
Otherwise, dequeue front message from the [[control message queue]].
-
2.3. Codec Work Parallel Queue
Each codec instance has an internal slot named [[codec work queue]] that is a parallel queue.
Each codec instance has an internal slot named [[codec implementation]] that refers to the underlying platform encoder or decoder. Except for assignment, any steps that reference [[codec implementation]] will be enqueued to the [[codec work queue]].
Each codec instance has a unique codec task source. Tasks queued from the [[codec work queue]] to the event loop will use the codec task source.
3. AudioDecoder Interface
[Exposed =(Window ,DedicatedWorker ),SecureContext ]interface :
AudioDecoder EventTarget {constructor (AudioDecoderInit );
init readonly attribute CodecState state ;readonly attribute unsigned long decodeQueueSize ;attribute EventHandler ondequeue ;undefined configure (AudioDecoderConfig );
config undefined decode (EncodedAudioChunk );
chunk Promise <undefined >flush ();undefined reset ();undefined close ();static Promise <AudioDecoderSupport >isConfigSupported (AudioDecoderConfig ); };
config dictionary {
AudioDecoderInit required AudioDataOutputCallback ;
output required WebCodecsErrorCallback ; };
error callback =
AudioDataOutputCallback undefined (AudioData );
output
3.1. Internal Slots
[[control message queue]]
-
A queue of control messages to be performed upon this codec instance. See [[control message queue]].
[[message queue blocked]]
-
A boolean indicating when processing the
[[control message queue]]
is blocked by a pending control message. See [[message queue blocked]]. [[codec implementation]]
-
Underlying decoder implementation provided by the User Agent. See [[codec implementation]].
[[codec work queue]]
-
A parallel queue used for running parallel steps that reference the
[[codec implementation]]
. See [[codec work queue]]. [[codec saturated]]
-
A boolean indicating when the
[[codec implementation]]
is unable to accept additional decoding work. [[output callback]]
-
Callback given at construction for decoded outputs.
[[error callback]]
-
Callback given at construction for decode errors.
[[key chunk required]]
-
A boolean indicating that the next chunk passed to
decode()
MUST describe a key chunk as indicated by[[type]]
. [[state]]
-
The current
CodecState
of thisAudioDecoder
. [[decodeQueueSize]]
-
The number of pending decode requests. This number will decrease as the underlying codec is ready to accept new input.
[[pending flush promises]]
-
A list of unresolved promises returned by calls to
flush()
. [[dequeue event scheduled]]
-
A boolean indicating whether a
dequeue
event is already scheduled to fire. Used to avoid event spam.
3.2. Constructors
AudioDecoder(init)
-
Let d be a new
AudioDecoder
object. -
Assign a new queue to
[[control message queue]]
. -
Assign
false
to[[message queue blocked]]
. -
Assign
null
to[[codec implementation]]
. -
Assign the result of starting a new parallel queue to
[[codec work queue]]
. -
Assign
false
to[[codec saturated]]
. -
Assign init.output to
[[output callback]]
. -
Assign init.error to
[[error callback]]
. -
Assign
true
to[[key chunk required]]
. -
Assign
"unconfigured"
to[[state]]
-
Assign
0
to[[decodeQueueSize]]
. -
Assign a new list to
[[pending flush promises]]
. -
Assign
false
to[[dequeue event scheduled]]
. -
Return d.
3.3. Attributes
state
, of type CodecState, readonly-
Returns the value of
[[state]]
. decodeQueueSize
, of type unsigned long, readonly-
Returns the value of
[[decodeQueueSize]]
. ondequeue
, of type EventHandler-
An event handler IDL attribute whose event handler event type is
dequeue
.
3.4. Event Summary
dequeue
-
Fired at the
AudioDecoder
when thedecodeQueueSize
has decreased.
3.5. Methods
configure(config)
-
Enqueues a control message to configure the audio decoder for decoding
chunks as described by config.
NOTE: This method will trigger a
NotSupportedError
if the User Agent does not support config. Authors are encouraged to first check support by callingisConfigSupported()
with config. User Agents don’t have to support any particular codec type or configuration.When invoked, run these steps:
-
If config is not a valid AudioDecoderConfig, throw a
TypeError
. -
If
[[state]]
is“closed”
, throw anInvalidStateError
. -
Set
[[state]]
to"configured"
. -
Set
[[key chunk required]]
totrue
. -
Queue a control message to configure the decoder with config.
Running a control message to configure the decoder means running these steps:
-
Let supported be the result of running the Check Configuration Support algorithm with config.
-
If supported is
true
, assign[[codec implementation]]
with an implementation supporting config. -
Otherwise, run the Close AudioDecoder algorithm with
NotSupportedError
and return"processed"
. -
Assign
true
to[[message queue blocked]]
. -
Enqueue the following steps to
[[codec work queue]]
:-
Configure
[[codec implementation]]
with config. -
Assign
false
to[[message queue blocked]]
.
-
-
Return
"processed"
.
-
decode(chunk)
-
Enqueues a control message to decode the given chunk.
When invoked, run these steps:
-
If
[[state]]
is not"configured"
, throw anInvalidStateError
. -
If
[[key chunk required]]
istrue
:-
Implementers SHOULD inspect the chunk’s
[[internal data]]
to verify that it is truly a key chunk. If a mismatch is detected, throw aDataError
. -
Otherwise, assign
false
to[[key chunk required]]
.
-
Increment
[[decodeQueueSize]]
. -
Queue a control message to decode the chunk.
Running a control message to decode the chunk means performing these steps:
-
If
[[codec saturated]]
equalstrue
, return"not processed"
. -
If decoding chunk will cause the
[[codec implementation]]
to become saturated, assigntrue
to[[codec saturated]]
. -
Decrement
[[decodeQueueSize]]
and run the Schedule Dequeue Event algorithm. -
Enqueue the following steps to the
[[codec work queue]]
:-
Attempt to use
[[codec implementation]]
to decode the chunk. -
If decoding results in an error, queue a task to run the Close AudioDecoder algorithm with
EncodingError
and return. -
If
[[codec saturated]]
equalstrue
and[[codec implementation]]
is no longer saturated, queue a task to perform the following steps:-
Assign
false
to[[codec saturated]]
.
-
-
Let decoded outputs be a list of decoded audio data outputs emitted by
[[codec implementation]]
. -
If decoded outputs is not empty, queue a task to run the Output AudioData algorithm with decoded outputs.
-
-
Return
"processed"
.
-
flush()
-
Completes all control messages in the control message queue and emits all outputs.
When invoked, run these steps:
-
If
[[state]]
is not"configured"
, return a promise rejected withInvalidStateError
DOMException
. -
Set
[[key chunk required]]
totrue
. -
Let promise be a new Promise.
-
Append promise to
[[pending flush promises]]
. -
Queue a control message to flush the codec with promise.
-
Return promise.
Running a control message to flush the codec means performing these steps with promise.
-
Enqueue the following steps to the
[[codec work queue]]
:-
Signal
[[codec implementation]]
to emit all internal pending outputs. -
Let decoded outputs be a list of decoded audio data outputs emitted by
[[codec implementation]]
. -
Queue a task to perform these steps:
-
If decoded outputs is not empty, run the Output AudioData algorithm with decoded outputs.
-
Remove promise from
[[pending flush promises]]
. -
Resolve promise.
-
-
-
reset()
-
Immediately resets all state including configuration, control messages in the control message queue, and all pending
callbacks.
When invoked, run the Reset AudioDecoder algorithm with an
AbortError
DOMException
. close()
-
Immediately aborts all pending work and releases system resources.
Close is final.
When invoked, run the Close AudioDecoder algorithm with an
AbortError
DOMException
. isConfigSupported(config)
-
Returns a promise indicating whether the provided config is supported by
the User Agent.
NOTE: The returned
AudioDecoderSupport
config
will contain only the dictionary members that User Agent recognized. Unrecognized dictionary members will be ignored. Authors can detect unrecognized dictionary members by comparingconfig
to their provided config.When invoked, run these steps:
-
If config is not a valid AudioDecoderConfig, return a promise rejected with
TypeError
. -
Let p be a new Promise.
-
Let checkSupportQueue be the result of starting a new parallel queue.
-
Enqueue the following steps to checkSupportQueue:
-
Let decoderSupport be a newly constructed
AudioDecoderSupport
, initialized as follows:-
Set
config
to the result of running the Clone Configuration algorithm with config. -
Set
supported
to the result of running the Check Configuration Support algorithm with config.
-
-
Resolve p with decoderSupport.
-
-
Return p.
-
3.6. Algorithms
- Schedule Dequeue Event
-
-
If
[[dequeue event scheduled]]
equalstrue
, return. -
Assign
true
to[[dequeue event scheduled]]
. -
Queue a task to run the following steps:
-
Assign
false
to[[dequeue event scheduled]]
.
-
- Output AudioData (with outputs)
-
Run these steps:
-
For each output in outputs:
-
Let data be an
AudioData
, initialized as follows:-
Assign
false
to[[Detached]]
. -
Let resource be the media resource described by output.
-
Let resourceReference be a reference to resource.
-
Assign resourceReference to
[[resource reference]]
. -
Let timestamp be the
[[timestamp]]
of theEncodedAudioChunk
associated with output. -
Assign timestamp to
[[timestamp]]
. -
If output uses a recognized
AudioSampleFormat
, assign that format to[[format]]
. Otherwise, assignnull
to[[format]]
. -
Assign values to
[[sample rate]]
,[[number of frames]]
, and[[number of channels]]
as determined by output.
-
-
Invoke
[[output callback]]
with data.
-
-
- Reset AudioDecoder (with exception)
-
Run these steps:
-
If
[[state]]
is"closed"
, throw anInvalidStateError
. -
Set
[[state]]
to"unconfigured"
. -
Signal
[[codec implementation]]
to cease producing output for the previous configuration. -
Remove all control messages from the
[[control message queue]]
. -
If
[[decodeQueueSize]]
is greater than zero:-
Set
[[decodeQueueSize]]
to zero. -
Run the Schedule Dequeue Event algorithm.
-
-
For each promise in
[[pending flush promises]]
:-
Reject promise with exception.
-
Remove promise from
[[pending flush promises]]
.
-
-
- Close AudioDecoder (with exception)
-
Run these steps:
-
Run the Reset AudioDecoder algorithm with exception.
-
Set
[[state]]
to"closed"
. -
Clear
[[codec implementation]]
and release associated system resources. -
If exception is not an
AbortError
DOMException
, queue a task to invoke the[[error callback]]
with exception.
-
4. VideoDecoder Interface
[Exposed =(Window ,DedicatedWorker ),SecureContext ]interface :
VideoDecoder EventTarget {constructor (VideoDecoderInit );
init readonly attribute CodecState state ;readonly attribute unsigned long decodeQueueSize ;attribute EventHandler ondequeue ;undefined configure (VideoDecoderConfig );
config undefined decode (EncodedVideoChunk );
chunk Promise <undefined >flush ();undefined reset ();undefined close ();static Promise <VideoDecoderSupport >isConfigSupported (VideoDecoderConfig ); };
config dictionary {
VideoDecoderInit required VideoFrameOutputCallback ;
output required WebCodecsErrorCallback ; };
error callback =
VideoFrameOutputCallback undefined (VideoFrame );
output
4.1. Internal Slots
[[control message queue]]
-
A queue of control messages to be performed upon this codec instance. See [[control message queue]].
[[message queue blocked]]
-
A boolean indicating when processing the
[[control message queue]]
is blocked by a pending control message. See [[message queue blocked]]. [[codec implementation]]
-
Underlying decoder implementation provided by the User Agent. See [[codec implementation]].
[[codec work queue]]
-
A parallel queue used for running parallel steps that reference the
[[codec implementation]]
. See [[codec work queue]]. [[codec saturated]]
-
A boolean indicating when the
[[codec implementation]]
is unable to accept additional decoding work. [[output callback]]
-
Callback given at construction for decoded outputs.
[[error callback]]
-
Callback given at construction for decode errors.
[[active decoder config]]
-
The
VideoDecoderConfig
that is actively applied. [[key chunk required]]
-
A boolean indicating that the next chunk passed to
decode()
MUST describe a key chunk as indicated bytype
. [[state]]
-
The current
CodecState
of thisVideoDecoder
. [[decodeQueueSize]]
-
The number of pending decode requests. This number will decrease as the underlying codec is ready to accept new input.
[[pending flush promises]]
-
A list of unresolved promises returned by calls to
flush()
. [[dequeue event scheduled]]
-
A boolean indicating whether a
dequeue
event is already scheduled to fire. Used to avoid event spam.
4.2. Constructors
VideoDecoder(init)
-
Let d be a new
VideoDecoder
object. -
Assign a new queue to
[[control message queue]]
. -
Assign
false
to[[message queue blocked]]
. -
Assign
null
to[[codec implementation]]
. -
Assign the result of starting a new parallel queue to
[[codec work queue]]
. -
Assign
false
to[[codec saturated]]
. -
Assign init.output to
[[output callback]]
. -
Assign init.error to
[[error callback]]
. -
Assign
null
to[[active decoder config]]
. -
Assign
true
to[[key chunk required]]
. -
Assign
"unconfigured"
to[[state]]
-
Assign
0
to[[decodeQueueSize]]
. -
Assign a new list to
[[pending flush promises]]
. -
Assign
false
to[[dequeue event scheduled]]
. -
Return d.
4.3. Attributes
state
, of type CodecState, readonly-
Returns the value of
[[state]]
. decodeQueueSize
, of type unsigned long, readonly-
Returns the value of
[[decodeQueueSize]]
. ondequeue
, of type EventHandler-
An event handler IDL attribute whose event handler event type is
dequeue
.
4.4. Event Summary
dequeue
-
Fired at the
VideoDecoder
when thedecodeQueueSize
has decreased.
4.5. Methods
configure(config)
-
Enqueues a control message to configure the video decoder for decoding
chunks as described by config.
NOTE: This method will trigger a
NotSupportedError
if the User Agent does not support config. Authors are encouraged to first check support by callingisConfigSupported()
with config. User Agents don’t have to support any particular codec type or configuration.When invoked, run these steps:
-
If config is not a valid VideoDecoderConfig, throw a
TypeError
. -
If
[[state]]
is“closed”
, throw anInvalidStateError
. -
Set
[[state]]
to"configured"
. -
Set
[[key chunk required]]
totrue
. -
Queue a control message to configure the decoder with config.
Running a control message to configure the decoder means running these steps:
-
Let supported be the result of running the Check Configuration Support algorithm with config.
-
If supported is
true
, assign[[codec implementation]]
with an implementation supporting config. -
Otherwise, run the Close VideoDecoder algorithm with
NotSupportedError
and return"processed"
. -
Assign
true
to[[message queue blocked]]
. -
Enqueue the following steps to
[[codec work queue]]
:-
Configure
[[codec implementation]]
with config. -
Set
[[active decoder config]]
toconfig
. -
Assign
false
to[[message queue blocked]]
.
-
-
Return
"processed"
.
-
decode(chunk)
-
Enqueues a control message to decode the given chunk.
NOTE: Authors are encouraged to call
close()
on outputVideoFrame
s immediately when frames are no longer needed. The underlying media resources are owned by theVideoDecoder
and failing to release them (or waiting for garbage collection) can cause decoding to stall.NOTE:
VideoDecoder
requires that frames are output in the order they expect to be presented, commonly known as presentation order. When using some[[codec implementation]]
s the User Agent will have to reorder outputs into presentation order.When invoked, run these steps:
-
If
[[state]]
is not"configured"
, throw anInvalidStateError
. -
If
[[key chunk required]]
istrue
:-
Implementers SHOULD inspect the chunk’s
[[internal data]]
to verify that it is truly a key chunk. If a mismatch is detected, throw aDataError
. -
Otherwise, assign
false
to[[key chunk required]]
.
-
Increment
[[decodeQueueSize]]
. -
Queue a control message to decode the chunk.
Running a control message to decode the chunk means performing these steps:
-
If
[[codec saturated]]
equalstrue
, return"not processed"
. -
If decoding chunk will cause the
[[codec implementation]]
to become saturated, assigntrue
to[[codec saturated]]
. -
Decrement
[[decodeQueueSize]]
and run the Schedule Dequeue Event algorithm. -
Enqueue the following steps to the
[[codec work queue]]
:-
Attempt to use
[[codec implementation]]
to decode the chunk. -
If decoding results in an error, queue a task to run the Close VideoDecoder algorithm with
EncodingError
and return. -
If
[[codec saturated]]
equalstrue
and[[codec implementation]]
is no longer saturated, queue a task to perform the following steps:-
Assign
false
to[[codec saturated]]
.
-
-
Let decoded outputs be a list of decoded video data outputs emitted by
[[codec implementation]]
in presentation order. -
If decoded outputs is not empty, queue a task to run the Output VideoFrame algorithm with decoded outputs.
-
-
Return
"processed"
.
-
flush()
-
Completes all control messages in the control message queue and emits all outputs.
When invoked, run these steps:
-
If
[[state]]
is not"configured"
, return a promise rejected withInvalidStateError
DOMException
. -
Set
[[key chunk required]]
totrue
. -
Let promise be a new Promise.
-
Append promise to
[[pending flush promises]]
. -
Queue a control message to flush the codec with promise.
-
Return promise.
Running a control message to flush the codec means performing these steps with promise.
-
Enqueue the following steps to the
[[codec work queue]]
:-
Signal
[[codec implementation]]
to emit all internal pending outputs. -
Let decoded outputs be a list of decoded video data outputs emitted by
[[codec implementation]]
. -
Queue a task to perform these steps:
-
If decoded outputs is not empty, run the Output VideoFrame algorithm with decoded outputs.
-
Remove promise from
[[pending flush promises]]
. -
Resolve promise.
-
-
-
reset()
-
Immediately resets all state including configuration, control messages in the control message queue, and all pending
callbacks.
When invoked, run the Reset VideoDecoder algorithm with an
AbortError
DOMException
. close()
-
Immediately aborts all pending work and releases system resources.
Close is final.
When invoked, run the Close VideoDecoder algorithm with an
AbortError
DOMException
. isConfigSupported(config)
-
Returns a promise indicating whether the provided config is supported by
the User Agent.
NOTE: The returned
VideoDecoderSupport
config
will contain only the dictionary members that User Agent recognized. Unrecognized dictionary members will be ignored. Authors can detect unrecognized dictionary members by comparingconfig
to their provided config.When invoked, run these steps:
-
If config is not a valid VideoDecoderConfig, return a promise rejected with
TypeError
. -
Let p be a new Promise.
-
Let checkSupportQueue be the result of starting a new parallel queue.
-
Enqueue the following steps to checkSupportQueue:
-
Let decoderSupport be a newly constructed
VideoDecoderSupport
, initialized as follows:-
Set
config
to the result of running the Clone Configuration algorithm with config. -
Set
supported
to the result of running the Check Configuration Support algorithm with config.
-
-
Resolve p with decoderSupport.
-
-
Return p.
-
4.6. Algorithms
- Schedule Dequeue Event
-
-
If
[[dequeue event scheduled]]
equalstrue
, return. -
Assign
true
to[[dequeue event scheduled]]
. -
Queue a task to run the following steps:
-
Assign
false
to[[dequeue event scheduled]]
.
-
- Output VideoFrames (with outputs)
-
Run these steps:
-
For each output in outputs:
-
Let timestamp and duration be the
timestamp
andduration
from theEncodedVideoChunk
associated with output. -
Let displayAspectWidth and displayAspectHeight be undefined.
-
If
displayAspectWidth
anddisplayAspectHeight
exist in the[[active decoder config]]
, assign their values to displayAspectWidth and displayAspectHeight respectively. -
Let colorSpace be the
VideoColorSpace
for output as detected by the codec implementation. If noVideoColorSpace
is detected, let colorSpace beundefined
.NOTE: The codec implementation can detect a
VideoColorSpace
by analyzing the bitstream. Detection is made on a best-effort basis. The exact method of detection is implementer defined and codec-specific. Authors can override the detectedVideoColorSpace
by providing acolorSpace
in theVideoDecoderConfig
. -
If
colorSpace
exists in the[[active decoder config]]
, assign its value to colorSpace. -
Let frame be the result of running the Create a VideoFrame algorithm with output, timestamp, duration, displayAspectWidth, displayAspectHeight, and colorSpace.
-
Invoke
[[output callback]]
with frame.
-
-
- Reset VideoDecoder (with exception)
-
Run these steps:
-
If
state
is"closed"
, throw anInvalidStateError
. -
Set
state
to"unconfigured"
. -
Signal
[[codec implementation]]
to cease producing output for the previous configuration. -
Remove all control messages from the
[[control message queue]]
. -
If
[[decodeQueueSize]]
is greater than zero:-
Set
[[decodeQueueSize]]
to zero. -
Run the Schedule Dequeue Event algorithm.
-
-
For each promise in
[[pending flush promises]]
:-
Reject promise with exception.
-
Remove promise from
[[pending flush promises]]
.
-
-
- Close VideoDecoder (with exception)
-
Run these steps:
-
Run the Reset VideoDecoder algorithm with exception.
-
Set
state
to"closed"
. -
Clear
[[codec implementation]]
and release associated system resources. -
If exception is not an
AbortError
DOMException
, queue a task to invoke the[[error callback]]
with exception.
-
5. AudioEncoder Interface
[Exposed =(Window ,DedicatedWorker ),SecureContext ]interface :
AudioEncoder EventTarget {constructor (AudioEncoderInit );
init readonly attribute CodecState state ;readonly attribute unsigned long encodeQueueSize ;attribute EventHandler ondequeue ;undefined configure (AudioEncoderConfig );
config undefined encode (AudioData );
data Promise <undefined >flush ();undefined reset ();undefined close ();static Promise <AudioEncoderSupport >isConfigSupported (AudioEncoderConfig ); };
config dictionary {
AudioEncoderInit required EncodedAudioChunkOutputCallback ;
output required WebCodecsErrorCallback ; };
error callback =
EncodedAudioChunkOutputCallback undefined (EncodedAudioChunk ,
output optional EncodedAudioChunkMetadata = {});
metadata
5.1. Internal Slots
[[control message queue]]
-
A queue of control messages to be performed upon this codec instance. See [[control message queue]].
[[message queue blocked]]
-
A boolean indicating when processing the
[[control message queue]]
is blocked by a pending control message. See [[message queue blocked]]. [[codec implementation]]
-
Underlying encoder implementation provided by the User Agent. See [[codec implementation]].
[[codec work queue]]
-
A parallel queue used for running parallel steps that reference the
[[codec implementation]]
. See [[codec work queue]]. [[codec saturated]]
-
A boolean indicating when the
[[codec implementation]]
is unable to accept additional encoding work. [[output callback]]
-
Callback given at construction for encoded outputs.
[[error callback]]
-
Callback given at construction for encode errors.
[[active encoder config]]
-
The
AudioEncoderConfig
that is actively applied. [[active output config]]
-
The
AudioDecoderConfig
that describes how to decode the most recently emittedEncodedAudioChunk
. [[state]]
-
The current
CodecState
of thisAudioEncoder
. [[encodeQueueSize]]
-
The number of pending encode requests. This number will decrease as the underlying codec is ready to accept new input.
[[pending flush promises]]
-
A list of unresolved promises returned by calls to
flush()
. [[dequeue event scheduled]]
-
A boolean indicating whether a
dequeue
event is already scheduled to fire. Used to avoid event spam.
5.2. Constructors
AudioEncoder(init)
-
Let e be a new
AudioEncoder
object. -
Assign a new queue to
[[control message queue]]
. -
Assign
false
to[[message queue blocked]]
. -
Assign
null
to[[codec implementation]]
. -
Assign the result of starting a new parallel queue to
[[codec work queue]]
. -
Assign
false
to[[codec saturated]]
. -
Assign init.output to
[[output callback]]
. -
Assign init.error to
[[error callback]]
. -
Assign
null
to[[active encoder config]]
. -
Assign
null
to[[active output config]]
. -
Assign
"unconfigured"
to[[state]]
-
Assign
0
to[[encodeQueueSize]]
. -
Assign a new list to
[[pending flush promises]]
. -
Assign
false
to[[dequeue event scheduled]]
. -
Return e.
5.3. Attributes
state
, of type CodecState, readonly-
Returns the value of
[[state]]
. encodeQueueSize
, of type unsigned long, readonly-
Returns the value of
[[encodeQueueSize]]
. ondequeue
, of type EventHandler-
An event handler IDL attribute whose event handler event type is
dequeue
.
5.4. Event Summary
dequeue
-
Fired at the
AudioEncoder
when theencodeQueueSize
has decreased.
5.5. Methods
configure(config)
-
Enqueues a control message to configure the audio encoder for
decoding chunks as described by config.
NOTE: This method will trigger a
NotSupportedError
if the User Agent does not support config. Authors are encouraged to first check support by callingisConfigSupported()
with config. User Agents don’t have to support any particular codec type or configuration.When invoked, run these steps:
-
If config is not a valid AudioEncoderConfig, throw a
TypeError
. -
If
[[state]]
is"closed"
, throw anInvalidStateError
. -
Set
[[state]]
to"configured"
. -
Queue a control message to configure the encoder using config.
Running a control message to configure the encoder means performing these steps:
-
Let supported be the result of running the Check Configuration Support algorithm with config.
-
If supported is
true
, assign[[codec implementation]]
with an implementation supporting config. -
Otherwise, run the Close AudioEncoder algorithm with
NotSupportedError
and return"processed"
. -
Assign
true
to[[message queue blocked]]
. -
Enqueue the following steps to
[[codec work queue]]
:-
Configure
[[codec implementation]]
with config. -
Set
[[active encoder config]]
toconfig
. -
Assign
false
to[[message queue blocked]]
.
-
-
Return
"processed"
.
-
encode(data)
-
Enqueues a control message to encode the given data.
When invoked, run these steps:
-
If the value of data’s
[[Detached]]
internal slot istrue
, throw aTypeError
. -
If
[[state]]
is not"configured"
, throw anInvalidStateError
. -
Let dataClone hold the result of running the Clone AudioData algorithm with data.
-
Increment
[[encodeQueueSize]]
. -
Queue a control message to encode dataClone.
Running a control message to encode the data means performing these steps:
-
If
[[codec saturated]]
equalstrue
, return"not processed"
. -
If encoding data will cause the
[[codec implementation]]
to become saturated, assigntrue
to[[codec saturated]]
. -
Decrement
[[encodeQueueSize]]
and run the Schedule Dequeue Event algorithm. -
Enqueue the following steps to the
[[codec work queue]]
:-
Attempt to use
[[codec implementation]]
to encode the media resource described by dataClone. -
If encoding results in an error, queue a task to run the Close AudioEncoder algorithm with
EncodingError
and return. -
If
[[codec saturated]]
equalstrue
and[[codec implementation]]
is no longer saturated, queue a task to perform the following steps:-
Assign
false
to[[codec saturated]]
.
-
-
Let encoded outputs be a list of encoded audio data outputs emitted by
[[codec implementation]]
. -
If encoded outputs is not empty, queue a task to run the Output EncodedAudioChunks algorithm with encoded outputs.
-
-
Return
"processed"
.
-
flush()
-
Completes all control messages in the control message queue and emits all outputs.
When invoked, run these steps:
-
If
[[state]]
is not"configured"
, return a promise rejected withInvalidStateError
DOMException
. -
Let promise be a new Promise.
-
Append promise to
[[pending flush promises]]
. -
Queue a control message to flush the codec with promise.
-
Return promise.
Running a control message to flush the codec means performing these steps with promise.
-
Enqueue the following steps to the
[[codec work queue]]
:-
Signal
[[codec implementation]]
to emit all internal pending outputs. -
Let encoded outputs be a list of encoded audio data outputs emitted by
[[codec implementation]]
. -
Queue a task to perform these steps:
-
If encoded outputs is not empty, run the Output EncodedAudioChunks algorithm with encoded outputs.
-
Remove promise from
[[pending flush promises]]
. -
Resolve promise.
-
-
-
reset()
-
Immediately resets all state including configuration, control messages in the control message queue, and all pending
callbacks.
When invoked, run the Reset AudioEncoder algorithm with an
AbortError
DOMException
. close()
-
Immediately aborts all pending work and releases system resources.
Close is final.
When invoked, run the Close AudioEncoder algorithm with an
AbortError
DOMException
. isConfigSupported(config)
-
Returns a promise indicating whether the provided config is supported by
the User Agent.
NOTE: The returned
AudioEncoderSupport
config
will contain only the dictionary members that User Agent recognized. Unrecognized dictionary members will be ignored. Authors can detect unrecognized dictionary members by comparingconfig
to their provided config.When invoked, run these steps:
-
If config is not a valid AudioEncoderConfig, return a promise rejected with
TypeError
. -
Let p be a new Promise.
-
Let checkSupportQueue be the result of starting a new parallel queue.
-
Enqueue the following steps to checkSupportQueue:
-
Let encoderSupport be a newly constructed
AudioEncoderSupport
, initialized as follows:-
Set
config
to the result of running the Clone Configuration algorithm with config. -
Set
supported
to the result of running the Check Configuration Support algorithm with config.
-
-
Resolve p with encoderSupport.
-
-
Return p.
-
5.6. Algorithms
- Schedule Dequeue Event
-
-
If
[[dequeue event scheduled]]
equalstrue
, return. -
Assign
true
to[[dequeue event scheduled]]
. -
Queue a task to run the following steps:
-
Assign
false
to[[dequeue event scheduled]]
.
-
- Output EncodedAudioChunks (with outputs)
-
Run these steps:
-
For each output in outputs:
-
Let chunkInit be an
EncodedAudioChunkInit
with the following keys: -
Let chunk be a new
EncodedAudioChunk
constructed with chunkInit. -
Let chunkMetadata be a new
EncodedAudioChunkMetadata
. -
Let encoderConfig be the
[[active encoder config]]
. -
Let outputConfig be a new
AudioDecoderConfig
that describes output. Initialize outputConfig as follows:-
Assign encoderConfig.
sampleRate
to outputConfig.sampleRate
. -
Assign to encoderConfig.
numberOfChannels
to outputConfig.numberOfChannels
. -
Assign outputConfig.
description
with a sequence of codec specific bytes as determined by the[[codec implementation]]
. The User Agent MUST ensure that the provided description could be used to correctly decode output.NOTE: The codec specific requirements for populating the
description
are described in the [WEBCODECS-CODEC-REGISTRY].
-
If outputConfig and
[[active output config]]
are not equal dictionaries:-
Assign outputConfig to chunkMetadata.
decoderConfig
. -
Assign outputConfig to
[[active output config]]
.
-
-
Invoke
[[output callback]]
with chunk and chunkMetadata.
-
-
- Reset AudioEncoder (with exception)
-
Run these steps:
-
If
[[state]]
is"closed"
, throw anInvalidStateError
. -
Set
[[state]]
to"unconfigured"
. -
Set
[[active encoder config]]
tonull
. -
Set
[[active output config]]
tonull
. -
Signal
[[codec implementation]]
to cease producing output for the previous configuration. -
Remove all control messages from the
[[control message queue]]
. -
If
[[encodeQueueSize]]
is greater than zero:-
Set
[[encodeQueueSize]]
to zero. -
Run the Schedule Dequeue Event algorithm.
-
-
For each promise in
[[pending flush promises]]
:-
Reject promise with exception.
-
Remove promise from
[[pending flush promises]]
.
-
-
- Close AudioEncoder (with exception)
-
Run these steps:
-
Run the Reset AudioEncoder algorithm with exception.
-
Set
[[state]]
to"closed"
. -
Clear
[[codec implementation]]
and release associated system resources. -
If exception is not an
AbortError
DOMException
, queue a task to invoke the[[error callback]]
with exception.
-
5.7. EncodedAudioChunkMetadata
The following metadata dictionary is emitted by theEncodedVideoChunkOutputCallback
alongside an associated EncodedVideoChunk
.
dictionary {
EncodedAudioChunkMetadata AudioDecoderConfig decoderConfig ; };
decoderConfig
, of type AudioDecoderConfig-
A
AudioDecoderConfig
that authors MAY use to decode the associatedEncodedAudioChunk
.
6. VideoEncoder Interface
[Exposed =(Window ,DedicatedWorker ),SecureContext ]interface :
VideoEncoder EventTarget {constructor (VideoEncoderInit );
init readonly attribute CodecState state ;readonly attribute unsigned long encodeQueueSize ;attribute EventHandler ondequeue ;undefined configure (VideoEncoderConfig );
config undefined encode (VideoFrame ,
frame optional VideoEncoderEncodeOptions = {});
options Promise <undefined >flush ();undefined reset ();undefined close ();static Promise <VideoEncoderSupport >isConfigSupported (VideoEncoderConfig ); };
config dictionary {
VideoEncoderInit required EncodedVideoChunkOutputCallback ;
output required WebCodecsErrorCallback ; };
error callback =
EncodedVideoChunkOutputCallback undefined (EncodedVideoChunk ,
chunk optional EncodedVideoChunkMetadata = {});
metadata
6.1. Internal Slots
[[control message queue]]
-
A queue of control messages to be performed upon this codec instance. See [[control message queue]].
[[message queue blocked]]
-
A boolean indicating when processing the
[[control message queue]]
is blocked by a pending control message. See [[message queue blocked]]. [[codec implementation]]
-
Underlying encoder implementation provided by the User Agent. See [[codec implementation]].
[[codec work queue]]
-
A parallel queue used for running parallel steps that reference the
[[codec implementation]]
. See [[codec work queue]]. [[codec saturated]]
-
A boolean indicating when the
[[codec implementation]]
is unable to accept additional encoding work. [[output callback]]
-
Callback given at construction for encoded outputs.
[[error callback]]
-
Callback given at construction for encode errors.
[[active encoder config]]
-
The
VideoEncoderConfig
that is actively applied. [[active output config]]
-
The
VideoDecoderConfig
that describes how to decode the most recently emittedEncodedVideoChunk
. [[state]]
-
The current
CodecState
of thisVideoEncoder
. [[encodeQueueSize]]
-
The number of pending encode requests. This number will decrease as the underlying codec is ready to accept new input.
[[pending flush promises]]
-
A list of unresolved promises returned by calls to
flush()
. [[dequeue event scheduled]]
-
A boolean indicating whether a
dequeue
event is already scheduled to fire. Used to avoid event spam.
6.2. Constructors
VideoEncoder(init)
-
Let e be a new
VideoEncoder
object. -
Assign a new queue to
[[control message queue]]
. -
Assign
false
to[[message queue blocked]]
. -
Assign
null
to[[codec implementation]]
. -
Assign the result of starting a new parallel queue to
[[codec work queue]]
. -
Assign
false
to[[codec saturated]]
. -
Assign init.output to
[[output callback]]
. -
Assign init.error to
[[error callback]]
. -
Assign
null
to[[active encoder config]]
. -
Assign
null
to[[active output config]]
. -
Assign
"unconfigured"
to[[state]]
-
Assign
0
to[[encodeQueueSize]]
. -
Assign a new list to
[[pending flush promises]]
. -
Assign
false
to[[dequeue event scheduled]]
. -
Return e.
6.3. Attributes
state
, of type CodecState, readonly-
Returns the value of
[[state]]
. encodeQueueSize
, of type unsigned long, readonly-
Returns the value of
[[encodeQueueSize]]
. ondequeue
, of type EventHandler-
An event handler IDL attribute whose event handler event type is
dequeue
.
6.4. Event Summary
dequeue
-
Fired at the
VideoEncoder
when theencodeQueueSize
has decreased.
6.5. Methods
configure(config)
-
Enqueues a control message to configure the video encoder for
decoding chunks as described by config.
NOTE: This method will trigger a
NotSupportedError
if the User Agent does not support config. Authors are encouraged to first check support by callingisConfigSupported()
with config. User Agents don’t have to support any particular codec type or configuration.When invoked, run these steps:
-
If config is not a valid VideoEncoderConfig, throw a
TypeError
. -
If
[[state]]
is"closed"
, throw anInvalidStateError
. -
Set
[[state]]
to"configured"
. -
Queue a control message to configure the encoder using config.
Running a control message to configure the encoder means performing these steps:
-
Let supported be the result of running the Check Configuration Support algorithm with config.
-
If supported is
true
, assign[[codec implementation]]
with an implementation supporting config. -
Otherwise, run the Close VideoEncoder algorithm with
NotSupportedError
and return"processed"
. -
Assign
true
to[[message queue blocked]]
. -
Enqueue the following steps to
[[codec work queue]]
:-
Configure
[[codec implementation]]
with config. -
Set
[[active encoder config]]
toconfig
. -
Assign
false
to[[message queue blocked]]
.
-
-
Return
"processed"
.
-
encode(frame, options)
-
Enqueues a control message to encode the given frame.
When invoked, run these steps:
-
If the value of frame’s
[[Detached]]
internal slot istrue
, throw aTypeError
. -
If
[[state]]
is not"configured"
, throw anInvalidStateError
. -
Let frameClone hold the result of running the Clone VideoFrame algorithm with frame.
-
Increment
[[encodeQueueSize]]
. -
Queue a control message to encode frameClone.
Running a control message to encode the frame means performing these steps:
-
If
[[codec saturated]]
equalstrue
, return"not processed"
. -
If encoding frame will cause the
[[codec implementation]]
to become saturated, assigntrue
to[[codec saturated]]
. -
Decrement
[[encodeQueueSize]]
and run the Schedule Dequeue Event algorithm. -
Enqueue the following steps to the
[[codec work queue]]
:-
Attempt to use
[[codec implementation]]
to encode the frameClone according to options. -
If encoding results in an error, queue a task to run the Close VideoEncoder algorithm with
EncodingError
and return. -
If
[[codec saturated]]
equalstrue
and[[codec implementation]]
is no longer saturated, queue a task to perform the following steps:-
Assign
false
to[[codec saturated]]
.
-
-
Let encoded outputs be a list of encoded video data outputs emitted by
[[codec implementation]]
. -
If encoded outputs is not empty, queue a task to run the Output EncodedVideoChunks algorithm with encoded outputs.
-
-
Return
"processed"
.
-
flush()
-
Completes all control messages in the control message queue and emits all outputs.
When invoked, run these steps:
-
If
[[state]]
is not"configured"
, return a promise rejected withInvalidStateError
DOMException
. -
Let promise be a new Promise.
-
Append promise to
[[pending flush promises]]
. -
Queue a control message to flush the codec with promise.
-
Return promise.
Running a control message to flush the codec means performing these steps with promise:
-
Enqueue the following steps to the
[[codec work queue]]
:-
Signal
[[codec implementation]]
to emit all internal pending outputs. -
Let encoded outputs be a list of encoded video data outputs emitted by
[[codec implementation]]
. -
Queue a task to perform these steps:
-
If encoded outputs is not empty, run the Output EncodedVideoChunks algorithm with encoded outputs.
-
Remove promise from
[[pending flush promises]]
. -
Resolve promise.
-
-
-
reset()
-
Immediately resets all state including configuration, control messages in the control message queue, and all pending
callbacks.
When invoked, run the Reset VideoEncoder algorithm with an
AbortError
DOMException
. close()
-
Immediately aborts all pending work and releases system resources.
Close is final.
When invoked, run the Close VideoEncoder algorithm with an
AbortError
DOMException
. isConfigSupported(config)
-
Returns a promise indicating whether the provided config is supported by
the User Agent.
NOTE: The returned
VideoEncoderSupport
config
will contain only the dictionary members that User Agent recognized. Unrecognized dictionary members will be ignored. Authors can detect unrecognized dictionary members by comparingconfig
to their provided config.When invoked, run these steps:
-
If config is not a valid VideoEncoderConfig, return a promise rejected with
TypeError
. -
Let p be a new Promise.
-
Let checkSupportQueue be the result of starting a new parallel queue.
-
Enqueue the following steps to checkSupportQueue:
-
Let encoderSupport be a newly constructed
VideoEncoderSupport
, initialized as follows:-
Set
config
to the result of running the Clone Configuration algorithm with config. -
Set
supported
to the result of running the Check Configuration Support algorithm with config.
-
-
Resolve p with encoderSupport.
-
-
Return p.
-
6.6. Algorithms
- Schedule Dequeue Event
-
-
If
[[dequeue event scheduled]]
equalstrue
, return. -
Assign
true
to[[dequeue event scheduled]]
. -
Queue a task to run the following steps:
-
Assign
false
to[[dequeue event scheduled]]
.
-
- Output EncodedVideoChunks (with outputs)
-
Run these steps:
-
For each output in outputs:
-
Let chunkInit be an
EncodedVideoChunkInit
with the following keys:-
Let
data
contain the encoded video data from output. -
Let
type
be theEncodedVideoChunkType
of output. -
Let
timestamp
be the[[timestamp]]
from theVideoFrame
associated with output. -
Let
duration
be the[[duration]]
from theVideoFrame
associated with output.
-
-
Let chunk be a new
EncodedVideoChunk
constructed with chunkInit. -
Let chunkMetadata be a new
EncodedVideoChunkMetadata
. -
Let encoderConfig be the
[[active encoder config]]
. -
Let outputConfig be a
VideoDecoderConfig
that describes output. Initialize outputConfig as follows:-
Assign
encoderConfig.codec
tooutputConfig.codec
. -
Assign
encoderConfig.width
tooutputConfig.visibleRect.width
. -
Assign
encoderConfig.height
tooutputConfig.visibleRect.height
. -
Assign
encoderConfig.displayWidth
tooutputConfig.displayWidth
. -
Assign
encoderConfig.displayHeight
tooutputConfig.displayHeight
. -
Assign the remaining keys of
outputConfig
as determined by[[codec implementation]]
. The User Agent MUST ensure that the configuration is completely described such that outputConfig could be used to correctly decode output.NOTE: The codec specific requirements for populating the
description
are described in the [WEBCODECS-CODEC-REGISTRY].
-
-
If outputConfig and
[[active output config]]
are not equal dictionaries:-
Assign outputConfig to chunkMetadata.
decoderConfig
. -
Assign outputConfig to
[[active output config]]
.
-
-
If encoderConfig.
scalabilityMode
describes multiple temporal layers:-
Let svc be a new
SvcOutputMetadata
instance. -
Let temporal_layer_id be the zero-based index describing the temporal layer for output.
-
Assign temporal_layer_id to svc.
temporalLayerId
. -
Assign svc to chunkMetadata.
svc
.
-
-
If encoderConfig.
alpha
is set to"keep"
:-
Let alphaSideData be the encoded alpha data in output.
-
Assign alphaSideData to chunkMetadata.
alphaSideData
.
-
-
Invoke
[[output callback]]
with chunk and chunkMetadata.
-
-
- Reset VideoEncoder (with exception)
-
Run these steps:
-
If
[[state]]
is"closed"
, throw anInvalidStateError
. -
Set
[[state]]
to"unconfigured"
. -
Set
[[active encoder config]]
tonull
. -
Set
[[active output config]]
tonull
. -
Signal
[[codec implementation]]
to cease producing output for the previous configuration. -
Remove all control messages from the
[[control message queue]]
. -
If
[[encodeQueueSize]]
is greater than zero:-
Set
[[encodeQueueSize]]
to zero. -
Run the Schedule Dequeue Event algorithm.
-
-
For each promise in
[[pending flush promises]]
:-
Reject promise with exception.
-
Remove promise from
[[pending flush promises]]
.
-
-
- Close VideoEncoder (with exception)
-
Run these steps:
-
Run the Reset VideoEncoder algorithm with exception.
-
Set
[[state]]
to"closed"
. -
Clear
[[codec implementation]]
and release associated system resources. -
If exception is not an
AbortError
DOMException
, queue a task to invoke the[[error callback]]
with exception.
-
6.7. EncodedVideoChunkMetadata
The following metadata dictionary is emitted by theEncodedVideoChunkOutputCallback
alongside an associated EncodedVideoChunk
.
dictionary {
EncodedVideoChunkMetadata VideoDecoderConfig decoderConfig ;SvcOutputMetadata svc ;BufferSource alphaSideData ; };dictionary {
SvcOutputMetadata unsigned long temporalLayerId ; };
decoderConfig
, of type VideoDecoderConfig-
A
VideoDecoderConfig
that authors MAY use to decode the associatedEncodedVideoChunk
. svc
, of type SvcOutputMetadata-
A collection of metadata describing this
EncodedVideoChunk
with respect to the configuredscalabilityMode
. alphaSideData
, of type BufferSource-
A
BufferSource
that contains theEncodedVideoChunk
's extra alpha channel data. temporalLayerId
, of type unsigned long-
A number that identifies the temporal layer for the associated
EncodedVideoChunk
.
7. Configurations
7.1. Check Configuration Support (with config)
Run these steps:-
If the codec string in config.codec is not a valid codec string or is otherwise unrecognized by the User Agent, return
false
. -
If config is an
AudioDecoderConfig
orVideoDecoderConfig
and the User Agent can’t provide a codec that can decode the exact profile (where present), level (where present), and constraint bits (where present) indicated by the codec string in config.codec, returnfalse
. -
If config is an
AudioEncoderConfig
orVideoEncoderConfig
:-
If the codec string in config.codec contains a profile and the User Agent can’t provide a codec that can encode the exact profile indicated by config.codec, return
false
. -
If the codec string in config.codec contains a level and the User Agent can’t provide a codec that can encode to a level less than or equal to the level indicated by config.codec, return
false
. -
If the codec string in config.codec contains constraint bits and the User Agent can’t provide a codec that can produce an encoded bitstream at least as constrained as indicated by config.codec, return
false
.
-
-
If the User Agent can provide a codec to support all entries of the config, including applicable default values for keys that are not included, return
true
.NOTE: The types
AudioDecoderConfig
,VideoDecoderConfig
,AudioEncoderConfig
, andVideoEncoderConfig
each define their respective configuration entries and defaults.NOTE: Support for a given configuration can change dynamically if the hardware is altered (e.g. external GPU unplugged) or if essential hardware resources are exhausted. User Agents describe support on a best-effort basis given the resources that are available at the time of the query.
-
Otherwise, return false.
7.2. Clone Configuration (with config)
NOTE: This algorithm will copy only the dictionary members that the User Agent recognizes as part of the dictionary type.
Run these steps:
-
Let dictType be the type of dictionary config.
-
Let clone be a new empty instance of dictType.
-
For each dictionary member m defined on dictType:
-
If
config[m]
is a nested dictionary, setclone[m]
to the result of recursively running the Clone Configuration algorithm withconfig[m]
. -
Otherwise, assign a copy of
config[m]
toclone[m]
.
Note: This implements a "deep-copy". These configuration objects are frequently used as the input of asynchronous operations. Copying means that modifying the original object while the operation is in flight won’t change the operation’s outcome.
7.3. Signalling Configuration Support
7.3.1. AudioDecoderSupport
dictionary {
AudioDecoderSupport boolean supported ;AudioDecoderConfig config ; };
supported
, of type boolean- A boolean indicating the whether the corresponding
config
is supported by the User Agent. config
, of type AudioDecoderConfig- An
AudioDecoderConfig
used by the User Agent in determining the value ofsupported
.
7.3.2. VideoDecoderSupport
dictionary {
VideoDecoderSupport boolean supported ;VideoDecoderConfig config ; };
supported
, of type boolean- A boolean indicating the whether the corresponding
config
is supported by the User Agent. config
, of type VideoDecoderConfig- A
VideoDecoderConfig
used by the User Agent in determining the value ofsupported
.
7.3.3. AudioEncoderSupport
dictionary {
AudioEncoderSupport boolean supported ;AudioEncoderConfig config ; };
supported
, of type boolean- A boolean indicating the whether the corresponding
config
is supported by the User Agent. config
, of type AudioEncoderConfig- An
AudioEncoderConfig
used by the User Agent in determining the value ofsupported
.
7.3.4. VideoEncoderSupport
dictionary {
VideoEncoderSupport boolean supported ;VideoEncoderConfig config ; };
supported
, of type boolean- A boolean indicating the whether the corresponding
config
is supported by the User Agent. config
, of type VideoEncoderConfig- A
VideoEncoderConfig
used by the User Agent in determining the value ofsupported
.
7.4. Codec String
A codec string describes a given codec format to be used for encoding or decoding.A valid codec string MUST meet the following conditions.
-
Is valid per the relevant codec specification (see examples below).
-
It describes a single codec.
-
It is unambiguous about codec profile, level, and constraint bits for codecs that define these concepts.
NOTE: In other media specifications, codec strings historically accompanied a MIME type as the "codecs=" parameter
(isTypeSupported()
, canPlayType()
) [RFC6381]. In this specification, encoded media is not containerized;
hence, only the value of the codecs parameter is accepted.
NOTE: Encoders for codecs that define level and constraint bits have flexibility around these parameters, but won’t produce bitstreams that have a higher level or are less constrained than requested.
The format and semantics for codec strings are defined by codec registrations listed in the [WEBCODECS-CODEC-REGISTRY]. A compliant implementation MAY support any combination of codec registrations or none at all.
7.5. AudioDecoderConfig
dictionary {
AudioDecoderConfig required DOMString codec ; [EnforceRange ]required unsigned long sampleRate ; [EnforceRange ]required unsigned long numberOfChannels ;BufferSource description ; };
To check if an AudioDecoderConfig
is a valid AudioDecoderConfig,
run these steps:
-
If
codec
is empty after stripping leading and trailing ASCII whitespace, returnfalse
. -
Return
true
.
codec
, of type DOMString- Contains a codec string in config.codec describing the codec.
sampleRate
, of type unsigned long- The number of frame samples per second.
numberOfChannels
, of type unsigned long- The number of audio channels.
description
, of type BufferSource-
A sequence of codec specific bytes, commonly known as extradata.
NOTE: The registrations in the [WEBCODECS-CODEC-REGISTRY] describe whether/how to populate this sequence, corresponding to the provided
codec
.
7.6. VideoDecoderConfig
dictionary {
VideoDecoderConfig required DOMString codec ;AllowSharedBufferSource description ; [EnforceRange ]unsigned long codedWidth ; [EnforceRange ]unsigned long codedHeight ; [EnforceRange ]unsigned long displayAspectWidth ; [EnforceRange ]unsigned long displayAspectHeight ;VideoColorSpaceInit colorSpace ;HardwareAcceleration hardwareAcceleration = "no-preference";boolean optimizeForLatency ; };
To check if a VideoDecoderConfig
is a valid VideoDecoderConfig,
run these steps:
-
If
codec
is empty after stripping leading and trailing ASCII whitespace, returnfalse
. -
If one of
codedWidth
orcodedHeight
is provided but the other isn’t, returnfalse
. -
If
codedWidth
= 0 orcodedHeight
= 0, returnfalse
. -
If one of
displayAspectWidth
ordisplayAspectHeight
is provided but the other isn’t, returnfalse
. -
If
displayAspectWidth
= 0 ordisplayAspectHeight
= 0, returnfalse
. -
Return
true
.
codec
, of type DOMString- Contains a codec string describing the codec.
description
, of type AllowSharedBufferSource-
A sequence of codec specific bytes, commonly known as extradata.
NOTE: The registrations in the [WEBCODECS-CODEC-REGISTRY] describes whether/how to populate this sequence, corresponding to the provided
codec
. codedWidth
, of type unsigned long- Width of the VideoFrame in pixels, potentially including non-visible padding, and prior to considering potential ratio adjustments.
codedHeight
, of type unsigned long-
Height of the VideoFrame in pixels, potentially including non-visible
padding, and prior to considering potential ratio adjustments.
NOTE:
codedWidth
andcodedHeight
are used when selecting a[[codec implementation]]
. displayAspectWidth
, of type unsigned long- Horizontal dimension of the VideoFrame’s aspect ratio when displayed.
displayAspectHeight
, of type unsigned long-
Vertical dimension of the VideoFrame’s aspect ratio when displayed.
NOTE:
displayWidth
anddisplayHeight
can both be different fromdisplayAspectWidth
anddisplayAspectHeight
, but have identical ratios, after scaling is applied when creating the video frame. colorSpace
, of type VideoColorSpaceInit- Configures the
VideoFrame
.colorSpace
forVideoFrame
s associated with thisVideoDecoderConfig
. IfcolorSpace
exists, the provided values will override any in-band values from the bitsream. hardwareAcceleration
, of type HardwareAcceleration, defaulting to"no-preference"
- Hint that configures hardware acceleration for this codec. See
HardwareAcceleration
. optimizeForLatency
, of type boolean-
Hint that the selected decoder SHOULD be configured to minimize the number
of
EncodedVideoChunk
s that have to be decoded before aVideoFrame
is output.NOTE: In addition to User Agent and hardware limitations, some codec bitstreams require a minimum number of inputs before any output can be produced.
7.7. AudioEncoderConfig
dictionary {
AudioEncoderConfig required DOMString codec ; [EnforceRange ]unsigned long sampleRate ; [EnforceRange ]unsigned long numberOfChannels ; [EnforceRange ]unsigned long long bitrate ;BitrateMode bitrateMode ; };
NOTE: Codec-specific extensions to AudioEncoderConfig
are described in
their registrations in the [WEBCODECS-CODEC-REGISTRY].
To check if an AudioEncoderConfig
is a valid AudioEncoderConfig,
run these steps:
-
If
codec
is empty after stripping leading and trailing ASCII whitespace, returnfalse
. -
If the
AudioEncoderConfig
has a codec-specific extension and the corresponding registration in the [WEBCODECS-CODEC-REGISTRY] defines steps to check whether the extension is a valid extension, return the result of running those steps. -
Return
true
.
codec
, of type DOMString- Contains a codec string describing the codec.
sampleRate
, of type unsigned long- The number of frame samples per second.
numberOfChannels
, of type unsigned long- The number of audio channels.
bitrate
, of type unsigned long long- The average bitrate of the encoded audio given in units of bits per second.
bitrateMode
, of type BitrateMode-
Configures the encoder to use a
constant
orvariable
bitrate as defined by [MEDIASTREAM-RECORDING].NOTE: Not all audio codecs support specific
BitrateMode
s, Authors are encouraged to check by callingisConfigSupported()
with config.
7.8. VideoEncoderConfig
dictionary {
VideoEncoderConfig required DOMString codec ; [EnforceRange ]required unsigned long width ; [EnforceRange ]required unsigned long height ; [EnforceRange ]unsigned long displayWidth ; [EnforceRange ]unsigned long displayHeight ; [EnforceRange ]unsigned long long bitrate ;double framerate ;HardwareAcceleration hardwareAcceleration = "no-preference";AlphaOption alpha = "discard";DOMString scalabilityMode ;VideoEncoderBitrateMode bitrateMode = "variable";LatencyMode latencyMode = "quality"; };
NOTE: Codec-specific extensions to VideoEncoderConfig
are described in their
registrations in the [WEBCODECS-CODEC-REGISTRY].
To check if a VideoEncoderConfig
is a valid VideoEncoderConfig,
run these steps:
-
If
codec
is empty after stripping leading and trailing ASCII whitespace, returnfalse
. -
If
displayWidth
= 0 ordisplayHeight
= 0, returnfalse
. -
Return
true
.
codec
, of type DOMString- Contains a codec string in config.codec describing the codec.
width
, of type unsigned long-
The encoded width of output
EncodedVideoChunk
s in pixels, prior to any display aspect ratio adjustments.The encoder MUST scale any
VideoFrame
whose[[visible width]]
differs from this value. height
, of type unsigned long-
The encoded height of output
EncodedVideoChunk
s in pixels, prior to any display aspect ratio adjustments.The encoder MUST scale any
VideoFrame
whose[[visible height]]
differs from this value.
displayWidth
, of type unsigned long- The intended display width of output
EncodedVideoChunk
s in pixels. Defaults towidth
if not present. displayHeight
, of type unsigned long- The intended display height of output
EncodedVideoChunk
s in pixels. Defaults towidth
if not present.
displayWidth
or displayHeight
that differs from width
and height
signals
that chunks are to be scaled after decoding to arrive at the final
display aspect ratio.
For many codecs this is merely pass-through information, but some codecs can sometimes include display sizing in the bitstream.
bitrate
, of type unsigned long long-
The average bitrate of the encoded video given in units of bits per second.
NOTE: Authors are encouraged to additionally provide a
framerate
to inform rate control. framerate
, of type double- The expected frame rate in frames per second, if known. This value, along
with the frame
timestamp
, SHOULD be used by the video encoder to calculate the optimal byte length for each encoded frame. Additionally, the value SHOULD be considered a target deadline for outputting encoding chunks whenlatencyMode
is set torealtime
. hardwareAcceleration
, of type HardwareAcceleration, defaulting to"no-preference"
- Hint that configures hardware acceleration for this codec. See
HardwareAcceleration
. alpha
, of type AlphaOption, defaulting to"discard"
- Whether the alpha component of the
VideoFrame
inputs SHOULD be kept or discarded prior to encoding. Ifalpha
is equal todiscard
, alpha data is always discarded, regardless of aVideoFrame
's[[format]]
. scalabilityMode
, of type DOMString- An encoding scalability mode identifier as defined by [WebRTC-SVC].
bitrateMode
, of type VideoEncoderBitrateMode, defaulting to"variable"
-
Configures encoding to use one of the rate control modes specified by
VideoEncoderBitrateMode
.NOTE: The precise degree of bitrate fluctuation in either mode is implementation defined.
latencyMode
, of type LatencyMode, defaulting to"quality"
- Configures latency related behaviors for this codec. See
LatencyMode
.
7.9. Hardware Acceleration
enum {
HardwareAcceleration "no-preference" ,"prefer-hardware" ,"prefer-software" , };
When supported, hardware acceleration offloads encoding or decoding to
specialized hardware. prefer-hardware
and prefer-software
are hints. While User Agents SHOULD
respect these values when possible, User Agents may ignore these values in some
or all circumstances for any reason.
To prevent fingerprinting, if a User Agent implements [media-capabilities],
the User Agent MUST ensure rejection or acceptance of a given HardwareAcceleration
preference reveals no additional information on top of
what is inherent to the User Agent and revealed by [media-capabilities]. If a
User Agent does not implement [media-capabilities] for reasons of
fingerprinting, they SHOULD ignore the HardwareAcceleration
preference.
prefer-hardware
or prefer-software
are for reasons of user privacy or
circumstances where the User Agent determines an alternative setting would
better serve the end user.
Most authors will be best served by using the default of no-preference
. This gives the User Agent flexibility
to optimize based on its knowledge of the system and configuration. A common
strategy will be to prioritize hardware acceleration at higher resolutions
with a fallback to software codecs if hardware acceleration fails.
Authors are encouraged to carefully weigh the tradeoffs when setting a hardware acceleration preference. The precise tradeoffs will be device-specific, but authors can generally expect the following:
-
Setting a value of
prefer-hardware
orprefer-software
can significantly restrict what configurations are supported. It can occur that the user’s device does not offer acceleration for any codec, or only for the most common profiles of older codecs. It can also occur that a given User Agent lacks a software based codec implementation. -
Hardware acceleration does not simply imply faster encoding / decoding. Hardware acceleration often has higher startup latency but more consistent throughput performance. Acceleration will generally reduce CPU load.
-
For decoding, hardware acceleration is often less robust to inputs that are mislabeled or violate the relevant codec specification.
-
Hardware acceleration will often be more power efficient than purely software based codecs.
-
For lower resolution content, the overhead added by hardware acceleration can yield decreased performance and power efficiency compared to purely software based codecs.
Given these tradeoffs, a good example of using "prefer-hardware" would be if an author intends to provide their own software based fallback via WebAssembly.
Alternatively, a good example of using "prefer-software" would be if an author is especially sensitive to the higher startup latency or decreased robustness generally associated with hardware acceleration.
no-preference
- Indicates that the User Agent MAY use hardware acceleration if it is available and compatible with other aspects of the codec configuration.
prefer-software
-
Indicates that the User Agent SHOULD prefer a
software codec implementation. User Agents may ignore this value for any
reason.
NOTE: This can cause the configuration to be unsupported on platforms where an unaccelerated codec is unavailable or is incompatible with other aspects of the codec configuration.
prefer-hardware
-
Indicates that the User Agent SHOULD prefer
hardware acceleration. User Agents may ignore this value for any reason.
NOTE: This can cause the configuration to be unsupported on platforms where an accelerated codec is unavailable or is incompatible with other aspects of the codec configuration.
7.10. Alpha Option
enum {
AlphaOption "keep" ,"discard" , };
Describes how the user agent SHOULD behave when dealing with alpha channels, for a variety of different operations.
keep
- Indicates that the user agent SHOULD preserve alpha
channel data for
VideoFrame
s, if it is present. discard
- Indicates that the user agent SHOULD ignore or
remove
VideoFrame
's alpha channel data.
7.11. Latency Mode
enum {
LatencyMode "quality" ,"realtime" };
quality
-
Indicates that the User Agent SHOULD optimize for encoding quality. In this mode:
realtime
-
Indicates that the User Agent SHOULD optimize for low latency. In this mode:
7.12. Configuration Equivalence
Two dictionaries are equal dictionaries if they contain the same keys and values. For nested dictionaries, apply this definition recursively.7.13. VideoEncoderEncodeOptions
dictionary {
VideoEncoderEncodeOptions boolean keyFrame =false ; };
NOTE: Codec-specific extensions to VideoEncoderEncodeOptions
are described in
their registrations in the [WEBCODECS-CODEC-REGISTRY].
keyFrame
, of type boolean, defaulting tofalse
- A value of
true
indicates that the given frame MUST be encoded as a key frame. A value offalse
indicates that the User Agent has flexibility to decide whether the frame will be encoded as a key frame.
7.14. VideoEncoderBitrateMode
enum {
VideoEncoderBitrateMode "constant" ,"variable" ,"quantizer" };
constant
- Encode at a constant bitrate. See
bitrate
. variable
- Encode using a variable bitrate, allowing more space to be used for
complex signals and less space for less complex signals.
See
bitrate
. quantizer
- Encode using a quantizer, that is specified for each video
frame in codec specific extensions of
VideoEncoderEncodeOptions
.
7.15. CodecState
enum {
CodecState "unconfigured" ,"configured" ,"closed" };
unconfigured
- The codec is not configured for encoding or decoding.
configured
- A valid configuration has been provided. The codec is ready for encoding or decoding.
closed
- The codec is no longer usable and underlying system resources have been released.
7.16. WebCodecsErrorCallback
callback =
WebCodecsErrorCallback undefined (DOMException );
error
8. Encoded Media Interfaces (Chunks)
These interfaces represent chunks of encoded media.8.1. EncodedAudioChunk Interface
[Exposed =(Window ,DedicatedWorker ),Serializable ]interface {
EncodedAudioChunk constructor (EncodedAudioChunkInit );
init readonly attribute EncodedAudioChunkType type ;readonly attribute long long timestamp ; // microsecondsreadonly attribute unsigned long long ?duration ; // microsecondsreadonly attribute unsigned long byteLength ;undefined copyTo (AllowSharedBufferSource ); };
destination dictionary {
EncodedAudioChunkInit required EncodedAudioChunkType ; [
type EnforceRange ]required long long ; // microseconds [
timestamp EnforceRange ]unsigned long long ; // microseconds
duration required BufferSource ; };
data enum {
EncodedAudioChunkType ,
"key" , };
"delta"
8.1.1. Internal Slots
[[internal data]]
-
An array of bytes representing the encoded chunk data.
[[type]]
-
Describes whether the chunk is a key chunk.
[[timestamp]]
-
The presentation timestamp, given in microseconds.
[[duration]]
-
The presentation duration, given in microseconds.
[[byte length]]
-
The byte length of
[[internal data]]
.
8.1.2. Constructors
EncodedAudioChunk(init)
-
Let chunk be a new
EncodedAudioChunk
object, initialized as follows-
Assign
init.type
to[[type]]
. -
Assign
init.timestamp
to[[timestamp]]
. -
If
init.duration
exists, assign it to[[duration]]
, or assignnull
otherwise. -
Assign a copy of
init.data
to[[internal data]]
. -
Assign
init.data.byteLength
to[[byte length]]
;
-
-
Return chunk.
8.1.3. Attributes
type
, of type EncodedAudioChunkType, readonly-
Returns the value of
[[type]]
. timestamp
, of type long long, readonly-
Returns the value of
[[timestamp]]
. duration
, of type unsigned long long, readonly, nullable-
Returns the value of
[[duration]]
. byteLength
, of type unsigned long, readonly-
Returns the value of
[[byte length]]
.
8.1.4. Methods
copyTo(destination)
-
When invoked, run these steps:
-
If the
[[byte length]]
of thisEncodedAudioChunk
is greater than in destination, throw aTypeError
. -
Copy the
[[internal data]]
into destination.
-
8.1.5. Serialization
- The
EncodedAudioChunk
serialization steps (with value, serialized, and forStorage) are: -
-
If forStorage is
true
, throw aDataCloneError
. -
For each
EncodedAudioChunk
internal slot in value, assign the value of each internal slot to a field in serialized with the same name as the internal slot.
-
- The
EncodedAudioChunk
deserialization steps (with serialized and value) are: -
-
For all named fields in serialized, assign the value of each named field to the
EncodedAudioChunk
internal slot in value with the same name as the named field.
-
NOTE: Since EncodedAudioChunk
s are immutable, User
Agents may choose to implement serialization using a reference counting
model similar to § 9.2.6 Transfer and Serialization.
8.2. EncodedVideoChunk Interface
[Exposed =(Window ,DedicatedWorker ),Serializable ]interface {
EncodedVideoChunk constructor (EncodedVideoChunkInit );
init readonly attribute EncodedVideoChunkType type ;readonly attribute long long timestamp ; // microsecondsreadonly attribute unsigned long long ?duration ; // microsecondsreadonly attribute unsigned long byteLength ;undefined copyTo (AllowSharedBufferSource ); };
destination dictionary {
EncodedVideoChunkInit required EncodedVideoChunkType ; [
type EnforceRange ]required long long ; // microseconds [
timestamp EnforceRange ]unsigned long long ; // microseconds
duration required AllowSharedBufferSource ; };
data enum {
EncodedVideoChunkType ,
"key" , };
"delta"
8.2.1. Internal Slots
[[internal data]]
-
An array of bytes representing the encoded chunk data.
[[type]]
-
The
EncodedVideoChunkType
of thisEncodedVideoChunk
; [[timestamp]]
-
The presentation timestamp, given in microseconds.
[[duration]]
-
The presentation duration, given in microseconds.
[[byte length]]
-
The byte length of
[[internal data]]
.
8.2.2. Constructors
EncodedVideoChunk(init)
-
Let chunk be a new
EncodedVideoChunk
object, initialized as follows-
Assign
init.type
to[[type]]
. -
Assign
init.timestamp
to[[timestamp]]
. -
If duration is present in init, assign
init.duration
to[[duration]]
. Otherwise, assignnull
to[[duration]]
. -
Assign a copy of
init.data
to[[internal data]]
. -
Assign
init.data.byteLength
to[[byte length]]
;
-
-
Return chunk.
8.2.3. Attributes
type
, of type EncodedVideoChunkType, readonly-
Returns the value of
[[type]]
. timestamp
, of type long long, readonly-
Returns the value of
[[timestamp]]
. duration
, of type unsigned long long, readonly, nullable-
Returns the value of
[[duration]]
. byteLength
, of type unsigned long, readonly-
Returns the value of
[[byte length]]
.
8.2.4. Methods
copyTo(destination)
-
When invoked, run these steps:
-
If
[[byte length]]
is greater than the[[byte length]]
of destination, throw aTypeError
. -
Copy the
[[internal data]]
into destination.
-
8.2.5. Serialization
- The
EncodedVideoChunk
serialization steps (with value, serialized, and forStorage) are: -
-
If forStorage is
true
, throw aDataCloneError
. -
For each
EncodedVideoChunk
internal slot in value, assign the value of each internal slot to a field in serialized with the same name as the internal slot.
-
- The
EncodedVideoChunk
deserialization steps (with serialized and value) are: -
-
For all named fields in serialized, assign the value of each named field to the
EncodedVideoChunk
internal slot in value with the same name as the named field.
-
NOTE: Since EncodedVideoChunk
s are immutable, User
Agents may choose to implement serialization using a reference counting
model similar to § 9.4.7 Transfer and Serialization.
9. Raw Media Interfaces
These interfaces represent unencoded (raw) media.9.1. Memory Model
9.1.1. Background
This section is non-normative.
Decoded media data MAY occupy a large amount of system
memory. To minimize the need for expensive copies, this specification defines a
scheme for reference counting (clone()
and close()
).
NOTE: Authors are encouraged to call close()
immediately when frames are
no longer needed.
9.1.2. Reference Counting
A media resource is storage for the actual pixel data or the audio
sample data described by a VideoFrame
or AudioData
.
The AudioData
[[resource reference]]
and VideoFrame
[[resource reference]]
internal slots hold a reference to a media resource.
VideoFrame
.clone()
and AudioData
.clone()
return new objects whose [[resource reference]]
points to the same media resource as the original
object.
VideoFrame
.close()
and AudioData
.close()
will clear their [[resource reference]]
slot, releasing the reference their media resource.
A media resource MUST remain alive at least as long
as it continues to be referenced by a [[resource reference]]
.
NOTE: When a media resource is no longer referenced by a [[resource reference]]
, the resource can be destroyed. User Agents are
encouraged to destroy such resources quickly to reduce memory pressure and
facilitate resource reuse.
9.1.3. Transfer and Serialization
This section is non-normative.
AudioData
and VideoFrame
are both transferable and serializable objects. Their transfer and
serialization steps are defined in § 9.2.6 Transfer and Serialization and § 9.4.7 Transfer and Serialization respectively.
Transferring an AudioData
or VideoFrame
moves its [[resource reference]]
to the destination object and closes (as in close()
)
the source object. Authors MAY use this facility
to move an AudioData
or VideoFrame
between realms without copying the
underlying media resource.
Serializing an AudioData
or VideoFrame
effectively clones (as in clone()
) the source object, resulting in two objects that
reference the same media resource. Authors MAY use
this facility to clone an AudioData
or VideoFrame
to another realm
without copying the underlying media resource.
9.2. AudioData Interface
[Exposed =(Window ,DedicatedWorker ),Serializable ,Transferable ]interface {
AudioData constructor (AudioDataInit );
init readonly attribute AudioSampleFormat ?format ;readonly attribute float sampleRate ;readonly attribute unsigned long numberOfFrames ;readonly attribute unsigned long numberOfChannels ;readonly attribute unsigned long long duration ; // microsecondsreadonly attribute long long timestamp ; // microsecondsunsigned long allocationSize (AudioDataCopyToOptions );
options undefined copyTo (AllowSharedBufferSource ,
destination AudioDataCopyToOptions );
options AudioData clone ();undefined close (); };dictionary {
AudioDataInit required AudioSampleFormat ;
format required float ; [
sampleRate EnforceRange ]required unsigned long ; [
numberOfFrames EnforceRange ]required unsigned long ; [
numberOfChannels EnforceRange ]required long long ; // microseconds
timestamp required BufferSource ;
data sequence <ArrayBuffer >= []; };
transfer
9.2.1. Internal Slots
[[resource reference]]
-
A reference to a media resource that stores the audio sample data for this
AudioData
. [[format]]
-
The
AudioSampleFormat
used by thisAudioData
. Will benull
whenever the underlying format does not map to anAudioSampleFormat
or when[[Detached]]
istrue
. [[sample rate]]
-
The sample-rate, in Hz, for this
AudioData
. [[number of frames]]
[[number of channels]]
-
The number of audio channels for this
AudioData
. [[timestamp]]
-
The presentation timestamp, in microseconds, for this
AudioData
.
9.2.2. Constructors
AudioData(init)
-
If init is not a valid AudioDataInit, throw a
TypeError
. -
If init.
transfer
contains more than one reference to the sameArrayBuffer
, then throw aDataCloneError
DOMException
. -
For each transferable in init.
transfer
:-
If
[[Detached]]
internal slot istrue
, then throw aDataCloneError
DOMException
.
-
-
Let frame be a new
AudioData
object, initialized as follows:-
Assign
false
to[[Detached]]
. -
Assign init.
format
to[[format]]
. -
Assign init.
sampleRate
to[[sample rate]]
. -
Assign init.
numberOfFrames
to[[number of frames]]
. -
Assign init.
numberOfChannels
to[[number of channels]]
. -
Assign init.
timestamp
to[[timestamp]]
. -
If init.
transfer
contains anArrayBuffer
referenced by init.data
the User Agent MAY choose to:-
Let resource be a new media resource referencing sample data in data.
-
-
Otherwise:
-
Let resource be a media resource containing a copy of init.
data
.
-
-
Let resourceReference be a reference to resource.
-
Assign resourceReference to
[[resource reference]]
.
-
-
For each transferable in init.
transfer
:-
Perform DetachArrayBuffer on transferable
-
-
Return frame.
9.2.3. Attributes
format
, of type AudioSampleFormat, readonly, nullable-
The
AudioSampleFormat
used by thisAudioData
. Will benull
whenever the underlying format does not map to aAudioSampleFormat
or when[[Detached]]
istrue
.The
format
getter steps are to return[[format]]
. sampleRate
, of type float, readonly-
The sample-rate, in Hz, for this
AudioData
.The
sampleRate
getter steps are to return[[sample rate]]
. numberOfFrames
, of type unsigned long, readonly-
The number of frames for this
AudioData
.The
numberOfFrames
getter steps are to return[[number of frames]]
. numberOfChannels
, of type unsigned long, readonly-
The number of audio channels for this
AudioData
.The
numberOfChannels
getter steps are to return[[number of channels]]
. timestamp
, of type long long, readonly-
The presentation timestamp, in microseconds, for this
AudioData
.The
numberOfChannels
getter steps are to return[[timestamp]]
. duration
, of type unsigned long long, readonly-
The duration, in microseconds, for this
AudioData
.The
duration
getter steps are to:-
Let microsecondsPerSecond be
1,000,000
. -
Let durationInSeconds be the result of dividing
[[number of frames]]
by[[sample rate]]
. -
Return the product of durationInSeconds and microsecondsPerSecond.
-
9.2.4. Methods
allocationSize(options)
-
Returns the number of bytes required to hold the samples as described by options.
When invoked, run these steps:
-
If
[[Detached]]
istrue
, throw anInvalidStateError
DOMException
. -
Let copyElementCount be the result of running the Compute Copy Element Count algorithm with options.
-
Let destFormat be the value of
[[format]]
. -
If options.
format
exists, assign options.format
to destFormat. -
Let bytesPerSample be the number of bytes per sample, as defined by the destFormat.
-
Return the product of multiplying bytesPerSample by copyElementCount.
-
copyTo(destination, options)
-
Copies the samples from the specified plane of the
AudioData
to the destination buffer.When invoked, run these steps:
-
If
[[Detached]]
istrue
, throw anInvalidStateError
DOMException
. -
Let copyElementCount be the result of running the Compute Copy Element Count algorithm with options.
-
Let destFormat be the value of
[[format]]
. -
If options.
format
exists, assign options.format
to destFormat. -
Let bytesPerSample be the number of bytes per sample, as defined by the destFormat.
-
If the product of multiplying bytesPerSample by copyElementCount is greater than
destination.byteLength
, throw aRangeError
. -
Let resource be the media resource referenced by
[[resource reference]]
. -
Let planeFrames be the region of resource corresponding to options.
planeIndex
. -
Copy elements of planeFrames into destination, starting with the frame positioned at options.
frameOffset
and stopping after copyElementCount samples have been copied. If destFormat does not equal[[format]]
, convert elements to the destFormatAudioSampleFormat
while making the copy.
-
clone()
-
Creates a new AudioData with a reference to the same media resource.
When invoked, run these steps:
-
If
[[Detached]]
istrue
, throw anInvalidStateError
DOMException
. -
Return the result of running the Clone AudioData algorithm with this.
-
close()
-
Clears all state and releases the reference to the media resource. Close is final.
When invoked, run the Close AudioData algorithm with this.
9.2.5. Algorithms
- Compute Copy Element Count (with options)
-
Run these steps:
-
Let destFormat be the value of
[[format]]
. -
If options.
format
exists, assign options.format
to destFormat. -
If destFormat describes an interleaved
AudioSampleFormat
and options.planeIndex
is greater than0
, throw aRangeError
. -
Otherwise, if destFormat describes a planar
AudioSampleFormat
and if options.planeIndex
is greater or equal to[[number of channels]]
, throw aRangeError
. -
If
[[format]]
does not equal destFormat and the User Agent does not support the requestedAudioSampleFormat
conversion, throw aNotSupportedError
DOMException
. Conversion tof32-planar
MUST always be supported. -
Let frameCount be the number of frames in the plane identified by options.
planeIndex
. -
If options.
frameOffset
is greater than or equal to frameCount, throw aRangeError
. -
Let copyFrameCount be the difference of subtracting options.
frameOffset
from frameCount. -
If options.
frameCount
exists:-
If options.
frameCount
is greater than copyFrameCount, throw aRangeError
. -
Otherwise, assign options.
frameCount
to copyFrameCount.
-
-
Let elementCount be copyFrameCount.
-
If destFormat describes an interleaved
AudioSampleFormat
, multiply elementCount by[[number of channels]]
-
return elementCount.
-
- Clone AudioData (with data)
-
Run these steps:
-
Let clone be a new
AudioData
initialized as follows:-
Let resource be the media resource referenced by data’s
[[resource reference]]
. -
Let reference be a new reference to resource.
-
Assign reference to
[[resource reference]]
. -
Assign the values of data’s
[[Detached]]
,[[format]]
,[[sample rate]]
,[[number of frames]]
,[[number of channels]]
, and[[timestamp]]
slots to the corresponding slots in clone.
-
-
Return clone.
-
- Close AudioData (with data)
-
Run these steps:
-
Assign
true
to data’s[[Detached]]
internal slot. -
Assign
null
to data’s[[resource reference]]
. -
Assign
0
to data’s[[sample rate]]
. -
Assign
0
to data’s[[number of frames]]
. -
Assign
0
to data’s[[number of channels]]
. -
Assign
null
to data’s[[format]]
.
-
- To check if a
AudioDataInit
is a valid AudioDataInit, run these steps: -
-
If
sampleRate
less than or equal to0
, returnfalse
. -
If
numberOfFrames
=0
, returnfalse
. -
If
numberOfChannels
=0
, returnfalse
. -
Verify
data
has enough data by running the following steps:-
Let totalSamples be the product of multiplying
numberOfFrames
bynumberOfChannels
. -
Let bytesPerSample be the number of bytes per sample, as defined by the
format
. -
Let totalSize be the product of multiplying bytesPerSample with totalSamples.
-
Let dataSize be the size in bytes of
data
. -
If dataSize is less than totalSize, return false.
-
-
Return
true
.
-
AudioDataInit
's data
's memory
layout matches the expectations of the planar or interleaved format
. There is no real way to verify whether the samples
conform to their AudioSampleFormat
. 9.2.6. Transfer and Serialization
- The
AudioData
transfer steps (with value and dataHolder) are: -
-
If value’s
[[Detached]]
istrue
, throw aDataCloneError
DOMException
. -
For all
AudioData
internal slots in value, assign the value of each internal slot to a field in dataHolder with the same name as the internal slot. -
Run the Close AudioData algorithm with value.
-
- The
AudioData
transfer-receiving steps (with dataHolder and value) are: -
-
For all named fields in dataHolder, assign the value of each named field to the
AudioData
internal slot in value with the same name as the named field.
-
- The
AudioData
serialization steps (with value, serialized, and forStorage) are: -
-
If value’s
[[Detached]]
istrue
, throw aDataCloneError
DOMException
. -
If forStorage is
true
, throw aDataCloneError
. -
Let resource be the media resource referenced by value’s
[[resource reference]]
. -
Let newReference be a new reference to resource.
-
Assign newReference to serialized.[[resource reference]].
-
For all remaining
AudioData
internal slots (excluding[[resource reference]]
) in value, assign the value of each internal slot to a field in serialized with the same name as the internal slot.
-
- The
AudioData
deserialization steps (with serialized and value) are: -
-
For all named fields in serialized, assign the value of each named field to the
AudioData
internal slot in value with the same name as the named field.
-
9.2.7. AudioDataCopyToOptions
dictionary { [
AudioDataCopyToOptions EnforceRange ]required unsigned long planeIndex ; [EnforceRange ]unsigned long frameOffset = 0; [EnforceRange ]unsigned long frameCount ;AudioSampleFormat format ; };
planeIndex
, of type unsigned long-
The index identifying the plane to copy from.
frameOffset
, of type unsigned long, defaulting to0
-
An offset into the source plane data indicating which frame to begin copying from. Defaults to
0
. frameCount
, of type unsigned long-
The number of frames to copy. If not provided, the copy will include all frames in the plane beginning with
frameOffset
. format
, of type AudioSampleFormat-
The output
AudioSampleFormat
for the destination data. If not provided, the resulting copy will use this AudioData’s[[format]]
. InvokingcopyTo()
will throw aNotSupportedError
if conversion to the requested format is not supported. Conversion from anyAudioSampleFormat
tof32-planar
MUST always be supported.NOTE: Authors seeking to integrate with [WEBAUDIO] can request
f32-planar
and use the resulting copy to create andAudioBuffer
or render viaAudioWorklet
.
9.3. Audio Sample Format
An audio sample format describes the numeric type used to represent a
single sample (e.g. 32-bit floating point) and the arrangement of samples from
different channels as either interleaved or planar. The audio
sample type refers solely to the numeric type and interval used to store
the data, this is u8
, s16
, s32
, or f32
for respectively
unsigned 8-bits, signed 16-bits, signed 32-bits, and 32-bits
floating point number. The audio buffer
arrangement refers solely to the way the samples are laid out in memory
(planar or interleaved).
A sample refers to a single value that is the magnitude of a signal at a particular point in time in a particular channel.
A frame or (sample-frame) refers to a set of values of all channels of a multi-channel signal, that happen at the exact same time.
NOTE: Consequently, if an audio signal is mono (has only one channel), a frame and a sample refer to the same thing.
All audio samples in this specification are using linear pulse-code modulation (Linear PCM): quantization levels are uniform between values.
NOTE: The Web Audio API, that is expected to be used with this specification, also uses Linear PCM.
enum {
AudioSampleFormat "u8" ,"s16" ,"s32" ,"f32" ,"u8-planar" ,"s16-planar" ,"s32-planar" ,"f32-planar" , };
u8
-
8-bit unsigned integer samples with interleaved channel arrangement.
s16
-
16-bit signed integer samples with interleaved channel arrangement.
s32
-
32-bit signed integer samples with interleaved channel arrangement.
f32
u8-planar
-
8-bit unsigned integer samples with planar channel arrangement.
s16-planar
-
16-bit signed integer samples with planar channel arrangement.
s32-planar
-
32-bit signed integer samples with planar channel arrangement.
f32-planar
9.3.1. Arrangement of audio buffer
When an AudioData
has an AudioSampleFormat
that is interleaved, the audio samples from different channels are laid out
consecutively in the same buffer, in the order described in the section § 9.3.3 Audio channel ordering. The AudioData
has a single plane, that contains a
number of elements therefore equal to [[number of frames]]
* [[number of channels]]
.
When an AudioData
has an AudioSampleFormat
that is planar, the audio samples from different channels are laid out
in different buffers, themselves arranged in an order described in the section § 9.3.3 Audio channel ordering. The AudioData
has a number of planes equal to the AudioData
's [[number of channels]]
. Each plane contains [[number of frames]]
elements.
NOTE: The Web Audio API currently uses f32-planar
exclusively.
AudioSampleFormat
s
9.3.2. Magnitude of the audio samples
The minimum value and maximum value of an audio sample, for a particular audio sample type, are the values below which (respectively above which) audio clipping might occur. They are otherwise regular types, that can hold values outside this interval during intermediate processing.
The bias value for an audio sample type is the value that often corresponds to the middle of the range (but often the range is not symmetrical). An audio buffer comprised only of values equal to the bias value is silent.
Sample type | IDL type | Minimum value | Bias value | Maximum value |
---|---|---|---|---|
u8
| octet | 0 | 128 | +255 |
s16
| short | -32768 | 0 | +32767 |
s32
| long | -2147483648 | 0 | +2147483647 |
f32
| float | -1.0 | 0.0 | +1.0 |
NOTE: There is no data type that can hold 24 bits of information conveniently, but audio content using 24-bit samples is common, so 32-bits integers are commonly used to hold 24-bit content.
AudioData
containing 24-bit samples SHOULD store those samples in s32
or f32
. When samples are stored in s32
, each sample MUST be left-shifted by 8
bits. By virtue of this process, samples outside of the valid 24-bit range
([-8388608, +8388607]) will be clipped. To avoid clipping and ensure lossless
transport, samples MAY be converted to f32
.
NOTE: While clipping is unavoidable in u8
, s16
, and s32
samples due
to their storage types, implementations SHOULD take care not to clip internally
when handling f32
samples.
9.3.3. Audio channel ordering
When decoding, the ordering of the audio channels in the resulting AudioData
MUST be the same as what is present in the EncodedAudioChunk
.
When encoding, the ordering of the audio channels in the resulting EncodedAudioChunk
MUST be the same as what is preset in the given AudioData
;
In other terms, no channel reordering is performed when encoding and decoding.
NOTE: The container either implies or specifies the channel mapping: the channel attributed to a particular channel index.
9.4. VideoFrame Interface
NOTE: VideoFrame
is a CanvasImageSource
. A VideoFrame
can be
passed to any method accepting a CanvasImageSource
, including CanvasDrawImage
's drawImage()
.
[Exposed =(Window ,DedicatedWorker ),Serializable ,Transferable ]interface {
VideoFrame constructor (CanvasImageSource ,
image optional VideoFrameInit = {});
init constructor (AllowSharedBufferSource ,
data VideoFrameBufferInit );
init readonly attribute VideoPixelFormat ?format ;readonly attribute unsigned long codedWidth ;readonly attribute unsigned long codedHeight ;readonly attribute DOMRectReadOnly ?codedRect ;readonly attribute DOMRectReadOnly ?visibleRect ;readonly attribute unsigned long displayWidth ;readonly attribute unsigned long displayHeight ;readonly attribute unsigned long long ?duration ; // microsecondsreadonly attribute long long timestamp ; // microsecondsreadonly attribute VideoColorSpace colorSpace ;VideoFrameMetadata metadata ();unsigned long allocationSize (optional VideoFrameCopyToOptions = {});
options Promise <sequence <PlaneLayout >>copyTo (AllowSharedBufferSource ,
destination optional VideoFrameCopyToOptions = {});
options VideoFrame clone ();undefined close (); };dictionary {
VideoFrameInit unsigned long long ; // microseconds
duration long long ; // microseconds
timestamp AlphaOption = "keep"; // Default matches image. May be used to efficiently crop. Will trigger // new computation of displayWidth and displayHeight using image’s pixel // aspect ratio unless an explicit displayWidth and displayHeight are given.
alpha DOMRectInit ; // Default matches image unless visibleRect is provided. [
visibleRect EnforceRange ]unsigned long ; [
displayWidth EnforceRange ]unsigned long ;
displayHeight VideoFrameMetadata ; };
metadata dictionary {
VideoFrameBufferInit required VideoPixelFormat ;
format required [EnforceRange ]unsigned long ;
codedWidth required [EnforceRange ]unsigned long ;
codedHeight required [EnforceRange ]long long ; // microseconds [
timestamp EnforceRange ]unsigned long long ; // microseconds // Default layout is tightly-packed.
duration sequence <PlaneLayout >; // Default visible rect is coded size positioned at (0,0)
layout DOMRectInit ; // Default display dimensions match visibleRect. [
visibleRect EnforceRange ]unsigned long ; [
displayWidth EnforceRange ]unsigned long ;
displayHeight VideoColorSpaceInit ;
colorSpace sequence <ArrayBuffer >= []; };
transfer dictionary { // Possible members are recorded in the VideoFrame Metadata Registry. };
VideoFrameMetadata
9.4.1. Internal Slots
[[resource reference]]
-
A reference to the media resource that stores the pixel data for this frame.
[[format]]
-
A
VideoPixelFormat
describing the pixel format of theVideoFrame
. Will benull
whenever the underlying format does not map to aVideoPixelFormat
or when[[Detached]]
istrue
. [[coded width]]
-
Width of the
VideoFrame
in pixels, potentially including non-visible padding, and prior to considering potential ratio adjustments. [[coded height]]
-
Height of the
VideoFrame
in pixels, potentially including non-visible padding, and prior to considering potential ratio adjustments. [[visible left]]
-
The number of pixels defining the left offset of the visible rectangle.
[[visible top]]
-
The number of pixels defining the top offset of the visible rectangle.
[[visible width]]
-
The width of pixels to include in visible rectangle, starting from
[[visible left]]
. [[visible height]]
-
The height of pixels to include in visible rectangle, starting from
[[visible top]]
. [[display width]]
-
Width of the
VideoFrame
when displayed after applying aspect ratio adjustments. [[display height]]
-
Height of the
VideoFrame
when displayed after applying aspect ratio adjustments. [[duration]]
-
The presentation duration, given in microseconds. The duration is copied from the
EncodedVideoChunk
corresponding to thisVideoFrame
. [[timestamp]]
-
The presentation timestamp, given in microseconds. The timestamp is copied from the
EncodedVideoChunk
corresponding to thisVideoFrame
. [[color space]]
-
The
VideoColorSpace
associated with this frame. [[metadata]]
-
The
VideoFrameMetadata
associated with this frame. Possible members are recorded in [webcodecs-video-frame-metadata-registry]. By design, allVideoFrameMetadata
properties are serializable.
9.4.2. Constructors
VideoFrame(image, init)
-
Check the usability of the image argument. If this throws an exception or returns bad, then throw an
InvalidStateError
DOMException
. -
If image is not origin-clean, then throw a
SecurityError
DOMException
. -
Let frame be a new
VideoFrame
. -
Switch on image:
NOTE: Authors are encouraged to provide a meaningful timestamp unless it is implicitly provided by the
CanvasImageSource
at construction. Interfaces that consumeVideoFrame
s can rely on this value for timing decisions. For example,VideoEncoder
can usetimestamp
values to guide rate control (seeframerate
).-
-
If image’s media data has no natural dimensions (e.g., it’s a vector graphic with no specified content size), then throw an
InvalidStateError
DOMException
. -
Let resource be a new media resource containing a copy of image’s media data. If this is an animated image, image’s bitmap data MUST only be taken from the default image of the animation (the one that the format defines is to be used when animation is not supported or is disabled), or, if there is no such image, the first frame of the animation.
-
Let width and height be the natural width and natural height of image.
-
Run the Initialize Frame With Resource and Size algorithm with init, frame, resource, width, and height
-
-
If image’s
networkState
attribute isNETWORK_EMPTY
, then throw anInvalidStateError
DOMException
. -
Let currentPlaybackFrame be the
VideoFrame
at the current playback position. -
If
metadata
does not exist in init, assign currentPlaybackFrame.[[metadata]]
to it. -
Run the Initialize Frame From Other Frame algorithm with init, frame, and currentPlaybackFrame.
-
-
-
Let resource be a new media resource containing a copy of image’s bitmap data.
NOTE: Implementers are encouraged to avoid a deep copy by using reference counting where feasible.
-
Let width be
image.width
and height beimage.height
. -
Run the Initialize Frame With Resource and Size algorithm with init, frame, resource, width, and height.
-
-
Run the Initialize Frame From Other Frame algorithm with init, frame, and image.
-
-
Return frame.
VideoFrame(data, init)
-
If init is not a valid VideoFrameBufferInit, throw a
TypeError
. -
Let defaultRect be «[ "x:" →
0
, "y" →0
, "width" → init.codedWidth
, "height" → init.codedWidth
]». -
Let overrideRect be
undefined
. -
If init.
visibleRect
exists, assign its value to overrideRect. -
Let parsedRect be the result of running the Parse Visible Rect algorithm with defaultRect, overrideRect, init.
codedWidth
, init.codedHeight
, and init.format
. -
If parsedRect is an exception, return parsedRect.
-
Let optLayout be
undefined
. -
Let combinedLayout be the result of running the Compute Layout and Allocation Size algorithm with parsedRect, init.
format
, and optLayout. -
If combinedLayout is an exception, throw combinedLayout.
-
If
data.byteLength
is less than combinedLayout’s allocationSize, throw aTypeError
. -
If init.
transfer
contains more than one reference to the sameArrayBuffer
, then throw aDataCloneError
DOMException
. -
For each transferable in init.
transfer
:-
If
[[Detached]]
internal slot istrue
, then throw aDataCloneError
DOMException
.
-
-
If init.
transfer
contains anArrayBuffer
referenced by data the User Agent MAY choose to:-
Let resource be a new media resource referencing pixel data in data.
-
-
Otherwise:
-
Let resource be a new media resource containing a copy of data. Use
visibleRect
andlayout
to determine where in data the pixels for each plane reside.The User Agent MAY choose to allocate resource with a larger coded size and plane strides to improve memory alignment. Increases will be reflected by
codedWidth
andcodedHeight
. Additionally, the User Agent MAY usevisibleRect
to copy only the visible rectangle. It MAY also reposition the visible rectangle within resource. The final position will be reflected byvisibleRect
.
-
-
For each transferable in init.
transfer
:-
Perform DetachArrayBuffer on transferable
-
-
Let resourceCodedWidth be the coded width of resource.
-
Let resourceCodedHeight be the coded height of resource.
-
Let resourceVisibleLeft be the left offset for the visible rectangle of resource.
-
Let resourceVisibleTop be the top offset for the visible rectangle of resource.
The spec SHOULD provide definitions (and possibly diagrams) for coded size, visible rectangle, and display size. See #166.
-
Let frame be a new
VideoFrame
object initialized as follows:-
Assign resourceCodedWidth, resourceCodedHeight, resourceVisibleLeft, and resourceVisibleTop to
[[coded width]]
,[[coded height]]
,[[visible left]]
, and[[visible top]]
respectively. -
If init.
visibleRect
exists:-
Let truncatedVisibleWidth be the value of
visibleRect
.width
after truncating. -
Assign truncatedVisibleWidth to
[[visible width]]
. -
Let truncatedVisibleHeight be the value of
visibleRect
.height
after truncating. -
Assign truncatedVisibleHeight to
[[visible height]]
.
-
-
Otherwise:
-
Assign
[[coded width]]
to[[visible width]]
. -
Assign
[[coded height]]
to[[visible height]]
.
-
-
If init.
displayWidth
exists, assign it to[[display width]]
. Otherwise, assign[[visible width]]
to[[display width]]
. -
If init.
displayHeight
exists, assign it to[[display height]]
. Otherwise, assign[[visible height]]
to[[display height]]
. -
Assign init’s
timestamp
andduration
to[[timestamp]]
and[[duration]]
respectively. -
Let colorSpace be
undefined
. -
If init.
colorSpace
exists, assign its value to colorSpace. -
Assign the result of running the Pick Color Space algorithm, with colorSpace and
[[format]]
, to[[color space]]
. -
Assign the result of calling Copy VideoFrame metadata with init’s
metadata
to frame.[[metadata]]
.
-
-
Return frame.
9.4.3. Attributes
format
, of type VideoPixelFormat, readonly, nullable-
Describes the arrangement of bytes in each plane as well as the number and order of the planes. Will be
null
whenever the underlying format does not map to aVideoPixelFormat
or when[[Detached]]
istrue
.The
format
getter steps are to return[[format]]
. codedWidth
, of type unsigned long, readonly-
Width of the
VideoFrame
in pixels, potentially including non-visible padding, and prior to considering potential ratio adjustments.The
codedWidth
getter steps are to return[[coded width]]
. codedHeight
, of type unsigned long, readonly-
Height of the
VideoFrame
in pixels, potentially including non-visible padding, and prior to considering potential ratio adjustments.The
codedHeight
getter steps are to return[[coded height]]
. codedRect
, of type DOMRectReadOnly, readonly, nullable-
A
DOMRectReadOnly
withwidth
andheight
matchingcodedWidth
andcodedHeight
andx
andy
at(0,0)
. Offered for convenience for use withallocationSize()
andcopyTo()
.The
codedRect
getter steps are:-
If
[[Detached]]
istrue
, returnnull
. -
Let rect be a new
DOMRectReadOnly
, initialized as follows:-
Assign
[[coded width]]
and[[coded height]]
towidth
andheight
respectively.
-
Return rect.
-
visibleRect
, of type DOMRectReadOnly, readonly, nullable-
A
DOMRectReadOnly
describing the visible rectangle of pixels for thisVideoFrame
.The
visibleRect
getter steps are:-
If
[[Detached]]
istrue
, returnnull
. -
Let rect be a new
DOMRectReadOnly
, initialized as follows:-
Assign
[[visible left]]
,[[visible top]]
,[[visible width]]
, and[[visible height]]
tox
,y
,width
, andheight
respectively.
-
-
Return rect.
-
displayWidth
, of type unsigned long, readonly-
Width of the VideoFrame when displayed after applying aspect ratio adjustments.
The
displayWidth
getter steps are to return[[display width]]
. displayHeight
, of type unsigned long, readonly-
Height of the VideoFrame when displayed after applying aspect ratio adjustments.
The
displayHeight
getter steps are to return[[display height]]
. timestamp
, of type long long, readonly-
The presentation timestamp, given in microseconds. The timestamp is copied from the
EncodedVideoChunk
corresponding to this VideoFrame.The
timestamp
getter steps are to return[[timestamp]]
. duration
, of type unsigned long long, readonly, nullable-
The presentation duration, given in microseconds. The duration is copied from the
EncodedVideoChunk
corresponding to this VideoFrame.The
duration
getter steps are to return[[duration]]
. colorSpace
, of type VideoColorSpace, readonly-
The
VideoColorSpace
associated with this frame.The
colorSpace
getter steps are to return[[color space]]
.
9.4.4. Internal Structures
A combined buffer layout is a struct that consists of:-
A allocationSize (an
unsigned long
) -
A computedLayouts (a list of computed plane layout structs).
A computed plane layout is a struct that consists of:
-
A destinationOffset (an
unsigned long
) -
A destinationStride (an
unsigned long
) -
A sourceTop (an
unsigned long
) -
A sourceHeight (an
unsigned long
) -
A sourceLeftBytes (an
unsigned long
) -
A sourceWidthBytes (an
unsigned long
)
9.4.5. Methods
allocationSize(options)
-
Returns the minimum byte length for a valid destination
BufferSource
to be used withcopyTo()
with the given options.When invoked, run these steps:
-
If
[[Detached]]
istrue
, throw anInvalidStateError
DOMException
. -
If
[[format]]
isnull
, throw aNotSupportedError
DOMException
. -
Let combinedLayout be the result of running the Parse VideoFrameCopyToOptions algorithm with options.
-
If combinedLayout is an exception, throw combinedLayout.
-
Return combinedLayout’s allocationSize.
-
copyTo(destination, options)
-
Asynchronously copies the planes of this frame into destination according to options. The format of the data is the same as this
VideoFrame
'sformat
.NOTE: Promises that are returned by several calls to
copyTo()
are not guaranteed to resolve in the order they were returned.When invoked, run these steps:
-
If
[[Detached]]
istrue
, throw anInvalidStateError
DOMException
. -
If
[[format]]
isnull
, throw aNotSupportedError
DOMException
. -
Let combinedLayout be the result of running the Parse VideoFrameCopyToOptions algorithm with options.
-
If combinedLayout is an exception, return a promise rejected with combinedLayout.
-
If
destination.byteLength
is less than combinedLayout’s allocationSize, return a promise rejected with aTypeError
. -
Let p be a new
Promise
. -
Let copyStepsQueue be the result of starting a new parallel queue.
-
Enqueue the following steps to copyStepsQueue:
-
Let resource be the media resource referenced by [[resource reference]].
-
Let numPlanes be the number of planes as defined by
[[format]]
. -
Let planeIndex be
0
. -
While planeIndex is less than combinedLayout’s numPlanes:
-
Let sourceStride be the stride of the plane in resource as identified by planeIndex.
-
Let computedLayout be the computed plane layout in combinedLayout’s computedLayouts at the position of planeIndex
-
Let sourceOffset be the product of multiplying computedLayout’s sourceTop by sourceStride
-
Add computedLayout’s sourceLeftBytes to sourceOffset.
-
Let destinationOffset be computedLayout’s destinationOffset.
-
Let rowBytes be computedLayout’s sourceWidthBytes.
-
Let row be
0
. -
While row is less than computedLayout’s sourceHeight:
-
Copy rowBytes bytes from resource starting at sourceOffset to destination starting at destinationOffset.
-
Increment sourceOffset by sourceStride.
-
Increment destinationOffset by computedLayout’s destinationStride.
-
Increment row by
1
.
-
-
Increment planeIndex by
1
.
-
-
Queue a task to resolve p.
-
-
Return p.
-
clone()
-
Creates a new
VideoFrame
with a reference to the same media resource.When invoked, run these steps:
-
If the value of frame’s
[[Detached]]
internal slot istrue
, throw anInvalidStateError
DOMException
. -
Return the result of running the Clone VideoFrame algorithm with this.
-
close()
-
Clears all state and releases the reference to the media resource. Close is final.
When invoked, run the Close VideoFrame algorithm with this.
metadata()
-
Gets the
VideoFrameMetadata
associated with this frame.When invoked, run these steps:
-
If
[[Detached]]
istrue
, throw anInvalidStateError
DOMException
. -
Return the result of calling Copy VideoFrame metadata with
[[metadata]]
.
-
9.4.6. Algorithms
- Create a VideoFrame (with output, timestamp, duration, displayAspectWidth, displayAspectHeight, and colorSpace)
-
-
Let frame be a new
VideoFrame
, constructed as follows:-
Assign
false
to[[Detached]]
. -
Let resource be the media resource described by output.
-
Let resourceReference be a reference to resource.
-
Assign resourceReference to
[[resource reference]]
. -
If output uses a recognized
VideoPixelFormat
, assign that format to[[format]]
. Otherwise, assignnull
to[[format]]
. -
Let codedWidth and codedHeight be the coded width and height of the output in pixels.
-
Let visibleLeft, visibleTop, visibleWidth, and visibleHeight be the left, top, width and height for the visible rectangle of output.
-
Let displayWidth and displayHeight be the display size of output in pixels.
-
If displayAspectWidth and displayAspectHeight are provided, increase displayWidth or displayHeight until the ratio of displayWidth to displayHeight matches the ratio of displayAspectWidth to displayAspectHeight.
-
Assign codedWidth, codedHeight, visibleLeft, visibleTop, visibleWidth, visibleHeight, displayWidth, and displayHeight to
[[coded width]]
,[[coded height]]
,[[visible left]]
,[[visible top]]
,[[visible width]]
, and[[visible height]]
respectively. -
Assign duration and timestamp to
[[duration]]
and[[timestamp]]
respectively. -
Assign
[[color space]]
with the result of running the Pick Color Space algorithm, with colorSpace and[[format]]
.
-
-
Return frame.
-
- Pick Color Space (with overrideColorSpace and format)
-
-
If overrideColorSpace is provided, return a new
VideoColorSpace
constructed with overrideColorSpace.User Agents MAY replace
null
members of the provided overrideColorSpace with guessed values as determined by implementer defined heuristics. -
Otherwise, if
[[format]]
is an RGB format return a new instance of the sRGB Color Space -
Otherwise, return a new instance of the REC709 Color Space.
-
- Validate VideoFrameInit (with format, codedWidth, and codedHeight):
-
-
If
visibleRect
exists:-
Let validAlignment be the result of running the Verify Rect Offset Alignment with format and visibleRect.
-
If validAlignment is
false
, returnfalse
. -
If any attribute of
visibleRect
is negative or not finite, returnfalse
. -
If
visibleRect
.width
==0
orvisibleRect
.height
==0
returnfalse
. -
If
visibleRect
.y
+visibleRect
.height
> codedHeight, returnfalse
. -
If
visibleRect
.x
+visibleRect
.width
> codedWidth, returnfalse
.
-
-
If codedWidth = 0 or codedHeight = 0,return
false
. -
If only one of
displayWidth
ordisplayHeight
exists, returnfalse
. -
If
displayWidth
==0
ordisplayHeight
==0
, returnfalse
. -
Return
true
.
-
- To check if a
VideoFrameBufferInit
is a valid VideoFrameBufferInit, run these steps: -
-
If
codedWidth
= 0 orcodedHeight
= 0,returnfalse
. -
If any attribute of
visibleRect
is negative or not finite, returnfalse
. -
If
visibleRect
.y
+visibleRect
.height
>codedHeight
, returnfalse
. -
If
visibleRect
.x
+visibleRect
.width
>codedWidth
, returnfalse
. -
If only one of
displayWidth
ordisplayHeight
exists, returnfalse
. -
If
displayWidth
= 0 ordisplayHeight
= 0, returnfalse
. -
Return
true
.
-
- Initialize Frame From Other Frame (with init, frame, and otherFrame)
-
-
Let format be otherFrame.
format
. -
If init.
alpha
isdiscard
, assign otherFrame.format
's equivalent opaque format format. -
Let validInit be the result of running the Validate VideoFrameInit algorithm with format and otherFrame’s
[[coded width]]
and[[coded height]]
. -
If validInit is
false
, throw aTypeError
. -
Let resource be the media resource referenced by otherFrame’s
[[resource reference]]
. -
Assign a new reference for resource to frame’s
[[resource reference]]
. -
Assign the following attributes from otherFrame to frame:
codedWidth
,codedHeight
,colorSpace
. -
Let defaultVisibleRect be the result of performing the getter steps for
visibleRect
on otherFrame. -
Let defaultDisplayWidth, and defaultDisplayHeight be otherFrame’s
[[display width]]
, and[[display height]]
respectively. -
Run the Initialize Visible Rect and Display Size algorithm with init, frame, defaultVisibleRect, defaultDisplayWidth, and defaultDisplayHeight.
-
If
duration
exists in init, assign it to frame’s[[duration]]
. Otherwise, assign otherFrame.duration
to frame’s[[duration]]
. -
If
timestamp
exists in init, assign it to frame’s[[timestamp]]
. Otherwise, assign otherFrame’stimestamp
to frame’s[[timestamp]]
. -
Assign format to frame.
[[format]]
. -
Assign the result of calling Copy VideoFrame metadata with init’s
metadata
to frame.[[metadata]]
.
-
- Initialize Frame With Resource and Size (with init, frame, resource, width and height)
-
-
Let format be
null
. -
If resource uses a recognized
VideoPixelFormat
, assign theVideoPixelFormat
of resource to format. -
Let validInit be the result of running the Validate VideoFrameInit algorithm with format, width and height.
-
If validInit is
false
, throw aTypeError
. -
Assign a new reference for resource to frame’s
[[resource reference]]
. -
If init.
alpha
isdiscard
, assign format’s equivalent opaque format to format. -
Assign format to
[[format]]
-
Assign width and height to frame’s
[[coded width]]
and[[coded height]]
respectively. -
Let defaultVisibleRect be a new
DOMRect
constructed with «[ "x:" →0
, "y" →0
, "width" → width, "height" → height ]» -
Run the Initialize Visible Rect and Display Size algorithm with init, frame, defaultVisibleRect, width, and height.
-
Assign
init
.duration
to frame’s[[duration]]
. -
Assign
init
.timestamp
to frame’s[[timestamp]]
. -
If resource has a known
VideoColorSpace
, assign its value to[[color space]]
. -
Otherwise, assign a new
VideoColorSpace
, constructed with an emptyVideoColorSpaceInit
, to[[color space]]
.
-
- Initialize Visible Rect and Display Size (with init, frame, defaultVisibleRect, defaultDisplayWidth and defaultDisplayHeight)
-
-
Let visibleRect be defaultVisibleRect.
-
If init.
visibleRect
exists, assign it to visibleRect. -
Assign visibleRect’s
x
,y
,width
, andheight
, to frame’s[[visible left]]
,[[visible top]]
,[[visible width]]
, and[[visible height]]
respectively. -
If
displayWidth
anddisplayHeight
exist in init, assign them to[[display width]]
and[[display height]]
respectively. -
Otherwise:
-
Let widthScale be the result of dividing defaultDisplayWidth by defaultVisibleRect.
width
. -
Let heightScale be the result of dividing defaultDisplayHeight by defaultVisibleRect.
height
. -
Multiply frame’s
[[visible width]]
by widthScale and round the result. Assign the rounded result to[[display width]]
. -
Multiply frame’s
[[visible height]]
by heightScale and round the result. Assign the rounded result to frame’s[[display height]]
.
-
-
- Clone VideoFrame (with frame)
-
-
Let clone be a new
VideoFrame
initialized as follows:-
Let resource be the media resource referenced by frame’s [[resource reference]].
-
Let newReference be a new reference to resource.
-
Assign newReference to clone’s
[[resource reference]]
. -
Assign all remaining internal slots of frame (excluding
[[resource reference]]
) to those of the same name in clone.
-
-
Return clone.
-
- Close VideoFrame (with frame)
-
-
Assign
null
to frame’s[[resource reference]]
. -
Assign
true
to frame’s[[Detached]]
. -
Assign
null
to frame’sformat
. -
Assign
0
to frame’s[[coded width]]
,[[coded height]]
,[[visible left]]
,[[visible top]]
,[[visible width]]
,[[visible height]]
,[[display width]]
, and[[display height]]
. -
Assign a new
VideoFrameMetadata
to frame.[[metadata]]
.
-
- Parse VideoFrameCopyToOptions (with options)
-
-
Let defaultRect be the result of performing the getter steps for
visibleRect
. -
Let overrideRect be
undefined
. -
-
Assign the value of options.
rect
to overrideRect. -
Let validAlignment be the result of running the Verify Rect Size Alignment algorithm with overrideRect and
[[format]]
. -
If validAlignment is
false
, throw aTypeError
.
-
-
Let parsedRect be the result of running the Parse Visible Rect algorithm with defaultRect, overrideRect,
[[coded width]]
,[[coded height]]
, and[[format]]
. -
If parsedRect is an exception, return parsedRect.
-
Let optLayout be
undefined
. -
Let combinedLayout be the result of running the Compute Layout and Allocation Size algorithm with parsedRect,
[[format]]
, and optLayout. -
Return combinedLayout.
-
- Verify Rect Offset Alignment (with format and rect)
-
-
If format is
null
, returntrue
. -
Let planeIndex be
0
. -
Let numPlanes be the number of planes as defined by format.
-
While planeIndex is less than numPlanes:
-
Let plane be the Plane identified by planeIndex as defined by format.
-
Let sampleWidth be the horizontal sub-sampling factor of each subsample for plane.
-
Let sampleHeight be the vertical sub-sampling factor of each subsample for plane.
-
If rect.
x
is not a multiple of sampleWidth, returnfalse
. -
If rect.
y
is not a multiple of sampleHeight, returnfalse
. -
Increment planeIndex by
1
.
-
-
Return
true
.
-
- Verify Rect Size Alignment (with format and rect)
-
-
If format is
null
, returntrue
. -
Let planeIndex be
0
. -
Let numPlanes be the number of planes as defined by format.
-
While planeIndex is less than numPlanes:
-
Let plane be the Plane identified by planeIndex as defined by format.
-
Let sampleWidth be the horizontal sub-sampling factor of each subsample for plane.
-
Let sampleHeight be the vertical sub-sampling factor of each subsample for plane.
-
If rect.
width
is not a multiple of sampleWidth, returnfalse
. -
If rect.
height
is not a multiple of sampleHeight, returnfalse
. -
Increment planeIndex by
1
.
-
-
Return
true
.
-
- Parse Visible Rect (with defaultRect, overrideRect, codedWidth, codedHeight, and format)
-
-
Let sourceRect be defaultRect
-
If overrideRect is not
undefined
:-
If either of overrideRect.
width
orheight
is0
, return aTypeError
. -
If the sum of overrideRect.
x
and overrideRect.width
is greater than codedWidth, return aTypeError
. -
If the sum of overrideRect.
y
and overrideRect.height
is greater than codedHeight, return aTypeError
. -
Assign overrideRect to sourceRect.
-
-
Let validAlignment be the result of running the Verify Rect Offset Alignment algorithm with format and sourceRect.
-
If validAlignment is
false
, throw aTypeError
. -
Return sourceRect.
-
- Compute Layout and Allocation Size (with parsedRect, format, and layout)
-
-
Let numPlanes be the number of planes as defined by format.
-
If layout is not
undefined
and its length does not equal numPlanes, throw aTypeError
. -
Let minAllocationSize be
0
. -
Let computedLayouts be a new list.
-
Let endOffsets be a new list.
-
Let planeIndex be
0
. -
While planeIndex < numPlanes:
-
Let plane be the Plane identified by planeIndex as defined by format.
-
Let sampleBytes be the number of bytes per sample for plane.
-
Let sampleWidth be the horizontal sub-sampling factor of each subsample for plane.
-
Let sampleHeight be the vertical sub-sampling factor of each subsample for plane.
-
Let sampleWidthBytes be the product of multiplying sampleWidth by sampleBytes.
-
Let computedLayout be a new computed plane layout.
-
Set computedLayout’s sourceTop to the result of the integer division of truncated parsedRect.
y
by sampleHeight. -
Set computedLayout’s sourceHeight to the result of the integer division of truncated parsedRect.
height
by sampleHeight -
Set computedLayout’s sourceLeftBytes to the result of the integer division of truncated parsedRect.
x
by sampleWidthBytes. -
Set computedLayout’s sourceWidthBytes to the result of the integer division of truncated parsedRect.
width
by sampleWidthBytes. -
If layout is not
undefined
:-
Let planeLayout be the
PlaneLayout
in layout at position planeIndex. -
If planeLayout.
stride
is less than computedLayout’s sourceWidthBytes, return aTypeError
. -
Assign planeLayout.
offset
to computedLayout’s destinationOffset. -
Assign planeLayout.
stride
to computedLayout’s destinationStride.
-
-
Otherwise:
NOTE: If an explicit layout was not provided, the following steps default to tight packing.
-
Assign minAllocationSize to computedLayout’s destinationOffset.
-
Assign computedLayout’s sourceWidthBytes to computedLayout’s destinationStride.
-
-
Let planeSize be the product of multiplying computedLayout’s destinationStride and sourceHeight.
-
Let planeEnd be the sum of planeSize and computedLayout’s destinationOffset.
-
If planeSize or planeEnd is greater than maximum range of
unsigned long
, return aTypeError
. -
Append planeEnd to endOffsets.
-
Assign the maximum of minAllocationSize and planeEnd to minAllocationSize.
NOTE: The above step uses a maximum to allow for the possibility that user specified plane offsets reorder planes.
-
Let earlierPlaneIndex be
0
. -
While earlierPlaneIndex is less than planeIndex.
-
Let earlierLayout be
computedLayouts[earlierPlaneIndex]
. -
If
endOffsets[planeIndex]
is less than or equal to earlierLayout’s destinationOffset or ifendOffsets[earlierPlaneIndex]
is less than or equal to computedLayout’s destinationOffset, continue.NOTE: If plane A ends before plane B starts, they do not overlap.
-
Otherwise, return a
TypeError
. -
Increment earlierPlaneIndex by
1
.
-
-
Append computedLayout to computedLayouts.
-
Increment planeIndex by
1
.
-
-
Let combinedLayout be a new combined buffer layout, initialized as follows:
-
Assign computedLayouts to computedLayouts.
-
Assign minAllocationSize to allocationSize.
-
-
Return combinedLayout.
-
- Copy VideoFrame metadata (with metadata)
-
-
Let metadataCopySerialized be StructuredSerialize(metadata).
-
Let metadataCopy be StructuredDeserialize(metadataCopySerialized, the current Realm).
-
return metadataCopy.
-
The goal of this algorithm is to ensure that metadata owned by a VideoFrame
is immutable.
9.4.7. Transfer and Serialization
- The
VideoFrame
transfer steps (with value and dataHolder) are: -
-
If value’s
[[Detached]]
istrue
, throw aDataCloneError
DOMException
. -
For all
VideoFrame
internal slots in value, assign the value of each internal slot to a field in dataHolder with the same name as the internal slot. -
Run the Close VideoFrame algorithm with value.
-
- The
VideoFrame
transfer-receiving steps (with dataHolder and value) are: -
-
For all named fields in dataHolder, assign the value of each named field to the
VideoFrame
internal slot in value with the same name as the named field.
-
- The
VideoFrame
serialization steps (with value, serialized, and forStorage) are: -
-
If value’s
[[Detached]]
istrue
, throw aDataCloneError
DOMException
. -
If forStorage is
true
, throw aDataCloneError
. -
Let resource be the media resource referenced by value’s
[[resource reference]]
. -
Let newReference be a new reference to resource.
-
Assign newReference to serialized.[[resource reference]].
-
For all remaining
VideoFrame
internal slots (excluding[[resource reference]]
) in value, assign the value of each internal slot to a field in serialized with the same name as the internal slot.
-
- The
VideoFrame
deserialization steps (with serialized and value) are: -
-
For all named fields in serialized, assign the value of each named field to the
VideoFrame
internal slot in value with the same name as the named field.
-
9.4.8. Rendering
When rendered, for example by CanvasDrawImage
drawImage()
, a VideoFrame
MUST be converted to a color space compatible with the rendering target, unless color
conversion is explicitly disabled.
Color space conversion during ImageBitmap
construction is controlled by ImageBitmapOptions
colorSpaceConversion
. Setting this
value to "none"
disables color space conversion.
9.5. VideoFrame CopyTo() Options
Options to specify a rectangle of pixels to copy and the offset and stride of planes in the destination buffer.dictionary {
VideoFrameCopyToOptions DOMRectInit rect ;sequence <PlaneLayout >layout ; };
copyTo()
or allocationSize()
will enforce the following requirements:
-
The coordinates of
rect
are sample-aligned as determined by[[format]]
. -
If
layout
exists, aPlaneLayout
is provided for all planes.
rect
, of type DOMRectInit-
A
DOMRectInit
describing the rectangle of pixels to copy from theVideoFrame
. If unspecified, thevisibleRect
will be used.NOTE: The coded rectangle can be specified by passing
VideoFrame
'scodedRect
.NOTE: The default
rect
does not necessarily meet the sample-alignment requirement and can result incopyTo()
orallocationSize()
rejecting. layout
, of type sequence<PlaneLayout>-
The
PlaneLayout
for each plane inVideoFrame
, affording the option to specify an offset and stride for each plane in the destinationBufferSource
. If unspecified, the planes will be tightly packed. It is invalid to specify planes that overlap.
9.6. DOMRects in VideoFrame
TheVideoFrame
interface uses DOMRect
s to specify the position and
dimensions for a rectangle of pixels. DOMRectInit
is used with copyTo()
and allocationSize()
to describe the
dimensions of the source rectangle. VideoFrame
defines codedRect
and visibleRect
for convenient copying
of the coded size and visible region respectively.
NOTE: VideoFrame pixels are only addressable by integer numbers. All floating
point values provided to DOMRectInit
will be truncated.
9.7. Plane Layout
APlaneLayout
is a dictionary specifying the offset and stride of a VideoFrame
plane once copied to a BufferSource
. A sequence of PlaneLayout
s MAY be provided to VideoFrame
's copyTo()
to specify how the plane is laid out in the destination BufferSource
}. Alternatively, callers can inspect copyTo()
's
returned sequence of PlaneLayout
s to learn the offset and stride for
planes as decided by the User Agent.
dictionary { [
PlaneLayout EnforceRange ]required unsigned long offset ; [EnforceRange ]required unsigned long stride ; };
offset
, of type unsigned long-
The offset in bytes where the given plane begins within a
BufferSource
. stride
, of type unsigned long-
The number of bytes, including padding, used by each row of the plane within a
BufferSource
.
9.8. Pixel Format
Pixel formats describe the arrangement of bytes in each plane as well as the number and order of the planes. Each format is described in its own sub-section.enum { // 4:2:0 Y, U, V
VideoPixelFormat "I420" , // 4:2:0 Y, U, V, A"I420A" , // 4:2:2 Y, U, V"I422" , // 4:4:4 Y, U, V"I444" , // 4:2:0 Y, UV"NV12" , // 32bpp RGBA"RGBA" , // 32bpp RGBX (opaque)"RGBX" , // 32bpp BGRA"BGRA" , // 32bpp BGRX (opaque)"BGRX" , };
Sub-sampling is a technique where a single sample contains information for multiple pixels in the final image. Sub-sampling can be horizontal, vertical or both, and has a factor, that is the number of final pixels in the image that are derived from a sub-sampled sample.
VideoFrame
is in I420
format, then the very first
component of the second plane (the U plane) corresponds to four pixels, that are
the pixels in the top-left angle of the image. Consequently, the first
component of the second row corresponds to the four pixels below those initial
four top-left pixels. The sub-sampling factor is 2 in both the horizontal
and vertical direction. If a VideoPixelFormat
has an alpha component, the format’s equivalent opaque format is the same VideoPixelFormat
, without an
alpha component. If a VideoPixelFormat
does not have an alpha component, it
is its own equivalent opaque format.
I420
-
This format is composed of three distinct planes, one plane of Luma and two
planes of Chroma, denoted Y, U and V, and present in this order. It is also
often refered to as Planar YUV 4:2:0.
The U an V planes are sub-sampled horizontally and vertically by a factor of 2 compared to the Y plane.
Each sample in this format is 8 bits.
There are
codedWidth
*codedHeight
samples (and therefore bytes) in the Y plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.There is
codedWidth
*codedHeight
/ 4 samples (and therefore bytes) in the two U and V planes, arranged starting at the top left in the image, incodedHeight
/ 2 lines ofcodedWidth
/ 2 samples.The
codedWidth
andcodedHeight
MUST be even. Similarly, the visible rectangle offset (visibleRect
.x
andvisibleRect
.y
) MUST be even. I420A
-
This format is composed of four distinct planes, one plane of Luma, two planes of Chroma, denoted Y, U and V, and one place of alpha values, all present in this order. It is also often refered to as Planar YUV 4:2:0 with an alpha channel.
The U an V planes are sub-sampled horizontally and vertically by a factor of 2 compared to the Y and Alpha planes.
Each sample in this format is 8 bits.
There are
codedWidth
*codedHeight
samples (and therefore bytes) in the Y and alpha plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.There are
codedWidth
*codedHeight
/ 4 samples (and therefore bytes) in the two U and V planes, arranged starting at the top left in the image, incodedHeight
/ 2 lines ofcodedWidth
/ 2 samples.The
codedWidth
andcodedHeight
MUST be even. Similarly, the visible rectangle offset (visibleRect
.x
andvisibleRect
.y
) MUST be even. I422
-
This format is composed of three distinct planes, one plane of Luma and two planes of Chroma, denoted Y, U and V, and present in this order. It is also often refered to as Planar YUV 4:2:2.
The U an V planes are sub-sampled horizontally by a factor of 2 compared to the Y plane, and not sub-sampled vertically.
Each sample in this format is 8 bits.
There are
codedWidth
*codedHeight
samples (and therefore bytes) in the Y plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.There are
codedWidth
*codedHeight
/ 2 samples (and therefore bytes) in the two U and V planes, arranged starting at the top left in the image, incodedHeight
/ 2 lines ofcodedWidth
samples.The
codedHeight
MUST be even. Similarly, the visible rectangle offset (visibleRect
.x
andvisibleRect
.y
) MUST be even. I444
-
This format is composed of three distinct planes, one plane of Luma and two planes of Chroma, denoted Y, U and V, and present in this order. It is also often refered to as Planar YUV 4:4:4.
Each sample in this format is 8 bits. This format does not use sub-sampling.
There are
codedWidth
*codedHeight
samples (and therefore bytes) in all three planes, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples. NV12
-
This format is composed of two distinct planes, one plane of Luma and then another plane for the two Chroma components. The two planes are present in this order, and are refered to as respectively the Y plane and the UV plane.
The U an V components are sub-sampled horizontally and vertically by a factor of 2 compared to the components in the Y planes.
Each sample in this format is 8 bits.
There are
codedWidth
*codedHeight
samples (and therefore bytes) in the Y plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.The UV planes is composed of interleaved U and V values, in
codedWidth
*codedHeight
/ 4 elements (of two bytes each), arranged starting at the top left in the image, incodedHeight
/ 2 lines ofcodedWidth
/ 2 elements. Each element is composed of a two Chroma values, the U and V value, in this order.The
codedWidth
andcodedHeight
MUST be even. Similarly, the visible rectangle offset (visibleRect
.x
andvisibleRect
.y
) MUST be even.An image in the NV12 pixel format that is 16 pixels wide and 10 pixels tall will be arranged like so in memory:YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY YYYYYYYYYYYYYYYY UVUVUVUVUVUVUVUV UVUVUVUVUVUVUVUV UVUVUVUVUVUVUVUV UVUVUVUVUVUVUVUV UVUVUVUVUVUVUVUV
All samples being linear in memory.
RGBA
-
This format is composed of a single plane, that encodes four components: Red, Green, Blue, and an alpha value, present in this order.
Each sample in this format is 8 bits, and each pixel is therefore 32 bits.
There are
codedWidth
*codedHeight
* 4 samples (and therefore bytes) in the single plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.RGBA
's equivalent opaque format isRGBX
. RGBX
-
This format is composed of a single plane, that encodes four components: Red, Green, Blue, and a padding value, present in this order.
Each sample in this format is 8 bits. The fourth element in each pixel is to be ignored, the image is always fully opaque.
There are
codedWidth
*codedHeight
* 4 samples (and therefore bytes) in the single plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples. BGRA
-
This format is composed of a single plane, that encodes four components: Blue, Green, Red, and an alpha value, present in this order.
Each sample in this format is 8 bits.
There are
codedWidth
*codedHeight
* 4 samples (and therefore bytes) in the single plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.BGRA
's equivalent opaque format isBGRX
. BGRX
-
This format is composed of a single plane, that encodes four components: Blue, Green, Red, and a padding value, present in this order.
Each sample in this format is 8 bits. The fourth element in each pixel is to be ignored, the image is always fully opaque.
There are
codedWidth
*codedHeight
* 4 samples (and therefore bytes) in the single plane, arranged starting at the top left in the image, incodedHeight
lines ofcodedWidth
samples.
9.9. Video Color Space Interface
[Exposed =(Window ,DedicatedWorker )]interface {
VideoColorSpace constructor (optional VideoColorSpaceInit = {});
init readonly attribute VideoColorPrimaries ?primaries ;readonly attribute VideoTransferCharacteristics ?transfer ;readonly attribute VideoMatrixCoefficients ?matrix ;readonly attribute boolean ?fullRange ; [Default ]VideoColorSpaceInit (); };
toJSON dictionary {
VideoColorSpaceInit VideoColorPrimaries ?=
primaries null ;VideoTransferCharacteristics ?=
transfer null ;VideoMatrixCoefficients ?=
matrix null ;boolean ?=
fullRange null ; };
9.9.1. Internal Slots
[[primaries]]
-
The color primaries.
[[transfer]]
-
The transfer characteristics.
[[matrix]]
-
The matrix coefficients.
[[full range]]
-
Indicates whether full-range color values are used.
9.9.2. Constructors
VideoColorSpace(init)
-
Let c be a new
VideoColorSpace
object, initialized as follows:-
Assign
init.primaries
to[[primaries]]
. -
Assign
init.transfer
to[[transfer]]
. -
Assign
init.matrix
to[[matrix]]
. -
Assign
init.fullRange
to[[full range]]
.
-
-
Return c.
9.9.3. Attributes
primaries
, of type VideoColorPrimaries, readonly, nullable-
The
primaries
getter steps are to return the value of[[primaries]]
. transfer
, of type VideoTransferCharacteristics, readonly, nullable-
The
transfer
getter steps are to return the value of[[transfer]]
. matrix
, of type VideoMatrixCoefficients, readonly, nullable-
The
matrix
getter steps are to return the value of[[matrix]]
. fullRange
, of type boolean, readonly, nullable-
The
fullRange
getter steps are to return the value of[[full range]]
.
9.10. Video Color Primaries
Color primaries describe the color gamut of video samples.enum {
VideoColorPrimaries "bt709" ,"bt470bg" ,"smpte170m" ,"bt2020" ,"smpte432" , };
bt709
- Color primaries used by BT.709 and sRGB, as described by [H.273] section 8.1 table 2 value 1.
bt470bg
- Color primaries used by BT.601 PAL, as described by [H.273] section 8.1 table 2 value 5.
smpte170m
- Color primaries used by BT.601 NTSC, as described by [H.273] section 8.1 table 2 value 6.
bt2020
- Color primaries used by BT.2020 and BT.2100, as described by [H.273] section 8.1 table 2 value 9.
smpte432
- Color primaries used by P3 D65, as described by [H.273] section 8.1 table 2 value 12.
9.11. Video Transfer Characteristics
Transfer characteristics describe the opto-electronic transfer characteristics of video samples.enum {
VideoTransferCharacteristics "bt709" ,"smpte170m" ,"iec61966-2-1" ,"linear" ,"pq" ,"hlg" , };
bt709
- Transfer characteristics used by BT.709, as described by [H.273] section 8.2 table 3 value 1.
smpte170m
- Transfer characteristics used by BT.601, as described by [H.273] section 8.2 table 3 value 6. (Functionally the same as "bt709".)
iec61966-2-1
- Transfer characteristics used by sRGB, as described by [H.273] section 8.2 table 3 value 13.
linear
- Transfer characteristics used by linear RGB, as described by [H.273] section 8.2 table 3 value 8.
pq
- Transfer characteristics used by BT.2100 PQ, as described by [H.273] section 8.2 table 3 value 16.
hlg
- Transfer characteristics used by BT.2100 HLG, as described by [H.273] section 8.2 table 3 value 18.
9.12. Video Matrix Coefficients
Matrix coefficients describe the relationship between sample component values and color coordinates.enum {
VideoMatrixCoefficients "rgb" ,"bt709" ,"bt470bg" ,"smpte170m" ,"bt2020-ncl" , };
rgb
- Matrix coefficients used by sRGB, as described by [H.273] section 8.3 table 4 value 0.
bt709
- Matrix coefficients used by BT.709, as described by [H.273] section 8.3 table 4 value 1.
bt470bg
- Matrix coefficients used by BT.601 PAL, as described by [H.273] section 8.3 table 4 value 5.
smpte170m
- Matrix coefficients used by BT.601 NTSC, as described by [H.273] section 8.3 table 4 value 6. (Functionally the same as "bt470bg".)
bt2020-ncl
- Matrix coefficients used by BT.2020 NCL, as described by [H.273] section 8.3 table 4 value 9.
10. Image Decoding
10.1. Background
Image codec definitions are typically accompanied by a definition for a
corresponding file format. Hence image decoders often perform both duties of
unpacking (demuxing) as well as decoding the encoded image data. The WebCodecs ImageDecoder
follows this pattern, which motivates an interface design that
is notably different from that of VideoDecoder
and AudioDecoder
.
In spite of these differences, ImageDecoder
uses the same codec processing model as the other codec interfaces. Additionally, ImageDecoder
uses the VideoFrame
interface to describe decoded outputs.
10.2. ImageDecoder Interface
[Exposed =(Window ,DedicatedWorker ),SecureContext ]interface {
ImageDecoder constructor (ImageDecoderInit );
init readonly attribute DOMString type ;readonly attribute boolean complete ;readonly attribute Promise <undefined >completed ;readonly attribute ImageTrackList tracks ;Promise <ImageDecodeResult >decode (optional ImageDecodeOptions = {});
options undefined reset ();undefined close ();static Promise <boolean >isTypeSupported (DOMString ); };
type
10.2.1. Internal Slots
[[control message queue]]
-
A queue of control messages to be performed upon this codec instance. See [[control message queue]].
[[message queue blocked]]
-
A boolean indicating when processing the
[[control message queue]]
is blocked by a pending control message. See [[message queue blocked]]. [[codec work queue]]
-
A parallel queue used for running parallel steps that reference the
[[codec implementation]]
. See [[codec work queue]]. [[ImageTrackList]]
-
An
ImageTrackList
describing the tracks found in[[encoded data]]
[[type]]
-
A string reflecting the value of the MIME
type
given at construction. [[complete]]
-
A boolean indicating whether
[[encoded data]]
is completely buffered. [[completed promise]]
-
The promise used to signal when
[[complete]]
becomestrue
. [[codec implementation]]
-
An underlying image decoder implementation provided by the User Agent. See [[codec implementation]].
[[encoded data]]
-
A byte sequence containing the encoded image data to be decoded.
[[prefer animation]]
-
A boolean reflecting the value of
preferAnimation
given at construction. [[pending decode promises]]
-
A list of unresolved promises returned by calls to decode().
[[internal selected track index]]
-
Identifies the image track within
[[encoded data]]
that is used by decoding algorithms. [[tracks established]]
-
A boolean indicating whether the track list has been established in
[[ImageTrackList]]
. [[closed]]
-
A boolean indicating that the
ImageDecoder
is in a permanent closed state and can no longer be used. [[progressive frame generations]]
-
A mapping of frame indices to Progressive Image Frame Generations. The values represent the Progressive Image Frame Generation for the
VideoFrame
which was most recently output by a call todecode()
with the given frame index.
10.2.2. Constructor
ImageDecoder(init)
-
NOTE: Calling
decode()
on the constructedImageDecoder
will trigger aNotSupportedError
if the User Agent does not support type. Authors are encouraged to first check support by callingisTypeSupported()
with type. User Agents don’t have to support any particular type.When invoked, run these steps:
-
If init is not valid ImageDecoderInit, throw a
TypeError
. -
If init.
transfer
contains more than one reference to the sameArrayBuffer
, then throw aDataCloneError
DOMException
. -
For each transferable in init.
transfer
:-
If
[[Detached]]
internal slot istrue
, then throw aDataCloneError
DOMException
.
-
-
Let d be a new
ImageDecoder
object. In the steps below, all mentions ofImageDecoder
members apply to d unless stated otherwise. -
Assign a new queue to
[[control message queue]]
. -
Assign
false
to[[message queue blocked]]
. -
Assign the result of starting a new parallel queue to
[[codec work queue]]
. -
Assign
[[ImageTrackList]]
a newImageTrackList
initialized as follows:-
Assign a new list to
[[track list]]
. -
Assign
-1
to[[selected index]]
.
-
-
Assign
null
to[[codec implementation]]
. -
If
init.preferAnimation
exists, assigninit.preferAnimation
to the[[prefer animation]]
internal slot. Otherwise, assign 'null' to[[prefer animation]]
internal slot. -
Assign a new list to
[[pending decode promises]]
. -
Assign
-1
to[[internal selected track index]]
. -
Assign
false
to[[tracks established]]
. -
Assign
false
to[[closed]]
. -
Assign a new map to
[[progressive frame generations]]
. -
If init’s
data
member is of typeReadableStream
:-
Assign a new list to
[[encoded data]]
. -
Assign
false
to[[complete]]
-
Queue a control message to configure the image decoder with init.
-
Let reader be the result of getting a reader for
data
. -
In parallel, perform the Fetch Stream Data Loop on d with reader.
-
-
Otherwise:
-
Assert that
init.data
is of typeBufferSource
. -
If init.
transfer
contains anArrayBuffer
referenced by init.data
the User Agent MAY choose to:-
Let
[[encoded data]]
reference bytes in data representing an encoded image.
-
-
Otherwise:
-
Assign a copy of
init.data
to[[encoded data]]
.
-
-
Assign
true
to[[complete]]
. -
Resolve
[[completed promise]]
. -
Queue a control message to configure the image decoder with init.
-
Queue a control message to decode track metadata.
-
-
For each transferable in init.
transfer
:-
Perform DetachArrayBuffer on transferable
-
-
return d.
Running a control message to configure the image decoder means running these steps:
-
Let supported be the result of running the Check Type Support algorithm with
init.type
. -
If supported is
false
, run the Close ImageDecoder algorithm with aNotSupportedError
DOMException
and return"processed"
. -
Otherwise, assign the
[[codec implementation]]
internal slot with an implementation supportinginit.type
-
Assign
true
to[[message queue blocked]]
. -
Enqueue the following steps to the
[[codec work queue]]
:-
Configure
[[codec implementation]]
in accordance with the values given forcolorSpaceConversion
,desiredWidth
, anddesiredHeight
. -
Assign
false
to[[message queue blocked]]
.
-
-
Return
"processed"
.
Running a control message to decode track metadata means running these steps:
-
Enqueue the following steps to the
[[codec work queue]]
:-
Run the Establish Tracks algorithm.
-
-
10.2.3. Attributes
type
, of type DOMString, readonly-
A string reflecting the value of the MIME
type
given at construction. complete
, of type boolean, readonly-
Indicates whether
[[encoded data]]
is completely buffered.The
complete
getter steps are to return[[complete]]
. completed
, of type Promise<undefined>, readonly-
The promise used to signal when
complete
becomestrue
.The
completed
getter steps are to return[[completed promise]]
. tracks
, of type ImageTrackList, readonly-
Returns a live
ImageTrackList
, which provides metadata for the available tracks and a mechanism for selecting a track to decode.The
tracks
getter steps are to return[[ImageTrackList]]
.
10.2.4. Methods
decode(options)
-
Enqueues a control message to decode the frame according to options.
When invoked, run these steps:
-
If
[[closed]]
istrue
, return aPromise
rejected with anInvalidStateError
DOMException
. -
If
[[ImageTrackList]]
's[[selected index]]
is '-1', return aPromise
rejected with anInvalidStateError
DOMException
. -
If options is
undefined
, assign a newImageDecodeOptions
to options. -
Let promise be a new
Promise
. -
Append promise to
[[pending decode promises]]
. -
Queue a control message to decode the image with options, and promise.
-
Return promise.
Running a control message to decode the image means running these steps:
-
Enqueue the following steps to the
[[codec work queue]]
:-
Wait for
[[tracks established]]
to becometrue
. -
If options.
completeFramesOnly
isfalse
and the image is a Progressive Image for which the User Agent supports progressive decoding, run the Decode Progressive Frame algorithm with options.frameIndex
and promise. -
Otherwise, run the Decode Complete Frame algorithm with options.
frameIndex
and promise.
-
-
reset()
-
Immediately aborts all pending work.
When invoked, run the Reset ImageDecoder algorithm with an
AbortError
DOMException
. close()
-
Immediately aborts all pending work and releases system resources. Close is final.
When invoked, run the Close ImageDecoder algorithm with an
AbortError
DOMException
. isTypeSupported(type)
-
Returns a promise indicating whether the provided config is supported by the User Agent.
When invoked, run these steps:
-
If type is not a valid image MIME type, return a
Promise
rejected withTypeError
. -
Let p be a new
Promise
. -
In parallel, resolve p with the result of running the Check Type Support algorithm with type.
-
Return p.
-
10.2.5. Algorithms
- Fetch Stream Data Loop (with reader)
-
Run these steps:
-
Let readRequest be the following read request.
- chunk steps, given chunk
-
-
If
[[closed]]
istrue
, abort these steps. -
If chunk is not a Uint8Array object, queue a task to run the Close ImageDecoder algorithm with a
DataError
DOMException
and abort these steps. -
Let bytes be the byte sequence represented by the Uint8Array object.
-
Append bytes to the
[[encoded data]]
internal slot. -
If
[[tracks established]]
isfalse
, run the Establish Tracks algorithm. -
Otherwise, run the Update Tracks algorithm.
-
Run the Fetch Stream Data Loop algorithm with reader.
-
- close steps
-
-
Assign
true
to[[complete]]
-
Resolve
[[completed promise]]
.
-
- error steps
-
-
Queue a task to run the Close ImageDecoder algorithm with a
NotReadableError
DOMException
-
-
Read a chunk from reader given readRequest.
-
- Establish Tracks
-
Run these steps:
-
Assert
[[tracks established]]
isfalse
. -
If
[[encoded data]]
does not contain enough data to determine the number of tracks:-
If
complete
istrue
, queue a task to run the Close ImageDecoder algorithm. -
Abort these steps.
-
-
If the number of tracks is found to be
0
, queue a task to run the Close ImageDecoder algorithm and abort these steps. -
Let newTrackList be a new list.
-
For each image track found in
[[encoded data]]
:-
Let newTrack be a new
ImageTrack
, initialized as follows:-
Assign this to
[[ImageDecoder]]
. -
Assign
tracks
to[[ImageTrackList]]
. -
If image track is found to be animated, assign
true
to newTrack’s[[animated]]
internal slot. Otherwise, assignfalse
. -
If image track is found to describe a frame count, assign that count to newTrack’s
[[frame count]]
internal slot. Otherwise, assign0
.NOTE: If this was constructed with
data
as aReadableStream
, theframeCount
can change as additional bytes are appended to[[encoded data]]
. See the Update Tracks algorithm. -
If image track is found to describe a repetition count, assign that count to
[[repetition count]]
internal slot. Otherwise, assign0
.NOTE: A value of
Infinity
indicates infinite repetitions. -
Assign
false
to newTrack’s[[selected]]
internal slot.
-
-
Append newTrack to newTrackList.
-
-
Let selectedTrackIndex be the result of running the Get Default Selected Track Index algorithm with newTrackList.
-
Let selectedTrack be the track at position selectedTrackIndex within newTrackList.
-
Assign
true
to selectedTrack’s[[selected]]
internal slot. -
Assign selectedTrackIndex to
[[internal selected track index]]
. -
Assign
true
to[[tracks established]]
. -
Queue a task to perform the following steps:
-
Assign newTrackList to the
tracks
[[track list]]
internal slot. -
Assign selectedTrackIndex to
tracks
[[selected index]]
. -
Resolve
[[ready promise]]
.
-
-
- Get Default Selected Track Index (with trackList)
-
Run these steps:
-
If
[[encoded data]]
identifies a Primary Image Track:-
Let primaryTrack be the
ImageTrack
from trackList that describes the Primary Image Track. -
Let primaryTrackIndex be position of primaryTrack within trackList.
-
If
[[prefer animation]]
isnull
, return primaryTrackIndex. -
If primaryTrack.
animated
equals[[prefer animation]]
, return primaryTrackIndex.
-
-
If any
ImageTrack
s in trackList haveanimated
equal to[[prefer animation]]
, return the position of the earliest such track in trackList. -
Return
0
.
-
- Update Tracks
-
A track update struct is a struct that consists of a track index (
unsigned long
) and a frame count (unsigned long
).Run these steps:
-
Assert
[[tracks established]]
istrue
. -
Let trackChanges be a new list.
-
Let trackList be a copy of
tracks
'[[track list]]
. -
For each track in trackList:
-
Let trackIndex be the position of track in trackList.
-
Let latestFrameCount be the frame count as indicated by
[[encoded data]]
for the track corresponding to track. -
Assert that latestFrameCount is greater than or equal to
track.frameCount
. -
If latestFrameCount is greater than
track.frameCount
:-
Let change be a track update struct whose track index is trackIndex and frame count is latestFrameCount.
-
Append change to tracksChanges.
-
-
-
If tracksChanges is empty, abort these steps.
-
Queue a task to perform the following steps:
-
For each update in trackChanges:
-
Let updateTrack be the
ImageTrack
at positionupdate.trackIndex
withintracks
'[[track list]]
. -
Assign
update.frameCount
to updateTrack’s[[frame count]]
.
-
-
-
- Decode Complete Frame (with frameIndex and promise)
-
-
Assert that
[[tracks established]]
istrue
. -
Assert that
[[internal selected track index]]
is not-1
. -
Let encodedFrame be the encoded frame identified by frameIndex and
[[internal selected track index]]
. -
Wait for any of the following conditions to be true (whichever happens first):
-
[[encoded data]]
contains enough bytes to completely decode encodedFrame. -
[[encoded data]]
is found to be malformed. -
complete
istrue
. -
[[closed]]
istrue
.
-
-
If
[[encoded data]]
is found to be malformed, run the Fatally Reject Bad Data algorithm and abort these steps. -
If
[[encoded data]]
does not contain enough bytes to completely decode encodedFrame, run the Reject Infeasible Decode algorithm with promise and abort these steps. -
Attempt to use
[[codec implementation]]
to decode encodedFrame. -
If decoding produces an error, run the Fatally Reject Bad Data algorithm and abort these steps.
-
If
[[progressive frame generations]]
contains an entry keyed by frameIndex, remove the entry from the map. -
Let output be the decoded image data emitted by
[[codec implementation]]
corresponding to encodedFrame. -
Let decodeResult be a new
ImageDecodeResult
initialized as follows:-
Assign 'true' to
complete
. -
Let duration be the presentation duration for output as described by encodedFrame. If encodedFrame does not have a duration, assign
null
to duration. -
Let timestamp be the presentation timestamp for output as described by encodedFrame. If encodedFrame does not have a timestamp:
-
If encodedFrame is a still image assign
0
to timestamp. -
If encodedFrame is a constant rate animated image and duration is not
null
, assign|frameIndex| * |duration|
to timestamp. -
If a timestamp can otherwise be trivially generated from metadata without further decoding, assign that to timestamp.
-
Otherwise, assign
0
to timestamp.
-
-
Assign
image
with the result of running the Create a VideoFrame algorithm with output, timestamp, and duration.
-
-
Run the Resolve Decode algorithm with promise and decodeResult.
-
- Decode Progressive Frame (with frameIndex and promise)
-
-
Assert that
[[tracks established]]
istrue
. -
Assert that
[[internal selected track index]]
is not-1
. -
Let encodedFrame be the encoded frame identified by frameIndex and
[[internal selected track index]]
. -
Let lastFrameGeneration be
null
. -
If
[[progressive frame generations]]
contains a map entry with the key frameIndex, assign the value of the map entry to lastFrameGeneration. -
Wait for any of the following conditions to be true (whichever happens first):
-
[[encoded data]]
contains enough bytes to decode encodedFrame to produce an output whose Progressive Image Frame Generation exceeds lastFrameGeneration. -
[[encoded data]]
is found to be malformed. -
complete
istrue
. -
[[closed]]
istrue
.
-
-
If
[[encoded data]]
is found to be malformed, run the Fatally Reject Bad Data algorithm and abort these steps. -
Otherwise, if
[[encoded data]]
does not contain enough bytes to decode encodedFrame to produce an output whose Progressive Image Frame Generation exceeds lastFrameGeneration, run the Reject Infeasible Decode algorithm with promise and abort these steps. -
Attempt to use
[[codec implementation]]
to decode encodedFrame. -
If decoding produces an error, run the Fatally Reject Bad Data algorithm and abort these steps.
-
Let output be the decoded image data emitted by
[[codec implementation]]
corresponding to encodedFrame. -
Let decodeResult be a new
ImageDecodeResult
. -
If output is the final full-detail progressive output corresponding to encodedFrame:
-
Assign
true
to decodeResult’scomplete
. -
If
[[progressive frame generations]]
contains an entry keyed by frameIndex, remove the entry from the map.
-
-
Otherwise:
-
Assign
false
to decodeResult’scomplete
. -
Let frameGeneration be the Progressive Image Frame Generation for output.
-
Add a new entry to
[[progressive frame generations]]
with key frameIndex and value frameGeneration.
-
-
Let duration be the presentation duration for output as described by encodedFrame. If encodedFrame does not describe a duration, assign
null
to duration. -
Let timestamp be the presentation timestamp for output as described by encodedFrame. If encodedFrame does not have a timestamp:
-
If encodedFrame is a still image assign
0
to timestamp. -
If encodedFrame is a constant rate animated image and duration is not
null
, assign|frameIndex| * |duration|
to timestamp. -
If a timestamp can otherwise be trivially generated from metadata without further decoding, assign that to timestamp.
-
Otherwise, assign
0
to timestamp.
-
-
Assign
image
with the result of running the Create a VideoFrame algorithm with output, timestamp, and duration. -
Remove promise from
[[pending decode promises]]
. -
Resolve promise with decodeResult.
-
- Resolve Decode (with promise and result)
-
-
Queue a task to perform these steps:
-
If
[[closed]]
, abort these steps. -
Assert that promise is an element of
[[pending decode promises]]
. -
Remove promise from
[[pending decode promises]]
. -
Resolve promise with result.
-
-
- Reject Infeasible Decode (with promise)
-
-
Assert that
complete
istrue
or[[closed]]
istrue
. -
If
complete
istrue
, let exception be aRangeError
. Otherwise, let exception be anInvalidStateError
DOMException
. -
Queue a task to perform these steps:
-
If
[[closed]]
, abort these steps. -
Assert that promise is an element of
[[pending decode promises]]
. -
Remove promise from
[[pending decode promises]]
. -
Reject promise with exception.
-
-
- Fatally Reject Bad Data
-
-
Queue a task to perform these steps:
-
If
[[closed]]
, abort these steps. -
Run the Close ImageDecoder algorithm with an
EncodingError
DOMException
.
-
-
- Check Type Support (with type)
-
-
If the User Agent can provide a codec to support decoding type, return
true
. -
Otherwise, return
false
.
-
- Reset ImageDecoder (with exception)
-
-
Signal
[[codec implementation]]
to abort any active decoding operation. -
For each decodePromise in
[[pending decode promises]]
:-
Reject decodePromise with exception.
-
Remove decodePromise from
[[pending decode promises]]
.
-
-
- Close ImageDecoder (with exception)
-
-
Run the Reset ImageDecoder algorithm with exception.
-
Assign
true
to[[closed]]
. -
Clear
[[codec implementation]]
and release associated system resources. -
Remove all entries from
[[ImageTrackList]]
. -
Assign
-1
to[[ImageTrackList]]
's[[selected index]]
.
-
10.3. ImageDecoderInit Interface
typedef (BufferSource or ReadableStream );
ImageBufferSource dictionary {
ImageDecoderInit required DOMString type ;required ImageBufferSource data ;ColorSpaceConversion colorSpaceConversion = "default"; [EnforceRange ]unsigned long desiredWidth ; [EnforceRange ]unsigned long desiredHeight ;boolean preferAnimation ;sequence <ArrayBuffer >= []; };
transfer
To determine if an ImageDecoderInit
is a valid ImageDecoderInit,
run these steps:
-
If type is not a valid image MIME type, return
false
. -
If data is of type
ReadableStream
and the ReadableStream is disturbed or locked, returnfalse
. -
If data is of type
BufferSource
:-
If the result of running IsDetachedBuffer (described in [ECMASCRIPT]) on data is
false
, returnfalse
. -
If data is empty, return
false
.
-
-
If
desiredWidth
exists anddesiredHeight
does not exist, returnfalse
. -
If
desiredHeight
exists anddesiredWidth
does not exist, returnfalse
. -
Return
true
.
A valid image MIME type is a string that is a valid MIME type
string and for which the type
, per Section 8.3.1 of [RFC9110], is image
.
type
, of type DOMString-
String containing the MIME type of the image file to be decoded.
data
, of type ImageBufferSource-
BufferSource
orReadableStream
of bytes representing an encoded image file as described bytype
. colorSpaceConversion
, of type ColorSpaceConversion, defaulting to"default"
-
Controls whether decoded outputs' color space is converted or ignored, as defined by
colorSpaceConversion
inImageBitmapOptions
. desiredWidth
, of type unsigned long-
Indicates a desired width for decoded outputs. Implementation is best effort; decoding to a desired width MAY not be supported by all formats/ decoders.
desiredHeight
, of type unsigned long-
Indicates a desired height for decoded outputs. Implementation is best effort; decoding to a desired height MAY not be supported by all formats/decoders.
preferAnimation
, of type boolean-
For images with multiple tracks, this indicates whether the initial track selection SHOULD prefer an animated track.
NOTE: See the Get Default Selected Track Index algorithm.
10.4. ImageDecodeOptions Interface
dictionary { [
ImageDecodeOptions EnforceRange ]unsigned long frameIndex = 0;boolean completeFramesOnly =true ; };
frameIndex
, of type unsigned long, defaulting to0
-
The index of the frame to decode.
completeFramesOnly
, of type boolean, defaulting totrue
-
For Progressive Images, a value of
false
indicates that the decoder MAY output animage
with reduced detail. Each subsequent call todecode()
for the sameframeIndex
will resolve to produce an image with a higher Progressive Image Frame Generation (more image detail) than the previous call, until finally the full-detail image is produced.If
completeFramesOnly
is assignedtrue
, or if the image is not a Progressive Image, or if the User Agent does not support progressive decoding for the given image type, calls todecode()
will only resolve once the full detail image is decoded.NOTE: For Progressive Images, settingcompleteFramesOnly
tofalse
can be used to offer users a preview an image that is still being buffered from the network (via thedata
ReadableStream
).Upon decoding the full detail image, the
ImageDecodeResult
'scomplete
will be set to true.
10.5. ImageDecodeResult Interface
dictionary {
ImageDecodeResult required VideoFrame image ;required boolean complete ; };
image
, of type VideoFrame-
The decoded image.
complete
, of type boolean-
Indicates whether
image
contains the final full-detail output.NOTE:
complete
is alwaystrue
whendecode()
is invoked withcompleteFramesOnly
set totrue
.
10.6. ImageTrackList Interface
[Exposed =(Window ,DedicatedWorker )]interface {
ImageTrackList getter ImageTrack (unsigned long );
index readonly attribute Promise <undefined >ready ;readonly attribute unsigned long length ;readonly attribute long selectedIndex ;readonly attribute ImageTrack ?selectedTrack ; };
10.6.1. Internal Slots
[[ready promise]]
-
The promise used to signal when the
ImageTrackList
has been populated withImageTrack
s.NOTE:
ImageTrack
frameCount
can receive subsequent updates untilcomplete
istrue
. [[track list]]
-
The list of
ImageTrack
s describe by thisImageTrackList
. [[selected index]]
-
The index of the selected track in
[[track list]]
. A value of-1
indicates that no track is selected.
10.6.2. Attributes
ready
, of type Promise<undefined>, readonly-
The
ready
getter steps are to return the[[ready promise]]
. length
, of type unsigned long, readonly-
The
length
getter steps are to return the length of[[track list]]
. selectedIndex
, of type long, readonly-
The
selectedIndex
getter steps are to return[[selected index]]
; selectedTrack
, of type ImageTrack, readonly, nullable-
The
selectedTrack
getter steps are:-
If
[[selected index]]
is-1
, returnnull
. -
Otherwise, return the ImageTrack from
[[track list]]
at the position indicated by[[selected index]]
.
-
10.7. ImageTrack Interface
[Exposed =(Window ,DedicatedWorker )]interface {
ImageTrack readonly attribute boolean animated ;readonly attribute unsigned long frameCount ;readonly attribute unrestricted float repetitionCount ;attribute boolean selected ; };
10.7.1. Internal Slots
[[ImageDecoder]]
-
The
ImageDecoder
instance that constructed thisImageTrack
. [[ImageTrackList]]
-
The
ImageTrackList
instance that lists thisImageTrack
. [[animated]]
-
Indicates whether this track contains an animated image with multiple frames.
[[frame count]]
-
The number of frames in this track.
[[repetition count]]
-
The number of times the animation is intended to repeat.
[[selected]]
-
Indicates whether this track is selected for decoding.
10.7.2. Attributes
animated
, of type boolean, readonly-
The
animated
getter steps are to return the value of[[animated]]
.NOTE: This attribute provides an early indication that
frameCount
will ultimately exceed 0 for images where theframeCount
starts at0
and later increments as new chunks of theReadableStream
data
arrive. frameCount
, of type unsigned long, readonly-
The
frameCount
getter steps are to return the value of[[frame count]]
. repetitionCount
, of type unrestricted float, readonly-
The
repetitionCount
getter steps are to return the value of[[repetition count]]
. selected
, of type boolean-
The
selected
getter steps are to return the value of[[selected]]
.The
selected
setter steps are:-
If
[[ImageDecoder]]
's[[closed]]
slot istrue
, abort these steps. -
Let newValue be the given value.
-
If newValue equals
[[selected]]
, abort these steps. -
Assign newValue to
[[selected]]
. -
Let parentTrackList be
[[ImageTrackList]]
-
Let oldSelectedIndex be the value of parentTrackList
[[selected index]]
. -
If oldSelectedIndex is not
-1
:-
Let oldSelectedTrack be the
ImageTrack
in parentTrackList[[track list]]
at the position of oldSelectedIndex. -
Assign
false
to oldSelectedTrack[[selected]]
-
-
If newValue is
true
, let selectedIndex be the index of thisImageTrack
within parentTrackList’s[[track list]]
. Otherwise, let selectedIndex be-1
. -
Assign selectedIndex to parentTrackList
[[selected index]]
. -
Run the Reset ImageDecoder algorithm on
[[ImageDecoder]]
. -
Queue a control message to
[[ImageDecoder]]
's control message queue to update the internal selected track index with selectedIndex. -
Process the control message queue belonging to
[[ImageDecoder]]
.
Running a control message to update the internal selected track index means running these steps:
-
Enqueue the following steps to
[[ImageDecoder]]
's[[codec work queue]]
:-
Assign selectedIndex to
[[internal selected track index]]
. -
Remove all entries from
[[progressive frame generations]]
.
-
-
11. Resource Reclamation
When resources are constrained, a User Agent MAY proactively reclaim codecs. This is particularly true in the case where hardware codecs are limited, and shared accross web pages or platform apps.
To reclaim a codec, a User Agent MUST run the appropriate
close algorithm (amongst Close AudioDecoder, Close AudioEncoder, Close VideoDecoder and Close VideoEncoder) with a QuotaExceededError
DOMException
.
The rules governing when a codec may be reclaimed depend on whether the codec is an active or inactive codec and/or a background codec.
An active codec is a codec that has
received a call to encode()
, decode()
, configure()
, flush()
or reset()
in the past 10 seconds
, or has called its output()
callback in the past 10 seconds
. Addionally, VideoEncoder
s are considered active if they are
making progress in encoding queued VideoFrame
s.
NOTE: Encoding large VideoFrame
s can take more than 10s
per frame. The
special case for VideoEncoder
s ensures that they are not reclaimed if more
than 10 seconds
elapses between each output()
callback.
An inactive codec is any codec that does not meet the definition of an active codec.
A background codec is a codec whose ownerDocument
(or owner set's Document
, for codecs in workers) has a attribute equal to
true
.
A User Agent MUST only reclaim a codec that is either an inactive codec, a background codec, or both. A User Agent MUST NOT reclaim a codec that is both active and in the foreground, i.e. not a background codec.
Additionally, User Agents MUST NOT reclaim an active background codec if it is:
-
An encoder, e.g. an
AudioEncoder
orVideoEncoder
.NOTE: This prevents long running encode tasks from being interrupted.
-
An
AudioDecoder
orVideoDecoder
, when there is, respectively, an activeAudioEncoder
orVideoEncoder
in the same global object.NOTE: This prevents prevents breaking long running transcoding tasks.
-
An
AudioDecoder
, when its tab is audibly playing audio.
12. Security Considerations
The primary security impact is that features of this API make it easier for an attacker to exploit vulnerabilities in the underlying platform codecs. Additionally, new abilities to configure and control the codecs can allow for new exploits that rely on a specific configuration and/or sequence of control operations.
Platform codecs are historically an internal detail of APIs like HTMLMediaElement
, [WEBAUDIO], and [WebRTC]. In this way, it has always
been possible to attack the underlying codecs by using malformed media
files/streams and invoking the various API control methods.
For example, you can send any stream to a decoder by first wrapping that stream
in a media container (e.g. mp4) and setting that as the src
of an HTMLMediaElement
. You can then cause the underlying video decoder to
be reset()
by setting a new value for <video>.currentTime
.
WebCodecs makes such attacks easier by exposing low level control when inputs are provided and direct access to invoke the codec control methods. This also affords attackers the ability to invoke sequences of control methods that were not previously possible via the higher level APIs.
The Working Group expects User Agents to mitigate this risk by extensively fuzzing their implementation with random inputs and control method invocations. Additionally, User Agents are encouraged to isolate their underlying codecs in processes with restricted privileges (sandbox) as a barrier against successful exploits being able to read user data.
An additional concern is exposing the underlying codecs to input mutation race conditions, such as allowing a site to mutate a codec input or output while the underlying codec is still operating on that data. This concern is mitigated by ensuring that input and output interfaces are immutable.
13. Privacy Considerations
The primary privacy impact is an increased ability to fingerprint users by querying for different codec capabilities to establish a codec feature profile. Much of this profile is already exposed by existing APIs. Such profiles are very unlikely to be uniquely identifying, but can be used with other metrics to create a fingerprint.
An attacker can accumulate a codec feature profile by calling IsConfigSupported()
methods with a number of different configuration
dictionaries. Similarly, an attacker can attempt to configure()
a codec with
different configuration dictionaries and observe which configurations are
accepted.
Attackers can also use existing APIs to establish much of the codec feature
profile. For example, the [media-capabilities] decodingInfo()
API
describes what types of decoders are supported and its powerEfficient
attribute can signal when a decoder uses hardware acceleration. Similarly, the [WebRTC] getCapabilities()
API can be used to determine what
types of encoders are supported and the getStats()
API can
be used to determine when an encoder uses hardware acceleration. WebCodecs will
expose some additional information in the form of low level codec features.
A codec feature profile alone is unlikely to be uniquely identifying. Underlying codecs are often implemented entirely in software (be it part of the User Agent binary or part of the operating system), such that all users who run that software will have a common set of capabilities. Additionally, underlying codecs are often implemented with hardware acceleration, but such hardware is mass produced and devices of a particular class and manufacture date (e.g. flagship phones manufactured in 2020) will often have common capabilities. There will be outliers (some users can be running outdated versions of software codecs or use a rare mix of custom assembled hardware), but most of the time a given codec feature profile is shared by a large group of users.
Segmenting groups of users by codec feature profile still amounts to a bit of entropy that can be combined with other metrics to uniquely identify a user. User Agents MAY partially mitigate this by returning an error whenever a site attempts to exhaustively probe for codec capabilities. Additionally, User Agents MAY implement a "privacy budget", which depletes as authors use WebCodecs and other identifying APIs. Upon exhaustion of the privacy budget, codec capabilities could be reduced to a common baseline or prompt for user approval.
14. Best Practices for Authors Using WebCodecs
While WebCodecs internally operates on background threads, authors working with
realtime media or in contended main thread environments are encouraged to ensure their
media pipelines operate in worker contexts entirely independent of the main
thread where possible. For example, realtime media processing of VideoFrame
s
are generally to be done in a worker context.
The main thread has significant potential for high contention and jank that can go unnoticed in development, yet degrade inconsistently across devices and User Agents in the field -- potentially dramatically impacting the end user experience. Ensuring the media pipeline is decoupled from the main thread helps provide a smooth experience for end users.
Authors using the main thread for their media pipeline ought to be sure of their target frame rates, main thread workload, how their application will be embedded, and the class of devices their users will be using.
15. Acknowledgements
The editors would like to thank Alex Russell, Chris Needham, Dale Curtis, Dan Sanders, Eugene Zemtsov, Francois Daoust, Guido Urdaneta, Harald Alvestrand, Jan-Ivar Bruaroey, Jer Noble, Mark Foltz, Peter Thatcher, Steve Anton, Matt Wolenetz, Rijubrata Bhaumik, Thomas Guilbert, Tuukka Toivonen, and Youenn Fablet for their contributions to this specification. Thank you also to the many others who contributed to the specification, including through their participation on the mailing list and in the issues.