WebCodecs

1. Definitions

Codec: Refers generically to an instance of AudioDecoder, AudioEncoder, VideoDecoder, or VideoEncoder.
Key Frame: An encoded frame that does not depend on any other frames for decoding.
Internal Pending Output: Codec outputs such as VideoFrames that currently reside in the internal pipeline of the underlying codec implementation. The underlying codec implementation may emit new outputs only when a 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 AudioFrame and VideoFrame objects. Such resources may be quickly exhuasted and should be released immediately when no longer in use.
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 indiciating a primary track is format defined.

2. Codec Processing Model

2.1. Background

This section is non-normative.

The codec interfaces defined by the specification are designed such that new codec tasks may be scheduled while previous tasks are still pending. For example, web authors may call decode() without waiting for a previous decode() to complete. This is achieved by offloading underlying codec tasks to a separate thread for parallel execution.

This section describes threading behaviors as they are visible from the perspective of web authors. Implementers may choose to use more or less threads as long the exernally visible behaviors of blocking and sequencing are maintained as follows.

2.2. Control Thread and Codec Thread

All steps in this specificaiton will run on either a control thread or a codec thread.

The control thread is the thread from which authors will construct a codec and invoke its methods. Invoking a codec’s methods will typically result in the creation of control messages which are later executed on the codec thread. Each global object has a separate control thread.

The codec thread is the thread from which a codec will dequeue control messages and execute their steps. Each codec instance has a separate codec thread. The lifetime of a codec thread matches that of its associated codec instance.

The control thread uses a traditional event loop, as described in [HTML].

The codec thread uses a specialized codec processing loop.

Communication from the control thread to the codec thread is done using control message passing. Communication in the other direction is done using regular event loop tasks.

Each codec instance has a single control message queue that is a queue of control messages.

Queuing a control message means enqueing the message to a codec’s control message queue. Invoking codec methods will often 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 control message may depend on injected state, supplied by the method that enqueued the message.

Resetting the control message queue means performing these steps:

For each control message in the control message queue:
1. If a control message’s injected state includes a promise, reject that promise.
2. Remove the message from the queue.

The codec processing loop must run these steps:

While true:
1. If the control message queue is emtpy, continue.
2. Dequeue front message from the control message queue.
3. Run control message steps described by front message.

3. AudioDecoder Interface

[Exposed=(Window,DedicatedWorker)]
interface AudioDecoder {
  constructor(AudioDecoderInit init);

  readonly attribute CodecState state;
  readonly attribute long decodeQueueSize;

  undefined configure(AudioDecoderConfig config);
  undefined decode(EncodedAudioChunk chunk);
  Promise<undefined> flush();
  undefined reset();
  undefined close();

  static Promise<AudioDecoderSupport> isConfigSupported(AudioDecoderConfig config);
};

dictionary AudioDecoderInit {
  required AudioFrameOutputCallback output;
  required WebCodecsErrorCallback error;
};

callback AudioFrameOutputCallback = undefined(AudioFrame output);

3.1. Internal Slots

[[codec implementation]]: Underlying decoder implementation provided by the User Agent.
[[output callback]]: Callback given at construction for decoded outputs.
[[error callback]]: Callback given at construction for decode errors.

3.2. Constructors

AudioDecoder(init)

Let d be a new AudioDecoder object.
Assign init.output to the [[output callback]] internal slot.
Assign init.error to the [[error callback]] internal slot.
Assign "unconfigured" to d.state.
Return d.

3.3. Attributes

state, of type CodecState, readonly: Describes the current state of the codec.
decodeQueueSize, of type long, readonly: The number of pending decode requests. This number will decrease as the underlying codec is ready to accept new input.

3.4. 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 should first check support by calling isConfigSupported() with config. User agents are not required 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 an InvalidStateError.
Set state to "configured".
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 algorith with config.
If supported is true, assign [[codec implementation]] with an implementation supporting config.
Otherwise, run the Close AudioDecoder algorithm with NotSupportedError.

decode(chunk)

Enqueues a control message to decode the given chunk.

When invoked, run these steps:

If state is not "configured", throw an InvalidStateError.
Increment decodeQueueSize.
Queue a control message to decode the chunk.

Running a control message to decode the chunk means performing these steps:

Attempt to use [[codec implementation]] to decode the chunk.
If decoding results in an error, queue a task on the control thread event loop to run the Close VideoDecoder algorithm with EncodingError.
Queue a task on the control thread event loop to decrement decodeQueueSize
Let decoded outputs be a list of decoded video data outputs emitted by [[codec implementation]].
If decoded outputs is not empty, queue a task on the control thread event loop to run the Output VideoFrames algorithm with decoded outputs.

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 with InvalidStateError DOMException.
Let promise be a new Promise.
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.

Signal [[codec implementation]] to emit all internal pending outputs.
Let decoded outputs be a list of decoded audio data outputs emitted by [[codec implementation]].
If decoded outputs is not empty, queue a task on the control thread event loop to run the Output AudioFrames algorithm with decoded outputs.
Queue a task on the control thread event loop to 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.

close()

Immediately aborts all pending work and releases system resources. Close is final.

When invoked, run the Close AudioDecoder algorithm.

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 memebers will be ignored. Authors may detect unrecognized dictionary members by comparinging config 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:
1. Let decoderSupport be a newly constructed AudioDecoderSupport, initialized as follows:
  1. Set config to the result of running the Clone Configuration algorithm with config.
  2. Set supported to the result of running the Check Configuration Support algorithm with config.
2. Resolve p with decoderSupport.
Return p.

3.5. Algorithms

Output AudioFrames (with outputs)

Run these steps:

For each output in outputs:
1. Let buffer be an AudioBuffer containing the decoded audio data in output.
2. Let frame be an AudioFrame containing buffer and a timestamp for the output.
3. Invoke [[output callback]] with frame.

Reset AudioDecoder

Run these steps:

If state is "closed", throw an InvalidStateError.
Set state to "unconfigured".
Signal [[codec implementation]] to cease producing output for the previous configuration.
Reset the control message queue.
Set decodeQueueSize to zero.

Close AudioDecoder (with error)

Run these steps:

Run the Reset AudioDecoder algorithm.
Set state to "closed".
Clear [[codec implementation]] and release associated system resources.
If error is set, queue a task on the control thread event loop to invoke the [[error callback]] with error.

4. VideoDecoder Interface

[Exposed=(Window,DedicatedWorker)]
interface VideoDecoder {
  constructor(VideoDecoderInit init);

  readonly attribute CodecState state;
  readonly attribute long decodeQueueSize;

  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

[[codec implementation]]: Underlying decoder implementation provided by the User Agent.
[[output callback]]: Callback given at construction for decoded outputs.
[[error callback]]: Callback given at construction for decode errors.

4.2. Constructors

VideoDecoder(init)

Let d be a new VideoDecoder object.
Assign init.output to the [[output callback]] internal slot.
Assign init.error to the [[error callback]] internal slot.
Assign "unconfigured" to d.state.
Return d.

4.3. Attributes

state, of type CodecState, readonly: Describes the current state of the codec.
decodeQueueSize, of type long, readonly: The number of pending decode requests. This number will decrease as the underlying codec is ready to accept new input.

4.4. Methods

configure(config)

Enqueues a control message to configure the video decoder for decoding chunks as described by config.

When invoked, run these steps:

If config is not a valid VideoDecoderConfig, throw a TypeError.
If state is “closed”, throw an InvalidStateError.
Set state to "configured".
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 algorith with config.
If supported is true, assign [[codec implementation]] with an implementation supporting config.
Otherwise, run the Close VideoDecoder algorithm with NotSupportedError.

decode(chunk)

Enqueues a control message to decode the given chunk.

When invoked, run these steps:

If state is not "configured", throw an InvalidStateError.
Increment decodeQueueSize.
Queue a control message to decode the chunk.

Running a control message to decode the chunk means performing these steps:

Attempt to use [[codec implementation]] to decode the chunk.
If decoding results in an error, queue a task on the control thread event loop to run the Close VideoDecoder algorithm with EncodingError.
Queue a task on the control thread event loop to decrement decodeQueueSize
Let decoded outputs be a list of decoded video data outputs emitted by [[codec implementation]].
If decoded outputs is not empty, queue a task on the control thread event loop to run the Output VideoFrames algorithm with decoded outputs.

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 with InvalidStateError DOMException.
Let promise be a new Promise.
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.

Signal [[codec implementation]] to emit all internal pending outputs.
Let decoded outputs be a list of decoded video data outputs emitted by [[codec implementation]].
If decoded outputs is not empty, queue a task on the control thread event loop to run the Output VideoFrames algorithm with decoded outputs.
Queue a task on the control thread event loop to 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.

close()

Immediately aborts all pending work and releases system resources. Close is final.

When invoked, run the Close VideoDecoder algorithm.

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 memebers will be ignored. Authors may detect unrecognized dictionary members by comparinging config 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:
1. Let decoderSupport be a newly constructed VideoDecoderSupport, initialized as follows:
  1. Set config to the result of running the Clone Configuration algorithm with config.
  2. Set supported to the result of running the Check Configuration Support algorithm with config.
2. Resolve p with decoderSupport.
Return p.

4.5. Algorithms

Output VideoFrames (with outputs)

Run these steps:

For each output in outputs:
1. Let timestamp and duration be the timestamp and duration from the EncodedVideoChunk associated with output.
2. Let frame be the result of running the Create a VideoFrame algorithm with output, timestamp, and duration.
3. Invoke [[output callback]] with frame.

Create a VideoFrame (with output, timestamp, and duration)

Let planes be a sequence of Planes containing the decoded video frame data from output.
Let pixelFormat be the PixelFormat of planes.
Let frameInit be a VideoFrameInit with the following keys:
1. Assign timestamp to timestamp.
2. Assign duration to duration.
3. Let codedWidth and codedHeight be the width and height of the decoded video frame output in pixels, prior to any cropping or aspect ratio adjustments.
4. Let cropLeft, cropTop, cropWidth, and cropHeight be the crop region of the decoded video frame output in pixels, prior to any aspect ratio adjustments.
5. Let displayWidth and displayHeight be the display size of the decoded video frame in pixels.
Return a new VideoFrame, constructed with pixelFormat, planes, and frameInit.

Reset VideoDecoder

Run these steps:

If state is "closed", throw an InvalidStateError.
Set state to "unconfigured".
Signal [[codec implementation]] to cease producing output for the previous configuration.
Reset the control message queue.
Set decodeQueueSize to zero.

Close VideoDecoder (with error)

Run these steps:

Run the Reset VideoDecoder algorithm.
Set state to "closed".
Clear [[codec implementation]] and release associated system resources.
If error is set, queue a task on the control thread event loop to invoke the [[error callback]] with error.

5. AudioEncoder Interface

[Exposed=(Window,DedicatedWorker)]
interface AudioEncoder {
  constructor(AudioEncoderInit init);

  readonly attribute CodecState state;
  readonly attribute long encodeQueueSize;

  undefined configure(AudioEncoderConfig config);
  undefined encode(AudioFrame frame);
  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);

5.1. Internal Slots

[[codec implementation]]: Underlying encoder implementation provided by the User Agent.
[[output callback]]: Callback given at construction for encoded outputs.
[[error callback]]: Callback given at construction for encode errors.

5.2. Constructors

AudioEncoder(init)

Let e be a new AudioEncoder object.
Assign init.output to the [[output callback]] internal slot.
Assign init.error to the [[error callback]] internal slot.
Assign "unconfigured" to e.state.
Return e.

5.3. Attributes

state, of type CodecState, readonly: Describes the current state of the codec.
encodeQueueSize, of type long, readonly: The number of pending encode requests. This number will decrease as the underlying codec is ready to accept new input.

5.4. Methods

configure(config)

Enqueues a control message to configure the audio encoder for decoding chunks as described by config.

When invoked, run these steps:

If config is not a valid AudioEncoderConfig, throw a TypeError.
If state is "closed", throw an InvalidStateError.
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 algorith with config.
If supported is true, assign [[codec implementation]] with an implementation supporting config.
Otherwise, run the Close AudioEncoder algorithm with NotSupportedError.

encode(frame)

Enqueues a control message to encode the given frame.

When invoked, run these steps:

If the value of frame’s [[detached]] internal slot is true, throw a TypeError.
If state is not "configured", throw an InvalidStateError.
Let frameClone hold the result of running the Clone Frame algorithm with frame.
Increment encodeQueueSize.
Queue a control message to encode frameClone.

Running a control message to encode the frame means performing these steps.

Attempt to use [[codec implementation]] to encode frameClone.
If encoding results in an error, queue a task on the control thread event loop to run the Close AudioEncoder algorithm with EncodingError.
Queue a task on the control thread event loop to decrement encodeQueueSize.
Let encoded outputs be a list of encoded audio data outputs emitted by [[codec implementation]].
If encoded outputs is not empty, queue a task on the control thread event loop to run the Output EncodedAudioChunks algorithm with encoded outputs.

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 with InvalidStateError DOMException.
Let promise be a new Promise.
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.

Signal [[codec implementation]] to emit all internal pending outputs.
Let encoded outputs be a list of encoded audio data outputs emitted by [[codec implementation]].
If encoded outputs is not empty, queue a task on the control thread event loop to run the Output EncodedAudioChunks algorithm with encoded outputs.
Queue a task on the control thread event loop to 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.

close()

Immediately aborts all pending work and releases system resources. Close is final.

When invoked, run the Close AudioEncoder algorithm.

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 memebers will be ignored. Authors may detect unrecognized dictionary members by comparinging config 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:
1. Let encoderSupport be a newly constructed AudioEncoderSupport, initialized as follows:
  1. Set config to the result of running the Clone Configuration algorithm with config.
  2. Set supported to the result of running the Check Configuration Support algorithm with config.
2. Resolve p with encoderSupport.
Return p.

5.5. Algorithms

Output EncodedAudioChunks (with outputs)

Run these steps:

For each output in outputs:
1. Let chunkInit be an EncodedAudioChunkInit with the following keys:
  1. Let data contain the encoded audio data from output.
  2. Let type be the EncodedAudioChunkType of output.
  3. Let timestamp be the timestamp from the AudioFrame associated with output.
2. Let chunk be a new EncodedAudioChunk constructed with chunkInit.
3. Invoke [[output callback]] with chunk.

Reset AudioEncoder

Run these steps:

If state is "closed", throw an InvalidStateError.
Set state to "unconfigured".
Signal [[codec implementation]] to cease producing output for the previous configuration.
Reset the control message queue.
Set encodeQueueSize to zero.

Close AudioEncoder (with error)

Run these steps:

Run the Reset AudioEncoder algorithm.
Set state to "closed".
Clear [[codec implementation]] and release associated system resources.
If error is set, queue a task on the control thread event loop invoke the [[error callback]] with error.

6. VideoEncoder Interface

[Exposed=(Window,DedicatedWorker)]
interface VideoEncoder {
  constructor(VideoEncoderInit init);

  readonly attribute CodecState state;
  readonly attribute long encodeQueueSize;

  undefined configure(VideoEncoderConfig config);
  undefined encode(VideoFrame frame, optional VideoEncoderEncodeOptions options = {});
  Promise<undefined> flush();
  undefined reset();
  undefined close();

  static Promise<boolean> isConfigSupported(VideoEncoderConfig config);
};

dictionary VideoEncoderInit {
  required EncodedVideoChunkOutputCallback output;
  required WebCodecsErrorCallback error;
};

callback EncodedVideoChunkOutputCallback = undefined(EncodedVideoChunk output, VideoDecoderConfig? output_config);

6.1. Internal Slots

[[codec implementation]]: Underlying encoder implementation provided by the User Agent.
[[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 emitted EncodedVideoChunk.

6.2. Constructors

VideoEncoder(init)

Let e be a new VideoEncoder object.
Assign init.output to the [[output callback]] internal slot.
Assign init.error to the [[error callback]] internal slot.
Assign "unconfigured" to e.state.
Return e.

6.3. Attributes

state, of type CodecState, readonly: Describes the current state of the codec.
encodeQueueSize, of type long, readonly: The number of pending encode requests. This number will decrease as the underlying codec is ready to accept new input.

6.4. Methods

configure(config)

Enqueues a control message to configure the video encoder for decoding chunks as described by config.

When invoked, run these steps:

If config is not a valid VideoEncoderConfig, throw a TypeError.
If state is "closed", throw an InvalidStateError.
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 algorith with config.
If supported is true, assign [[codec implementation]] with an implementation supporting config.
Otherwise, run the Close VideoEncoder algorithm with NotSupportedError and abort these steps.
Set [[active encoder config]] to config.

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 is true, throw a TypeError.
If state is not "configured", throw an InvalidStateError.
Let frameClone hold the result of running the Clone Frame algorithm with frame.
Increment encodeQueueSize.
Queue a control message to encode frameClone.

Running a control message to encode the frame means performing these steps.

Attempt to use [[codec implementation]] to encode frameClone according to options.
If encoding results in an error, queue a task on the control thread event loop to run the Close VideoEncoder algorithm with EncodingError.
Queue a task on the control thread event loop to decrement encodeQueueSize.
Let encoded outputs be a list of encoded video data outputs emitted by [[codec implementation]].
If encoded outputs is not empty, queue a task on the control thread event loop to run the Output EncodedVideoChunks algorithm with encoded outputs.

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 with InvalidStateError DOMException.
Let promise be a new Promise.
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.

Signal [[codec implementation]] to emit all internal pending outputs.
Let encoded outputs be a list of encoded video data outputs emitted by [[codec implementation]].
If encoded outputs is not empty, queue a task on the control thread event loop to run the Output EncodedVideoChunks algorithm with encoded outputs.
Queue a task on the control thread event loop to 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.

close()

Immediately aborts all pending work and releases system resources. Close is final.

When invoked, run the Close VideoEncoder algorithm.

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 memebers will be ignored. Authors may detect unrecognized dictionary members by comparinging config 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:
1. Let encoderSupport be a newly constructed VideoEncoderSupport, initialized as follows:
  1. Set config to the result of running the Clone Configuration algorithm with config.
  2. Set supported to the result of running the Check Configuration Support algorithm with config.
2. Resolve p with encoderSupport.
Return p.

6.5. Algorithms

Output EncodedVideoChunks (with outputs)

Run these steps:

For each output in outputs:
1. Let encoder_config be the [[active encoder config]].
  
  The intent is for encoder_config to be the [[active encoder config]] that was used to encode output. But, as written, it may occur that output was encoded using a previous VideoEncoderConfig that has since been replaced by a later call to configure(). See #138.
2. Let output_config be a VideoDecoderConfig that describes output. Initialize output_config as follows:
  1. Assign encoder_config.codec to output_config.codec.
  2. Assign encoder_config.width to output_config.cropWidth.
  3. Assign encoder_config.height to output_config.cropHeight.
  4. Assign encoder_config.displayWidth to output_config.displayWidth.
  5. Assign encoder_config.displayHeight to output_config.displayHeight.
  6. Assign the remaining keys of output_config as determined by [[codec implementation]]. The user agent must ensure that the configuration is completely described such that output_config could be used to correctly decode output.
    
    NOTE: This includes supplying the description to describe codec specific "extradata", the use of which may be further described in codec registrations listed in the [WEBCODECS-CODEC-REGISTRY].
3. If output_config and [[active output config]] are equal dictionaries, set output_config to null. Otherwise, set [[active output config]] to output_config.
  
  NOTE: The VideoDecoderConfig output_config will be null if the configuration hasn’t changed from previous outputs. The first output will always include a non-null output_config.
4. Let chunkInit be an EncodedVideoChunkInit with the following keys:
  1. Let data contain the encoded video data from output.
  2. Let type be the EncodedVideoChunkType of output.
  3. Let timestamp be the [[timestamp]] from the VideoFrame associated with output.
  4. Let duration be the [[duration]] from the VideoFrame associated with output.
5. Let chunk be a new EncodedVideoChunk constructed with chunkInit.
6. Invoke [[output callback]] with chunk.

Reset VideoEncoder

Run these steps:

If state is "closed", throw an InvalidStateError.
Set state to "unconfigured".
Set [[active encoder config]] to null.
Set [[active output config]] to null.
Signal [[codec implementation]] to cease producing output for the previous configuration.
Reset the control message queue.
Set encodeQueueSize to zero.

Close VideoEncoder (with error)

Run these steps:

Run the Reset VideoEncoder algorithm.
Set state to "closed".
Clear [[codec implementation]] and release associated system resources.
If error is set, queue a task on the control thread event loop invoke the [[error callback]] with error.

7. Configurations

7.1. Check Configuration Support (with `config`)

Run these steps:

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, and VideoEncoderConfig each define their respective configuration entries and defaults.

NOTE: Support for a given configuration may change dynamically if the hardware is altered (e.g. external GPU unplugged) or if required hardware resources are exhausted. User agents should 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:
1. If m does not exist in config, then continue.
2. If config[m] is a nested dictionary, set clone[m] to the result of recursively running the Clone Configuration algorithm with config[m].
3. Otherwise, assign the value of config[m] to clone[m].

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 of supported.

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 of supported.

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 of supported.

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 of supported.

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 and level 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.

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;
  required unsigned long sampleRate;
  required unsigned long numberOfChannels;
  BufferSource description;
};

To check if an AudioDecoderConfig is a valid AudioDecoderConfig, run these steps:

If codec is not a valid codec string, return false.
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.
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;
  BufferSource description;
  unsigned long codedWidth;
  unsigned long codedHeight;
  unsigned long cropLeft;
  unsigned long cropTop;
  unsigned long cropWidth;
  unsigned long cropHeight;
  unsigned long displayWidth;
  unsigned long displayHeight;
  HardwareAcceleration hardwareAcceleration = "allow";
};

To check if a VideoDecoderConfig is a valid VideoDecoderConfig, run these steps:

If codec is not a valid codec string, return false.
If codedWidth = 0 or codedHeight = 0, return false.
If cropWidth = 0 or cropHeight = 0, return false.
If cropTop + cropHeight >= codedHeight, return false.
If cropLeft + cropWidth >= codedWidth, return false.
If displayWidth = 0 or displayHeight = 0, return false.
Return true.

codec, of type DOMString: Contains a codec string describing the codec.
description, of type BufferSource: A sequence of codec specific bytes, commonly known as extradata.
NOTE: The registrations in the [WEBCODECS-CODEC-REGISTRY] may describe whether/how to populate this sequence, corresponding to the provided codec.
codedWidth, of type unsigned long: Width of the VideoFrame in pixels, prior to any cropping or aspect ratio adjustments.
codedHeight, of type unsigned long: Height of the VideoFrame in pixels, prior to any cropping or aspect ratio adjustments.
cropLeft, of type unsigned long: The number of pixels to remove from the left of the VideoFrame, prior to aspect ratio adjustments. Defaults to zero if not present.
cropTop, of type unsigned long: The number of pixels to remove from the top of the VideoFrame, prior to aspect ratio adjustments. Defaults to zero if not present.
cropWidth, of type unsigned long: The width in pixels to include in the crop, starting from cropLeft. Defaults to codedWidth if not present.
cropHeight, of type unsigned long: The height in pixels to include in the crop, starting from cropLeft. Defaults to codedHeight if not present.
displayWidth, of type unsigned long: Width of the VideoFrame when displayed. Defaults to cropWidth if not present.
displayHeight, of type unsigned long: Height of the VideoFrame when displayed. Defaults to cropHeight if not present.
hardwareAcceleration, of type HardwareAcceleration, defaulting to "allow": Configures hardware acceleration for this codec. See HardwareAcceleration.

7.7. AudioEncoderConfig

dictionary AudioEncoderConfig {
  required DOMString codec;
  unsigned long sampleRate;
  unsigned long numberOfChannels;
  unsigned long long bitrate;
};

NOTE: Codec-specific extensions to AudioEncoderConfig may be defined by the registrations in the [WEBCODECS-CODEC-REGISTRY].

To check if an AudioEncoderConfig is a valid AudioEncoderConfig, run these steps:

If codec is not a valid codec string, return false.
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.

7.8. VideoEncoderConfig

dictionary VideoEncoderConfig {
  required DOMString codec;
  unsigned long long bitrate;
  required unsigned long width;
  required unsigned long height;
  unsigned long displayWidth;
  unsigned long displayHeight;
  HardwareAcceleration hardwareAcceleration = "allow";
};

NOTE: Codec-specific extensions to VideoEncoderConfig may be defined by the registrations in the [WEBCODECS-CODEC-REGISTRY].

To check if a VideoEncoderConfig is a valid VideoEncoderConfig, run these steps:

If codec is not a valid codec string, return false.
If width = 0 or height = 0, return false.
If displayWidth = 0 or displayHeight = 0, return false.
Return true.

codec, of type DOMString: Contains a codec string describing the codec.
bitrate, of type unsigned long long: The average bitrate of the encoded video given in units of bits per second.
width, of type unsigned long: The encoded width of output EncodedVideoChunks in pixels, prior to any display aspect ratio adjustments.
The encoder must scale any VideoFrame who’s [[crop width]] differs from this value.
height, of type unsigned long: The encoded height of output EncodedVideoChunks in pixels, prior to any display aspect ratio adjustments.
The encoder must scale any VideoFrame who’s [[crop height]] differs from this value.

displayWidth, of type unsigned long: The intended display width of output EncodedVideoChunks in pixels. Defaults to width if not present.
displayHeight, of type unsigned long: The intended display height of output EncodedVideoChunks in pixels. Defaults to width if not present.

NOTE: Providing a displayWidth or displayHeight that differs from width and height signals that chunks should be scaled after decoding to arrive at the final display aspect ratio.

For many codecs this is merely pass-through information, but some codecs may optionally include display sizing in the bitstream.

hardwareAcceleration, of type HardwareAcceleration, defaulting to "allow": Configures hardware acceleration for this codec. See HardwareAcceleration.

7.9. Hardware Acceleration

enum HardwareAcceleration {
  "allow",
  "deny",
  "require",
};

When supported, hardware acceleration offloads encoding or decoding to specialized hardware.

NOTE: Most authors will be best served by using the default of allow. 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 should carefully weigh the tradeoffs setting a hardware acceleration preference. The precise trade-offs will be device-specific, but authors should generally expect the following:

Setting a value of require may significantly restrict what configurations are supported. It may occur that the user’s device does not offer acceleration for any codec, or only for the most common profiles of older codecs.
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 may yield decreased performance and power efficiency compared to purely software based codecs.

Given these tradeoffs, a good example of using "require" would be if an author intends to provide their own software based fallback via WebAssembly.

Alternatively, a good example of using "disallow" would be if an author is especially sensitive to the higher startup latency or decreased robustness generally associated with hardware acceleration.

allow: Indicates that the user agent may use hardware acceleration if it is available and compatible with other aspects of the codec configuration.
deny: Indicates that the user agent must not use hardware acceleration.
NOTE: This will cause the configuration to be unsupported on platforms where an unaccelerated codec is unavailable or is incompatible with other aspects of the codec configuration.
require: Indicates that the user agent must use hardware acceleration.
NOTE: This will 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. Configuration Equivalence

Two dictionaries are equal dictionaries if they contain the same keys and values. For nested dictionaries, apply this definition recursively.

7.11. VideoEncoderEncodeOptions

dictionary VideoEncoderEncodeOptions {
  boolean keyFrame = false;
};

keyFrame, of type boolean, defaulting to false: A value of true indicates that the given frame MUST be encoded as a key frame. A value of false indicates that the user agent has flexibility to decide whether the frame will be encoded as a key frame.

7.12. 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.

7.13. 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)]
interface EncodedAudioChunk {
  constructor(EncodedAudioChunkInit init);
  readonly attribute EncodedAudioChunkType type;
  readonly attribute unsigned long long timestamp;  // microseconds
  readonly attribute ArrayBuffer data;
};

dictionary EncodedAudioChunkInit {
  required EncodedAudioChunkType type;
  required unsigned long long timestamp;
  required BufferSource data;
};

enum EncodedAudioChunkType {
    "key",
    "delta",
};

8.1.1. Constructors

EncodedAudioChunk(init)

Let chunk be a new EncodedAudioChunk object, initialized as follows
1. Assign init.type to chunk.type.
2. Assign init.timestamp to chunk.timestamp.
3. Assign a copy of init.data to chunk.data.
Return chunk.

8.1.2. Attributes

type, of type EncodedAudioChunkType, readonly: Describes whether the chunk is a key frame.
timestamp, of type unsigned long long, readonly: The presentation timestamp, given in microseconds.
data, of type ArrayBuffer, readonly: A sequence of bytes containing encoded audio data.

8.2. EncodedVideoChunk Interface

[Exposed=(Window,DedicatedWorker)]
interface EncodedVideoChunk {
  constructor(EncodedVideoChunkInit init);
  readonly attribute EncodedVideoChunkType type;
  readonly attribute unsigned long long timestamp;  // microseconds
  readonly attribute unsigned long long? duration;  // microseconds
  readonly attribute ArrayBuffer data;
};

dictionary EncodedVideoChunkInit {
  required EncodedVideoChunkType type;
  required unsigned long long timestamp;
  unsigned long long duration;
  required BufferSource data;
};

enum EncodedVideoChunkType {
    "key",
    "delta",
};

8.2.1. Constructors

EncodedVideoChunk(init)

Let chunk be a new EncodedVideoChunk object, initialized as follows
1. Assign init.type to chunk.type.
2. Assign init.timestamp to chunk.timestamp.
3. If duration is present in init, assign init.duration to chunk.duration. Otherwise, assign null to chunk.duration.
Assign a copy of init.data to chunk.data.
Return chunk.

8.2.2. Attributes

type, of type EncodedVideoChunkType, readonly: Describes whether the chunk is a key frame or not.
timestamp, of type unsigned long long, readonly: The presentation timestamp, given in microseconds.
duration, of type unsigned long long, readonly, nullable: The presentation duration, given in microseconds.
data, of type ArrayBuffer, readonly: A sequence of bytes containing encoded video data.

9. Raw Media Interfaces (Frames)

These interfaces represent unencoded (raw) media.

9.1. AudioFrame Interface

[Exposed=(Window,DedicatedWorker)]
interface AudioFrame {
  constructor(AudioFrameInit init);
  readonly attribute unsigned long long timestamp;
  readonly attribute AudioBuffer? buffer;
  undefined close();
};

dictionary AudioFrameInit {
  required unsigned long long timestamp;
  required AudioBuffer buffer;
};

9.1.1. Internal Slots

[[detached]]: Boolean indicating whether close() was invoked and underlying resources have been released.

9.1.2. Constructors

AudioFrame(init)

Let frame be a new AudioFrame object.
Assign init.timestamp to frame.timestamp.
Assign init.buffer to frame.buffer.
Assign false to the [[detached]] internal slot.
Return frame.

9.1.3. Attributes

timestamp, of type unsigned long long, readonly: The presentation timestamp, given in microseconds.
buffer, of type AudioBuffer, readonly, nullable: The buffer containing decoded audio data.

9.1.4. Methods

close()

Immediately frees system resources. When invoked, run these steps:

Release system resources for buffer and set its value to null.
Assign true to the [[detached]] internal slot.

NOTE: This section needs work. We should use the name and semantics of VideoFrame destroy(). Similarly, we should add clone() to make a deep copy.

9.2. VideoFrame Interface

[Exposed=(Window,DedicatedWorker)]
interface VideoFrame {
  constructor(ImageBitmap imageBitmap, optional VideoFrameInit frameInit = {});
  constructor(PixelFormat pixelFormat, sequence<(Plane or PlaneInit)> planes,
              optional VideoFrameInit frameInit = {});

  readonly attribute PixelFormat format;
  readonly attribute FrozenArray<Plane> planes;
  readonly attribute unsigned long codedWidth;
  readonly attribute unsigned long codedHeight;
  readonly attribute unsigned long cropLeft;
  readonly attribute unsigned long cropTop;
  readonly attribute unsigned long cropWidth;
  readonly attribute unsigned long cropHeight;
  readonly attribute unsigned long displayWidth;
  readonly attribute unsigned long displayHeight;
  readonly attribute unsigned long long? duration;
  readonly attribute unsigned long long? timestamp;

  undefined destroy();
  VideoFrame clone();

  Promise<ImageBitmap> createImageBitmap(
    optional ImageBitmapOptions options = {});

};

dictionary VideoFrameInit {
  unsigned long codedWidth;
  unsigned long codedHeight;
  unsigned long cropLeft;
  unsigned long cropTop;
  unsigned long cropWidth;
  unsigned long cropHeight;
  unsigned long displayWidth;
  unsigned long displayHeight;
  unsigned long long duration;
  unsigned long long timestamp;
};

9.2.1. Internal Slots

[[detached]]: A boolean indicating whether destroy() was invoked and underlying resources have been released.
[[format]]: A PixelFormat describing the pixel format of the VideoFrame.
[[planes]]: A list of Planes describing the memory layout of the pixel data in VideoFrame. The number of Planes and their semantics are determined by [[format]].
[[coded width]]: Width of the VideoFrame in pixels, prior to any cropping or aspect ratio adjustments.
[[coded height]]: Height of the VideoFrame in pixels, prior to any cropping or aspect ratio adjustments.
[[crop left]]: The number of pixels to remove from the left of the VideoFrame, prior to aspect ratio adjustments.
[[crop top]]: The number of pixels to remove from the top of the VideoFrame, prior to aspect ratio adjustments.
[[crop width]]: The width of pixels to include in the crop, starting from cropLeft.
[[crop height]]: The height of pixels to include in the crop, starting from cropLeft.
[[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 this VideoFrame.
[[timestamp]]: The presentation timestamp, given in microseconds. The timestamp is copied from the EncodedVideoChunk corresponding to this VideoFrame.

9.2.2. Constructors

NOTE: this section needs work. Current wording assumes a VideoFrame can always be easily represented using one of the known pixel formats. In practice, the underlying UA resources may be GPU backed or formatted in such a way that conversion to an allowed pixel format requires expensive copies and translation. When this occurs, we should allow planes to be null and format to be "opaque" to avoid early optimization. We should make conversion explicit and user controlled by offering a videoFrame.convertTo(format) that returns a Promise containing a new VideoFrame for which the copies/translations are performed.

VideoFrame(imageBitmap, frameInit)

If frameInit is not a valid VideoFrameInit, throw a TypeError.
If the value of imageBitmap’s' [[Detached]] internal slot is set to true, then throw an InvalidStateError DOMException.
Let frame be a new VideoFrame.
Assign false to frame’s [[detached]] internal slot.
Use a copy of the pixel data in imageBitmap to initialize to following frame internal slots:
1. Initialize [[format]] be the underlying format of imageBitmap.
2. Initialize [[planes]] to describe the arrangement of memory of the copied pixel data.
3. Assign regions of the copied pixel data to the [[plane buffer]] internal slot of each plane as appropriate for the pixel format.
4. Initialize [[coded width]] and [[coded height]] to describe the width and height of the imageBitamp prior to any cropping or aspect ratio adjustments.
Use frameInit to initialize the remaining frame internal slots:
1. If frameInit.cropLeft is present, assign it to [[crop left]]. Otherwise, assign 0 to [[crop left]].
2. If frameInit.cropTop is present, assign it to [[crop top]]. Otherwise, assign 0 to [[crop top]]
3. If frameInit.cropWidth is present, assign it to [[crop width]]. Otherwise, assign [[coded width]] to [[crop width]].
4. If frameInit.cropHeight is present, assign it to [[crop height]]. Otherwise, assign [[coded height]] to [[crop height]].
5. If frameInit.displayWidth is present, assign it to [[display width]]. Otherwise, assign [[crop width]] to [[display width]].
6. If frameInit.displayHeight is present, assign it to [[display height]]. Otherwise, assign [[crop height]] to [[display height]].
7. If frameInit.duration is present, assign it to [[duration]]. Otherwise, assign null to [[duration]].
8. If frameInit.timestamp is present, assign it to [[timestamp]]. Otherwise, assign null to [[timestamp]].
Return frame.

VideoFrame(pixelFormat, planes, frameInit)

If either codedWidth or codedHeight is not present in frameInit, throw a TypeError.
If frameInit is not a valid VideoFrameInit, throw a TypeError.
If the length of planes is incompatible with the given pixelFormat, throw a TypeError.
Let frame be a new VideoFrame object.
Assign false to frame’s [[detached]] internal slot.
Assign init.format to frame’s [[format]].
For each element p in planes:
1. If p is a Plane, append a copy of p to frame’s [[planes]] and continue.
2. If p is a PlaneInit, append a new Plane q to frame’s [[planes]], initialized as follows:
  1. Assign a copy of p.src to q’s [[plane buffer]] internal slot.
    
    NOTE: the samples should be copied exactly, but the user agent may add row padding as needed to improve memory alignment.
  2. Assign the width of each row in [[plane buffer]], including any padding, to q.stride.
  3. Assign p.rows to q.rows.
  4. Assign the product of (q.rows * q.stride) to q.length
Assign frameInit.codedWidth to frame’s [[coded width]].
Assign frameInit.codedHeight to frame’s [[coded height]].
If frameInit.cropLeft is present, assign it frame’s [[crop left]]. Otherwise, assign 0 to [[crop left]].
If frameInit.cropTop is present, assign it to frame’s [[crop top]]. Otherwise, assign 0 to [[crop top]].
If frameInit.cropWidth is present, assign it to frame’s [[crop width]]. Otherwise, assign [[coded width]] to [[crop width]].
If frameInit.cropHeight is present, assign it to frame’s [[crop height]]. Otherwise, assign [[coded height]] to [[crop height]].
If frameInit.displayWidth is present, assign it to frame’s [[display width]]. Otherwise, assign [[crop width]] to [[display width]].
If frameInit.displayHeight is present, assign it to frame’s [[display height]]. Otherwise, assign [[crop height]] to [[display height]].
If frameInit.duration is present, assign it to [[duration]]. Otherwise, assign null to [[duration]].
If frameInit.timestamp is present, assign it to [[timestamp]]. Otherwise, assign null to [[timestamp]].
Return frame.

9.2.3. Attributes

format, of type PixelFormat, readonly

Describes the arrangement of bytes in each plane as well as the number and order of the planes.

The format getter steps are to return [[format]].

planes, of type FrozenArray<Plane>, readonly

Holds pixel data data, laid out as described by format and Plane attributes.

The planes getter steps are to return [[planes]].

codedWidth, of type unsigned long, readonly

Width of the VideoFrame in pixels, prior to any cropping or aspect ratio adjustments.

The codedWidth getter steps are to return [[coded width]].

codedHeight, of type unsigned long, readonly

Height of the VideoFrame in pixels, prior to any cropping or aspect ratio adjustments.

The codedHeight getter steps are to return [[coded height]].

cropLeft, of type unsigned long, readonly

The number of pixels to remove from the left of the VideoFrame, prior to aspect ratio adjustments.

The cropLeft getter steps are to return [[crop left]].

cropTop, of type unsigned long, readonly

The number of pixels to remove from the top of the VideoFrame, prior to aspect ratio adjustments.

The cropTop getter steps are to return [[crop top]].

cropWidth, of type unsigned long, readonly

The width of pixels to include in the crop, starting from cropLeft.

The cropWidth getter steps are to return [[crop width]].

cropHeight, of type unsigned long, readonly

The height of pixels to include in the crop, starting from cropLeft.

The cropHeight getter steps are to return [[crop height]].

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 unsigned long long, readonly, nullable

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]].

9.2.4. Methods

destroy() Immediately frees system resources. Destruction applies to all references, including references that are serialized and passed across Realms.

NOTE: Authors should take care to manage frame lifetimes by calling destroy() immediately when frames are no longer needed.

NOTE: Use clone() to create a deep copy. Cloned frames have their own lifetime and will not be affected by destroying the original frame.

When invoked, run these steps:

If [[detached]] is true, throw an InvalidStateError.
Remove all Planes from [[planes]] and release associated memory.
Assign true to the [[detached]] internal slot.

clone() Creates a new VideoFrame with a separate lifetime containing a deep copy of this frame’s resources.

NOTE: VideoFrames may require a large amount of memory. Use clone() sparingly.

When invoked, run the following steps:

If the value of the [[detached]] slot is true, return a promise rejected with InvalidStateError DOMException.
Let p be a new Promise.
In parallel, resolve p with the result of running the Clone Frame algorithm with this.
Return p.

createImageBitmap(options) Creates an ImageBitmap from this VideoFrame.

When invoked, run these steps:

Let p be a new Promise.
If either options’s resizeWidth or resizeHeight is present and is 0, then return p rejected with an InvalidStateError DOMException.
If the this' [[detached]] internal slot is set to true, then return p rejected with an InvalidStateError DOMException.
Let imageBitmap be a new ImageBitmap object.
Set imageBitmap’s bitmap data to a copy of the VideoFrame pixel data, at the frame’s intrinsic width and intrinsic height (i.e., after any aspect-ratio correction has been applied), cropped to the source rectangle with formatting.
If the origin of imageBitmap’s image is not same origin with entry settings object’s origin, then set the origin-clean flag of imageBitmap’s bitmap to false.
Run this step in parallel:
Resolve p with imageBitmap.

9.2.5. Algorithms

To check if a VideoFrameInit is a valid VideoFrameInit, run these steps:

If codedWidth = 0 or codedHeight = 0, return false.
If cropWidth = 0 or cropHeight = 0, return false.
If cropTop + cropHeight >= codedHeight, return false.
If cropLeft + cropWidth >= codedWidth, return false.
If displayWidth = 0 or displayHeight = 0, return false.
Return true.

9.3. Plane Interface

A Plane acts like a thin wrapper around an ArrayBuffer, but may actually be backed by a texture. Planes hide any padding before the first sample or after the last row.

A Plane is solely constructed by its VideoFrame. During construction, the User Agent may use knowledge of the frame’s PixelFormat to add padding to the Plane to improve memory alignment.

A Plane cannot be used after the VideoFrame is destroyed. A new VideoFrame can be assembled from existing Planes, and the new VideoFrame will remain valid when the original is destroyed. This makes it possible to efficiently add an alpha plane to an existing VideoFrame.

[Exposed=(Window,DedicatedWorker)]
interface Plane {
  readonly attribute unsigned long stride;
  readonly attribute unsigned long rows;
  readonly attribute unsigned long length;

  undefined readInto(ArrayBufferView dst);
};

dictionary PlaneInit {
  required BufferSource src;
  required unsigned long stride;
  required unsigned long rows;
};

9.3.1. Internal Slots

[[parent frame]]: Refers to the VideoFrame that constructed and owns this plane.
[[plane buffer]]: Internal storage for the plane’s pixel data.

9.3.2. Attributes

stride, of type unsigned long, readonly: The width of each row including any padding.
rows, of type unsigned long, readonly: The number of rows.
length, of type unsigned long, readonly: The total byte length of the plane (stride * rows).

9.3.3. Methods

readInto(dst)

Copies the plane data into dst.

When invoked, run these steps:

If [[parent frame]] has been destroyed, throw an InvalidStateError.
If length is greater than |dst.byteLength|, throw a TypeError.
Copy the [[plane buffer]] into dst.

9.4. Pixel Format

Pixel formats describe the arrangement of bytes in each plane as well as the number and order of the planes.

NOTE: This section needs work. We expect to add more pixel formats and offer much more verbose definitions. For now, please see http://www.fourcc.org/pixel-format/yuv-i420/ for a more complete description.

enum PixelFormat {
  "I420"
};

I420: Planar 4:2:0 YUV.

9.5. Algorithms

Clone Frame (with frame)

Let cloneFrame be a new object of the same type as frame (either AudioFrame or VideoFrame).
Initialize each attribute and internal slot of clone with a copy of the value from the corresponding attribute of this frame.

NOTE: User Agents are encouraged to avoid expensive copies of large objects (for instance, VideoFrame pixel data). Frame types are immutable, so the above step may be implemented using memory sharing techniques such as reference counting.
Return cloneFrame.

10. Image Decoding

10.1. Background

This section is non-normative.

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)]
interface ImageDecoder {
  constructor(ImageDecoderInit init);

  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

[[ImageTrackList]]: An ImageTrackList describing the tracks found in [[encoded data]]
[[complete]]: A boolean indicating whether [[encoded data]] is completely buffered.
[[completed promise]]: The promise used to signal when [[complete]] becomes true.
[[codec implementation]]: An underlying image decoder implementation provided by the User Agent.
[[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 on the codec thread.
[[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 to decode() with the given frame index.

10.2.2. Constructor

ImageDecoder(init)

NOTE: Calling decode() on the constructed ImageDecoder will trigger a NotSupportedError if the user agent does not support type. Authors should first check support by calling isTypeSupported() with type. User agents are not required to support any particular type.

When invoked, run these steps:

If init is not valid ImageDecoderInit, throw a TypeError.
Let d be a new ImageDecoder object. In the steps below, all mentions of ImageDecoder members apply to d unless stated otherwise.
Assign [[ImageTrackList]] a new ImageTrackList initialized as follows:
1. Assign a new list to [[track list]].
2. Assign -1 to [[selected index]].
Assign null to [[codec implementation]].
If init.preferAnimation exists, assign init.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 type ReadableStream:
1. Assign a new list to [[encoded data]].
2. Assign false to complete
3. Queue a control message to configure the image decoder with init.
4. Let reader be the result of getting a reader for data.
5. In parallel, perform the Fetch Stream Data Loop on d with reader.
Otherwise:
1. Assert that init.data is of type BufferSource.
2. Assign a copy of init.data to [[encoded data]].
3. Assign true to complete.
4. Reslove [[completed promise]].
5. Queue a control message to configure the image decoder with init.
6. Queue a control message to decode track metadata.
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, queue a task on the control thread event loop to run the Close ImageDecoder algorithm with a NotSupportedError DOMException and abort these steps.
If supported is true, assign the [[codec implementation]] internal slot with an implementation supporting init.type
Configure [[codec implementation]] in accordance with the values given for premultiplyAlpha, colorSpaceConversion, desiredWidth, and desiredHeight.

Running a control message to decode track metadata means running these steps:

Run the Establish Tracks algorithm.

10.2.3. Attributes

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 becomes true.

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]] is true, return a Promise rejected with an InvalidStateError DOMException.
If [[ImageTrackList]]'s [[selected index]] is '-1', return a Promise rejected with an InvalidStateError DOMException.
If options is undefined, assign a new ImageDecodeOptions to options.
Let promise be a new Promise.
Queue a control message to decode the the image with options, and promise.
Append promise to [[pending decode promises]].
Return promise.

Running a control message to decode the image means running these steps:

Wait for [[tracks established]] to become true.
If options.completeFramesOnly is false 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 and AbortError DOMException.

close()

Immediately aborts all pending work and releases system resources. Close is final.

When invoked, run the Close ImageDecoder algorithm with and 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 with TypeError.
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
1. If [[closed]] is true, abort these steps.
2. If chunk is not a Uint8Array object, queue a task on the control thread event loop to run the Close ImageDecoder algorithm with a DataError DOMException and abort these steps.
3. Let bytes be the byte sequence represented by the Uint8Array object.
4. Append bytes to the [[encoded data]] internal slot.
5. If [[tracks established]] is false, run the Establish Tracks algorithm.
6. Otherwise, run the Update Tracks algorithm.
7. Run the Fetch Stream Data Loop algorithm with reader.
close steps
1. Assign true to complete
2. Resolve [[completed promise]].
error steps
1. Queue a task on the control thread event loop 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]] is false.
If [[encoded data]] does not contain enough data to determine the number of tracks:
1. If complete is true, queue a task on the control thread event loop to run the Close ImageDecoder algorithm.
2. Abort these steps.
If the number of tracks is found to be 0, queue a task on the control thread event loop to run the Close ImageDecoder algorithm and abort these steps.
Let newTrackList be a new list.
For each image track found in [[encoded data]]:
1. Let newTrack be a new ImageTrack, initialized as follows:
  1. Assign this to [[ImageDecoder]].
  2. Assign tracks to [[ImageTrackList]].
  3. If image track is found to be animated, assign true to newTrack’s [[animated]] internal slot. Otherwise, assign false.
  4. If image track is found to describe a frame count, assign that count to newTrack’s [[frame count]] internal slot. Otherwise, assign 0.
    
    NOTE: If this was constructed with data as a ReadableStream, the frameCount may change as additional bytes are appended to [[encoded data]]. See the Update Tracks algorithm.
  5. If image track is found to describe a repetition count, assign that count to [[repetition count]] internal slot. Otherwise, assign 0.
    
    NOTE: A value of Infinity indicates infinite repetitions.
  6. Assign false to newTrack’s [[selected]] internal slot.
2. 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 on the control thread event loop to perform the following steps:
1. Assign newTrackList to the tracks [[track list]] internal slot.
2. Assign selectedTrackIndex to tracks [[selected index]].
3. Resolve [[ready promise]].

Get Default Selected Track Index (with trackList)

Run these steps:

If [[encoded data]] identifies a Primary Image Track:
1. Let primaryTrack be the ImageTrack from trackList that describes the Primary Image Track.
2. Let primaryTrackIndex be position of primaryTrack within trackList.
3. If [[prefer animation]] is null, return primaryTrackIndex.
4. If primaryTrack.animated equals [[prefer animation]], return primaryTrackIndex.
If any ImageTracks in trackList have animated 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]] is true.
Let trackChanges be a new list.
Let trackList be a copy of tracks' [[track list]].
For each track in trackList:
1. Let trackIndex be the position of track in trackList.
2. Let latestFrameCount be the frame count as indicated by [[encoded data]] for the track corresponding to track.
3. Assert that latestFrameCount is greater than or equal to track.frameCount.
4. If latestFrameCount is greater than track.frameCount:
  1. Let change be a track update struct whose track index is trackIndex and frame count is latestFrameCount.
  2. Append change to tracksChanges.
If tracksChanges is empty, abort these steps.
Queue a task on the control thread event loop to perform the following steps:
1. For each update in trackChanges:
  1. Let updateTrack be the ImageTrack at position update.trackIndex within tracks' [[track list]].
  2. Assign update.frameCount to updateTrack’s [[frame count]].
  3. Fire a simple event named change at the tracks object.

Decode Complete Frame (with frameIndex and promise)

Assert that [[tracks established]] is true.
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):
1. [[encoded data]] contains enough bytes to completely decode encodedFrame.
2. [[encoded data]] is found to be malformed.
3. complete is true.
4. [[closed]] is true.
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:
1. Assign 'true' to complete.
2. Let timestamp and duration be the presentation timestamp and duration for output as described by encodedFrame. If encodedFrame does not describe a timestamp or duration, assign null to the corresponding variable.
3. 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]] is true.
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):
1. [[encoded data]] contains enough bytes to decode encodedFrame to produce an output who’s Progressive Image Frame Generation exceeds lastFrameGeneration.
2. [[encoded data]] is found to be malformed.
3. complete is true.
4. [[closed]] is true.
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 who’s 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:
1. Assign true to decodeResult’s complete.
2. If [[progressive frame generations]] contains an entry keyed by frameIndex, remove the entry from the map.
Otherwise:
1. Assign false to decodeResult’s complete.
2. Let frameGeneration be the Progressive Image Frame Generation for output.
3. Add a new entry to [[progressive frame generations]] with key frameIndex and value frameGeneration.
Let timestamp and duration be the presentation timestamp and duration for output as described by encodedFrame. If encodedFrame does not describe a timestamp or duration, assign null to the corresponding variable.
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 on the control thread event loop to run these steps:
1. If [[closed]], abort these steps.
2. Assert that promise is an element of [[pending decode promises]].
3. Remove promise from [[pending decode promises]].
4. Resolve promise with result.

Reject Infeasible Decode (with promise)

Assert that complete is true or [[closed]] is true.
If complete is true, let exception be a RangeError. Otherwise, let exception be an InvalidStateError DOMException.
Queue a task on the control thread event loop to run these steps:
1. If [[closed]], abort these steps.
2. Assert that promise is an element of [[pending decode promises]].
3. Remove promise from [[pending decode promises]].
4. Reject promise with exception.

Fatally Reject Bad Data

Queue a task on the control thread event loop to run these steps:
1. If [[closed]], abort these steps.
2. 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]]:
1. Reject decodePromise with exception.
2. 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;
  PremultiplyAlpha premultiplyAlpha = "default";
  ColorSpaceConversion colorSpaceConversion = "default";
  unsigned long desiredWidth;
  unsigned long desiredHeight;
  boolean preferAnimation;
};

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, return false.
If data is of type BufferSource:
1. If the result of running IsDetachedBuffer (described in [ECMASCRIPT]) on data is false, return false.
2. If data is empty, return false.
If desiredWidth exists and desiredHeight does not exist, return false.
If desiredHeight exists and desiredWidth does not exist, return false.
Return true.

A valid image MIME type is a string that is a valid MIME type string and for which the type, per Section 3.1.1.1 of [RFC7231], is image.

type, of type DOMString

String containing the MIME type of the image file to be decoded.

data, of type ImageBufferSource

BufferSource or ReadableStream of bytes representing an encoded image file as described by type.

premultiplyAlpha, of type PremultiplyAlpha, defaulting to "default"

Controls whether decoded outputs' color channels are to be premultiplied by their alpha channel, as defined by premultiplyAlpha in ImageBitmapOptions.

colorSpaceConversion, of type ColorSpaceConversion, defaulting to "default"

Controls whether decoded outputs' color space is converted or ignored, as defined by colorSpaceConversion in ImageBitmapOptions.

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 {
  unsigned long frameIndex = 0;
  boolean completeFramesOnly = true;
};

frameIndex, of type unsigned long, defaulting to 0

The index of the frame to decode.

completeFramesOnly, of type boolean, defaulting to true

For Progressive Images, a value of false indicates that the decoder may output an image with reduced detail. Each subsequent call to decode() for the same frameIndex 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 assigned true, 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 to decode() will only resolve once the full detail image is decoded.

NOTE: For Progressive Images, setting completeFramesOnly to false may be used to offer users a preview an image that is still being buffered from the network (via the data ReadableStream).

Upon decoding the full detail image, the ImageDecodeResult's complete 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 always true when decode() is invoked with completeFramesOnly set to true.

10.6. ImageTrackList Interface

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 with ImageTracks.

NOTE: ImageTrack frameCount may receive subsequent updates until complete is true.

[[track list]]

The list of ImageTracks describe by this ImageTrackList.

[[selected index]]

The index of the selected track in [[track list]]. A value of -1 indeicates 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, return null.
Otherwise, return the ImageTrack from [[track list]] at the position indicated by [[selected index]].

10.7. ImageTrack Interface

interface ImageTrack : EventTarget {
  readonly attribute boolean animated;
  readonly attribute unsigned long frameCount;
  readonly attribute unrestricted float repetitionCount;
  attribute EventHandler onchange;
  attribute boolean selected;
};

10.7.1. Internal Slots

[[ImageDecoder]]: The ImageDecoder instance that constructed this ImageTrack.
[[ImageTrackList]]: The ImageTrackList instance that lists this ImageTrack.
[[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 the frameCount starts at 0 and later increments as new chunks of the ReadableStream 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]].

onchange, of type EventHandler

An event handler IDL attribute whose event handler event type is change.

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 is true, 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:
1. Let oldSelectedTrack be the ImageTrack in parentTrackList [[track list]] at the position of oldSelectedIndex.
2. Assign false to oldSelectedTrack [[selected]]
If newValue is true, let selectedIndex be the index of this ImageTrack 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.

Running a control message to update the internal selected track index means running these steps:

Assign selectedIndex to [[internal selected track index]].
Remove all entries from [[progressive frame generations]].

10.7.3. Event Summary

change: Fired at the ImageTrack when the frameCount is altered.

11. 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 may 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.

User agents should 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. Specifically, it should not be possible for a site to mutate a codec input or output while the underlying codec may still be operating on that data. This concern is mitigated by ensuring that input and output interfaces are immutable.

EncodedVideoChunk and EncodedAudioChunk currently expose a mutable data. See #80.

12. 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 may be used with other metrics to create a fingerprint.

An attacker may accumulate a codec feature profile by calling IsConfigSupported() methods with a number of different configuration dictionaries. Similarly, an attacker may attempt to configure() a codec with different configuration dictionaries and observe which configurations are accepted.

Attackers may 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 may signal when a decoder uses hardware acceleration. Similarly, the [WebRTC] getCapabilities() API may be used to determine what types of encoders are supported and the getStats() API may 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 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 may run 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.

WebCodecs

W3C Working Draft, 3 May 2021

Abstract

Status of this document

1. Definitions

2. Codec Processing Model

2.1. Background

2.2. Control Thread and Codec Thread

3. AudioDecoder Interface

3.1. Internal Slots

3.2. Constructors

3.3. Attributes

3.4. Methods

3.5. Algorithms

4. VideoDecoder Interface

4.1. Internal Slots

4.2. Constructors

4.3. Attributes

4.4. Methods

4.5. Algorithms

5. AudioEncoder Interface

5.1. Internal Slots

5.2. Constructors

5.3. Attributes

5.4. Methods

5.5. Algorithms

6. VideoEncoder Interface

6.1. Internal Slots

6.2. Constructors

6.3. Attributes

6.4. Methods

6.5. Algorithms

7. Configurations

7.1. Check Configuration Support (with config)

7.2. Clone Configuration (with config)

7.3. Signalling Configuration Support

7.3.1. AudioDecoderSupport

7.3.2. VideoDecoderSupport

7.3.3. AudioEncoderSupport

7.3.4. VideoEncoderSupport

7.4. Codec String

7.5. AudioDecoderConfig

7.6. VideoDecoderConfig

7.7. AudioEncoderConfig

7.8. VideoEncoderConfig

7.9. Hardware Acceleration

7.10. Configuration Equivalence

7.11. VideoEncoderEncodeOptions

7.12. CodecState

7.13. WebCodecsErrorCallback

8. Encoded Media Interfaces (Chunks)

8.1. EncodedAudioChunk Interface

8.1.1. Constructors

8.1.2. Attributes

8.2. EncodedVideoChunk Interface

8.2.1. Constructors

8.2.2. Attributes

9. Raw Media Interfaces (Frames)

9.1. AudioFrame Interface

9.1.1. Internal Slots

9.1.2. Constructors

9.1.3. Attributes

9.1.4. Methods

9.2. VideoFrame Interface

9.2.1. Internal Slots

9.2.2. Constructors

9.2.3. Attributes

9.2.4. Methods

9.2.5. Algorithms

9.3. Plane Interface

9.3.1. Internal Slots

9.3.2. Attributes

9.3.3. Methods

9.4. Pixel Format

9.5. Algorithms

10. Image Decoding

10.1. Background

10.2. ImageDecoder Interface

10.2.1. Internal Slots

10.2.2. Constructor

7.1. Check Configuration Support (with `config`)

7.2. Clone Configuration (with `config`)