Media Capabilities

2.1.1. MediaConfiguration

dictionary MediaConfiguration {
  VideoConfiguration video;
  AudioConfiguration audio;
};

dictionary MediaDecodingConfiguration : MediaConfiguration {
  required MediaDecodingType type;
  MediaCapabilitiesKeySystemConfiguration keySystemConfiguration;
};

dictionary MediaEncodingConfiguration : MediaConfiguration {
  required MediaEncodingType type;
};

The input to the decoding capabilities is represented by a MediaDecodingConfiguration dictionary and the input of the encoding capabilities by a MediaEncodingConfiguration dictionary.

For a MediaConfiguration to be a valid MediaConfiguration, all of the following conditions MUST be true:

1. audio and/or video MUST be present.
2. audio MUST be a valid audio configuration if present.
3. video MUST be a valid video configuration if present.

For a MediaDecodingConfiguration to be a valid MediaDecodingConfiguration, all of the following conditions MUST be true:

1. It MUST be a valid MediaConfiguration.
2. If keySystemConfiguration is present:
   1. If keySystemConfiguration.audio is present, audio MUST also be present.
   2. If keySystemConfiguration.video is present, video MUST also be present.

For a MediaDecodingConfiguration to describe [ENCRYPTED-MEDIA], a keySystemConfiguration MUST be present.

2.1.2. MediaDecodingType

enum MediaDecodingType {
  "file",
  "media-source",
};

A MediaDecodingConfiguration has two types:

• file is used to represent a configuration that is meant to be used for a plain file playback.
• media-source is used to represent a configuration that is meant to be used for playback of a MediaSource as defined in the [media-source] specification.

2.1.3. MediaEncodingType

enum MediaEncodingType {
  "record",
  "transmission"
};

A MediaEncodingConfiguration can have one of two types:

• record is used to represent a configuration for recording of media, e.g. using MediaRecorder as defined in [mediastream-recording].
• transmission is used to represent a configuration meant to be transmitted over electronic means (e.g. using RTCPeerConnection as defined in [webrtc]).

2.1.4. MIME types

In the context of this specification, a MIME type is also called content type. A valid media MIME type is a string that is a valid MIME type per [mimesniff]. If the MIME type does not imply a codec, the string MUST also have one and only one parameter that is named codecs with a value describing a single media codec. Otherwise, it MUST contain no parameters.

A valid audio MIME type is a string that is valid media MIME type and for which the type per [RFC7231] is either audio or application.

A valid video MIME type is a string that is a valid media MIME type and for which the type per [RFC7231] is either video or application.

2.1.5. VideoConfiguration

dictionary VideoConfiguration {
  required DOMString contentType;
  required unsigned long width;
  required unsigned long height;
  required unsigned long long bitrate;
  required double framerate;
  boolean hasAlphaChannel;
  HdrMetadataType hdrMetadataType;
  ColorGamut colorGamut;
  TransferFunction transferFunction;
};

The contentType member represents the MIME type of the video track.

To check if a VideoConfiguration configuration is a valid video configuration, the following steps MUST be run:

1. If configuration’s contentType is not a valid video MIME type, return false and abort these steps.
2. If framerate is not finite or is not greater than 0, return false and abort these steps.
3. Return true.

The width and height members represent respectively the visible horizontal and vertical encoded pixels in the encoded video frames.

The bitrate member represents the average bitrate of the video track given in units of bits per second. In the case of a video stream encoded at a constant bit rate (CBR) this value should be accurate over a short term window. For the case of variable bit rate (VBR) encoding, this value should be usable to allocate any necessary buffering and throughput capability to provide for the un-interrupted decoding of the video stream over the long-term based on the indicated contentType.

The framerate member represents the framerate of the video track. The framerate is the number of frames used in one second (frames per second). It is represented as a double.

The hasAlphaChannel member represents whether the video track contains alpha channel information. If true, the encoded video stream can produce per-pixel alpha channel information when decoded. If false, the video stream cannot produce per-pixel alpha channel information when decoded. If undefined, the UA should determine whether the video stream encodes alpha channel information based on the indicated contentType, if possible. Otherwise, the UA should presume that the video stream cannot produce alpha channel information.

If present, the hdrMetadataType member represents that the video track includes the specified HDR metadata type, which the UA needs to be capable of interpreting for tone mapping the HDR content to a color volume and luminance of the output device. Valid inputs are defined by HdrMetadataType.

If present, the colorGamut member represents that the video track is delivered in the specified color gamut, which describes a set of colors in which the content is intended to be displayed. If the attached output device also supports the specified color, the UA needs to be able to cause the output device to render the appropriate color, or something close enough. If the attached output device does not support the specified color, the UA needs to be capable of mapping the specified color to a color supported by the output device. Valid inputs are defined by ColorGamut.

If present, the transferFunction member represents that the video track requires the specified transfer function to be understood by the UA. Transfer function describes the electro-optical algorithm supported by the rendering capabilities of a user agent, independent of the display, to map the source colors in the decoded media into the colors to be displayed. Valid inputs are defined by TransferFunction.

2.1.6. HdrMetadataType

enum HdrMetadataType {
  "smpteSt2086",
  "smpteSt2094-10",
  "smpteSt2094-40"
};

If present, HdrMetadataType describes the capability to interpret HDR metadata of the specified type.

The VideoConfiguration may contain one of the following types:

• smpteSt2086, representing the static metadata type defined by [SMPTE-ST-2086].
• smpteSt2094-10, representing the dynamic metadata type defined by [SMPTE-ST-2094].
• smpteSt2094-40, representing the dynamic metadata type defined by [SMPTE-ST-2094].

2.1.7. ColorGamut

enum ColorGamut {
  "srgb",
  "p3",
  "rec2020"
};

The VideoConfiguration may contain one of the following types:

• srgb, representing the [sRGB] color gamut.
• p3, representing the DCI P3 Color Space color gamut. This color gamut includes the srgb gamut.
• rec2020, representing the ITU-R Recommendation BT.2020 color gamut. This color gamut includes the p3 gamut.

2.1.8. TransferFunction

enum TransferFunction {
  "srgb",
  "pq",
  "hlg"
};

The VideoConfiguration may contain one of the following types:

• srgb, representing the transfer function defined by [sRGB].
• pq, representing the "Perceptual Quantizer" transfer function defined by [SMPTE-ST-2084].
• hlg, representing the "Hybrid Log Gamma" transfer function defined by BT.2100.

2.1.9. AudioConfiguration

dictionary AudioConfiguration {
  required DOMString contentType;
  DOMString channels;
  unsigned long long bitrate;
  unsigned long samplerate;
  boolean spatialRendering;
};

The contentType member represents the MIME type of the audio track.

To check if a AudioConfiguration configuration is a valid audio configuration, the following steps MUST be run:

1. If configuration’s contentType is not a valid audio MIME type, return false and abort these steps.
2. Return true.

The channels member represents the audio channels used by the audio track.

The channels needs to be defined as a double (2.1, 4.1, 5.1, ...), an unsigned short (number of channels) or as an enum value. The current definition is a placeholder.

The bitrate member represents the number of average bitrate of the audio track. The bitrate is the number of bits used to encode a second of the audio track.

The samplerate represents the samplerate of the audio track in. The samplerate is the number of samples of audio carried per second.

The samplerate is expressed in Hz (ie. number of samples of audio per second). Sometimes the samplerates value are expressed in kHz which represents the number of thousands of samples of audio per second.
44100 Hz is equivalent to 44.1 kHz.

The spatialRendering member indicates that the audio SHOULD be renderered spatially. The details of spatial rendering SHOULD be inferred from the contentType. If not present, the UA MUST presume spatialRendering is not required. When true, the user agent SHOULD only report this configuration as supported if it can support spatial rendering *for the current audio output device* without failing back to a non-spatial mix of the stream.

2.3. Algorithms

2.4. Navigator and WorkerNavigator extension

[Exposed=Window]
partial interface Navigator {
  [SameObject] readonly attribute MediaCapabilities mediaCapabilities;
};

[Exposed=Worker]
partial interface WorkerNavigator {
  [SameObject] readonly attribute MediaCapabilities mediaCapabilities;
};

2.5. Media Capabilities Interface

[Exposed=(Window, Worker)]
interface MediaCapabilities {
  [NewObject] Promise<MediaCapabilitiesDecodingInfo> decodingInfo(MediaDecodingConfiguration configuration);
  [NewObject] Promise<MediaCapabilitiesInfo> encodingInfo(MediaEncodingConfiguration configuration);
};

The decodingInfo() method method MUST run the following steps:

1. If configuration is not a valid MediaDecodingConfiguration, return a Promise rejected with a newly created TypeError.
2. If configuration.keySystemConfiguration is present, run the following substeps:
   1. If the global object is of type WorkerGlobalScope, return a Promise rejected with a newly created DOMException whose name is InvalidStateError.
   2. If the result of running Is the environment settings object settings a secure context? [secure-contexts] with the global object’s relevant settings object is not "Secure", return a Promise rejected with a newly created DOMException whose name is SecurityError.
3. Let p be a new promise.
4. In parallel, run the Create a MediaCapabilitiesDecodingInfo algorithm with configuration and resolve p with its result.
5. Return p.

Note, calling decodingInfo() with a keySystemConfiguration present may have user-visible effects, including requests for user consent. Such calls should only be made when the author intends to create and use a MediaKeys object with the provided configuration.

The encodingInfo() method MUST run the following steps:

1. If configuration is not a valid MediaConfiguration, return a Promise rejected with a newly created TypeError.
2. Let p be a new promise.
3. In parallel, run the Create a MediaCapabilitiesEncodingInfo algorithm with configuration and resolve p with its result.
4. Return p.

4.1. Query recording capabilities with encodingInfo()

<script>
  const configuration = {
      type : 'record',
      video : {
        contentType : 'video/webm;codecs=vp8',
        width : 640,
        height : 480,
        bitrate : 10000,
        framerate : 29.97
    }
  };
  navigator.mediaCapabilities.encodingInfo(configuration)
      .then((result) => {
        console.log(result.contentType + ' is:'
            + (result.supported ? '' : ' NOT') + ' supported,'
            + (result.smooth ? '' : ' NOT') + ' smooth and'
            + (result.powerEfficient ? '' : ' NOT') + ' power efficient');
      })
      .catch((err) => {
        console.error(err, ' caused encodingInfo to throw');
      });
</script>