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 or 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.audioRubstness is present , audio MUST also be present .
   2. If keySystemConfiguration.videoRubstness 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 ).

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 DOMString framerate;
};

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 none of the following is true, return false and abort these steps:
   • Applying the rules for parsing floating-point number values to configuration ’s framerate results in a number that is finite and greater than 0.
   • configuration ’s framerate contains one occurence occurrence of U+002F SLASH character (/) and the substrings before and after this character, when applying the rules for parsing floating-point number values results in a number that is finite and greater than 0.
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 either as a double or as a fraction.

2.1.6. AudioConfiguration

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

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 .

2.3. Algorithms

2.3. 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;
};

2.4. 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 and the encodingInfo() method method MUST run the following steps:

1. If configuration is not a valid MediaConfiguration MediaDecodingConfiguration , return a Promise rejected with a newly created TypeError .
2. If configuration.video configuration.keySystemConfiguration is present , run the following substeps:
   1. If the global object and is not a valid video configuration , of type WorkerGlobalScope , return a Promise rejected with a newly created TypeError .
   2. If configuration.audio 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 present 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 audio configuration MediaConfiguration , return a Promise rejected with a newly created TypeError .
2. Let p be a new promise.
3. In parallel , run the create Create a MediaCapabilitiesInfo algorithm algorithm with configuration and resolve p with its result.
4. Return p .

3.1. Screen Luminance

interface ScreenLuminance {
  readonly attribute double min;
  readonly attribute double max;
  readonly attribute double maxAverage;
};

The ScreenLuminance object represents the known luminance characteristics of the screen.

The min attribute MUST return the minimal screen luminance that a pixel of the screen can emit in candela per square metre. The minimal screen luminance is the luminance used when showing the darkest color a pixel on the screen can display.

The max attribute MUST return the maximal screen luminance that a pixel of the screen can emit in candela per square metre. The maximal screen luminance is the luminance used when showing the whitest color a pixel on the screen can display.

The maxAverage attribute MUST return the maximal average screen luminance that the screen can emit in candela per square metre. The maximal average screen luminance is the maximal luminance value such as all the pixels of the screen emit the same luminance. The value returned by maxAverage is expected to be different from max as screens usually can’t apply the maximal screen luminance to the entire panel.

3.2. Screen Color Gamut

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

The ScreenColorGamut represents the color gamut supported by a Screen , that means the range of color that the screen can display.

The ScreenColorGamut values are:

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

3.3. Screen extension

Part of this section is 🐵 patching of the CSSOM View Module. Issue #4 is tracking merging the changes. This partial interface requires the Screen interface to become an EventTarget .

partial interface Screen {
  readonly attribute ScreenColorGamut colorGamut;
  readonly attribute ScreenLuminance? luminance;
  attribute EventHandler onchange;
};

The colorGamut attribute SHOULD return the ScreenColorGamut approximately supported by the screen. In other words, the screen does not need to fully support the given color gamut but needs to be close enough. If the user agent does not know the color gamut supported by the screen, if the supported color gamut is lower than srgb , or if the user agent does not want to expose this information for privacy consideration, it SHOULD return srgb as a default value. The value returned by colorGamut MUST match the value returned by the color-gamut CSS media query.

The luminance attribute SHOULD return a ScreenLuminance object that will expose the luminance characteristics of the screen. If the user agent has no access to the luminance characteristics of the screen, it MUST return null . The user agent MAY also return null if it does not want to expose the luminance information for privacy reasons.

The onchange attribute is an event handler whose corresponding event handler event type is change .

Whenever the user agent is aware that the state of the Screen object has changed, that is if one the value exposed on the Screen object or in an object exposed on the Screen object, it MUST queue a task to fire an event named change on Screen .

5.1. Query recording capabilities with encodingInfo()

<script>
  const configuration = {
      type : 'record',
      video : {
        contentType : 'video/webm;codecs=vp8',
        width : 640,
        height : 480,
        bitrate : 10000,
        framerate : '30'
    }
  };
  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>