WebRTC Encoded Transform

1. Introduction

The [WEBRTC-NV-USE-CASES] document describes several functions that can only be achieved by access to media (requirements N20-N22), including, but not limited to:

• Funny Hats

• Machine Learning

• Virtual Reality Gaming

These use cases further require that processing can be done in worker threads (requirement N23-N24).

Furthermore, the "trusted JavaScript cloud conferencing" use case requires such processing to be done on encoded media, not just the raw media.

This specification gives an interface inspired by [WEB-CODECS] to provide access to such functionality while retaining the setup flow of RTCPeerConnection.

This iteration of the specification provides access to encoded media, which is the output of the encoder part of a codec and the input to the decoder part of a codec.

2. Terminology

3. Specification

The Streams definition doesn’t use WebIDL much, but the WebRTC spec does. This specification shows the IDL extensions for WebRTC.

It uses an extension to RTCConfiguration in order to notify the RTCPeerConnection that insertable streams will be used, and uses an additional API on RTCRtpSender and RTCRtpReceiver to insert the processing into the pipeline.

// New dictionary
dictionary RTCInsertableStreams {
    ReadableStream readable;
    WritableStream writable;
};

// New fields in RTCConfiguration
partial dictionary RTCConfiguration {
    boolean encodedInsertableStreams = false;
};

typedef (SFrameTransform or RTCRtpScriptTransform) RTCRtpTransform;

// New methods for RTCRtpSender and RTCRtpReceiver
partial interface RTCRtpSender {
    RTCInsertableStreams createEncodedStreams();
    attribute RTCRtpTransform? transform;
};

partial interface RTCRtpReceiver {
    RTCInsertableStreams createEncodedStreams();
    attribute RTCRtpTransform? transform;
};

3.1. Extension operation

At the time when a codec is initialized as part of the encoder, and the corresponding flag is set in the RTCPeerConnection's RTCConfiguration argument, ensure that the codec is disabled and produces no output.

3.1.1. Stream creation

At construction of each RTCRtpSender or RTCRtpReceiver, run the following steps:

1. Initialize this.[[Streams]] to null.

2. Initialize this.[[transform]] to null.

3. Initialize this.[[readable]] to a new ReadableStream.

4. Set up this.[[readable]]. this.[[readable]] is provided frames using the readEncodedData algorithm given this as parameter.

5. Set this.[[readable]].[[owner]] to this.

6. Initialize this.[[writable]] to a new WritableStream.

7. Set up this.[[writable]] with its writeAlgorithm set to writeEncodedData given this as parameter.

8. Set this.[[writable]].[[owner]] to this.

9. Initialize this.[[pipeToController]] to null.

10. Initialize this.[[lastReceivedFrameTimestamp]] to zero.

11. If the RTCPeerConnection's configuration does not have encodedInsertableStreams set to "true", queue a task to run the following steps:

    1. If this.[[pipeToController]] is not null, abort these steps.

    2. Set this.[[pipeToController]] to a new AbortController.

    3. Call pipeTo with this.[[readable]], this.[[writable]], preventClose equal to true, preventAbort equal to true, preventCancel equal to true and this.[[pipeToController]].signal.

The createEncodedStreams() method steps are:

1. If the RTCPeerConnection's configuration does not have encodedInsertableStreams set to "true", throw an "InvalidAccessError" DOMException and abort these steps.

2. If the data source does not permit access, throw an "InvalidAccessError" DOMException and abort these steps.

3. If this.[[Streams]] is not null, throw an "InvalidAccessError" DOMException.

4. If this.[[pipeToController]] is not null, throw an "InvalidAccessError" DOMException.

5. Set this.[[Streams]] to an RTCInsertableStreams object.

6. Set this.[[Streams]].readable to this.[[readable]].

7. Set this.[[Streams]].writable to this.[[writable]].

8. Enable the encoded data source.

9. Return this.[[Streams]].

3.1.2. Stream processing

The readEncodedData algorithm is given a rtcObject as parameter. It is defined by running the following steps:

1. Wait for a frame to be produced by rtcObject’s encoder if it is a RTCRtpSender or rtcObject’s packetizer if it is a RTCRtpReceiver.

2. Let frame be the newly produced frame.

3. Set frame.[[owner]] to rtcObject.

4. Enqueue frame in rtcObject.[[readable]].

The writeEncodedData algorithm is given a rtcObject as parameter and a frame as input. It is defined by running the following steps:

1. If frame.[[owner]] is not equal to rtcObject, abort these steps and return a promise resolved with undefined. A processor cannot create frames, or move frames between streams.

2. If the frame’s timestamp is equal to or larger than rtcObject.[[lastReceivedFrameTimestamp]], abort these steps and return a promise resolved with undefined. A processor cannot reorder frames, although it may delay them or drop them.

3. Set rtcObject.[[lastReceivedFrameTimestamp]] to the frame’s timestamp.

4. Enqueue the frame for processing as if it came directly from the encoded data source, by running one of the following steps:

   • If rtcObject is a RTCRtpSender, enqueue it to rtcObject’s packetizer, to be processed in parallel.

   • If rtcObject is a RTCRtpReceiver, enqueue it to rtcObject’s decoder, to be processed in parallel.

5. Return a promise resolved with undefined.

3.2. Extension attribute

A RTCRtpTransform has two private slots called [[readable]] and [[writable]].

The transform getter steps are:

1. Return this.[[transform]].

The transform setter steps are:

2. Let transform be the argument to the setter.

3. Let checkedTransform set to transform if it is not null or to an identity transform stream otherwise.

4. Let reader be the result of getting a reader for checkedTransform.[[readable]].

5. Let writer be the result of getting a writer for checkedTransform.[[writable]].

6. Initialize newPipeToController to a new AbortController.

7. If this.[[pipeToController]] is not null, run the following steps:

   1. Add the chain transform algorithm to this.[[pipeToController]].signal.

   2. signal abort this.[[pipeToController]].signal.

8. Else, run the chain transform algorithm steps.

9. Set this.[[pipeToController]] to newPipeToController.

10. Set this.[[transform]] to transform.

The chain transform algorithm steps are defined as:

1. If newPipeToController’s aborted flag is true, abort these steps.

2. Release reader.

3. Release writer.

4. Assert that newPipeToController is the same object as rtcObject.[[pipeToController]].

5. Call pipeTo with rtcObject.[[readable]], checkedTransform.[[writable]], preventClose equal to false, preventAbort equal to false, preventCancel equal to true and newPipeToController.signal.

6. Call pipeTo with checkedTransform.[[readable]], rtcObject.[[writable]], preventClose equal to true, preventAbort equal to true, preventCancel equal to false and newPipeToController.signal.

This algorithm is defined so that transforms can be updated dynamically. There is no guarantee on which frame will happen the switch from the previous transform to the new transform.

If a web application sets the transform synchronously at creation of the RTCRtpSender (for instance when calling addTrack), the transform will receive the first frame generated by the RTCRtpSender's encoder. Similarly, if a web application sets the transform synchronously at creation of the RTCRtpReceiver (for instance when calling addTrack, or at track event handler), the transform will receive the first full frame generated by the RTCRtpReceiver's packetizer.

4. SFrameTransform

enum SFrameTransformRole {
    "encrypt",
    "decrypt"
};

dictionary SFrameTransformOptions {
    SFrameTransformRole role = "encrypt";
};

typedef [EnforceRange] unsigned long long SmallCryptoKeyID;
typedef (SmallCryptoKeyID or bigint) CryptoKeyID;

[Exposed=(Window,DedicatedWorker)]
interface SFrameTransform {
    constructor(optional SFrameTransformOptions options = {});
    Promise<undefined> setEncryptionKey(CryptoKey key, optional CryptoKeyID keyID);
};
SFrameTransform includes GenericTransformStream;

The new SFrameTransform(options) constructor steps are:

1. Let transformAlgorithm be an algorithm which takes a frame as input and runs the SFrame transform algorithm with this and frame.

2. Set this.[[transform]] to a new TransformStream.

3. Set up this.[[transform]] with transformAlgorithm set to transformAlgorithm.

4. Let options be the method’s first argument.

5. Set this.[[role]] to options["role"].

6. Set this.[[readable]] to this.[[transform]].[[readable]].

7. Set this.[[writable]] to this.[[transform]].[[writable]].

4.1. Algorithm

The SFrame transform algorithm, given sframe as a SFrameTransform object and frame, runs these steps:

1. Let role be sframe.[[role]].

2. If frame.[[owner]] is a RTCRtpSender, set role to 'encrypt'.

3. If frame.[[owner]] is a RTCRtpReceiver, set role to 'decrypt'.

4. Let data be undefined.

5. If frame is a BufferSource, set data to frame.

6. If frame is a RTCEncodedAudioFrame, set data to frame.data

7. If frame is a RTCEncodedVideoFrame, set data to frame.data

8. If data is undefined, abort these steps.

9. Let buffer be the result of running the SFrame algorithm with data and role as parameters. This algorithm is defined by the SFrame specification and returns an ArrayBuffer.

10. If frame is a BufferSource, set frame to buffer.

11. If frame is a RTCEncodedAudioFrame, set frame.data to buffer.

12. If frame is a RTCEncodedVideoFrame, set frame.data to buffer.

13. Enqueue frame in sframe.[[transform]].

4.2. Methods

1. Let promise be a new promise.

2. If keyID is a bigint which cannot be represented as a integer between 0 and 2⁶⁴-1 inclusive, reject promise with a RangeError exception.

3. Otherwise, in parallel, run the following steps:

   1. Set key with its optional keyID as key material to use for the SFrame transform algorithm, as defined by the SFrame specification.

   2. If setting the key material fails, reject promise with an InvalidModificationError exception and abort these steps.

   3. Resolve promise with undefined.

4. Return promise.

5. RTCRtpScriptTransform

interface mixin EncodedMediaChunk {
  readonly attribute unsigned long long timestamp;  // microseconds
  readonly attribute ArrayBuffer data;
};

// WebCodecs definitions with introduction of EncodedMediaChunk to more easily refer to timestamp and data.
// They should be removed from this spec at some point.
[Exposed=(Window,DedicatedWorker)]
interface EncodedVideoChunk {
  constructor(EncodedVideoChunkInit init);
  readonly attribute EncodedVideoChunkType type;
  readonly attribute unsigned long long? duration;  // microseconds
};
EncodedVideoChunk includes EncodedMediaChunk;

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

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

[Exposed=(Window,DedicatedWorker)]
interface EncodedAudioChunk {
  constructor(EncodedAudioChunkInit init);
  readonly attribute EncodedAudioChunkType type;
};
EncodedAudioChunk includes EncodedMediaChunk;

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

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

dictionary RTCEncodedVideoFrameMetadata {
    long long frameId;
    sequence<long long> dependencies;
    unsigned short width;
    unsigned short height;
    long spatialIndex;
    long temporalIndex;
    long synchronizationSource;
    sequence<long> contributingSources;
};

[Exposed=(Window,DedicatedWorker)]
interface RTCEncodedVideoFrame : EncodedVideoChunk {
    RTCEncodedVideoFrameMetadata getMetadata();
};

dictionary RTCEncodedAudioFrameMetadata {
    long synchronizationSource;
    sequence<long> contributingSources;
};

[Exposed=(Window,DedicatedWorker)]
interface RTCEncodedAudioFrame : EncodedAudioChunk {
    RTCEncodedAudioFrameMetadata getMetadata();
};

// New interfaces to expose JavaScript-based transforms.

[Exposed=DedicatedWorker]
interface RTCTransformEvent : Event {
    readonly attribute RTCRtpScriptTransformer transformer;
};

partial interface DedicatedWorkerGlobalScope {
    attribute EventHandler onrtctransform;
};

[Exposed=DedicatedWorker]
interface RTCRtpScriptTransformer {
    readonly attribute ReadableStream readable;
    readonly attribute WritableStream writable;
    readonly attribute any options;
};

[Exposed=Window]
interface RTCRtpScriptTransform {
    constructor(Worker worker, optional any options, optional sequence<object> transfer);
};

5.1. Operations

The new RTCRtpScriptTransform(worker, options, transfer) constructor steps are:

1. Set t1 to an identity transform stream.

2. Set t2 to an identity transform stream.

3. Set this.[[writable]] to t1.[[writable]].

4. Set this.[[readable]] to t2.[[readable]].

5. Let serializedOptions be the result of StructuredSerializeWithTransfer(options, transfer).

6. Let serializedReadable be the result of StructuredSerializeWithTransfer(t1.[[readable]], « t1.[[readable]] »).

7. Let serializedWritable be the result of StructuredSerializeWithTransfer(t2.[[writable]], « t2.[[writable]] »).

8. Queue a task on the DOM manipulation task source worker’s global scope to run the following steps:

   1. Let transformerOptions be the result of StructuredDeserialize(serializedOptions, the current Realm).

   2. Let readable be the result of StructuredDeserialize(serializedReadable, the current Realm).

   3. Let writable be the result of StructuredDeserialize(serializedWritable, the current Realm).

   4. Let transformer be a new RTCRtpScriptTransformer.

   5. Set transformer.[[options]] to transformerOptions.

   6. Set transformer.[[readable]] to readable.

   7. Set transformer.[[writable]] to writable.

   8. Let event be the result of creating an event with RTCTransformEvent.

   9. Set event.type attribute to "rtctransform".

   10. Set event.transformer to transformer.

   11. Dispatch event on worker’s global scope.

// FIXME: Describe error handling (worker closing flag true at RTCRtpScriptTransform creation time. And worker being terminated while transform is processing data).

5.2. Attributes

A RTCRtpScriptTransformer has three private slots called [[options]], [[readable]] and [[writable]].

The options getter steps are:

1. Return this.[[options]].

The readable getter steps are:

1. Return this.[[readable]].

The writable getter steps are:

1. Return this.[[writable]].

6. Privacy and security considerations

This API gives Javascript access to the content of media streams. This is also available from other sources, such as Canvas and WebAudio.

However, streams that are isolated (as specified in [WEBRTC-IDENTITY]) or tainted with another origin, cannot be accessed using this API, since that would break the isolation rule.

The API will allow access to some aspects of timing information that are otherwise unavailable, which allows some fingerprinting surface.

7. Examples

See the explainer document.