The sendKeyFrameRequest() method of the RTCRtpScriptTransformer interface may be called by a WebRTC Encoded Transform that is processing incoming encoded video frames, in order to request a key frame from the sender.

The method may only be called when receiving video (not audio) frames and if, for whatever reason, a recipient will not be able to decode the video without a new key frame. Note that the user agent can decide that the request for a key frame is not necessary, in which case the returned promise will fulfill even though the request was not actually sent.

None

A Promise that fulfills with undefined once the request is sent, or the user agent decides that it does not need to.

InvalidStateError

The de-packetizer is not processing video packets, or is undefined.

The example below shows how the main thread of a WebRTC application that is receiving encoded video might pass a decryption key to a receiver transform, and request the sender emit a key frame.

Note that the main thread doesn't have direct access to the RTCRtpScriptTransformer object, so it needs to pass the key to the worker. Here we do that with a MessageChannel, transferring the second port to the transformer code running in the worker. The code assumes there is already a peer connection, and videoReceiver is an RTCRtpReceiver.

const worker = new Worker("worker.js");
const channel = new MessageChannel();

videoReceiver.transform = new RTCRtpScriptTransform(
  worker,
  { name: "receiverTransform", port: channel.port2 },
  [channel.port2],
);

// Post new key to the receiver
channel.port1.start();
channel.port1.postMessage({
  key: "93ae0927a4f8e527f1gce6d10bc6ab6c",
});

The rtctransform event handler in the worker gets the port as event.transformer.options.port. The code snippet below shows how that is used to listen for message events on the channel. If an event is received the handler gets the key, and then calls sendKeyFrameRequest() on the transformer.

event.transformer.options.port.onmessage = (event) => {
  const { key } = event.data;
  // key is used by the transformer to decrypt frames (not shown)

  // Request sender to emit a key frame.
  // Here 'rcevent' is the rtctransform event.
  rcevent.transformer.sendKeyFrameRequest();
};

• Using WebRTC Encoded Transforms
• RTCRtpScriptTransformer