RTCRtpSender: transform property

The transform property of the RTCRtpSender object is used to insert a transform stream (TransformStream) running in a worker thread into the sender pipeline. This allows stream transforms to be applied to encoded video and audio frames after they are output by a codec, and before they are sent.

The transform that is to be added is defined using an RTCRtpScriptTransform and its associated Worker. If the transform is set synchronously immediately after creating the RTCRtpSender it will receive the first full frame generated by the sender's encoder.


A RTCRtpScriptTransform, or null if the sender has no associated transform stream.


This example shows how you might stream video from a user's webcam over WebRTC, adding a WebRTC encoded transform to modify the outgoing streams. Note that this is is part of a larger example in the guide topic Using WebRTC Encoded Transforms.

The code assumes that there is an RTCPeerConnection called peerConnection that is already connected to a remote peer. It first gets a MediaStreamTrack, using getUserMedia() to get a video MediaStream from a media device, and then the MediaStream.getTracks() method to get the first MediaStreamTrack in the stream.

The track is added to the peer connection using addTrack(). This returns a new RTCRtpSender that will be used to send it.


const mediaStream = await navigator.mediaDevices.getUserMedia({ video: true });
const [track] = mediaStream.getTracks();
const videoSender = peerConnection.addTrack(track, mediaStream);

The code above sets up the connection and starts sending the track. To add a transform stream into the pipeline we need to construct an RTCRtpScriptTransform and assign it to the sender's transform property. As the transform is constructed immediately after creation of the RTCRtpSender, it will receive the first frame generated by the sender's encoder, before it is sent.


const worker = new Worker("worker.js");
videoSender.transform = new RTCRtpScriptTransform(worker, {
  name: "senderTransform",

Note that you can add the transform at any time. However by adding it immediately after calling addTrack() the transform will get the first encoded frame that is sent.


WebRTC Encoded Transform
# dom-rtcrtpsender-transform

Browser compatibility

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See also