Limited availability

This feature is not Baseline because it does not work in some of the most widely-used browsers.

The RTCTransformEvent of the WebRTC API represent an event that is fired in a dedicated worker when an encoded frame has been queued for processing by a WebRTC Encoded Transform.

The interface has a transformer property that exposes a readable stream and a writable stream. A worker should read encoded frames from transformer.readable, modify them as needed, and write them to transformer.writable in the same order and without any duplication.

At time of writing there is just one event based on RTCTransformEvent: rtctransform.

Event RTCTransformEvent

Instance properties

Since RTCTransformEvent is based on Event, its properties are also available.

RTCTransformEvent.transformer Read only

Returns the RTCRtpScriptTransformer associated with the event.

Transform event types

There is only one type of transform event.


The rtctransform event is fired at the worker global scope on construction of an associated RTCRtpScriptTransform, and whenever a new encoded video or audio frame is enqueued for processing.

You can add a rtctransform event listener to be notified when the new frame is available using either DedicatedWorkerGlobalScope.addEventListener() or the onrtctransform event handler property.


This example creates an event listener for the rtctransform event.

The example assumes we have a TransformStream with an options object passed from a RTCRtpScriptTransform constructor in the main-thread. The code at the end shows how the stream is piped through the transform stream from the readable to the writable.

addEventListener("rtctransform", (event) => {
  let transform;
  // Select a transform based on passed options
  if (event.transformer.options.name == "senderTransform") {
    transform = createSenderTransform(); // A TransformStream (not shown)
  } else if (event.transformer.options.name == "receiverTransform") {
    transform = createReceiverTransform(); // A TransformStream (not shown)
  // Pipe frames from the readable to writeable through TransformStream

Note that this code is part of a more complete example provided in Using WebRTC Encoded Transforms.


WebRTC Encoded Transform
# rtctransformevent

Browser compatibility

BCD tables only load in the browser