The readable read-only property of the RTCRtpScriptTransformer interface returns a ReadableStream instance is a source for encoded media frames.

When the corresponding RTCRtpScriptTransform is inserted into the WebRTC sender and receiver pipelines, this stream may be enqueued with outgoing or incoming encoded media frames (RTCEncodedVideoFrame or RTCEncodedAudioFrame). A WebRTC encoded transform can read the frames, modify them as needed, and then send them back into the WebRTC pipeline by writing them to RTCRtpScriptTransformer.writable. A common way to perform this operation is to pipe the frames through a TransformStream.

A ReadableStream.

The following example shows how readable is piped through a TransformStream to RTCRtpScriptTransformer.writable.

addEventListener("rtctransform", (event) => {
  let transform;
  // Select a transform based on passed options
  if (event.transformer.options.name == "senderTransform")
    transform = createSenderTransform(); // A TransformStream
  else if (event.transformer.options.name == "receiverTransform")
    transform = createReceiverTransform(); // A TransformStream
  else return;

  // Pipe frames from the readable to writeable through TransformStream
  event.transformer.readable
    .pipeThrough(transform)
    .pipeTo(event.transformer.writable);
});

The code implements a handler for the rtctransform event, which is fired at the global worker object on construction of the corresponding RTCRtpScriptTransform, and when new frames are enqueued for processing. event.transformer is the RTCRtpScriptTransformer that has a readable and writable property.

An different TransformStream is created to process outgoing and incoming frames, using createSenderTransform() or createReceiverTransform(), respectively (implementations not shown). The event handler chooses the correct transform stream to use based on options passed through from the RTCRtpScriptTransform constructor and assigns it to transform.

The code calls ReadableStream.pipeThrough() on the readable to pipe encoded frames through the selected TransformStream, and then ReadableStream.pipeTo() to pipe them to the RTCRtpScriptTransformer.writable.

• Using WebRTC Encoded Transforms