ImageDecoder: decode() method

Secure contextAvailable in workers

The decode() method of the ImageDecoder interface enqueues a control message to decode the frame of an image.

Syntax

decode()
decode(options)

Parameters

• options Optional
  • : An object containing the following members:
    • frameIndex Optional
      • : An integer representing the index of the frame to decode. Defaults to 0 (the first frame).
    • completeFramesOnly Optional
      • : A Boolean defaulting to true. When true, the Promise returned by the method resolves only when the image is fully decoded. When false, the method will return a new Promise that may resolve with a partially decoded image. The method can be called repeatedly until result.complete is true, with each step providing an image with the next available level of detail.

Return value

A Promise that resolves with an object containing the following members:

• image
  • : A VideoFrame containing the decoded image.
• complete
  • : A Boolean, if true indicates that image contains the final full-detail output.

Exceptions

If an error occurs, the promise will resolve with following exception:

• InvalidStateError DOMException
  • : Returned if any of the following conditions apply:
    • close is true, meaning close() has already been called.
    • The requested frame does not exist.

Examples

Synchronous decoding of a completed image frame

The following example decodes the second frame (at index 1) and prints the resulting VideoFrame to the console.

let result = await imageDecoder.decode({ frameIndex: 1 });
console.log(result.image);

Partial decoding of a progressive image frame

The following example decodes the first frame repeatedly until its complete:

let complete = false;
while (!complete) {
  // The promise returned by `decode()` will only resolve when a new
  // level of detail is available or the frame is complete. I.e.,
  // calling `decode()` in a loop like this won't needlessly spin.
  let result = await imageDecoder.decode({ completeFramesOnly: false });

  // Do something with `result.image`.

  complete = result.complete;
}

Specifications

Browser compatibility