ImageDecoder

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The ImageDecoder interface of the WebCodecs API provides a way to unpack and decode encoded image data.

Constructor

ImageDecoder() Experimental

Creates a new ImageDecoder object.

Properties

ImageDecoder.complete Read only Experimental

Returns a boolean value indicating whether encoded data is completely buffered.

ImageDecoder.completed Read only Experimental

Returns a Promise that resolves once complete is true.

ImageDecoder.tracks Read only Experimental

Returns an ImageTrackList object listing the available tracks and providing a method for selecting a track to decode.

ImageDecoder.type Read only Experimental

Returns a string reflecting the MIME type configured during construction.

Methods

ImageDecoder.close() Experimental

Ends all pending work and releases system resources.

ImageDecoder.decode() Experimental

Enqueues a control message to decode the frame of an image.

ImageDecoder.reset() Experimental

Aborts all pending decode() operations.

Static methods

ImageDecoder.isTypeSupported() Experimental

Indicates if the provided MIME type is supported for unpacking and decoding.

Examples

Given a <canvas> element:

<canvas></canvas>

the following code decodes and renders an animated image to that canvas:

let imageDecoder = null;
let imageIndex = 0;

function renderImage(result) {
  const canvas = document.querySelector('canvas');
  const canvasContext = canvas.getContext('2d');

  canvasContext.drawImage(result.image, 0, 0);

  const track = imageDecoder.tracks.selectedTrack;

  // We check complete here since `frameCount` won't be stable until all
  // data has been received. This may cause us to receive a RangeError
  // during the decode() call below which needs to be handled.
  if (imageDecoder.complete) {
    if (track.frameCount === 1)
      return;

    if (imageIndex + 1 >= track.frameCount)
      imageIndex = 0;
  }

  // Decode the next frame ahead of display so it's ready in time.
  imageDecoder.decode({frameIndex: ++imageIndex})
      .then(
          (nextResult) => setTimeout(
              () => {
                renderImage(nextResult);
              },
              result.image.duration / 1000.0))
      .catch((e) => {
        // We can end up requesting an imageIndex past the end since
        // we're using a ReadableStream from fetch(), when this happens
        // just wrap around.
        if (e instanceof RangeError) {
          imageIndex = 0;
          imageDecoder.decode({frameIndex: imageIndex})
              .then(renderImage);
        } else {
          throw e;
        }
      });
}

function decodeImage(imageByteStream) {
  imageDecoder = new ImageDecoder(
      {data: imageByteStream, type: 'image/gif'});
  imageDecoder.decode({frameIndex: imageIndex}).then(renderImage);
}

fetch('fancy.gif').then((response) => decodeImage(response.body));

Specifications

Specification
WebCodecs
# imagedecoder-interface

Browser compatibility

BCD tables only load in the browser