OffscreenCanvas

img NOTE: OffscreenCanvas is supported since API version 7. Updates will be marked with a superscript to indicate their earliest API version.

OffscreenCanvas defines a canvas object that can be rendered off the screen.

Attributes

Name Type Description
width number Width of the OffscreenCanvas object.
height number Height of the OffscreenCanvas object.

Methods

getContext

getContext(type: string, options?: CanvasRenderingContext2DSettings): OffscreenCanvasRenderingContext2D

Obtains the OffscreenCanvas context. This method returns a 2D drawing object.

  • Parameters
Name Type Mandatory Description
contextId string Yes The value can only be "2d".
options CanvasRenderingContext2DSettings No 2D drawing object, which can be used to draw rectangles, images, and texts, on the OffscreenCanvas.
  • Return values
Type Description
OffscreenCanvasRenderingContext2D 2D drawing object, which can be used to draw rectangles, images, and texts, on the OffscreenCanvas.

toDataURL

toDataURL(type?: string, quality?:number):

Generates a URL containing image display information.

  • Parameters
Name Type Mandatory Description
type string No Image format. The default value is image/png.
quality number No Image quality, which ranges from 0 to 1, when the image format is image/jpeg or image/webp. If the set value is beyond the value range, the default value 0.92 is used.
  • Return values
Type Description
string Image URL.

transferToImageBitmap

transferToImageBitmap(): ImageBitmap

Creates an ImageBitmap object on the most recently rendered image of the OffscreenCanvas.

  • Return values
Type Description
ImageBitmap Pixel data rendered on the OffscreenCanvas.

Example

<!-- xxx.hml -->
<div>
  <canvas ref="canvasId" style="width: 200px; height: 150px; background-color: #ffff00;"></canvas>
</div>
//xxx.js
export default {
  onShow() {
    var canvas = this.$refs.canvasId.getContext('2d');
    var offscreen = new OffscreenCanvas(500,500);
    var offscreenCanvasCtx = offscreen.getContext("2d");

    // ... some drawing for the canvas using the offscreenCanvasCtx ...

    var dataURL = offscreen.toDataURL();
    console.log(dataURL); //data:image/png;base64,xxxxxx

    var bitmap = offscreen.transferToImageBitmap();
    canvas.transferFromImageBitmap(bitmap);
  }
}