Function blobAlphaChannelFromCanvas

blobAlphaChannelFromCanvas(canvasOrId: CanvasOrId): Promise<Blob>
Encode just the alpha channel of a canvas as a PNG Blob.

The encoded image has RGB = 0 everywhere and A = sourceAlpha. Read the alpha channel of the decoded image to recover the mask.
Parameters
- canvasOrId: CanvasOrId
  Canvas element or its DOM id.
Returns Promise<Blob>
PNG Blob, or null on failure.
- Defined in util/canvas.ts:191