HTML5 Canvas, 2nd Edition

Compositing on the Canvas

Compositing refers to how finely we can control the transparency and layering effects of objects as we draw them to the canvas. There are two attributes we can use to control Canvas compositing operations: globalAlpha and globalCompositeOperation.

globalAlpha

The globalAlpha Canvas property defaults to 1.0 (completely opaque) and can be set from 0.0 (completely transparent) through 1.0. This Canvas property must be set before a shape is drawn to the canvas.

globalCompositeOperation

The globalCompositeOperation value controls how shapes are drawn into the current Canvas bitmap after both globalAlpha and any transformations have been applied. (See the next section, Simple Canvas Transformations, for more information.)

In the following list, the “source” is the shape we are about to draw to the canvas, and the “destination” refers to the current bitmap displayed on the canvas:

copy

Where they overlap, displays the source and not the destination.

destination-atop

Destination atop the source. Where the source and destination overlap and both are opaque, displays the destination image. Displays the source image wherever the source image is opaque but the destination image is transparent. Displays transparency elsewhere.

destination-in

Destination in the source. Displays the destination image wherever both the destination image and source image are opaque. Displays transparency elsewhere.

destination-out

Destination out source. Displays the destination image wherever the destination ...