Composite operations in Android Canvas

问题:

I'm just starting with Android development and I'm coming from JavaScript/HTML world so I'm currently investigating the possibilities of the Android SDK.

The HTML 5 canvas supports composite operations (See here).

Is this possible in an Android Canvas? I scanned the API of the Canvas class but couldn't find anything useful. I need at least the composite operation "source-in" or (if this isn't possible) "source-atop".

回答1:

Composition is handled by drawing on a Canvas with a Paint that uses a PorterDuffXfermode.

Paint p=new Paint();
p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DARKEN));
canvas.drawBitmap(bitmap, 0, 0, p);

See here for more info. I also managed to override onPaint so that we can apply this composition to elements that draw themeselves.

回答2:

I haven't looked into doing those kinds of operations before, but I think there's a way to do them using the Rect shape. You can define rectangles to mark out which areas should be drawn into the canvas, or which to take out of the source image, and the Rect class has some comparison features built-in. For example to imitate a source-in, you could use setIntersect(Rect boundingBoxOfSource, Rect boundingBoxOfDestination) to make your new rectangle only have the intersected area of the two images. Then you can use that rectangle as an argument in one of the Canvas's draw commands, telling it to only take the pixels from the source image that are inside that rectangle. You then control the drawing in the Z-axis by the order in which the draw commands are executed.

回答3:

Found it myself. The PorterDuffXfermode implements all these composite modes.