Jesse Wilson Jesse Wilson - 2 months ago 15
Android Question

How do I antialias the clip boundary on Android's canvas?

I'm using Android's android.graphics.Canvas class to draw a ring. My onDraw method clips the canvas to make a hole for the inner circle, and then draws the full outer circle over the hole:

clip = new Path();
clip.addRect(outerCircle, Path.Direction.CW);
clip.addOval(innerCircle, Path.Direction.CCW);

canvas.save();
canvas.clipPath(clip);
canvas.drawOval(outerCircle, lightGrey);
canvas.restore();


The result is a ring with a pretty, anti-aliased outer edge and a jagged, ugly inner edge:

aliased

What can I do to antialias the inner edge?

I don't want to cheat by drawing a grey circle in the middle because the dialog is slightly transparent. (This transparency isn't as subtle on on other backgrounds.)

Answer

AFAIK, you can't antialias clip regions.

I'd suggest using bitmap masking instead. Render the the pink, white, and light gray foreground to one bitmap, render the outer/inner circle mask (the grayscale alpha channel) to another bitmap, and then use Paint.setXfermode to render the foreground bitmap with the mask as its alpha channel.

An example can be found in the ApiDemos source code here:


EDIT: for good measure, here's a screenshot of that activity in API Demos, illustrating masking:

Xfermodes screenshot