PorterDuffXfermode DST_IN not working as expected

Question

So I'm trying to speed up some drawing we're doing (drawing a portion of an arc with alpha transparency) and was attempting to cache the entire arc into a separate bitmap, and show it selectively with an alpha mask.

From the research I've done (the Xfermodes API demo for Android, this example, and this tool), if I have for example the following two graphics:

and draw using the following:

Xfermode DST_IN = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);

canvas.drawBitmap(circle, 0, 0, paint);
paint.setXfermode(DST_IN);
canvas.drawBitmap(arc, 0, 0, paint);
paint.setXfermode(null);

I should get this result:

Where the destination image (the circle) is clipped to the area where the source image (the arc) is drawn. Instead, I get the full circle. If I just draw the arc, it appears in the correct location, and if I use DST_OUT instead, I get the inverse of the expected result (the other three quadrants of the circle).

I've also made sure to disable hardware rendering for this view, in case there was an issue with this Xfermode, but it doesn't make a difference.

I broke it out into a separate project at the simplest level trying to get it to work, and using the following code, I still have the same problem:

public class ClippedView extends View {
    private Xfermode DST_IN, DST_OUT;
    private Paint paint;

    public ClippedView(Context context) {
        super(context);
        init();
    }

    private void init() {
        setLayoutParams(new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));
        this.setLayerType(View.LAYER_TYPE_SOFTWARE, null);
        paint = new Paint(Paint.ANTI_ALIAS_FLAG);
        DST_IN = new PorterDuffXfermode(PorterDuff.Mode.DST_IN);
        DST_OUT = new PorterDuffXfermode(PorterDuff.Mode.DST_OUT);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        paint.setColor(Color.GREEN);
        canvas.drawRect(0, 0, getWidth() / 2, getHeight() / 2, paint);
        paint.setColor(Color.BLACK);
        paint.setXfermode(DST_IN);
        canvas.drawCircle(getWidth() / 2, getHeight() / 2, getWidth() / 2, paint);
        paint.setXfermode(null);
    }
}

Am I using it wrong? Am I just missing something? Have I found a bug? :)

Answer 1

There's a much cheaper and easier way to achieve this: use clipping. Canvas.clipRect() is enough. Your solution is burning a lot of fillrate. You can get the effect you want by using SRC_IN instead of DST_IN. Be careful though: it will work only in a transparent Bitmap or in layer. When you draw directly on screen, the destination is already filled by the window background.