Android Alpha Masking question

Question

in my overridden onDraw() function i have a canvas with two gauges in it (gauges are PNG's). The gauges are stacked on top of eachother. I would like only half of the gauge on the top to show while revealing the bottom gauge on the other half. Here's what i have so far:

@Override
public void onDraw(Canvas c){

    int targetWidth=200;
    int targetHeight=200;

    Paint p = new Paint();

    Bitmap bottom = BitmapFactory.decodeResource(getResources(), R.drawable.dashboard_rpm_bottom);
    Bitmap top = BitmapFactory.decodeResource(getResources(), R.drawable.dashboard_rpm_top);

    p.setFilterBitmap(false);

    c.translate(55,320);

    c.drawBitmap(
            bottom,
            new Rect(0, 0, bottom.getWidth(), bottom.getHeight()),
            new Rect(0, 0, targetWidth, targetHeight),
            p);


    p.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.SRC_OUT));

    c.drawBitmap(
            top,
            new Rect(0, 0, top.getWidth(), top.getHeight()),
            new Rect(0, 0, targetWidth, targetHeight),
            p);



    }

doesn't seem to work, anyone have any ideas?

Answer 1

I was able to do this using the xfermode DST_OUT method