How to apply dynamic alpha mask to a Text on Android

Question

I want to make a dynamic alpha mask with drawable shapes as circles or whatever, and apply it to a drawed text on Android. Here is an example of what i want :

I am trying to make it with setXfermode(new PorterDuffXfermode(Mode.SRC_IN)), but i can't get it work. Here is the code I have in onDraw(Canvas canvas) method :

Paint paint = new Paint();
paint.setAntiAlias(true);
canvas.drawARGB(0, 0, 0, 0);
paint.setColor(Color.WHITE);
canvas.drawCircle(50, 50, 50, paint);

paint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));
paint.setColor(Color.RED);
canvas.drawText("hello", 0, 50, paint);

Thanks in advance for your help

Answer 1

Try creating your source and mask bitmaps separately. Most of the examples I have seen involve using two bitmaps and using drawBitmap to perform the masking.

I use PorterDuff.Mode.DST_IN for the paint, then draw the source image (with no paint) followed by the mask image (with the paint). Something like this:

        Bitmap bitmapOut = Bitmap.createBitmap(sizeX, sizeY,
                Bitmap.Config.ARGB_8888);
        Canvas canvas = new Canvas(bitmapOut);

        Paint xferPaint = new Paint(Paint.ANTI_ALIAS_FLAG);
        xferPaint.setColor(Color.BLACK);

        xferPaint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_IN));

        canvas.drawBitmap(sourceImage, 0, 0, null);
        canvas.drawBitmap(alphaMask, 0, 0, xferPaint);

At this point, bitmapOut contains my masked image.