Masking a Drawable/Bitmap on Android

Question

I\'m currently looking for a way to use a black and white bitmap to mask the alpha channel of another bitmap or Drawable on Android. I\'m curious as to what the best way to

Answer 1

My solution is close to @over_optimistic's solution, less one saveLayer() call. I use a Drawable mask instead of a path, in my case it was a disc.

I declared these variables as fields (it's good practice to allocate memory outside of onDraw method):

private Paint maskingPaint = new Paint();
private Drawable mask = 

Then, somewhere outside of onDraw(), setup the objects:

// Xfermode won't work if hardware accelerated
setLayerType(View.LAYER_TYPE_SOFTWARE, null);

// Using destination shape as a mask
// For a good explanation of PorterDuff transfer modes : http://ssp.impulsetrain.com/porterduff.html
maskingPaint.setXfermode(new PorterDuffXfermode(Mode.SRC_IN));
maskingPaint.setAntiAlias(true);

// Position the mask
mask.setBounds();

Then finally, the onDraw() method applies the mask:

@Override
protected synchronized void onDraw(Canvas canvas)
{
    // Draw the mask first, making it the PorterDuff destination
    mask.draw(canvas);

    // Save the layer with the masking paint, that will be applied on restore()
    // Using CLIP_TO_LAYER_SAVE_FLAG to avoid any overflow of the masked image outside the mask bounds.
    Rect bounds = mask.getBounds();
    canvas.saveLayer(bounds.left, bounds.top, bounds.right, bounds.bottom, maskingPaint, 
            Canvas.CLIP_TO_LAYER_SAVE_FLAG);

    // Draw the shape offscreen, making it the PorterDuff source
    super.onDraw(canvas);

    // Apply the source to the destination, using SRC_IN transfer mode
    canvas.restore();
}

For a better understanding of the transfer modes, I referred to http://ssp.impulsetrain.com/porterduff.html. That page is pretty interesting to read. After that, with the same kind of code you'll be able to acheive much more than a simple mask!