Masking a Drawable/Bitmap on Android

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 do this is. I certainly have a couple of ideas for how to do this, but they are not optimal.

I need to be able to apply a new mask to the image every so often (the black and white bitmap will change every few seconds).

Any feedback on how to achieve this would be greatly appreciated.

Solution

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 = <insert your drawable here>

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(<insert your mask bounds here>);

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!