Alpha Scratch
An example behaviour for 'painting' into the alpha channel of a bitmap. In this example, the behaviour is applied to sprite 2. As you click and 'scratch' away at this sprite, the bottom sprite (a picture of Eliza modelling the latest in PAL9000 street wear) is revealed.
The 'brush' that paints into the alpha of the target image has an alpha channel. This means we can apply colour to the alpha channel with 'transparency' effects. The brush is also continuously rotating its quad, for added shtick!