Looking within the birds.p64 demo:

-- draw mask -- display pixels that have one of these bits set are drawn
-- 0 means draw all pixels
poke(0x547d, 0b00111111)

For a windowed app, this poke means that black (0x0) is not drawn to the screen at all. If we use cls(0) at the top of _draw(), then our entire background is effectively transparent, though the underlying color at those pixels is still 0. if we use a custom color table, gfx ops over the background will consider the "target color" to be 0 as well.

Is there a way to use the underlying desktop pixels as the target instead? This could be used to create some fun effects on the desktop: i'm imagining translucent windows, sophisticated shadows, additive blending...