Is there an efficient compositing mode or technique that would allow a primitive to be applied such that the destination layer's alpha becomes more transparent? This would have the effect of rubbing out, as an eraser might, previously rendered data on a layer.
Examining the spec seems to suggest a way of doing this by creating a soft mask for each "erasing" primitive and applying this mask to the entire layer, but this is going to be deadly slow at runtime. Is there a better method?
Thanks.