Specifically subtract so I can clear the screen with alpha (motion blur) and not leave artefacts, but others would allow for some really lovely effects.
That's probably doable.
Added blend modes:
I adopted the naming scheme, but it should be changed to:
Wow that looks amazingly easy, have you fully tested this?
No, I did only one test. Seemed to work with this code though: http://gist.github.com/503407
Images are here: http://www.stud.uni-karlsruhe.de/~upbll/top.png http://www.stud.uni-karlsruhe.de/~upbll/bottom.jpg
Original patch missed multiply blend mode. Fixed that.
The multiply mode is only a pseudo multiply but looks close enough.
Added subtractive and multiplicative (the pseudo one vrld supplied).