First off, my apologies for bothering you folk with what may well be a ridiculously simple problem. I'm sure there's an obvious solution that I just keep overlooking, but I can't for the life of me see it.
Alright, so I have a sprite('A'), which has a pure magenta (transparency) background. I want to draw another image ('B') over it translucently, in such a way as to only draw over the non-transparent pixels in A, whilst leaving the transparent pixels their current colour.
I can't think of a clearer way to word that, so perhaps an example would be better.
(please excuse the atrocious placeholder graphics, they're just for example's sake)
A is the original sprite, B the image I want to draw over it. When I use draw_trans_sprite to do this, I get C as a result, whilst the result I want is labelled D.
If anyone can please tell me how I can get that result, short of writing my own blitting function, I'd be grateful.