[SDL] blitting from-to surface with per-pixel alpha doesn't work...

D. Gyimesi dgyimesi at gmail.com
Wed Nov 16 11:15:55 PST 2005


Hi there!

I know SDL's problem about blitting with per-pixel alpha surfaces into
each other.

In the pygame (SDL binding for Python) documentation one can find the
following:
'Surface alphas can be mixed with colorkeys, but an image with per pixel
alphas cannot use the other modes.'

I can interpret this like one can blit pixel data from any number of
surfaces to another with "only" one restriction: all of the surfaces'
transparency must be set to per-pixel alpha.

So, my problem is the following:
I have four surfaces: screen, background, tank and its parts.
The order of blitting: screen<-background and in another cycle
screen<-tank<-parts. All surfaces are having per-pixel-data of course.

Despite of this, the result in the tank surface (the surface I blit into
the parts) draws a strange black 'belt' around the parts, but other
transparent areas of the surfaces are showing up well.

I know it's a little hard to imagine, but anyone meet a problem similar
to this?

Thank you, and sorry for my english!





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