I was going to write a simple fireworks demo, using small RGBA sparks
with decreasing per-surface alpha, but “per-pixel and per-surface alpha
cannot be combined; the per-pixel alpha is always used if available”
(from http://sdldoc.csn.ul.ie/sdlsetalpha.php).
So I think I could write some code for SDL to support combination of
per-pixel with per-surface alpha. For per-surface alpha 0, there’d be
no blitting, for 255 normal alpha blitting would be used, and for
everything between 1 and 254 every pixel’s alpha value would be
multiplied by per-surface alpha and shifted 8 bits right (yes, it
should be divided by 255, not 256 but it’d be much faster that way),
except maybe 128 and other powers of two where single right shift could
be used.
Well, it would be a nice feature to have, but 3 or 4 multiplications and
the same number of shifts extra per pixel (plus the extra shuffling
around between registers for crap CPUs like x86) will result in quite a
performance hit - and alpha blending is slow enough as it is.
However, if it got in, I could remove the “RGBA XOR surface alpha” logic
in glSDL without breaking SDL compatibility. 
(Actually, I can probably remove it anyway, as any code that would be
affected is broken.)
I’m also thinking about other “modes” of blitting surfaces, similar to
other than normal modes of displaying layers in Gimp, from which the
addition mode would be best for fireworks.
Additive blending is indeed useful, but unfortunately the required
saturation checks could be even slower than multiplications! (On Pentium
MMX and later, with 3 cycle MUL, that is.)
An MMX implementation could easilly be very, very fast, though, as MMX
has both saturating operations and test instructions that generate masks
rather than change the instruction flow.
What do you think? I know it’d be slower and hardware acceleration
couldn’t be used (especially for such addition mode etc.),
Currently, h/w acceleration cannot be used even for plain alpha, so
that’s not really an argument.
Anyway, h/w acceleration can be used for various blending operations
other than alpha. All you need is a driver and an API that support it.
Look at the OpenGL blending control - most 3D cards accelerate most of
the modes you can set up with it. (At least I know for sure that both
alpha and additive works on the G400. 
but it still could be useful. Maybe there’s already someone working on
it?
Well… I can just remove two or three lines on glSDL, and voila!
//David Olofson — Programmer, Reologica Instruments AB
.- M A I A -------------------------------------------------.
| Multimedia Application Integration Architecture |
| A Free/Open Source Plugin API for Professional Multimedia |
----------------------------> http://www.linuxdj.com/maia -' .- David Olofson -------------------------------------------. | Audio Hacker - Open Source Advocate - Singer - Songwriter |-------------------------------------> http://olofson.net -'On Wednesday 26 December 2001 18:00, RaFaL Pocztarski wrote: