Erlend Strømsvik wrote:

the original game you most likely uses 'addative' blending for the 
explosions. this sort of blending looks fantastic for smoke and fire 
effects, but it not at all supported by SDL.

since this 'add' blending just adds the two colors together, any black 
pixels have no effect on the other image. therefore no need for alpha.


and now for some Numeric wizardry lessons ;)

as for your code, using Numeric we can do this sort of thing without 
needing to loop "pixel by pixel" in python. here's something that 
should be similar and considerable faster (hopefully, an email type-in 
here). also note by using the "pixels" instead of "array" functions we 
are modifying the image 'in-place' and no longer need the blit_array 
function at the end

     if needalpha == 1:
         colors = surfarray.pixels3d(surface)
         alphas = surfarray.pixels_alpha(surface)
         r = colors[:,:,0]
         g = colors[:,:,1]
	b = colors[:,:,2]
	alphas[:] = r | g | b

or, if you prefer the 'packed pixel' method, you can still do it all 
without a pixel by pixel loop.

     if needalpha == 1:
         masks = surface.get_masks()
         shifts = surface.get_shifts()
         losses = surface.get_losses()
         pixels = surfarray.pixels2d(surface)
	
	r = (pixels&masks[0]) > > shifts[0]
	g = (pixels&masks[1]) > > shifts[1]
	b = (pixels&masks[2]) > > shifts[2]
	a = r | g | b

	pixels |= a > > losses[3] << shifts[3]