[SDL] BUG?: RGBA->screen blit has 1-off error with opaque colors

[David Olofson]

On Wednesday 06 June 2007, Markus Laire wrote:
> The attached program should print "First pixel: 00FFFFFF" but
> instead of that I get "First pixel: 00FEFEFE"
> 
> Am I right in thinking that this is a bug in SDL?
> 
> OS is Linux (Knoppix 4.0.2)
> SDL version is 1.2.11

I think this is the result of the blitters multiplying by the alpha 
value, and then dividing by 256 (8 bit right shift) rather than 255. 
(Actual division is *very* expensive, compared to shift operations!)

One way around this is to scale the alpha value from [0, 255] to [0, 
256]. In the case of full surface alpha, this costs virtually 
nothing, but it would probably slow down alpha channel (RGBA) blits 
quite a bit - although not nearly as much as actually dividing by 255 
for each channel.

One could cheat by scaling the alpha channel to [0, 128] and shifting 
7 bits, but obviosuly, that results in a "funny" pixel format, and 
less accurate blending... Another hack is to add 1 somewhere, but 
then you just get 1 instead of 0 for "fully" transparent.

I'm afraid there is no easy, low cost solution - which is why it is 
the way it is, I would think.


//David Olofson - Programmer, Composer, Open Source Advocate

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