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Updated blend semantics so blending uses the following formula:

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dstRGB = (srcRGB * srcA) + (dstRGB * (1-srcA))
    dstA = srcA + (dstA * (1-srcA))
This allows proper compositing semantics without requiring premultiplied alpha.

Needs full unit test coverage and bug fixes!
This commit is contained in:
Sam Lantinga 2013-07-23 08:06:49 -07:00
parent a99edf3519
commit 32188834b5
17 changed files with 362 additions and 1001 deletions
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30
include/SDL_surface.h
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@ -399,44 +399,42 @@ extern DECLSPEC int SDLCALL SDL_FillRects
*
* The blit function should not be called on a locked surface.
*
* The blit semantics for surfaces with and without alpha and colorkey
* The blit semantics for surfaces with and without blending and colorkey
* are defined as follows:
* \verbatim
RGBA->RGB:
SDL_SRCALPHA set:
alpha-blend (using alpha-channel).
Source surface blend mode set to SDL_BLENDMODE_BLEND:
alpha-blend (using the source alpha-channel and per-surface alpha)
SDL_SRCCOLORKEY ignored.
SDL_SRCALPHA not set:
Source surface blend mode set to SDL_BLENDMODE_NONE:
copy RGB.
if SDL_SRCCOLORKEY set, only copy the pixels matching the
RGB values of the source color key, ignoring alpha in the
comparison.
RGB->RGBA:
SDL_SRCALPHA set:
alpha-blend (using the source per-surface alpha value);
set destination alpha to opaque.
SDL_SRCALPHA not set:
Source surface blend mode set to SDL_BLENDMODE_BLEND:
alpha-blend (using the source per-surface alpha)
Source surface blend mode set to SDL_BLENDMODE_NONE:
copy RGB, set destination alpha to source per-surface alpha value.
both:
if SDL_SRCCOLORKEY set, only copy the pixels matching the
source color key.
RGBA->RGBA:
SDL_SRCALPHA set:
alpha-blend (using the source alpha channel) the RGB values;
leave destination alpha untouched. [Note: is this correct?]
Source surface blend mode set to SDL_BLENDMODE_BLEND:
alpha-blend (using the source alpha-channel and per-surface alpha)
SDL_SRCCOLORKEY ignored.
SDL_SRCALPHA not set:
Source surface blend mode set to SDL_BLENDMODE_NONE:
copy all of RGBA to the destination.
if SDL_SRCCOLORKEY set, only copy the pixels matching the
RGB values of the source color key, ignoring alpha in the
comparison.
comparison.
RGB->RGB:
SDL_SRCALPHA set:
alpha-blend (using the source per-surface alpha value).
SDL_SRCALPHA not set:
Source surface blend mode set to SDL_BLENDMODE_BLEND:
alpha-blend (using the source per-surface alpha)
Source surface blend mode set to SDL_BLENDMODE_NONE:
copy RGB.
both:
if SDL_SRCCOLORKEY set, only copy the pixels matching the