Fixed bug 4903 - Lack of color multiply with alpha (SDL_BLENDMODE_MOD + SDL_BLENDMODE_BLEND) blending mode for all renderers

Konrad

This kind of blending is rather quite useful and in my opinion should be available for all renderers. I do need it myself, but since I didn't want to use a custom blending mode which is supported only by certain renderers (e.g. not in software which is quite important for me) I did write implementation of SDL_BLENDMODE_MUL for all renderers altogether.

SDL_BLENDMODE_MUL implements following equation:

dstRGB = (srcRGB * dstRGB) + (dstRGB * (1-srcA))
dstA = (srcA * dstA) + (dstA * (1-srcA))

Background:

https://i.imgur.com/UsYhydP.png

Blended texture:

https://i.imgur.com/0juXQcV.png

Result for SDL_BLENDMODE_MOD:

https://i.imgur.com/wgNSgUl.png

Result for SDL_BLENDMODE_MUL:

https://i.imgur.com/Veokzim.png

I think I did cover all possibilities within included patch, but I didn't write any tests for SDL_BLENDMODE_MUL, so it would be lovely if someone could do it.

src/video/SDL_blit.h

@@ -36,6 +36,7 @@ extern Uint8* SDL_expand_byte[9];
 #define SDL_COPY_BLEND              0x00000010
 #define SDL_COPY_ADD                0x00000020
 #define SDL_COPY_MOD                0x00000040
+#define SDL_COPY_MUL                0x00000080
 #define SDL_COPY_COLORKEY           0x00000100
 #define SDL_COPY_NEAREST            0x00000200
 #define SDL_COPY_RLE_DESIRED        0x00001000