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.

include/SDL_blendmode.h

@@ -50,6 +50,9 @@ typedef enum
     SDL_BLENDMODE_MOD = 0x00000004,      /**< color modulate
                                               dstRGB = srcRGB * dstRGB
                                               dstA = dstA */
+    SDL_BLENDMODE_MUL = 0x00000008,      /**< color multiply
+                                              dstRGB = (srcRGB * dstRGB) + (dstRGB * (1-srcA))
+                                              dstA = (srcA * dstA) + (dstA * (1-srcA)) */
     SDL_BLENDMODE_INVALID = 0x7FFFFFFF
 
     /* Additional custom blend modes can be returned by SDL_ComposeCustomBlendMode() */