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_slow.c

@@ -118,7 +118,7 @@ SDL_Blit_Slow(SDL_BlitInfo * info)
                     srcB = (srcB * srcA) / 255;
                 }
             }
-            switch (flags & (SDL_COPY_BLEND | SDL_COPY_ADD | SDL_COPY_MOD)) {
+            switch (flags & (SDL_COPY_BLEND | SDL_COPY_ADD | SDL_COPY_MOD | SDL_COPY_MUL)) {
             case 0:
                 dstR = srcR;
                 dstG = srcG;
@@ -147,6 +147,20 @@ SDL_Blit_Slow(SDL_BlitInfo * info)
                 dstG = (srcG * dstG) / 255;
                 dstB = (srcB * dstB) / 255;
                 break;
+            case SDL_COPY_MUL:
+                dstR = ((srcR * dstR) + (dstR * (255 - srcA))) / 255;
+                if (dstR > 255)
+                    dstR = 255;
+                dstG = ((srcG * dstG) + (dstG * (255 - srcA))) / 255;
+                if (dstG > 255)
+                    dstG = 255;
+                dstB = ((srcB * dstB) + (dstB * (255 - srcA))) / 255;
+                if (dstB > 255)
+                    dstB = 255;
+                dstA = ((srcA * dstA) + (dstA * (255 - srcA))) / 255;
+                if (dstA > 255)
+                    dstA = 255;
+                break;
             }
             if (dst_fmt->Amask) {
                 ASSEMBLE_RGBA(dst, dstbpp, dst_fmt, dstR, dstG, dstB, dstA);