This is implemented for accelerated renderers in this commit:
https://hg.libsdl.org/SDL/rev/180e8906dc3c

I think this is what you want:

        blendMode = SDL_ComposeCustomBlendMode(SDL_BLENDFACTOR_SRC_ALPHA,
                                               SDL_BLENDFACTOR_ONE,
                                               SDL_BLENDOPERATION_SUBTRACT,
                                               SDL_BLENDFACTOR_ZERO,
                                               SDL_BLENDFACTOR_ONE,
                                               SDL_BLENDOPERATION_SUBTRACT)

Hi Sam,
Thank you for taking care of my bug report. This is exactly what I was looking for. However a certain feature seems to be missing from the custom blend modes.

When using a custom blend mode, SDL_SetTextureAlphaMod seems to have no effect. I feel this should be taken into consideration. Example code below:

// I set up my subtractive blend mode like so
SDL_BlendMode SubBlendMode = SDL_ComposeCustomBlendMode(SDL_BLENDFACTOR_ONE, SDL_BLENDFACTOR_ONE, SDL_BLENDOPERATION_SUBTRACT, SDL_BLENDFACTOR_SRC_ALPHA, SDL_BLENDFACTOR_ONE, SDL_BLENDOPERATION_SUBTRACT);
SDL_SetTextureBlendMode(tempbuffer, SubBlendMode);

// Then I set an alpha mod for the entire texture, let's say 50%
SDL_SetTextureAlphaMod(tempbuffer, 128);

// And render it
SDL_RenderCopy(main_renderer, tempbuffer, &srcRect, &dstRect);


It doesn't matter what alpha mod I set, it still always subtracts it at 100% intensity. And in this case my source texture doesn't have an alpha channel, but even if it did i feel the alpha mod should be taken into consideration?
The subtract operation should be, in my opinion:
dstRGB = dstRGB - (srcRGB * (srcAlpha * srcAlphaMod))

This way you have more flexibility, you can set the TextureAlphaMod to 255 and use the source texture alpha channel. Another option would be to add another BlendFactor called "SDL_BLENDFACTOR_SRC_ALPHAMOD". Might be a better solution? Either way, there should be a way to use the AlphaMod, right? Thoughts?