Blending textures in SDL2

Question

I have 2 textures, both of them are created with SDL_TEXTUREACCESS_STREAMING flag. They have blend mode set to SDL_BLENDMODE_BLEND. I'm updating both of them in each frame from 2 different surfaces. One of them has mostly alpha values equal to 0 (mostly transparent). When I put one of the textures on the top of another, the blending doesn't happen.

For example - one surface is going to have RGBA values = 255, 255, 255, 0 for all pixels. When I update a texture with it, and put the same texture on the top of another image I get only black pixels.

How do I accomplish blending on the 2 textures in such a way that pixels with 0 alpha will be treated as transparent instead of being turned to black pixels? What is happening? Are Textures ignore alpha values?

Answer 1

This has the formulae for blending modes:

SDL.org: SDL_BlendMode

and this has visual examples of each of the ways of combining two textures via blending (for openGL, but the results should be the same for the few modes SDL implements):

StackExchange GameDev: OpenGL Blending GUI Textures: Answer 1

SDL_BLENDMODE_BLEND has a (1-alpha) term in it, which, if alpha is near 0, will be near 1, which could be causing opacity where you're expecting transparency. It depends on which you're using as src and which as dest, also.

(nb: I'd just post this as a comment, but I don't have the reputation yet.)