Opengl - appending to a texture

Question

I want to create a texture system where I add to a texture, not overwrite it. My texture has integer values (32 bit). What I want: Ex. I have an integer pixel with bits 100, I want to add 10 to it so it becomes 110.

My current implementation has two textures, one with the previous texture, and a texture to write on. The previous texture's values are read and then rewritten with the new data. Is there a better method to do so because using two textures feel very inefficient?

Answer 1

Depending on what you mean by "appending", you could use additive blending :

glEnable(GL_BLEND);
glBlendEquation(GL_FUNC_ADD);
glBlendFunc(GL_ONE, GL_ONE);

then, the routput of your fragment shader will by added to the current contents of the color buffer. If you use a FBO to render into the texture, you can directly add to this texture.

You should just be careful to not create any feedback loops, so your fragment shader's result should not depend on any sample of the very same texture you render to.

UPDATE

As noted in the comment, the texture in question has GL_RED_INTEGER format. Unfortunately, the blending is only applied on floating-point color buffers (including normalized integers), and never on unnormalized integers.

However, there is another potential approach. The rules for the "feedback loops" I mentioned before have been relaxed with recent OpenGL. The extension GL_ARB_texture_barrier explicitely allowes a fragment shader to read pixels from the same texture it is writing to:

Specifically, the values of rendered fragments are undefined if any shader stage fetches texels and the same texels are written via fragment shader outputs, even if the reads and writes are not in the same Draw call, unless any of the following exceptions apply :

• The reads and writes are from/to disjoint sets of texels (after accounting for texture filtering rules).

• There is only a single read and write of each texel, and the read is in the fragment shader invocation that writes the same texel (eg using "texelFetch2D(sampler, ivec2(gl_FragCoord.xy), 0);").

• [...]

This extension has been promoted to a core feature of OpenGL 4.5. This is quite new and not available on a lot of platforms, so it is unclear if you can use it...