Going through some documents I came across this extension for GLES. In a nutshell:
Now, I've searched for something similar on desktop GL and there's nothing I could find even in the latest 4.5 specs. I'm wondering if there's anything stopping Khronos from adding this extension (or something similar) to desktop GL.This extension provides a mechanism whereby a fragment shader may read existing framebuffer data as input.
I realize the same (and much more) can be achieved with render targets so it is somewhat redundant, but often times one finds the need to simply use blending operations and this seems like the perfect way to detach blending from the application logic, which might be a desirable choice for some designs.