Can I read from a framebuffer while drawing to it?

[CodeBunny]

Yes, I know I cannot read and write to a target from within a shader. This is slightly different. Can I have a single framebuffer, render different passes to different sections of it, and then render the final version to
another section?

A simple graphic illustrating what I'm trying to do:

[spasi]

Two options:

- NV_texture_barrier: Widely supported on NV/AMD, has limitations.
- ARB_shader_image_load_store: More advanced, but supported on DX11+ GPUs only.

[CodeBunny]

So, in a generally-supported sense, no?

[spasi]

Why do you want to render this way anyway? Why not have a texture that holds the 3 render passes (like [1][2][3]) and then perform the combined render pass on the backbuffer directly?

[CodeBunny]

Efficiency experiments. That's actually what I'm doing currently, and I'm trying to improve my render process.

For one thing, I'm using pixel scaling, so the actual scene I'm rendering is 2-4x smaller than the view. The shader I use to combine the render passes is expensive, so it occurred to me that rendering to a smaller texture first, and then just using a straight replace blend function would probably be the fastest method.

Additionally, it seemed like I could be most efficient memory-wise if I combined my framebuffers onto a single texture. I try to minimize PoT overflow as much as I can.