OpenGL4 Multisample Anti-Aliasing and Render To Texture

Question

I have a working OpenGL multipass rendering example, and need to add multisampling.

I tried two approaches. Both approaches do not use depth or stencil buffers, but only render colors.

Multisampling during principal rendering

1. Bind Multisampling Fbo
2. Render geometry 
3. Blit / resolve multisampling
4. [1..n] Bind normal Fbo and apply post-process

Multisampling at the end

1. Bind normal Fbo
2. Render geometry
3. [1..n] Bind normal Fbo and apply post-process
4. Bind Multisample Fbo
5. Render result texture onto a quad into the multisample fbo
6. Blit / resolve multisampling

Problem

While both approaches seem to work (complete framebuffers etc), I can clearly see hard edges in the second approach. Only using the first approach I actually get antialiased soft edges.

Is there a conceptual problem with the second approach? Are there some specific prerequisites to multisampling so that it won't work as described?

I would greatly prefer using the second approach.

Answer 1

Your second approach cannot work. Multisampling uses coverage information to write different values for each sample belonging to a pixel. You lose this information when you draw your geometry into a non-multisample buffer. Rendering the non-multisample buffer into the multi-sample buffer can't recover information which is no longer there (the aliasing has already happened).