Mixing render threads?

Is there any way to render a scene multiple times with different settings, and then automatically composite the results?

I have a scene that I want to render using the GPU, but hair rendering only works on the CPU. Is there any way to set up two separate render passes, one on CPU, the other on GPU, and then composite them?

I know there was a way a while back to set up a cycles and and internal render pass, and to composite them, I just hope there’s a way to do the same with two separate cycles passes.

Have you tried setting up a separate scene for each render. One set to cpu cycles rendering and another for gpu cycles rendering. Then a third scene used to composite the renders from each. The same method you’d use for combining BI and Cycles renders.

I’ll try that, thanks! I wasn’t sure that the settings would be different in each scene, but it does make sense.
Can I just link the scenes together, so that any changes I make in one appear in the other, or do I have to actually copy everything over?

It’s just like any other compositing situation, although the setups will be different to suit the technical requirements of each kind of render (and the output data produced also will be different). What’s the same is what’s important: the geometry, the camera point-of-view, and the moves made by everything. See to it that they coincide exactly.

I think that the real “eye-opening moment” for me was to realize that the final output of a render isn’t the final version of a finished picture that you eventually see. It’s a single piece of exposed film, so to speak, waiting to go into the “digital darkroom” where all of the real magic happens. Just as songs are produced in their final form (or forms) by a “mix-down” process after all the individual recordings are done.

Once you’ve decided what you want the final shot to be, “sneak up on it slowly.” Produce individual components that are “clean and isolated.” Use the most-expedient process to create each one.