Hi Martin,
It is a question of speed; in complex scenes with a lot of relections, Optix is usually faster; it is specially designed for faster ray tracing. If your scene is simpler, with simple lighting and materials, CUDA might be faster.
I don’t bother and leave it on Optix all the time, but if you want you can compare the two and choose the best one for your specific scene. (This might be useful, when rendering an animation, then you can test one frame…)
Remember however, that this only applies to rendering with Cycles!
And you need a compatible NVIDiA GPU…
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