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> Extracting sufficient coherency from path tracing in order to be able to get good SIMD utilization is a surprisingly difficult problem

Huh, I'd have assumed SIMD would just be exploited to improve quality without a perf hit, by turning individual paths into ever so slightly dispersed path-packets likely to still intersect the same objects. More samples per path traced...

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If you only ever-so-slightly perturb paths, you generally don't get anywhere near as much of a benefit from monte carlo integration, especially for things like light transport at a global non-local scale (might plausibly be useful for splitting for scattering bounces or something in some cases).

So it's often worth paying the penalty of having to sort rays/hitpoints into batches to intersect/process them more homogeneously, at least in terms of noise variance reduction per progression.

But very much depends on overall architecture and what you're trying to achieve (i.e. interactive rendering, or batch rendering might also lead to different solutions, like time to first useful pixel or time to final pixel).

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