aec72c0a9a
Numerical instability mainly comes from adding values together which have very different magnitude. There are algorithms to keep the error small like "Kahan Summation", however those are also slower because of additional code in the hot loop. This patch implements a simpler approach that is slightly less accurate, but still seems to solve the cases that people commonly run into while being simpler and faster. The approach is to simply compute a couple partial sums first, and to add those up in the end. The individual partial sums can also be computed in parallel. Care has to be taken to maintain determinism with floating point values. If accuracy is still not enough for some use cases, we can revisit this later and e.g. use doubles or a better summation algorithm. Pull Request: https://projects.blender.org/blender/blender/pulls/132759
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