Intel® Embree Ray Tracing Kernels
Discussion forum on the open source ray tracing kernels for fast photo-realistic rendering on Intel® CPUs.

Bug in ray traversal

Max_Liani
Beginner
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Hi, I have spotted a bug in ray traversal (not sure if it had been already reported/fixed).

Ray constructor precomputes reciprocal ray dir usig approximate SSE _mm_rcp_ps with improved precision through one iteration of Newton method. Problem revelas if one of the ray dir component is zero. This would normally gives rcp=inf, which would be correctly handled during ray traversal. But improving precision using Newton-Raphson method turns that inf into a NaN and that will cause the tree traversal to visit every single node of the tree with very strong performance degradation when those rare case happens (in my test case a 70ms render was turning into a 2000ms one).

A full precision _mm_div_(__mm_set1_ps(one), m128)) instead fixes the problem with apparently negligible performance drawbacks.

Max

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