On Sat, Jun 18, 2016 at 12:42 PM, Jason Ekstrand <ja...@jlekstrand.net> wrote: > While mathematically correct, these two optimizations result in an > expression with substantially lower precision than the original. For any > positive finite floating-point value, log2(x) is well-defined and finite. > More precisely, it is in the range [-150, 150] so any sum of logarithms > log2(a) + log2(b) is also well-defined and finite as long as a and b are > both positive and finite. However, if a and b are either very small or > very large, their product may get flushed to infinity or zero causing > log2(a * b) to be nowhere close to log2(a) + log2(b). > > This imprecision was causing incorrect rendering in Talos Principal because > part of its HDR rendering process involves doing 8 texture operations, > clamping the result to [0, 65000], taking a dot-product with a constant, > and then taking the log2. This is done 6 or 8 times and summed to produce > the final result which is written to a red texture. In cases where you > have a region of the screen that is very dark, it can end up getting a > result value of -inf which is not what is intended. > > Signed-off-by: Jason Ekstrand <ja...@jlekstrand.net> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96425 > Cc: Matt Turner <matts...@gmail.com>
Ah, that's too bad. Reviewed-by: Matt Turner <matts...@gmail.com> _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
