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> --- src/compiler/nir/nir_opt_algebraic.py | 2 -- 1 file changed, 2 deletions(-) diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py index 011263a..fd22801 100644 --- a/src/compiler/nir/nir_opt_algebraic.py +++ b/src/compiler/nir/nir_opt_algebraic.py @@ -233,8 +233,6 @@ optimizations = [ (('~flog2', ('frcp', a)), ('fneg', ('flog2', a))), (('~flog2', ('frsq', a)), ('fmul', -0.5, ('flog2', a))), (('~flog2', ('fpow', a, b)), ('fmul', b, ('flog2', a))), - (('~fadd', ('flog2', a), ('flog2', b)), ('flog2', ('fmul', a, b))), - (('~fadd', ('flog2', a), ('fneg', ('flog2', b))), ('flog2', ('fdiv', a, b))), (('~fmul', ('fexp2', a), ('fexp2', b)), ('fexp2', ('fadd', a, b))), # Division and reciprocal (('~fdiv', 1.0, a), ('frcp', a)), -- 2.5.0.400.gff86faf _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev
