From d86fddc87630c5a61d6b170dd6e213a16034bff5 Mon Sep 17 00:00:00 2001 From: Roland Scheidegger Date: Sat, 27 Jul 2013 03:45:30 +0200 Subject: [PATCH] util: don't flush overflowing values to infinity in half-float conversion I am not able to find _any_ rounding behavior specified for OpenGL for float to half-float conversions. However, it is specified for fp11/fp10 which suggests round to next finite value but round-to-zero would also be allowed, but finite values must not be flushed to infinity in either case. Hence I believe it makes sense to do the same for half-floats too. We could probably also use round-to-zero consistently, which is in fact required by d3d10 (but it doesn't seem to matter much). Does not match the mesa core function doing the same though (which is saying it was built to match intel gpus which I don't believe for a second as it would cause failures in d3d10, moreover the PRM (for ivy bridge, not listed in older manuals) while not specifying rounding behavior clearly states finite numbers are never flushed to infinity). Reviewed-by: Jose Fonseca --- src/gallium/auxiliary/util/u_format_r11g11b10f.h | 14 +++++++------- src/gallium/auxiliary/util/u_half.h | 12 ++++++++++-- 2 files changed, 17 insertions(+), 9 deletions(-) diff --git a/src/gallium/auxiliary/util/u_format_r11g11b10f.h b/src/gallium/auxiliary/util/u_format_r11g11b10f.h index b883b318e03..57516c39c6e 100644 --- a/src/gallium/auxiliary/util/u_format_r11g11b10f.h +++ b/src/gallium/auxiliary/util/u_format_r11g11b10f.h @@ -69,10 +69,10 @@ static INLINE unsigned f32_to_uf11(float val) */ uf11 = UF11_MAX_EXPONENT; if (mantissa) { - uf11 |= 1; /* NaN */ + uf11 |= 1; /* NaN */ } else { - if (sign) - uf11 = 0; /* 0.0 */ + if (sign) + uf11 = 0; /* 0.0 */ } } else if (sign) { return 0; @@ -155,14 +155,14 @@ static INLINE unsigned f32_to_uf10(float val) */ uf10 = UF10_MAX_EXPONENT; if (mantissa) { - uf10 |= 1; /* NaN */ + uf10 |= 1; /* NaN */ } else { - if (sign) - uf10 = 0; /* 0.0 */ + if (sign) + uf10 = 0; /* 0.0 */ } } else if (sign) { return 0; - } else if (val > 64512.0f) { /* Overflow - flush to Infinity */ + } else if (val > 64512.0f) { /* From the GL_EXT_packed_float spec: * * "Likewise, finite positive values greater than 64512 (the maximum diff --git a/src/gallium/auxiliary/util/u_half.h b/src/gallium/auxiliary/util/u_half.h index f7009f54844..d340b9a7aef 100644 --- a/src/gallium/auxiliary/util/u_half.h +++ b/src/gallium/auxiliary/util/u_half.h @@ -75,9 +75,17 @@ util_float_to_half(float f) f32.f *= magic.f; f32.ui -= round_mask; - /* Clamp to infinity if overflowed */ + /* + * Clamp to max finite value if overflowed. + * OpenGL has completely undefined rounding behavior for float to + * half-float conversions, and this matches what is mandated for float + * to fp11/fp10, which recommend round-to-nearest-finite too. + * (d3d10 is deeply unhappy about flushing such values to infinity, and + * while it also mandates round-to-zero it doesn't care nearly as much + * about that.) + */ if (f32.ui > f16inf) - f32.ui = f16inf; + f32.ui = f16inf - 1; f16 = f32.ui >> 13; } -- 2.30.2