Mesa (13.0): glsl: Improve accuracy of alpha scaling in advanced blend lowering.
Emil Velikov
evelikov at kemper.freedesktop.org
Tue Nov 1 16:10:25 UTC 2016
Module: Mesa
Branch: 13.0
Commit: 0ff597c39b7330bef99fd76d0a2db5c1d8e7fb85
URL: http://cgit.freedesktop.org/mesa/mesa/commit/?id=0ff597c39b7330bef99fd76d0a2db5c1d8e7fb85
Author: Kenneth Graunke <kenneth at whitecape.org>
Date: Thu Oct 27 20:12:56 2016 -0700
glsl: Improve accuracy of alpha scaling in advanced blend lowering.
When blending with GL_COLORBURN_KHR and these colors:
dst = <0.372549027, 0.372549027, 0.372549027, 0.372549027>
src = <0.09375, 0.046875, 0.0, 0.375>
the normalized dst value became 0.99999994 (due to precision problems
in the floating point divide of rgb by alpha). This caused the color
burn equation to fail the dst >= 1.0 comparison. The blue channel would
then fall through to the dst < 1.0 && src >= 0 comparison, which was
true, since src.b == 0. This produced a factor of 0.0 instead of 1.0.
This is an inherent numerical instability in the color burn and dodge
equations - depending on the precision of alpha scaling, the value can
be either 0.0 or 1.0. Technically, GLSL floating point division doesn't
even guarantee that 0.372549027 / 0.372549027 = 1.0. So arguably, the
CTS should allow either value. I've filed a bug at Khronos for further
discussion (linked below).
In the meantime, this patch improves the precision of alpha scaling by
replacing the division with (rgb == alpha ? 1.0 : rgb / alpha). We may
not need this long term, but for now, it fixes the following CTS tests:
ES31-CTS.blend_equation_advanced.blend_specific.GL_COLORBURN_KHR
ES31-CTS.blend_equation_advanced.blend_all.GL_COLORBURN_KHR_all_qualifier
Cc: currojerez at riseup.net
Cc: mesa-stable at lists.freedesktop.org
Bugzilla: https://cvs.khronos.org/bugzilla/show_bug.cgi?id=16042
Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima at igalia.com>
Reviewed-by: Francisco Jerez <currojerez at riseup.net>
(cherry picked from commit e6aeeace6953a7007d98082e3f44bff40a44106d)
---
src/compiler/glsl/lower_blend_equation_advanced.cpp | 10 ++++++++--
1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/src/compiler/glsl/lower_blend_equation_advanced.cpp b/src/compiler/glsl/lower_blend_equation_advanced.cpp
index 1d03392..f8210e3 100644
--- a/src/compiler/glsl/lower_blend_equation_advanced.cpp
+++ b/src/compiler/glsl/lower_blend_equation_advanced.cpp
@@ -308,12 +308,18 @@ calc_blend_result(ir_factory f,
f.emit(assign(dst_alpha, swizzle_w(fb)));
f.emit(if_tree(equal(dst_alpha, imm1(0)),
assign(dst_rgb, imm3(0)),
- assign(dst_rgb, div(swizzle_xyz(fb), dst_alpha))));
+ assign(dst_rgb, csel(equal(swizzle_xyz(fb),
+ swizzle(fb, SWIZZLE_WWWW, 3)),
+ imm3(1),
+ div(swizzle_xyz(fb), dst_alpha)))));
f.emit(assign(src_alpha, swizzle_w(src)));
f.emit(if_tree(equal(src_alpha, imm1(0)),
assign(src_rgb, imm3(0)),
- assign(src_rgb, div(swizzle_xyz(src), src_alpha))));
+ assign(src_rgb, csel(equal(swizzle_xyz(src),
+ swizzle(src, SWIZZLE_WWWW, 3)),
+ imm3(1),
+ div(swizzle_xyz(src), src_alpha)))));
ir_variable *factor = f.make_temp(glsl_type::vec3_type, "__blend_factor");
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