[Mesa-dev] [PATCH] glsl: Make blend_colordodge compare against 1.0 - FLT_EPSILON.

[Kenneth Graunke]

This fixes a numerical precision issue that was causing two CTS
failures:

ES31-CTS.blend_equation_advanced.blend_specific.GL_COLORBURN_KHR
ES31-CTS.blend_equation_advanced.blend_all.GL_COLORBURN_KHR_all_qualifier

When blending with GL_COLORDODGE_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 imprecisions in the
alpha scaling, presumably), which failed 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.

To work around this, compare with 1.0 - FLT_EPSILON.

Signed-off-by: Kenneth Graunke <kenneth at whitecape.org>
---
 src/compiler/glsl/lower_blend_equation_advanced.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/compiler/glsl/lower_blend_equation_advanced.cpp b/src/compiler/glsl/lower_blend_equation_advanced.cpp
index a998df1..f8b0261 100644
--- a/src/compiler/glsl/lower_blend_equation_advanced.cpp
+++ b/src/compiler/glsl/lower_blend_equation_advanced.cpp
@@ -28,6 +28,7 @@
 #include "program/prog_instruction.h"
 #include "program/prog_statevars.h"
 #include "util/bitscan.h"
+#include <float.h>
 
 using namespace ir_builder;
 
@@ -101,7 +102,7 @@ blend_colorburn(ir_variable *src, ir_variable *dst)
     *   1 - min(1,(1-Cd)/Cs), if Cd < 1 and Cs > 0
     *   0, if Cd < 1 and Cs <= 0
     */
-   return csel(gequal(dst, imm3(1)), imm3(1),
+   return csel(gequal(dst, imm3(1 - FLT_EPSILON)), imm3(1),
                csel(lequal(src, imm3(0)), imm3(0),
                     sub(imm3(1), min2(imm3(1), div(sub(imm3(1), dst), src)))));
 }
-- 
2.9.3