[Mesa-dev] [PATCH] glsl: Improve accuracy of alpha scaling in advanced blend lowering.

[Kenneth Graunke]

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>
---
 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");
 
-- 
2.10.1