From e6aeeace6953a7007d98082e3f44bff40a44106d Mon Sep 17 00:00:00 2001 From: Kenneth Graunke Date: Thu, 27 Oct 2016 20:12:56 -0700 Subject: [PATCH] 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@riseup.net Cc: mesa-stable@lists.freedesktop.org Bugzilla: https://cvs.khronos.org/bugzilla/show_bug.cgi?id=16042 Signed-off-by: Kenneth Graunke Reviewed-by: Eduardo Lima Mitev Reviewed-by: Francisco Jerez --- 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 1d033924159..f8210e3aaa5 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"); -- 2.30.2