Pre-multiply gradient colors the right way

Alpha pre-multiplication must be done after applying the
opto-electronic transfer function when linear blending is
disabled. The correct way would be to pre-multiply before
gamma encoding but this leads to improper blending which
cannot be corrected without using sRGB frame buffers and
texture sampling.

Bug: 33010587
Test: cts-tradefed run singleCommand cts-dev --module CtsUiRenderingTestCases --test android.uirendering.cts.testclasses.GradientTests
Change-Id: I5f04bda4cb9f63674537aef5931621c14d601884

1 files changed, 5 insertions, 3 deletions

diff --git a/libs/hwui/ProgramCache.cpp b/libs/hwui/ProgramCache.cpp
index 0c2309faf4ea..2688ba484212 100644
--- a/libs/hwui/ProgramCache.cpp
+++ b/libs/hwui/ProgramCache.cpp
@@ -175,10 +175,11 @@ const char* gFS_Gradient_Functions =
 const char* gFS_Gradient_Preamble[2] = {
         // Linear framebuffer
         "\nvec4 dither(const vec4 color) {\n"
-        "    return vec4(color.rgb + (triangleNoise(gl_FragCoord.xy * screenSize.xy) / 255.0), color.a);"
+        "    return vec4(color.rgb + (triangleNoise(gl_FragCoord.xy * screenSize.xy) / 255.0), color.a);\n"
         "}\n"
         "\nvec4 gammaMix(const vec4 a, const vec4 b, float v) {\n"
-        "    return pow(mix(a, b, v), vec4(vec3(1.0 / 2.2), 1.0));"
+        "    vec4 c = pow(mix(a, b, v), vec4(vec3(1.0 / 2.2), 1.0));\n"
+        "    return vec4(c.rgb * c.a, c.a);\n"
         "}\n",
         // sRGB framebuffer
         "\nvec4 dither(const vec4 color) {\n"
@@ -186,7 +187,8 @@ const char* gFS_Gradient_Preamble[2] = {
         "    return vec4(dithered * dithered, color.a);\n"
         "}\n"
         "\nvec4 gammaMix(const vec4 a, const vec4 b, float v) {\n"
-        "    return mix(a, b, v);"
+        "    vec4 c = mix(a, b, v);\n"
+        "    return vec4(c.rgb * c.a, c.a);\n"
         "}\n"
 };