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
diff --git a/libs/hwui/FloatColor.h b/libs/hwui/FloatColor.h
index 9df7338..d8afa35 100644
--- a/libs/hwui/FloatColor.h
+++ b/libs/hwui/FloatColor.h
@@ -38,13 +38,13 @@
}
// "color" is a gamma-encoded sRGB color
- // After calling this method, the color is stored as a pre-multiplied linear color
- // if linear blending is enabled.
- void setSRGB(uint32_t color) {
+ // After calling this method, the color is stored as a linear color. The color
+ // is not pre-multiplied.
+ void setUnPreMultipliedSRGB(uint32_t color) {
a = ((color >> 24) & 0xff) / 255.0f;
- r = a * EOCF_sRGB(((color >> 16) & 0xff) / 255.0f);
- g = a * EOCF_sRGB(((color >> 8) & 0xff) / 255.0f);
- b = a * EOCF_sRGB(((color ) & 0xff) / 255.0f);
+ r = EOCF_sRGB(((color >> 16) & 0xff) / 255.0f);
+ g = EOCF_sRGB(((color >> 8) & 0xff) / 255.0f);
+ b = EOCF_sRGB(((color ) & 0xff) / 255.0f);
}
bool isNotBlack() {
diff --git a/libs/hwui/GradientCache.cpp b/libs/hwui/GradientCache.cpp
index 0972ac1..1dad58f 100644
--- a/libs/hwui/GradientCache.cpp
+++ b/libs/hwui/GradientCache.cpp
@@ -188,26 +188,28 @@
void GradientCache::mixBytes(const FloatColor& start, const FloatColor& end,
float amount, uint8_t*& dst) const {
float oppAmount = 1.0f - amount;
- *dst++ = uint8_t(OECF_sRGB(start.r * oppAmount + end.r * amount) * 255.0f);
- *dst++ = uint8_t(OECF_sRGB(start.g * oppAmount + end.g * amount) * 255.0f);
- *dst++ = uint8_t(OECF_sRGB(start.b * oppAmount + end.b * amount) * 255.0f);
- *dst++ = uint8_t( (start.a * oppAmount + end.a * amount) * 255.0f);
+ float a = start.a * oppAmount + end.a * amount;
+ *dst++ = uint8_t(a * OECF_sRGB((start.r * oppAmount + end.r * amount)) * 255.0f);
+ *dst++ = uint8_t(a * OECF_sRGB((start.g * oppAmount + end.g * amount)) * 255.0f);
+ *dst++ = uint8_t(a * OECF_sRGB((start.b * oppAmount + end.b * amount)) * 255.0f);
+ *dst++ = uint8_t(a * 255.0f);
}
void GradientCache::mixFloats(const FloatColor& start, const FloatColor& end,
float amount, uint8_t*& dst) const {
float oppAmount = 1.0f - amount;
+ float a = start.a * oppAmount + end.a * amount;
float* d = (float*) dst;
#ifdef ANDROID_ENABLE_LINEAR_BLENDING
- *d++ = start.r * oppAmount + end.r * amount;
- *d++ = start.g * oppAmount + end.g * amount;
- *d++ = start.b * oppAmount + end.b * amount;
+ *d++ = a * (start.r * oppAmount + end.r * amount);
+ *d++ = a * (start.g * oppAmount + end.g * amount);
+ *d++ = a * (start.b * oppAmount + end.b * amount);
#else
- *d++ = OECF_sRGB(start.r * oppAmount + end.r * amount);
- *d++ = OECF_sRGB(start.g * oppAmount + end.g * amount);
- *d++ = OECF_sRGB(start.b * oppAmount + end.b * amount);
+ *d++ = a * OECF_sRGB(start.r * oppAmount + end.r * amount);
+ *d++ = a * OECF_sRGB(start.g * oppAmount + end.g * amount);
+ *d++ = a * OECF_sRGB(start.b * oppAmount + end.b * amount);
#endif
- *d++ = start.a * oppAmount + end.a * amount;
+ *d++ = a;
dst += 4 * sizeof(float);
}
@@ -217,16 +219,19 @@
uint8_t pixels[rowBytes * height];
static ChannelMixer gMixers[] = {
- &android::uirenderer::GradientCache::mixBytes, // colors are stored gamma-encoded
- &android::uirenderer::GradientCache::mixFloats, // colors are stored in linear
+ // colors are stored gamma-encoded
+ &android::uirenderer::GradientCache::mixBytes,
+ // colors are stored in linear (linear blending on)
+ // or gamma-encoded (linear blending off)
+ &android::uirenderer::GradientCache::mixFloats,
};
ChannelMixer mix = gMixers[mUseFloatTexture];
FloatColor start;
- start.setSRGB(colors[0]);
+ start.setUnPreMultipliedSRGB(colors[0]);
FloatColor end;
- end.setSRGB(colors[1]);
+ end.setUnPreMultipliedSRGB(colors[1]);
int currentPos = 1;
float startPos = positions[0];
@@ -241,7 +246,7 @@
currentPos++;
- end.setSRGB(colors[currentPos]);
+ end.setUnPreMultipliedSRGB(colors[currentPos]);
distance = positions[currentPos] - startPos;
}
diff --git a/libs/hwui/ProgramCache.cpp b/libs/hwui/ProgramCache.cpp
index 0c2309f..2688ba4 100644
--- a/libs/hwui/ProgramCache.cpp
+++ b/libs/hwui/ProgramCache.cpp
@@ -175,10 +175,11 @@
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 @@
" 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"
};
diff --git a/libs/hwui/SkiaShader.cpp b/libs/hwui/SkiaShader.cpp
index 34e6a06..0f6651b 100644
--- a/libs/hwui/SkiaShader.cpp
+++ b/libs/hwui/SkiaShader.cpp
@@ -81,7 +81,7 @@
}
///////////////////////////////////////////////////////////////////////////////
-// gradient shader matrix helpers
+// Gradient shader matrix helpers
///////////////////////////////////////////////////////////////////////////////
static void toLinearUnitMatrix(const SkPoint pts[2], SkMatrix* matrix) {
@@ -161,7 +161,7 @@
gradInfo.fColorOffsets = &colorOffsets[0];
shader.asAGradient(&gradInfo);
- if (CC_UNLIKELY(!isSimpleGradient(gradInfo))) {
+ if (CC_UNLIKELY(!description->isSimpleGradient)) {
outData->gradientSampler = (*textureUnit)++;
#ifndef SK_SCALAR_IS_FLOAT
@@ -174,8 +174,8 @@
outData->gradientSampler = 0;
outData->gradientTexture = nullptr;
- outData->startColor.setSRGB(gradInfo.fColors[0]);
- outData->endColor.setSRGB(gradInfo.fColors[1]);
+ outData->startColor.setUnPreMultipliedSRGB(gradInfo.fColors[0]);
+ outData->endColor.setUnPreMultipliedSRGB(gradInfo.fColors[1]);
}
return true;