Pre-multiply color components for 2-stop gradients

Bug #7033344

Change-Id: Ia168501f1dc56ba7a1bb0c55078320432309a66a

Author: Romain Guy

core/java/android/view/View.java

@@ -17610,23 +17610,27 @@ public ScrollabilityCache(ViewConfiguration configuration, View host) {
             // use use a height of 1, and then wack the matrix each time we
             // actually use it.
             shader = new LinearGradient(0, 0, 0, 1, 0xFF000000, 0, Shader.TileMode.CLAMP);
-
             paint.setShader(shader);
             paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
+
             this.host = host;
         }
 
         public void setFadeColor(int color) {
-            if (color != 0 && color != mLastColor) {
+            if (color != mLastColor) {
                 mLastColor = color;
-                color |= 0xFF000000;
-
-                shader = new LinearGradient(0, 0, 0, 1, color | 0xFF000000,
-                        color & 0x00FFFFFF, Shader.TileMode.CLAMP);
 
-                paint.setShader(shader);
-                // Restore the default transfer mode (src_over)
-                paint.setXfermode(null);
+                if (color != 0) {
+                    shader = new LinearGradient(0, 0, 0, 1, color | 0xFF000000,
+                            color & 0x00FFFFFF, Shader.TileMode.CLAMP);
+                    paint.setShader(shader);
+                    // Restore the default transfer mode (src_over)
+                    paint.setXfermode(null);
+                } else {
+                    shader = new LinearGradient(0, 0, 0, 1, 0xFF000000, 0, Shader.TileMode.CLAMP);
+                    paint.setShader(shader);
+                    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
+                }
             }
         }
 

libs/hwui/GradientCache.cpp

@@ -217,10 +217,12 @@ void GradientCache::generateTexture(uint32_t* colors, float* positions,
         float amount = (pos - start) / distance;
         float oppAmount = 1.0f - amount;
 
-        *p++ = uint8_t(startR * oppAmount + endR * amount);
-        *p++ = uint8_t(startG * oppAmount + endG * amount);
-        *p++ = uint8_t(startB * oppAmount + endB * amount);
-        *p++ = uint8_t(startA * oppAmount + endA * amount);
+        const float alpha = startA * oppAmount + endA * amount;
+        const float a = alpha / 255.0f;
+        *p++ = uint8_t(a * (startR * oppAmount + endR * amount));
+        *p++ = uint8_t(a * (startG * oppAmount + endG * amount));
+        *p++ = uint8_t(a * (startB * oppAmount + endB * amount));
+        *p++ = uint8_t(alpha);
     }
 
     for (int i = 1; i < GRADIENT_TEXTURE_HEIGHT; i++) {

libs/hwui/SkiaShader.cpp

@@ -46,11 +46,12 @@ static inline bool isPowerOfTwo(unsigned int n) {
 }
 
 static inline void bindUniformColor(int slot, uint32_t color) {
+    const float a = ((color >> 24) & 0xff) / 255.0f;
     glUniform4f(slot,
-            ((color >> 16) & 0xff) / 255.0f,
-            ((color >>  8) & 0xff) / 255.0f,
-            ((color      ) & 0xff) / 255.0f,
-            ((color >> 24) & 0xff) / 255.0f);
+            a * ((color >> 16) & 0xff) / 255.0f,
+            a * ((color >>  8) & 0xff) / 255.0f,
+            a * ((color      ) & 0xff) / 255.0f,
+            a);
 }
 
 ///////////////////////////////////////////////////////////////////////////////
@@ -154,10 +155,6 @@ void SkiaBitmapShader::setupProgram(Program* program, const mat4& modelView,
 
     // Uniforms
     bindTexture(texture, mWrapS, mWrapT);
-    // Assume linear here; we should really check the transform in
-    // ::updateTransforms() but we don't have the texture object
-    // available at that point. The optimization is not worth the
-    // effort for now.
     texture->setFilter(GL_LINEAR);
 
     glUniform1i(program->getUniform("bitmapSampler"), textureSlot);
@@ -166,14 +163,6 @@ void SkiaBitmapShader::setupProgram(Program* program, const mat4& modelView,
     glUniform2f(program->getUniform("textureDimension"), 1.0f / width, 1.0f / height);
 }
 
-void SkiaBitmapShader::updateTransforms(Program* program, const mat4& modelView,
-        const Snapshot& snapshot) {
-    mat4 textureTransform;
-    computeScreenSpaceMatrix(textureTransform, modelView);
-    glUniformMatrix4fv(program->getUniform("textureTransform"), 1,
-            GL_FALSE, &textureTransform.data[0]);
-}
-
 ///////////////////////////////////////////////////////////////////////////////
 // Linear gradient shader
 ///////////////////////////////////////////////////////////////////////////////
@@ -257,13 +246,6 @@ void SkiaLinearGradientShader::setupProgram(Program* program, const mat4& modelV
     glUniformMatrix4fv(program->getUniform("screenSpace"), 1, GL_FALSE, &screenSpace.data[0]);
 }
 
-void SkiaLinearGradientShader::updateTransforms(Program* program, const mat4& modelView,
-        const Snapshot& snapshot) {
-    mat4 screenSpace;
-    computeScreenSpaceMatrix(screenSpace, modelView);
-    glUniformMatrix4fv(program->getUniform("screenSpace"), 1, GL_FALSE, &screenSpace.data[0]);
-}
-
 ///////////////////////////////////////////////////////////////////////////////
 // Circular gradient shader
 ///////////////////////////////////////////////////////////////////////////////
@@ -384,13 +366,6 @@ void SkiaSweepGradientShader::setupProgram(Program* program, const mat4& modelVi
     glUniformMatrix4fv(program->getUniform("screenSpace"), 1, GL_FALSE, &screenSpace.data[0]);
 }
 
-void SkiaSweepGradientShader::updateTransforms(Program* program, const mat4& modelView,
-        const Snapshot& snapshot) {
-    mat4 screenSpace;
-    computeScreenSpaceMatrix(screenSpace, modelView);
-    glUniformMatrix4fv(program->getUniform("screenSpace"), 1, GL_FALSE, &screenSpace.data[0]);
-}
-
 ///////////////////////////////////////////////////////////////////////////////
 // Compose shader
 ///////////////////////////////////////////////////////////////////////////////

libs/hwui/SkiaShader.h

@@ -82,10 +82,6 @@ struct SkiaShader {
         mGradientCache = gradientCache;
     }
 
-    virtual void updateTransforms(Program* program, const mat4& modelView,
-            const Snapshot& snapshot) {
-    }
-
     uint32_t getGenerationId() {
         return mGenerationId;
     }
@@ -148,7 +144,6 @@ struct SkiaBitmapShader: public SkiaShader {
     void describe(ProgramDescription& description, const Extensions& extensions);
     void setupProgram(Program* program, const mat4& modelView, const Snapshot& snapshot,
             GLuint* textureUnit);
-    void updateTransforms(Program* program, const mat4& modelView, const Snapshot& snapshot);
 
 private:
     SkiaBitmapShader() {
@@ -172,7 +167,6 @@ struct SkiaLinearGradientShader: public SkiaShader {
     void describe(ProgramDescription& description, const Extensions& extensions);
     void setupProgram(Program* program, const mat4& modelView, const Snapshot& snapshot,
             GLuint* textureUnit);
-    void updateTransforms(Program* program, const mat4& modelView, const Snapshot& snapshot);
 
 private:
     SkiaLinearGradientShader() {
@@ -197,7 +191,6 @@ struct SkiaSweepGradientShader: public SkiaShader {
     virtual void describe(ProgramDescription& description, const Extensions& extensions);
     void setupProgram(Program* program, const mat4& modelView, const Snapshot& snapshot,
             GLuint* textureUnit);
-    void updateTransforms(Program* program, const mat4& modelView, const Snapshot& snapshot);
 
 protected:
     SkiaSweepGradientShader(Type type, float x, float y, uint32_t* colors, float* positions,