display: Render screen-off fade animation in linear sRGB space

The sRGB transfer function is a piecewise function with linear and gamma
2.4 parts, not involving cosine or other magic constants.

Fade colors in linear sRGB instead of non-linear sRGB + magic gamma to
minimize color distortion as the animation progresses.

Change-Id: I57db834f938cc63b7298af1c9dfe8c284dc6abe2

core/res/res/raw/color_fade_frag.frag

@@ -3,12 +3,34 @@
 precision mediump float;
 uniform samplerExternalOES texUnit;
 uniform float opacity;
-uniform float gamma;
 varying vec2 UV;
 
-void main()
-{
-    vec4 color = texture2D(texUnit, UV);
-    vec3 rgb = pow(color.rgb * opacity, vec3(gamma));
-    gl_FragColor = vec4(rgb, 1.0);
+vec3 mixBvec3(vec3 a, vec3 b, bvec3 sel) {
+    return vec3(
+        sel.x ? b.x : a.x,
+        sel.y ? b.y : a.y,
+        sel.z ? b.z : a.z
+    );
+}
+
+vec3 srgbTransfer(vec3 c) {
+    vec3 gamma = 1.055 * pow(c, vec3(1.0/2.4)) - 0.055;
+    vec3 linear = 12.92 * c;
+    bvec3 selectParts = lessThan(c, vec3(0.0031308));
+    return mixBvec3(gamma, linear, selectParts);
+}
+
+vec3 srgbTransferInv(vec3 c) {
+    vec3 gamma = pow((c + 0.055)/1.055, vec3(2.4));
+    vec3 linear = c / 12.92;
+    bvec3 selectParts = lessThan(c, vec3(0.04045));
+    return mixBvec3(gamma, linear, selectParts);
+}
+
+void main() {
+    vec3 inRgb = srgbTransferInv(texture2D(texUnit, UV).rgb);
+    vec3 fade = inRgb * opacity * opacity;
+    vec3 outRgb = srgbTransfer(fade);
+
+    gl_FragColor = vec4(outRgb, 1.0);
 }

services/core/java/com/android/server/display/ColorFade.java

@@ -112,7 +112,7 @@ final class ColorFade {
     private final float mProjMatrix[] = new float[16];
     private final int[] mGLBuffers = new int[2];
     private int mTexCoordLoc, mVertexLoc, mTexUnitLoc, mProjMatrixLoc, mTexMatrixLoc;
-    private int mOpacityLoc, mGammaLoc;
+    private int mOpacityLoc;
     private int mProgram;
 
     // Vertex and corresponding texture coordinates.
@@ -288,7 +288,6 @@ private boolean initGLShaders(Context context) {
         mTexMatrixLoc = GLES20.glGetUniformLocation(mProgram, "tex_matrix");
 
         mOpacityLoc = GLES20.glGetUniformLocation(mProgram, "opacity");
-        mGammaLoc = GLES20.glGetUniformLocation(mProgram, "gamma");
         mTexUnitLoc = GLES20.glGetUniformLocation(mProgram, "texUnit");
 
         GLES20.glUseProgram(mProgram);
@@ -432,12 +431,7 @@ public boolean draw(float level) {
             GLES20.glClear(GLES20.GL_COLOR_BUFFER_BIT);
 
             // Draw the frame.
-            double one_minus_level = 1 - level;
-            double cos = Math.cos(Math.PI * one_minus_level);
-            double sign = cos < 0 ? -1 : 1;
-            float opacity = (float) -Math.pow(one_minus_level, 2) + 1;
-            float gamma = (float) ((0.5d * sign * Math.pow(cos, 2) + 0.5d) * 0.9d + 0.1d);
-            drawFaded(opacity, 1.f / gamma);
+            drawFaded(level);
             if (checkGlErrors("drawFrame")) {
                 return false;
             }
@@ -449,9 +443,9 @@ public boolean draw(float level) {
         return showSurface(1.0f);
     }
 
-    private void drawFaded(float opacity, float gamma) {
+    private void drawFaded(float opacity) {
         if (DEBUG) {
-            Slog.d(TAG, "drawFaded: opacity=" + opacity + ", gamma=" + gamma);
+            Slog.d(TAG, "drawFaded: opacity=" + opacity);
         }
         // Use shaders
         GLES20.glUseProgram(mProgram);
@@ -460,7 +454,6 @@ private void drawFaded(float opacity, float gamma) {
         GLES20.glUniformMatrix4fv(mProjMatrixLoc, 1, false, mProjMatrix, 0);
         GLES20.glUniformMatrix4fv(mTexMatrixLoc, 1, false, mTexMatrix, 0);
         GLES20.glUniform1f(mOpacityLoc, opacity);
-        GLES20.glUniform1f(mGammaLoc, gamma);
 
         // Use textures
         GLES20.glActiveTexture(GLES20.GL_TEXTURE0);