Linear blending, step 1

NOTE: Linear blending is currently disabled in this CL as the
      feature is still a work in progress

Android currently performs all blending (any kind of linear math
on colors really) on gamma-encoded colors. Since Android assumes
that the default color space is sRGB, all bitmaps and colors
are encoded with the sRGB Opto-Electronic Conversion Function
(OECF, which can be approximated with a power function). Since
the power curve is not linear, our linear math is incorrect.
The result is that we generate colors that tend to be too dark;
this affects blending but also anti-aliasing, gradients, blurs,
etc.

The solution is to convert gamma-encoded colors back to linear
space before doing any math on them, using the sRGB Electo-Optical
Conversion Function (EOCF). This is achieved in different
ways in different parts of the pipeline:

- Using hardware conversions when sampling from OpenGL textures
  or writing into OpenGL frame buffers
- Using software conversion functions, to translate app-supplied
  colors to and from sRGB
- Using Skia's color spaces

Any type of processing on colors must roughly ollow these steps:

[sRGB input]->EOCF->[linear data]->[processing]->OECF->[sRGB output]

For the sRGB color space, the conversion functions are defined as
follows:

OECF(linear) :=
linear <= 0.0031308 ? linear * 12.92 : (pow(linear, 1/2.4) * 1.055) - 0.055

EOCF(srgb) :=
srgb <= 0.04045 ? srgb / 12.92 : pow((srgb + 0.055) / 1.055, 2.4)

The EOCF is simply the reciprocal of the OECF.
While it is highly recommended to use the exact sRGB conversion
functions everywhere possible, it is sometimes useful or beneficial
to rely on approximations:

- pow(x,2.2) and pow(x,1/2.2)
- x^2 and sqrt(x)

The latter is particularly useful in fragment shaders (for instance
to apply dithering in sRGB space), especially if the sqrt() can be
replaced with an inversesqrt().

Here is a fairly exhaustive list of modifications implemented
in this CL:

- Set TARGET_ENABLE_LINEAR_BLENDING := false in BoardConfig.mk
  to disable linear blending. This is only for GLES 2.0 GPUs
  with no hardware sRGB support. This flag is currently assumed
  to be false (see note above)
- sRGB writes are disabled when entering a functor (WebView).
  This will need to be fixed at some point
- Skia bitmaps are created with the sRGB color space
- Bitmaps using a 565 config are expanded to 888
- Linear blending is disabled when entering a functor
- External textures are not properly sampled (see below)
- Gradients are interpolated in linear space
- Texture-based dithering was replaced with analytical dithering
- Dithering is done in the quantization color space, which is
  why we must do EOCF(OECF(color)+dither)
- Text is now gamma corrected differently depending on the luminance
  of the source pixel. The asumption is that a bright pixel will be
  blended on a dark background and the other way around. The source
  alpha is gamma corrected to thicken dark on bright and thin
  bright on dark to match the intended design of fonts. This also
  matches the behavior of popular design/drawing applications
- Removed the asset atlas. It did not contain anything useful and
  could not be sampled in sRGB without a yet-to-be-defined GL
  extension
- The last column of color matrices is converted to linear space
  because its value are added to linear colors

Missing features:
- Resource qualifier?
- Regeneration of goldeng images for automated tests
- Handle alpha8/grey8 properly
- Disable sRGB write for layers with external textures

Test: Manual testing while work in progress
Bug: 29940137

Change-Id: I6a07b15ab49b554377cd33a36b6d9971a15e9a0b

1 files changed, 116 insertions, 56 deletions

diff --git a/libs/hwui/ProgramCache.cpp b/libs/hwui/ProgramCache.cpp
index 59225e108ac7..315c60eccb3e 100644
--- a/libs/hwui/ProgramCache.cpp
+++ b/libs/hwui/ProgramCache.cpp
@@ -17,8 +17,8 @@
 #include <utils/String8.h>
 
 #include "Caches.h"
-#include "Dither.h"
 #include "ProgramCache.h"
+#include "Properties.h"
 
 namespace android {
 namespace uirenderer {
@@ -69,22 +69,16 @@ const char* gVS_Header_Varyings_HasBitmap =
         "varying highp vec2 outBitmapTexCoords;\n";
 const char* gVS_Header_Varyings_HasGradient[6] = {
         // Linear
-        "varying highp vec2 linear;\n"
-        "varying vec2 ditherTexCoords;\n",
-        "varying float linear;\n"
-        "varying vec2 ditherTexCoords;\n",
+        "varying highp vec2 linear;\n",
+        "varying float linear;\n",
 
         // Circular
-        "varying highp vec2 circular;\n"
-        "varying vec2 ditherTexCoords;\n",
-        "varying highp vec2 circular;\n"
-        "varying vec2 ditherTexCoords;\n",
+        "varying highp vec2 circular;\n",
+        "varying highp vec2 circular;\n",
 
         // Sweep
-        "varying highp vec2 sweep;\n"
-        "varying vec2 ditherTexCoords;\n",
-        "varying highp vec2 sweep;\n"
-        "varying vec2 ditherTexCoords;\n",
+        "varying highp vec2 sweep;\n",
+        "varying highp vec2 sweep;\n",
 };
 const char* gVS_Header_Varyings_HasRoundRectClip =
         "varying highp vec2 roundRectPos;\n";
@@ -98,22 +92,16 @@ const char* gVS_Main_OutTransformedTexCoords =
         "    outTexCoords = (mainTextureTransform * vec4(texCoords, 0.0, 1.0)).xy;\n";
 const char* gVS_Main_OutGradient[6] = {
         // Linear
-        "    linear = vec2((screenSpace * position).x, 0.5);\n"
-        "    ditherTexCoords = (transform * position).xy * " STR(DITHER_KERNEL_SIZE_INV) ";\n",
-        "    linear = (screenSpace * position).x;\n"
-        "    ditherTexCoords = (transform * position).xy * " STR(DITHER_KERNEL_SIZE_INV) ";\n",
+        "    linear = vec2((screenSpace * position).x, 0.5);\n",
+        "    linear = (screenSpace * position).x;\n",
 
         // Circular
-        "    circular = (screenSpace * position).xy;\n"
-        "    ditherTexCoords = (transform * position).xy * " STR(DITHER_KERNEL_SIZE_INV) ";\n",
-        "    circular = (screenSpace * position).xy;\n"
-        "    ditherTexCoords = (transform * position).xy * " STR(DITHER_KERNEL_SIZE_INV) ";\n",
+        "    circular = (screenSpace * position).xy;\n",
+        "    circular = (screenSpace * position).xy;\n",
 
         // Sweep
+        "    sweep = (screenSpace * position).xy;\n",
         "    sweep = (screenSpace * position).xy;\n"
-        "    ditherTexCoords = (transform * position).xy * " STR(DITHER_KERNEL_SIZE_INV) ";\n",
-        "    sweep = (screenSpace * position).xy;\n"
-        "    ditherTexCoords = (transform * position).xy * " STR(DITHER_KERNEL_SIZE_INV) ";\n",
 };
 const char* gVS_Main_OutBitmapTexCoords =
         "    outBitmapTexCoords = (textureTransform * position).xy * textureDimension;\n";
@@ -147,12 +135,11 @@ const char* gFS_Uniforms_TextureSampler =
         "uniform sampler2D baseSampler;\n";
 const char* gFS_Uniforms_ExternalTextureSampler =
         "uniform samplerExternalOES baseSampler;\n";
-const char* gFS_Uniforms_Dither =
-        "uniform sampler2D ditherSampler;";
 const char* gFS_Uniforms_GradientSampler[2] = {
-        "%s\n"
+        "uniform vec2 screenSize;\n"
         "uniform sampler2D gradientSampler;\n",
-        "%s\n"
+
+        "uniform vec2 screenSize;\n"
         "uniform vec4 startColor;\n"
         "uniform vec4 endColor;\n"
 };
@@ -172,18 +159,51 @@ const char* gFS_Uniforms_HasRoundRectClip =
         "uniform vec4 roundRectInnerRectLTRB;\n"
         "uniform float roundRectRadius;\n";
 
+// Dithering must be done in the quantization space
+// When we are writing to an sRGB framebuffer, we must do the following:
+//     EOCF(OECF(color) + dither)
+// We approximate the transfer functions with gamma 2.0 to avoid branches and pow()
+// The dithering pattern is generated with a triangle noise generator in the range [-0.5,1.5[
+// TODO: Handle linear fp16 render targets
+const char* gFS_Dither_Functions =
+        "\nmediump float triangleNoise(const highp vec2 n) {\n"
+        "    highp vec2 p = fract(n * vec2(5.3987, 5.4421));\n"
+        "    p += dot(p.yx, p.xy + vec2(21.5351, 14.3137));\n"
+        "    highp float xy = p.x * p.y;\n"
+        "    return fract(xy * 95.4307) + fract(xy * 75.04961) - 0.5;\n"
+        "}\n";
+const char* gFS_Dither_Preamble[2] = {
+        // Linear framebuffer
+        "\nvec4 dither(const vec4 color) {\n"
+        "    return vec4(color.rgb + (triangleNoise(gl_FragCoord.xy * screenSize.xy) / 255.0), color.a);"
+        "}\n",
+        // sRGB framebuffer
+        "\nvec4 dither(const vec4 color) {\n"
+        "    vec3 dithered = sqrt(color.rgb) + (triangleNoise(gl_FragCoord.xy * screenSize.xy) / 255.0);\n"
+        "    return vec4(dithered * dithered, color.a);\n"
+        "}\n"
+};
+
+// Uses luminance coefficients from Rec.709 to choose the appropriate gamma
+// The gamma() function assumes that bright text will be displayed on a dark
+// background and that dark text will be displayed on bright background
+// The gamma coefficient is chosen to thicken or thin the text accordingly
+// The dot product used to compute the luminance could be approximated with
+// a simple max(color.r, color.g, color.b)
+const char* gFS_Gamma_Preamble =
+        "\n#define GAMMA (%.2f)\n"
+        "#define GAMMA_INV (%.2f)\n"
+        "\nfloat gamma(float a, const vec3 color) {\n"
+        "    float luminance = dot(color, vec3(0.2126, 0.7152, 0.0722));\n"
+        "    return pow(a, luminance < 0.5 ? GAMMA_INV : GAMMA);\n"
+        "}\n";
+
 const char* gFS_Main =
         "\nvoid main(void) {\n"
-        "    lowp vec4 fragColor;\n";
+        "    vec4 fragColor;\n";
 
-const char* gFS_Main_Dither[2] = {
-        // ES 2.0
-        "texture2D(ditherSampler, ditherTexCoords).a * " STR(DITHER_KERNEL_SIZE_INV_SQUARE),
-        // ES 3.0
-        "texture2D(ditherSampler, ditherTexCoords).a"
-};
-const char* gFS_Main_AddDitherToGradient =
-        "    gradientColor += %s;\n";
+const char* gFS_Main_AddDither =
+        "    fragColor = dither(fragColor);\n";
 
 // Fast cases
 const char* gFS_Fast_SingleColor =
@@ -202,24 +222,32 @@ const char* gFS_Fast_SingleA8Texture =
         "\nvoid main(void) {\n"
         "    gl_FragColor = texture2D(baseSampler, outTexCoords);\n"
         "}\n\n";
+const char* gFS_Fast_SingleA8Texture_ApplyGamma =
+        "\nvoid main(void) {\n"
+        "    gl_FragColor = vec4(0.0, 0.0, 0.0, pow(texture2D(baseSampler, outTexCoords).a, GAMMA));\n"
+        "}\n\n";
 const char* gFS_Fast_SingleModulateA8Texture =
         "\nvoid main(void) {\n"
         "    gl_FragColor = color * texture2D(baseSampler, outTexCoords).a;\n"
         "}\n\n";
+const char* gFS_Fast_SingleModulateA8Texture_ApplyGamma =
+        "\nvoid main(void) {\n"
+        "    gl_FragColor = color * gamma(texture2D(baseSampler, outTexCoords).a, color.rgb);\n"
+        "}\n\n";
 const char* gFS_Fast_SingleGradient[2] = {
         "\nvoid main(void) {\n"
-        "    gl_FragColor = %s + texture2D(gradientSampler, linear);\n"
+        "    gl_FragColor = dither(texture2D(gradientSampler, linear));\n"
         "}\n\n",
         "\nvoid main(void) {\n"
-        "    gl_FragColor = %s + mix(startColor, endColor, clamp(linear, 0.0, 1.0));\n"
+        "    gl_FragColor = dither(mix(startColor, endColor, clamp(linear, 0.0, 1.0)));\n"
         "}\n\n",
 };
 const char* gFS_Fast_SingleModulateGradient[2] = {
         "\nvoid main(void) {\n"
-        "    gl_FragColor = %s + color.a * texture2D(gradientSampler, linear);\n"
+        "    gl_FragColor = dither(color.a * texture2D(gradientSampler, linear));\n"
         "}\n\n",
         "\nvoid main(void) {\n"
-        "    gl_FragColor = %s + color.a * mix(startColor, endColor, clamp(linear, 0.0, 1.0));\n"
+        "    gl_FragColor = dither(color.a * mix(startColor, endColor, clamp(linear, 0.0, 1.0)));\n"
         "}\n\n"
 };
 
@@ -239,11 +267,13 @@ const char* gFS_Main_FetchTexture[2] = {
         // Modulate
         "    fragColor = color * texture2D(baseSampler, outTexCoords);\n"
 };
-const char* gFS_Main_FetchA8Texture[2] = {
+const char* gFS_Main_FetchA8Texture[4] = {
         // Don't modulate
         "    fragColor = texture2D(baseSampler, outTexCoords);\n",
+        "    fragColor = texture2D(baseSampler, outTexCoords);\n",
         // Modulate
         "    fragColor = color * texture2D(baseSampler, outTexCoords).a;\n",
+        "    fragColor = color * gamma(texture2D(baseSampler, outTexCoords).a, color.rgb);\n",
 };
 const char* gFS_Main_FetchGradient[6] = {
         // Linear
@@ -271,29 +301,38 @@ const char* gFS_Main_BlendShadersBG =
         "    fragColor = blendShaders(gradientColor, bitmapColor)";
 const char* gFS_Main_BlendShadersGB =
         "    fragColor = blendShaders(bitmapColor, gradientColor)";
-const char* gFS_Main_BlendShaders_Modulate[3] = {
+const char* gFS_Main_BlendShaders_Modulate[6] = {
         // Don't modulate
         ";\n",
+        ";\n",
         // Modulate
         " * color.a;\n",
+        " * color.a;\n",
         // Modulate with alpha 8 texture
         " * texture2D(baseSampler, outTexCoords).a;\n",
+        " * gamma(texture2D(baseSampler, outTexCoords).a, color.rgb);\n",
 };
-const char* gFS_Main_GradientShader_Modulate[3] = {
+const char* gFS_Main_GradientShader_Modulate[6] = {
         // Don't modulate
         "    fragColor = gradientColor;\n",
+        "    fragColor = gradientColor;\n",
         // Modulate
         "    fragColor = gradientColor * color.a;\n",
+        "    fragColor = gradientColor * color.a;\n",
         // Modulate with alpha 8 texture
         "    fragColor = gradientColor * texture2D(baseSampler, outTexCoords).a;\n",
+        "    fragColor = gradientColor * gamma(texture2D(baseSampler, outTexCoords).a, gradientColor.rgb);\n",
     };
-const char* gFS_Main_BitmapShader_Modulate[3] = {
+const char* gFS_Main_BitmapShader_Modulate[6] = {
         // Don't modulate
         "    fragColor = bitmapColor;\n",
+        "    fragColor = bitmapColor;\n",
         // Modulate
         "    fragColor = bitmapColor * color.a;\n",
+        "    fragColor = bitmapColor * color.a;\n",
         // Modulate with alpha 8 texture
         "    fragColor = bitmapColor * texture2D(baseSampler, outTexCoords).a;\n",
+        "    fragColor = bitmapColor * gamma(texture2D(baseSampler, outTexCoords).a, bitmapColor.rgb);\n",
     };
 const char* gFS_Main_FragColor =
         "    gl_FragColor = fragColor;\n";
@@ -385,7 +424,8 @@ const char* gBlendOps[18] = {
 ///////////////////////////////////////////////////////////////////////////////
 
 ProgramCache::ProgramCache(Extensions& extensions)
-        : mHasES3(extensions.getMajorGlVersion() >= 3) {
+        : mHasES3(extensions.getMajorGlVersion() >= 3)
+        , mHasSRGB(extensions.hasSRGB()) {
 }
 
 ProgramCache::~ProgramCache() {
@@ -518,6 +558,7 @@ String8 ProgramCache::generateVertexShader(const ProgramDescription& description
 static bool shaderOp(const ProgramDescription& description, String8& shader,
         const int modulateOp, const char** snippets) {
     int op = description.hasAlpha8Texture ? MODULATE_OP_MODULATE_A8 : modulateOp;
+    op = op * 2 + description.hasGammaCorrection;
     shader.append(snippets[op]);
     return description.hasAlpha8Texture;
 }
@@ -570,13 +611,16 @@ String8 ProgramCache::generateFragmentShader(const ProgramDescription& descripti
         shader.append(gFS_Uniforms_ExternalTextureSampler);
     }
     if (description.hasGradient) {
-        shader.appendFormat(gFS_Uniforms_GradientSampler[description.isSimpleGradient],
-                gFS_Uniforms_Dither);
+        shader.append(gFS_Uniforms_GradientSampler[description.isSimpleGradient]);
     }
     if (description.hasRoundRectClip) {
         shader.append(gFS_Uniforms_HasRoundRectClip);
     }
 
+    if (description.hasGammaCorrection) {
+        shader.appendFormat(gFS_Gamma_Preamble, Properties::textGamma, 1.0f / Properties::textGamma);
+    }
+
     // Optimization for common cases
     if (!description.hasVertexAlpha
             && !blendFramebuffer
@@ -607,18 +651,26 @@ String8 ProgramCache::generateFragmentShader(const ProgramDescription& descripti
             fast = true;
         } else if (singleA8Texture) {
             if (!description.modulate) {
-                shader.append(gFS_Fast_SingleA8Texture);
+                if (description.hasGammaCorrection) {
+                    shader.append(gFS_Fast_SingleA8Texture_ApplyGamma);
+                } else {
+                    shader.append(gFS_Fast_SingleA8Texture);
+                }
             } else {
-                shader.append(gFS_Fast_SingleModulateA8Texture);
+                if (description.hasGammaCorrection) {
+                    shader.append(gFS_Fast_SingleModulateA8Texture_ApplyGamma);
+                } else {
+                    shader.append(gFS_Fast_SingleModulateA8Texture);
+                }
             }
             fast = true;
         } else if (singleGradient) {
+            shader.append(gFS_Dither_Functions);
+            shader.append(gFS_Dither_Preamble[mHasSRGB]);
             if (!description.modulate) {
-                shader.appendFormat(gFS_Fast_SingleGradient[description.isSimpleGradient],
-                        gFS_Main_Dither[mHasES3]);
+                shader.append(gFS_Fast_SingleGradient[description.isSimpleGradient]);
             } else {
-                shader.appendFormat(gFS_Fast_SingleModulateGradient[description.isSimpleGradient],
-                        gFS_Main_Dither[mHasES3]);
+                shader.append(gFS_Fast_SingleModulateGradient[description.isSimpleGradient]);
             }
             fast = true;
         }
@@ -652,6 +704,10 @@ String8 ProgramCache::generateFragmentShader(const ProgramDescription& descripti
     if (description.isBitmapNpot) {
         generateTextureWrap(shader, description.bitmapWrapS, description.bitmapWrapT);
     }
+    if (description.hasGradient) {
+        shader.append(gFS_Dither_Functions);
+        shader.append(gFS_Dither_Preamble[mHasSRGB]);
+    }
 
     // Begin the shader
     shader.append(gFS_Main); {
@@ -659,7 +715,8 @@ String8 ProgramCache::generateFragmentShader(const ProgramDescription& descripti
         if (description.hasTexture || description.hasExternalTexture) {
             if (description.hasAlpha8Texture) {
                 if (!description.hasGradient && !description.hasBitmap) {
-                    shader.append(gFS_Main_FetchA8Texture[modulateOp]);
+                    shader.append(
+                            gFS_Main_FetchA8Texture[modulateOp * 2 + description.hasGammaCorrection]);
                 }
             } else {
                 shader.append(gFS_Main_FetchTexture[modulateOp]);
@@ -671,7 +728,6 @@ String8 ProgramCache::generateFragmentShader(const ProgramDescription& descripti
         }
         if (description.hasGradient) {
             shader.append(gFS_Main_FetchGradient[gradientIndex(description)]);
-            shader.appendFormat(gFS_Main_AddDitherToGradient, gFS_Main_Dither[mHasES3]);
         }
         if (description.hasBitmap) {
             if (!description.isBitmapNpot) {
@@ -715,6 +771,10 @@ String8 ProgramCache::generateFragmentShader(const ProgramDescription& descripti
             }
         }
 
+        if (description.hasGradient) {
+            shader.append(gFS_Main_AddDither);
+        }
+
         // Output the fragment
         if (!blendFramebuffer) {
             shader.append(gFS_Main_FragColor);