libkwineffects/kwinglutils: fix saturation calculation with wide color gamut and document it

src/libkwineffects/kwinglutils.cpp

@@ -430,6 +430,8 @@ void GLShader::resolveLocations()
 
     mVec2Location[Offset] = uniformLocation("offset");
 
+    m_vec3Locations[Vec3Uniform::PrimaryBrightness] = uniformLocation("primaryBrightness");
+
     mVec4Location[ModulationConstant] = uniformLocation("modulation");
 
     mFloatLocation[Saturation] = uniformLocation("saturation");
@@ -470,6 +472,12 @@ bool GLShader::setUniform(GLShader::Vec2Uniform uniform, const QVector2D &value)
     return setUniform(mVec2Location[uniform], value);
 }
 
+bool GLShader::setUniform(Vec3Uniform uniform, const QVector3D &value)
+{
+    resolveLocations();
+    return setUniform(m_vec3Locations[uniform], value);
+}
+
 bool GLShader::setUniform(GLShader::Vec4Uniform uniform, const QVector4D &value)
 {
     resolveLocations();
@@ -783,6 +791,7 @@ QByteArray ShaderManager::generateFragmentSource(ShaderTraits traits) const
         }
         if (traits & ShaderTrait::AdjustSaturation) {
             stream << "uniform float saturation;\n";
+            stream << "uniform vec3 primaryBrightness;\n";
         }
 
         stream << "\n"
@@ -841,9 +850,6 @@ QByteArray ShaderManager::generateFragmentSource(ShaderTraits traits) const
     } else if (traits & ShaderTrait::UniformColor) {
         stream << "    result = geometryColor;\n";
     }
-    if (traits & ShaderTrait::AdjustSaturation) {
-        stream << "    result.rgb = mix(vec3(dot(result.rgb, vec3(0.2126, 0.7152, 0.0722))), result.rgb, saturation);\n";
-    }
     if (traits & ShaderTrait::TransformColorspace) {
         // sRGB -> output colorspace & linear
         stream << "    if (sourceNamedTransferFunction == 0) {\n";
@@ -851,6 +857,12 @@ QByteArray ShaderManager::generateFragmentSource(ShaderTraits traits) const
         stream << "    }\n";
         stream << "    result.rgb = doTonemapping(colorimetryTransform * result.rgb, maxHdrBrightness);\n";
     }
+    if (traits & ShaderTrait::AdjustSaturation) {
+        // this calculates the Y component of the XYZ color representation for the color,
+        // which roughly corresponds to the brightness of the RGB tuple
+        stream << "    float Y = dot(result.rgb, primaryBrightness);\n";
+        stream << "    result.rgb = mix(vec3(Y), result.rgb, saturation);\n";
+    }
     if (traits & ShaderTrait::Modulate) {
         stream << "    result *= modulation;\n";
     }

src/libkwineffects/kwinglutils.h

@@ -121,6 +121,10 @@ public:
         Vec2UniformCount
     };
 
+    enum class Vec3Uniform {
+        PrimaryBrightness = 0
+    };
+
     enum Vec4Uniform {
         ModulationConstant,
         Vec4UniformCount
@@ -150,6 +154,7 @@ public:
     bool setUniform(MatrixUniform uniform, const QMatrix3x3 &value);
     bool setUniform(MatrixUniform uniform, const QMatrix4x4 &matrix);
     bool setUniform(Vec2Uniform uniform, const QVector2D &value);
+    bool setUniform(Vec3Uniform uniform, const QVector3D &value);
     bool setUniform(Vec4Uniform uniform, const QVector4D &value);
     bool setUniform(FloatUniform uniform, float value);
     bool setUniform(IntUniform uniform, int value);
@@ -177,6 +182,7 @@ private:
     bool mExplicitLinking : 1;
     int mMatrixLocation[MatrixCount];
     int mVec2Location[Vec2UniformCount];
+    QHash<Vec3Uniform, int> m_vec3Locations;
     int mVec4Location[Vec4UniformCount];
     int mFloatLocation[FloatUniformCount];
     int mIntLocation[IntUniformCount];

src/libkwineffects/kwinoffscreeneffect.cpp

@@ -155,7 +155,6 @@ void OffscreenData::paint(const RenderTarget &renderTarget, const RenderViewport
 {
     GLShader *shader = m_shader ? m_shader : ShaderManager::instance()->shader(ShaderTrait::MapTexture | ShaderTrait::Modulate | ShaderTrait::AdjustSaturation | ShaderTrait::TransformColorspace);
     ShaderBinder binder(shader);
-    shader->setColorspaceUniformsFromSRGB(renderTarget.colorDescription());
 
     const double scale = viewport.scale();
 
@@ -185,11 +184,14 @@ void OffscreenData::paint(const RenderTarget &renderTarget, const RenderViewport
     QMatrix4x4 mvp = data.projectionMatrix();
     mvp.translate(window->x() * scale, window->y() * scale);
 
+    const auto toXYZ = renderTarget.colorDescription().colorimetry().toXYZ();
     shader->setUniform(GLShader::ModelViewProjectionMatrix, mvp * data.toMatrix(scale));
     shader->setUniform(GLShader::ModulationConstant, QVector4D(rgb, rgb, rgb, a));
     shader->setUniform(GLShader::Saturation, data.saturation());
+    shader->setUniform(GLShader::Vec3Uniform::PrimaryBrightness, QVector3D(toXYZ(1, 0), toXYZ(1, 1), toXYZ(1, 2)));
     shader->setUniform(GLShader::TextureWidth, m_texture->width());
     shader->setUniform(GLShader::TextureHeight, m_texture->height());
+    shader->setColorspaceUniformsFromSRGB(renderTarget.colorDescription());
 
     const bool clipping = region != infiniteRegion();
     const QRegion clipRegion = clipping ? viewport.mapToRenderTarget(region) : infiniteRegion();

src/scene/itemrenderer_opengl.cpp

@@ -322,7 +322,11 @@ void ItemRendererOpenGL::renderItem(const RenderTarget &renderTarget, const Rend
     vbo->bindArrays();
 
     GLShader *shader = ShaderManager::instance()->pushShader(shaderTraits);
-    shader->setUniform(GLShader::Saturation, data.saturation());
+    if (shaderTraits & ShaderTrait::AdjustSaturation) {
+        const auto toXYZ = renderTarget.colorDescription().colorimetry().toXYZ();
+        shader->setUniform(GLShader::Saturation, data.saturation());
+        shader->setUniform(GLShader::Vec3Uniform::PrimaryBrightness, QVector3D(toXYZ(1, 0), toXYZ(1, 1), toXYZ(1, 2)));
+    }
     shader->setColorspaceUniformsFromSRGB(renderTarget.colorDescription());
 
     if (renderContext.hardwareClipping) {