Adding new uniform to enforce that alpha becomes 1.0

When rendering opaque (RGB-only) windows the alpha ends up to be 0
with blending disabled. This breaks subsequent rendering steps which
require blenden (e.g. Lanczos). Therefore a uniform is used to ensure
that the alpha channel is set to 1.

kwin/lib/kwinglutils.cpp

@@ -1377,6 +1377,7 @@ void ShaderManager::resetShader(ShaderType type)
         shader->setUniform("opacity", 1.0f);
         shader->setUniform("brightness", 1.0f);
         shader->setUniform("saturation", 1.0f);
+        shader->setUniform("u_forceAlpha", 0);
         break;
     }
     case GenericShader: {
@@ -1408,6 +1409,7 @@ void ShaderManager::resetShader(ShaderType type)
         shader->setUniform("opacity", 1.0f);
         shader->setUniform("brightness", 1.0f);
         shader->setUniform("saturation", 1.0f);
+        shader->setUniform("u_forceAlpha", 0);
         break;
     }
     case ColorShader: {

kwin/lib/kwinglutils.h

@@ -346,6 +346,9 @@ class KWIN_EXPORT ShaderManager
              * normalized texture coordinates. Defaults to @c (1.0/1.0). And expects a @c vec3
              * uniform @c colorManiuplation, with @c x being opacity, @c y being brightness and
              * @c z being saturation. All three values default to @c 1.0.
+             * The fragment shader takes a uniform @c u_forceAlpha to force the alpha component
+             * to @c 1.0 if it is set to a value not @c 0. This uniform is useful when rendering
+             * a RGB-only window and the alpha channel is required in a later step.
              * The sampler uniform is @c sample and defaults to @c 0.
              * The shader uses two vertex attributes @c vertex and @c texCoord.
              **/

kwin/scene-fragment.glsl

@@ -5,6 +5,7 @@ uniform float opacity;
 uniform float brightness;
 uniform float saturation;
 uniform int debug;
+uniform int u_forceAlpha;
 
 varying vec2 varyingTexCoords;
 
@@ -25,6 +26,9 @@ void main() {
     }
     tex.rgb = tex.rgb * opacity * brightness;
     tex.a = tex.a * opacity;
+    if (u_forceAlpha > 0) {
+        tex.a = 1.0;
+    }
     /*if (debug != 0) {
         tex.g += 0.5;
     }*/

kwin/scene_opengl.cpp

@@ -836,6 +836,7 @@ void SceneOpenGL::Window::prepareShaderRenderStates( TextureType type, double op
     shader->setUniform("opacity", (float)opacity);
     shader->setUniform("saturation", (float)saturation);
     shader->setUniform("brightness", (float)brightness);
+    shader->setUniform("u_forceAlpha", opaque ? 1 : 0);
 
     // setting texture width and heiht stored in shader
     // only set if it is set by an effect that is not negative
@@ -1043,6 +1044,7 @@ void SceneOpenGL::Window::restoreShaderRenderStates( TextureType type, double op
         {
         glDisable( GL_BLEND );
         }
+    ShaderManager::instance()->getBoundShader()->setUniform("u_forceAlpha", 0);
 #ifndef KWIN_HAVE_OPENGLES
     glPopAttrib();  // ENABLE_BIT
 #endif
@@ -1210,6 +1212,7 @@ void SceneOpenGL::EffectFrame::render( QRegion region, double opacity, double fr
             shader->setUniform("offset", QVector2D(0, 0));
         shader->setUniform("saturation", 1.0f);
         shader->setUniform("brightness", 1.0f);
+        shader->setUniform("u_forceAlpha", 0);
 
         shader->setUniform("textureWidth", 1.0f);
         shader->setUniform("textureHeight", 1.0f);