Fix composition for windows with alpha

While we will cover the entire surface with our textured quads, the clear
is still necessary in order to make top-levels with TranslucentBackground
render correctly: We don't want to blend transparent areas with undefined
content that is in the surface's framebuffer.

Blending is problematic for alpha values. We now prevent the blended alpha
from being written out. This ensures that in examples like qquickviewcomparison,
where the backingstore image contains an alpha of 0.5 while the QQuickWidget
texture 1.0, the result is still an alpha value of 1.0 in the final image.
Writing out an alpha of 0.5 would break on systems where windows get an alpha
buffer by default.

hellogl2 can now take a --transparent parameter which makes the QOpenGLWidget
being cleared to transparent in order to verify it works in combination
with Qt::WA_TranslucentBackground.

The swapped red and blue problem is also corrected. RGBA8888 does not need
swizzling. The only format that needs this is RGB32.

Task-number: QTBUG-40716
Change-Id: I54a9fd3a91a1b59575b38cdb908835315514e40f
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

2 files changed, 7 insertions, 0 deletions

diff --git a/examples/opengl/hellogl2/glwidget.cpp b/examples/opengl/hellogl2/glwidget.cpp
index c8db3047a1..7764fa8dfc 100644
--- a/examples/opengl/hellogl2/glwidget.cpp
+++ b/examples/opengl/hellogl2/glwidget.cpp
@@ -52,6 +52,11 @@ GLWidget::GLWidget(QWidget *parent)
       m_program(0)
 {
     m_core = QCoreApplication::arguments().contains(QStringLiteral("--coreprofile"));
+    // --transparent causes the clear color to be transparent. Therefore, on systems that
+    // support it, the widget will become transparent apart from the logo.
+    m_transparent = QCoreApplication::arguments().contains(QStringLiteral("--transparent"));
+    if (m_transparent)
+        setAttribute(Qt::WA_TranslucentBackground);
 }
 
 GLWidget::~GLWidget()
@@ -183,6 +188,7 @@ void GLWidget::initializeGL()
     connect(context(), &QOpenGLContext::aboutToBeDestroyed, this, &GLWidget::cleanup);
 
     initializeOpenGLFunctions();
+    glClearColor(0, 0, 0, m_transparent ? 0 : 1);
 
     m_program = new QOpenGLShaderProgram;
     m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, m_core ? vertexShaderSourceCore : vertexShaderSource);
diff --git a/examples/opengl/hellogl2/glwidget.h b/examples/opengl/hellogl2/glwidget.h
index fcc6225999..2d642649f1 100644
--- a/examples/opengl/hellogl2/glwidget.h
+++ b/examples/opengl/hellogl2/glwidget.h
@@ -98,6 +98,7 @@ private:
     QMatrix4x4 m_proj;
     QMatrix4x4 m_camera;
     QMatrix4x4 m_world;
+    bool m_transparent;
 };
 
 #endif