Fix up QOpenGLWidget transparency support

The glColorMask call was troublesome. In addition, the Qt::WA_TranslucentBackground
was misinterpreted and recommended misleadingly in the documentation. The hellogl2
example's --transparent argument was disfunctional in practice.

Replace glColorMask with glBlendFuncSeparate. The hellogl2 example and the docs are
now corrected wrt enabling semi-transparency in a QOpenGLWidget that is not a top-level
(which is the most common case).

Task-number: QTBUG-47276
Change-Id: I6f40e732d455f5efcf158649ac9a52ff9f240e85
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

2 files changed, 9 insertions, 2 deletions

diff --git a/examples/opengl/hellogl2/glwidget.cpp b/examples/opengl/hellogl2/glwidget.cpp
index 9738dcda20..6505ad3141 100644
--- a/examples/opengl/hellogl2/glwidget.cpp
+++ b/examples/opengl/hellogl2/glwidget.cpp
@@ -55,8 +55,11 @@ GLWidget::GLWidget(QWidget *parent)
     // --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);
+    if (m_transparent) {
+        QSurfaceFormat fmt = format();
+        fmt.setAlphaBufferSize(8);
+        setFormat(fmt);
+    }
 }
 
 GLWidget::~GLWidget()
diff --git a/examples/opengl/hellogl2/main.cpp b/examples/opengl/hellogl2/main.cpp
index 4dc67f6e15..554e37e19b 100644
--- a/examples/opengl/hellogl2/main.cpp
+++ b/examples/opengl/hellogl2/main.cpp
@@ -59,6 +59,10 @@ int main(int argc, char *argv[])
     QSurfaceFormat::setDefaultFormat(fmt);
 
     MainWindow mainWindow;
+    if (QCoreApplication::arguments().contains(QStringLiteral("--transparent"))) {
+        mainWindow.setAttribute(Qt::WA_TranslucentBackground);
+        mainWindow.setAttribute(Qt::WA_NoSystemBackground, false);
+    }
     mainWindow.resize(mainWindow.sizeHint());
     int desktopArea = QApplication::desktop()->width() *
                      QApplication::desktop()->height();