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>

1 files changed, 4 insertions, 0 deletions

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();