diff options
authorLaszlo Agocs <>2019-03-08 10:55:14 +0100
committerLaszlo Agocs <>2019-03-30 19:22:17 +0000
commit28a264cfe2d161f2eba3b1efa3ea4985445cd2de (patch)
parent387f120bf9d6c718796efe5d0db772abc71ab442 (diff)
Fix alpha blending regression with WA_StacksOnTop QOpenGLWidgets
0bc42886898 in Qt 5.6 introduced support for premultiplied alpha in the raster-rendered QWidget content. Unfortunately this introduced a regression for OpenGL content from QOpenGLWidgets with WA_StacksOnTop set: these used standard alpha blending in 5.5 and earlier, and switching them to premultiplied (in case the - unrelated - raster content has a _Premultiplied QImage format) breaks all content that was done with non-premultiplied alpha in mind, for example the qopenglwidget example's "Transparent background" checkbox. Restore the pre-5.6 behavior. Fixes: QTBUG-74285 Change-Id: I76fcadd53cd436efa2b619b8d6739270995d044f Reviewed-by: Allan Sandfeld Jensen <>
1 files changed, 5 insertions, 0 deletions
diff --git a/src/gui/painting/qplatformbackingstore.cpp b/src/gui/painting/qplatformbackingstore.cpp
index afb4613ba5..c71d82546a 100644
--- a/src/gui/painting/qplatformbackingstore.cpp
+++ b/src/gui/painting/qplatformbackingstore.cpp
@@ -446,6 +446,11 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
+ // There is no way to tell if the OpenGL-rendered content is premultiplied or not.
+ // For compatibility, assume that it is not, and use normal alpha blend always.
+ if (d_ptr->premultiplied)
+ funcs->glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE);
// Textures for renderToTexture widgets that have WA_AlwaysStackOnTop set.
for (int i = 0; i < textures->count(); ++i) {
if (textures->flags(i).testFlag(QPlatformTextureList::StacksOnTop))