Resurrect QGLWidget::renderPixmap()

This function has been completely broken since Qt 5.0. Unfortunately
the autotest's verification steps were somewhat faulty so returning
a null pixmap from renderPixmap() did not trigger a failure.

The implementation is now done with framebuffer objects and pixel
readback. This is not ideal performance-wise, but is the only option.
In Qt 4 pixmaps were often backed by platform dependent native pixmaps
that could be used as rendering targets for OpenGL content. This is not
an option anymore since pixmaps are raster backed on all the major platforms.

Task-number: QTBUG-33186
Change-Id: I8f558e33bf7967ac3be439fd5a3eac07b6444be5
Reviewed-by: Jørgen Lind <jorgen.lind@digia.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/opengl/qgl_qpa.cpp b/src/opengl/qgl_qpa.cpp
index 6e698bf939..8b66c891bb 100644
--- a/src/opengl/qgl_qpa.cpp
+++ b/src/opengl/qgl_qpa.cpp
@@ -138,6 +138,9 @@ bool QGLContext::chooseContext(const QGLContext* shareContext)
 {
     Q_D(QGLContext);
     if(!d->paintDevice || d->paintDevice->devType() != QInternal::Widget) {
+        // Unlike in Qt 4, the only possible target is a widget backed by an OpenGL-based
+        // QWindow. Pixmaps in particular are not supported anymore as paint devices since
+        // starting from Qt 5 QPixmap is raster-backed on almost all platforms.
         d->valid = false;
     }else {
         QWidget *widget = static_cast<QWidget *>(d->paintDevice);