I am having problems displaying overlays on Linux with my NVIDIA card. The same code works on windows.

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Answer:

The NVIDIA driver only offers RGBA overlays. The default setting in QGLFormat is to use an index-based overlays. You can switch to RGBA overlays using this code:

	QGLFormat f = QGLFormat::defaultOverlayFormat(); 
  	f.setRgba(true); 
  	QGLFormat::setDefaultOverlayFormat(f);