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Is it possible to view background color or other widgets behind QGLWidget by using transparent GL background (glClearColor)?

  • I have a QGLWidget where I want to set transparent background in GL, and in order to get proper blending I end up premultiplying the alpha of background color (for details of what/why on "premultiplying" here is an example blog post I found). This means in the case of a completely transparent background, the glClearColor becomes completely black(with 0 alpha): RGBA = 0,0,0,0

    I want to have a solid background color visible for user convenience, but in a different color than the black that I get from pre-multiplying. The reason for wanting separate "displayed" background vs whats in GL frame buffer is because I need to be able to grabFrameBuffer and export images which can have transparent backgrounds.

    I have tried setting the QGLWidget's background color via its QPalette, and even the palette of its parentWidget, and tried dozens of combinations of different widget attributes etc, but I basically only have seen one of 2 results:

    1. The black background of QGLWidget shows completely opaque, can't see the palette bg color.
    2. The QGLWidget is completely transparent and views the desktop or other windows behind my application, but still no palette bg color.

    So is it possible to see through a QGLWidget onto a Qt-defined (not GL) background color? Or to view some other opaque widget behind the QGLWidget?