R – Qt: transparent QRubberBand

Do you set up the correct clear color (glClearColor) and actually do a clear (glClear)? Are you making sure that your color write (glColorMask) mask is fully enabled (it also affects clears). Next you can check if QT sets up some weird pixel copy transfer (other than the default, see glPixelStore, glPixelTransfer and glPixelMap). Are you sure you are getting (and reading into) an RGBA buffer in both cases, not just RGB? Lastly, it is very well possible that the framebuffer object extension is buggy on your particular graphics card/driver combination, more so if there is no primary visible window. At least check cards from both vendors. And of course always check glGetError after all potentially failing operations. After re-reading your question, do you get any other valid rendering in the non QGLWidget case at all? Does clear to say green, return green? Because it might very well be that you just did not initialize any valid OpenGL context to begin with in that case. Framebuffer objects, unlike pbuffers need an external context.

That link (and sorting on CPU) is for alpha blending. If you need only Alpha Testing (not Blending), then you don't need to sort anything. Just enable alpha test, keeping depth test enabled, and everything will be rendered fine.

See here: http://www.opengl.org/wiki/Transparency_Sorting You need "Alpha test" that requires alpha testing, not "Standard translucent" that requires sorting.