Wednesday, August 29, 2007

OpenGL redraw bug on Windows

A while ago, "glguy" reported an issue with Factor 0.89: the UI was not being redrawn properly on Windows. I borrowed a Windows laptop and reproduced the exact same issue, and it turns out both me and glguy have the same graphics adapter: an Intel Express GM915.

I found a workaround: if I disable double buffering and don't call SwapBuffers after redrawing the window, then the issue disappears, and furthermore, it appears as if double buffering is still performed!

This is really strange, and I don't think this workaround will work elsewhere, unless WGL always double buffers, and uses hardware page flipping if you explicitly ask for it, or something.

If anybody has UI redraw issues and they're not using this adapter, or not running Windows at all, let me know!

Update: I found the right fix. On Windows, the glSwapHintRectWIN function must be called before SwapBuffers when doing a partial redraw. Turns out this function is not necessary with most drivers, but Intel drivers are picky when it comes to this. Good to resolve this one, finally. The fix is in the darcs repository.

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