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A matter of state

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I end up usually having a lot of rectangular 2D layers and most of the time, rather than having multiple VBOs, I use a single full screen quad VBO, a vertex shader that does _no_ transforms, and the only thing that changes is the texture and viewport. So my question is regarding the overhead of a moderate number of glViewport calls. How serious is it? Can I assume this function doesn't result in kernel mode call by the driver? For cases where some objects overlap exactly and the new coordinates are the same ones as the old: does the driver check whether there is a change or should I do such a check manually before calling glViewport to improve performance?

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Hi,

Whatever the case, glViewport(...) will not cause any serious overhead. If you're just looking at a few passes, worry about binding your shaders/textures a LOT more.

You may be better off not checking for state changes, although, as always, try to avoid it. If it's still a serious concern for you, you can always just benchmark your code with and without the checks.

-G

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