i hope this can help other newbies like me: building the terrain engine for my flight simulation, i noticed fps where dropping from 100 to 50 when, moving the camera, i was looking at where land and water intersected each other (land and water are two separate triangle strips). I tought that was the reason of the fps drop, i.e. the depth buffer calculations or perhaps the z-buffer fighting. But just casually i reduced the water texture from 128x128 to 64x64 and wow ... now i get 100 fps everywhere !!! glHorizon_Project

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this is normal behaviour. and part of the reason that you use mipmapping. smaller textures equals greater speed

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The sudden 50fps speed drop also sounds suspicously like vsync is enabled. Try disabling it and the fps drop wouldnt be nearly as big.