From Wikibooks, open books for an open world. A typical 3D rasterizer will perform perspective-correct interpolation, which is meant to take perspective distortion into account when drawing a polygon that has been projected to 2D screen space. As an optimization, it is possible to render detail from a complex, high-resolution model or expensive process such as global illumination into a surface texture possibly on a low-resolution model. In OpenGL assuming you haven't hacked the heck out of your ST coordinates per pixel in a fragment shader this always comes from a "perspective" division somewhere in the pipeline. Moreover, clipping fails in some cases.
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