Backward mapping (also known as inverse mapping or screen order ) is a technique used in texture mapping to create a 2D image from 3D data. Because a texel (the smallest graphical element in a texture map) does not correspond exactly to a screen pixel , the developer must apply a filter computation to map the texels to the screen. Forward mapping steps through the texture map and computes the screen location of each texel. In comparison, backward mapping steps through the screen pixels and computes a pixel's color according to the texels that map to it.
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Real time video effects systems use forward mapping. However, since many texels are likely to map to a single pixel, performing the filter computation at each pixel for every frame is very expensive. Most systems that don't have to produce real time effects use backward mapping.