Transformations : Direct or Inverse

Inverse scanning is used most of the time...

Forward mapping is stepping through the texture map and computing the screen location of each texel. This is done in real-time video effects systems. Most other systems use inverse mapping, which is done by stepping through the screen pixels and computing the color of each by determining which texels map to this pixel.

A disadvantage of mapping from texture space to pixel space is that a selected texture patch usually does not match up with the pixels boundaries, thus requiring calculation of the fractional area of pixel coverage. Therefore, mapping from pixel space to texture space is the most commonly used texture mapping method. This avoid pixel subdivision calculations and allows antialiasing (filtering) procedures to be easily applied.