Adaptive tri-linear interpolation for use when switching to a new level-of-detail map

Abstract

A 3D-graphics engine has several texture maps with different levels of detail (LOD). The largest of the four derivatives of the u,v texture-map coordinates with respect to the x,y screen coordinates determines which LOD texture map to select. Using bi-linear interpolation, the four nearest texture pixels or texels are fetched from the texture map in a texture memory and a weighted-average texel generated. Distortion in space and time can be visible when a triangle transitions from one LOD texture map to the next LOD map. Tri-linear interpolation eliminates this LOD-transitioning distortion by generating weighted-average texels for both the LOD map and for four texels from a next LOD map. Unfortunately the calculational complexity is more than doubled for tri-linear rather than bi-linear interpolation. Tri-linear interpolation is employed only near a transition to a next LOD map. When the derivatives are not near an LOD-map transition, only bi-linear interpolation is performed. When the derivatives are near the LOD transition, tri-linear interpolation smoothes out the transition, eliminating visible distortions. The tri-linear interpolation performs linear LOD-blending with various slopes larger than typical, or with a non-linear blending function read from a lookup table. An interpolation factor read from the lookup table or generated by shifting the LOD fraction is multiplied by the weighted-texel average for the tri-linear weighting.