Derivative Calculation (cont)
In per fragment operation, before Texel is read from map, must correct S’,T’ back to original S,T range
- S’’ = S’ / w’, T’’ = T’ / w’
- S’’, T’’ used to access texture map
- The w’ at a fragment operation has been incrementally computed using dw’/dx, dw’/dy, dw’/dscan values
Computation cost on a per fragment level is two divisions, one for S and one for T.
Computation cost at setup/rasterizer level is extra derivative calculations for w, correction of original S,T vertex values.