OpenImageIO/oiio — pull request #246 by cmstein (2012-03-09 00:45:45+00:00)
Title: if texture derivatives are colinear, then orthogonalize them

If we perform a texture lookup where dsdx,dsdy..dtdy=0, then the options.sblur parameter won't get applied.

This is because adjust_width_blur will produce colinear derivative components (dsdx=dtdx=dsdy=dtdy) which then produces numerical problems with the ellipse axes and effectively results in point-lookups.

This fix just "orthogonalizes" the texture derivatives and the blur seems to get applied correctly.