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.

You can merge this Pull Request by running:

  git pull https://github.com/cmstein/oiio fix_colinear_derivs

Or you can view, comment on it, or merge it online at:

  https://github.com/OpenImageIO/oiio/pull/246

-- Commit Summary --

* if texture derivatives are colinear, then orthogonalize them

-- File Changes --

M src/libtexture/texturesys.cpp (5)

-- Patch Links --

  https://github.com/OpenImageIO/oiio/pull/246.patch
  https://github.com/OpenImageIO/oiio/pull/246.diff

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Reply to this email directly or view it on GitHub:
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