[Sidefx-houdini-list] dPds, dPdt troubleshooting...

Dan Seddon dan.seddon at framestore-cfc.com
Thu Aug 31 06:32:24 EDT 2006

In the case of mantra s and t is the primitive  parameterization - so 
this can be discontinuous from one face to another in the case of 
poly/subd's. I don't use mantra so much at the mo, but in prman it's 
often better to find the derivative of your texture coordinates if you 
want them to be fairly continuous (as much as your uvs are anyway). But, 
there are easier ways if you're just after vectors for an anisotropic 
shader - you could take a cross products of your normals with some other 
vector (either fixed or varying by noise in some way), though this can 
be problematic with deforming geometry. An adaptation of this is to 
"comb" a vector with the Comb Sop and pass this to your shader (you can 
orthogonalize  it  at render time if you like),  this enables you to use 
Houdini to perform the necessary deformations and, in the case of 
something like a brushed steel shader control, gives you more control 
over the look. Use the AttribReorient Sop to move your combed vectors 
with a deforming object.


> Hey guys!
> Can you explain me how mantra calculates derivatives? I know they 
> depend on topology of geometry. Does they depend on vertex order? I'm 
> playing with anisotropic shader based on dPds, dPdt and do some test. 
> Each test gives me an unpredictable result.
> Don't let me die from this headache, please!
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