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

hoknamahn at cgtalk.ru hoknamahn at cgtalk.ru
Thu Aug 31 07:18:03 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'm trying to figure out when the polygonal geometry behaves as 
parametric surface and when as polygonal (with discontinuity).
In some cases the whole piece of polygonal geometry looks like 
parametric surface (i.e. with nice anisotropy) in other case the whole 
geometry or the part of geometry looks weird.

 > 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
Thanks for the idea :)

Dan Seddon wrote:

> 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.
>
> Cheers
>
> Dan
>
>> 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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