Renderman dPdu & dPdv

[noizFACTORY]

I'll give it a shot.

From what I remember of the calculus classes in school, dxdy are derivatives of x with respect to y. Basically it tells you how a value x changes with a change in y and that too at really minuscule level. More detailed info on derivatives:
http://en.wikipedia.org/wiki/Derivative

dPdu - change in position P with respect to the u value (u as in uv)

dPdv - change in position P with respect to the v value (v as in uv)

So this allows you to calculating stuff like shading and even manipulate it across a surface at a very fine level. From the RiSpec (Page 119):

The geometry is characterized by the surface position P which is a function of the surface
parameters (u,v). The rate of change of surface parameters are available as (du,dv). The
parametric derivatives of position are also available as dPdu and dPdv.

You would have also seen this image numerous times that illustrates this:

http://sachinmukti.com/images/Figure.12.1.big.gif

One of the most common places I've seen this being used is in anisotropic specular. This is all very simple and common info but hope it helps.

-Sachin

[ndeboar]

ah, got it!

[playmesumch00ns]

It's basically the 3d vector pointing to the next micropolygon vertex. As sachin mentioned it can be used for getting a coordinate frame for anisotropic shading... however on anything but nurbs the result is usually fubar.

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