filterwidth() D.D.Derivatives for a Cel Shader

[Mcronin]

I am building a cel shader in VOPs based on a Renderman Shader I found. The Renderman Shader uses this macro, filterwidth. It's supposed to filter the outline I guess, and I need some help figuring out how to implement this in VOPs. I have all the other pieces for the shader pretty much, but my edges are pretty harsh, and I understand this macro presumably will fix that problem. I will gladly submit the shader to the codex section if some one can help me out.

Some explanation;

x is the result of the absolute value of the dot product of N and I. I guess Du() and Dv() return derivatives of x, but Calc wasn't exactly my strongest subject:angry: This is my main sticking point, the derivative thing. I'm guessing du and dv are analogous to dPds and dPdt, and I can make MINFILTWIDTH a constant, make all this crap a vector and use Max Vector Component. I'm going to stop typing now, because I'm giving myself a headache just by thinking about the word "derivative". Can anyone help? I'm so close to getting this done and time is short.

#ifndef MINFILTWIDTH
#  define MINFILTWIDTH 1.0e-6
#endif


/* The filterwidth macro takes a float and returns the approximate 
 * amount that the float changes from pixel to adjacent pixel.
 */
#define filterwidth(x)  max (abs(Du(x)*du) + abs(Dv(x)*dv), MINFILTWIDTH)

[Mcronin]

Hold the phone, think I just figured it out.

[edward]

I think you can use the Shading Derivative vop for Du(x). The equivalent vex expression should be this (without the additional multpies of du & dv)

max(abs(Du(x))+abs(Dv(x)), MINFILTWIDTH)

[Mcronin]

Thanks Edward, I just discovered Shading Derivative before you posted, but I may be having another problem... I'll probably be posting again shortly.