How to calculate a distance?

[hoknamahn]

Hi guys!

Imagine what we have some object... Maybe sphere or cube. I want to calculate a thickness of this object in the surface shader. I mean to calculate a distance from one side of object to other side along view vector. How can i do this?

[Mario Marengo]

Hi guys!

Imagine what we have some object... Maybe sphere or cube. I want to calculate  a thickness of this object in the surface shader. I mean to calculate a distance from one side of object to other side along view vector. How can i do this?

If you want to do this for an arbitrarily-shaped object, then you might want to enlist the ray tracer for some help. Something like this:

#pragma hint alongN  toggle

surface RTDepth(
      int    alongN = 0;      // ON: trace along -N; OFF: trace along I
      float  dmax   = 1e+6;   // max length of intersection test
      float  rbias  = 0.005;  // ray bias to avoid self-intersections
   )
{
   string oname = getobjectname();
   vector D     = alongN ? normalize(-frontface(N,I)) : normalize(I);
   float  dhit  = rayhittest(P,D*dmax,rbias,"scope",oname);

   // Final coloring to show missed rays ...
   // -- look at the red and green channels separately --
   Cf = dhit<0. ? {0,30,0} : set(dhit,0,0);
}

This will show you depth in the red channel and missed hits (boosted X30) in the green.

Two things to note here. First, it works And second, it doesn't work as well as you might think (try it on a NURBs torus with everything at default... lots of "stars" over in the green channel).

I've tried doing my own super-sampling over the patch to get rid of the ray misses but, alas, to no avail -- I don't think it has anything to do with sampling density (well...past a certain minimum threshold, that is), and everything to do with micropolys, patch boundaries, dicing, etc.

So.... what to do... the only remedy I've found so far is to turn off micropoly rendering, up the "Ray Tracing Level of Detail" to at least 2, and set "Super Sample" to a minimum of 4X4.... but your mileage may vary since we're heading squarely into "voodoo" land at that point

Cheers!

-- Mario.