Houdini Blobby Shader

Surface Color Test

Using ray tracing to define nearby surfaces, we can use the distance to nearby objects to affect the color of the surface being shaded.   In the VEX shading language I used the rayhittest() function.

Basic function
float rayhittest(vector P, vectorD, pHit, nHit, float bias)
This function returns the distance from the surface position P to the object in direction D.  The length of direction D will determine how far the rayhittest function checks for objects and returns a negative number if no hits occur.  The position and normal of the hit surface are pHit and nHit.

Added function options
scope: You can limit what objects in your scene affect the shaded surface by specifying which objects in the scene will be used to calculate the distance.
Get VEX Code

I wrote this surface shader in order to determine if it was possible for nearby surfaces to affect the displacement of the surface being shaded.  After doing some research, I discovered that the VEX ray tracing function rayhittest() does not work in displacement shading.  None of the raytracing functions of VEX operate in the VEX displacement shader.  This is because the displacement shader is run to figure out where point P is. In order to raytrace an object you have to already know this information, therefore it will not work by this method.

surface RTDepth05(
     int    alongN = 0;      
// ON: trace along -N; OFF: trace along I
     float  dmax   = 8;   
// max length of intersection test
     float  rbias  = 0.005;  
// ray bias to avoid self-intersections
	string oname = "";
  )
{
 
vector n = normalize(N);
vector i = normalize(I);
vector D = alongN ? normalize(frontface(n,i)) : n;
vector hitpoint;
vector hitnormal;

float  dhit  = rayhittest(P, D * dmax, hitpoint,
 hitnormal, rbias, "scope", oname);

float angle = dot(n, normalize(hitnormal));


vector red = set(1, 0, 0);
vector yellow = set(1, 1, 0);

Cf = lerp(yellow, red, lerp(angle); 
}