RenderMan Displacement Shader

From K-3D

Global Variables

The following table lists the set of global variables that can be used by displacement shaders.

Name	Type	Storage	Description
P	point	varying	Displaced surface position (output)
N	normal	varying	Displaced surface shading normal (output)
P	point	varying	Surface position
dPdu	vector	varying	Change in surface position along u
dPdv	vector	varying	Change in surface position along v
N	normal	varying	Surface shading normal
Ng	normal	varying	Surface geometric normal
I	vector	varying	Direction of ray stricking a surface point (from the camera)
E	point	uniform	Position of the camera
u,v	float	varying	Surface parameters
du,dv	float	varying	Change in surface parameters
s,t	float	varying	Surface texture coordinates
ncomps	float	uniform	Number of color components
time	float	uniform	Current shutter time
dtime	float	uniform	The amount of time covered by this shading sample.
dPdtime	vector	varying	How the surface position P is changing per unit time, as described by motion blur in the scene.

Geometry of Shading

Displacement shaders alter the smoothness of a surface. Unlike bump mapping which mimics the appearance of bumpiness by reorientating surface normals, RenderMan's displacement shading technique genuinly effects the geometry of a surface. Generally a displacement shader 'pushes' or 'pulls' each point on a surface parallel to the surface normal, after which it recalculates the normal so that any surface shader that has been 'attached' to the geometry can assign the appropriate lighting effects.

         N
         ^                    
_________|_________
           
                N 
         ____  /  
        /    \/
_______/      \______

The displacement shader environment is very similar to a surface shader, except that it only has access to the geometric surface parameters. It computes a new P and/or a new N. In rendering implementations that do not support the Displacement capability, modifications to P will not actually move the surface (change the hidden surface elimination calculation); however, modifications to N will still occur correctly.

Examples

The examples below are taken from RManNotes' Writing RenderMan Shaders.

You can download the source files used to generate pictures below, modified to work with Aqsis, here: k3d.sourceforge.net/wiki-data/aqsis_rmannotes_displacement.tar.gz .

The RIB file uses a RenderMan sphere because it has automatic mapping coordinates. If you try with a polygonal primitive or any other mesh, you'll get unexpected results: K-3D doesn't automatically set texture coordinates for them.

disp1.1.sl :

displ.2.sl :

displ.3.sl :

#include "rmannotes.sl"

displacement disp1_1(float freq = 5, Km = 0.01)
{
  float surface_mag, layer_mag;
  float ss, tt;
  float fuzz = 0.05;

  /* background layer */

  surface_mag = 0;

  /* rotate & repeat layers */

  rotate2d(s, t, radians(45), .5, .5, ss, tt);
  ss = repeat(ss, freq);
  tt = repeat(tt, freq);

  /* layer 1 */

  layer_mag = pulse(0.35, 0.65, fuzz, ss);
  surface_mag += layer_mag;

  /* layer 2 */

  layer_mag = pulse(0.35, 0.65, fuzz, tt);

  surface_mag += layer_mag;
  /* surface_mag = max(surface_mag, layer_mag); */ /* disp1_2 */
  /* surface_mag = max(surface_mag, layer_mag) + layer_mag; */ /* disp1_3 */

  /* displace */

  P += Km * surface_mag * normalize(N);
  N = calculatenormal(P);
}