An OSL Wood Shader for Blender Cycles

Good looking procedural wood is not that simple to implement but fortunately for us their is a whole host of renderman shading experience available online. Based on a shader by Larry Gritz we implement a fairly realistic wood shader in OSL.

Both the dark wood of Suzanne and the torus and the lighter wood of the planks they are resting on was done with the shader we present in this blog article. The algorithms used to produce the ring and the grain are essentialy the same as those implemented in Larry Gritz' oak shader. However, Open Shading Language (OSL) is not quite the same as Renderman so there was in the end quite some adaptation needed.

Beside syntactic differences between RSL and OSL, the main differences are that OSL provides us natively with snoise() functions but on the other hand, a function like area() doesn't seem to work well and neither are the derivative functions. However, because of the inherent antialiasing in Cycles we don't have to bother much about filter width (as we would have in other types of render engines) so we can do without I think. Functionally the biggest adaptation was that the original implementation provided a complete lighting model (a closure) for the wood shader while we adopt a more modular approach provinding color and displacement output that can be combined with existing closures for maximum flexibility. See the node setup at the end of the article for an example.

The code is for the OSL shader is shown below. Implementation mistakes are mine but the credits go to Larry Gritz:

// for the original renderman shader, check http://www.larrygritz.com/arman/materials.html

// adapted from larry gritz advanced renderman patterns.h
float smoothpulse (float e0, float e1, float e2, float e3, float x)
{
    return smoothstep(e0,e1,x) - smoothstep(e2,e3,x);
}

/* A pulse train of smoothsteps: a signal that repeats with a given
 * period, and is 0 when 0 <= mod(x/period,1) < edge, and 1 when
 * mod(x/period,1) > edge.  
 */
float smoothpulsetrain (float e0, float e1, float e2, float e3, float period, float x)
{
    return smoothpulse (e0, e1, e2, e3, mod(x,period));
}

// adapted from larry gritz advanced renderman noises.h
/* fractional Brownian motion
 * Inputs: 
 *    p     position
 *    octaves        max # of octaves to calculate
 *    lacunarity     frequency spacing between successive octaves
 *    gain           scaling factor between successive octaves
 */

/* A vector-valued antialiased fBm. */
vector vfBm (point p, float octaves, float lacunarity, float gain)
{
    float amp = 1;
    point pp = p;
    vector sum = 0;
    float i;

    for (i = 0;  i < octaves;  i += 1) {
  vector d = snoise(pp);
  sum += amp * d;
  amp *= gain;  
  pp *= lacunarity;  
    }
    return sum;
}

// adapted from larry gritz oak.sl and oak.h
// original comments between /* ... */
// my comments start with //
// note that I dropped the whole filterwidth stuff, partly 
// because I don't think it necessary in Blender Cycles, partly
// because the derivatives and area() function doesn't seem to work (yet)
// all specialized snoise defines are replaced by snoise() function calls
float oaktexture (point Pshad,
  float dPshad,
  float ringfreq, 
  float ringunevenness, 
  float grainfreq,
     float ringnoise, 
  float ringnoisefreq,
     float trunkwobble, 
  float trunkwobblefreq,
     float angularwobble,
  float angularwobblefreq,
     float ringy, 
  float grainy)
{
    /* We shade based on Pshad, but we add several layers of warping: */
    /* Some general warping of the domain */
    vector offset = vfBm(Pshad*ringnoisefreq, 2, 4, 0.5);
    
 point Pring = Pshad + ringnoise*offset;
    /* The trunk isn't totally steady xy as you go up in z */
 vector d = snoise(Pshad[2]*trunkwobblefreq) ;
    Pring += trunkwobble * d * vector(1,1,0);

    /* Calculate the radius from the center. */
    float r = hypot(Pring[0], Pring[1]) * ringfreq;
    /* Add some noise around the trunk */
    r += angularwobble * smoothstep(0,5,r)
    * snoise (angularwobblefreq*(Pring)*vector(1,1,0.1));

    /* Now add some noise so all rings are not equal width */
    r += ringunevenness*snoise(r);

    float inring = smoothpulsetrain (.1, .55, .7, .95, 1, r);

    point Pgrain = Pshad*grainfreq*vector(1,1,.05);
    float dPgrain = dPshad; //dropped filterwidthp(Pgrain);
    float grain = 0;
    float i, amp=1;
    for (i = 0;  i < 2;  i += 1) {
 float grain1valid = 1-smoothstep(.2,.6,dPgrain);
 if (grain1valid > 0) {
     float g = grain1valid * snoise (Pgrain);
     g *= (0.3 + 0.7*inring);
     g = pow(clamp(0.8 - (g),0,1),2);
     g = grainy * smoothstep (0.5, 1, g);
     if (i == 0)
  inring *= (1-0.4*grain1valid);
     grain = max (grain, g);
 }
 Pgrain *= 2;
 dPgrain *= 2;
 amp *= 0.5;
    }

    return mix (inring*ringy, 1, grain);
}

// larry gritz' original shader was a closure but this shader
// provides different outputs that you can plug into your own
// closures/shaders
surface oak( 
   point Pos = P,
   float Sharpness = 0.01, // sharpness of the grain. hand tweaked because we lack derivatives.
   float ringfreq = 8, 
   float ringunevenness = 0.5,
      float ringnoise = 0.02, 
   float ringnoisefreq = 1,
      float grainfreq = 25,
      float trunkwobble = 0.15, 
   float trunkwobblefreq = 0.025,
      float angularwobble = 1, 
   float angularwobblefreq = 1.5,
      color Clightwood = color(.5, .2, .067),
      color Cdarkwood = color(0.15, 0.077, 0.028),
      float ringy = 1, 
   float grainy = 1,
      output color Color = 0,
   output float Spec = 0.1,
   output float Roughness = 0.1,
   output float Disp = 0
    )
{
    float wood = oaktexture (Pos, Sharpness, ringfreq, ringunevenness, grainfreq, 
         ringnoise, ringnoisefreq, trunkwobble, trunkwobblefreq,
         angularwobble, angularwobblefreq, ringy, grainy);
    
 Color = mix (Clightwood, Cdarkwood, wood);
    Disp = -wood;  // lightwood = 0, darkwood is deeper/lower = -1
 Spec = 0.1*(1-0.5*wood); // darkwood is less specular
 Roughness = 0.1+0.1*wood; // and rougher
}

Example node setup

The node setup for the Suzanne and the torus looks like this: