Irregular stone patterns in OSL, a first attempt

On the BlenderArtists forum a member was analyzing irregular stone walls and posed the interesting question: does this resemble a tree pattern and can it be done in OSL? Quite a lot of thinking and tinkering is needed for a full solution put in this post we explore some of the basic requirements.

Irregular stone patterns

A sample pattern with primary colors, Mondrian eat your heart out :-)
The sample image shows that we have generated a pattern consisting of rows with different heights, each consiting of stones of varying width. Additionally, some stones within a row are further split horizontally. The code to generate the pattern is shown below:

shader stones(
  point p = P,
  vector Scale = 1,
  float w = 0.02,
  float s = 2,
    
  output float Fac = 0
){
  point Pos = p * Scale;
  
  float bot = floor(Pos[1]-1)+cellnoise(Pos[1]-1);
  float lev = floor(Pos[1])+cellnoise(Pos[1]);
  float top = floor(Pos[1]+1)+cellnoise(Pos[1]+1);

  if( Pos[1] < lev ){
    Pos[0] += s*cellnoise(Pos[1]);
  }else{
    Pos[0] += s*cellnoise(Pos[1]+1);
  }
  float left = floor(Pos[0]-1)+cellnoise(Pos[0]-1);
  float mid = floor(Pos[0])+cellnoise(Pos[0]);
  float right = floor(Pos[0]+1)+cellnoise(Pos[0]+1);
  if( 
    ((Pos[0] > left+w) && ( Pos[0] < mid - w )) 
    || 
    ((Pos[0] > mid+w ) && ( Pos[0] < right - w))
  ){
    if( 
      ((Pos[1] > bot+w) && ( Pos[1] < lev - w )) 
      || 
      ((Pos[1] > lev+w ) && ( Pos[1] < top - w))
    ){
      int stoneindex=0;
      float seeda = left;
      float seedb = bot;
      float bounda = mid;
      float boundb = lev;
      
      if( Pos[0] > mid ){ stoneindex += 2; seeda = mid; bounda = right; }
      if( Pos[1] > lev ){ stoneindex += 1; seedb = lev; boundb = top; }
      int pattern = (int)floor(cellnoise(seeda,seedb)*4);
      if( pattern == 0 ){
            // horizontally halved
            float nlev = (seedb + boundb)/2;
            if( (Pos[1] > nlev - w) &&  (Pos[1] < nlev + w) ){
              Fac = 0;
            } else {
              Fac = cellnoise(vector(seeda,seedb,Pos[1]>nlev));
            }
      } else {
        Fac = cellnoise(vector(seeda,seedb,-1));
      }
    }
  }
}

(The code is also available on GitHub.)

Sample node setup

The example image at the top of this article was made with the following node setup (click to enlarge)
:

Further work

Obviously we need more sub patterns for the individual stones and add some distortion to the underlying coordinates to make really random stones. It would probably also be a good idea to vary the spacing between the stones and use the output value to drive displacement and bump maps and the code could certainly do with a bit of cleanup but I thinks this approach is at least shows promiss.