Programmable shading - idea by Rob Cook. "Connect the blocks" to hook up shader pieces into complex functionality.

Pat Hanrahan formalized the idea into RSL.
Overall philosophy of shader-writing:

* you'd write them "blind", ie. not knowing when on a surface it gets called

* RenderMan will provide detailed neighborhood info. for current shading point

* you'd write the shader in ASCII (.sl), compile it, generate a more efficient machine-readable version (.slo)

* the shader (.slo file) will be referenced in the RIB file containing the surface that we wish to shade

* shaders can be thought of as external (to RIB) shading "plugins" which RenderMan uses during rendering
RenderMan specifies five shader types:

Simplest possible (surface) shader: 
surface shaderskel()
{
}/* shaderskel */

surface constant()
{
    Oi = Os;
    Ci = Os * Cs;
}
What are Oi, Os, Ci and Cs? They are "magic" (pre-defined) variables..
A shader has three sources of data: 

surface matte( float Ka=1, Kd=1 )
{
    normal Nf; // 'local' variable, normal 'type'

    Nf = faceforward(normalize(N),I);

    Oi = Os;
    Ci = Os * Cs * ( Ka*ambient() + Kd*diffuse(Nf) ) ;
}
Variables are of specific types in RSL. Available types are float, color, point, vector, normal, matrix, string.

ambient() and diffuse() are 'built-in functions'. RSL has a rich, expressive set of such built-ins.
Neighborhood of a shading point:

Here are the main variables available to you inside a shader:

For a complete list of globals, see the Spec. 
surface metal (float Ka=1, Ks=1, roughness=.1)
{
    normal Nf;
    vector V;

    Nf = faceforward(normalize(N), I) ;
    V = normalize(-I) ;

    Oi = Os;
    Ci = Os * Cs * ( Ka*ambient() + Ks*specular(Nf,V,roughness) );
}
Note that incoming parameters ("arguments") have (need) default values. Also, note that there is no formal return value! All output occurs through pre-determined globals (eg. Ci, Oi..).
In addition to built-in functions, you can write your own, and call them (make use of them) repeatedly in your shader. Functions let you isolate, package, share/reuse shader blocks.

Our next shader uses noise() calls to create a turbulence pattern. 

// not used, is here just for studying - see turbul() below which *is* used
float turb(varying point PP)
{
  varying point PPP = PP/2;	/* frequency adjustment (S-shaped curve) */
  float pixelsize = sqrt(area(PPP));
  float twice = 2 * pixelsize;
  float scale, weight, turbulence;
  
  /* compute turbulence */
  turbulence = 0;
  for (scale = 1; scale > twice; scale /= 2) {
    turbulence += scale * abs(noise(PPP/scale)-0.5);
  }
  /* gradually fade out of highest freq component near visibility limit */
  if (scale > pixelsize) {
    weight = (scale / pixelsize) - 1;
    weight = clamp(weight, 0, 1);
    turbulence += weight * scale * abs(noise(PPP/scale)-0.5);
  }

  float ct = turbulence;
  // float ct = clamp(turbulence,0.75,1);
  // ct -= 0.75;
  // ct *= 4.0;

  // printf("Turb: %f\n",ct);

  return ct;

}

// this simpler version is used instead of turb() above
float turbul(varying point PP;float n)
{
  float t;
  if(n==4)
   t = 0.5333*(noise(PP) + 0.5*(noise(2*PP)) + 0.25*(noise(4*PP)) + 0.125*(noise(8*PP)));
  else if (n==3)
   t = 0.5714*(noise(PP) + 0.5*(noise(2*PP)) + 0.25*(noise(4*PP)));
  else if (n==2)
   t = 0.6667*(noise(PP) + 0.5*(noise(2*PP)));
  else if (n==1)
   t = noise(PP);

   return t;
}


surface smarble(float Ka=1, Kd=.6, Ks=0.4, roughness=.2,Ksin=1.0,freq=0.1,n=4;)
{
  varying point PP;
  varying float cmi;
  varying normal Nf;
  varying vector V;
  color diffusecolor ;
  


  Nf = faceforward( normalize(N), I);
  V = -normalize(I);
  PP = transform("shader",P) * freq;

  // float t = 0.5*(1+sin(Ksin*3.1415*turbul(PP,n)));
  float u = turbul(PP,n);
  // print(u);

color base = color spline(u,
color (0.8, 0.2, 0.05),
color (0.8, 0.2, 0.05),
color (0.8,0.5,0.3),
color (0.6,0.594,0.58),
color (0.3,0.3,0.4),
color (0.05, 0.05, 0.1),
color (0.8,0.79,0.77),
color (0.8,0.8,0.79)
);

  Ci = Os*Cs*  ((base*(Ka*ambient() + Kd*diffuse(Nf) + Ks * specular(Nf,V,roughness)))) ;


}// smarble()

// Surface "smarble" "freq" 1.5 "Kd" .46 "Ks" 2.5 "Ka" .4 "roughness" .2

Here is a RIB file with an 'smarble' call. Note that the 'n' parameter in the shader is what specifies how many octaves of frequencies to add (n can be ints between 1 through 4). For n=1,2,3,4 (specified via the RIB file) we get the following renders. You can see how by layering add'l higher frequencies (with corresponding lower amplitudes) we evolve pure noise into turbulence.


n=1


n=2


n=3


n=4


animated GIF version of the above four images

This shader colors based on normal at shading point. 
surface showN()
{
  point PP;
  normal Nf;
  vector V;
  color Ct, Ot;
 
  normal NN = normalize(N);
  vector posNorm = 0.5*(vector(1,1,1)+NN);
  color posCol = color(comp(posNorm,0),comp(posNorm,1),comp(posNorm,2));

  Oi = Os;
  Ci = Oi*posCol;
}
This shader reads a texture map (a specially processed image file) and uses the map colors to set Ci. 
surface tex(string tmap="generic.tex";)
{
  

  if(tmap!="") 
  {
    float alpha;
    Ci = color texture(tmap,s,t);
    
    /* alpha = texture(tmap[3],s,t); 
    Ci = alpha*Ci; */
  }

  Oi = Os;
  Ci *= (Cs*Oi);
 
}// tex()
Compile the above into tex.slo, then use this RIB file to render it. You will also need a hues.tex texture map, which you would have to create starting with hues.tif (a standard TIFF image file), using the PRMan command 'txmake', like so: 
txmake hues.tif hues.tex
The hues.tif image looks like this:

Our rendered surface that uses the above texture map (via our 'tex' shader shown earlier) looks like this:

RSL lets you "write your own lights". 
light
noisypointlt(
    float  intensity=1 ;
    color  lightcolor=1 ;
    point from = point "shader" (0,0,0) ;
    float freq=1.0, mixf=0.5;
)
{
  vector ldir = vtransform("shader",vector(Ps-from));
  illuminate(from)
    {
      vector Ls = vtransform("shader",L);
      float n = (noise(Ls*freq)); //
      Cl = (mixf*n+intensity)*lightcolor;
    }
}

Sample RIB to test our 'noisypointlt' light:

# teapot.rib
# Author: Scott Iverson 
# Date: 6/7/95
#
Display "TeapotAfter.tif" "framebuffer" "rgb"
Format 900 600 1

Projection "perspective" "fov" 30

Translate 0 0 25

Rotate -22 1 0 0
Rotate 19 0 1 0

Translate 0 -3 0

WorldBegin

LightSource "ambientlight" 1 "intensity" .26
LightSource "distantlight" 2 "intensity" .26 "from" [-4 6 -7] "to" [0 0 0]  "lightcolor" [1.0 0.4 1.0]
LightSource "distantlight" 3 "intensity" .36 "from" [14 6 7] "to" [0 -2 0] "lightcolor" [0.0 1.0 1.0]
# our noisypointlt 'shader'
LightSource "noisypointlt" 4 "intensity" 0 "from" [0 10 0] "freq" 1

Surface "plastic"

Color [1 .6 1]


# spout

AttributeBegin
  Sides 2
  Translate 3 1.3 0
  Rotate 30 0 0 1
  Rotate 90 0 1 0
  Hyperboloid  1.2 0 0 .4 0 5.7  360
AttributeEnd


# handle

AttributeBegin
Translate -4.3 4.2 0
TransformBegin
Rotate 180 0 0 1
Torus 2.9 .26 0 360 90
TransformEnd
TransformBegin
Translate -2.38 0 0
Rotate 90 0 0 1
Torus 0.52 .26 0 360 90
TransformEnd
Translate -2.38 0.52 0
Rotate 90 0 1 0
Cylinder .26 0 3.3 360
AttributeEnd

# body

AttributeBegin
Rotate -90 1 0 0
TransformBegin
Translate 0 0 1.7
Scale 1 1 1.05468457
Sphere 5 0 3.12897569 360
TransformEnd
TransformBegin
Translate 0 0 1.7
Scale 1 1 0.463713017
Sphere 5 -3.66606055 0 360
TransformEnd
AttributeEnd

# top

AttributeBegin
Rotate -90 1 0 0
Translate 0 0 5
  AttributeBegin
    Scale 1 1 0.2051282
    Sphere 3.9 0 3.9 360
  AttributeEnd

  Translate 0 0 .8
  AttributeBegin
    Orientation "rh"
    Sides 2
     Torus 0.75 0.45 90 180 360
  AttributeEnd
  Translate 0 0 0.675
  Torus 0.75 0.225 -90 90 360

  Disk 0.225 0.75 360
AttributeEnd

WorldEnd
We don't have time to introduce the other shader types.. Please read Ch08 ('Shading') of the RfB book to learn more about these (especially 'displacement' shaders).