{
        Carpet function.

        Scale pattern to tuft size.

        A1      - tuft red color
        A2      - tuft green color
        A3      - tuft blue color
        A4      - degree of variation
}
                                        { carpet tuft position }
{ Varies from [(1-A4),1] based on Perlin 3D noise function, a random wave
  function over 3D space, with scalar value everywhere between about -1 and 1.
  Sense of this is 0 means bottom of tuft and 1 means top (carpet surface). }
carpt = 1 - A4*(.5+.5*noise3(Px,Py,Pz));
                                        { exponent for carpet color }
{ Takes [(1-A4),1] range and maps it to [1,(1+4*A4)]  for # interreflections }
carpe = 1 + 4*(1-carpt);
                                        { coefficient (with fuzzies) }
{ Takes square of carpt as general shading coefficient (self-shadowing)
  and multiplies by some fractal noise hovering around 1 with a base frequency
  10 times the tuft frequency (fuzzies).  The noneg() function guarantees
  positive results for outlying values of the noise functions. }
coef = noneg( carpt*carpt * (1 + .25*A4*fnoise3(Px*10,Py*10,Pz*10)) );
                                        { carpet color }
{ Final color is shading coefficient times carpet color raised to the power
  of the number of interreflections, which causes both darkening and
  deepening of color in between tufts. }
carpr = coef * A1^carpe;
carpg = coef * A2^carpe;
carpb = coef * A3^carpe;