woodgrain effects easily simulated using embedded cylinder equations

eqn of cylinder: r1 = (u^2 + v^2)^0.5

perturb (deviate) wood growth

r2 = r1 + 2*sin(a*A)

twist axis of cylinders:

r2 = r1 + 2*sin(a*A + v/b)
where u, v, w are parametric (local) coordinates; a, b are constants
A = arctan(u/w)

then use mod function to simulate embedded cylinders, and map r2 size to a colour (eg. dark, light)

eg. Code from Watt, Program 7.1, p. 248 :

wood_grain(u,v,w:real; var r,g,b:real);
   var radius, angle: real;
   grain:integer;

   radius := sqrt(u*u+v*v);
   if w=0 then angle := pi / 2
          else angle := arctan(u,w);
   /* arctan evaluates arctan(u,w), but uses ingo to return
      a value betwen 0 and 2*pi */
   radius := radius + 2*sin(20*angle+v/150);
   grain := round(radius) mod 60;
   if grain < 40 then
      r := R_LIGHT; g := G_LIGHT; b := B_LIGHT;
   else
      r := R_DARK; g := G_DARK; b := B_DARK;
end;