// The U() macro creates the union of a <0,0,0>,<0,1,0> cylinder
// and a subtree at the <0,1,0> end of the cylinder (by making a
// recursive T() call).
#macro U()
  union
  { cylinder
    { y, 0 I/70 // Radius according to the recursion level
      pigment { rgb <1 I/6> } // Colored according to the recursion level
    }
    #if(I)
      // Recursively create a subtree and translate it to <0,1,0>
      T(I-1)
      translate y }
    #end
    rotate // The 'rotate' keyword of the three rotates needed in T()
#end

// The T() macro creates the union of three subtrees, rotating each one
// in different directions.
#macro T(I)
  union
  { U() <-30,40> }
    U() -24*z }
    U() <22,15> }
#end

// The F() macro creates the "forest", ie. J+1 trees translated randomly.
#macro F(J, R)
  T(6)
  translate <rand(R)-.5, -.1 rand(R)>*20 }
  #if(J) F(J-1 R) #end
#end

// Instantiate the forest (10 trees):
F(9 seed(2))

// Light source:
light_source{x,1}