Fractal 3d Solenoid in G'MIC

# Create a random fractal solenoid, as shown in http://paulbourke.net/fractals/solenoid/
# using the G'MIC script language.
solenoid3d :
  -torus3d 100,10,64,32 -col3d. ${-RGB} -nm. 0,0,0,1,0,0,0,1,0,0,0,1,100,10 # Create initial torus
  -repeat 5 # Number of scales
    subd={round(1.25^-($>+1)*[64,32])} # Torus resolution for this scale (decreasing).
    col=${-RGB} # Same random color for a single scale
    N={round(u(4,8))} # Random number of subdivision for this scale
    -repeat $! -l[$<] # For each existing torus, let's add children !
      nm={n}
      -if {narg($nm)>1}
        X={([$nm])[0,3]}  # Current center
        R={([$nm])[3,9]}  # Current rotation matrix
        r={([$nm])[12,2]} # Current radii
        nr={[$r]/2.5}     # New radii (decreasing).
        -repeat $N # Start the loop to add children
          ang={$>*360/$N}
          nX={rad=$ang*pi/180;[$X]+([$r])[0]*[$R]*[cos(rad),sin(rad),0]}  # New center
          nR={mul([$R],mul(rot(0,0,1,$ang),rot(1,0,0,90),3),3)}           # New rotation matrix
          -torus3d $nr,$subd nbp={i[6]} # Add new torus
          -sh. 8,{8+3*$nbp-1},0,0 -r. 3,$nbp,1,1,-1 3,3,1,1,$nR -transpose. -m*[-2,-1] -rm. # Small hack to orient 3d torus with matrix nR.
          -+3d. $nX -col3d. $col # Translate torus and assign color
          -nm. $nX,$nR,$nr # Associate its new parameters.
        -done
        -nm[0] 0 # Disable recursion for the parent torus in next iterations
      -endif
    -endl -done
  -done
  -+3d # Add all 3d object togethers
  -m3d 5 -db3d 0 -md3d -1 # Define reasonable rendering mode