3D Repulsive Particle System [G'MIC]

# A G'MIC implementation of a system of 3D repulsive particules
# (Author: David Tschumperlé, April 2020).
#-----------------------------------------------------------------
particles3d :
  512,512,1,3
  1,900,1,3,u(w#-1-1)

  t=0
  do
    xrep,yrep,zrep={w#0*(0.5+0.3*[cos(4.2*$t),sin(2.7*$t),cos(1.9*$t)])}
    t+=0.01

    # Render particles.
    N=$!
    repeat h circle3d {"I[#1,"$>"]"},4 done
    +3d[$N--1] col3d. 255 o3d. 0.75
    circle3d $xrep,$yrep,$zrep,20 col3d. 255,0,0
    +3d[-2,-1]
    box3d 512,512,512 p3d. 1 o3d. 0.4 +3d[-2,-1]

    -3d. {0,[w,w,w]/2} r3d. 1,1,0,34 #r3d. 0,1,0,{80*$t}
    f3d 400
    +f[0] 0 j3d. ..,50%,50%,150,1,2
    *[0] 'd=y/h;[0.24+0.75*d,0.92,0.99-0.3*d]' max[0,-1]
    rm. w[0]

    # Compute and apply particle velocity.
    +. '"*
      add_veloc(Q,strength,sigma) = (
        U = P - Q;
        nU = norm(U);
        U/=max(1e-5,nU);
        w = strength*(sigma>0?gauss(nU,sigma,0):1/(1e-5+nU));
        sumw+=w;
        V+=w*U;
      );

      P = I(#1);
      V = [ 0,0,0 ];
      sumw = 0;

      for (p = 0, p<h, ++p, p!=y?add_veloc(I[#1,p],5,20));

      const W = w#0-1;

      add_veloc([0,P[1],P[2]],10,0);
      add_veloc([P[0],0,P[2]],10,0);
      add_veloc([P[0],P[1],0],10,0);
      add_veloc([W,P[1],P[2]],10,0);
      add_veloc([P[0],W,P[2]],10,0);
      add_veloc([P[0],P[1],W],10,0);

      add_veloc(["$xrep,$yrep,$zrep"],50,70);
      2.8*V/sumw"'
    wait 30

  while {*}" && "!{*,ESC}
  rm w[] 0