G'MIC Hilbert Curve Animation

# Generate an animation that draws the Hilbert curve.
anim_hilbert :

  # Precompute Hilbert curve.
  0 eval "
    const n = 3 + 1;
    const N = 1<<n;
    repeat (N^2,d,
      for (x = y = 0; t = d; s = 1, s<N, s<<=1,
        rx = 1&(t>>1); ry = 1&xor(t,rx);
        !ry?(rx==1?(x = s - 1 - x; y = s - 1 - y); swap(x,y));
        x+=s*rx; y+=s*ry; t>>=2;
      );
      da_push([x,y]);
    ); da_freeze()" => pts
  r. 1,{h*5},1,2,5 b. y,0.1% n. -0.5,0.5

  # Generate frames for curve generation.
  repeat {pts,h} {
    +rows[pts] 0,$> 600,600
    eval.. "
      begin(
        const zoom = lerp(15,0.7,$%^0.15);
        const fact = 0.95*w#-1*zoom;
        angle = lerp(0,360*4,$%)°;
        R = rot(angle);
        Pc = I[-1,2];
      );
      pP = J[-1,1]; P = I;
      pP = Pc + R*(pP - Pc); P = Pc + R*(P - Pc);
      Off = lerp(Pc,0,$%^10); pP-=Off; P-=Off;
      polygon(#-1,2,round(w#-1/2 + pP*fact),round(w#-1/2 + P*fact),1,1)"

    dilate_circ. {lerp(60,5,$%^0.25)}
    +b. {lerp(8,0.5,$%^0.25)}%,3 *. 600 c. 0,255
    *.. 255 to_rgb.. map. hot
    max[-2,-1]
    r. 500,500,1,3,0,0,0.5,0.5
    w. wait 10
    rs. 50% on. frame.jpg,$>
    rm[-2,-1]
  }