Double Pendulum

// Original code from Daniel Shiffman, I've tweaked some things. :)

// Daniel Shiffman

// http://codingtra.in

// http://patreon.com/codingtrain

​

// Double Pendulum

// https://youtu.be/uWzPe_S-RVE

​

let r1 = 150;

let r2 = 150;

let m1 = 10;

let m2 = 10;

let a1 = 0;

let a2 = 0;

let a1_v = 0;

let a2_v = 0;

let g = 1;

​

let px2 = -1;

let py2 = -1;

let cx, cy;

​

let buffer;

let inkleft = 100;

​

function mouseClicked() {

  saveCanvas('DoublePendulum', 'png');  

}

​

function setup() {

  createCanvas(windowWidth, windowHeight);

  a1 = PI + random()/100-.005; //random a1

  a2 = PI + random()-.5; //random a2

  cx = width / 2;

  cy = height / 2;

  buffer = createGraphics(width, height);

  buffer.background(0);

  buffer.colorMode(HSB, 256);

  buffer.translate(cx, cy);

  buffer.strokeWeight(3);

}

​

function draw() {

  background(175);

  imageMode(CORNER);

  image(buffer, 0, 0, width, height);

  if(inkleft > 0) {

    for(let i = 0; i < 100; i++) {  //How many times per draw loop to progress forward  

      let num1 = -g * (2 * m1 + m2) * sin(a1);

      let num2 = -m2 * g * sin(a1 - 2 * a2);

      let num3 = -2 * sin(a1 - a2) * m2;

      let num4 = a2_v * a2_v * r2 + a1_v * a1_v * r1 * cos(a1 - a2);

      let den = r1 * (2 * m1 + m2 - m2 * cos(2 * a1 - 2 * a2));

      let a1_a = (num1 + num2 + num3 * num4) / den;

​

      num1 = 2 * sin(a1 - a2);

      num2 = (a1_v * a1_v * r1 * (m1 + m2));

      num3 = g * (m1 + m2) * cos(a1);

      num4 = a2_v * a2_v * r2 * m2 * cos(a1 - a2);

      den = r2 * (2 * m1 + m2 - m2 * cos(2 * a1 - 2 * a2));

      let a2_a = (num1 * (num2 + num3 + num4)) / den;

​

      translate(cx, cy);

      stroke(255);

      strokeWeight(2);

​

      let x1 = r1 * sin(a1);

      let y1 = r1 * cos(a1);

​

      let x2 = x1 + r2 * sin(a2);

      let y2 = y1 + r2 * cos(a2);

​

      //line(0, 0, x1, y1);

      //fill(0);

      //ellipse(x1, y1, m1, m1);

​

      //line(x1, y1, x2, y2);

      //fill(0);

      //ellipse(x2, y2, m2, m2);

​

      a1_v += a1_a;

      a2_v += a2_a;

      a1 += a1_v;

      a2 += a2_v;

​

      a1_v *= 0.9999;

      a2_v *= 0.9999;

​

      buffer.stroke(0);

      if (frameCount > 1) {

        buffer.stroke(map(dist(px2, py2, x2, y2), 0, 30, 0, 256), 256, 256, map(inkleft, 100, 0, 256, 0)); //stroke color changes with speed.

        buffer.strokeWeight(map(dist(px2, py2, x2, y2), 0, 30, 5, 1)); //StrokeWeight inversly proportional to thinner when faster.

        buffer.line(px2, py2, x2, y2);

      }

​

      px2 = x2;

      py2 = y2;

    }

    inkleft--;

  }

}