​

let shoulder = {

  x: 100,

  y: 300

}

​

let elbow = {

  x: 200,

  y: 200

}

​

let hand = {

  x: 300,

  y: 250

}

​

function setup() {

  createCanvas(400, 400);

  upperArm = dist(shoulder.x, shoulder.y, elbow.x, elbow.y)

  forearm = dist(elbow.x, elbow.y, hand.x, hand.y)

}

​

function draw() {

  background(220);

  strokeWeight(2)

  fill(255)

  line(shoulder.x, shoulder.y, elbow.x, elbow.y)

  line(elbow.x, elbow.y, hand.x, hand.y)

  circle(shoulder.x, shoulder.y, 20)

  circle(elbow.x, elbow.y, 20)

  fill(100, 255, 100)

  circle(hand.x, hand.y, 20)

  if(mouseIsPressed){

    if(dist(mouseX, mouseY, hand.x, hand.y) < 20){

      hand.x = mouseX;

      hand.y = mouseY

      elbow = ik(shoulder, hand, forearm, upperArm)

    }

  }

  strokeWeight(0.5)

  line(shoulder.x, shoulder.y, hand.x, hand.y)

  line(shoulder.x, shoulder.y, 400, shoulder.y)

}

​

function ik(a, b, A, B) {

  let C = dist(a.x, a.y, b.x, b.y)

  let th = acos( (B**2 + C**2 - A**2) / (2*B*C) )

  let phi = atan2(-(b.y - a.y), b.x - a.x)

  return {

    x: a.x + B*cos(th + phi),

    y: a.y - B*sin(th + phi)

  }

}

​

​

​

​

​

​