procedural_anim/sketch.js

const vec = p5.Vector;

class Leg {
  constructor() {
    this.base = new vec(0, 0);
    this.end = new vec(0, 0);
    this.ikJoint = new IKJoint(4, 30);
  }

  moveTo(startX, startY, endX, endY) {
    this.base = new vec(startX, startY);
    let target = new vec(endX, endY);
    
    if (vec.dist(target, this.end) > 80) {
      this.end = new vec(endX, endY);
    }
    this.ikJoint.moveTo(startX, startY, this.end.x, this.end.y);
  }

  getPoints() {
    return this.ikJoint.points;
  }

  draw() {
    let points = this.ikJoint.points;
    let numPts = points.length;
    let px = this.base.x;
    let py = this.base.y;
    
    for (let i = 0; i < numPts; i++) {
      let pt = points[i];
      line(px, py, pt.pos.x, pt.pos.y);
      px = pt.pos.x;
      py = pt.pos.y;
    }
  }
}

class IKJoint {
  constructor(numPoints, pointDist) {
    this.origin = new vec(0, 0);
    this.points = [];
    for (let i = 0; i < numPoints + 1; i++) {
      this.points.push(new ConstraintPoint(0, 0, pointDist, 10));
    }
    this.pointDist = pointDist;
    this.iterations = 15;
  }

  moveStart(targetX, targetY) {
    let numPts = this.points.length;
    this.points[0].pos.x = targetX;
    this.points[0].pos.y = targetY;
    this.points[0].follow(targetX, targetY);
    
    for (let i = 1; i < numPts; i++) {
      let prevPoint = this.points[i - 1];
      this.points[i].follow(prevPoint.pos.x, prevPoint.pos.y, prevPoint.forward);
    }
  }

  moveEnd(targetX, targetY) {
    let numPts = this.points.length;
    this.points[numPts - 1].pos.x = targetX;
    this.points[numPts - 1].pos.y = targetY;
    
    for (let i = numPts - 2; i >= 0; i--) {
      let prevPoint = this.points[i + 1];
      this.points[i].follow(prevPoint.pos.x, prevPoint.pos.y, prevPoint.forward);
    }
  }
  
  moveTo(origX, origY, targetX, targetY) {
    for (let i = 0; i < this.iterations; i++) {
      this.moveEnd(targetX, targetY);
      this.moveStart(origX, origY);
    }
  }
}

class ConstraintPoint {
  constructor(x, y, size, angleConstraint = 0.2) {
    this.size = size;
    this.pos = new vec(x, y);
    this.forward = new vec(0, 0);
    this.angleConstraint = angleConstraint;
  }

  getRel(angleOffset, dist) {
    let dir = vec.rotate(this.forward, angleOffset);
    dir.setMag(dist);
    return vec.add(this.pos, dir);
  }
  
  follow(targetX, targetY, parentForward = null) {
    let target = new vec(targetX, targetY);
    let targetForward = vec.sub(target, this.pos);
    let targetAngle = targetForward.heading();

    if (parentForward) {
      let parentAngle = parentForward.heading();
      let diff = targetAngle - parentAngle;

      while (diff < -PI) diff += TWO_PI;
      while (diff > PI) diff -= TWO_PI;

      let maxAngle = this.angleConstraint; 
      diff = constrain(diff, -maxAngle, maxAngle);

      targetAngle = parentAngle + diff;
    }

    this.forward = p5.Vector.fromAngle(targetAngle);

    let dir = p5.Vector.mult(this.forward, -this.size);
    this.pos = vec.add(target, dir);
  }
}

let numPoints = 0;
const sizes = [60, 55, 50, 40, 45, 47, 50, 47, 45, 42, 39, 35, 30, 25, 22, 17, 13];
const bodyPoints = [];
const leg1 = new Leg();
const leg2 = new Leg();
const leg3 = new Leg();
const leg4 = new Leg();

function setup() {
  createCanvas(800, 800);
  numPoints = sizes.length;
  for (let i = 0; i < numPoints; i++) {
    let pt = new ConstraintPoint(400, 400, 20);
    bodyPoints.push(pt);
  }
}

function drawPoints(points, sizes) {
  beginShape();

  let numPoints = points.length;
  let head = points[0];
  let tail = points[numPoints - 1];

  for (let angle = -HALF_PI; angle <= HALF_PI; angle += PI / 10) {
    let p = head.getRel(angle, sizes[0]);
    vertex(p.x, p.y);
  }

  for (let i = 1; i < numPoints - 1; i++) {
    let p = points[i].getRel(HALF_PI, sizes[i]);
    vertex(p.x, p.y);
  }

  for (let angle = HALF_PI; angle <= HALF_PI + PI; angle += PI / 10) {
    let p = tail.getRel(angle, sizes[numPoints - 1]);
    vertex(p.x, p.y);
  }

  for (let i = numPoints - 2; i >= 1; i--) {
    let p = points[i].getRel(-HALF_PI, sizes[i]);
    vertex(p.x, p.y);
  }
  endShape(CLOSE);

}

function draw() {
  background(150);
  
  // fisica
  bodyPoints[0].follow(mouseX, mouseY);
  for (let i = 1; i < numPoints; i++) {
    let prevPoint = bodyPoints[i - 1];
    bodyPoints[i].follow(prevPoint.pos.x, prevPoint.pos.y, prevPoint.forward);
  }
  let baseJoint = bodyPoints[2];
  let baseL1 = baseJoint.getRel(-HALF_PI, 20);
  let endL1 = baseJoint.getRel(-HALF_PI, 100);
  let baseR1 = baseJoint.getRel(HALF_PI, 20);
  let endR1 = baseJoint.getRel(HALF_PI, 100);

  leg1.moveTo(baseL1.x, baseL1.y, endL1.x, endL1.y);
  leg2.moveTo(baseR1.x, baseR1.y, endR1.x, endR1.y);

  // dibujar
  fill(249, 182, 249);
  stroke(0);
  strokeWeight(2);
  

  leg2.draw();

  drawPoints(leg1.getPoints(), [20, 17, 15, 10, 5]);
  drawPoints(bodyPoints, sizes);

  let eye1 = bodyPoints[0].getRel(HALF_PI / 2, 45);
  let eye2 = bodyPoints[0].getRel(-HALF_PI / 2, 45);
  fill(0);
  circle(eye1.x, eye1.y, 12);
  circle(eye2.x, eye2.y, 12);
}