​

function setup() {

  createCanvas(1280, 720);

  points = [

    createVector(0, -300),

    createVector(300, 0),

    createVector(-100, 300),

    createVector(0, 0),

    createVector(-100, 0),

    createVector(-300, -100)

  ];

}

​

function draw() {

  background(0);

  push();

  translate(width/2, height/2);

  stroke(255);

  strokeWeight(2);

  noFill();

  let pt = pointPolygonCollision(mouseX - width / 2, mouseY - height / 2, points);

  beginShape();

  for (let i = 0; i < points.length; i++) {

    vertex(points[i].x, points[i].y);

  }

  endShape(CLOSE);

  if (pt != null) {

    stroke(255, 0, 0);

    strokeWeight(10);

    point(pt.x, pt.y);

  }

  pop();

}

​

function pointPolygonCollision(px, py, points) {

  let num = 0;

  let closestPt = createVector();

  let closestDSq = Infinity;

  for (let i = 0; i < points.length; i++) {

    let x1 = points[i].x;

    let y1 = points[i].y;

    let x2 = points[(i + 1) % points.length].x;

    let y2 = points[(i + 1) % points.length].y;

    if (y1 > y2) {

      [x1, y1, x2, y2] = [x2, y2, x1, y1];

    }

    let grad = (y2 - y1) / (x2 - x1);

    let nx = (1/grad * px + grad * x1 + py - y1) / (1/grad + grad);

    let ny = grad * (nx - x1) + y1;

    let dSq = (nx - px) * (nx - px) + (ny - py) * (ny - py);

    if (dSq < closestDSq) {

      closestDSq = dSq;

      closestPt = createVector(nx, ny);

    }

    if (y1 == y2) continue;

    let xl = (py - y1) / (y2 - y1) * (x2 - x1) + x1;

    if (y1 < py && py < y2 && xl > px) num++;

  }

  if (num % 2 == 0) return null;

  else return closestPt;

}