const b = 75;

let theta = 0;

const points = [];

​

function setup() {

  createCanvas(480, 480);

  angleMode(DEGREES);

}

​

function draw() {

  background(255);

  strokeWeight(3);

  noFill();

  stroke(255, 0, 0);

  line(width/2 - a, height/2, width/2 + a, height/2);

  stroke(0, 0, 255);

  line(width/2, height/2 - b, width/2, height/2 + b);

  const x = width/2  + a * cos(-theta);

  const y = height/2 + b * sin(-theta);

  const xa = width/2  + b * cos(theta);

  const yb = height/2 + b * sin(theta);

  stroke(128);

  strokeWeight(8);

  point(xa, height/2);

  point(width/2, yb);

  strokeWeight(3);

  line(width/2, yb, x, y);

  stroke(0, 204, 0);

  strokeWeight(8);

  point(x, y);

  points.push(createVector(x, y));

  strokeWeight(3);

  beginShape();

  for (const p of points) {

    vertex(p.x, p.y);

  }

  endShape();

  theta += 1;

  if (theta == 360) {

    points.splice(0);

    theta = 0;

  }

}