var gap = 0.4;

​

function setup() { 

  createCanvas(windowWidth, windowHeight);

    var diam = windowWidth/2.5;

  dir = createVector(random(-width,width), random(-height,height));

  background(0);

  stroke(255);

  strokeWeight(5);

  translate(width/2, height/2);

  drawArrow(dir.heading(), diam);

    noStroke();

    fill(255);

    ellipse(0, 0, diam, diam);

    fill(255, 0, 0);

    arc(0, 0, diam, diam, dir.heading() + gap, dir.heading() - gap, PIE);

  stroke(255, 0, 0);

  for(var a = dir.heading() + gap; a < dir.heading() - gap + TWO_PI; a+= gap) {

    drawArrow(a, diam/1.5);

  }

​

}

​

function drawArrow(heading, len) {

  push();

  rotate(heading);

  line(0, 0, len, 0);

  translate(len, 0);

  rotate(-3*PI/4);

  line(0, 0, 10, 0);

  rotate(6*PI/4);

  line(0, 0, 10, 0);

    pop();

}