Coding Train Starfield

The Starfield

This is another p5.js hello-world, this time taken from Daniel Schiffman's Starfield in Processing coding challenge. It's a rough version of traveling through the stars at warp speed. He managed to do it in about 15 minutes, if I remember correctly. It starts static but if you move your mouse back and forth horizontally it adjusts the speed.

The Main Sketch

The most basic processing/p5 sketch uses two functions setup to initially set up your canvas and draw to update the frames over time. This function creates our canvas and star objects in the setup and then calculates a speed based on the mouse position of the user in order to update the stars. It gets passed a p5 element, called p, in order to get access to the p5.js code.

/** The main sketch
 * this gets passed to p5 so it defines the setup and draw functions
 * that p5 expects
let starfield_sketch = function(p) {
  let star_count = 800;
  let parent_div_id = "schiffman-starfield";
  p.BLACK = 0;
  p.WHITE = 255;
  p.ALPHA = 100;

The Setup Function

Not too much voodoo here, other than the use of the JQuery outerWidth method which gets the width of the DIV that we're using to hold the sketch and uses it as the width for the canvas.

p.setup = function() {
  p.stars = [];
  this.canvas = p.createCanvas($("#" + parent_div_id).outerWidth(true), 800);
  for (let i=0; i < star_count; i++) {
    p.stars[i] = new Star(p);
} // end setup


Again, not too fancy. The draw function:

  • Sets the background to black to wipe out our previous drawing
  • gets the "speed" of the stars using the x-position of the mouse and mapping this location from 0 to the width of the canvas to a smaller range from 0 to 50
  • Translate the (0, 0) of the coordinate system from the top left of the canvas to the middle so our stars emerge from the center instead of the top left
  • update all the stars with the speed and re-draw them
  p.draw = function() {
    p.background(p.BLACK, p.ALPHA);
    speed =, 0, p.width, 0, 50);
    p.translate(p.width/2, p.height/2);
    for (let i=0; i < p.stars.length; i++){
  } //end draw
}; //end starfield_sketch

The Star Class

The Star stores the position of a "star" and updates it based on the speed that it's given. Our initial constructor sets up the coordinates of the star using random values.

function Star(p) {
  this.x = p.random(-p.width, p.width);
  this.y = p.random(-p.height, p.height);
  this.z = p.random(p.width);

The Update

Most of the time the update reduces the z value by the current speed, but since we don't want the stars to go off the canvas and disappear, if it gets too small we re-randomize the position of the star.

this.update = function(speed) {
  this.z = this.z - speed

  if (this.z < 1) {
    this.x = p.random(-p.width, p.width);
    this.y = p.random(-p.height, p.height);
    this.z = p.random(p.width);
} //end update

The Show Function

The show function is where most of the work is done. It calculates the proprotions of x and y to z and then maps that to the width and height of the canvas and then draws an ellipse. To get the radius of the ellipse we map the current z-value using an inverted target of 16, 0. This means that as z gets smaller our radius gets bigger. = function() {
    var x_now =, 0, 1, 0, p.width);
    var y_now =, 0, 1, 0, p.height);

    var radius =, 0, p.width, 16, 0);
    p.ellipse(x_now, y_now, radius, radius);

  } // end show
}; //end class Star

Attaching the Sketch

This next bit attaches our sketch to a specific DIV defined in the HTML. You don't have to do this, you could just use the parts as universal functions like the examples show, but if you have more than one sketch on a page sometimes things get funky so I prefer this pattern to keep everything in place.

// Attach the starfield_sketch function at the top to the div with ID
// schiffman-starfield
sketch_container = new p5(starfield_sketch, 'schiffman-starfield');