Henry Smith

🚀 make things
📝 write about it

Apple Pie From Scratch

This is a quick write-up of my solution to one of the first problems I bumped into in my latest project. I’m building a JavaScript model of the solar system, which in the end will hopefully feature all the planets orbiting the Sun according to some approximation of Newtonian gravity.

The problem arose because it’s easier to implement the equations if your data model uses SI units: meters, kilograms and seconds. Plus, if you adopt a nice convention like SI units, it makes the code so much more readable. I find names like distanceInKm or massInKg super ugly. It’s nicer if you can use distance and mass and just infer the units from the convention.

class Planet {
  constructor ({ mass, position, radius }) {
    this.mass = mass;
    this.position = position; // { x, y }
    this.radius = radius;

This is lovely, but the CanvasRenderingContext2D API works in pixels. Take it or leave it. So the code in charge of rendering planets needs to know how to convert between meters and pixels.

It’s the kind of problem that’s tempting to sort of hack your way through. Just keep throwing code at the wall and eventually something will stick. Easy enough, but the result is usually not great. It’s bad enough not understanding your own code a few weeks after writing it. Not understanding it while you’re still writing it is just… no fun.

So I made a cup of tea, cleared a whiteboard, and had a bit of a think. The resulting diagram is a bit incomprehensible, but I sometimes think it’d be nice if people were more open about how lost they get with even simple stuff like this, so here it is.


The gist of the solution is to have a scale property somewhere, denoting the conversion rate between pixels and meters. Next, I needed there to be a “thing” in my system to actually own that property and apply it. The name I settled on for that “thing” was Viewport.

The Viewport needs to a few things to do its job. It needs that scale value, for starters. It needs a reference to the rendering context, too. And I figured it also needs to know which part of the universe it’s centered on, which I pass in as an object called center with an x and a y property. Because x and y are spatial values, you can just assume they’re in meters. See how nice that is?

class Viewport {
  constructor({ center, context, scale }) {
    this.center = center; // { x, y }
    this.context = context;
    this.scale = scale;

The last thing that’s missing is a Viewport method to render planets. Rendering circles is nice and simple because it’s part of the browser API, so that’s a freebie. What’s less simple is that I’ve designed the Viewport to accept the coordinates of its center point in meters, whereas the built-in arc method expects the coordinates of the circle in pixels from the top-left corner.

This is important, because I need it to be easy to center the viewport on a given planet. I want it to be as simple as viewport.center = planet.position. As usual, the price of simplicity somewhere is complexity elsewhere.

Converting between those two points is a two-step calculation. First, you have to calculate the position of the top-left pixel of the canvas in imaginary space meters. Then you subtract the coordinates of that position from the coordinates of the planet, and multiply the results by the scale property from earlier. This gives you the pixel position of the planet within the canvas. The resulting code is really heavy on arithmetic and took a while to get right.

drawPlanet (planet) {
  const { width, height } = this.context.canvas;
  const topLeft = new Point(
    this.center.x - ((width / 2) * (1 / this.scale)),
    this.center.y - ((height / 2) * (1 / this.scale))

  this.context.fillStyle = planet.color;

    (planet.position.x - topLeft.x) * this.scale,
    (planet.position.y - topLeft.y) * this.scale,
    planet.radius * this.scale,
    Math.PI * 2


With that in place, I now have a working graphical layer that understands how to render my SI units data model onto a 2D canvas element.

It’s a really fundamental part of the system, so you don’t end up with much to show for all the effort, but I did manage to squeeze one cool thing out of it. By gradually shrinking the value of the scale property, I can generate a “flying backwards through space” effect. Check it out. Right at the very end you can see the Moon whizz by on the right once we’re far enough from Earth.

Now that the basics are in place to display what’s going on, the next step is to get everything moving. Right now I’m still struggling to make gravity work right. It’s a lot harder than this was.