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Comment by rehevkor5 | Hacker News Reader

Yeah he should be getting the starting & ending position of the ball during the tick, and determining whether that line intersects the other line. If it does, the intersection point should be found and the segment that goes past the line should be reflected over the normal, and the ball placed at the new endpoint of the reflected segment. And, of course, the velocity vector needs to be changed as well.

But... I guess this won't be the first time someone posts bad physics code on the internet and poses it as a good example.

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Hmmm, you appear to have a high opinion of your ability to determne 'good physics code'. Unfortunately, just a high opinion, not a high ability.

The OPs code is fine. And was written by a she, not a he.

Your suggestion is what is sometimes called 'continuous' collision detection. It is needed in some cases, particular when the OP approach might miss collisions (less often for when knock-on-collisions are missed, since those are rarer). But that approach is not 'better' - it is far more time consuming (vastly more for 3d rigid bodies with angular velocity), and has a visual benefit in some cases (not in the OPs case - or at least you wouldn't see the difference).

But, of course, both approaches are very approximate and only intended to give the right feel. If you wanted to be even more accurate you would process your collisions by subdividing updates around the collision (a good pool-ball simulation does this, to model proper breaks). And you'd have to use different integrators than the 1st degree Newton update.

This process can keep going as far as you like. For physics systems I've worked on for modelling the behavior of rigid components in MotoGP bikes, the kinds of approximations you allow are very different to the physics engines I've built for commercial games. Neither are 'better' or 'worse' - a good programmer understands the requirements of their domain . To that extent, the OP's approach is fine, and powers the vast majority of the physics in current generation video games (most engines do support continuous collision detection, but it will be switched off for most rigid bodies in a scene, for performance).

You're right, there's a gradation of compromise, and I wasn't fully polite in my comment (nor did I think to attempt to determine OPs sex... then again, I'm a guy with a "girl's name", so who's to say "Mary" is necessarily a girl? aaaanyway...). I'm not suggesting that all software must compute integrals in order to have convincing physics. However, when I see code like this I involuntarily get itchy:

      while (trig.isLineIntersectingCircle(circle, line)) {
        physics.moveCircle(circle);
      }

Thanks for that, sorry for my impoliteness in response. HN is sometimes frustrating in its density of people with Dunning Kruger problems. I am sometimes one of them.

You bring up a slightly different issue now, which I'm not sure if you are suggesting the problem is with the unbounded loop or with the serial strategy for resolution.

On the former, I agree with in the narrow (interpenetration resolution isn't usually guaranteed to converge), but taking

  while (has_interpenetration()) {
    resolve_interpenetration();
  }

and adding a limit

  for (int i = relaxation_steps; i > 0 && has_interpenetration(); --i) {
    resolve_interpenetration();
  }

gives you code that is in most game physics engines, in some form.

On the latter, modern game engines do some steps globally, rather than just running through each rigid body (often collision detection, interpenetration resolution, resting contact resolution, but not normally collision resolution or integration). But the OP approach of running everything in series wasn't unusual 15 years ago when game physics systems started to become ubiquitous. Before companies like Mathengine and Havok perfected LCP approaches, folks like Ipion and FastCar were doing impulse-based calculations that were heavily serial. So for a very simple JS experiment, it didn't strike me as a problem. I'd have written it that way too, if it were me.

The challenge of the more accurate method you describe is balancing how much CPU is used to find the point in game time where a collision occurs because it is almost certainly at some point between your tick times. You have to perform the calculation at many units of time across the tick delta with the significance level you care about, with more calculations necessary as velocities of bodies increases.

Of course, this is what physics engines do, and why you should probably use one instead of reinventing the wheel.

In terms of showing how a game loop and really basic game physics works, however, this is a pretty clean and understandable example.

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