Possible restitution/force bug

[And-re]

I'm applying some force to 6 circles in a world without any gravity.
I'm rerunning the same code multiple times and I get 3 completely different results (the last one is at the end).
http://recordit.co/Amjrsw7ct9

Source code (you may need to adjust the value of defaultForceY):
http://codepen.io/and_re/pen/YZaBXV?editors=0010#0

@liabru is it because circles aren't normal circles but approximated using polygons?
http://brm.io/matter-js/docs/files/src_factory_Bodies.js.html#l121

[liabru]

Strange. My guess is that it has to do with slight fluctuations in FPS for a few frames after page load (e.g. while the browser is doing stuff like JIT), which will cause different results because Matter.Runner has a dynamic timestep by default (something I'd like to improve, but it's very hard to eliminate totally).

Try starting the runner after letting the framerate settle for a second or two:

setTimeout(function() {
   Runner.run(runner, engine);
}, 2000);

Seems to give the same result every time for me after doing that. Alternatively use a fixed delta for the runner:

var runner = Runner.create({
  isFixed: true
});

Runner.run(runner, engine);

In fact this shows there's a pretty huge difference! I had to increase the force to make them reach the bottom, but it remains consistent every time.

I wonder if I should somehow integrate these findings into the runner or examples.

[And-re]

@liabru Thanks for the reply. This is just an example to present the problem using only restitution (no custom mass, friction, etc.). I have the same problem in my game that isn't using Matter.Runner but Matter.Engine only.

I tried playing around with slop, friction, setting restitution to static bodies, etc. but I couldn't find any solution.

I don't see such a problem in P2: http://codepen.io/and_re/pen/evrgLG but I had to set stiffness to Number.MAX_VALUE
Is there something like stiffness in Matter JS? Slop = 0 wasn't helping.

[liabru]

@liabru Thanks for the reply. This is just an example to present the problem using only restitution (no custom mass, friction, etc.).

Can you explain exactly what the problem is then? Since with 0 restitution they should always just hit the wall and stop. Is it that they move differently?

I have the same problem in my game that isn't using Matter.Runner but Matter.Engine only.

How are you calling Matter.Engine.update? With a fixed value for delta?

[And-re]

I'm expecting circle bodies with restitution 0, 0.5 and 1 to bounce like this:

(1st case in this gif http://recordit.co/Amjrsw7ct9)
But they are randomly not bouncing at all (behaving as if they have suddenly 0 restitution)

I'm using https://github.com/sethvincent/gameloop on the server (delta in ms):

const gameloop = createGameloop({
    fps: 60,
})

gameloop.on('update', function (delta) { 
   Matter.Engine.update(engine, delta * 1000)
})

[liabru]

Is delta guaranteed to be fixed or does it vary? Otherwise the issue exists there too and the same possible solutions apply as above I think.

[And-re]

From this file it looks like delta is always consistent (the same value): https://github.com/sethvincent/gameloop/blob/master/index.js

But is it really possible that even a small difference in delta could cause 3 completely different results as you can see on my gif?

[And-re]

I just updated my codepen according to your suggestions but the problem still exists:
http://imgur.com/a/qUzdo
I expect the 3rd and the 5th circle bodies to bounce more like 4th and 6th

[liabru]

Thanks for the update! Looks like you are right and I think I've tracked down the cause. It appears to be because the resolver considers slow collisions to be 'resting' under a certain threshold. It's possible to lower the threshold and it seems to fix the issue:

Matter.Resolver._restingThresh = 1; // default is 4

It's entirely possible that the default is set a little too high, so maybe I need to review it. Tell me if that works for you.

[And-re]

@liabru Thank you so much :D
Using:

Matter.Resolver._restingThresh = 0.001

really helps.

If you are curious why it was so important for my game you can check it out here (it's still under development):
https://haxarena.herokuapp.com/ (EU server)
https://haxarena-us.herokuapp.com/ (US server)
You move with arrow keys and kick a ball with X or Spacebar.

[liabru]

No problem, neat game! Send me a message when you release it.