Prevent object passing through walls / tunneling (implement CCD)

[liabru]

Currently there is no continuous collision detection (CCD), so fast moving objects pass through other objects.

A description of the problem:
http://www.stencyl.com/help/view/continuous-collision-detection

Solution is to implement a CCD algorithm.

One I'm currently considering is speculative contacts, see here and here.

"we compute the closest distance d between the two objects; the idea is that we want to remove exactly the right amount of velocity from A such that it will be exactly in touching contact with B on the next frame"

I've already implemented the code that extends the AABB based on object velocity.

The next part is to find the closest distance between two objects and remove the required velocity.

[abataille]

I just added a poor man's solution to the tunneling problem. It detects if a body is outside the world bounds and then reverses the velocity and translates the body back. I tried via an event. Here is the code. Please have a look.
Events.on(_engine, 'afterTick afterRender', function (event)
{
// avoid tunneling
for (var i = 0; i < _world.bodies.length; i++)
{
var bodyA = _world.bodies[i];
if (bodyA.isStatic || bodyA.isSleeping)
{
continue;
}
var outside = false;
var adjustX = 0;
var adjustY = 0;
var epsilon = 0.01;
if (bodyA.bounds.min.y + epsilon >= _world.bounds.max.y)
{ // bottom
adjustY = Math.abs(bodyA.bounds.max.y - _world.bounds.max.y);
Body.translate(bodyA, {
x: 0,
y: -adjustY
});
bodyA.velocity.y = -1.0;
outside = true;
}
if (bodyA.bounds.max.x - epsilon<= _world.bounds.min.x)
{ // left
bodyA.velocity.x = 1.0;
adjustX = Math.abs(bodyA.bounds.min.x - _world.bounds.min.x);
Body.translate(bodyA, {
x: adjustX,
y: 0
});
outside = true;
}
if (bodyA.bounds.max.y - epsilon <= _world.bounds.min.y)
{ // Top
adjustY = Math.abs(bodyA.bounds.min.y - _world.bounds.min.y);
Body.translate(bodyA, {
x: 0,
y: adjustY // since body.bounds.min.y is negative
});
bodyA.velocity.y = 1.0;
outside = true;
}
if (bodyA.bounds.min.x + epsilon > _world.bounds.max.x)
{ // right
bodyA.velocity.x = -1.0;adjustX = Math.abs(bodyA.bounds.max.x - _world.bounds.max.x);
Body.translate(bodyA, {
x: -adjustX,
y: 0
});
outside = true;
}
if (outside)
{
Bounds.update(bodyA.bounds, bodyA.vertices, bodyA.velocity);
}
}
});

[liabru]

Thanks for sharing, but does this work?

I ask because body.velocity is technically read only - the engine uses position Verlet meaning velocity is implicit to changes in position (see here)!

So what you need to do instead is simply set the body position to be inside the world, the engine will handle the velocity change for you.

Either way, as you say this is indeed a poor mans solution, as it's only a solution to going outside of the world bounds.

BTW note the event name is not 'afterTick afterRender', those are two separate events (that happen to relate to the same point in the step). You only need to use one here (e.g. it's like jQuery's events), otherwise this will run twice per step.

What we really need implementing is speculative contacts. I'll get round to it at some point hopefully!

[abataille]

You are right. velocity change is not necessary. Just the translation does it.
I know, it does not work for inside objects, but at least it leaves the bodies at the table.

[shakiba]

+1

[Danetag]

Still experiencing this issue, I guess it's still unsolved?

[liabru]

It's a pretty big feature to implement really, I've played around with some ideas for this but I've not yet got much working. It's a tricky one.

I'm considering implementing a simple version for now (ray casting) that should prevent some of the more obvious tunneling cases.

[Danetag]

Ok I totally understand, thank you so much for all your hard work :)

[anjiro]

Note that increasing engine.timing.timeScale can cause this issue as well.

[isaac-jordan]

Still having this issue using latest master as of e698b6b. I have somewhat mitigated it by halving the engine.timing.timeScale value. Not sure of the repercussions of this, but works for my simple use case.

[homerjam]

+1

[slaskis]

@liabru for https://cubeslam.com I ported a few SAT implementations to javascript but finally fell for this brilliant SAT implementation and converted it to be a part of this library.

It's very similar to your implementation but will also project ahead based on the polygons relative velocity to see if it will intersect and then returns a minimum translation vector which may be applied to keep two polygons from colliding if desired.

I found it to be quite stable and fast even in the pace of cube slam. Unfortunately google code just shut down so the implementation code for it is not viewable online anymore. But you can download it and have a look at the narrowphase implementation in lib/sim/physics.js#L74.

[liabru]

@slaskis I remember playing with cubeslam, awesome project!

Thanks for the info. I actually played with using SAT for continuous collisions but I couldn't get it working at the time, so I'll take a look at your implementation for some pointers.

[slaskis]

@liabru thanks! I hope it can help you get this sweet library even better

[dcrockwell]

+1 Seeing lots of tunneling with bodies passing through svg generated bodies.

[ser10us]

Is there any workaround for this issue? The proposal from @abataille doesn't work for me.

[liabru]

Not yet, I have a work in progress implementation. Depending on your needs, you could try doing some ray casts between updates (e.g. between centroids of bodies moving over a certain speed), if there's an intersection then move the body back. This won't be perfect but might help for some cases.

[slaskis]

@liabru would you mind sharing your in progress implementation as a PR or branch? I'm curious what it looks like and maybe we can help out?

[IceReaper]

Will this also allow collision with simple lines? To be more specific imagine a force field, which limits you to not pass from both sides but has no thickness.
Which comes to the second idea of force fields coliding only in one direction, so you might move into the forcefield, but not leave it again.
Imagine it as a simple polygon which has only a one-sided face, you walk through it as you cannot see it. If you try to move back you see a wall there.
Used that feature to build some nasty tricky puzzles in prey a while ago, where the back-side was a portal placed inside a door. Which results in a door bringing you from a to b, but the same door back brings you from b to c :D

[liabru]

@slaskis it is in a very unstable state, I was trying a different approach than using SAT. But it looks like it's not going to be viable, so I'm going to try implement it again in the same way you did. Looking at your code, the key part seems to be these lines right?

[spiderworm]

@liabru Any update on this? It looks like you abandoned the CCD branch and maybe are implementing your new approach in a different branch? Perhaps I can help code it up. I'm trying to make a game for mobile and this is the most performant 2D physics JS library I've found so far. Just need to get this bit fixed.

[zxb5102]

Does that solve the problem now?

[zxb5102]

I have a similar problem with my project

[liabru]

I have a partially working solution in a branch I've not yet pushed, but it still has some major issues. It's a tricky one. Maybe I should push it if other people are interested in helping?

[spiderworm]

@liabru Sounds like that might be a good idea? Along with a brief description of your approach, and of the problems that still need solving might be helpful.

[billylo1]

In my use case, I managed to work around it by listening to collisionEnd event, check if the bodies were forced out of the viewport. If so, re-position that body to bring it back. (this is good enough for my use case because I only need to make sure no objects can get thrown out of the bounding rectangles.

Hope this helps.

[qwertyquerty]

Is this ever going to be continued?

[pixelscripter]

I was also wondering about that @qwertyquerty. Any progress on implementing CCD @liabru ?

[soulofmischief]

Also interested in seeing this feature.

[whaqzhzd]

Any progress on implementing CCD

[cheneyweb]

object passing through walls，I met this problem, too.
I'm really looking forward to the problem being solved

[7ammer]

^^^ Me too

[yungzhu]

Is there any way?

[fabienjuif]

EDIT 2: https://gist.github.com/fabienjuif/a7d9fc3e34e23f6000bcfc185dc0e341

EDIT: I fall into FPS issues, the engine run objects faster on 60FPS than on 30FPS. So don't try that at home :(

---

I did this trick for now:

  // engine
  // - run the engine several times, so we have the feeling the game is fast
  // - also, this avoid collision detecting issue since CCD is not implemented yet in matter-js
  let lastLoop = Date.now()
  for (let i = 0; i < RERUN; ++i) {
    Engine.update(engine, i === 0 ? delta : (Date.now() - lastLoop))
    lastLoop = Date.now()
  }

And divide all my velocities by the RERUN amount :)
It works, but I assume this is less performant

[osfa]

+1

[maltenuhn]

Question: could a bounty help push this issue forward? We'd be happy to contribute.

[qwertyquerty]

This really should be completed, considering it's such a big issue, and it's been open for so long.

[stefannew]

Is there any work in-progress to address tunneling issues?

[notchris]

I'd also like to see the tunneling issues resolved.

[dzcpy]

Any updates?