Tilemap Collider 2D Bug (Mesh not generating correctly, falling through floors/walls)

The original problem was that the (great) libtess2 library that the CompositeCollider2D uses was producing very small polygons with almost collinear sides and depending on the polygon itself, Box2Ds collision detection algorithms were not producing contacts. We knew this already and detected absolute collinear verts and even allow a small amount of wiggle room so "nearly" collinear but the original bug report shows verts that were outside of this but still Box2D thought they were colinear so we increased it slightly and used an addition metric of polygon area and this fixed all the reports we had where under a special configuration you would pass through the shape.

Maybe you're creating another one. To be honest, it's a tricky balancing act.

Whilst these cases are rare, the CompositeCollider2D can produce all sorts of weird configurations so it's why it's appearing there and not in the PolygonCollider2D which also uses libtess2.

The way forword I think is to perform a better edge-merging for these cases but it can become non-trivial to do.

We do have another report of this which we've accepted but if you do have a project you could share with the bug reported then please go ahead and created it and send me the case number and I'll see if it's the same.

As a workaround, maybe you'll get better results if you were to use Outline mode (assuming you can) and/or change the Vertex Distance to smooth out the resultant shapes. As I say though, this is a workaround to keep you moving forward.

 

I don't have a link for you as it's not public. Let's not assume it's the same thing as that happens all the time. Please submit a reproduction case and send me the case number.

 

The original case wasn't about performing queries but was found to be Box2D producing poor collision detection when some polygons were almost collinear. We increased the threshold slightly to remove the ones we encountered in the original case but increasing it too much results in other problems.

It's not as simple as saying the original bug report wasn't fixed; it was, however there are edge cases for certain types of collision detection in Box2D which are not easy to fix. I am not sure what specific issue you're encountering in your new case but I'll wait until QA have gone through the case you submitted. It might be yet another case of something being near-collinear or just as easily, something else.

In the end, Box2D doesn't know about any of the Unity colliders, it only sees Polygon, Capsule, Circle and Edge and we rely on it for casting and its ability to produce contacts and collision response. We have found that it can be sensitive to certain polygon set-ups and the CompositeCollider2D is unfortunately good at producing them by the very nature of what it does. Our "fixes" nearly always come in the form of filtering out stuff it doesn't like.

We've been discussing switching to a different triangulation method such as Delaunay that will reduce these long thin polygons etc.

 

If you can afford the cost of continuous col-det or just use collision queries then you should not see the issue with outline mode which uses edge primitives. The added advantage is that this produces continuous edges unlike polygon mode.

 

So today I went through your bug report and sure enough, it's related to a collinear issue where your specific case was unfortunately only just beyond the threshold.

I've gone ahead and fixed the issue by improving the collinearity check (scale invariant & larger threshold). This should ensure that the mesh that comes out of our polygon decomposition library is something that Box2D is happy with.

The reason why this collinearity is appearing though is coming from the Tilemap and how it makes incremental changes to the tile mesh as you paint tiles. Without going into too much detail, it has to do various operations to ensure it can quickly and cleanly update the mesh when painting in the editor or tiles being modified at runtime. This is why causing a full recreate by disabling/enabling the TilemapCollider2D or modifying the CompositeCollider2D etc will change it and often remove the issue).

Anyway, I'll get this fix in ASAP.

EDIT: If you're interested, it might be worth me creating a custom build based upon 2019.3 so you can adequately test it out to ensure it fixes your problem is all scenarios. I need to figure out how to get your character moving too in the demo you provided.

 

No, the level you supplied demonstrates the issue well.

I've sent you a DM with more information on the custom build.

 

A fix has to go through a lot of processes so unfortunately not. We’re still working on verifying that it does indeed fix all output from the composite. We do know what the issue is though so it’s being resolved.

 

Whilst I cannot tell you when it'll land, I have today pushed a PR with a fix. This actually solves a number of related issues which manifest as unstable collision-detection (objects never quite settling), physics queries not detecting edges and in extreme cases (when using the CompositeCollider2D which produces much more varied and complex set-ups), geometry missing.