Collisions with thin instances

Questions
1

How can I set checkCollisions for thin instances? In the docs, it it mentioned that thin instances support collisions. However, I couldn’t find any example on how to enable them. What I tried is setting mesh.checkCollisions = true on the mesh that I then call mesh.thinInstanceAdd() on. However, this didn’t work for me.

For picking, there’s an example in the article where picking is enabled by doing

box.isPickable = true;
box.thinInstanceEnablePicking = true;

However, I couldn’t find a similar attribute for enabling thinInstanceCheckCollsions.
One thing I found is thinInstanceRegisterAttribute and thinInstanceSetAttributeAt. However, I have no idea if I can use those to enable collisions.

2

I have just set up an example PG, if this is what you look for, like docs say it only uses one boundingInfo for all thinInstances:

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3

So there’s no way to checkCollisions with the individual instances, right? This occured to me too when I looked at boundingBox of my thinInstance and it was a box that contained every instance.
Sadly that is not enough for my use case, since they’re spread all over the scene, but I think I have another idea that might work for my scene.

4

As a follow-up question: The thin instances in my scene are spread throughout the scene, but they are clustered, so there are a few points where there’s alot of them and in the rest of the scene, there’s none. Is there any way to “draw an invisible box / bounding-box” around each cluster (I know the position of the clusters). I suppose I could just create a normal box and set it’s alpha to 0, but I’m sure there’s a better way.

5

The collider box mesh idea is very used, I’d go with that :grin: And Mesh only supports one Bounding Box so you can’t set multiple of them

6

Instead alpha = 0, won’t isVisible = false be better in terms of performance?

Or this might help in deeper section: Drawing Bounding Boxes | Babylon.js Documentation

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7

I have had similar issues with “clusters” of meshes and the accuracy of my collision detection has been limited by the bounding boxes being cuboids.

I have resorted to doing collision detection via CSG intersections. It’s reliable but for large number of meshes or meshes with a lot of facets it may be slow:

    const csg1 = BABYLON.CSG.FromMesh(mesh1);
    const csg2 = BABYLON.CSG.FromMesh(mesh2);
    const intersecting =  (csg1.intersect(csg2) as any).polygons.length > 0

polygons is not exposed in the API which is why the ‘as any’ it there.

To make cluster meshes do unionInPlace

I hope that’s useful.

-Flex

8

Great, that’w how I implemented it now. I was wondering how that would impact performance/FPS. Do you think this could have a noticable effect on my scene? I have approximately n/6 collider boxes (set to invisible) for n meshes and n instanced meshes. I didn’t really notice any performance decrease when just viewing the scene.

9

Yeah that’s a good point, isVisible does seem like the better fit for this. I ended up using it over alpha, so thank you for the suggestion.

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10

It won’t impact anything at GPU level since invisible meshes are not passed, so if you’re not CPU bound it’s going to be just fine :smiley:

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11

Great, thank you!

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