this post was submitted on 04 Jul 2023
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Is there any way to get ray intersections with meshes that don't have collisions? (lemmy.blahaj.zone)
submitted 1 year ago* (last edited 1 year ago) by [email protected] to c/godot
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So far (outside of somehow adding it into Godot via C++ myself) this doesn't seem possible but wanted to ask to see if i maybe missed something.

The most i could find was this issue and a couple of related ones.

My specific use case would be for drawing bullet holes onto meshes (via decals). Ideally also being able to determine the texture used on the faces of the meshes that are intersected (to determine an appropriate decal to use).

Having everything that is possible to hit duplicated as a static body or area seems excessive to me. Since the only data required for ray intersection should be the transform of the meshes faces/edges/vertices etc which i would think are already contained in the mesh itself?

Failing being able to make intersecting a ray with meshes work i will probably try experimenting with using areas. They seem the lowest performance cost out of all the options the existing ray cast can detect. I would still end up with most of the games geometry being duplicated though, once as actual meshes and once as collisions for all the areas.

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[–] [email protected] 1 points 1 year ago (1 children)

If you really want to use different decals for different materials you can put them on different layers and spawn multiple decals on those layers.

Ooh, yeah that might be an option, ill keep it in mind when i get there, thanks :D.

I might be wrong but decals dont really care about collisions so you can just spawn a single decal regardless what it hits

Ive already been using a single decal to begin with and i'm having problems that being able to get ray intersections with meshes could solve. The decals need to be close to the mesh they are supposed to be projected on to look right. Otherwise they seem to fade with distance.

For example here:

The bullet is colliding with a triangle shaped collider that goes over these small stairs since that is more convenient and simpler for collisions with the player. But because the collision is simplified compared to the stairs actual mesh the decal can get placed far away from the mesh and not show up or only show up in some places.

If i can only get ray intersections with colliders i'd have to create a much more complex collider that matches the stair mesh much closer to be able to place the decals closer to the mesh. But creating that many colliders that have no other uses besides this one seems like a waste. If i could get ray intersections with meshes directly i could just use that to determine where the decal should be located.

[–] [email protected] 1 points 1 year ago (1 children)

You can adjust the size of the decal and the vertical fade to avoid these issues. You can use the mesh's AABB to approximate the size so that it always hits the mesh. I dont know of a way to intersect per primitive, I think that is a fairly expensive operation. That is essentially the same as GPU raytracing except not on the GPU. Another idea would be to use a simplified collision mesh that is on a completely different layer to everything else. Hopefully the engine optimizes that in a way that it is not checked against other colliders during the phyiscs step. But I think the better approach is to fiddle with the decals in a way that the end result is satisfying.

[–] [email protected] 1 points 1 year ago (1 children)

You can use the mesh’s AABB to approximate the size so that it always hits the mesh [...] But I think the better approach is to fiddle with the decals in a way that the end result is satisfying.

I've been trying this approach now for a while (at least if i understood you correctly). Though im approximating the size of the decal by finding the location where the raycast for the bullet exits the collider instead of using a bounding box. Currently struggling a bit with sizing and positioning the decal correctly for it to show up properly😅. But im also starting to think this approach isn't going to be satisfying when i do get it to work.

The problem is that i am not sure along which direction to project the decal. If the decal projects along the path of the bullet (which would make the most physical sense i think) it can end up incredibly stretched when it encounters the mesh at a very shallow angle. Eg:

So i'm back to the original problem of not being able to determine where/which face of the "visual" mesh the bullet would hit and what its normal is, at least not without making a lot of unnecessary colliders that match it more closely. And if i use the one normal i do have access to (the one from the collider) i cannot guarantee that it is similar enough to the normal of the mesh face the decal will be projected onto to not be stretched anyways.

[–] [email protected] 1 points 1 year ago (1 children)

I dont think there is anything wrong with that picture. That makes perfect physical sense to me.

[–] [email protected] 1 points 1 year ago (1 children)

Well yeah physically its sort of right, but for one (though im not sure if this is just a difference of opinion or if its not clear from the picture) imo the streched hole does seem abnormally large. But also if i use any decal more complicated than just this black circle it would seem off. Since the bullet hole texture would be modeled off a "clean" 90 degree impact bullet hole it would look off if streched like that.

Here a simple example that makes it a bit more clear where i put a white x inside the bullet hole.

Decals that make sense for (near) 90 degree impacts would look bad (or at least very different) when stretched like this.

[–] [email protected] 1 points 1 year ago

Oh yeah, that makes sense. You can try capping the angle between the collision normal and the incoming ray. But it will always be an approximation. I dont think you should get too hung up on it. People always see malformed decals in games. Or footprints on hard surfaces etc. Its not a big deal.