Fix debug line drawing with tiny convex hull mesh colliders

[aaronfranke]

Godot's convex hulls support lines and planes, but the code to draw debug lines currently disables itself if there are fewer than 4 points. This means that the debug lines are not able to draw valid convex hulls with 2 or 3 points, even though there is no reason the code couldn't do that.

[lawnjelly]

In terms of a lot of usage, if a convex hull has no planes, it is undefined. So e.g. a point or line segment is not strictly speaking a valid convex hull - there can be infinite planes for both, and they have no volume. So whether these will work in many convex hull routines I don't know (in most hull generation / sliding code I have written in the past this would be an error, but it's just possible the existing godot physics routines might deal with it gracefully). But very dodgy territory.

A tetrahedron should be okay though, it has volume.

[aaronfranke]

@lawnjelly The current Godot physics is able to deal with points and lines gracefully aside from the few places in this PR that I modified. More details: The uint32_t size_to_reserve change is needed to support tris/tetrahedrons, and the editor if (points.size() > 1) { change is a no brainer to keep (if there is physics data, it makes sense to display it).

If we wanted to change the minimum to 3 points, we would remove the if (unlikely(ch.edges.size() < 3)) { bit, and change the check in ConvexPolygonShape3D::set_points to check size() < 3, but then it would be an almost arbitrary limitation since supporting in Godot just needs modifying one check and adding one check.

[aaronfranke]

@lawnjelly Updated the PR. #70403 fixed the worst offender, and I removed the changes I made to support convex hulls with 1 or 2 points. Now it's just a no-brainer update to the editor code to support drawing debug shapes for fewer points (only two points are needed to draw lines in the editor).

[lawnjelly]

This PR fixes a few problems I found with tiny convex hull meshes.

Probably needs a physics guy to decide on, I'm not sure from the title of this class ConvexPolygonShape3D what it is meant to represent, and there are no comments, so it is effectively defined by its usage.

If it is meant to be a convex hull, my earlier comment was to caution that a convex hull is not really valid with less than 4 points (and these must form a tetrahedron with non-zero volume). This is the smallest form that can be changed from the 3 plane intersection form to point form back and forth, and lots of convex hull stuff will fail with an invalid hull. This may be the reason for the previous if (poly_points.size() > 3) checks which are removed by this PR.

On the other hand, there is an argument to support less than 4 points for editing purposes, and let the client code deal with the case of it being invalid (or store a bool in the class to indicate whether or not it is in a valid state, because checking for zero volume is expensive). This is commonly done (I do this in the Portal class for example to allow editing).

It might alternatively be intended to handle both convex hulls and non-convex hulls, but that would need a physics guy to confirm.

So in summary:

• If a physics guy can confirm that it is intended to work with non-"convex hulls" that would make it ok.
• If not, at the least I would be tempted to add a valid / invalid flag and check this before use (and reject and possibly flag errors if attempting to use a non-valid hull).
• If the convex hulls are not meant to be editable, then perhaps the existing method of disallowing non-valid hulls to be set could still be appropriate.

[aaronfranke]

@lawnjelly For the replacement of if (points.size() > 3) { with if (points.size() > 1) {, this is done because this is the minimum requirement for the debug code to draw lines. Regardless of what we want to enforce as the minimum, if some data with only 2 or 3 points gets turned into a convex hull, we should still show it in the debug lines, instead of having it be invisible.

[lawnjelly]

I guess as aaron has been so carefully keeping up this PR we should consider trying it, and in the absence of a full time physics maintainer I guess we have to decide as best we can.

As I say, I'm not sure about the logic of being able to have convex hulls with less than 4 points, but if this is already allowed, then I guess displaying it makes sense.

[fire]

@lawnjelly Let's do the best we can!

[Repiteo]

Thanks!