Why does the UE4 manual recommend that multiple collision convex hulls in a static mesh should not intersect with one another?

According to this page in the official UE4 manual, “[i]n the case of an object whose collision is defined by multiple convex hulls, results are best when the hulls do not intersect with one another”.

Why is this the case? And in what way are the results “better” when collision meshes for static world objects do not overlap? For example, does it affect performance, the accuracy of collision detection, or something else entirely?

The reason that I ask is that I am trying to make a seamless collision mesh for a long and bendy tunnel created using spline meshes. It is important that there are no gaps whatsoever - I am using raycasting to keep a spaceship a minimum distance from the surface - and therefore it is inevitable that the collision meshes will overlap each other, both within a single model (i.e. a single tunnel section), and between separate models (i.e. overlapping between the repeated tunnel sections which are deformed along the spline curve).

If this overlap truly is a problem, can anyone explain why, and how best to deal with it? Thanks!

It’s a best practices kind of thing. It’s not to say that you cannot do it, but it’s just not the best way to go about it.

So for something where you’re doing some line traces and you have overlapping collision hulls you may be more likely for it to read the wrong collision hull in that type of instance. The more collision hulls you have in your scene or even just on your mesh will take away from performance. So by optimizing and keeping hulls from overlapping you’re preventing the wasted use of these collision hulls.For some assets it may not matter as much, but something where you wouldn’t want to risk that miscalculation adhering to the suggestion in the documentation would be best.

To add to what Tim said; not to mention that your collision meshes are for blocking the player and in most cases, there’s no need for the collision meshes to be intersecting because the player isn’t small enough to fit between the meshes. Also, if you’re rendering a physics object with the mesh, if the collision is intersecting it can sometimes cause weird issues where the meshes are forced apart.