I am having issues with unions in studio, more specifically in script.
What is happening? There are two issues that are happening, one relating to pathfinding and the other relating to hitboxes.
Hitboxes are inaccurate (20%-30%~ of the time), even when using PreciseConvexDecomposition (or whatever the collision fidelity is called) This typically happens when unioning in script. I am making a game that relies on unions and this is a big issue because when unioning out a piece of a wall to create a door, the hitboxes would mess up.
Pathfinding thinks it can go through unions that were created in script (in-game), which is an issue because pathfinding is a key aspect of my game which uses unions. (This issue usually happens when the part that is being subtracted from size is less than or equal to 0.5)
As you can see, there is a random wedge blocking part of the doorway. This example is a minor case, but the position and size of this buggy collision triangle is random, so you could get a doorway path that is completely untraversable.
Using Enum.CollisionFidelity.PreciseConvexDecomposition significantly reduces the chance of this happing, but it does not stop it either.
Pretty sure this is expected behavior with Unions. The collision geometry is collapsed to reduce physics performance impact. Precise convex decomposition reduces the extent that this happens, but does not completely eliminate it. This is documented.
You should not drill holes in parts using CSG and expect the collision on those holes to be completely accurate. This is a very common problem with people trying to make doorways. You should use a doorway prefab section to splice into the wall, or use three-four parts to create the gap.
This option is the most accurate representation of collision geometry for complex objects with built-in tolerances to reduce the total number of convex-hulls by compromising on accuracy when justified.
The phrasing of this says (translated to plain english) that perfect precision is not guaranteed. I’ve also seen this mentioned on the forum in an old announcement before.
As far as that question goes: you cannot modify the physics data yourself. You could make the unions for visuals and then approximate the collisions using invisible parts though. Unfortunate and difficult situation, but not much to do about it.