Is there a way I can fix this collide issue?
I tried turning the union’s collide into hulls and boxes both but neither work. I can’t figure out if I can, and if so, how to fix it so you can stand on the ground behind the barricade without having to float on it.
I’m having the same problem with some trees.
7 Likes
Hey @Vallorum
Understanding the Problem
Use this plugin to inspect the true collision of the model:
Plugin: https://www.roblox.com/library/414923656/Show-Decomposition-Geometry
Explanation: If your collisions are bad (Which from the picture they are), that’s because of a few things, but I’ll list the most important three things.
Factor 1: Unions/CSG are meant for small models and intricate objects, when you’re testing something as strenuous and as ambiguous as that tree model or whatever that model is, you’re letting a computer algorithm decide your collisions which can more than likely be wrong. This is why people abstain from CSG for world environment models | and basically everything else that involves CSG
Factor 2: You’re using the wrong collision fidelity for a tree, the collision should either be default or hull, while default being least optimal, it is the most suited for an organic model as a tree. Hull creates collisions around your geometry.
Factor 3: You can’t control your geometry by hand, so you’re somewhat gambling. I’ll say this again and before, use *any 3D modeling program please.
Conclusion
Use the plugin, play with the collisions now since you have a visual understanding. Otherwise consider using an external 3D software to have better control of your collisions.
Hope I helped!
5 Likes
You can’t control collisions with a 3D software easily. A mesh made in Blender or with unions will have similar collisions. The only thing a 3rd party software offers is the ability to divide a single mesh into convex shaped meshes, giving you a nearly perfect collision.
2 Likes
You don’t necessarily need a plugin to show decomposition geometry - pretty sure there’s a setting near the bottom in Studio Settings > Physics. Doesn’t require a restart, just hit F5 and then “Stop” to see the changes.
Hello @Vallorum,
What happens here is that Roblox’s CSG system does not support “holes” because when importing a mesh or creating a UnionOperation it is simplified into geometry decompositions, meaning it will lose some collision details. The best you can do to get around this is to separate your object into some parts, thus avoiding problems. It is also important to note that when importing a mesh you can triangulate its faces and make it easier to process the Roblox collision system, it will automatically triangulate when importing, but this is an overload that can become detrimental.
How to get around?
It’s worth knowing that it varies from operation to operation, and the best way to do it is to make your own collision as Normal CSG parts of Roblox Studio.
-
Meshs: A good job is to understand that the Roblox system only supports a triangular topology mesh, so as stated earlier. In Blender 2.79, just select the entire mesh in Edit Mode and press
CTRL + Tto triangulate, then your mesh is ready to export. -
UnionOperations: there is not much to do, but separate your union into some parts can help.
Viewing the problems
ShowDecompositionGeometry is not very important to me because I am not a fan of UnionOperations, but it can be enabled in Roblox Studio settings.
Settings> Physics> Display> ShowDecompositionGeometry
There is also the collision mesh that I am always using, it works with any collision-enabled 3D object in your Workspace, it will show you where a player can move. Thus, literally showing the collisions of objects as a whole.
Settings> Studio> Visualization> ShowNavigationMesh
I tried to be as clear as possible, I hope I helped! 
If you have doubts, let me know!
4 Likes
Hey thanks for the replies @nooneisback and @Ultimate_Miner64!
Quick Replies To Address your Statements
Nooneisback: You can’t control collisions with a 3D software easily. A mesh made in Blender or with unions will have similar collisions. The only thing a 3rd party software offers is the ability to divide a single mesh into convex shaped meshes, giving you a nearly perfect collision.
You’re already describing a method of control when you’re using a 3D Software, sectioning off parts of a mesh that’d make sense and giving better control of collision shape (unions can’t do this without . At the end of the day they’re both form of geometries, yes. However it can be argued that clean geometry converted with collisions will have better collisions. If you want to argue against, it would be nice to see a complex model made with unions have better collisions than that of a mesh.
@nooneisback You don’t necessarily need a plugin to show decomposition geometry - pretty sure there’s a setting near the bottom in Studio Settings > Physics. Doesn’t require a restart, just hit F5 and then “Stop” to see the changes.
The plugin is superior, of course you don’t need it, it works like a light switch. On and off, it’s incredibly convenient and from what you’re describing, it’d be more convenient in this scenario. I’m not going try to persuade you otherwise, I’m just giving you another way to the solution.
Thanks.
1 Like
A mesh with concave surfaces will have to be uploaded as multiple meshes. That’s the only way to get a perfect collision without using invisible parts. I’m speaking of ease of use here, not whether it is possible or not.
Try making a flat donut shaped mesh in Blender and Roblox. They will have the same collision quality. You could argue about the convex shape splitting, but you can also practically do that with unions. Duplicate your union multiple times and negate them down to multiple convex shapes. You will get the same result.
Ah, Unions and Negates in roblox can be a bit fiddly.
- Make sure the part you are wishing is selected first.
- If the negated object is a union itself this can cause issues.
- Alter properties of negated parts, I believe you should turn off collision in the negated part but am unsure as I just use Maya and Blender so it’s been a while since i’ve had a use for Unions inside Roblox.
Tip for future use, Unions are CPU intensive, use with caution. Meshs are a better alternative.
3 Likes