An implementation and explanation for slicing an ArrayMesh with a Plane.
There is a class called Slicer with one static function get_sliced_meshes that takes an ArrayMesh and a Plane as inputs and returns an array with two ArrayMesh's.
The first one is the "upper" mesh, and the second is the "lower" mesh.
There is also an example project. The example slices the MeshInstance in the scene tree by the SlicePlane. These can be moved in the editor.
To use the utility, I recommend downloading make_mesh_slices.gd directly and adding it to your project. This file contains the Slicer class.
To use the example, pull this repository and import it as a Godot project.
As long as make_mesh_slices.gd is in your project directory, you can use the slicing function by calling Slicer.get_sliced_meshes from any script.
As stated above, this function takes an ArrayMesh and Plane as its two parameters, and it returns an array with two new ArrayMesh's.
This function was written to be as time-efficient as possible, intending for real-time slicing of meshes. As such, there may be redundant faces created as the result of a slice. I recommend using meshes that have lower polygon counts.
The slices created by this function have normals generated before being returned. However, they currently do not have UV coordinates. I hope to implement this in the future.
When creating the Plane, assume the mesh is centered at (0, 0, 0) and is not rotated. This is because mesh vertices have their own local coordinates.
See the _ready function in SlicerExample.gd for a way to get the Plane based on global coordinates and rotations of a mesh and a slicing plane.
There is a step-by-step explanation of how this implementation works in the in-line comments of make_mesh_slices.gd. This includes helpful links and higher-level interpretations.
I recommend having the following pages open while reading:
