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# How to detect separated parts in a mesh

I have a mesh that consists of two submeshes. I want to select each submesh separately.

The problem with Unity-submeshes is you have to assign separate material-ids in the 3D program (such as Blender or 3dsMax). My submeshes however use the same material and the same texture space. I don't want to say to our artists that they have to assign different material ids every time a mesh is comprised of different separated parts. Instead I want to detect this myself with a script.

So what the script should do is very similar to the "select element" tool in 3dsMax: select a triangle, find all connected triangles, and return the triangle-list back to me.

Now I've tried this by -> 1. selecting the first vertex-id in the triangle-list -> 2. find the next two vertex-ids (so I've selected my triangle) -> 3. searching through the triangle-list of other triangles contain at least one of the vertex-ids in my selected triangle(s) -> 4. add those triangles to my selected triangles-list -> 5. repeat step 3 and 4 until I get no more feedback

This method however returns only parts of a submesh for me, cutting submeshes on (mostly) hard edges. This is logical, as hard edges create two separate vertices with distinct normals. However, there should be a method to detect this. How would 3dsMax or Blender otherwise be able to do this?

**Answer** by Dante_CyberdeckGames
·
Mar 28, 2017 at 04:40 AM

@Syndias you have the right approach. My only guess is that there is a bug in your implementation, maybe you aren't recursively checking all of the connected faces found?

I just had to solve this problem for myself as well, here is the solution I just posted on my blog (more detailed explanation there and also editor code for picking faces in the scene view). http://dantefalcone.name/finding-linked-faces-elements-within-a-3d-model-mesh-with-unity3d-and-c/

```
/// <summary>
/// Given two faces, return whether they share any common vertices.
/// </summary>
/// <param name="faceA">Face represented as array of vertex indices.</param>
/// <param name="faceB">Face represented as array of vertex indices.</param>
/// <returns>bool - whether the faces are connected. </returns>
bool IsConnected(int[] faceA, int[] faceB)
{
for (int i = 0; i < faceA.Length; i++)
for (int j = 0; j < faceB.Length; j++)
if (faceA[i] == faceB[j])
return true;
return false;
}
/// <summary>
/// Given a single triangle face of vertex indices,
/// returns a list of all the vertices of all linked faces.
/// </summary>
/// <param name="pickedTriangle">The known triangle to find linked faces from.</param>
/// <param name="triangles">The index buffer triangle list of all vertices in the mesh.</param>
/// <returns>List<int> - List of all the triangle face indices that represent the surface element.</returns>
public List<int> GetElement(int[] pickedTriangle, List<int> triangles)
{
// Create the return result list, starting with the current picked face
List<int> result = new List<int>(pickedTriangle);
// Iterate through the triangle list index buffer by triangle (iterations of 3)
for (int i = 0; i < triangles.Count; i += 3)
{
// Select the (i)th triangle in the index buffer
int[] curTriangle = new int[3] { triangles[i], triangles[i + 1], triangles[i + 2] };
// Check if faces are linked
if (IsConnected(curTriangle, pickedTriangle))
{
// Recursively add all the linked faces to the result
result.AddRange(GetElement(curTriangle, triangles));
}
}
return result;
}
```

**Answer** by felipin
·
Jun 25, 2018 at 05:51 AM

Ok, I can be a bit later, but I've written a method that does it and I wanna share it. So, basically It returns a list with all vertices indices connected to the `vertexId`

(including itselft).

```
public List<int> GetConnectedVertices(int vertexId, out Vector3 centerOfMass, int[] triangles, Vector3[] vertices) {
var result = new List<int>(vertices.Length);
var selectedVertices = new bool[vertices.Length];
var ignoreTriangles = new bool[triangles.Length / 3];
var v = new int[3];
selectedVertices[vertexId] = true;
result.Add(vertexId);
centerOfMass = new Vector3();
bool isSearching = true;
while (isSearching) {
isSearching = false;
for (int i = 0; i < triangles.Length; i += 3) {
if (ignoreTriangles [i / 3]) {
continue;
}
v[0] = triangles[i];
v[1] = triangles[i + 1];
v[2] = triangles[i + 2];
if (selectedVertices[v[0]] || selectedVertices[v[1]] || selectedVertices[v[2]]) {
for (int j = 0; j < 3; j++) {
if (!selectedVertices[v[j]]) {
result.Add(v[j]);
centerOfMass += vertices[v[j]];
selectedVertices[v[j]] = true;
isSearching = true;
}
}
ignoreTriangles [i / 3] = true;
}
}
if (!isSearching) {
break;
}
for (int i = 0; i < vertices.Length; i++) {
for (int j = i + 1; j < vertices.Length; j++) {
if (selectedVertices[j] != selectedVertices[i]) {
if ((vertices[i] - vertices[j]).sqrMagnitude < 0.00001f) {
selectedVertices[j] = selectedVertices[i] = true;
}
}
}
}
}
centerOfMass /= result.Count;
return result;
}
```

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