Deleting faces or triangles from an object by material on load scene

Question

I have objects that have multiple materials imported from blender as a .FBX.

I'd like to find the objects where materials are used, and destroy faces when a certain material name is used ("!null", in my case).

I know this is not the correct way to work. Normally I should delete the faces in Blender, but as I am building quite complex geometry using faces that I need in Blender but don't want the Unity engine to render, it seems the only way to go.

I tried "cutout", but although they are invisible, it does not make any difference in rendering times.

Is this at all possible? If it is, how do I do it?

Answer 1

I have had this question answered on Unity Answers, though I needed to modify the original code to work properly.

using UnityEngine;
using System.Collections.Generic;

/// <summary>
/// Class: ClearNull Derived from MonoBehaviour
/// 
/// Purpose: Debugging of objects not yet completely finished in blender.
///          This class will destroy any face marked with !NULL to avoid  them to render
///          Restore the Original settings once destroyed
///          
/// Parameters DestroyString by default "!NULL" *** Warning this is case insensitive***
/// 
/// Notes: The code was inspired by Bunny83 in thread 
///        http://answers.unity3d.com/questions/1232964/
///        
///        This code should only be use for debugging and not for final products
///        
///        It will not work on children viewed from the parent.
///         
/// Further develepment ideas: 
///         - Make scene wide check on load to save and destroy 
///               every !NULLL face in the scene.
///         - Make it so it supports children 
/// 
/// Copyright: Code originally by Bunny83 adapted by Napivo on 2016/08/23 
///            free for anyone... Hope It help you as much as it does me
/// </summary>
public class ClearNull : MonoBehaviour 
{
    List<Material> oMats; // used to store the orriginal Materials
    List<int[]> oTris;    // used to store the orriginal triangles  
    public string DestroyString = "!NULL";

    void Start() 
    {
        //get the renderer
        MeshRenderer renderer = GetComponent<MeshRenderer>(); 
        //GetComponent the mesh
        Mesh mesh = GetComponent<MeshFilter>().sharedMesh;
        //Get the list of materials and save them for restore
        oMats = new List<Material>(renderer.sharedMaterials);
        List<Material> mats = new List<Material>(renderer.sharedMaterials);
        //Create a list to store the old and new triangles
        oTris = new List<int[]>();
        List<int[]> tris = new List<int[]>();

        int i = 0; // stores current number minus the one(s) lost
        int j = 0; // stores the current number 

        while (i < mats.Count) 
        {
            if (mats[i].name.ToUpper() == DestroyString.ToUpper()) 
            {
                Tris.Add(mesh.GetTriangles(j));  //Make a backup
                mats.Remove(mats[i]);
                j++;
            }
            else 
            {
                oTris.Add(mesh.GetTriangles(j)); //Make a backup
                tris.Add(mesh.GetTriangles(j));  //Copy to new triangles
                j++;
                i++;
            }
        }

        mesh.subMeshCount = tris.Count;         //Set new triangle count

        for (int n = 0; n < tris.Count; n++)    //Set new triangles without those lost 
        {  
            mesh.SetTriangles(tris[n], n);
        }

        renderer.sharedMaterials = mats.ToArray();  //Set the material without those lost
    }

    /// <summary>
    /// Purpose: Reset the object to the stati it was before
    /// </summary>
    void OnDestroy()
    {
        MeshRenderer renderer = GetComponent<MeshRenderer>();
        Mesh mesh = GetComponent<MeshFilter>().sharedMesh;

        mesh.subMeshCount = oTris.Count;
        for (int n = 0; n < oTris.Count; n++)
        {
            mesh.SetTriangles(oTris[n], n);
        }

        renderer.sharedMaterials = oMats.ToArray();
    }
}