Unity, Blender and Instancing Models

As many of you may know, Blender files can be imported directly by Unity as models (teeechnically it's just telling the blender file to export itself as FBX, then importing that, but it's defacto Blender -> Unity for the end user). Honestly, I find it impossible not to want to use Blender as my level / modelling tool rather than trying to build levels out of prefabs in Unity. But with the default importer, there's a catch; instancing.

Instancing is simply the reuse of a model in multiple places. If you have a room full of pillars, and all those pillars are identical, it makes sense to just have one pillar model that you can change and edit with the other copies catching up. Equally, it makes sense to only store that one pillar model in memory, rather than paying the memory cost in full for every single copy of the pillar. Both Blender and Unity support doing this, but it all falls down on the importation; a set of instances in Blender becoming a set of "unique" meshes in Unity.

Thankfully, there is a way to fiddle imported assets automatically as they come in; the AssetPostprocessor. This can actually be levied to serve a wide variety of purposes and automation, where model imports are involved, but for now I'll be happy with simple instancing support. So here it is:

using UnityEngine;
using UnityEditor;
using System.Collections;
using System.Collections.Generic;

public class ModelPostprocess : AssetPostprocessor {

    Dictionary<string, List<Mesh>> meshLookup;

    void OnPostprocessModel (GameObject root)
    {
        if (meshLookup == null) meshLookup = new Dictionary<string, List<Mesh>>();
        else meshLookup.Clear();

        Apply(root.transform);
    }

    //removes the _001 etc after instance names
    string CleanMeshName(string name)
    {
        int end = name.LastIndexOf('_');
        if (end == -1) return name;

        return name.Remove(end, name.Length - end);
    }

    void Apply(Transform transform)
    {
        MeshFilter mF = transform.GetComponent<MeshFilter>();
        if (mF)
        {
            Mesh mesh = mF.sharedMesh;
            string name = CleanMeshName(mesh.name);

            List<Mesh> relatives;
            if(!meshLookup.TryGetValue(name, out relatives)) {
                mesh.name = name;
                relatives = new List<Mesh>();
                relatives.Add(mesh);
                meshLookup.Add(name, relatives);
            }
            else
            {
                //Debug.LogFormat("mesh {0} has {1} relatives", name, relatives.Count);
                for(int i = 0; i < relatives.Count; i++)
                {
                    Mesh relMesh = relatives[i];
                    if (CompareForMeshEquality(relMesh, mesh))
                    {
                        //Debug.LogFormat("mesh {0} is instance of mesh {1}; replacing", mesh.name, relMesh.name);
                        mF.sharedMesh = relMesh;

                        MeshCollider mC = transform.GetComponent<MeshCollider>();
                        if (mC)
                        {
                            mC.sharedMesh = relMesh;
                        }

                        GameObject.DestroyImmediate(mesh); //ensures the duplicate is completely removed
                    }
                }
            }
        }

        for (int i = 0; i < transform.childCount; i++) Apply(transform.GetChild(i));
    }

    const float maxBoundsError = 0.001f; //trying to evade floating point shenanigans
    bool CompareForMeshEquality(Mesh a, Mesh b)
    {
        //check if we're actually comparing a mesh object against itself.
        //since we delete 'b' if equal, we actually want to return false here or we'll delete 'a' as well
        if (a == b) return false;

        //check submeshes and vertex counts; this should early-out most non-duplicates
        if (a.subMeshCount != b.subMeshCount || a.vertexCount != b.vertexCount) return false;

        //check bounds centers
        if (Vector3.SqrMagnitude(a.bounds.center - b.bounds.center) > maxBoundsError) return false;
        //check bounds sizes
        if (Vector3.SqrMagnitude(a.bounds.size - b.bounds.size) > maxBoundsError) return false;

        //if not exited as false by now, they're very probably identical
        return true;
    }
}

It's only a simple postprocessor intended for that one thing; catching instance duplicates and clearing them out. The checks it makes for duplicity are only rudimentary though; if two meshes have the same name once instance numbers are stripped (ie 'Cube', 'Cube_001' and 'Cube_UnderscoresAreMyLife' are all considered the "same" name; everything after the last '_' is ignored) they will be compared on their vertex count, number of submeshes (material IDs) and their AABB bounds; if they are equal they are declared 'identical' and the latter is replaced by the former for both MeshFilters and MeshColliders. The script will do nothing at all with SkinnedMeshRenderers.

It does support having multiple meshes that share the same name but are not considered identical, so if you create some objects in Blender whilst forgetting to change them from their default names it probably won't eat them all outside of some very odd coincidences.

modelinstancepostprocessor.zip
File Size:	1 kb
File Type:	zip

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