VertexAnimation/Runtime/Scripts/ModelBaker/MeshCombiner.cs

// References:
// https://forum.unity.com/threads/help-combining-and-manipulating-skinned-mesh-renderers-imported-from-blender.505078/
// http://wiki.unity3d.com/index.php/CombineSkinnedMeshes
// http://wiki.unity3d.com/index.php/SkinnedMeshCombiner

// TODO:
// ---Bake ALL the MeshRenderers/SkinnedMeshRenderers and merge them together.---
// ---Bake multiple animations.---
// ---Get the longest animation to calculate the texture height, so all the textures have the same height for the 3D texture.---
// Add options and previews for texture size, animation phasing/fps.
// Either merge with the animation books or generate them from this and maybe store them as child (and then don't destroy them on re-bake to keep the reference but replace it).

using UnityEngine;
using System.Collections.Generic;
using System.Linq;

namespace TAO.VertexAnimation
{
    public static class MeshCombiner
    {
        private struct MaterialMeshGroup
        {
            public List<SkinnedMeshRenderer> skinnedMeshes;
            public List<(MeshFilter mf, MeshRenderer mr)> meshes;
            public Material material;
        }

        public static SkinnedMeshRenderer Combine(this SkinnedMeshRenderer target, List<SkinnedMeshRenderer> skinnedMeshes, List<(MeshFilter mf, MeshRenderer mr)> meshes)
        {
            List<MaterialMeshGroup> groups = new List<MaterialMeshGroup>();

            // Group skinnedMeshes.
            foreach (var sm in skinnedMeshes)
            {
                bool hasGroup = false;
                foreach (var g in groups)
                {
                    if (sm.sharedMaterial == g.material)
                    {
                        hasGroup = true;
                        g.skinnedMeshes.Add(sm);
                    }
                }

                if (!hasGroup)
                {
                    groups.Add(new MaterialMeshGroup()
                    { 
                        skinnedMeshes = new List<SkinnedMeshRenderer>()
                        {
                            sm
                        },
                        meshes = new List<(MeshFilter mf, MeshRenderer mr)>(),
                        material = sm.sharedMaterial
                    });
                }
            }

            // Group Meshes.
            foreach (var m in meshes)
            {
                bool hasGroup = false;
                foreach (var g in groups)
                {
                    if (m.mr.sharedMaterial == g.material)
                    {
                        hasGroup = true;
                        g.meshes.Add(m);
                    }
                }

                if (!hasGroup)
                {
                    groups.Add(new MaterialMeshGroup()
                    {
                        skinnedMeshes = new List<SkinnedMeshRenderer>(),
                        meshes = new List<(MeshFilter mf, MeshRenderer mr)>()
                        {
                            m
                        },
                        material = m.mr.sharedMaterial
                    });
                }
            }

            List<GameObject> tmp = new List<GameObject>();
            for (int i = 0; i < groups.Count; i++)
            {
                tmp.Add(new GameObject("tmpChild", typeof(SkinnedMeshRenderer)));
                tmp[i].transform.parent = target.transform;

                MaterialMeshGroup mmg = groups[i];
                tmp[i].GetComponent<SkinnedMeshRenderer>().Combine(mmg.skinnedMeshes, mmg.meshes, mmg.material);
            }

            // TODO: Merge materialMergedObjects.
            // TEMP: Remove when materialMergedObjects.
            SkinnedMeshRenderer newSkinnedMeshRenderer = tmp[0].GetComponent<SkinnedMeshRenderer>();
            target.sharedMesh = newSkinnedMeshRenderer.sharedMesh;
            target.sharedMaterial = newSkinnedMeshRenderer.sharedMaterial;
            target.bones = newSkinnedMeshRenderer.bones;

            foreach (var go in tmp)
            {
                GameObject.DestroyImmediate(go);
            }

            // Set a name to make it more clear.
            target.sharedMesh.name = target.transform.name.Replace("(Clone)", "");
            return target;
        }

        public static SkinnedMeshRenderer Combine(this SkinnedMeshRenderer target, List<SkinnedMeshRenderer> skinnedMeshes, List<(MeshFilter mf, MeshRenderer mr)> meshes, Material mainMaterial)
        {
            List<Transform> bones = new List<Transform>();
            List<BoneWeight> boneWeights = new List<BoneWeight>();
            List<Matrix4x4> bindPoses = new List<Matrix4x4>();
            List<CombineInstance> combineInstances = new List<CombineInstance>();

            // Combine SkinnedMeshes.
            int boneOffset = 0;
            for (int s = 0; s < skinnedMeshes.Count; s++)
            {
                SkinnedMeshRenderer smr = skinnedMeshes[s];

                //if the skinned mesh renderer has a material other than the default
                //we assume it's a one-off face material and deal with it later
                if (smr.sharedMaterial != mainMaterial)
                {
                    continue;
                }

                BoneWeight[] meshBoneweight = smr.sharedMesh.boneWeights;

                // May want to modify this if the renderer shares bones as unnecessary bones will get added.
                // We don't care since it is going to be converted into vertex animations later anyways.
                for (int i = 0; i < meshBoneweight.Length; ++i)
                {
                    BoneWeight bWeight = meshBoneweight[i];

                    bWeight.boneIndex0 += boneOffset;
                    bWeight.boneIndex1 += boneOffset;
                    bWeight.boneIndex2 += boneOffset;
                    bWeight.boneIndex3 += boneOffset;

                    boneWeights.Add(bWeight);
                }

                boneOffset += smr.bones.Length;

                Transform[] meshBones = smr.bones;
                for (int i = 0; i < meshBones.Length; ++i)
                {
                    bones.Add(meshBones[i]);

                    //we take the old bind pose that mapped from our mesh to world to bone,
                    //and take out our localToWorldMatrix, so now it's JUST the bone matrix
                    //since our skinned mesh renderer is going to be on the root of our object that works
                    bindPoses.Add(smr.sharedMesh.bindposes[i] * smr.transform.worldToLocalMatrix);
                }

                CombineInstance ci = new CombineInstance
                {
                    mesh = smr.sharedMesh,
                    transform = smr.transform.localToWorldMatrix
                };
                combineInstances.Add(ci);

                GameObject.DestroyImmediate(smr);
            }

            // Combine Meshes.
            for (int s = 0; meshes != null && s < meshes.Count; s++)
            {
                MeshFilter filter = meshes[s].mf;
                MeshRenderer renderer = meshes[s].mr;
            
                //if the skinned mesh renderer has a material other than the default
                //we assume it's a one-off face material and deal with it later
                if (renderer.sharedMaterial != mainMaterial)
                {
                    continue;
                }

                // May want to modify this if the renderer shares bones as unnecessary bones will get added.
                // We don't care since it is going to be converted into vertex animations later anyways.
                int vertCount = filter.sharedMesh.vertexCount;
                for (int i = 0; i < vertCount; ++i)
                {
                    BoneWeight bWeight = new BoneWeight
                    {
                        boneIndex0 = boneOffset,
                        boneIndex1 = boneOffset,
                        boneIndex2 = boneOffset,
                        boneIndex3 = boneOffset,
                        weight0 = 1
                    };

                    boneWeights.Add(bWeight);
                }

                boneOffset += 1;

                bones.Add(filter.transform);

                // TODO: figure out what this should be.
                bindPoses.Add(filter.transform.worldToLocalMatrix);

                CombineInstance ci = new CombineInstance
                {
                    mesh = filter.sharedMesh,
                    transform = filter.transform.localToWorldMatrix
                };
                combineInstances.Add(ci);

                GameObject.DestroyImmediate(filter);
                GameObject.DestroyImmediate(renderer);
            }

            // Actually combine and recalculate mesh.
            Mesh skinnedMesh = new Mesh();
            skinnedMesh.CombineMeshes(combineInstances.ToArray(), true, true);
            skinnedMesh.RecalculateBounds();

            // Copy settings to target.
            target.sharedMesh = skinnedMesh;
            target.sharedMaterial = mainMaterial;
            target.bones = bones.ToArray();
            target.sharedMesh.boneWeights = boneWeights.ToArray();
            target.sharedMesh.bindposes = bindPoses.ToArray();

            return target;
        }

        public static void ConbineAndConvertGameObject(this GameObject gameObject)
        {
            // Get Skinned Meshes.
            List<SkinnedMeshRenderer> skinnedMeshes = gameObject.GetComponentsInChildren<SkinnedMeshRenderer>(true).ToList();
            // Get Meshes.
            List<(MeshFilter, MeshRenderer)> meshes = new List<(MeshFilter, MeshRenderer)>();
            foreach (var mf in gameObject.GetComponentsInChildren<MeshFilter>(true))
            {
                if (mf.TryGetComponent(out MeshRenderer mr))
                {
                    meshes.Add((mf, mr));
                }
            }

            // Add target mesh.
            SkinnedMeshRenderer target = gameObject.AddComponent<SkinnedMeshRenderer>();
            target.Combine(skinnedMeshes, meshes);
        }
    }
}