镜像网格和错误的 UV 贴图运行时导出

Question

编辑：在与 Assimp 开发人员短暂接触后，我被指向了导入过程。当我从别人那里接手代码时，我没有想到看那部分：

using (var importer = new AssimpContext())
{
   scene = importer.ImportFile(file, PostProcessSteps.Triangulate | PostProcessSteps.FlipUVs | PostProcessSteps.JoinIdenticalVertices);
}

FlipUVs 完全按照它说的做，它在 y 轴上翻转，所以原点现在在左上角。所以现在我能够得到具有适当 UV 但仍然是镜像网格的模型。将父对象设置为 scale x = -1 会将其翻转回正常状态并使其看起来不错，但我想这不是故意的。所以我一直在寻找。

看图片，有两种吊车模型。左边的是在运行时通过序列化和重建加载的，而右边的是原始的，只是简单地拖到场景中。序列化发生在 Assimp 库中。

地板恰好先创建，似乎得到了正确的 uv 贴图。而其他项目得到错误的 uv 贴图。虽然我正在打印 uv 贴图的值，但它们似乎与原始贴图匹配。

这是序列化的方法，这是 Assimp 的 Mesh class，不是 Unity Mesh class，应用序列化是 Windows UWP 中构建的应用程序：

    private static void SerializeMeshes(BinaryWriter writer, IEnumerable<Mesh> meshes)
    {
        foreach (Mesh mesh in meshes)
        {
            ICollection<int> triangles = MeshLoadTriangles(mesh);
            MeshSerializeHeader(writer, mesh.Name, mesh.VertexCount, triangles.Count, mesh.MaterialIndex);
            MeshSerializeVertices(writer, mesh.Vertices);
            MeshSerializeUVCoordinate(writer, mesh.TextureCoordinateChannels);
            MeshSerializeTriangleIndices(writer, triangles);                       
        }
    }

    private static void MeshSerializeUVCoordinate(BinaryWriter writer, List<Vector3D>[] textureCoordinateChannels)
    {
        // get first channel and serialize to writer. Discard z channel
        // This is Vector3D since happening outside Unity
        List<Vector3D> list = textureCoordinateChannels[0];
        foreach (Vector3D v in list)
        {
            float x = v.X;
            float y = v.Y;
            writer.Write(x);
            writer.Write(y);
        }
    }
    private static void MeshSerializeVertices(BinaryWriter writer, IEnumerable<Vector3D> vertices)
    {
        foreach (Vector3D vertex in vertices)
        {
            Vector3D temp = vertex;
            writer.Write(temp.X);
            writer.Write(temp.Y);
            writer.Write(temp.Z);
        }
    }
    private static void MeshSerializeTriangleIndices(BinaryWriter writer, IEnumerable<int> triangleIndices)
    {
          foreach (int index in triangleIndices)  { writer.Write(index); }
    }

这是反转过程：

    private static void DeserializeMeshes(BinaryReader reader, SceneGraph scene)
    {           
        MeshData[] meshes = new MeshData[scene.meshCount];
        for (int i = 0; i < scene.meshCount; i++)
        {
             meshes[i] = new MeshData();
             MeshReadHeader(reader, meshes[i]);
             MeshReadVertices(reader, meshes[i]);
             MeshReadUVCoordinate(reader, meshes[i]);
             MeshReadTriangleIndices(reader, meshes[i]);
        }
        scene.meshes = meshes as IEnumerable<MeshData>;
    }
private static void MeshReadUVCoordinate(BinaryReader reader, MeshData meshData)
    {
        bool hasUv = reader.ReadBoolean();
        if(hasUv == false) { return; }
        Vector2[] uvs = new Vector2[meshData.vertexCount];
        for (int i = 0; i < uvs.Length; i++)
        {
            uvs[i] = new Vector2();
            uvs[i].x = reader.ReadSingle();
            uvs[i].y = reader.ReadSingle();
        }
        meshData.uvs = uvs;
    }
    private static void MeshReadHeader(BinaryReader reader, MeshData meshData)
    {
        meshData.name = reader.ReadString();
        meshData.vertexCount = reader.ReadInt32();
        meshData.triangleCount = reader.ReadInt32();
        meshData.materialIndex = reader.ReadInt32();
    }
    private static void MeshReadVertices(BinaryReader reader, MeshData meshData)
    {
        Vector3[] vertices = new Vector3[meshData.vertexCount];

        for (int i = 0; i < vertices.Length; i++)
        {
            vertices[i] = new Vector3();
            vertices[i].x = reader.ReadSingle();
            vertices[i].y = reader.ReadSingle();
            vertices[i].z = reader.ReadSingle();
        }
        meshData.vertices = vertices;
    }
    private static void MeshReadTriangleIndices(BinaryReader reader, MeshData meshData)
    {
        int[] triangleIndices = new int[meshData.triangleCount];

        for (int i = 0; i < triangleIndices.Length; i++)
        {
            triangleIndices[i] = reader.ReadInt32();
        }
        meshData.triangles = triangleIndices;
    }

MeshData 只是一个临时容器，其中包含来自 fbx 的反序列化值。然后，创建网格：

private static Mesh[] CreateMeshes(SceneGraph scene)
{
    Mesh[] meshes = new Mesh[scene.meshCount];
    int index = 0;
    foreach (MeshData meshData in scene.meshes)
    {
        meshes[index] = new Mesh();           
        Vector3[] vec = meshData.vertices;
        meshes[index].vertices = vec;
        meshes[index].triangles = meshData.triangles;
        meshes[index].uv = meshData.uvs;   
        meshes[index].normals = meshData.normals;  
        meshes[index].RecalculateNormals();
        index++;
    }
    return meshes;
}

我没有在代码中看到任何导致这种行为的原因，我想说如果值是错误的，它会完全搞砸网格。

我可以看到我拥有的 fbx 文件使用四边形而不是三角形进行索引。

难道 Assimp 不适合这个？

Answer 1

我没有通过 Assimp 以正确的方式解决问题。

我们使用的基本解决方案是对对象变换中翻转的轴进行负缩放。

更合适的解决方案是将所有顶点提供给 Unity 端的矩阵，以便正确解析顶点的位置。

获取顶点列表
foreach顶点与旋转矩阵相乘
将数组分配给网格
使用网格渲染

镜像网格和错误的 UV 贴图运行时导出

Mirrored mesh and wrong UV map runtime export

c#

unity3d

assimp