通过 C# 从 USD 提取资产

//需要提取资产的源文件
string file = "template.usd";
Scene scene = new Scene(file);  

//输出为压缩文件格式，Directory 表示现有文件夹的名称
var zipOutput = Path.Combine("Directory", "OutputFile.zip");
using var output = new FileStream(zipOutput, FileMode.Create);
using var za = new Zip(output);

//调用 Extract 方法执行资产提取操作
Extract(scene,za,true);

//可调用Extract方法，参数texture表示：是否提取纹理
private void Extract(Scene scene, Zip za,bool texture)
{
    var extractor = new Extractor(za,texture);
    extractor.Extract(scene);
}

//创建压缩文件处理类
class Zip : IDisposable
{
    private ZipArchive archive;
    private HashSet<string> entries = new HashSet<string>();

    public Zip(Stream stream)
    {
        archive = new ZipArchive(stream, ZipArchiveMode.Create);
    }
    public void Dispose()
    {
        archive.Dispose();
    }

    public void Add(string fileName, byte[] content, bool enableCompression)
    {
        var entryName = PickName(fileName);
        var compressionLevel = enableCompression ? CompressionLevel.Fastest : CompressionLevel.NoCompression;
        var entry = archive.CreateEntry(entryName, compressionLevel);
        using var stream = entry.Open();
        stream.Write(content, 0, content.Length);
    }
    
    private string PickName(string fileName)
    {
        if (!entries.Contains(fileName))
        {
            entries.Add(fileName);
            return fileName;
        }
        var baseName = Path.GetFileNameWithoutExtension(fileName);
        var ext = Path.GetExtension(fileName);
        for (var idx = 2; ; idx++)
        {
            var newName = baseName + "_" + idx;
            if (!string.IsNullOrEmpty(ext))
                newName += ext;
            if (entries.Contains(newName))
                continue;
            entries.Add(newName);
            return newName;
        }
    }
}

//创建资产提取处理类
class Extractor
{
    private Zip zip;
    private bool texture;
    HashSet<A3DObject> visited = new HashSet<A3DObject>();
    public Extractor(Zip zip,bool texture)
    {
        this.zip = zip;
        this.texture = texture;
    }

    private bool CanVisit(A3DObject obj)
    {
        if (visited.Contains(obj))
            return false;
        visited.Add(obj);
        return true;
    }
    public void Extract(Scene scene)
    {
        if (scene.Library != null && scene.Library.Count > 0)
        {
            foreach (var obj in scene.Library)
            {
                Visit(obj);
            }
        }
        VisitNode(scene.RootNode);
    }
    private void VisitNode(Node node)
    {
        if (!CanVisit(node))
            return;
        if (texture)
        {
            foreach (var mat in node.Materials)
            {
                VisitMaterial(mat);
            }
        }

        foreach (var entity in node.Entities)
        {
            if (entity is Mesh)
                Save((Mesh)entity, node.Name);
        }
        
        foreach (var child in node.ChildNodes)
        {
            VisitNode(child);
        }
    }
    private void VisitMaterial(Material mat)
    {
        if (!CanVisit(mat))
            return;
        if (!texture)
            return;
        foreach (var tslot in mat)
        {
            if (tslot.Texture is Texture)
            {
                Save((Texture)tslot.Texture);
            }
        }
    }
    private void Visit(A3DObject obj)
    {
        if (texture && obj is Texture)
        {
            Save((Texture)obj);
        }
        else if (obj is Mesh)
        {
            Save((Mesh)obj, null);
        }
        else if (obj is Node)
        {
            VisitNode((Node)obj);
        }
    }
    private void Save(Mesh mesh, string? nodeName)
    {
        if (!CanVisit(mesh))
            return;
        Scene scene = new Scene(mesh);
        using (var ms = new MemoryStream())
        {
            scene.Save(ms, FileFormat.FBX7400ASCII);
            var name = nodeName;
            if (string.IsNullOrEmpty(name))
                name = mesh.Name;
            if (string.IsNullOrEmpty(name))
                name = "mesh";
            var ext = ".fbx";
            zip.Add(name + ext, ms.ToArray(), true);
        }
    }
    private void Save(Texture tex)
    {
        if (tex.Content == null || !CanVisit(tex))
            return;
        var fileName = tex.FileName != null ? Path.GetFileName(tex.FileName) : null;
        zip.Add(fileName, tex.Content, false);
    }
}