SceneKit框架中SCNScene场景手动导出STL格式模型

一、SCNScene系统API导出STL格式模型会出现细节丢失问题

- (BOOL)writeToURL:(NSURL *)url options:(nullable NSDictionary<NSString *, id> *)options delegate:(nullable id <SCNSceneExportDelegate>)delegate progressHandler:(nullable SCNSceneExportProgressHandler)progressHandler;

二、STL文件格式：二进制格式

起始有80个字节文件头用于存储文件名，接着4个字节表示三角面片的数量【数量一定要正确，否则导出的模型会显示异常】，每个三角面片占用固定的50个字节：法向矢量---3个4字节浮点数、三角面的三个顶点：每个顶点为3个4字节浮点数，接着是2个字节描述三角形基本属性【暂时没有用到】

三、实现

/// 保存多个模型到一个文件：异步操作
/// @param nodes 模型节点列表
/// @param filePath <#filePath description#>
/// @param complete <#complete description#>
+(void)xlSaveSCNNodes:(NSArray *)nodes
               toFile:(NSString *)filePath
             complete:(void(^)(NSError *error))complete{
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        // 写文件句柄
        NSFileHandle *fileHandler = [self xlFileHandleForWritingAtPath:filePath];
        
        // 写入文件头
        NSError *error = nil;
        NSString *name = filePath.lastPathComponent;
        name = [name stringByDeletingPathExtension];
        NSData *data = [name dataUsingEncoding:NSUTF8StringEncoding];
        
        if (data.length > 80) {
            data = [data subdataWithRange:NSMakeRange(0, 80)];
            [self xlFileHandler:fileHandler writeData:data error:&error];
        } else {
            [self xlFileHandler:fileHandler writeData:data error:&error];
            NSInteger datCcount = 80 - data.length;
            NSString *space = @"";
            for (int index = 0; index < datCcount; index ++) {
                space = [space stringByAppendingString:@" "];
            }
            data = [space dataUsingEncoding:NSUTF8StringEncoding];
            [self xlFileHandler:fileHandler writeData:data error:&error];
        }
        
        // 写入面的个数
        NSInteger allCount = [self xlNormalsWithNodes:nodes];
        data = [NSData dataWithBytes:&allCount length:sizeof(int)];
        [self xlFileHandler:fileHandler writeData:data error:&error];
        
        // 保存法线与顶点信息
        for (SCNNode *node in nodes) {
            SCNGeometry *geometry = node.geometry;
            // 法线信息
            NSArray<SCNGeometrySource *> *normals = [geometry geometrySourcesForSemantic:SCNGeometrySourceSemanticNormal];
            SCNGeometrySource *normal = normals[0];
            
            // 顶点信息
            NSArray<SCNGeometrySource *> *vertices = [geometry geometrySourcesForSemantic:SCNGeometrySourceSemanticVertex];
            SCNGeometrySource *vertice = vertices[0];
            
            NSInteger normalCount = (normal.data.length - normal.dataOffset) / normal.dataStride / 3;
            NSMutableData *mData = [NSMutableData data];
            char a[2];
            NSData *vectorData = nil;
            SCNMatrix4 matrix4 = node.worldTransform;
            GLKMatrix4 glkMatrix4 = SCNMatrix4ToGLKMatrix4(matrix4);
            for (int i = 0; i < normalCount; i ++) {
                NSData *data = [normal.data subdataWithRange:NSMakeRange(normal.dataOffset + i * normal.dataStride * 3, normal.dataStride)];
                SCNVector3 nVector3 = [self xlVector3FromData:data];
                vectorData = [NSData dataWithBytes:&(nVector3.x) length:4];
                [mData appendData:vectorData];
                vectorData = [NSData dataWithBytes:&(nVector3.y) length:4];
                [mData appendData:vectorData];
                vectorData = [NSData dataWithBytes:&(nVector3.z) length:4];
                [mData appendData:vectorData];
                for (int j = 0; j <= 2; j ++) {
                    data = [vertice.data subdataWithRange:NSMakeRange(i * normal.dataStride * 3 + j*normal.dataStride, normal.dataStride)];
                    nVector3 = [self xlVector3FromData:data];
                    GLKVector3 glkVector3 = SCNVector3ToGLKVector3(nVector3);
                    GLKVector3 nGLKVector3 = GLKMatrix4MultiplyAndProjectVector3(glkMatrix4, glkVector3);

                    nVector3 = SCNVector3FromGLKVector3(nGLKVector3);
                    vectorData = [NSData dataWithBytes:&(nVector3.x) length:4];
                    [mData appendData:vectorData];
                    vectorData = [NSData dataWithBytes:&(nVector3.y) length:4];
                    [mData appendData:vectorData];
                    vectorData = [NSData dataWithBytes:&(nVector3.z) length:4];
                    [mData appendData:vectorData];
                }
                [mData appendBytes:(char*)(a) length:sizeof(char) * 2];
            }
            [self xlFileHandler:fileHandler writeData:mData error:&error];
        }
        
        if (@available(iOS 13.0, *)) {
            NSError *closeError = nil;
            [fileHandler closeAndReturnError:&closeError];
            if (complete) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    complete(nil);
                });
            }
        } else {
            [fileHandler closeFile];
            if (complete) {
                dispatch_async(dispatch_get_main_queue(), ^{
                    complete(nil);
                });
            }
        }
    });
}

// 计算面个数
+(NSInteger)xlNormalsWithNodes:(NSArray *)nodes{
    NSInteger allCount = 0;
    for (SCNNode *node in nodes) {
        SCNGeometry *geometry = node.geometry;
        // 获取法线信息
        NSArray<SCNGeometrySource *> *normals = [geometry geometrySourcesForSemantic:SCNGeometrySourceSemanticNormal];
        // 获取法线信息
        SCNGeometrySource *normal = normals[0];
        allCount = allCount + (normal.data.length - normal.dataOffset) / normal.dataStride / 3;
    }
    return allCount;
}

// 文件句柄
+(NSFileHandle *)xlFileHandleForWritingAtPath:(NSString *)filePath{
    NSFileHandle *fileHandler = [NSFileHandle fileHandleForWritingAtPath:filePath];
    if(fileHandler == nil) {
         [[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil];
        fileHandler = [NSFileHandle fileHandleForWritingAtPath:filePath];
    }
    return fileHandler;
}

// NSData转坐标
+(SCNVector3)xlVector3FromData:(NSData *)data{
    float tmpValue[1];
    float v0, v1, v2;
    NSData *tmpData = [data subdataWithRange:NSMakeRange(0, 4)];
    [tmpData getBytes:tmpValue length:4];
    v0 = tmpValue[0];
    tmpData = [data subdataWithRange:NSMakeRange(4, 4)];
    [tmpData getBytes:tmpValue length:4];
    v1 = tmpValue[0];
    tmpData = [data subdataWithRange:NSMakeRange(4 * 2, 4)];
    [tmpData getBytes:tmpValue length:4];
    v2 = tmpValue[0];
    return SCNVector3Make(v0, v1, v2);
}

// SCN坐标系转OpenGL坐标系
+(SCNVector3)xlSCNMatrix4:(SCNMatrix4)scnMatrix4 multiplyAndProjectVector3:(SCNVector3)scnVector3{
    GLKMatrix4 glkMatrix4 = SCNMatrix4ToGLKMatrix4(scnMatrix4);
    GLKVector3 glkVector3 = SCNVector3ToGLKVector3(scnVector3);
    GLKVector3 nGLKVector3 = GLKMatrix4MultiplyAndProjectVector3(glkMatrix4, glkVector3);

    return SCNVector3FromGLKVector3(nGLKVector3);
}

// 写文件
+(void)xlFileHandler:(NSFileHandle *)fileHandler writeData:(NSData *)data error:(NSError **)error{
    if (@available(iOS 13.0, *)) {
        [fileHandler writeData:data error:error];
    } else {
        // Fallback on earlier versions
        [fileHandler writeData:data];
    }
}

四、其他

ASCII编码格式的STL文件导出未实现，可以参考STL文件解析来处理ASCII编码格式的STL文件导出。

函数中计算法向量数使用的normal.dataOffset，为数据的起始偏移值，不同的加载方式该值可能不同【暂时遇到过0、12】，normal.dataStride为单个顶点数据的字节长度。

刚接触3D渲染框架，难免有错漏，欢迎评论留言指出。

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