iOS 基于Metal的本地视频渲染流程及详细解析

1.本地视频的渲染流程与AVFoundation采集的视频流渲染流程完全一致，只不过视频流的来源不同，一个是AVFoundation采集的CMSampleBufferRef和AVAssetReader读取本地视频文件的CMSampleBufferRef， AVFoundation这个采集的是异步的所以数据来了生成准备好的Y和UV纹理，AVAssetReader可以用的时候再读取

2. AVAssetReader 相关的本地视频文件读取类代码实现

#import 
@interface CCAssetReader : NSObject

//初始化
- (instancetype)initWithUrl:(NSURL *)url;

//从MOV文件读取CMSampleBufferRef 数据
- (CMSampleBufferRef)readBuffer;

@implementation CCAssetReader
{
    //轨道
    AVAssetReaderTrackOutput *readerVideoTrackOutput;
    //AVAssetReader可以从原始数据里获取解码后的音视频数据
    AVAssetReader   *assetReader;
    //视频地址
    NSURL *videoUrl;
    //锁
    NSLock *lock;
}

//初始化
- (instancetype)initWithUrl:(NSURL *)url{
    
    self = [super init];
    if(self != nil)
    {
        videoUrl = url;
        lock = [[NSLock alloc]init];
        [self setUpAsset];
    }
    return self;
}

//Asset 相关设置
-(void)setUpAsset{
   
    //AVURLAssetPreferPreciseDurationAndTimingKey 默认为NO,YES表示提供精确的时长
    NSDictionary *inputOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
    
    //1. 创建AVURLAsset 是AVAsset 子类,用于从本地/远程URL初始化资源
    AVURLAsset *inputAsset = [[AVURLAsset alloc] initWithURL:videoUrl options:inputOptions];
    
    //2.异步加载资源
    //weakSelf 解决循环引用
    __weak typeof(self) weakSelf = self;
    
    //定义属性名称
    NSString *tracks = @"tracks";
   
    //对资源所需的键执行标准的异步载入操作,这样就可以访问资源的tracks属性时,就不会受到阻碍.
    [inputAsset loadValuesAsynchronouslyForKeys:@[tracks] completionHandler: ^{
        
            //延长self 生命周期
            __strong typeof(self) strongSelf = weakSelf;
       
      //开辟子线程并发队列异步函数来处理读取的inputAsset
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSError *error = nil;
    
            //获取状态码.
            AVKeyValueStatus tracksStatus = [inputAsset statusOfValueForKey:@"tracks" error:&error];
            //如果状态不等于成功加载,则返回并打印错误信息
            if (tracksStatus != AVKeyValueStatusLoaded)
            {
                NSLog(@"error %@", error);
                return;
            }
            //处理读取的inputAsset
            [weakSelf processWithAsset:inputAsset];
        });
    }];
    
}

//处理获取到的asset
- (void)processWithAsset:(AVAsset *)asset
{
    //锁定
    [lock lock];
    NSLog(@"processWithAsset");
    NSError *error = nil;
    
    //1.创建AVAssetReader
    assetReader = [AVAssetReader assetReaderWithAsset:asset error:&error];
    
    //2.kCVPixelBufferPixelFormatTypeKey 像素格式.
    /*
     kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange : 420v
     kCVPixelFormatType_32BGRA : iOS在内部进行YUV至BGRA格式转换
     */
    NSMutableDictionary *outputSettings = [NSMutableDictionary dictionary];
    [outputSettings setObject:@(kCVPixelFormatType_420YpCbCr8BiPlanarFullRange) forKey:(id)kCVPixelBufferPixelFormatTypeKey];
    
    /*3. 设置readerVideoTrackOutput
     assetReaderTrackOutputWithTrack:(AVAssetTrack *)track outputSettings:(nullable NSDictionary *)outputSettings
     参数1: 表示读取资源中什么信息
     参数2: 视频参数
     */
    readerVideoTrackOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:[[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] outputSettings:outputSettings];
    
    //alwaysCopiesSampleData : 表示缓存区的数据输出之前是否会被复制.YES:输出总是从缓存区提供复制的数据,你可以自由的修改这些缓存区数据
    readerVideoTrackOutput.alwaysCopiesSampleData = NO;
    
    //4.为assetReader 填充输出
    [assetReader addOutput:readerVideoTrackOutput];
    
    //5.assetReader 开始读取.并且判断是否开始.
    if ([assetReader startReading] == NO)
    {
        NSLog(@"Error reading from file at URL: %@", asset);
    }
    
    //取消锁
    [lock unlock];
}

//读取Buffer 数据
- (CMSampleBufferRef)readBuffer {
    //锁定
    [lock lock];
    CMSampleBufferRef sampleBufferRef = nil;
    
    //1.判断readerVideoTrackOutput 是否创建成功.
    if (readerVideoTrackOutput) {
       
        //复制下一个缓存区的内容到sampleBufferRef
        sampleBufferRef = [readerVideoTrackOutput copyNextSampleBuffer];
    }
    
    //2.判断assetReader 并且status 是已经完成读取 则重新清空readerVideoTrackOutput/assetReader.并重新初始化它们
    if (assetReader && assetReader.status == AVAssetReaderStatusCompleted) {
        NSLog(@"customInit");
        readerVideoTrackOutput = nil;
        assetReader = nil;
        [self setUpAsset];
    }
    
    //取消锁
    [lock unlock];
    
    //3.返回读取到的sampleBufferRef 数据
    return sampleBufferRef;
}

3.渲染代码的差异，直接读取CMSampleBuffer

- (void)drawInMTKView:(MTKView *)view {
  
    //1.每次渲染都要单独创建一个CommandBuffer
    id commandBuffer = [self.commandQueue commandBuffer];
    //获取渲染描述信息
    MTLRenderPassDescriptor *renderPassDescriptor = view.currentRenderPassDescriptor;
   
    //2. 从CCAssetReader中读取图像数据
    CMSampleBufferRef sampleBuffer = [self.reader readBuffer];
    
    //3.判断renderPassDescriptor 和 sampleBuffer 是否已经获取到了?
    if(renderPassDescriptor && sampleBuffer)
    {
        //4.设置renderPassDescriptor中颜色附着(默认背景色)
        renderPassDescriptor.colorAttachments[0].clearColor = MTLClearColorMake(0.0, 0.5, 0.5, 1.0f);
        
        //5.根据渲染描述信息创建渲染命令编码器
        id renderEncoder = [commandBuffer renderCommandEncoderWithDescriptor:renderPassDescriptor];
        
        //6.设置视口大小(显示区域)
        [renderEncoder setViewport:(MTLViewport){0.0, 0.0, self.viewportSize.x, self.viewportSize.y, -1.0, 1.0 }];
        
        //7.为渲染编码器设置渲染管道
        [renderEncoder setRenderPipelineState:self.pipelineState];
        
        //8.设置顶点缓存区
        [renderEncoder setVertexBuffer:self.vertices
                                offset:0
                               atIndex:CCVertexInputIndexVertices];
        
        //9.设置纹理(将sampleBuffer数据 设置到renderEncoder 中)
        [self setupTextureWithEncoder:renderEncoder buffer:sampleBuffer];
        
        //10.设置片元函数转化矩阵
        [renderEncoder setFragmentBuffer:self.convertMatrix
                                  offset:0
                                 atIndex:CCFragmentInputIndexMatrix];
        
        //11.开始绘制
        [renderEncoder drawPrimitives:MTLPrimitiveTypeTriangle
                          vertexStart:0
                          vertexCount:self.numVertices];
        
        //12.结束编码
        [renderEncoder endEncoding];
        
        //13.显示
        [commandBuffer presentDrawable:view.currentDrawable];
    }
    
    //14.提交命令
    [commandBuffer commit];
    
}