短视频从无到有 (八)视频速率调节

录制好的视频我们可以处理成慢速或者快速的效果。还是十分简单，AVMutableCompositionTrack这个类已经开放了接口scaleTimeRange: toDuration:,我们只需要处理成所想要的效果即可。不多说，还是直接上代码：

  // 获取视频
    AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:_fileURL options:nil];
                              
    // 视频混合
    AVMutableComposition* mixComposition = [AVMutableComposition composition];
   // 视频轨道
   AVMutableCompositionTrack *compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    // 音频轨道
  AVMutableCompositionTrack *compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
  // 视频的方向
 CGAffineTransform videoTransform = [videoAsset tracksWithMediaType:AVMediaTypeVideo].lastObject.preferredTransform;
if (videoTransform.a == 0 && videoTransform.b == 1.0 && videoTransform.c == -1.0 && videoTransform.d == 0) {
    NSLog(@"垂直拍摄");
    videoTransform = CGAffineTransformMakeRotation(M_PI_2);
    
 }else if (videoTransform.a == 0 && videoTransform.b == -1.0 && videoTransform.c == 1.0 && videoTransform.d == 0) {
    NSLog(@"倒立拍摄"); videoTransform = CGAffineTransformMakeRotation(-M_PI_2);
    
}else if (videoTransform.a == 1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == 1.0) {
     NSLog(@"Home键右侧水平拍摄");
     videoTransform = CGAffineTransformMakeRotation(0);
    
}else if (videoTransform.a == -1.0 && videoTransform.b == 0 && videoTransform.c == 0 && videoTransform.d == -1.0) {
        NSLog(@"Home键左侧水平拍摄");
        videoTransform = CGAffineTransformMakeRotation(M_PI);
        
}
   // 根据视频的方向同步视频轨道方向
   compositionVideoTrack.preferredTransform = videoTransform;
    //时间刻度
    compositionVideoTrack.naturalTimeScale = 600;
// 插入视频轨道
[compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, CMTimeMake(videoAsset.duration.value, videoAsset.duration.timescale)) ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] firstObject] atTime:kCMTimeZero error:nil];
// 插入音频轨道
[compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, CMTimeMake(videoAsset.duration.value, videoAsset.duration.timescale)) ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeAudio] firstObject] atTime:kCMTimeZero error:nil];
 // 适配视频速度比率 譬如慢速4 快速0.3
    //10s的视频会处理成40s
    CGFloat scale = 4;
    
    [compositionVideoTrack scaleTimeRange:CMTimeRangeMake(kCMTimeZero, CMTimeMake(videoAsset.duration.value,videoAsset.duration.timescale)) toDuration:CMTimeMake(videoAsset.duration.value*scale, videoAsset.duration.timescale)];
    
    
    [compositionAudioTrack scaleTimeRange:CMTimeRangeMake(kCMTimeZero, CMTimeMake(videoAsset.duration.value,videoAsset.duration.timescale)) toDuration:CMTimeMake(videoAsset.duration.value*scale, videoAsset.duration.timescale)];
    
    NSLog(@"输出时间结构体");
    CMTimeShow(videoAsset.duration);
    //合成视频输出路径
    NSString *path =[self getVideoSaveFilePathString:@".MOV" addPathArray:NO];
    //输出
    AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:mixComposition
                                                                      presetName:AVAssetExportPreset1280x720];
    
    exporter.outputURL = [NSURL fileURLWithPath:path];
    exporter.outputFileType = AVFileTypeQuickTimeMovie;
    exporter.shouldOptimizeForNetworkUse = YES;
    [exporter exportAsynchronouslyWithCompletionHandler:^{
       
        dispatch_async(dispatch_get_main_queue(), ^{
           
            switch (exporter.status) {
                case AVAssetExportSessionStatusCompleted:{
                    
                    //保存到相册
                    [self savePhotosAlbum:exporter.outputURL];
                    
                    
                }
                    
                    break;
                    
                default:
                    break;
            }
            
            
            
            
        });
        
        
    }];
    

注意这里与实时慢速拍摄视频不同。实时慢速拍摄视频也十分简单，查阅GPUImage的头文件发现GPUImageVideoCamera下发现了frameRate这个属性，譬如我们可以设置videoCamera.frameRate =5;这样就可以实现实时慢速拍摄视频了，简单吧。经过实验，发现frameRate的取值范围在0-30之间。

有什么问题，欢迎留言讨论。