8

我正在录制小视频剪辑（大约一秒钟左右，前后摄像头都有，可能有不同的方向）。然后尝试使用 AVAssetExportSession 合并它们。我基本上用适当的变换和音频和视频轨道制作合成和视频合成。

问题是，在 iOS 5 上，如果您有 4 个以上的视频剪辑，它会失败，而在 iOS 6 上，限制似乎是 16 个剪辑。

这对我来说似乎真的很令人费解。AVAssetExportSession 是在做一些奇怪的事情，还是对可以传递给它的剪辑数量有一些未记录的限制？以下是我的代码的一些摘录：

-(void)exportVideo
{
    AVMutableComposition *composition = video.composition;
    AVMutableVideoComposition *videoComposition = video.videoComposition;
    NSString * presetName = AVAssetExportPresetMediumQuality;

    AVAssetExportSession *_assetExport = [[AVAssetExportSession alloc] initWithAsset:composition presetName:presetName];
    self.exportSession = _assetExport;

    videoComposition.renderSize = CGSizeMake(640, 480);
    _assetExport.videoComposition = videoComposition;

    NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent: @"export.mov"];
    NSURL *exportUrl = [NSURL fileURLWithPath:exportPath];

    // Delete the currently exported files if it exists
    if([[NSFileManager defaultManager] fileExistsAtPath:exportPath])
        [[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];

    _assetExport.outputFileType = AVFileTypeQuickTimeMovie;
    _assetExport.outputURL = exportUrl;
    _assetExport.shouldOptimizeForNetworkUse = YES;

    [_assetExport exportAsynchronouslyWithCompletionHandler:^{
        switch (_assetExport.status)
        {
            case AVAssetExportSessionStatusCompleted:
                NSLog(@"Completed exporting!");
                break;
            case AVAssetExportSessionStatusFailed:
                NSLog(@"Failed:%@", _assetExport.error.description);
                break;
            case AVAssetExportSessionStatusCancelled:
                NSLog(@"Canceled:%@", _assetExport.error);
                break;
            default:
                break;
        }
    }];
}

以下是这些作品的制作方式：

-(void)setVideoAndExport
{
    video = nil;
    video = [[VideoComposition alloc] initVideoTracks];

    CMTime localTimeline = kCMTimeZero;

    // Create the composition of all videofiles
    for (NSURL *url in outputFileUrlArray) {
        AVAsset *asset = [[AVURLAsset alloc]initWithURL:url options:nil];
        [video setVideo:url at:localTimeline];
        localTimeline = CMTimeAdd(localTimeline, asset.duration); // Increment the timeline
    }
    [self exportVideo];
}

这是 VideoComposition 类的主要内容：

-(id)initVideoTracks
{
    if((self = [super init]))
    {
        composition = [[AVMutableComposition alloc] init];
        addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
        mainInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
        instructions = [[NSMutableArray alloc] init];
        videoComposition = [AVMutableVideoComposition videoComposition];
    }
    return self;
}


-(void)setVideo:(NSURL*) url at:(CMTime)to
{
    asset = [[AVURLAsset alloc]initWithURL:url options:nil];

    AVAssetTrack *assetTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];

    AVMutableCompositionTrack *compositionTrackVideo = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [compositionTrackVideo insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack: assetTrack atTime:to error:nil];

    AVMutableCompositionTrack *compositionTrackAudio = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    [compositionTrackAudio insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack:[[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:to error:nil];

    mainInstruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeAdd(to, asset.duration));

    AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionTrackVideo];

    [layerInstruction setTransform: assetTrack.preferredTransform atTime: kCMTimeZero];
    [layerInstruction setOpacity:0.0 atTime:CMTimeAdd(to, asset.duration)];
    [instructions addObject:layerInstruction];

    mainInstruction.layerInstructions = instructions;
    videoComposition.instructions = [NSArray arrayWithObject:mainInstruction];
    videoComposition.frameDuration = CMTimeMake(1, 30);
}
4

2 回答 2

8

Okay, I also contacted Apple about this issue and they gave a response:

"This is a known condition. You are hitting the decoder limit set in AVFoundation."

They also asked me to file a bug report on the issue, since the error message that AVAssetExportSession gives if vague and misleading. So I filed a bug report to apple complaining about the fact that the error message is bad.

So these limits in AVAssetExportSession are confirmed. In iOS 5 the decoder limit is 4 and in iOS 6 it was raised to 16. The main issue here is that the error reported by AVAssetExportSession is bad as it only reports: 11820 "Cannot Complete Export" instead of actually telling us that we have hit a limit.

于 2013-01-04T08:45:30.237 回答
3

我也遇到过类似的问题。我已经设法通过将资产插入组合而不是将轨道插入可变轨道来修复它。因此，在您的“setVideo”代码中，而不是这一行：

[compositionTrackVideo insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofTrack: assetTrack atTime:to error:nil];

尝试这个：

[self insertTimeRange:CMTimeRangeMake(kCMTimeZero, asset.duration) ofAsset:asset atTime:to error:nil]
于 2012-12-30T17:30:11.797 回答