ios - AVMutableVideoComposition を使用してテキスト オーバーレイを特定の時間範囲に追加する

Question

動画にテキスト オーバーレイを追加する方法を示す例をいくつか見つけました。

レイのチュートリアル - http://www.raywenderlich.com/30200/avfoundation-tutorial-adding-overlays-and-animations-to-videos

この SO 回答 - iOS でキャプチャしたビデオに透かしを追加するにはどうすればよいですか

他にもいくつかありましたので参考にさせていただきました。私のコードはその答えとほぼ同じに見えます.ビデオの冒頭でテキストオーバーレイが1〜2秒間だけ表示されるように微調整しようとしています. どうすればそれを達成できるかについての助けはありますか?

これが私のコードです。これは、オーバーレイが全体にわたって表示された状態でビデオをエクスポートするためにそのまま機能します。

if(vA) {
    videoCompositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [videoCompositionTrack insertTimeRange:videoTimerange ofTrack:[[vA tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:&error];
    [videoCompositionTrack setPreferredTransform:[[[vA tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] preferredTransform]];
    if(error)
        NSLog(@"%@", error);
}

if(aA) {
    audioCompositionTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    [audioCompositionTrack insertTimeRange:audioTimerange ofTrack:[[aA tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:&error];
    if(error)
        NSLog(@"%@", error);
}

CGSize videoSize = [videoCompositionTrack naturalSize];

UIImage *myImage = [UIImage imageNamed:@"cover.png"];
CALayer *aLayer = [CALayer layer];
aLayer.contents = (id)myImage.CGImage;
aLayer.frame = CGRectMake(5, 25, 100, 56);
aLayer.opacity = 0.7;

CATextLayer *titleLayer = [CATextLayer layer];
titleLayer.string = titleText;
titleLayer.fontSize = 18;
titleLayer.foregroundColor = titleColor.CGColor;
titleLayer.alignmentMode = kCAAlignmentCenter;
titleLayer.frame = CGRectMake(20, 10, videoSize.width - 40, 20);
[titleLayer displayIfNeeded];

CALayer *parentLayer = [CALayer layer];
CALayer *videoLayer = [CALayer layer];
parentLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
videoLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
[parentLayer addSublayer:videoLayer];
[parentLayer addSublayer:aLayer];
if(titleText && [titleText length] > 0) {
    [parentLayer addSublayer:titleLayer];
}

AVMutableVideoComposition *videoComp = [AVMutableVideoComposition videoComposition];
videoComp.renderSize = videoSize;
videoComp.frameDuration = CMTimeMake(1, 30);
videoComp.animationTool = [AVVideoCompositionCoreAnimationTool videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];

AVAssetTrack *videoTrack = [[mixComposition tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
AVMutableVideoCompositionLayerInstruction *layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
[instruction setTimeRange:CMTimeRangeMake(kCMTimeZero, [mixComposition duration])];
instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction];
videoComp.instructions = [NSArray arrayWithObject:instruction];

AVAssetExportSession *_assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
_assetExport.videoComposition = videoComp;
_assetExport.outputFileType = AVFileTypeMPEG4;
_assetExport.outputURL = outputFileUrl;

[_assetExport exportAsynchronouslyWithCompletionHandler:^{
    AVAssetExportSessionStatus status = [_assetExport status];
    switch (status) {
        case AVAssetExportSessionStatusFailed:
            NSLog(@"Export Failed");
            NSLog(@"Export Error: %@", [_assetExport.error localizedDescription]);
            NSLog(@"Export Error Reason: %@", [_assetExport.error localizedFailureReason]);
            break;
        case AVAssetExportSessionStatusCompleted:
            NSLog(@"Export Completed");
            [self performSelectorOnMainThread:@selector(updateProgressIndicator:) withObject:[NSNumber numberWithFloat:2] waitUntilDone:YES];
            break;
        case AVAssetExportSessionStatusUnknown:
            NSLog(@"Export Unknown");
            break;
        case AVAssetExportSessionStatusExporting:
            NSLog(@"Export Exporting");
            break;
        case AVAssetExportSessionStatusWaiting:
            NSLog(@"Export Waiting");
            break;
    }
}];