Quero cortar um vídeo para o quadrado, mas não consegui fazer isso.

Eu convertiesse código rápido, mas eu só fico com tela preta depois de exportar o vídeo

        var videoComposition: AVMutableVideoComposition = AVMutableVideoComposition()
        videoComposition.frameDuration = CMTimeMake(1, 60)
        videoComposition.renderSize = CGSizeMake(clipVideoTrack.naturalSize.height, clipVideoTrack.naturalSize.height)

        var instruction: AVMutableVideoCompositionInstruction = AVMutableVideoCompositionInstruction()
        instruction.timeRange = CMTimeRangeMake(kCMTimeZero, CMTimeMakeWithSeconds(60, 30))

        var transformer: AVMutableVideoCompositionLayerInstruction =
        AVMutableVideoCompositionLayerInstruction(assetTrack: clipVideoTrack)

        var t1: CGAffineTransform = CGAffineTransformMakeTranslation(clipVideoTrack.naturalSize.height, 0)
        var t2: CGAffineTransform = CGAffineTransformRotate(t1, CGFloat(M_PI_2))

        var finalTransform: CGAffineTransform = t2

        transformer.setTransform(finalTransform, atTime: kCMTimeZero)

        instruction.layerInstructions = NSArray(object: transformer)
        videoComposition.instructions = NSArray(object: instruction)

        var documentsPath: NSString = NSSearchPathForDirectoriesInDomains(.DocumentDirectory,.UserDomainMask,true)[0] as NSString

        var exportPath: NSString = documentsPath.stringByAppendingFormat("/xvideo.mov")
        var exportUrl: NSURL = NSURL.fileURLWithPath(exportPath)!


        var exporter = AVAssetExportSession(asset: asset!, presetName: AVAssetExportPresetHighestQuality)
        exporter.videoComposition = videoComposition
        exporter.outputFileType = AVFileTypeQuickTimeMovie
        exporter.outputURL = exportUrl

O que estou perdendo?