如何通过CAAffineTranslation(或其他方式)将视频定位从山水到人像?

I am merging the video using AVMutableComposition and the videos gets merged.But the video gets stored in the landscape (which is default mode for AVAsset) after merging.So i want the video to be fix in portrait mode only...Is there any way to set the video orientation to portrait mode. Thanx in advanceI am merging the video using AVMutableCompositi