thanks guys. now i know what happened. This did come from a low complexity region being masked out almost completely, and I did mistake the option '-F' for '-f'. Thanks all of the help! _________________________________________________________________ ÓëÁª»úµÄÅóÓѽøÐн»Á÷£¬ÇëʹÓà MSN Messenger: http://messenger.msn.com/cn