那个是自动阈值的啊
追答你仔细的看看那个函数吧。可以指定阈值的。
BW = EDGE(I,'prewitt',THRESH) specifies the sensitivity threshold for
the Prewitt method. EDGE ignores all edges that are not stronger than
THRESH. If you do not specify THRESH, or if THRESH is empty ([]),
EDGE chooses the value automatically.