On the stage of describing the shape of airplane and extracting feature , we derive five - coplanar - point invariant from the basic projective invariant - cross ratio , and resolve the problem of stabilization of five - coplanar - point when there is three points are nearly collinear 在對飛機外型提取特征進行描述的階段,首先從基本的射影不變量交比不變量出發(fā),推導出了平面五點不量,并解決了當有三點接近共線時不變量不穩(wěn)定的問題。