错误如下：
    Unknown error code -28 (The input arrays should have at least 4 corresponding point sets to calculate Homography) in findHomography, file /build/master_pack-android/opencv/modules/calib3d/src/fundam.cpp, line 385

    这是因为，一个矩形需要4个点，如果优化后的点小于4但还是调用findHomography方法，那就会报错