按书本代码运行出现错误代码：
[ INFO:0] Initialize OpenCL runtime...
OpenCV(3.4.1) Error: Assertion failed (K == 1 && update == 0 && mask.empty()) in
tance.cpp, line 265
Traceback (most recent call last):
cv2.error: OpenCV(3.4.1) D:\Build\OpenCV\opencv-3.4.1\modules\core\src\batch_dis
tance.cpp:265: error: (-215) K == 1 && update == 0 && mask.empty() in function c
v::batchDistance
更改脚本代码为：
import numpy as np
import cv2
from matplotlib import pyplot as plt
img1 = cv2.imread('logo.jpg',0)
img2 = cv2.imread('sing.jpg',0)
orb = cv2.ORB_create()
kp1,des1 = orb.detectAndCompute(img1,None)
kp2,des2 = orb.detectAndCompute(img2,None)
bf = cv2.BFMatcher(cv2.NORM_HAMMING,crossCheck=True)
matches = bf.knnMatch(des1,des2,k=1)#此处课本为2，更改为1后可运行
img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,matches,img2,flags=2)
plt.imshow(img3)
plt.show()
参考：http://answers.opencv.org/question/42714/some-problem-about-knnmatch/