问题：在用python3使用img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,flags=2)的时候，可能会产生错误：TypeError: Required argument 'outImg' (pos 6) not found

1. import numpy as np

2. import cv2

3. from matplotlib import pyplot as plt

4. img1 = cv2.imread('data/queryImage.jpg', 0) # queryImage

5. img2 = cv2.imread('data/trainImage.jpg', 0) # trainImage

6. # Initiate SIFT detector

7. sift = cv2.xfeatures2d.SIFT_create()

8. # find the keypoints and descriptors with SIFT

9. kp1, des1 = sift.detectAndCompute(img1,None)

10. kp2, des2 = sift.detectAndCompute(img2,None)

11. # BFMatcher with default params

12. bf = cv2.BFMatcher()

13. matches = bf.knnMatch(des1,des2, k=2)

14. # Apply ratio test

15. good = []

16. for m,n in matches:

17. if m.distance < 0.75*n.distance:

18. good.append([m])

19. # cv2.drawMatchesKnn expects list of lists as matches.

20. img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,flags=2)

21. ....

报错信息：

1. ---------------------------------------------------------------------------

2. TypeError Traceback (most recent call last)

3. <ipython-input-54-8a7d0ce88553> in <module>()

4. 18 good.append([m])

5. 19 # cv2.drawMatchesKnn expects list of lists as matches.

6. ---> 20 img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,flags=2)

7. 21 plt.imshow(img3),plt.show()

8.  

9. TypeError: Required argument 'outImg' (pos 6) not found

解决：将img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,flags=2) 替换为：

          img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,None,flags=2) 即：如果没有outImg参数则传入None。

分析：

        在最后一个demo中：是将outImg参数以None的形式传入。

1. import cv2

2. cv2.drawMatchesKnn?

3. ------------------------------------------------------------------------------------------------------

4. Docstring:

5. drawMatchesKnn(img1, keypoints1, img2, keypoints2, matches1to2, outImg[, matchColor[, singlePointColor[, matchesMask[, flags]]]]) -> outImg

6. . @overload

7. Type: builtin_function_or_method

8.  
9.  

10. demo:

11. draw_params = dict(matchColor = (0,255,0),

12. singlePointColor = (255,0,0),

13. matchesMask = matchesMask,

14. flags = 0)

15.  

16. img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,matches,None,**draw_params)

        在python3中，如果没有outImg参数则传入None就没问题了。

变更样例：

img3 = cv2.drawMatchesKnn(img1,kp1,img2,kp2,good,None,flags=2)

 可参考文章：https://www.e-learn.cn/content/wangluowenzhang/309127

说明：问题产生的环境
  Python版本：3.6.5
  OpenCV版本：3.4.2