4
themap = cv.CreateMat(8,8,cv.CV_8UC1)
cv.SetZero(themap)
cv.Line(themap,(0,0),(7,7),(10),1,cv.CV_AA) #draw a line
print np.asarray(themap[:,:])

#######output
[[ 0  0  0  0  0  0  0  0]
 [ 0  0  0  0  0  0  0  0]
 [ 0  2  7  2  0  0  0  0]
 [ 0  0  3 10  3  0  0  0]
 [ 0  0  0  3 10  3  0  0]
 [ 0  0  0  0  2  7  2  0]
 [ 0  0  0  0  0  0  0  0]
 [ 0  0  0  0  0  0  0  0]]

However, when I change the image type to cv.CV_16UC1, the result is totally different:

themap = cv.CreateMat(8,8,cv.CV_16UC1)
cv.SetZero(themap)
cv.Line(themap,(0,0),(7,7),(10),1,cv.CV_AA) #draw a line
print np.asarray(themap[:,:])

#######output
[[10  0  0  0  0  0  0  0]
 [ 0 10  0  0  0  0  0  0]
 [ 0  0 10  0  0  0  0  0]
 [ 0  0  0 10  0  0  0  0]
 [ 0  0  0  0 10  0  0  0]
 [ 0  0  0  0  0 10  0  0]
 [ 0  0  0  0  0  0 10  0]
 [ 0  0  0  0  0  0  0 10]]

I'm totally confused by such results. Since the pixel value is only 10, there should not be overflow problems for cv.CV_8UC1 image type. Why the results of cv.Line can be so different?

The opencv in installed in /usr/local/Cellar/opencv@2/2.4.13.6_2/lib/python2.7/site-packages/cv.py with brew install opencv@2.

Haoran Shi
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  • Don't use `cv.CreateMat`. This is part of the old deprecated API. Use `np.zeros((8,8), dtype=np.uint8)` instead. – Sunreef Jun 12 '18 at 08:38

1 Answers1

1

Don't use the old, deprecated OpenCV API. Use the cv2 module instead and create your images with Numpy directly. With the following code, the result is as expected and is the same for both 8-bit and 16-bit images.

themap = np.zeros((8,8), dtype=np.uint16)                                                                                                                                           
cv2.line(themap, (0,0), (7,7), (10), lineType=cv2.LINE_AA)                                                                                                                                                 
print themap
Sunreef
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  • Thanks! The reason I test this function is that I found an old algorithm source code using this API and I need to use it. And this problem should be a bug of the deprecated openCV API? – Haoran Shi Jun 13 '18 at 03:51
  • @HaoranShi Considering that it works without problem with the new API, I'd say it's a bug of the old API. Probably some changes have been made and the deprecated functions don't work as expected anymore. For your algorithm, it should be a good exercise for you to translate it to the new API. – Sunreef Jun 13 '18 at 07:14
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    the flag should be cv2.LINE_AA – kursun Jun 22 '20 at 09:13