Ambiguous Exception with calcHist in OpenCV

Question

I've been getting the following ambiguous error when trying to use cv2.calcHist()

>>> img
array([ 1.,  2.,  3.,  4.,  5.])
>>> cv2.calcHist( [img], channels = [0], mask = np.ones(img.size), histSize = [6], ranges = [(0,6)])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: error return without exception set

The error is so generic that I have absolutely no way of knowing what could be going wrong. You can find the function documented here. I am currently running OpenCV v2.3.1 as installed through MacPorts.

Thanks!

mathematical.coffee · Accepted Answer · 2012-01-11 23:57:52Z

11

I find the online documentation a bit sparse for the Python interface. One resource I find invaluable are the samples (OpenCV-2.3.x/samples/python2), in which you can find example use of almost all the functions in the Python interface.

However looking at the documentation:

the mask argument must be 8-bit (mask.astype('uint8'))
the input img should be CV_8U or CV_32F (so img.astype('uint8') or img.astype('float32'))

And from looking at the python2 samples (camshift.py,color_histogram.py):

the ranges argument doesn't need to be a tuple, just a list (although the documentation suggests it should be a list of lists or array of arrays...)

So:

cv2.calcHist( [img.astype('float32')],                 # <-- convert to float32
              channels=[0], 
              mask=np.ones(img.size).astype('uint8'),  # <-- convert to uint8
              histSize=[6], 
              ranges=[0,6] )                           # <-- flat list

answered Jan 11, 2012 at 23:57

mathematical.coffee

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2 Comments

duckworthd Over a year ago

I didn't realize there were examples! MacPorts is pretty quiet about what it installs. In the end, I realized that it would be faster and easier to use numpy.histogram instead of OpenCV's solution.

UXkQEZ7 Over a year ago

I had the same issue. The ranges argument not being a flat list was a problem for me.

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