Converting a wrapped NumPy array to a CvMat type in Python using cv.fromarray
I have a problem where some numpy arrays are not converted to cvMat using cv.fromarray (). The problem seems to occur whenever the numpy array is moved.
import numpy as np
import cv
# This works fine:
b = np.arange(6).reshape(2,3).astype('float32')
B = cv.fromarray(b)
print(cv.GetSize(B))
# But this produces an error:
a = np.arange(6).reshape(3,2).astype('float32')
b = a.T
B = cv.fromarray(b)
print(cv.GetSize(B))
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "test_err.py", line 17, in <module>
B = cv.fromarray(b)
TypeError: cv.fromarray array can only accept arrays with contiguous data
Any suggestions? Many of my arrays have been migrated at some point, so the error often occurs.
I am using Python2.7 on MacOS X Lion with NumPy 1.6.2 and OpenCV 2.4.2.1 installed from MacPorts.
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1 answer
You can check your arrays with an attribute flags.contiguous
, and if not, force them to use copy()
:
>>> a = np.arange(16).reshape(4,4)
>>> a.flags.contiguous
True
>>> b = a.T
>>> b.flags.contiguous
False
>>> b = b.copy()
>>> b.flags.contiguous
True
When you ask for a transpose, numpy doesn't actually carry the data, just the steps used to access it, unless you specifically start the copy with copy()
.
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