how to transform a OpenCV cvMat back to ndarray in numpy ？

Question

I follow the code in OpenCV cookbook for python interface to transform cvMat to numpy array:

mat = cv.CreateMat(3,5,cv.CV_32FC1)
cv.Set(mat,7)
a = np.asarray(mat)

but with OpenCV 2.1 on my PC, it does not work. The result a here is a object array, using "print a" does not print all element in a, only print <cvmat(type=42424005 rows=3 cols=5 step=20 )>. so how to fully transform a OpenCV Mat object to original numpy.ndarray object.

Answer 1

Try using appending [:,:] to the matrix (ie. use mat[:,:] instead of mat) in your call to np.asarray - doing this will also allows asarray to work on images.

Your example:

>>> import cv
>>> import numpy as np
>>> mat = cv.CreateMat( 3 , 5 , cv.CV_32FC1 )
>>> cv.Set( mat , 7 )
>>> a = np.asarray( mat[:,:] )
>>> a
array([[ 7.,  7.,  7.,  7.,  7.],
       [ 7.,  7.,  7.,  7.,  7.],
       [ 7.,  7.,  7.,  7.,  7.]], dtype=float32)

And for an image:

>>> im = cv.CreateImage( ( 5 , 5 ) , 8 , 1 )
>>> cv.Set( im , 100 )
>>> im_array = np.asarray( im )
>>> im_array
array(<iplimage(nChannels=1 width=5 height=5 widthStep=8 )>, dtype=object)
>>> im_array = np.asarray( im[:,:] )
>>> im_array
array([[100, 100, 100, 100, 100],
       [100, 100, 100, 100, 100],
       [100, 100, 100, 100, 100],
       [100, 100, 100, 100, 100],
       [100, 100, 100, 100, 100]], dtype=uint8)

Answer 2

You are right, the cookbook example is not working either for me and I get the same output as yours (win xp, python 2.6.6, opencv 2.1., numpy 1.5.1).

Maybe you can use something similar to:

>>> mat = cv.CreateMat(3,5,cv.CV_32FC1)
>>> cv.Set(mat,7)
>>> mylist = [[mat[i,j] for i in range(3)] for j in range(5)]
>>> ar = np.array(mylist)
>>> ar
array([[ 7.,  7.,  7.],
       [ 7.,  7.,  7.],
       [ 7.,  7.,  7.],
       [ 7.,  7.,  7.],
       [ 7.,  7.,  7.]])