[Numpy-discussion] Trying to implement the array interface
Wed Jan 14 05:17:40 CST 2009
I'm currently extending the Python wrapper for the Open Computer
Vision Library (opencv) with the goal to interface numerical libraries
as seemless as possible. Unfortunately, it doesn't seem to be that
What I've done so far:
- Added an __array_interface__ property to the Python side of OpenCV
data structures (matrices and images) that uses version 3 of the
interface definition and supplies the keys 'version', 'shape',
'typestr', 'data' and in some cases 'strides' when we have non-
continuos memory layout. I think, I'm compatible to http://numpy.scipy.org/array_interface.shtml
- Added parsing the __array_interface__ of Python objects passed to
OpenCV methods. I'm a bit unsure of how to use the C/C++ side (array
struct) and if I can expect it to be there (for example: I don't
provide one with OpenCV). Since I intend to keep OpenCV independent of
numpy, calling functions from numpy.h is not an option, as far as I
The stuff described above is in the head revision of OpenCV,
accessible via "svn co https://opencvlibrary.svn.sourceforge.net/svnroot/opencvlibrary/trunk/opencv
I've tried using the following packages with OpenCV this way:
- numpy (1.0.4): everything works as expected. This is the most
important library for OpenCV users, so this is a good sign.
- pylab/matplotlib (0.91.2): seems to use numpy / scipy-core.
- PIL (1.1.6): the array interface (Python side) doesn't adhere to the
definition -> no 'version' key, 'data' is string, not a tuple holding
the pointer. What to do with this?
- Numeric (24.2): I can create arrays from OpenCV datatypes and print
them. Converting to other types however always yields 'Cannot convert
scalar to float' or 'a float is required'. Strange :-/ Numeric.array
instances also don't carry an __array_interface__. I can however
convert by using numpy.arrays as intermediate step.
- Gnuplot (1.7): uses Numeric, so doesn't work as well
- pymat: didn't check. Seems to use Numeric, test results cover
Numeric 23 and Matlab 6.5 only, so this package might be dead?
- numarray: didn't check. Is there still any relevance of this package?
Institut für Nachrichtentechnik, RWTH Aachen University
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