Using minEnclosingCircle(...) in pyopencv - what does it want?

[Drmn4ea]

Hi,
I am working on the beginnings of an open-source pick and place
system, but new to OpenCV and pyOpenCv in particular. Right now I'm
trying to find the bounding circle enclosing each object on a table by
using minEnclosingCircle(...) on the external contours returned from
findContours(img, RETR_EXTERNAL, CHAIN_APPROX_SIMPLE, Point(0,0))

Does anyone know what the pyOpenCV implementation of
minEnclosingCircle expects as input? I can't seem to find a
combination of logical input arguments that do not return, e.g:

Traceback (most recent call last):
File "C:\data\projects\pickplace\sticker\segmentation\contours.py",
line 147, in <module>
minEnclosingCircle(contour_mat, center, radius)
ArgumentError: Python argument types in
pyopencv.pyopencvext.minEnclosingCircle(Mat, Point2f, float)
did not match C++ signature:
minEnclosingCircle(cv::Mat points, cv::Point_<float> {lvalue}
center, float {lvalue} radius)



The autogenerated API document has this to say:

minEnclosingCircle( (Mat)points, (Point2f)center, (object)radius) ->
None :

C++ signature :
void minEnclosingCircle(cv::Mat,cv::Point_<float>
{lvalue},float {lvalue})


and in opencv, it returns the result in the passed-by-reference
Point2f and float passed in. The 2nd argument is pretty obvious (I
think). But passing a float for radius and expecting a return value
seems suspect for Python - aren't the basic numeric types immutable?
Also, what is the shape/dimensionality/type(s) of the Mat expected for
the first argument?

Any help would be appreciated.
Thanks!

[Minh-Tri Pham]

Hi,

I've double-checked the wrapping code and it was indeed a bug. The radius and
the center point should be output arguments rather than input arguments. I
will fix it soon. Thanks for raising the issue.

Cheers,
Minh-Tri

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[Drmn4ea]

Minh-Tri,
Thanks!!

It looks like openCV passes return values this way fairly frequently.

[gblaha]

I just posted a new (related) topic then came across this one. I've
having what seems like a similar problem with the cornerSubPix
function, which also accepts an argument with the offending {lvalue}
signature.

[Minh-Tri Pham]

I've just checked the cornerSubPix function. Although it accepts an lvalue
argument, the argument is passed by reference. Therefore the method works
just fine in this case.

By the way, the issue with the minEnclosingCircle function has been fixed in
one of recent revisions.

Cheers,
Minh-Tri