Digitization errors in small circles

[Adam Hughes]

Hi,

I was trying to measure the circularity as a function of circle radius
using the underlying circle object from skimage.draw().  I got some strange
results, and tried to outline the exercise as well as the confusing outcome
in a notebook.  Essentially, the circularity as a function of particle
radius does not seem to converge to 1.0, and in addition, at very small
radii, the circularity does not agree with what one would expect from the
circularity of a pixel (ie rectangle).

I was hoping that an expert could have a glance at the notebook and offer
some feedback?  Hoping that I made an obvious mistake or assertion
somewhere.

The notebook actually uses my "pyparty" library that I posted a while back
for drawing and storing the descriptors; however, I verified that these are
the same descriptors that one gets using skimage.labels() directly (this is
not show in the notebook).

Here's the static view:

http://nbviewer.ipython.org/github/hugadams/pyparty/blob/master/examples/Notebooks/digitization.ipynb

Thanks.
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