refs

http://pythongeek.blogspot.com/2012/06/canny-edge-detection.html
https://github.com/rishimukherjee/Canny-Python/blob/master/canny-slow.py

https://code.google.com/p/python-for-matlab-users/source/browse/Examples/SciPy-Canny.py?r=65c99a42f0a901ba8aec66fd9d392a6b495c889c

http://scikit-image.org/docs/dev/auto_examples/plot_canny.html

http://www.pyimagesearch.com/2014/05/05/building-pokedex-python-opencv-perspective-warping-step-5-6/

https://nabinsharma.wordpress.com/2012/12/26/linear-hough-transform-using-python/

https://github.com/ahojnnes/numpy-snippets/blob/master/imgproc.py

http://scriptogr.am/alyssa/post/understanding-hough-transform-with-python

http://opencv-code.com/tutorials/automatic-perspective-correction-for-quadrilateral-objects/

http://stackoverflow.com/questions/14177744/how-does-perspective-transformation-work-in-pil