Hi plaidfuji! I did try HoughLinesP during experimentation. I vaguely remember (since this was almost 2 years back) getting the actual line segment as a combination of multiple smaller line segments in all cases (which could then be combined to form the actual segment using some heuristic). It came down to getting the actual table line segment out which a combination morphological transformations and cv2.findContours provided (without the need for another combining step).
Interesting. I noticed you mentioned below that you're trying to get rid of OpenCV as a dependency - that's really tough. I came from a Matlab background where image processing was really well-packaged and Python is a total mess.
If you managed to vendor a small portion of OpenCV that contained image i/o, basic colorspace conversion, thresholding, scaling/rotating, shape drawing/insertion, HoughLines and findContours, I think you could release that as its own package and it would be quite popular. OpenCV is such a bloated dependency...
