skimage.transform.hough_line: new parameter to set bin size in the rho dimension

[bennlich]

From #1433:

it is possible to specify a custom angle range, but not a custom distance range. Is there a good reason why one can't stipulate a distance range.

Other implementations of the hough transform seem to use a "rho_resolution" parameter that represents how many bins to use in the rho dimension of the accumulator, so I thought I'd give that a try.

I'm new to Cython and C, and discovered that I can't use np.searchsorted in the with nogil block. Any suggestions for how I would go about doing something similar in C? I'm just trying to determine what bin index of the accumulator to increment (I think it should be pretty obvious if you take a look at my changes).

[bennlich]

I think it would also make more sense to use a rho_resolution parameter that represents the width of a bin in the rho direction, rather than a num_rho parameter (as I've currently done), which represents the number of bins to use.

[stefanv]

You cannot use numpy functions inside a with nogil block, because that takes you back to the Python layer (away from C). Have a look at my Cython tutorial if you want to know more: https://github.com/stefanv/teaching

[bennlich]

@stefanv Yes, I figured that out. I'm wondering: what's a good way to accomplish np.searchsorted in C? (Or any better way of determining a bin index.)

[stefanv]

You can make a call to searchsorted, you just have to do it outside with nogil. In the end, you'll have to time the new approach at what is already there.