Add scanline algorithm for drawing polygons

[stefanv]

Implementation of a scanline algorithm for rendering polygons in response to #928

[stefanv]

Suggestions for API and placement within skimage welcome. Code may have to be turned to Cython code to compete with existing polygon drawing, even though the algorithm should be much faster for large polygons.

[ahojnnes]

I assume this was for testing purposes.

[ahojnnes]

Is there a reason why we not replace the existing function altogether? Does it produce the same result?

[JDWarner]

@ahojnnes 👍 agreed, if possible the ideal change would be a drop-in replacement. That function gets used directly or indirectly quite a bit, so existing test coverage should be very good.

[stefanv]

It depends on which one you want to replace, I guess. It gives identical results to the existing draw.polygon, but differs from morphology._pnpoly.grid_points_inside_poly. Also, it edits the image directly, so it returns a mask, not coordinates.

[JDWarner]

Hmm, perhaps both should be replaced? I haven't delved into this deeply to see if a single backend is reasonable/possible; if not (or if the overhead would be undesirable) perhaps grid_points_inside_poly should be dealt with separately.

[stefanv]

I just realised that it may very well be that the difference in these
algorithms is simply due to a (.5,.5) offset. I will investigate further.
On 14 Mar 2014 23:12, "Josh Warner" notifications@github.com wrote:

Hmm, perhaps both should be replaced? I haven't delved into this deeply to
see if a single backend is reasonable/possible; if not (or if the overhead
would be undesirable) perhaps grid_points_inside_poly should be dealt
with separately.

Reply to this email directly or view it on GitHubhttps://github.com//pull/931#issuecomment-37697017
.

[jni]

@stefanv thanks for resurrecting this! Looks great! Care to change it to rows/cols and Cythonize the loop? ;) I would also think that using lists is unnecessary. Pairs is simple enough to implement with looping, and you can pre-allocate an array of the same size as the number of segments for intersections.