Cut inner ring from outer ring when edge is colinear

[e-n-f]

@springmeyer says that when mapnik-vt clips a polygon with a hole along a line that cuts through both the inner and outer ring, it turns it into a single ring with the hole cut out instead of two rings with a colinear edge, and that tippecanoe should do this too.

[e-n-f]

For what it's worth, geojson-vt doesn't do this, and leaves both rings on the edge, so if this is a requirement, it probably needs to do it too.

Example donut and as clipped by geojson-vt and (not yet merged to master) tippecanoe: https://gist.github.com/ericfischer/ea8e7c6545390871a705

cc @mourner @jakepruitt

[mourner]

@ericfischer do you mean there should be no degenerate edges at the top and the bottom in the example? This is what full clipping with StrictlySimple would do, although I don't think this is required for Earcut tessellation. "Artifacts" like this are not harmful and often expected on clipping algorithms like Sutherland-Hodgeman.

[e-n-f]

@mourner I am satisfied with the current (Sutherland-Hodgeman) behavior but got the impression from @springmeyer that StrictlySimple-style clipping to remove the degenerate edges was now required. @springmeyer can you clarify?

[mourner]

@ericfischer I mean, for guaranteed valid polygons, you have to run StrictlySimple-style clipping post-processing on all tiles. There's no point doing something specifically for the case above without addressing any other possible artifacts (mainly caused by simplification).

[e-n-f]

OK. But is strict simplicity for polygons now required, or isn't it?

[mourner]

@ericfischer it will be if we want to rip the benefits of much faster tessellation on both GL JS and GL Native.

[e-n-f]

Thanks @mourner. I'm changing tippecanoe now to use Clipper instead of my own clipping implementation.

[e-n-f]

Hmm. I am using Clipper with StrictlySimple now, and getting exactly the same result back: an outer polygon and inner hole, not two outer rings. I wonder how mapnik is seeing anything different.

[springmeyer]

Hmm. I am using Clipper with StrictlySimple now, and getting exactly the same result back: an outer polygon and inner hole, not two outer rings. I wonder how mapnik is seeing anything different.

Okay, then perhaps I'm wrong. Let's lean on @jakepruitt to confirm exactly what mapnik-vt is producing.

[e-n-f]

Doing another StrictlySimple after simplifying and scaling produces two outer rings.