polygons that cross the bbox multiple times return joined results

[Tim Channell]

I would expect 2 triangles, but I get back a single polygon. Is this expected behavior with Sutherland-Hodgman?

https://gist.github.com/tcql/ab60a9fecd8e9691d0a7

[Volodymyr Agafonkin]

Yes, this is expected behavior with Sutherland-Hodgeman. It produces degenerate edges. This is a price to pay for exceptional performance of the algorithm.

[Tim Channell]

👍 cool. Just making sure. I'll move ahead on pulling together turf-clip, then ;)

[Volodymyr Agafonkin]

@tcql awesome, looking forward to it!

[Andreas Lorer]

Hey there, I got the very same issue but actually can not find a library or another algorithm which does solve this issues problem.

Is there any solution for this problem available or do you @mourner or @tcql know of any?
I am currently on a project where we want to visualize ship routes. For doing that we want to clip a world map containing all coastlines by a bounding box (depending on the envelope of the route). When lineclip produces degenerate edges we really run in to trouble when we want to draw the clipped polygon to the html canvas. E.g. like this is the clipped output when it cuts the same polygon multiple times:

And when we want to draw it (iterating over all features of the featureCollection)

Do you know any solution to this problem? We basically would need for each clipping a new feature should be created. Then we would know when we would need to "lift the pen and go to the new feature location and then start drawing again".

For any sort of help you would help our routing project big times!

[Volodymyr Agafonkin]

@Andi-Lo the picture look more like you're trying to clip a polygon with a polyline clipping algorithm. Are you using lineclip.polygon?

[Andreas Lorer]

@mourner yes I'm using lineclip.polygon

This is my clipping function:

let clipPolygon = function(fc, bbox) {
  let points;
  let polygon;
  bbox = turf.size(turf.square(bbox), 2);
  let clipped = turf.featureCollection([]);
  turf.meta.featureEach(fc, function(feature) {
    points = turf.meta.coordAll(feature);
    if(points.length > 0)
      polygon = lineclip.polygon(points, bbox);
    if(polygon.length > 0)
      clipped.features.push(turf.lineString(polygon));
  });
  return clipped;
};

Would be happy to provide more detailed information if needed!