Fixing self-intersections in rings

[funchal]

I have a set of unoriented rings, possibly (self-)intersecting, and I want to construct a multi_polygon from them, by just simply assuming that:

• each outer ring is a polygon.
• a ring inside of another is a hole.
• a ring inside of a hole is another polygon, etc...

I figured that, if I can convert each ring on its own to a polygon, then sym_difference should resolve intersections nicely, and give me what I want. However, sym_difference can't work with self-intersecting rings (assertion hit), so I am trying to figure out the easiest way to fix the self-intersections first.

I think the best way to handle self-intersections in my case is to split the rings, so looking around I found split_rings.hpp. However:

• this line seems to have a bug since detail::overlay::turn_operation is used without its template parameters.
• also, I understand that code is an extension, so I am asking whether there are any better/easier ways to deal with this problem. I have considered using self_turns directly.

Thanks!

[barendgehrels]

Hi,
Did you also consider dissolve ? (which is also an extension, currently)
Current released code does not provide this functionality...

[funchal]

Hi there.

Yes I did look at dissolve, but to be honest I don't really understand what exactly dissolve is doing (and my best recollection was that it also failed to compile unfortunately). Can I read about dissolve somewhere?

I think fixing split_rings itself is not very difficult and I can try to submit a patch. I also think split_rings is a very intuitive way to resolve self-intersections in rings and perhaps it should be in the main library. In contrast, I think dissolve has a few heuristics in places and may be much harder to make a case of it being general purpose, which is why I was leaning towards using split_rings. Perhaps a part of this is fundamental and due to dissolve operating on polygons instead of rings.