Add Planar Graph #269
Add Planar Graph #269
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Adds a PlanarGraph implementation based on a Doubly Connected Edge List (DCEL). A DCEL is a graph data structure of vertices and half-edges. Half-edges are directed edges where one segment can be represented by a pair of half-edges. Half-edges are linked so that faces can be traversed (although faces are not stored). The PlanarGraph accepts an array of segments to build it and additional edges can be added via the add_edge and add_edges methods. When edges are added, intersections are automatically recomputed and handled. When possible, existing half-edges are preserved so that geometries can reference those half-edges. Also adds GeometryGraph which is a convenient way to interface geometries to a planar graph and provides ways to find important half-edges in each geometry. I haven't done a full performance test yet, but one notable area of improvement is that everytime an edge or edges is added, all of the half-edges are re-linked when we likely only need to re-link the half-edges that originate at new vertices.
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Thank you for that complete implementation, at this point I'm just waiting for the third PR to see usage of all that!
I've already made a few comments, tried to answer you questions, and ask some questions of my own. I think I'll have to review that completely a second time just to be sure I've fully understood it.
lib/rgeo/cartesian/planar_graph.rb
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| # It is possible that the half-edge will be nil if a suitable one cannot | ||
| # be found. This indicates the polygon is likely invalid. |
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likely here is a bit scary. Is there a possibility it is valid ?
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There's no scenario I can think of where this would happen and the polygon would be valid, but I suppose that may not be the most robust test :)
…alfEdge and Cartesian::GeometryGraph
… if no block is given
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Thank you for all the reviews and help with this @BuonOmo. I'm going to merge this and get the validation checking PR up soon. |
Summary
Adds a PlanarGraph implementation based on a Doubly Connected Edge List (DCEL). A DCEL is a graph data structure of
vertices and half-edges. Half-edges are directed edges where one segment can be represented by a pair of half-edges.
Half-edges are linked so that faces can be traversed (although faces are not stored).
The PlanarGraph accepts an array of segments to build it and additional edges can be added via the add_edge and add_edges methods. When edges are added, intersections are automatically recomputed and handled. When possible, existing half-edges are preserved so that geometries can reference those half-edges.
Also adds GeometryGraph which is a convenient way to interface geometries to a planar graph and provides ways to find important half-edges in each geometry.
Algorithm. I can provide more info about it if needed. It's not too complex, just a lot of bookkeeping.
Purpose
Notes
GEOS does something slightly different where their edges are an entire linestring, not just a single edge. This reduces the size and can help simplify computing relationships between geometries, but updating and finding split edges becomes more tedious.