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ManifoldEdges is a list of edges that have exactly two neighbor triangles, and BorderEdges is a list of edges that have exactly one neighbor triangle. These are crucial for rendering shadows using shadow volumes.
Edges with more than two neighbors are allowed. If an edge has an odd number of neighbors, it will be placed in BorderEdges. Every other pair of neighbors will be "paired" and placed as one manifold edge inside ManifoldEdges. So actually edge with exactly 1 neighbor (odd number, so makes one BorderEdges item) and edge with exactly 2 neighbors (even number, one pair of triangles, makes one item in ManifoldEdges) — they are just a special case of a general rule, that allows any neighbors number.
Note that vertexes must be consistently ordered in triangles. For two neighboring triangles, if one triangle's edge has order V0, V1, then on the neighbor triangle the order must be reversed (V1, V0). This is true in almost all situations, for example if you have a closed solid object and all outside faces are ordered consistently (all CCW or all CW). Failure to order consistently will result in edges not being "paired", i.e. we will not recognize that some 2 edges are in fact one edge between two neighboring triangles — and this will result in more edges in BorderEdges.
Both of these lists are calculated at once, i.e. when you call ManifoldEdges or BorderEdges for the 1st time, actually both ManifoldEdges and BorderEdges are calculated at once. If all edges are in ManifoldEdges, then the scene is a correct closed manifold, or rather it's composed from any number of closed manifolds.
Results of these functions are cached, and are also owned by this object. So don't modify it, don't free it.
This uses TrianglesList(false).
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