function getEdge( a, b, map ) {

		var vertexIndexA = Math.min( a, b );
		var vertexIndexB = Math.max( a, b );

		var key = vertexIndexA + "_" + vertexIndexB;

		return map[ key ];

	}
@author zz85 / http://twitter.com/blurspline / http://www.lab4games.net/zz85/blog
	@author centerionware / http://www.centerionware.com

	Subdivision Geometry Modifier
		using Loop Subdivision Scheme

	References:
		http://graphics.stanford.edu/~mdfisher/subdivision.html
		http://www.holmes3d.net/graphics/subdivision/
		http://www.cs.rutgers.edu/~decarlo/readings/subdiv-sg00c.pdf

	Known Issues:
		- currently doesn't handle "Sharp Edges"