Half-edges

Overview ¶

	In Houdini, edges are normally treated as undirected and shared between faces. However, for some tasks (such as simplifying or cleaning geometry) it is useful to view faces as each having their own edges.
	You can think of this as splitting each shared undirected edge along its length into two half edges. (Boundary edges of course will only have one “half-edge”.)
	Each half-edge is directed (it has a start vertex and an end vertex).
	The half-edge functions let you quickly find a half-edge’s source and destination vertex, the next half-edge, get the other half-edges from the same edge, find all half-edges sharing a given point, and other manipulations.

Equivalence ¶

Two half-edges are equivalent if they are “split” from the same shared edge. In the following figure, three primitives share an edge. The half-edges e, f, and g are equivalent because they are from the same edge. Directionality does not matter – e is considered equivalent to f and g even though they have opposite directions.

The source (and destination) vertex of a half-edge uniquely identify it, meaning that there can be at most one half-edge with a given vertex as the source. This is because a vertex can belong to only one primitive. However, because several vertices can be wired to the same point, half-edges can have shared source and destination points. Another way of thinking about equivalence is that two half-edges are equivalent if their vertices are shared by the same two points.

One of a set of equivalent half-edges is considered the primary half-edge in that class. The VEX functions let you check if a given half-edge is primary, or given a non-primary half-edge, find the equivalent primary half-edge. In the case of an unshared edge, it only has a single primary half-edge.

Functions ¶

hedge_isvalid	Check whether a half-edge is valid
hedge_isprimary	Check whether a half-edge is primary
hedge_primary	Find the primary equivalent half-edge. If it belongs to an unshared edge, then the same half-edge is returned.
hedge_isequiv	Test whether two half-edges are equivalent
hedge_nextequiv	Iterate over the half-edges in an equivalence class
hedge_equivcount	Get the number of half-edges in an equivalence class
hedge_srcvertex	Get the source vertex of a half-edge
hedge_dstvertex	Get the destination vertex of a half-edge
hedge_presrcvertex	Get the vertex preceding the source vertex in half-edge primitive
hedge_postdstvertex	Get the vertex following the destination vertex in half-edge primitive
hedge_prim	Get the primitive of a half-edge
hedge_srcpoint	Get the source point of a half-edge
hedge_dstpoint	Get the destination point of a half-edge
hedge_presrcpoint	Get the pre-source point of a half-edge
hedge_postdstpoint	Get the post-destination point of a half-edge
hedge_next	Get the next half-edge in primitive
hedge_prev	Get the previous half-edge in primitive
pointedge	Find half-edge between two points
pointhedge	Find half-edge with given source and destination
pointhedge	Find a half-edge with a given source point
pointhedgenext	Iterate over half-edges with a given source point
vertexhedge	Get the half-edge with a source vertex
primhedge	Get a half-edge on a given primitive

Half-edges

Overview ¶

Equivalence ¶

Functions ¶

VEX

Language ¶

Next steps ¶

Reference ¶