Outlining Geometry with Unshared Edges in Houdini

Identifying Edges using point numbers in Houdini

Houdini’s geometry is made up of points, vertices, and primitives. Sure there are edges, but the edges are actually made of points and are identified uniquely by these same points. An edge is a straight line with two points on both ends. In order to identify this edge or be able to reference it, the two points on both ends must be used.

To identify an edge that has the two point numbers 0 & 1:

p0-1

That’s the edge. This is how we reference the edge in Houdini, using the point numbers of both ends of the edge.

The edge highlighted in the screenshot on the right is being shared by two primitives, primitive 4 & 0, therefore this is a shared edge.

Outline Geometry (Orange Highlight) in Houdini

Geometry Outline

In a tutorial video titled Destruction Optimization & Delay Load & Swap, I needed to identify all the edges or points that were outlining a piece of geometry.

Outline Geometry (Orange Highlight) in Houdini

The outlining edges of a geometry means that the edges aren’t being shared by multiple primitives. This doesn’t mean the edge isn’t being used by a primitive, the edge is just being used by one primitive.

Outline Geometry created by Houdini Group with Unshared Edges (Orange Highlight)

In order to achieve this, I used Unshared Edges.