How to Close Polygons and Curves in Houdini: Ends, PolyCap, and PolyFill
09 Sep 26 (1d ago)
Closing geometry in SideFX Houdini often causes confusion because "closing" means two completely different things depending on whether you are working with a 1D curve (connecting start to end) or a 3D surface (patching an open topological boundary hole).
Depending on your mesh type and topology requirements, here is the breakdown of the three primary nodes used to close polygons and curves, along with key troubleshooting steps.
1. The Ends SOP (Closing 1D Curves & Splines)
If you have an open line of points (such as a curve drawn with the Curve SOP, an imported SVG profile, or an edge chain extracted from a mesh) and want to connect the final point back to the first point:
- Append an Ends SOP.
- Set the Close U parameter to Always (or Close Unclosed).
Important Detail: Closing a curve with the Ends SOP bridges the start and end points to form a continuous closed loop, but it remains a hollow polyline border. If you need it to become an actual solid, renderable 2D polygon face, route it into a PolyFill SOP or a Planar Patch SOP downstream.
2. The PolyCap SOP (Fast Planar Capping)
When working with hollow procedural volumes (such as an open tube, an extruded font boundary, or an unclosed cylinder):
- Append a PolyCap SOP.
- By default, it automatically evaluates unshared boundary half-edges and creates a flat polygon cap over each open perimeter loop.
Best for: Clean, flat cross-sections (like pipes, architectural columns, or mechanical pins) where a simple n-gon cap is acceptable.
3. The PolyFill SOP (Advanced Boundary & Quad Meshing)
If your open boundary is irregular, non-planar, or needs clean internal edge flow for subdivision surfaces and deformation:
- Append a PolyFill SOP.
- In the Fill Mode dropdown, pick the topology layout that fits your geometry:
- Triangles / Triangle Fan: Lightweight triangulation for non-planar holes.
- Quadrilaterals: Generates quad-dominant internal topology.
- Grid Fill: Produces an even, uniform quad grid across the patch—ideal for organic surfaces or preparing geometry for deformation.
Quick Comparison: Which Node to Choose?
| Scenario | Primary SOP | Result |
|---|---|---|
| Open spline / curve chain | Ends SOP | Closed wire loop (polyline perimeter) |
| Flat cross-section hole (tube, cylinder) | PolyCap SOP | Fast planar n-gon cap |
| Irregular/non-planar hole on 3D mesh | PolyFill SOP (Triangles / Quads) | Watertight patch matching border curvature |
| Generating a face from raw point lists | Add SOP (Polygons > Closed) | Direct planar polygon face |
Troubleshooting Common Capping Issues
Inverted or Black Faces (Normal Flip)
When Houdini generates a new cap polygon, the winding order might not match adjoining primitives, causing the face to appear pitch black or shaded inside out.
- Append a Normal SOP (set to Compute Face Normals).
- Alternatively, append a Facet SOP and enable Orient Polygons to guarantee unified face winding across all connected primitives.
Generating Faces Directly from an Add SOP
If you are generating procedural shapes from scratch using an Add SOP:
- Switch to the Polygons tab.
- Choose By Group.
- Check the Closed checkbox. This instructs Houdini to treat the points as an enclosed polygon boundary and build the face in a single step.