# Points and vertices

A point is simply a point in space as defined by four numbers (X, Y, Z, W).

A vertex is a reference to a point. Primitives use vertices to reference points. For example, the corners of a polygon, the center of a sphere, or a control vertex of a spline curve.

Primitives can share points, while vertices are unique to a primitive.

For example, you can have polygons that have corners at an identical location in space. The polygons may share a single point, in which case each polygon’s vertex at that corner would be a reference to the same point. Or, those vertices may each point to unique points that happen to have the same spacial location.

This is not merely an academic distinction. Unique-ing the points (giving each vertex its own point instead of sharing points at shared corners) with the Facet SOP has several important effects on polygons:

• Because OpenGL no longer computes shared point normals, the polygons are individually shaded with a flat look.

However, you can pre-compute the normals to preserve the smooth shading of the original surface.

• You can independently scale the polygon faces to create interesting effects.

If you pre-compute the normals , the individual pieces retain their shading from the original surface.

Similarly to unique-ing all points, you can use the Facet SOP to cusp polygons, unique-ing points selectively based on the angle at which the shared edges meet.

# Geometry

## Understanding

• Describes how Houdini represents geometry using details, primitives, points, vertices, and attributes.

## Modeling

• In Houdini you can not only edit the parameters of a surface node, you can reselect the geometry the operation applies to.

• Tools on the Polygon tab let you modify polygons in the scene view.

• Tools on the Model tab let you edit objects in the scene view.

## Terrain

• How to use Houdini’s height field tools to generate realistic terrain.

• A heuristic workflow using the heightfield tools, based on experience generating realistic-looking terrain.

## Fracturing

See RBD Fracturing in Dynamics.

## Next steps

• The appearance editor mode of the data tree pane lets you edit various controls for object viewport/rendered appearance in one place.

## Guru level

• You can mark a chain of compilable SOPs as a compiled block which can execute much faster in parallel.

• Many geometry (SOP) nodes allow you to use them in Python scripts to generate geometry programmatically.