= Quadratic primitive shapes =

Houdini supports primitive spheres/ellipsoids, circles/ellipses, and tubes/cones. The shapes are defined by very simple mathematical parameters (for example, by height, and width for an ellipsoid) and a single center-point vertex. They can be moved, rotated, and scaled.

[Image:/images/model_primitives.png]

Quadratic primitives are extremely lightweight, and so can be used for applications such as:

* Showing hundreds of thousands or millions of copies of simple shapes, as in [particle replacement|part_replace] .

* Stand-in geometry.

* Envelope geometry (for example, for bouncing particles off of).

Many modeling tools will work on quadratic primitives, but the primitives do not have any topology so, for example, operators that act on surface points will not work with quadratic primitives.