Facets Introduction

A facet is a special kind of polygon that can be made subject to the current lighting and shading environment (see Lighting and Shading). Without lighting and shading, the facet is displayed in the current colour as either a filled polygon or its boundary depending on its fill style. The facets reaction to light is defined in terms of its material properties which are used instead of its colour when lighting and shading is enabled (see Material Properties).

In the same way as a normal polygon, a facet is defined by a set of vertices in 3D space, but because of its association with lighting and shading the facet should be defined as either triangular (3 vertices) or quadrilateral (4 vertices). The  primitive is not actually limited to this number or vertices, but in order to be able to correctly calculate the true effect of light shining on the facet, the vertices should be planar (i.e. All the vertices lie in one plane), hence the preference for triangular and quadrilateral facets. It is also often true that graphics hardware is also tuned to the display of these simpler facets, giving a much improved performance when objects are limited to them.

In addition to the facet vertices, this primitive can have additional information associated with the vertices which determine the actual appearance of the facet in the current lighting and shading environment. These include non-planar normals, texture coordinates or vertex colours. These are described in the following sections.