A 3-D primitive is a collection of vertices that form a single 3-D entity. The simplest primitive is a collection of points in a 3-D coordinate system, which is called a point list in Direct3D.
Often, 3-D primitives are polygons. A polygon in Direct3D is a closed 3-D figure delineated by at least three vertices. The simplest polygon is a triangle. Direct3D uses triangles to compose most of its polygons because all three of the vertices in a triangle are guaranteed to be coplanar. Rendering non-planar vertices is inefficient. You can composite triangles together to form large, complex polygons and meshes.
The following illustration shows a cube. Two triangles form each face of the cube. The entire set of triangles taken together forms one cubic primitive. You can apply textures and materials to the surfaces of primitives to make them appear to be a single solid form. For details, see Materials and Textures.
You can also use triangles to create primitives whose surfaces appear to be smooth curves. The following illustration shows how a sphere can be simulated with triangles. After a material is applied, it looks curved when it is rendered. This is especially true if using Gouraud shading. For details, see Gouraud Shading
