A Catmull-Clark polygon mesh (ccmesh) is defined very similar to a regular polygon mesh, however it is further subdivided to get a smooth limit surface.

The polygons in the ccmesh can have an arbitrary number of edges. In the subdivision process each polygon is split into quadrilaterals using generalized Catmull-Clark subdivision rules.

Similar to subdivision surfaces a fractional sharpness value can be assigned to the polygon edges and vertices can be tagged with a corner feature. The sharpness values introduces a local crease on the surface.

There are two main differences between a ccmesh and a subdivision surface. First, a ccmesh cannot define hierarchies, and second, a ccmesh can define polygons with an arbitrary number of edges, not just triangles and quads.

object "object_name" ... # flags, boxes, data, etc. groupvector listvertex listccmesh... # more surfaces [approximation list] end group end object

The *vector list* in the group is a list of *(x, y,
z)* vectors used for polygon vertex positions, texture and
motion vectors.

The *vertex list* that follows the vector list builds
control vertices from the vectors. This works like the vertex list
of polygonal geometry, except that normals should not be defined.
Here position vectors, texture
coordinates and motion vectors can be referenced.

A corner features may be associated with vertices by specifying
`corner` behind the regular vertex definition:

vvec_ref[tex_list] [ mmotion_ref] [ corner [ levellevel] ]

Here *vec_ref* is the reference of a position,
*tex_list* is a list of
texture coordinates with `t`
keywords and corresponding coordinate indices, and
*motion_ref* is an optional motion vector.
Up to 15 motion vectors may be specified for each vertex.

The optional **vertex feature** follows, and
an optional level can be specified if
the feature is active on a level above vertex definition level. The
corner vertex feature is described in more detail in a separate
section below.

The ccmesh geometry list consists of `ccmesh` statements,
much like Free-Form geometry consists of `surface`
statements. For a description of vector lists and vertex lists,
refer to page vector.

The approximation statements are very similar to the free-form
case, except that `approximate surface` is replaced by
`approximate ccmesh`. See section
approx for details.

A ccmesh specifies a name, size options and a list of polygons, followed by optional derivative request:

ccmesh "surface_name" [polygonnp_{int}vertexnv_{int}] [mesh] [derivative_request] end ccmesh

The *polygon* statement specifies the total number of
polygons which are defined in the mesh, the *vertex* statement
specifies the total number of vertices used by the polygons.

The mesh is very similar to polygons, it uses the same syntax.

p [ "material_name" ]vertex_ref_list[ creasesharpness_list]

The `p` keyword begins definition of a polygon. An
optional material name may follow, otherwise the material of the
previous polygon is assigned to the current polygon. If the current
object is marked as `tagged`, a label integer must be given
instead of the material name (this is not shown here). Polygon
vertices must be specified in the *vertex_ref_list*.

The optional `crease` statement allows specification of
crease edges. If the crease statement is given, then for each
polygon edge a floating point sharpness values follows in the
*sharpness_list*. Any edge not creased in this list must be
given with a zero sharpness.

Vertices may be tagged with a corner feature to modify the
subdivision rule. At a `corner` vertex more than two crease
or boundary edges are incident, the limit surface has no tangent
plane at this location, and the vertex is interpolated. The surface
is partitioned into smooth patches separated by the crease lines
and converge at the corner vertex.

Creases are discontinuities introduced into the surface in oder to define sharp geometric details such as wrinkles. Crease edges may be specified for edges which are not on the geometric boundary.

A vertex with exactly two incident crease edges is internally marked as a crease vertex. Normal vectors for vertices on an infinite sharp crease line are not shared; instead, two normal vectors are created for the polygons on each of the two sides on the crease line. A crease sharpness value of 0 assigned to an edge will result in smooth subdivision, a value of 1 results will generate an infinite sharp crease, and fractional values between 0 and 1 will generate smooth creases. Sharpness values of subsequent higher subdivision levels are computed using a quadratic B-Spline function applied to the sharpness values of the parent edges.

mental ray can
generate **surface
derivative** vectors for ccmesh if requested. They are
computed and stored only if requested by *derivative_request*
statements in the ccmesh definition:

derivativenumber_{int}[space_{int}]

For *number* 1 must be specified, higher derivatives are
not supported. Since a ccmesh allow arbitrary topology, there is no
intrinsic surface parameterization available, so a texture space
which defines orientation of the derivative vectors must always be
specified with the *space* number. The first `t`
vectors in the vertex definition is space 0, the second is space 1,
and so on.

In the example below a cylinder with hexagonal caps is created. Infinitely sharp creases are assigned to the cap polygons.

object "hexacylinder" group 2 0 0 1.5 -2 0 -1.5 -2 0 -2 0 0 -1.5 2 0 1.5 2 0 2 0 6 1.5 -2 6 -1.5 -2 6 -2 0 6 -1.5 2 6 1.5 2 6 v 0 v 1 v 2 v 3 v 4 v 5 v 6 v 7 v 8 v 9 v 10 v 11 ccmesh "surf1" polygon 8 vertex 36 p 0 1 2 3 4 5 crease 1 1 1 1 1 1 p 6 11 10 9 8 7 crease 1 1 1 1 1 1 p 2 1 7 8 p 1 0 6 7 p 0 5 11 6 p 5 4 10 11 p 4 3 9 10 p 3 2 8 9 end ccmesh approximate ccmesh parametric 2 "surf1" end group end objectCopyright © 1986-2011 by mental images GmbH