OFF Files
---------

       The conventional suffix for `OFF' files is `.off'.

       Syntax:

            [C][N][4]OFF        # Header keyword (optional)
            NVERTICES  NFACES  NEDGES   # NEdges not used or checked

            X[0]  Y[0]  Z[0]    # Vertices, possibly with colors
                           # and/or normals if `COFF' or `NOFF'
            ...
            X[NVERTICES-1]  Y[NVERTICES-1]  Z[NVERTICES-1]

                           # Faces
                           # NV = # vertices on this face
                           # V[0] ... V[NV-1]: vertex indices
                           #         in range 0..NVERTICES-1

            NV  V[0] V[1] ... V[NV-1]  COLORSPEC
            ...
                           # COLORSPEC continues past V[NV-1]
                           # to end-of-line; may be 0 to 4 numbers
                           # nothing: default
                           #  integer: colormap index
                           # 3 or 4 integers: RGB[A] values 0..255
                                # 3 or 4 floats: RGB[A] values 0..1

       `OFF' files (name for "object file format") represent collections
       of planar polygons with possibly shared vertices, a convenient way to
       describe polyhedra.  The polygons may be concave but there's no
       provision for polygons containing holes.

       An `OFF' file may begin with the keyword `OFF'; it's
       recommended but optional, as many existing files lack this keyword.

       Three ASCII integers follow: NVERTICES, NFACES, and
       NEDGES.  Thse are the number of vertices, faces, and edges,
       respectively.  Current software does not use nor check NEDGES; it
       needn't be correct but must be present.

       The vertex coordinates follow: dimension * NVERTICES
       floating-point values.  They're implicitly numbered 0 through
       NVERTICES-1.  dimension is either 3 (default) or 4 (specified by
       the key character `4' directly before `OFF' in the keyword).

       Following these are the face descriptions, typically written
       with one line per face.  Each has the form
            N  VERT1 VERT2 ... VERTN  [COLOR]
       Here N is the number of vertices on this face,
       and VERT1 through VERTN are indices into the list of
       vertices (in the range 0..NVERTICES-1).

       The optional COLOR may take several forms.  Line breaks are
       significant here: the COLOR description begins after VERTN
       and ends with the end of the line (or the next # comment).  A
       COLOR may be:

       nothing
            the default color
       one integer
            index into "the" colormap; see below
       three or four integers
            RGB and possibly alpha values in the range 0..255
       three or four floating-point numbers
            RGB and possibly alpha values in the range 0..1

       For the one-integer case, the colormap is currently read from the file
       `cmap.fmap' in Geomview's `data' directory.  Some better
       mechanism for supplying a colormap is likely someday.

       The meaning of "default color" varies.  If no face of the object has a
       color, all inherit the environment's default material color.  If some
       but not all faces have colors, the default is gray (R,G,B,A=.666).