#### 4.2.7 SKEL Files

`SKEL` files represent collections of points and polylines, with shared vertices. The conventional suffix for `SKEL` files is `.skel`.

Syntax:

         [C][4][n]SKEL
         [NDim]                    # Vertex dimension, present only if nSKEL
         NVertices  NPolylines

         x[0]  y[0]  z[0]  # Vertices
                                             # if 4SKEL, each vertex has 4 components
                                             # if nSKEL, each vertex has NDim components
                                             # if C[4][n]SKEL vertex coordinates are
                                             # followed by an RGBA color specification
         ...
         x[NVertices-1]  y[NVertices-1]  z[NVertices-1]

                                 # Polylines
                                 # Nv = vertices on this polyline (1 = point)
                                 # 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 nothing, or 3 or 4 numbers.
                                 # nothing: default color
                                 # 3 or 4 floats: RGB[A] values 0..1

The syntax resembles that of `OFF` files, with a table of vertices followed by a sequence of polyline descriptions, each referring to vertices by index in the table. Each polyline has an optional color.

For `nSKEL` objects, each vertex has NDim components. For `4nSKEL` objects, each vertex has NDim+1 components; the final component is the homogeneous divisor.

A [4][n]SKEL BINARY format is accepted; See Binary format. It resembles the ASCII format in almost the way you'd expect, with 32-bit integers for all counters and vertex indices and 32-bit floats for vertex positions.

Exception: each polyline's vertex indices are followed by an integer indicating how many color components accompany it. Polyline color components must be floats, not integer values. Thus a colorless polyline with 3 vertices might be represented as

         int int int int int
         3   17   5   9   0

while the same polyline colored red might be

         int int int int int float float float float
          3  17   5   9   4   1.0   0.0   0.0   1.0