4.2.5 OFF Files

The conventional suffix for OFF files is .off.

Syntax:

     [ST][C][N][4][n]OFF	# Header keyword
     [Ndim]		# Space dimension of vertices, present only if nOFF
     NVertices  NFaces  NEdges   # NEdges not used or checked
     
     x[0]  y[0]  z[0]	# Vertices, possibly with normals,
     			# colors, and/or texture coordinates, in that order,
     			# if the prefixes N, C, ST
     			# are present.
     			# If 4OFF, each vertex has 4 components,
     			# including a final homogeneous component.
     			# If nOFF, each vertex has Ndim components.
     			# If 4nOFF, each vertex has Ndim+1 components.
     ...
     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).

A [ST][C][N][n]OFF 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 (and texture coordinates or vertex colors or normals if COFF/NOFF/CNOFF/STCNOFF/etc. format).

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

     int int int int int
     3   17   5   9   0

while the same face 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