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4.2.4 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 section 4.1.6 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


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This document was generated by Steve M. Robbins on September, 12 2004 using texi2html