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OFF format description

OFF Files in Antiprism

Syntax:

[C]OFF	# Header keyword
NVertices  NFaces  NEdges   # NEdges not used or checked

x[0]  y[0]  z[0]	# Vertices, possibly with normals,
			# colors if the prefix C is present.
...
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 3 (default). or 4 (specified by.

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 index numbers can be converted to color values using the off_color program.

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).

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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