OFF format description

OFF Files in Antiprism

Syntax:

OFF                        # Header keyword
NVertices  NFaces  NEdges  # NEdges not used or checked

x[0]  y[0]  z[0]           # Vertices, any extra data is ignored
...
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 must begin with a header line that includes the keyword OFF;

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.

The vertex coordinates follow: 3 floating-point values. They are implicitly numbered 0 through NVertices-1.

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

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

Antiprism treats the colour (0.0, 0.0, 0.0, 0.0), completely transparent black, as a non-display color.

A value of '1' is valid in both the integer and floating point ranges, which allows more than one interpretation of some colors. The color '1 0 0' may be a near-black written for the integer range, or red written for the floating point range.

When reading a file an integer interpretation is prefered over a floating point interpretation. However, if all the integer color values in the file are either 1 or 0 then they are all converted to 0.0 and 1.0 in the floating point range.

To avoid ambiguities color values are written in the floating point range and always with a decimal point.

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