Graphics Reference
In-Depth Information
APPENDIX C
IGES
C.1
What Is IGES?
The goal of this appendix is not to provide an authoritative account of the IGES
product definition interface but simply to give the reader an idea of what IGES files
look like and what it takes to generate them. For anything more, the reader is referred
to the official manuals from the National Bureau of Standards. The functionality of
IGES has changed as CAD and CAGD has evolved and anyone doing serious work
with IGES would have to get the latest manual; however, the core of the standard has
stayed the same and the specific reference for the discussion here is [IGES88].
To use IGES one must write and read files that follow a very special format. There
are two basic file types consisting of a binary or ASCII format. Binary IGES files are
not readable and the less common of the two. We shall have nothing to say about
them here. The ASCII file format, which is readable and can be created using a text
editor, comes in two flavors. There is a compressed format and one that consists of
fixed 80 character lines. We shall restrict ourselves to describing the latter.
C.2
A Sample IGES File
Figure C.1 shows some excerpts of a sample IGES file in ASCII format. A “.” at the
left end of an otherwise blank line means that one or more lines in the actual file were
omitted in the sample listing. Our goal is to describe the parts in this file and a few
other aspects of IGES. This should be adequate to give the reader a good idea of what
is involved with IGES. The next two sections will discuss a few additional geometric
and nongeometric type formats.
To begin with every IGES file in ASCII format consists of a sequence of 80 char-
acter lines divided into five nonempty sections that appear in the following order:
Start section
Global section
Directory Entry section
Parameter Data section
Terminate section
