.IGES File Format – File structure

Lines and sections

In the Fixed Format, which almost every file uses, a file is a series of 80-column lines from its first character. Columns 1 to 72 hold the line’s data, column 73 a letter that names the section, and columns 74 to 80 a sequence number that starts at 1 in each section and rises by 1 per line, right-justified with space or zero fill. The sections appear in this order:

Within a section, the data belonging to one entity is a record and may run over several lines. Completely blank lines (without a sequence number) may not appear before the Terminate line and no sequenced line may follow it. Blank lines may follow it, when the sending system pads the file to a block size, and a reader ignores everything after the Terminate line. A Start Section line may contain any characters except the control characters 00 to 1F and 7F.

Data types

Six data types appear in Global and Parameter Data fields.

A blank (empty) field is a defaulted field, except inside a string or language statement. Leading blanks in numbers are ignored, trailing or embedded blanks are not allowed, and a number may not contain a comma even when the delimiter has been changed or the sender’s country writes decimals with a comma. A string or language statement may continue over lines: the count and H stay on the first line, the text runs to the last data column (72 in Global, 64 in Parameter Data) and carries on in column 1 of the next line.

Free-formatted data

The Global and Parameter Data sections are free-formatted. The parameter delimiter (Global parameter 1, default comma) ends a field and the record delimiter (parameter 2, default semicolon) ends the record. Two delimiters in a row, or a parameter delimiter followed by the record delimiter, mean an empty field. When a record delimiter comes early, every remaining required field takes its fixed or default value. The last data column of a line is not a delimiter, so a number must end at least one column earlier. A Parameter Data record may end before its two trailing pointer groups, because the counts in front of them default to zero.

The delimiters may not be control characters (00–1F, 7F), the space (20), the digits (30–39), +, -, . (2B, 2D, 2E) or the letters D, E and H (44, 45, 48). Only four combinations are allowed:

New delimiters take effect immediately, so the rest of the Global Section already uses them.

Field categories

Every field falls into one of these categories, and a reader treats an empty value accordingly.

In a fixed-width section a default is eight spaces. A numeric zero or a space-filled string such as 4H     is not a defaulted field.

Global Section

The Global Section is one delimited record of 26 parameters. Strings use the Hollerith form.

Units (parameters 14 and 15)

Version flag (parameter 23)

A reader that finds an unrecognised value below 1 assumes 3, and one above 11 assumes 11.

Appendix J of the specification gives the full citation for each reference.

Drafting standard flag (parameter 24)

Directory Entry Section

Every entity has one Directory Entry record: two lines, each of nine 8-column fields (columns 1 to 72), with values right-justified. Fields 1, 2, 10, 11, 14 and 20 may not be defaulted in the Fixed Format. Apart from fields 10, 16, 17, 18 and 20, every field is an integer or pointer. The order of the records in the section is arbitrary. Where a field offers either an attribute value or a pointer, a positive value is the attribute and a negated value is a pointer (its absolute value is the Directory Entry sequence number).

Fields are numbered 1 to 20 across the two lines. Entity descriptions use <n.a.> for a field with no meaning for that entity: it is empty or zero and a reader ignores it.

Status number (field 9)

The field is eight digits with no spaces, made of four two-digit values in this order.

When a parent is positioned in model space, the transformations apply in order: the entity’s own matrix, then the matrices of each parent in turn, any subfigure scale and position, and finally the view’s matrix, scale and drawing position. The specification’s worked example is Section 2.2.4.4.9.2.

Parameter Data Section

Each entity’s record is free-formatted, ends at column 64, has a space in column 65, the sequence number of the entity’s first Directory Entry line in columns 66 to 72, the letter P in column 73 and its own sequence number in columns 74 to 80. The first value is always the entity type number followed by a parameter delimiter, even though the entity tables do not show it. Text after the record delimiter is a comment; extra comment lines keep the same Directory Entry pointer and count in Directory Entry field 14.

After an entity’s own parameters, two optional groups of pointers may follow, written before the record delimiter. If NV is the number of the last defined parameter:

Pointers to associativities are called back pointers and appear only when the associativity’s definition requires them. A pointer to a General Note gives an entity its associated text, and a pointer to a Text Display Template gives display parameters for the first string in the entity. Either group, or both, may be empty.

Terminate Section

One line, T with sequence number 1, made of ten fields of eight columns. Each of the first four holds a section letter left-justified and the last sequence number used in that section, right-justified; leading zeros are optional.

S0000020G0000003D0000500P0000261                                        T0000001

Compressed Format

The Compressed Format is an alternative for large files that the specification says has not been widely implemented. A file in it starts with one Flag line (C in column 73, sequence number 1). The Start, Global and Terminate Sections are as in the Fixed Format, and the Directory Entry and Parameter Data sections merge into a variable-length Data section with no section letter.

• Each entity’s record starts with D and the unsigned sequence number of its Directory Entry, followed directly by zero or more field specifiers @n_value, where n is the Directory Entry field number and the value follows the underscore (the specification calls it an underscore, though its text shows a hyphen). There is no delimiter between specifiers, the group ends with the record delimiter, and it may continue on further lines but only between complete specifiers.
• Fields 2, 10, 11 and 20 are never given. A field keeps its value from the previous entity until a new one is stated, so only the first entity has a complete set.
• The Parameter Data of the entity follows on a new line, as in the Fixed Format, on lines that end before column 65.

C
(Start Section lines as in the Fixed Format)                           S
(Global Section lines as in the Fixed Format)                          G
D1@1_100 ...more field specifiers...;
(parameter data of entity 1, ending with the record delimiter)
D3@ ...;
(parameter data of entity 3)
(Terminate Section as in the Fixed Format)                             T

Not on this page

• The deprecated Binary Format (Appendix H of the specification, “shall not be used to create new files”).
• Appendix A’s part-file examples and Appendix E’s ASCII form conversion utility.

Sources

• Initial Graphics Exchange Specification, IGES 5.3 – U.S. Product Data Association (IGES/PDES Organization), 23 September 1996; the version read here is the copy US PRO published as a PDF, kept by the Internet Archive. Sections 2.1 to 2.3