IGES file Specification
Contents
 1 What is IGES
 2 File Format
 2.1 File Header (Start and Global Sections
 2.2 File Data (DE and PD Sections)
 2.3 Entities
 2.3.1 Circular Arc (Type 100)
 2.3.2 Composite Curve (Type 102)
 2.3.3 Conic Arc (Type 104)
 2.3.4 Copious Data (Type 106)
 2.3.5 Plane (Type 108)
 2.3.6 Line (Type 110)
 2.3.7 Parametric Spline Curve (Type 112)
 2.3.8 Parametric Spline Surface (Type 114)
 2.3.9 Point (Type 116)
 2.3.10 Ruled Surface (Type 118)
 2.3.11 Surface of Revolution (Type 120)
 2.3.12 Tabulated Cylinder (Type 122)
 2.3.13 Direction (Type 123)
 2.3.14 Transformation Matrix (Type 124)
 2.3.15 Rational BSpline Curve (Type 126)
 2.3.16 Rational BSpline Surface (Type 128)
 2.3.17 Offset Curve (Type 130)
 2.3.18 Offset Surface (Type 140)
 2.3.19 Boundary (Type 141)
 2.3.20 Curve on a Parametric Surface (Type 142)
 2.3.21 Bounded Surface (Type 143)
 2.3.22 Trimmed Surface (Type 144)
 2.3.23 Block (Type 150)
 2.3.24 Right Angular Wedge (Type 152)
 2.3.25 Right Circular Cylinder (Type 154)
 2.3.26 Right Circular Cone (Type 156)
 2.3.27 Sphere (Type 158)
 2.3.28 Torus (Type 160)
 2.3.29 Manifold Solid BRep Object (Type 186)
 2.3.30 Plane Surface (Type 190)
 2.3.31 Right Circular Cylindrical Surface (Type 192)
 2.3.32 Right Circular Conical Surface (Type 194)
 2.3.33 Toroidal Surface (Type 198)
 2.3.34 Subfigure Definition (Type 308)
 2.3.35 Singular Subfigure Instance (Type 408)
 2.3.36 Vertex List (Type 502)
 2.3.37 Edge List (Type 504)
 2.3.38 Loop (Type 508)
 2.3.39 Face (Type 510)
 2.3.40 Shell (Type 514)
What is IGES
The Initial Graphics Exchange Specification (IGES) outlines a file format for the transfer of geometry data and CAD models. It is an older specification, and was most recently published as version 6 here. It can be used to represent both BoundaryRepresentation (BRep) and Constructive Solid Geometry (CSG) geometries, as well as two dimensional CAD diagrams.
File Format
The file is an ASCII text based format, having each line be exactly 80 characters long. As explained in the Wikipedia article on IGES, the file is split into five sections, denoted by the specific upper case letter in the 73rd column. Those sections are Start (S), Global (G), Data Entry (D), Parameter Data (P), and Terminate (T) sections. The Data Entry and Parameter Data sections are commonly abbreviated DE and PD, respectively.
File Header (Start and Global Sections
The Start and Global sections contain basic information about the name of the file and its source, the delimiters for the Parameter Data section, the author of the file, and other general information. The start field contains human readable descriptions of the file, and my have any characters in columns 172, with the line ending with the section header and section line number. There must be at least 1 line of the Start section. The global section contains preprocessor data. It also must be present in the file and end with the G000000# format. For example, here is the Start and Global sections from the example document on Wikipedia:
S 1 1H,,1H;,4HSLOT,37H$1$DUA2:[IGESLIB.BDRAFT.B2I]SLOT.IGS;, G 1 17HBravo3 BravoDRAFT,31HBravo3>IGES V3.002 (02Oct87),32,38,6,38,15, G 2 4HSLOT,1.,1,4HINCH,8,0.08,13H871006.192927,1.E06,6., G 3 31HD. A. Harrod, Tel. 313/9956333,24HAPPLICON  Ann Arbor, MI,4,0; G 4
Note that the strings are expressed in Hollerith format, meaning that every string has the number of characters it contains followed by an H directly preceding it. For example, the string IGES would be 4HIGES.
File Data (DE and PD Sections)
The Data Entry and Parameter Data sections contain the information on the basic data of the IGES file format: it’s entities. There are around 150 different defined entities in IGES (including differing ‘forms’ of some entity types). We will focus on the more common and geometry centered entities. An entity is described in the Data Entry section as shown here:
116 1 0 1 0 0 0 0 1D 1 116 1 5 1 0 0D 2
First, the file lists all of the entities it contains in the Data entry section. This section is specified by a D in the 73rd column and lists properties about the entity it describes. Each line in this section is split into 10 8 character fields, and each entity is given two lines of the section. This indicates that every entity has 20 fields in the Data Entry section, which are usually right justified. These fields map to the following properties:
Col 18  Col 916  Col 1724  Col 2532  Col 3340  Col 4148  Col 4956  Col 5764  Col 6572  Col 7380 

Entity Type  PD pointer  Structure  Line Font Pattern  Level  View  Transformation matrix pointer  Label Display Associativity  Status Number  Section Code and Sequence Number 
Entity Type  Line Weight Number  Color Number  Parameter Line Count  Form Number  Reserved  Reserved  Entity Label  Entity Subscript Number  Section Code and Sequence Number 
These fields indicate the following properties about the entity being declared:
 Entity Type This is the type of entity being described. For example, 116 describes a Point entity.
 PD pointer This gives the location for this entities data in the Parameter Data section. This location is simply the line number inside the PD section that has the first line of this entity data.
 Structure Zero or pointer to definition entity. Not applicable for most entities
 Line Font Pattern Number or pointer to line font pattern entity. Number signifies:
 0 No pattern specified (default)
 1 Solid
 2 Dashed
 3 Phantom
 4 Centerline
 5 Dotted
 Level Specifies levels to be associated with this entity. Allows entity to appear on more than one level
 View Specifies viewing options. These are:
0 Indicates equal visibility and characteristics in all views. Default Pointer to the View entity (Type 410) that it can be viewed from Reference a View Visible Associativity entity (Type 402, Form 3)
 Transformation Matrix pointer References a transformation matrix entity (Type 124) or is zero by default (no transformation)
 Label Display Associativity’’’ References a Label Display Associativity (Type 402, Form 5) which defines how the entity label appears.
 Status Number Contains four sections of two numbers. 12: Blank status. Either 00 for normal or 01 for blanked. 34: Subordinate entity switch: is 00 for independent, 01 for physically dependent, 02 for logically dependent, and 03 for both. 56: Entity Use flag: is either 00 for Geometry, 01 for annotation, 02 for definition, 03 for Other, 04 for Logical, 05 for 2D parametric, and 06 for Construction geometry. Finally, 78 is the hierarchy, where 00 indicates global top down (use this entity’s characteristics), 01 is global defer(do not use this entity’s characteristics), and 02 is use hierarchy property(use Hierarchy Entity (Type 406, Form 10)to determine characteristics of hierarchical grouping).
 Sequence Number Specified by D#, where # is the line number for this section (not from the top of the file). This is also used to point to this Data Entry entity.
 Entity Type See above it is specified twice per entity listing
 Line Weight Number Specifies thickness when displaying entity. Smallest is 1, 0 is default
 Color Number Specifies the entity color. Allowed integer values are:
 0 No color (default)
 1 Black
 2 Red
 3 Green
 4 Blue
 5 Yellow
 6 Magenta
 7 Cyan
 8 White
 Parameter Line Count Number Specifies the number of lines this entity takes up in the Parameter Data Section
 Form Number Indicates the form, or the representation of this entity. Changes how the parameter data is interpreted. Default is 0
 Reserved Field Not used
 Reserved Field Not used
 Entity Label Application specified identifier right justified
 Subscript Number Numeric qualifier for the entity label. Both together form a unique identifier for the entity
 Sequence Number See above. This will be D#+1, as each entity is specified on two lines.
The Parameter Data section comes right after the Data Entries, and lists the data for each respective entry. A typical entry looks something like this:
126,1,1,1,0,1,0,5.,5.,5.,5.,1.,1.,10.,0.,0.,10.,10.,0.,5.,5., 3P 3 0.,0.,1.; 3P 4
Note the corresponding Data Entry:
126 3 0 0 0 0 0 000010501D 3 126 0 0 2 0 0 0 0D 4
The parameter data section uses the delimiters specified in the Global section to list parameters for the entity. These delimiters are usually commas to separate parameters and a semicolon to end the listing. The parameter data section listing starts with the entity type followed by parameter data in columns 464. Columns 65 to 72 contain the Data Entry pointer number, which gives the index of the data entry listing for this entity (must be an odd number, as the even numbers contain the other half of the Data Entry). The last columns, 7380, contain the Sequence Number, being P#, similar to the Data Entry section.
Entities
Circular Arc (Type 100)
Simple circular arc of constant radius. Usually defined with a Transformation Matrix Entity (Type 124).
Index in list  Type of data  Name  Description 

1  REAL  Z  z displacement on XT,YT plane 
2  REAL  X  x coordinate of center 
3  REAL  Y  y coordinate of center 
4  REAL  X1  x coordinate of start 
5  REAL  Y1  y coordinate of start 
6  REAL  X2  x coordinate of end 
7  REAL  Y2  y coordinate of end 
Composite Curve (Type 102)
Groups other curves to form a composite. Can use Ordered List, Point, Connected Point, and Parameterized Curve entities.
Index in list  Type of data  Name  Description 

1  REAL  N  Number of curves comprising this entity 
2  Pointer  DE(1)  Pointer to first curve 
. . 
. . 
. . 

1+N  Pointer  DE(N)  Pointer to last curve 
Conic Arc (Type 104)
Arc defined by the equation: Axt^2 + Bxtyt + Cyt^2 + Dxt + Eyt + F = 0, with a Transformation Matrix (Entity 124). Can define an ellipse, parabola, or hyperbola.
The definitions of the terms ellipse, parabola, and hyperbola are given in terms of the quantities Q1,Q2, andQ3. These quantities are:
 A B/2 D/2   A B/2  Q1=  B/2 C E/2  Q2 =  B/2 C  Q3 = A + C  D/2 E/2 F 
A parent conic curve is:
 An ellipse if Q2 > 0 and Q1Q3 < 0.
 A hyperbola if Q2 < 0 and Q1 != 0.
 A parabola if Q2 = 0 and Q1 != 0.
Index in list  Type of data  Name  Description 

1  REAL  A  coefficient of xt^2 
2  REAL  B  coefficient of xtyt 
3  REAL  C  coefficient of yt^2 
4  REAL  D  coefficient of xt 
5  REAL  E  coefficient of yt 
6  REAL  F  scalar coefficient 
7  REAL  X1  x coordinate of start point 
8  REAL  Y1  y coordinate of start point 
9  REAL  Z1  z coordinate of start point 
10  REAL  X2  x coordinate of end point 
11  REAL  Y2  y coordinate of end point 
12  REAL  Z2  z coordinate of end point 
Copious Data (Type 106)
The Copious Data entity defines a set of points. There are three different interpretations, depending on first parameter. It is an INT, and is either 1,2, or 3. 1 Indicates that the points are couples(x,y), 2 indicates the points are triples (x,y,z), and 3 indicates the points are sextuplets(x,y,z,i,j,k). If the format is 1, then the first parameter after gives the common Z value for the ordered xy pairs.
Index in list  Type of data  Name  Description 

1  INT  Type  Either 1,2, or 3 
2  REAL  Z / XP1  If 1 is above, common z if 2 or 3, first value 
. . 
. . 
. . 

N  REAL  YPN / ZPN / KPN  Last value for last point 
Plane (Type 108)
Defines a plane by Ax + By +Cz = D, and a curve pointer that gives the plane its bounds. Also gives a display symbol at a specified vertex and with a specified size.
Index in list  Type of data  Name  Description 

1  REAL  A  coefficient of x 
2  REAL  B  coefficient of y 
3  REAL  C  coefficient of z 
4  REAL  D  scalar coefficient 
5  Pointer  Bounds  Pointer to bounding curve 
6  REAL  X  x coordinate of display symbol 
7  REAL  Y  y coordinate of display symbol 
8  REAL  Z  z coordinate of display symbol 
11  REAL  Size  size of display symbol 
Line (Type 110)
Defines a line using an end point and a start point
Index in list  Type of data  Name  Description 

1  REAL  X1  x coordinate of start point 
2  REAL  Y1  y coordinate of start point 
3  REAL  Z1  z coordinate of start point 
4  REAL  X2  x coordinate of end point 
5  REAL  Y2  y coordinate of end point 
6  REAL  Z2  z coordinate of end point 
Parametric Spline Curve (Type 112)
Defines a curve as a series of parametric polynomials, given as Ax(i) + sBx(i) + s^2 Cx(i) + s^3 Dx(i), for the x component in the i'th section. The same function is used for y and z.
Index in list  Type of data  Name  Description 

1  INT  Type  Spline type: 1=Linear 2=Quadratic 3=Cubic 4=WilsonFowler 5=Modified WilsonFowler 6=Bspline 
2  INT  K  Degree of continuity 0=Curvature 1=Slope 2=Both 
3  INT  Dim  Dimensions (3 or 2) 
4  INT  N  Number of segments 
5  REAL  T1  First break point 
. . 
. . 
. . 

5+N  REAL  TN  Last break point 
6+N  REAL  AX1  X coordinate polynomial 
7+N  REAL  BX1  
. . 
. . 
. . 

5+13N  REAL  DZN  Last z coordinate polynomial 
6+13N  REAL  XN  Last x coordinate 
7+13N  REAL  XN'  Last n curve x derivative 
8+13N  REAL  XN''/2!  Second xn derivative 
9+13N  REAL  XN'''/3!  Third xn derivative 
10+13N  REAL  YN  Last y coordinate 
. . 
. . 
. . 

17+13N  REAL  ZN'''/3!  Third zn derivative 
Parametric Spline Surface (Type 114)
Defines a surface as a series of parametric surfaces, splitting them into a grid (i by j). They are described by the equation Ax(i,j) + sBx(i,j) + s^2 Cx(i,j) + s^3 Dx(i,j) + tEx(i,j) + tsEx(i,j) + ts^2 Fx(i,j) + ts^3 Gx(i,j) + t^2 Kx(i,j) + t^2 s Lx(i,j) + t^2 s^2 Mx(i,j) + t^2 s^3 Nx(i,j) + t^3 Px(i,j) + t^3 s Qx(i,j) + t^3 s^2 Rx(i,j) + t^3 s^2 Sx(i,j). Note that that equation is the description of the X element in the i,j section of the spline surface.
Index in list  Type of data  Name  Description 

1  INT  Type  Spline type: 1=Linear 2=Quadratic 3=Cubic 4=WilsonFowler 5=Modified WilsonFowler 6=Bspline 
2  INT  K  Patch type 0=Unspecified 1=Cartesian Product 
3  INT  M  Number of u segments 
4  INT  N  Number of v segments 
5  REAL  TU1  First break point in u 
. . 
. . 
. . 

5+M  REAL  TUM  Last break point in u 
6+M  REAL  TV1  First break point in v 
. . 
. . 
. . 

5+M+N  REAL  TVN  Last break point in v 
7+M+N  REAL  Ax(1,1)  First coefficient for i,j = 1,1 
. . 
. . 
. . 

...  REAL  Sz(M,N)  Last coefficient for i,j = M,N 
Point (Type 116)
Defines a point in 3D space.
Index in list  Type of data  Name  Description 

1  REAL  X  x coordinate of point 
2  REAL  Y  y coordinate of point 
3  REAL  Z  z coordinate of point 
4  Pointer  P  Pointer to subfigure entity, specifies display symbol 
Ruled Surface (Type 118)
This is a surface formed by sweeping over an area between defined curves. The sweep can be done by lines connecting points of equal arc length (Form 0) or equal parametric values (Form1). Valid curves would be points, lines, circles, conics, parametric splines, rational Bsplines, composite curves, or any parametric curves.
Index in list  Type of data  Name  Description 

1  Pointer  P1  pointer to first curve 
2  Pointer  P2  pointer to second curve 
3  INT  DirFlag  Direction. 0=First to first, last to last 1=First to last, last to first 
4  INT  DevFlag  Developable: 0=Possibly not 1=Yes 
Surface of Revolution (Type 120)
This solid is formed by rotating a bounded surface on a specified axis and recording the area it passes through.
Index in list  Type of data  Name  Description 

1  Pointer  Axis  Pointer to Line describing axis of rotation 
2  Pointer  Surface  Pointer to generatrix entity 
3  REAL  SA  Start angle (Rad) 
4  REAL  EA  End angle (Rad) 
Tabulated Cylinder (Type 122)
Formed by moving a line segment parallel to itself along a curve called the directrix. Curve may be any of: a line, a circular arc, a conic arc, a parametric spline curve, or a rational Bspline curve.
Index in list  Type of data  Name  Description 

1  Pointer  Curve  Pointer to directrix 
2  REAL  Lx  x coordinate of line end 
3  REAL  Ly  y coordinate of line end 
4  REAL  Lz  z coordinate of line end 
Direction (Type 123)
Gives a direction in 3 Dimensions, where x^2 + y^2 + z^2 > 0
Index in list  Type of data  Name  Description 

1  REAL  X1  x component 
2  REAL  Y1  y component 
3  REAL  Z1  z component 
Transformation Matrix (Type 124)
Transforms entities by matrix multiplication and vector addition to give a translation, as shown below:
 R11 R12 R13   T1  R=  R21 R22 R23  T =  T2  ET = RE + T, where E is the entity coordinate  R31 R32 R33   T3 
Index in list  Type of data  Name  Description 

1  REAL  R11  First row 
2  REAL  R12  .. 
3  REAL  R13  .. 
4  REAL  T1  First T vector value 
5  REAL  R21  Second row.. 
. . 
. . 
. . 

12  REAL  T3  Third T vector value 
Rational BSpline Curve (Type 126)
Composes analytic curves. Form: 0=Determined by data 1=Line 2=Circular arc 3=Elliptical arc 4=Parabolic arc 5=Hyperbolic arc
Index in list  Type of data  Name  Description 

1  INT  K  Upper index of sum 
2  INT  M  Degree of basis functions 
3  INT  Flag1  0=nonplanar, 1=planar 
4  INT  Flag2  0=open curve, 1=closed curve 
5  INT  Flag3  0=rational, 1=polynomial 
6  INT  Flag4  0=nonperiodic , 1=periodic 
7  REAL  T1  First value of knot sequence 
. . 
. . 
. . 

8+K+M  REAL  T(1+K+M)  Last value of knot sequence 
9+K+M  REAL  W0  First weight 
. . 
. . 
. . 

9+2K+M  REAL  WK  Last weight 
10+2K+M  REAL  X0  x of first control point 
. . 
. . 
. . 

12+5*K+M  REAL  ZK  z of last control point 
13+5*K+M  REAL  V0  Start parameter value 
14+5*K+M  REAL  V1  End parameter value 
14+5*K+M  REAL  XN  Unit normal x (if planar) 
15+5*K+M  REAL  YN  Unit normal y (if planar) 
16+5*K+M  REAL  ZN  Unit normal z (if planar) 
Rational BSpline Surface (Type 128)
This is a surface entity defined by multiple surfaces. The form number describes the general type: 0=determined from data, 1=Plane, 2=Right circular cylinder, 3=Cone, 4=Sphere, 5=Torus, 6=Surface of revolution, 7=Tabulated cylinder, 8=Ruled surface, 9=General quadratic surface.
Index in list  Type of data  Name  Description 

1  INT  K2  Upper index of first sum 
2  INT  K1  Upper index of second sum 
3  INT  M1  Degree of first basis functions 
4  INT  M2  Degree of second basis functions 
5  INT  Flag1  0=closed in first direction, 1=not closed 
6  INT  Flag2  0=closed in second direction, 1=not closed 
7  INT  Flag3  0=rational, 1=polynomial 
8  INT  Flag4  0=nonperiodic in first direction , 1=periodic 
9  INT  Flag5  0=nonperiodic in second direction , 1=periodic 
10  REAL  T1(0)  First value of first knot sequence 
. . 
. . 
. . 

11+K1+M1  REAL  T1(1+K1+M1)  Last value of first knot sequence 
12+K1+M1  REAL  T2(0)  First value of second knot sequence 
. . 
. . 
. . 

13+K1+M1+K2+M2  REAL  T2(1+K2+M2)  Last value of second knot sequence 
14+K1+M1+K2+M2  REAL  W(0,0)  First weight 
. . 
. . 
. . 

14+K1+K2+M1+M2+(1+K1)(1+K2)  REAL  W(K1,K2)  Last weight 
^+1  REAL  X(0,0)  x of first control point 
. . 
. . 
. . 

^+9+3(1+K1)(1+K2)  REAL  K(K1)(K2)  z of last control point 
^+1  REAL  U0  Start first parameter value 
^+1  REAL  U1  End first parameter value 
^+1  REAL  V0  Start second parameter value 
^+1  REAL  V1  End second parameter value 
Offset Curve (Type 130)
Contains the data to determine curve offsets
Index in list  Type of data  Name  Description 

1  Pointer  Curve  Curve to be offset 
2  INT  Flag1  Offset distance 1=Single value offset 2=Offset distance varying linearly 3=Offset distance as a function 
3  Pointer  Offset  Pointer to curve describing offset (Flag1=3) 
4  INT  Dim  Coordinate of offset curve to use (Flag1=3) 
5  INT  Flag2  Tapered offset type: 1=Function of arc length 2=Function of parameter (Flag1=2, 3) 
6  REAL  D1  First offset distance(Flag1=1, 2) 
7  REAL  TD1  Arc length/parameter value (Flag1=2) 
8  REAL  D2  Second offset distance 
9  REAL  TD2  Second arc length/parameter value (Flag1=2) 
10  REAL  X  X value of normal vector 
11  REAL  Y  Y value of normal vector 
12  REAL  Z  Z value of normal vector 
13  REAL  Offset curve start parameter value  
14  REAL  Offset curve end parameter value 