Open CASCADE Technology: IGES Support

2015/6/4 Open CASCADE Technology: IGES Support

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Table of Contents

Introduction

Reading IGES

Procedure

Domain covered

Translatable entities

Attributes

Administrative data

Description of the process

Loading the IGES file

Checking the IGES

file

Setting translation

parameters

Selecting entities

Performing the IGES

file translation

Getting the

translation results

Mapping of IGES entities to

Open CASCADE Technology

shapes

Points

Curves

Surfaces

Boundary

Representation Solid

Entities

Structure Entities

Subfigures

Transformation

Matrix

Messages

Tolerance management

Values used for

tolerances during

reading IGES

Initial setting of

tolerances in

translating objects

Transfer process

Code architecture

Example

Writing IGES

Procedure

Domain covered

IGES Support

Introduction

This manual explains how to convert an IGES file to an

Open CASCADE Technology (OCCT) shape and vice versa.

It provides basic documentation on conversion. For

advanced information on conversion, see our offerings on

our web site at www.opencascade.org/support/training/

IGES files up to and including IGES version 5.3 can be read.

IGES files that are produced by this interface conform to

IGES version 5.3 (Initial Graphics Exchange Specification,

IGES 5.3. ANS US PRO/IPO-100-1996).

This manual principally deals with two OCCT classes:

The Reader class, which loads IGES files and

translates their contents to OCCT shapes,

The Writer class, which translates OCCT shapes to

IGES entities and then writes these entities to IGES

files.

File translation is performed in the programming mode, via

C++ calls, and the resulting OCCT objects are shapes.

All definitions in IGES version 5.3 are recognized but only

3D geometric entities are translated. When the processor

encounters data, which is not translated, it ignores it and

writes a message identifying the types of data, which was

not handled. This message can be written either to a log

file or to screen output.

Reading IGES

Procedure

You can translate an IGES file to an OCCT shape by

following the steps below:

1. Load the file,

2. Check file consistency,

3. Set the translation parameters,

4. Perform the file translation,

5. Fetch the results.

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Description of the process

Initializing the

process

Setting the

translation

parameters

Performing the Open

CASCADE

Technology shape

translation

Writing the IGES file

Mapping Open CASCADE

Technology shapes to IGES

entities

Curves

Surfaces

Topological entities ­

Translation in Face

mode

Topological entities ­

Translation in BRep

mode

Tolerance management

Setting resolution in

an IGES file

Code architecture

Graph of calls

Example

Using XSTEPDRAW

Setting interface

parameters

Reading IGES files

Analyzing the transferred

data

Checking file

contents

Estimating the

results of reading

IGES

Writing an IGES file

Reading from and writing to

XDE

Loading an IGES file

Checking the loaded IGES

file

Setting parameters for

translation to XDE

Performing the translation

of an IGES file to XDE

Initializing the process of

translation from XDE to

IGES

Setting parameters for

translation from XDE to

IGES

Domain covered

Translatable entities

The types of IGES entities, which can be translated, are:

Points

Lines

Curves

Surfaces

B-Rep entities

Structure entities (groups). Each entity in the group

outputs a shape. There can be a group of groups.

Subfigures. Each entity defined in a sub-figure

outputs a shape

Transformation Matrix.

Note that all non-millimeter length unit values in the IGES

file are converted to millimeters.

Attributes

Entity attributes in the Directory Entry Section of the IGES

file (such as layers, colors and thickness) are translated to

Open CASCADE Technology using XDE.

Administrative data

Administrative data, in the Global Section of the IGES file

(such as the file name, the name of the author, the date

and time a model was created or last modified) is not

translated to Open CASCADE Technology. Administrative

data can, however, be consulted in the IGES file.

Description of the process

Loading the IGES file

Before performing any other operation, you have to load

the file using the syntax below.

IGESControl_Reader reader;

IFSelect_ReturnStatus stat =

reader.ReadFile(“filename.igs”);

The loading operation only loads the IGES file into

computer memory; it does not translate it.

Checking the IGES file

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Performing the translation

of an XDE document to

IGES

Writing an IGES file

This step is not obligatory. Check the loaded file with:

Standard_Boolean ok = reader.Check(Standard_True);

The variable “ok is True” is returned if no fail message was

found; “ok is False” is returned if there was at least one fail

message.

reader.PrintCheckLoad (failsonly, mode);

Error messages are displayed if there are invalid or incomplete IGES entities, giving you information on

the cause of the error.

Standard_Boolean failsonly = Standard_True or Standard_False;

If you give True, you will see fail messages only. If you give False, you will see both fail and warning

messages.

Your analysis of the file can be either message-oriented or entity-oriented. Choose your preference with

IFSelect_PrintCount mode = IFSelect_xxx, where xxx can be any of the following:

ItemsByEntity gives a sequential list of all messages per IGES entity.

CountByItem gives the number of IGES entities with their types per message.

ShortByItem gives the number of IGES entities with their types per message and displays rank

numbers of the first five IGES entities per message.

ListByItem gives the number of IGES entities with their type and rank numbers per message.

EntitiesByItem gives the number of IGES entities with their types, rank numbers and Directory

Entry numbers per message.

Setting translation parameters

The following parameters can be used to translate an IGES file to an OCCT shape. If you give a value

that is not within the range of possible values, it will be ignored.

read.iges.bspline.continuity

manages the continuity of BSpline curves (IGES entities 106, 112 and 126) after translation to Open

CASCADE Technology (Open CASCADE Technology requires that the curves in a model be at least C1

continuous; no such requirement is made by IGES).

0: no change; the curves are taken as they are in the IGES file. C0 entities of Open CASCADE

Technology may be produced.

1: if an IGES BSpline, Spline or CopiousData curve is C0 continuous, it is broken down into pieces

of C1 continuous Geom_BSplineCurve.

2: This option concerns IGES Spline curves only. IGES Spline curves are broken down into pieces

of C2 continuity. If C2 cannot be ensured, the Spline curves will be broken down into pieces of C1

continuity.

Read this parameter with:

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Standard_Integer ic = Interface_Static::IVal(“read.iges.bspline.continuity”);

Modify this value with:

if (!Interface_Static::SetIVal (“read.iges.bspline.continuity”,2))

.. error ..;

Default value is 1.

This parameter does not change the continuity of curves that are used in the construction of IGES BRep

entities. In this case, the parameter does not influence the continuity of the resulting OCCT curves (it is

ignored).

read.precision.mode

reads the precision value.

File (0) the precision value is read in the IGES file header (default).

User (1) the precision value is that of the read.precision.val parameter.

Read this parameter with:

Standard_Integer ic = Interface_Static::IVal(“read.precision.mode”);

Modify this value with:

if (!Interface_Static::SetIVal (“read.precision.mode”,1))

.. error ..;

Default value is File (0).

read.precision.val

User defined precision value. This parameter gives the precision for shape construction when the

read.precision.mode parameter value is 1. By default it is 0.0001, but can be any real positive (non null)

value.

This value is in the measurement unit defined in the IGES file header.

Read this parameter with:

Standard_Real rp = Interface_Static::RVal(“read.precision.val”);

Modify this parameter with:

if (!Interface_Static::SetRVal (“read.precision.val”,0.001))

.. error ..;

Default value is 0.0001.

The value given to this parameter is a target value that is applied to TopoDS_Vertex, TopoDS_Edge and

TopoDS_Face entities. The processor does its best to reach it. Under certain circumstances, the value

you give may not be attached to all of the entities concerned at the end of processing. IGES-to-OCCT

translation does not improve the quality of the geometry in the original IGES file. This means that the

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value you enter may be impossible to attain the given quality of geometry in the IGES file.

Value of tolerance used for computation is calculated by multiplying the value of read.precision.val and

the value of coefficient of transfer from the file units to millimeters.

read.maxprecision.mode

defines the mode of applying the maximum allowed tolerance. Its possible values are:

Preferred(0) maximum tolerance is used as a limit but sometimes it can be exceeded (currently,

only for deviation of a 3D curve of an edge from its pcurves and from vertices of such edge) to

ensure shape validity;

Forced(1) maximum tolerance is used as a rigid limit, i.e. it can not be exceeded and, if this

happens, tolerance is trimmed to suit the maximum-allowable value.

Read this parameter with:

Standard_Integer mv = Interface_Static::IVal(“read.maxprecision.mode”);

Modify this parameter with:

if (!Interface_Static::SetIVal (“read.maxprecision.mode”,1))

.. error ..;

Default value is Preferred (0).

read.maxprecision.val

defines the maximum allowable tolerance (in mm) of the shape. It should be not less than the basis

value of tolerance set in processor (either Resolution from the file or read.precision.val). Actually, the

maximum between read.maxprecision.val and basis tolerance is used to define maximum allowed

tolerance. Read this parameter with:

Standard_Real rp = Interface_Static::RVal(“read.maxprecision.val”);

Modify this parameter with:

if (!Interface_Static::SetRVal (“read.maxprecision.val”,0.1))

.. error ..;

Default value is 1.

read.stdsameparameter.mode

defines the using of BRepLib::SameParameter. Its possible values are:

0 (Off) - BRepLib::SameParameter is not called,

1 (On) - BRepLib::SameParameter is called. BRepLib::SameParameter is used through

ShapeFix_Edge::SameParameter. It ensures that the resulting edge will have the lowest tolerance

taking pcurves either unmodified from the IGES file or modified by BRepLib::SameParameter.

Read this parameter with:

Standard_Integer mv = Interface_Static::IVal(“read.stdsameparameter.mode”);

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Modify this parameter with:

if (!Interface_Static::SetIVal (“read.stdsameparameter.mode”,1))

.. error ..;

Deafault value is 0 (Off).

read.surfacecurve.mode

preference for the computation of curves in case of 2D/3D inconsistency in an entity which has both 2D

and 3D representations.

Here we are talking about entity types 141 (Boundary), 142 (CurveOnSurface) and 508 (Loop). These are

entities representing a contour lying on a surface, which is translated to a TopoDS_Wire, formed by

TopoDS_Edges. Each TopoDS_Edge must have a 3D curve and a 2D curve that reference the surface.

The processor also decides to re-compute either the 3D or the 2D curve even if both curves are

translated successfully and seem to be correct, in case there is inconsistency between them. The

processor considers that there is inconsistency if any of the following conditions is satisfied:

the number of sub-curves in the 2D curve is different from the number of sub-curves in the 3D

curve. This can be either due to different numbers of sub-curves given in the IGES file or because

of splitting of curves during translation.

3D or 2D curve is a Circular Arc (entity type 100) starting and ending in the same point (note that

this case is incorrect according to the IGES standard).

The parameter read.surfacecurve.mode defines which curve (3D or 2D) is used for re-computing the

other one:

Default(0) use the preference flag value in the entity’s Parameter Data section. The flag values

are:

0: no preference given,

1: use 2D for 142 entities and 3D for 141 entities,

2: use 3D for 142 entities and 2D for 141 entities,

3: both representations are equally preferred.

2DUse_Preferred (2) : the 2D is used to rebuild the 3D in case of their inconsistency,

2DUse_Forced (-2): the 2D is always used to rebuild the 3D (even if 3D is present in the file),

3DUse_Preferred (3): the 3D is used to rebuild the 2D in case of their inconsistency,

3DUse_Forced (-3): the 3D is always used to rebuild the 2D (even if 2D is present in the file),

If no preference is defined (if the value of read.surfacecurve.mode is Default and the value of the

preference flag in the entity’s Parameter Data section is 0 or 3), an additional analysis is performed.

The 3D representation is preferred to the 2D in two cases:

if 3D and 2D contours in the file have a different number of curves,

if the 2D curve is a Circular Arc (entity type 100) starting and ending in the same point and the 3D

one is not.

In any other case, the 2D representation is preferred to the 3D.

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If either a 3D or a 2D contour is absent in the file or cannot be translated, then it is re-computed from

another contour. If the translation of both 2D and 3D contours fails, the whole curve (type 141 or 142) is

not translated. If this curve is used for trimming a face, the face will be translated without this trimming

and will have natural restrictions.

Read this parameter with:

Standard_Integer ic = Interface_Static::IVal(“read.surfacecurve.mode”);

Modify this value with:

if (!Interface_Static::SetIVal (“read.surfacecurve.mode”,3))

.. error ..;

Default value is Default (0).

read.encoderegularity.angle

This parameter is used within the BRepLib::EncodeRegularity() function which is called for a shape read

from an IGES or a STEP file at the end of translation process. This function sets the regularity flag of an

edge in a shell when this edge is shared by two faces. This flag shows the continuity, which these two

faces are connected with at that edge.

Read this parameter with:

Standard_Real era = Interface_Static::RVal(“read.encoderegularity.angle”);

Modify this parameter with:

if (!Interface_Static::SetRVal (“read.encoderegularity.angle”,0.1))

.. error ..;

Default value is 0.01.

read.iges.bspline.approxd1.mode

This parameter is obsolete (it is rarely used in real practice). If set to True, it affects the translation of

bspline curves of degree 1 from IGES: these curves (which geometrically are polylines) are split by

duplicated points, and the translator attempts to convert each of the obtained parts to a bspline of a

higher continuity.

Read this parameter with:

Standard_Real bam = Interface_Static::CVal(“read.iges.bspline.approxd1.mode”);

Modify this parameter with:

if (!Interface_Static::SetRVal (“read.encoderegularity.angle”,”On”))

.. error ..;

Default value is Off.

read.iges.resource.name and read.iges.sequence

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These two parameters define the name of the resource file and the name of the sequence of operators

(defined in that file) for Shape Processing, which is automatically performed by the IGES translator. The

Shape Processing is a user-configurable step, which is performed after the translation and consists in

application of a set of operators to a resulting shape. This is a very powerful tool allowing to customize

the shape and to adapt it to the needs of a receiving application. By default, the sequence consists of a

single operator ShapeFix that calls Shape Healing from the IGES translator.

Please find an example of the resource file for IGES (which defines parameters corresponding to the

sequence applied by default, i.e. if the resource file is not found) in the Open CASCADE Technology

installation, by the path CASROOT%/src/XSTEPResource/IGES .

In order for the IGES translator to use that file, you have to define the environment variable

CSF_IGESDefaults, which should point to the directory where the resource file resides. Note that if you

change parameter read.iges.resource.name, you should change the name of the resource file and the

name of the environment variable correspondingly. The variable should contain a path to the resource

file.

Default values:

read.iges.resource.name - IGES,

read.iges.sequence - FromIGES.

read.scale.unit

This parameter is obsolete (the parameter xstep.cascade.unit should be used instead when necessary).

If it is set to ‘M’, the shape is scaled 0.001 times (as if it were in meters) after translation from IGES or

STEP.

Default value is MM.

xstep.cascade.unit

This parameter defines units to which a shape should be converted when translated from IGES or STEP

to CASCADE. Normally it is MM; only those applications that work internally in units other than MM

should use this parameter.

Default value is MM.

Selecting entities

A list of entities can be formed by invoking the method IGESControl_Reader::GiveList.

Handle(TColStd_HSequenceOfTransient) list = reader.GiveList();

Several predefined operators can be used to select a list of entities of a specific type. To make a

selection, you use the method IGESControl_Reader::GiveList with the selection type in quotation marks

as an argument. You can also make cumulative selections. For example, you would use the following

syntax:

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1. Requesting the faces in the file:

faces = Reader.GiveList(“iges-faces”);

2. Requesting the visible roots in the file:

visibles = Reader.GiveList(iges-visible-roots);

3. Requesting the visible faces:

visfac = Reader.GiveList(iges-visible-roots,faces);

Using a signature, you can define a selection dynamically, filtering the string by means of a

criterion. When you request a selection using the method GiveList, you can give either a

predefined selection or a selection by signature. You make your selection by signature using the

predefined signature followed by your criterion in parentheses as shown in the example below.

The syntaxes given are equivalent to each other.

faces = Reader.GiveList(“xst-type(SurfaceOfRevolution)”);

faces = Reader.GiveList(“iges-type(120)”);

You can also look for:

values returned by your signature which match your criterion exactly

faces = Reader.GiveList(“xst-type(=SurfaceOfRevolution)”);

values returned by your signature which do not contain your criterion

faces = Reader.GiveList(“xst-type(!SurfaceOfRevolution)”);

values returned by your signature which do not exactly match your criterion.

faces = Reader.GiveList(“xst-type(!=SurfaceOfRevolution)”);

List of predefined operators that can be used:

xst-model-all - selects all entities.

xst-model-roots - selects all roots.

xst-transferrable-all - selects all translatable entities.

xst-transferrable-roots - selects all translatable roots (default).

xst-sharing + selection - selects all entities sharing at least one entity selected by selection.

xst-shared + selection - selects all entities shared by at least one entity selected by selection.

iges-visible-roots - selects all visible roots, whether translatable or not.

iges-visible-transf-roots - selects all visible and translatable roots.

iges-blanked-roots - selects all blank roots, whether translatable or not.

iges-blanked-transf-roots - selects all blank and translatable roots.

iges-status-independant - selects entities whose IGES Subordinate Status = 0.

iges-bypass-group Selects all root entities. If a root entity is a group (402/7 or 402/9), the entities

in the group are selected.

iges-bypass-subfigure Selects all root entities. If a root entity is a subfigure definition (308), the

entities in the subfigure definition are selected.

* iges-bypass-group-subfigure* Selects all root entities. If a root entity is a group (402/7 or

402/9) or a subfigure definition (308), the entities in the group and in the subfigure definition are

selected.

iges-curves-3d - selects 3D curves, whether they are roots or not (e.g. a 3D curve on a surface).

iges-basic-geom - selects 3D curves and untrimmed surfaces.

iges-faces - selects face-supporting surfaces (trimmed or not).

iges-surfaces - selects surfaces not supporting faces (i.e. with natural bounds).

iges-basic-curves-3d selects the same entities as iges-curves-3d. Composite Curves are broken

down into their components and the components are selected.

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Performing the IGES file translation

Perform translation according to what you want to translate:

1. Translate an entity identified by its rank with:

Standard_Boolean ok = reader.Transfer (rank);

2. Translate an entity identified by its handle with:

Standard_Boolean ok = reader.TransferEntity (ent);

3. Translate a list of entities in one operation with:

Standard_Integer nbtrans = reader.TransferList (list);

reader.IsDone();

where nbtrans returns the number of items in the list that produced a shape and reader.IsDone()

indicates whether at least one entity was translated.

4. Translate a list of entities, entity by entity:

Standard_Integer i,nb = list-Length();

for (i = 1; i = nb; i ++) {

Handle(Standard_Transient) ent = list-Value(i);

Standard_Boolean OK = reader.TransferEntity (ent);

}

5. Translate the whole file (all entities or only visible entities) with:

Standard_Boolean onlyvisible = Standard_True or Standard_False;

reader.TransferRoots(onlyvisible)

Getting the translation results

Each successful translation operation outputs one shape. A series of translations gives a series of

shapes. Each time you invoke TransferEntity, Transfer or Transferlist, their results are accumulated and

NbShapes increases. You can clear the results (Clear function) between two translation operations, if

you do not do this, the results from the next translation will be added to the accumulation.

TransferRoots operations automatically clear all existing results before they start.

Standard_Integer nbs = reader.NbShapes();

returns the number of shapes recorded in the result.

TopoDS_Shape shape = reader.Shape(num);,

returns the result num, where num is an integer between 1 and NbShapes.

TopoDS_Shape shape = reader.Shape();

returns the first result in a translation operation.

TopoDS_Shape shape = reader.OneShape();

returns all results in a single shape which is:

a null shape if there are no results,

in case of a single result, a shape that is specific to that result,

a compound that lists the results if there are several results.

reader.Clear();

erases the existing results.

reader.PrintTransferInfo (failsonly, mode);

displays the messages that appeared during the last invocation of Transfer or TransferRoots.

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If failsonly is IFSelect_FailOnly, only fail messages will be output, if it is IFSelect_FailAndWarn, all

messages will be output. Parameter “mode” can have IFSelect_xxx values where xxx can be:

GeneralCount - gives general statistics on the transfer (number of translated IGES entities,

number of fails and warnings, etc)

CountByItem - gives the number of IGES entities with their types per message.

ListByItem - gives the number of IGES entities with their type and DE numbers per message.

ResultCount - gives the number of resulting OCCT shapes per type.

Mapping gives mapping between roots of the IGES file and the resulting OCCT shape per IGES and

OCCT type.

Mapping of IGES entities to Open CASCADE Technology shapes

NOTE that IGES entity types that are not given in the following tables are not translatable.

Points

IGES entity type CASCADE shape Comments

116: Point TopoDS_Vertex

Curves

Curves, which form the 2D of face boundaries, are translated as Geom2D_Curves (Geom2D circles, etc.).

IGES

entity

type

CASCADE

shape Comments

100:

Circular

Arc

TopoDS_Edge

The geometrical support is a Geom_Circle or a Geom_TrimmedCurve (if

the arc is not closed).

102:

Composite

Curve

TopoDS_Wire

The resulting shape is always a TopoDS_Wire that is built from a set of

TopoDS_Edges. Each TopoDS_Edge is connected to the preceding and

to the following edge by a common TopoDS_Vertex.

104: Conic

Arc

TopoDS_Edge

The geometric support depends on whether the IGES entity’s form is 0

(Geom_Circle), 1 (Geom_Ellipse), 2 (Geom_Hyperbola), or 3

(Geom_Parabola). A Geom_TrimmedCurve is output if the arc is not

closed.

106:

Copious

Data

TopoDS_Edge

or

TopoDS_Wire

IGES entity Copious Data (type 106, forms 1-3) is translated just as the

IGES entities Linear Path (106/11-13) and the Simple Closed Planar

Curve (106/63). Vectors applying to forms other than 11,12 or 63 are

ignored. The Geom_BSplineCurve (geometrical support) has C0

continuity. If the Copious Data has vectors (DataType = 3) they will be

ignored.

The supporting curve is a Geom_TrimmedCurve whose basis curve is a

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110: Line TopoDS_Edge Geom_Line.

112:

Parametric

Spline

Curve

TopoDS_Edge

or

TopoDS_Wire

The geometric support is a Geom_BsplineCurve.

126:

BSpline

Curve

TopoDS_Edge

or

TopoDS_Wire

130: Offset

Curve

TopoDS_Edge

or

TopoDS_Wire

The resulting shape is a TopoDS_Edge or a TopoDS_Wire (depending

on the translation of the basis curve) whose geometrical support is a

Geom_OffsetCurve built from a basis Geom_Curve. Limitation: The IGES

Offset Type value must be 1.

141:

Boundary

TopoDS_Wire

Same behavior as for the Curve On Surface (see below). The translation

of a non-referenced Boundary IGES entity in a BoundedSurface IGES

entity outputs a TopoDS_Edge or a TopoDS_Wire with a Geom_Curve.

142: Curve

On Surface

TopoDS_Wire

Each TopoDS_Edge is defined by a 3D curve and by a 2D curve that

references the surface.

The type of OCCT shapes (either TopDS_Edges or TopoDS_Wires) that result from the translation of

IGES entities 106, 112 and 126 depends on the continuity of the curve in the IGES file and the value of

the read.iges.bspline.continuity translation parameter.

Surfaces

Translation of a surface outputs either a TopoDS_Face or a TopoDS_Shell. If a TopoDS_Face is output,

its geometrical support is a Geom_Surface and its outer and inner boundaries (if it has any) are

TopoDS_Wires.

IGES

entity

type

CASCADE

shape Comments

108: Plane TopoDS_Face

The geometrical support for the TopoDS_Face is a Geom_Plane and the

orientation of its TopoDS_Wire depends on whether it is an outer

TopoDS_Wire or whether it is a hole.

114:

Parametric

Spline

Surface

TopoDS_Face The geometrical support of a TopoDS_Face is a Geom_BSplineSurface.

118: Ruled

Surface

TopoDS_Face

or

TopoDS_Shell

The translation of a Ruled Surface outputs a TopoDS_Face if the profile

curves become TopoDS_Edges, or a TopoDS_Shell if the profile curves

become TopoDS_Wires. Limitation: This translation cannot be

completed when these two TopoDS_Wires are oriented in different

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directions.

120:

Surface Of

Revolution

TopoDS_Face

or

TopoDS_Shell

The translation of a Surface Of Revolution outputs: a TopoDS_Face if

the generatrix becomes a TopoDS_Edge, a TopoDS_Shell if the

generatrix becomes a TopoDS_Wire. The geometrical support may be:

Geom_CylindricalSurface, Geom_ConicalSurface,

Geom_SphericalSurface, Geom_ToroidalSurface or a

Geom_SurfaceOfRevolution depending on the result of the CASCADE

computation (based on the generatrix type).

122:

Tabulated

Cylinder

TopoDS_Face

or

TopoDS_Shell

The translation outputs a TopoDS_Face if the base becomes a

TopoDS_Edge or a TopoDS_Shell if the base becomes a TopoDS_Wire.

The geometrical support may be Geom_Plane, Geom_Cylindrical

Surface or a Geom_SurfaceOfLinearExtrusion depending on the result

of the CASCADE computation (based on the generatrix type). The

Geom_Surface geometrical support is limited according to the

generatrix.

128:

BSpline

Surface

TopoDS_Face

The geometrical support of the TopoDS_Face is a

Geom_BsplineSurface.

140: Offset

Surface

TopoDS_Face

The translation of an Offset Surface outputs a TopoDS_Face whose

geometrical support is a Geom_OffsetSurface. Limitations: For OCCT

algorithms, the original surface must be C1-continuous so that the

Geom_OffsetSurface can be created. If the basis surface is not C1-

continuous, its translation outputs a TopoDS_Shell and only the first

TopoDS_Face in the TopoDS_Shell is offset.

143:

Bounded

Surface

TopoDS_Face

or

TopoDS_Shell

If the basis surface outputs a TopoDS_Shell (that has more than one

TopoDS_Face), the IGES boundaries are not translated. Limitations: If

the bounding curves define holes, natural bounds are not created. If the

orientation of the contours is wrong, it is not corrected.

144:

Trimmed

Surface

TopoDS_Face

or

TopoDS_Shell

For the needs of interface processing, the basis surface must be a face.

Shells are only processed if they are single-face. The contours (wires

that are correctly oriented according to the definition of the IGES 142:

Curve On Surface entity) are added to the face that is already created. If

the orientation of the contours is wrong, it is corrected.

190: Plane

Surface

TopoDS_Face

This type of IGES entity can only be used in BRep entities in place of an

IGES 108 type entity. The geometrical support of the face is a

Geom_Plane.

Boundary Representation Solid Entities

IGES entity

type

CASCADE

shape Comments

186:

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ManifoldSolid TopoDS_Solid

514: Shell TopoDS_Shell

510: Face TopoDS_Face

This is the lowest IGES entity in the BRep structure that can be

specified as a starting point for translation.

508: Loop TopoDS_Wire

504: Edge

List

502: Vertex

List

Structure Entities

IGES entity

type

CASCADE shape Comments

402/1:

Associativity

Instance:

Group with

back pointers

TopoDS_Compound

402/7:

Associativity

Instance:

Group without

back pointers

TopoDS_Compound

402/9:

Associativity

Instance:

Single Parent

TopoDS_Face

The translation of a SingleParent entity is only performed for

402 form 9 with entities 108/1 and 108/-1. The geometrical

support for the TopoDS_Face is a Geom_Plane with

boundaries: the parent plane defines the outer boundary; the

child planes define the inner boundaries.

Subfigures

IGES entity

type

CASCADE shape Comments

308: Subfigure

Definition

TopoDS_Compound

This IGES entity is only translated when there are no

Singular Subfigure Instance entities.

408: Singular

Subfigure

Instance

TopoDS_Compound

This shape has the Subfigure Definition Compound as its

origin and is positioned in space by its translation vector

and its scale factor.

Transformation Matrix

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IGES entity type CASCADE shape Comments

124:

Transformation

Matrix

Geom_Transformation

This entity is never translated alone. It must be

included in the definition of another entity.

Messages

Messages are displayed concerning the normal functioning of the processor (transfer, loading, etc.).

You must declare an include file:

includeInterface_DT.hxx

You have the choice of the following options for messages:

IDT_SetLevel (level);

level modifies the level of messages:

0: no messages

1: raise and fail messages are displayed, as are messages concerning file access,

2: warnings are also displayed.

IDT_SetFile (“tracefile.log”);

prints the messages in a file,

IDT_SetStandard();

restores screen output.

Tolerance management

Values used for tolerances during reading IGES

During the transfer of IGES to Open CASCADE Technology several parameters are used as tolerances

and precisions for different algorithms. Some of them are computed from other using specific functions.

3D (spatial) tolerances

Package method Precision::Confusion equal to 10-7 is used as a minimal distance between

points, which are considered distinct.

Resolution in the IGES file is defined in the Global section of an IGES file. It is used as a

fundamental value of precision during the transfer.

User-defined variable read.precision.val can be used instead of resolution from the file when

parameter read.precision.mode is set to 1 (“User”).

Field EpsGeom of the class IGESToBRep_CurveAndSurface is a basic precision for translating an

IGES object. It is set for each object of class IGESToBRep_CurveAndSurface and its derived

classes. It is initialized for the root of transfer either by value of resolution from the file or by value

of read.precision.val, depending on the value of read.precision.mode parameter. It is returned by

call to method IGESToBRep_CurvAndSurface::GetEpsGeom. As this value belongs to

measurement units of the IGES file, it is usually multiplied by the coefficient UnitFactor (returned

by method IGESToBRep_CurvAndSurface::GetUnitFactor) to convert it to Open CASCADE

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Technology units.

Field MaxTol of the class IGESToBRep_CurveAndSurface is used as the maximum tolerance for

some algorithms. Currently, it is computed as the maximum between 1 and

GetEpsGeom*GetUnitFactor. This field is returned by method

IGESToBRep_CurvAndSurface::GetMaxTol.

2D (parametric) tolerances

Package method Precision::PConfusion equal to 0.01*Precision::Confusion, i.e. 10-9. It is used to

compare parametric bounds of curves.

Field EpsCoeff of the class IGESToBRep_CurveAndSurface is a parametric precision for

translating an IGES object. It is set for each object of class IGESToBRep_CurveAndSurface and its

derived classes. Currently, it always has its default value 10-6. It is returned by call to method

IGESToBRep_CurvAndSurface::GetEpsCoeff. This value is used for translating 2d objects (for

instance, parametric curves).

Methods UResolution(tolerance3d) and VResolution(tolerance3d) of the class

GeomAdaptor_Surface or BRepAdaptor_Surface return tolerance in parametric space of a surface

computed from 3D tolerance. When one tolerance value is to be used for both U and V parametric

directions, the maximum or the minimum value of UResolution and VResolution is used.

Methods Resolution(tolerance3d) of the class GeomAdaptor_Curve or BRepAdaptor_Curve return

tolerance in the parametric space of a curve computed from 3d tolerance.

Zero-dimensional tolerances

Field Epsilon of the class IGESToBRep_CurveAndSurface is set for each object of class

IGESToBRep_CurveAndSurface and returned by call to method GetEpsilon. It is used in comparing

angles and converting transformation matrices. In most cases, it is reset to a fixed value (10-5 -

10-3) right before use. The default value is 10-4.

Initial setting of tolerances in translating objects

Transfer starts from one entity treated as a root (either the actual root in the IGES file or an entity

selected by the user). The function which performs the transfer (that is IGESToBRep_Actor::Transfer or

IGESToBRep_Reader::Transfer) creates an object of the type IGESToBRep_CurveAndSurface, which is

intended for translating geometry.

This object contains three tolerances: Epsilon, EpsGeom and EpsCoeff.

Parameter Epsilon is set by default to value 10-4. In most cases when it is used in the package

IGESToBRep, it is reset to a fixed value, either 10-5 or 10-4 or 10-3. It is used as precision when

comparing angles and transformation matrices and does not have influence on the tolerance of the

resulting shape.

Parameter EpsGeom is set right after creating a IGESToBRep_CurveAndSurface object to the value of

resolution, taken either from the Global section of an IGES file, or from the XSTEP.readprecision.val

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parameter, depending on the value of XSTEP.readprecision.mode.

Parameter EpsCoeff is set by default to 10-6 and is not changed.

During the transfer of a shape, new objects of type IGESToBRep_CurveAndSurface are created for

translating subshapes. All of them have the same tolerances as the root object.

Transfer process

Translating into Geometry

Geometrical entities are translated by classes IGESToBRep_BasicCurve and IGESToBRep_BasicSurface.

Methods of these classes convert curves and surfaces of an IGES file to Open CASCADE Technology

geometry objects: Geom_Curve, Geom_Surface, and Geom_Transformation.

Since these objects are not BRep objects, they do not have tolerances. Hence, tolerance parameters are

used in these classes only as precisions: to detect specific cases (e.g., to distinguish a circle, an ellipse,

a parabola and a hyperbola) and to detect bad cases (such as coincident points).

Use of precision parameters is reflected in the following classes:

IGESToBRep_BasicCurve - all parameters and points are compared with precision EpsGeom. All

transformations (except IGESToBRep_BasicCurve::TransferTransformation) are fulfilled with

precision Epsilon which is set to 10-3 (in the IGESToBRep_BasicCurve::TransferTransformation

the value 10-5 is used).

IGESToBRep_BasicCurve::TransferBSplineCurve - all weights of BSplineCurve are assumed to be

more than Precision::PConfusion (else the curve is not translated).

IGESToBRep_BasicSurface all parameters and points are compared with precision EpsGeom. All

transformations are fulfilled with precision Epsilon, which is set to 10-3.

IGESToBRep_BasicSurface::TransferBSplineSurface - all weights of BSplineSurface are assumed

to be more than Precision::PConfusion (else the surface is not translated).

Translating into Topology

IGES entities represented as topological shapes and geometrical objects are translated into OCCT

shapes by use of the classes IGESToBRep_TopoCurve, IGESToBRep_TopoSurface,

IGESToBRep_BRepEntity and ShapeFix_Wire.

Class IGESToBRep_BRepEntity is intended for transferring BRep entities (IGES version 5.1) while the two

former are used for translating geometry and topology defined in IGES 5.1. Methods from

IGESToBRep_BRepEntity call methods from IGESToBRep_TopoCurve and IGESToBRep_TopoSurface,

while those call methods from IGESToBRep_BasicCurve and IGESToBRep_BasicSurface to translate

IGES geometry into OCCT geometry.

Although the IGES file contains only one parameter for tolerance in the Global Section, OCCT shapes are

produced with different tolerances. As a rule, updating the tolerance is fulfilled according to local

distances between shapes (distance between vertices of adjacent edges, deviation of edge’s 3D curve

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and its parametric curve and so on) and may be less or greater than precision in the file.

The following classes show what default tolerances are used when creating shapes and how they are

updated during transfer.

Class IGESToBRep_TopoCurve

All methods are in charge of transferring curves from IGES curve entities *(TransferCompositeCurve,

Transfer2dCompositeCurve, TransferCurveOnFace, TransferBoundaryOnFace, TransferOffsetCurve,

TransferTopoBasicCurve)* if an entity has transformation call to

IGESData_ToolLocation::ConvertLocation with Epsilon value set to 10-4.

IGESToBRep_TopoCurve::TransferPoint - vertex is constructed from a Point entity with tolerance

EpsGeom*UnitFactor.

IGESToBRep_TopoCurve::Transfer2dPoint - vertex is constructed from a Point entity with

tolerance EpsCoeff.

IGESToBRep_TopoCurve::TransferCompositeCurveGeneral - obtains shapes (edges or wires) from

other methods and adds them into the resulting wire. Two adjacent edges of the wire can be

connected with tolerance up to MaxTol.

IGESToBRep_TopoCurve::TransferCurveOnFace and

IGESToBRep_TopoCurve::TransferBoundaryOnFace build a wire from 3D and 2D representations

of a curve on surface. Edges and vertices of the wire cannot have tolerance larger than MaxTol.

The value EpsGeom*UnitFactor is passed into ShapeFix_Wire::SetPrecision and MaxTol - into

ShapeFix_Wire::MaxTolerance. To find out how these parameters affect the resulting tolerance

changes, please, refer to class ShapeFix_Wire.

IGESToBRep_TopoCurve::TransferTopoBasicCurve and

IGESToBRep_TopoCurve::Transfer2dTopoBasicCurve - the boundary vertices of an edge (or a

wire if a curve was of C0 continuity) translated from a basis IGES curve (BSplineCurve,

CopiousData, Line, etc.) are built with tolerance EpsGeom*UnitFactor, the edge tolerance is

Precision::Confusion. If a curve was divided into several edges, the common vertices of such

adjacent edges have tolerance Precision::Confusion.

Class IGESToBRep_TopoSurface

All faces created by this class have tolerance Precision::Confusion.

Class IGESToBRep_BRepEntity

IGESToBRep_BRepEntity::TransferVertex - the vertices from the VertexList entity are constructed

with tolerance EpsGeom*UnitFactor.

IGESToBRep_BRepEntity::TransferEdge - the edges from the EdgeList entity are constructed with

tolerance Precision::Confusion.

IGESToBRep_BRepEntity::TransferLoop - this function works like

IGESToBRep_TopoCurve::TransferCurveOnFace and

IGESToBRep_TopoCurve::TransferBoundaryOnFace.

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IGESToBRep_BRepEntity::TransferFace the face from the Face IGES entity is constructed with

tolerance Precision::Confusion.

Shape Healing classes

After performing a simple mapping, shape-healing algorithms are called (class ShapeFix_Shape) by

IGESToBRep_Actor::Transfer(). Shape-healing algorithm performs the correction of the resulting OCCT

shape. Class ShapeFix_Wire can increase the tolerance of a shape. This class is used in

IGESToBRep_BRepEntity::TransferLoop, IGESToBRep_TopoCurve::TransferBoundaryOnFace and

IGESToBRep_TopoCurve::TransferCurveOnFace for correcting a wire. The maximum possible tolerance

applied to the edges or vertices after invoking the methods of this class is MaxTolerance (set by method

ShapeFix_Wire::MaxTolerance() ).

Code architecture

The following diagram illustrates the structure of calls in reading IGES. The highlighted classes produce

OCCT geometry.

The structure of calls in reading IGES

Example

include “IGESControl_Reader.hxx”

include “TColStd_HSequenceOfTransient.hxx”

include “TopoDS_Shape.hxx”

{I

GESControl_Reader myIgesReader;

Standard_Integer nIgesFaces,nTransFaces;

myIgesReader.ReadFile (“MyFile.igs”);

//loads file MyFile.igs

Handle(TColStd_HSequenceOfTransient) myList = myIgesReader.GiveList(“iges-faces”);

//selects all IGES faces in the file and puts them into a list called //MyList,

nIgesFaces = myList-Length();

nTransFaces = myIgesReader.TransferList(myList);

//translates MyList,

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cout“IGES Faces: “nIgesFaces“ Transferred:”nTransFacesendl;

TopoDS_Shape sh = myIgesReader.OneShape();

//and obtains the results in an OCCT shape.

} W

riting IGES

Procedure

You can translate OCCT shapes to IGES entities in the following steps:

1. Initialize the process.

2. Set the translation parameters,

3. Perform the model translation,

4. Write the output IGES file.

You can translate several shapes before writing a file. Each shape will be a root entity in the IGES model.

Domain covered

There are two families of OCCT objects that can be translated:

geometrical,

topological.

Description of the process

Initializing the process

Choose the unit and the mode you want to use to write the output file as follows:

IGESControl_Controller::Init performs standard initialization. Returns False if an error occurred.

IGESControl_Writer writer uses the default unit (millimeters) and the default write mode (Face).

IGESControl_Writer writer (UNIT) uses the Face write mode and any of the units that are accepted

by IGES.

IGESControl_Writer writer (UNIT,modecr) uses the unit (accepted by IGES) and the write mode of

your choice.

0: Faces,

1: BRep The result is an IGESControl_Writer object.

Setting the translation parameters

The following parameters are used for the OCCT-to-IGES translation.

write.iges.brep.mode: allows choosing the write mode:

“Faces” (0): OCCT TopoDS_Faces will be translated into IGES 144 (Trimmed Surface)

entities, no B-Rep entities will be written to the IGES file,

“BRep” (1): OCCT TopoDS_Faces will be translated into IGES 510 (Face) entities, the IGES

file will contain B-Rep entities. Read this parameter with:

Standard_Integer byvalue = Interface_Static::IVal(“write.iges.brep.mode”);

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Modify this parameter with:

Interface_Static::SetIVal (“write.iges.brep.mode”, 1);

Default value is “Faces” (0).

write.convertsurface.mode when writing to IGES in the BRep mode, this parameter indicates

whether elementary surfaces (cylindrical, conical, spherical, and toroidal) are converted into

corresponding IGES 5.3 entities (if the value of a parameter value is On), or written as surfaces of

revolution (by default).

write.iges.unit: allows choosing the unit. The default unit for Open CASCADE Technology is “MM”

(millimeter). You can choose to write a file into any unit accepted by IGES.

Read this parameter with Standard_String byvalue =

Interface_Static::CVal(“write.iges.unit”);

Modify this parameter with Interface_Static::SetCVal (“write.iges.unit”, “INCH”);

write.iges.header.autor: gives the name of the author of the file. The default value is the system

name of the user.

Read this parameter with Standard_String byvalue =

Interface_Static::CVal(“write.iges.header.author”);

Modify this value with Interface_Static::SetCVal (“write.iges.header.author”, “name”);

write.iges.header.company: gives the name of the sending company. The default value is “”

(empty).

Read this parameter with Standard_String byvalue =

Interface_Static::CVal(“write.iges.header.company”);

Modify this value with Interface_Static::SetCVal (“write.iges.header.company”, “Open

CASCADE”);

write.iges.header.product: gives the name of the sending product. The default value is “CAS.CADE

IGES processor Vx.x”, where x.x means the current version of Open CASCADE Technology.

Read this parameter with Standard_String byvalue =

Interface_Static::CVal(“write.iges.header.product”);

Modify this value with Interface_Static::SetCVal (“write.iges.header.product”, “product

name”);

write.iges.header.receiver: - gives the name of the receiving company. The default value is “”

(empty).

Read this parameter with Standard_String byvalue =

Interface_Static::CVal(“write.iges.header.receiver”);

Modify this value with Interface_Static::SetCVal (“write.iges.header.receiver”, “reciever

name”);

write.precision.mode: specifies the mode of writing the resolution value into the IGES file.

“Least” (-1): resolution value is set to the minimum tolerance of all edges and all vertices in

an OCCT shape.

“Average” (0): resolution value is set to average between the average tolerance of all edges

and the average tolerance of all vertices in an OCCT shape. This is the default value.

“Greatest” (1): resolution value is set to the maximum tolerance of all edges and all vertices

in an OCCT shape.

“Session” (2): resolution value is that of the write.precision.val parameter.

Read this parameter with Standard_Integer ic =

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Interface_Static::IVal(“write.precision.mode”);

Modify this parameter with if (!Interface_Static::SetIVal(“write.precision.mode”,1)) .. error ..

*

*write.precision.val: is the user precision value. This parameter gives the resolution value for an

IGES file when the write.precision.mode parameter value is 1. It is equal to 0.0001 by default, but

can take any real positive (non null) value.

Read this parameter with:

Standard_Real rp = Interface_Static::RVal(;write.precision.val;);

Modify this parameter with:

if (!Interface_Static::SetRVal(;write.precision.val;,0.01))

.. error ..

Default value is 0.0001.

write.iges.resource.name

and

write.iges.sequence

are the same as the corresponding read.iges.* parameters, please, see above. Note that the default

sequence for writing contains DirectFaces operator, which converts elementary surfaces based on lefthand axes (valid in CASCADE) to right-hand axes (which are valid only in IGES).

Default values :

write.iges.resource.name – IGES,

write.iges.sequence – ToIGES.

Performing the Open CASCADE Technology shape translation

You can perform the translation in one or several operations. Here is how you translate topological and

geometrical objects:

Standard_Boolean ok = writer.AddShape (TopoDS_Shape);

ok is True if translation was correctly performed and False if there was at least one entity that was not

translated.

Standard_Boolean ok = writer.AddGeom (geom);

where geom is Handle(Geom_Curve) or Handle(Geom_Surface); ok is True if the translation was

correctly performed and False if there was at least one entity whose geometry was not among the

allowed types.

Writing the IGES file

Write the IGES file with:

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Standard_Boolean ok = writer.Write (“filename.igs”);

to give the file name.

Standard_Boolean ok = writer.Write (S);

where S is Standard_OStream ok is True if the operation was correctly performed and False if an error

occurred (for instance, if the processor could not create the file).

Mapping Open CASCADE Technology shapes to IGES entities

Translated objects depend on the write mode that you chose. If you chose the Face mode, all of the

shapes are translated, but the level of topological entities becomes lower (geometrical one). If you

chose the BRep mode, topological OCCT shapes become topological IGES entities.

Curves

CASCADE shape IGES entity

type

Comments

Geom_BsplineCurve

126: BSpline

Curve

Geom_BezierCurve

126: BSpline

Curve

Geom_TrimmedCurve

All types of

translatable

IGES curves

The type of entity output depends on the type of the basis

curve. If the curve is not trimmed, limiting points will be

defined by the CASCADE RealLast value.

Geom_Circle

100: Circular

Arc or 126:

BSpline Curve

A BSpline Curve is output if the Geom_Circle is closed

Geom_Ellipse

104: Conic

Arc or 126:

BSpline Curve

A Conic Arc has Form 1. A BSpline Curve is output if the

Geom_Ellipse is closed.

Geom_Hyperbola

104: Conic

Arc

Form 2

Geom_Parabola

104: Conic

Arc

Form 3

Geom_Line 110: Line

Geom_OffsetCurve

130: Offset

Curve

Surfaces

CASCADE shapes IGES

entity type Comments

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Geom_BSplineSurface

128:

BSpline

Surface

Geom_BezierSurface

128:

BSpline

Surface

Geom_RectangularTrimmedSurface

All types of

translatable

IGES

surfaces.

The type of entity output depends on the type of

the basis surface. If the surface is not trimmed

and has infinite edges/sides, the coordinates of

the sides in IGES will be limited to the CASCADE

RealLast value.

Geom_Plane

128:

BSpline

Surface or

190: Plane

Surface

A BSpline Surface (of degree 1 in U and V) is

output if you are working in the face mode. A

Plane Surface is output if you are working in the

BRep mode.

Geom_CylindricalSurface

120:

Surface Of

Revolution

Geom_ConicalSurface

120:

Surface Of

Revolution

Geom_SphericalSurface

120:

Surface Of

Revolution

Geom_ToroidalSurface

120:

Surface Of

Revolution

Geom_SurfaceOfLinearExtrusion

122:

Tabulated

Cylinder

Geom_SurfaceOfRevolution

120:

Surface Of

Revolution

Geom_OffsetSurface

140: Offset

Surface

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Topological entities - Translation in Face mode

CASCADE shapes IGES

entity type Comments

Single

TopoDS_Vertex

116: 3D

Point

TopoDS_Vertex in a

TopoDS_Edge

No

equivalent

Not transferred.

TopoDS_Edge

All types of

translatable

IGES curves

The output IGES curve will be the one that corresponds to the

Open CASCADE Technology definition.

Single

TopoDS_Wire

102:

Composite

Curve

Each TopoDS_Edge in the TopoDS_Wire results in a curve.

TopoDS_Wire in a

TopoDS_Face

142: Curve

On Surface

Both curves (3D and pcurve) are transferred if they are defined

and result in a simple curve or a composite curve depending on

whether there is one or more edges in the wire.Note: if the basis

surface is a plane (108), only the 3D curve is used.

TopoDS_Face

144:

Trimmed

Surface

TopoDS_Shell

402: Form 1

Group or no

equivalent

Group is created only if TopoDS_Shell contains more than one

TopoDS_Face. The IGES group contains Trimmed Surfaces.

TopoDS_Solid

402: Form 1

Group or no

equivalent

Group is created only if TopoDS_Solid contains more than one

TopoDS_Shell. One IGES entity is created per TopoDS_Shell.

TopoDS_CompSolid

402: Form 1

Group or no

equivalent

Group is created only if TopoDS_CompSolid contains more than

one TopoDS_Solid. One IGES entity is created per TopoDS_Solid.

TopoDS_Compound

402: Form 1

Group or no

equivalent

Group is created only if TopoDS_Compound contains more than

one item. One IGES entity is created per TopoDS_Shape in the

TopoDS_Compound. If TopoDS_Compound is nested into

another TopoDS_Compound, it is not mapped.

Topological entities - Translation in BRep mode

CASCADE shapes

IGES

entity

type

Comments

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Single

TopoDS_Vertex

No

equivalent

Not transferred.

TopoDS_Vertex in a

TopoDS_Edge

One item

in a 502:

VertexList

TopoDS_Edge

No

equivalent

Not transferred as such. This entity serves as a part of a Loop

entity.

TopoDS_Edge in a

TopoDS_Wire

One item

in a 504:

EdgeList

TopoDS_Wire 508: Loop

TopoDS_Face 510: Face

If the geometrical support of the face is a plane, it will be

translated as a 190 entity PlaneSurface.

TopoDS_Shell 514: Shell

TopoDS_Solid

186:

Manifold

Solid

TopoDS_CompSolid

402

Form1

Group or

no

equivalent

Group is created only if TopoDS_Compound contains more than

one item. One IGES Manifold Solid is created for each

TopoDS_Solid in the TopoDS_CompSolid.

TopoDS_Compound

402

Form1

Group or

no

equivalent

Group is created only if TopoDS_Compound contains more than

one item. One IGES entity is created per TopoDS_Shape in the

TopoDS_Compound. If TopoDS_Compound is nested into another

TopoDS_Compound it is not mapped.

Tolerance management

Setting resolution in an IGES file

There are several possibilities to set resolution in an IGES file. They are controlled by

write.precision.mode parameter; the dependence between the value of this parameter and the set

resolution is described in paragraph Setting the translation parameters.

If the value of parameter write.precision.mode is -1, 0 or 1, resolution is computed from tolerances of

sub-shapes inside the shape to be translated. In this computation, only tolerances of TopoDS_Edges

and TopoDS_Vertices participate since they reflect the accuracy of the shape. TopoDS_Faces are

ignored in computations since their tolerances may have influence on resulting computed resolution

while IGES resolution mainly concerns points and curves but not surfaces.

Code architecture

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Graph of calls

The following diagram illustrates the class structure in writing IGES. The highlighted classes are

intended to translate geometry.

The class structure in writing IGES

Example

{c++}

include IGESControl_Controller.hxx

include IGESControl_Writer.hxx

include TopoDS_Shape.hxx

Standard_Integer main()

{

IGESControl_Controller::Init();

IGESControl_Writer ICW (;MM;, 0);

//creates a writer object for writing in Face mode with millimeters

TopoDS_Shape sh;

ICW.AddShape (sh);

//adds shape sh to IGES model

ICW.ComputeModel();

Standard_Boolean OK = ICW.Write (;MyFile.igs;);

//writes a model to the file MyFile.igs

} U

sing XSTEPDRAW

XSTEPDRAW UL is intended for creating executables for testing XSTEP interfaces interactively in the

DRAW environment. It provides an additional set of DRAW commands specific for the data exchange

tasks, which allow loading and writing data files and analysis of resulting data structures and shapes.

In the description of commands, square brackets ([]) are used to indicate optional parameters.

Parameters given in the angle brackets (<>) and sharps (#) are to be substituted by an appropriate

value. When several exclusive variants are possible, vertical dash (|) is used.

Setting interface parameters

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A set of parameters for importing and exporting IGES files is defined in the XSTEP resource file. In

XSTEPDRAW, these parameters can be viewed or changed using command Draw> param

[ [

]] Command param with no arguments gives a list of all parameters with their values. When argument

parameter_name is specified, information about this parameter is printed (current value and short

description).

The third argument is used to set a new value of the given parameter. The result of the setting is printed

immediately.

During all interface operations, the protocol of the process (fail and warning messages, mapping of the

loaded entities into OCCT shapes etc.) can be output to the trace file. Two parameters are defined in the

DRAW session: trace level (integer value from 0 to 9, default is 0), and trace file (default is a standard

output).

Command xtrace is intended to view and change these parameters:

Draw> xtrace - prints current settings (e.g.: “Level=0 - Standard Output”);

Draw> xtrace # - sets the trace level to the value #;

Draw> xtrace tracefile.log - sets the trace file as tracefile.log;

Draw xtrace - directs all messages to the standard output.

Reading IGES files

For a description of parameters used in reading an IGES file refer to Setting the translation parameters.

These parameters are set by command param :

Description Name Values

Precision for input entities read.precision.mode 0 or 1

read.precision.val real

Continuity of B splines read.iges.bspline.continuity 0-2

Surface curves read.surfacecurve.mode 2, 3 or 0

It is possible either only to load an IGES file into memory (i.e. to fill the model with data from the file), or

to read it (i.e. to load and convert all entities to OCCT shapes).

Loading is done by the command

Draw> xload

Once the file is loaded, it is possible to investigate the structure of the loaded data. To learn how to do it

see Analyzing the transferred

Reading of an IGES file is done by the command

Draw> igesbrep []

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Here a dot can be used instead of a filename if the file is already loaded by xload or igesbrep command.

In that case, only conversion of IGES entities to OCCT shapes will be done.

Command igesbrep will interactively ask the user to select a set of entities to be converted:

N Mode Description

0 End finish conversion and exit igesbrep

1 Visible roots convert only visible roots

2 All roots convert all roots

3 One entity convert entity with number provided by the user

4 Selection convert only entities contained in selection

After the selected set of entities is loaded the user will be asked how loaded entities should be

converted into OCCT shapes (e.g., one shape per root or one shape for all the entities). It is also possible

to save loaded shapes in files, and to cancel loading.

The second parameter of the igesbrep command defines the name of the loaded shape. If several

shapes are created, they will get indexed names. For instance, if the last parameter was ‘s’, they will be

*s_1, … s_N.

specifies the scope of selected entities in the model, it is xst-transferrable-roots by

default. An asterisk “*” can be specified instead of iges-visible-transf-roots. For possible values for

selection refer to Selecting entities section.

Instead of igesbrep it is possible to use commands:

Draw> trimport

which outputs the result of translation of each selected entity into one shape, or

Draw> trimpcomp

which outputs the result of translation of all selected entities into one shape (TopoDS_Compound for

several entities).

An asterisk “*” can be specified instead of selection, it means xst-transferrable-roots.

During the IGES translation, a map of correspondence between IGES entities and OCCT shapes is

created. The following commands are available:

Draw> tpent # * - provides information on the result of translation of the given IGES entity;

*Draw> tpdraw # - creates an OCCT shape corresponding to an IGES entity;

Draw> fromshape provides the number of an IGES entity corresponding to an

OCCT shape;

Draw> tpclear clears the map of correspondences between IGES entities and OCCT shapes.

Analyzing the transferred data

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The procedure of analysis of the data import can be divided into two stages:

1. Checking the file contents;

2. Estimation of translation results (conversion and validated ratios).

Checking file contents

General statistics on the loaded data can be obtained by using command

Draw> data

The information printed by this command depends on the symbol specified:

Symbol Output

g Prints information contained in the header of the file (Start and Global sections)

c or f

Runs check procedure of the integrity of the loaded data and prints the resulting statistics

(f works only with fails while c with both fail and warning messages)

t The same as c or f, with a list of failed or warned entities

m or l The same as t but also prints a status for each entity

e Lists all entities of the model with their numbers, types, status of validity etc.

r The same as e but lists only root entities

There is a set of special objects, which can be used to operate with the loaded model. They can be of

the following types:

Special object type Operation

Selection Filters allow selecting subsets of entities of the loaded model

Counters Calculate statistics on the model data

A list of these objects defined in the current session can be printed in DRAW by command

Draw> listitems

In the following commands if several arguments are specified the results of each following

selection are applied to the results of the previous one.

Draw> givelist []

prints a list of loaded entities defined by selection argument.

Draw> givecount []

prints a number of loaded entities defined by selection argument.

Three commands are used to calculate statistics on the entities in the model:

Draw> count [ …] - prints only a number of entities per each type matching

the criteria defined by arguments.

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Draw> sumcount [ …] - prints the total number of entities of all types

matching the criteria defined by arguments and the largest number corresponding to one type.

Draw> listcount [ …] prints a list of entities per each type matching the

criteria defined by arguments.

Optional argument, if specified, defines a subset of entities, which are to be taken into

account. Argument should be one of the currently defined counters:

Counter Operation

xst-types Calculates how much entities of each OCCT type exist

iges-types Calculates how much entities of each IGES type and form exist

iges-levels Calculates how much entities lie in different IGES levels

The command:

Draw> listtypes …

gives a list of entity types which were encountered in the last loaded file (with a number of IGES entities

of each type). The list can be shown not for all entities but for a subset of them. This subset is defined

by an optional selection argument.

Entities in the IGES file are numbered in the succeeding order. An entity can be identified either by its

number (#) or by its label. Label is the letter ‘D’ followed by the index of the first line with the data for

this entity in the Directory Entry section of the IGES file. The label can be calculated on the basis of the

number as ‘D(2*# -1)’. For example, entity # 6 has label D11.

Draw> elab # - provides a label for an entity with a known number;

Draw> enum # - prints a number for an entity with the given label;

Draw> entity # - gives the content of an IGES entity;

Draw> estat # - provides the list of entities referenced by a given entity and the list of entities

referencing to it.

Estimating the results of reading IGES

All of the following commands are available only after the data are converted into OCCT shapes (i.e.

after command igesbrep).

Draw> tpstat [*|?] []

provides all statistics on the last transfer, including the list of transferred entities with mapping from

IGES to OCCT types, as well as fail and warning messages. The parameter defines what

information will be printed:

G - General statistics (list of results and messages)

C - Count of all warning and fail messages

C - List of all warning and fail messages

F - Count of all fail messages

F - List of all fail messages

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N - List of all transferred roots

S - The same, with types of source entity and result type

B - The same, with messages

T - Count of roots for geometrical types

R - Count of roots for topological types

l - The same, with a type of the source entity

The sign ‘*’ before the parameters n, s, b, t, r makes it work on all entities (not only on roots). The sign ‘?’

before n, s, b, t limits the scope of information to invalid entities.

Optional argument can limit the action of the command with a selected subset of entities.

To get help, run this command without arguments.

For example, to get translation ratio on IGES faces, you can use.

Draw:> tpstat *l iges-faces

The second version of the same command is TPSTAT (not capital spelling).

Draw:> TPSTAT symbol

Symbol can be of the following values:

g - General statistics (list of results and messages)

c - Count of all warning and fail messages

C - List of all warning and fail messages

r - Count of resulting OCCT shapes per each type

s - Mapping of IGES roots and resulting OCCT shapes

Sometimes the trimming contours of IGES faces (i.e., entity 141 for 143, 142 for 144) can be lost during

translation due to fails.

The number of lost trims and the corresponding IGES entities can be obtained by the command:

Draw> tplosttrim []

It outputs the rank and DE numbers of faces that lost their trims and their numbers for each type (143,

144, 510) and their total number. If a face lost several of its trims it is output only once.

Optional parameter can be TrimmedSurface, BoundedSurface or Face to specify the only

type of IGES faces.

For example, to get untrimmed 144 entities, use command

Draw> tplosttrim TrimmedSurface

To get the information on OCCT shape contents, use command

Draw> statshape

It outputs the number of each kind of shapes (vertex, edge, wire, etc.) in a shape and some geometrical

data (number of C0 surfaces, curves, indirect surfaces, etc.).

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Note. The number of faces is returned as a number of references. To obtain the number of single

instances the standard command (from TTOPOLOGY executable) nbshapes can be used.

To analyze the internal validity of a shape, use command

Draw> checkbrep

It checks the geometry and topology of a shape for different cases of inconsistency, like selfintersecting wires or wrong orientation of trimming contours. If an error is found, it copies bad parts of

the shape with the names “expurged_subshape_name _#” and generates an appropriate message. If

possible, this command also tries to find IGES entities the OCCT shape was produced from.

will contain the original shape without invalid subshapes.

To get information on tolerances of subshapes, use command

Draw> tolerance [ [] []]

It outputs maximum, average and minimum values of tolerances for each kind of subshapes having

tolerances or it can output tolerances of all subshapes of the whole shape.

When specifying and arguments this command outputs shapes with names

… and their total number with tolerances in the range [min, max].

is used for specifying the kind of sub-shapes to analyze:

v - for vertices,

e - for edges,

f - for faces,

c - for shells and faces.

Writing an IGES file

Refer to Setting the translation parameters for a description of parameters used in reading an IGES file.

The parameters are set by command param:

Description Name Values

Author XSTEP.iges.header.author String

Company XSTEP.iges.header.company String

Receiver XSTEP.iges.header.receiver String

Write mode for shapes XSTEP.iges.writebrep.mode 0/Faces or 1/BRep

Measurement units XSTEP.iges.unit 1-11 (or a string value)

Several shapes can be written in one file. To start writing a new file, enter command

Draw> newmodel

This command clears the InterfaceModel to make it empty.

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Draw> brepiges [