Okino IGES Importer ('Trimmed Surfaces')

Overview

This complex geometry import converter reads in IGES files including the most used NURBS entities, their related trim curve information, 3D point sets, independent lines, independent NURBS curves, geometry hierarchy connectivity and polygonal data. This is a "surface based" IGES importer that has been under constant development at Okino since 1989 and hence it is "rock stable" - we know IGES like the back of our hands!

!IMPORTANT! This importer was designed for "surfaced" IGES files. Most IGES files after 2003, roughly speaking, should be converted via the alternative "BREP solids IGES" format instead. Both of these variations of IGES are handled by Okino's "Granite/Pack" module which is the main back-bone of most Okino customers' CAD conversions. The Granite/Pack handles both of the surfaced IGES and BREP solids IGES file formats. You will always want to use the "BREP solids" version of IGES since it provides topological information in the file, allowing for crack-free meshing of the NURBS data.

In our experience with customers, many have come to believe that IGES is fraught with problems and that it is a sub-optimal file format for conversion of CAD data. This is very far from the truth. In this day and age all major CAD packages output clean IGES files (with Pro/Engineer creating the cleanest IGES files in all our experience). It is often the IGES reader in a program which can give IGES conversions a bad name, as a bad reader will just end up creating bad imported data. Okino's IGES importer has seen almost every conceivable bad IGES file over the years and hence has been tweaked to regonize and tweak up such odd files. One use of our IGES importer is to take a surfaced IGES file (with contents such as extrusions, surfaces of revolution, etc) and re-export it again as IGES in a cleaner and simpler form (such as just all trimmed NURBS patches).

Please note that this converter is sold as an optional add-on module to the NuGraf and PolyTrans software as part of the "CAD/Pack" or "Granite/Pack" licenses.

It has been said that their are over 200 different variations of IGES files that are currently being written by different CAD and 3D software packages. These 'flavors' of IGES, as they are typically referred by, come about because of the way each software manufacturer interprets the 620 page IGES specification. As such, a user of this converter must be careful to choose the correct settings when writing out an IGES file from another program before importing the IGES file into this converter; this steps will be outlined below.

See also

Specific information for Pro/Engineer users.
Case studies of Okino customers using PolyTrans & NuGraf in the 3D industry

How to successfully read an IGES File into this IGES Converter

As mentioned, the IGES file specification is carefully detailed in a manual of over 620 pages, many of which detail countless numbers of different geometric entities and attributes. In general only about a quarter of the specification is used for translation of 3D polygonal and NURB geometry while the remainder of the specification is not applicable (B-Rep geometry, 2d entities such as lines, hatch symbols, text entities, etc).

To successfully input an IGES file into this converter the following options should be specified in the program which is writing out the IGES file:

• Output trimmed NURB patches as entities 144 (trimmed parametric surface entity) and 142 (curve on a parametric surface). Do not output as entities 143 (bounded surface entity) and 141 (boundary entity).
• Please make sure that you specify 'uv-space' or 'parameter space' trimming curves to be output to the 144/142 trimmed surface entity as well as 'model space' trimming curves. If you do not specify 'parameter space' curves then this converter will not be able to cut holes (trim) the NURB surfaces.
• No not enable the output of any B-Rep (boundary representation) entities # 180-186.

Supported IGES Entities

The following are the supported 3D entities:

• 106 - 2D and 3D point set data (forms 1, 2, 3)
• 106 - 2D and 3D polygonal or lineset data (forms 12, 13, 14)
• 108 - Bounded plane (form 1)
• 110 - Independent line sets
• 114 - Parametric spline surface
• 118 - Ruled surface
• 120 - Surface of revolution
• 122 - Tabulated surface
• 128 - Rational B-Spline surface

All of the above entities will be converted into legal and clean trimmed NURB patches by the import converter.

For the 144 trimmed surface entity, the following 3d surface types are supported as surfaces which can be trimmed by a NURB curve:

• 108 - Bounded plane (form 0, unbounded plane)
• 114 - Parametric spline surface
• 118 - Ruled surface
• 120 - Surface of revolution
• 122 - Tabulated surface
• 128 - Rational B-Spline surface

The following are the supported 2D curve types:

• 100 - Circular arc
• 102 - Composite curve
• 104 - Conic arc
• 106/12 - Piecewise linear 3d curve (3d line segments)
• 110 - Line
• 112 - Parametric B-Spline curve
• 126 - Rational B-Spline curve

All of the above entities will be converted into legal and clean NURB trim curves by the import converter.

And the following are the remaining supported IGES entities:

• 124 - Transformation matrix. All inherited hierarchy methods supported as well.
• 308 - Subfigure definition
• 314 - Color definition
• 402 - Group associativity
• 408 - Subfigure instance
• 416 - External file reference

If any of the following 2d curve types are closed and not referenced by another entity then they will be turned into trimmed planar regions if the corresponding option is enabled on the dialog box:

• 100 - Circular arc
• 102 - Composite curve
• 104 - Conic arc
• 112 - Parametric B-Spline curve
• 126 - Rational B-Spline curve

Unsupported IGES Entities

There are many IGES entities not supported (and not required) by this converter. The main IGES entities which are not supported are:

• 108 - Forms 0 and -1, unbounded plane (unless trimmed by curve)
• 130 - Offset curve
• 140 - Offset surface
• 141/143 - Bounded surface (use 142/144 trimmed surface instead)

The main IGES entities which use CSG and B-Rep geometry, but which are not supported by this IGES reader are:

• 150-169, Constructive Solid Geometry entities
• 180-186, Manifold solid B-Rep entities

Features of this IGES Reader

• All of the major 3d geometry entities, such as tabulated surface, surface of revolution, parametric spline surface and all NURB entities are imported and converted to clean and legal trimmed NURB patches.
  
  
• The reader fully recreates the geometrical hierarchy of an IGES file via entities 308 (subfigure definitive) and 408 (subfigure instance) in an efficient manner using the instance/object capabilities of the NuGraf/PolyTrans internal database manager. This allows the IGES reader to define an object once (such as a bolt) then have it instanced many times; thus, there only has to be one copy of the geometry in memory even though there are many virtual copies of it in the scene.
  
  
• The IGES specification does not provide an efficient method to store meshed polygonal data. Rather, each single polygon is typically written to an IGES file within its own entity (#106, copious data). When reading in an IGES file, this converter will flag all of these polygons which belong to a single subfigure definition and accumulate them into an efficient 'Indexed Polygon' mesh primitive rather than create a new object for each polygon read in.
  
  
• All polygons which do not belong to a subfigure definition will be accumulated and stored in a single indexed polygon mesh with an instance name of 'lone polygons'.
  
  
• All 3D points which do not belong to a subfigure definition will be accumulated and stored in a single pointset primitive with an instance name of ‘3d pointset’.
  
  
• All lines which do not belong to a subfigure definition will be accumulated and stored in a single indexed line primitive with an instance name of ‘lone lines’.
  
  
• If a circular arc is not subordinate to another entity (ie: it is not used as a trim curve on a NURB surface) then it is turned into a planar NURB patch surface with a circular boundary. The same holds true for composite curves and conic arcs which are closed.
  
  
• External file references are handled, in a recursive manner. This allows sets of IGES files, hierarchically linked via one main "root" IGES file, to be imported and its hierarchy recreated. This is most often used for files coming from Pro/Engineer.

Dialog Box Options

The following information explains the various options on the dialog box:

Optimize Scene (** Important Option **)

Many IGES files structure their contents so that an object from the source modeling program is represented by 1 or more parametric or NURBS surfaces. For example, a single solid box with 2 holes in it will actually be stored in the IGES file as 5 trimmed NURBS patches, as shown here.

By enabling this "Optimize Scene" checkbox a series of algorithms will be invoked to combine all sub-parts contained within grouping nodes (under the yellow folder shown above) into respective single objects. In other words, the hundreds of sub-parts contained within grouping nodes will be reduced to single parts within each grouping node. In addition, once all the parts have been combined together, redundant grouping and hierarchy nodes will be deleted or merged.

This is a VERY IMPORTANT option to enable in many cases. In general you will want to keep it enabled so that the minimal number of parts and hierarchy nodes are imported into the destination program. It is disabled by default, however, because it will convert all the imported NURBS patches to polygons which may not be the behavior expected by some users who will want to re-export pure NURBS and not polygons.

NOTES:

• If you enable this option then all of the trimmed NURBS objects from the IGES file will be converted to polygons prior to the parts being combined together. If you don't enable this option then the parts will be imported unmodified as pure trimmed NURBS objects (unless they are being imported into a program such as 3DS MAX or Maya which can't handle the import of trimmed NURBS).
• This option should always be enabled when converting IGES files to Lightwave since Lightwave creates one .lwo file on disk per IGES object definition. Thus, you should always reduce the part count.

Fix Pro-E "All Levels" Assembly Import If No Geometry Imported

This fixes a problem when importing a top-level IGES file which references multiple sub-IGES files via "external reference" nodes (#416). In particular this fixes the case where an assembly has been exported from the Pro-E® modeler as "All Levels"; in this case the top-level external reference nodes are marked as "Logical Entity" and not the expected "Geometry Use Entity" (the same test file in question would not load into 4 other standard CAD modeling packages, saying that the file was empty according to the "entity use" flags defined in the top-level file).

If you import a top-level assembly IGES file and find no data in it, then enable this option and try again.

Flip Model so that Y Axis is 'Up'

If this checkbox is enabled then the model will be reoriented so that the 'Up' axis of the model will be aligned with the positive Y axis (which is used by the converter) instead of the positive 'Z' axis as used by some IGES files.

Create and Assign Colored Materials

If an entity has been assigned an explicit color in the IGES file then enabling this option will cause a NuGraf/PolyTrans compatible 'surface definition' (a material) to be created and assigned to the imported geometry which uses the assigned IGES color. If this option is not enabled then no surface definitions will be created.

Create Tessellated NURB Data While Inputting

Before a NURB patch can be displayed within the NuGraf or PolyTrans software it must first be tessellated into triangles. If you enable this checkbox then the tessellation process will be performed as the IGES file is being parsed and read into memory. If you disable the checkbox then the tessellation process will not happen until the file has been completely imported into the program.

Report Statistics about the Parsed File

If this checkbox is enabled then statistics about the parsed IGES file will be displayed in the message window after the file has been completely parsed and processed.

Convert Closed Circular Arcs Into Planar Faces

If this checkbox is enabled then any circular arc curves (which are not subordinate to another entity, such as a trimmed surface) will be converted into a solid planar NURB patch object with the outline of the original circular arc. This will allow 2D circular arcs to become 3D in nature so that they can be made visible in a rendered image. If this checkbox is not enabled then all lone circular arcs will not be processed unless they are used as trim curves of 3D entities. Please note that a circular arc will not be turned into a solid patch object if the angle of the arc is less than 355 degrees.

Convert Closed Curves into Planar Faces

If this checkbox is enabled then any closed 2D curves (which are not subordinate to another entity, such as a trimmed surface) will be converted into a solid planar NURB patch object with the outline of the original closed curve. This will allow 2D closed curves to become 3D in nature so that they can be made visible in a rendered image. If this checkbox is not enabled then all lone closed curves will not be processed unless they are used as trim curves of 3D entities. The following IGES curve entities are currently supported by this conversion routine:

• 102 - Composite curve
• 104 - Conic arc
• 112 - Parametric B-Spline curve
• 126 - Rational B-Spline curve

Import Independent 3D Curves

If this checkbox is enabled then any curves found in the file, which are not used for NURBS surface trimming and which are considered 'independent' within the IGES file, will be imported as raw 3D curve objects. If this option is not available on the IGES importer dialog box then this feature has not been publicly released yet. The following IGES curve entities are currently imported; all will be imported as a single NURBS curve:

• 102 - Composite curve
• 104 - Conic arc
• 112 - Parametric B-Spline curve
• 126 - Rational B-Spline curve

Import Composite Curve as Single Curve

If this checkbox is enabled then composite curves will be imported as a single NURBS curve. For example, if the IGES curve consists of a line, an arc and a NURBS curve then enabling this option will weld all these curve segments together into a single continuous NURBS curve. If this option is disabled then the final curve object will consist of the individual curve segments.

Units Options

"Units conversion" is often an important aspect of importing IGES files. People who create 3D models in a modeling package will do so using a specific units system, such as meters or feet. However, when these models are imported into another 3D package that uses a different default units system (such as Lightwave which uses centimeters), a "units conversion" operation needs to occur (the imported model geometry needs to be rescaled).

This IGES import panel determines what will be done when the units defined in the IGES file do not match current internal system units. If the units do not match, the IGES importer will rescale the imported geometry based on the ratio between the IGES units and the internal system units.

Match IGES File Units To Internal System Units

If this checkbox is enabled then the IGES importer will rescale all imported geometry so that the units of the IGES file can be made to match the units of the internal NuGraf/PolyTrans database. If the checkbox is disabled then the geometry will be imported unchanged and untouched, meaning that its scale may be "incorrect".

Internal System Units

This drop-down combo box specifies the current units system in effect within the internal NuGraf/PolyTrans 3d database. It defaults to meters. What you need to do is select an appropriate entry from the drop-down combo box which matches the system units used by your destination program: for example, set it to Centimeters for Lightwave, Inches for 3DS MAX, Centimeters for Maya, etc.

None,		No units; never do any geometry scaling.
Custom,		Custom units are in effect. See below.
Microinches,	1e-6 inches
Mils,		1e-3 inches
Inches,		0.0254 meters
Feet,		12 inches
Miles,		63360 inches
Microns,		1e-6 meters
Millimeters,	1e-3 meters
Centimeters,	1e-2 meters
Meters,
Kilometers,	1e+3 meters

Custom Units in "Units Per Meter"

If the drop-down "Internal System Units" combo box is set to "Custom", then this edit text control defines the "units per meter" scale factor for the custom unit system.

List Known 3D Packages' Unit Systems

Pressing this button will display the online help page describing known internal system units of various popular 3d packages.

Other Main Options

Import NURBs Surface Trimming Curves

IGES files typically have outer (boundary) and inner (hole) trimming curves on their NURBs surfaces. This is how they achieve their trimmed shape from their original rectangular form.

If this option is disabled then none of these outer and inner trim curves will be imported. Normally you would want to keep this option enabled.

Import Inner "Hole" Trimming Curves

Likewise, disabling this option will prevent the inner (hole) trim curves from being imported. Normally you would want to keep this option enabled.

Import 3D Pointsets

If this checkbox is enabled then any 3D points found in the file, which are considered 'independent' within the IGES file, will be imported as a single, unified geometric pointset primitive named "3D pointset".

Weld Pointset Vertices

Enabling this option will cause all imported 3D points to be welded together, meaning that redundant 3D points will be removed from the dataset.

Import Independent 3D Line Segments

If this checkbox is enabled then any lines found in the file, which are considered 'independent' within the IGES file, will be imported as a single, unified geometric indexed lineset primitive named "lone lines".

Import #106, 11-12-13 as: Polygons, Line Set

Entity # 106, forms 11/12/13 define a closed path entity. Few IGES files ever use this entity to convey data. However, the Okino IGES exporter allows 3D polygons and linesets to be embedded in IGES files using this entity.

If this combo box is set to "Polygons" then the imported paths will be created as a polygon mesh.

If this combo box is set to "Line Set" then the imported paths will be created as an indexed line set. Line sets have no area and thus will not render in a 3D rendering program.

Create Smoothing Information for Polygon Data

This converter imports and accumulates Entity # 106 (copious data) as an optimized mesh of polygons. If the accumulated polygon mesh forms a single uniform surface then this option can be enabled to automatically create vertex normals for the new mesh. These normals will make the polygonal surface appear smooth when it is rendered. There are two values that affect the create of smoothed vertex normals:

• Vertex Welding Threshold. Before the polygons can be smoothed out all redundant vertices must be removed from the polygon mesh so that all vertices are 'welded' together. If two or more vertices are closer together than the value shown in this type-in box then these vertices will be welded together into a single vertex. Larger values will make more vertices weld together into a single vertex.
• Smoothing Cutoff Angle. The smoothing criterion is based on the angle between abutting polygons; common smoothed vertex normals will be computed if the angle between their geometric surfaces normals is less than the angle specified in this type-in value (which defaults to 45 degrees) - in other words, higher values for the smoothing angle will make the surface appear to be more smooth and less angular.

Output Verbose Information to file 'igesinfo.txt'

If this checkbox is enabled then the parser will output detailed information about each parsed entity to the ASCII file 'igesinfo.txt' located in the current working directory.

Output Raw Debug Information to file 'debugigs.txt'

If this checkbox is enabled then the parser will output raw debugging information to the ASCII file 'debugigs.txt' located in the current working directory. This information is rarely used.

NURBS Patch Tesselation Method (Subdivisions)

The parameters in this section of the dialog box specify how a NURB patch is to be subdivided into polygons. Please note that these parameters are actually a mirror copy of those found in the main NuGraf/PolyTrans program in which they are found on the 'Geometric Subdivisions' dialog box; they are duplicated on the IGES reader dialog box for convenience.

There are two NURB subdivision methods supported by the NuGraf & PolyTrans software:

Uniform Subdivision, No Trim Curves

This subdivision method simply subdivides the NURB patch into a uniform mesh of U by V number of rectangular polygons. It produces the fastest tessellation of a NURB patch but it does not allow for NURB patches to be trimmed by trimming curves.

Adaptive Subdivision (with trim curves)

This method adaptively subdivides the NURB patch based on its curvature. Areas with high curvature will be subdivided more finely. This method also allows the NURB patches to be trimmed by trimming curves (if there are any associated with the NURB patches) at the expense of making the tessellation process slower.

The number of polygons used to approximate the true NURB surface (or, its smoothness) is controlled by the 'Surface Tolerance' slider. Lower values will make the surface smoother but at the expense of a longer tessellation time and more resultant polygons. This slider represents the maximum allowable distance (tolerance) between the true NURB patch and the tessellated polygonal surface; it is measured as a percentage of an object's maximum bounding box size. For example, if a NURB patch is being tessellated which is 10x10x10 units in size, and this slider is set to 0.2% then the resultant polygonalized surface will not deviate from the ideal NURB surface by more than 0.02 units (2% of 10).

Also, the 'Curve Tolerance' slider is used to control the smoothness of the NURB trimming curves. Lower values will make trimmed curve regions smoother, but again at the expense of a longer tessellation time and more resultant polygons. This slider represents the maximum allowable distance (tolerance) between the true NURB curve and the tessellated polygonal curve; it is measured as a percentage of an object's maximum bounding box size (the object on which the curve lies). For example, if a NURB patch is being tessellated which is 10x10x10 units in size, and this slider is set to 0.2% then the resultant polygonalized NURB curve will not deviate from the ideal NURB curve by more than 0.02 units (2% of 10).

IGES Processing & Tolerance Parameters

These parameters affect the processing of the IGES data as it is being transformed into internal NURB data format.

Combine Surface Knots Based on Adjacency

A 'knot' is a single floating point value which defines where a patch's world-space control point is located within the patch's (u,v) parameter space. Sometimes two or more knots can be very close together which might cause numerical instability problems. If this option is enabled and the type-in parameter value is set greater than 0.0 then any knots which are closer together than the specified distance will be combined into a single knot value.

Combine Curve Knots Based on Adjacency

This checkbox and its associated type-in numeric value performs the same function as described above except that the knots are used in NURB curves instead of NURB patches.

Minimum Distance Between Control Points of Entity # 102

IGES entity # 102 allows a complex curve to be created from a composite of a series of simpler curves. During the input process this converter stitches all of these simpler curves into a single NURB curve. If the stitching process finds one or more control points that are within the distance specified by this option then all of the control points will be collapsed into a single control point.

Minimum Radius of a Circle

This type-in numeric value specifies the minimum legal radius for an IGES circle. If a circle has a smaller radius then an error will be reported.