Overview Of Convergent Modeling

82.1 What is Convergent Modeling?

Parasolid provides the ability to perform topological operations, such as booleans, sectioning and offsetting, on bodies containing any combination of classic and facet geometry.

The different combinations of facet and “Classic Geometry” that may be found in a body are described below:

Type of body	Description
Classic body	A classic body is a body that contains only classic Parasolid surface and curve geometry.
Facet body	A facet body is a body that contains only facet geometry. Facet geometry comprises geometry composed of mesh and polyline data (often referred to collectively as facet data), rather than classic Parasolid surface and curve geometry. A polyline is a curve consisting of a series of line segments. In a facet model, polylines are attached to model edges in place of classic curves. A mesh is a type of facet surface consisting of a connected collection of triangular facets. In a facet model, meshes are attached to model faces in place of classic surfaces.
Mixed body	A mixed body is a body that contains a combination of both classic and facet geometry.

Type of body

Description

Classic body

A classic body is a body that contains only classic Parasolid surface and curve geometry.

Facet body

A facet body is a body that contains only facet geometry. Facet geometry comprises geometry composed of mesh and polyline data (often referred to collectively as facet data), rather than classic Parasolid surface and curve geometry.

A polyline is a curve consisting of a series of line segments. In a facet model, polylines are attached to model edges in place of classic curves.
A mesh is a type of facet surface consisting of a connected collection of triangular facets. In a facet model, meshes are attached to model faces in place of classic surfaces.

Mixed body

A mixed body is a body that contains a combination of both classic and facet geometry.

Within a mixed body you may find topology with combinations of facet or classic geometry as follows:

Geometry	Topology	Definition
Classic	Edge	A classic edge is an edge that has only classic geometry, and all of whose faces (if any) have only classic geometry.
Facet	Edge	A facet edge is an edge that has only facet geometry, and all of whose faces (if any) have only facet geometry.
Mixed	Edge	A mixed edge is an edge that has a mixture of facet and classic geometry, and/or whose faces (if any) have a mixture of facet and classic geometry.
Classic	Vertex	A classic vertex is a vertex whose faces (if any) and/or edges have only classic geometry.
Facet	Vertex	A facet vertex is a vertex whose faces (if any) and/or edges have only facet geometry.
Mixed	Vertex	A mixed vertex is a vertex whose faces (if any) and/or edges have a mixture of facet and classic geometry.

Note: In a mixed body, Parasolid still supports rubber topology (that is, topology that has no geometry attached), as an interim stage between modelling operations. For example:

An edge with a classic surface attached to one of its faces, and no geometry on the other face is considered a classic edge.
An edge with a mesh attached to one of its faces, and no geometry on the other face is considered a facet edge.

See Chapter 83, “Facet Model Structure” for more information on polylines and meshes.

See Chapter 84.6, “Creating a mixture of facet and classic geometry” for more information on combining mixed geometry.

Note: In the context of this volume, a mesh is not the same as a finite element mesh.

The following areas of Parasolid functionality can receive and operate on mesh data:

Booleans
Sheet Trim and Extend
Extrude
Sectioning
Imprinting
Hollow/Offset/Thicken
Hole-filling
Rendering
Faceting Output
Swept and Spun Outline
Topological and Geometric Enquiries
Clash detection
Mass properties
Min/Max distance (closest approach)
Read and Write XT
Repair of meshes
Replacing surfaces of selected faces
Edge blending
Deformation operations
Sewing an array of facet sheet bodies

Figure 82-1 Performing a deformation operation on a mesh

Figure 82-2 Thickening and hollowing a facet body

82.1.1 Mesh and facet data requirements

Parasolid places some minimal requirements on the mesh and facet data that it can work with. This section explains what those requirements are.

Requirement	Notes
Triangular facets	All facets in a mesh must be triangular. Any attempt to pass mesh data that contains non-triangular facets to Parasolid will be unsuccessful.
Manifold meshes	All meshes must be manifold, that is, they must have only two facets meeting along each mfin. If your application needs to pass non-manifold models to Parasolid, then you should use non-manifold topology with individual meshes that are themselves manifold.
Vertex matched meshes	Meshes that are attached to models must be vertex matched.This means that along the polyline dividing the two meshes, the facet vertices of both meshes must lie in the same location as the position vectors on the polyline, so that they match one-to-one with those position vectors. Such models will always be watertight (i.e.there are no gaps between facets along the polyline), as shown in Figure 82-3 (a). Figure 82-3 (b) shows an example where some facet vertices along the polyline do not match one-to-one with the position vectors.

Figure 82-3 Ensuring face meshes are vertex matched with the polyline of the edge

Disjoint meshes, that is, meshes with separate components, can be passed to Parasolid. These components must be split up before attaching them to Parasolid faces.

Self-intersecting meshes can also be passed to Parasolid, but you should treat these in the same way that you would treat self-intersecting B-surfaces: leave them as orphan geometry until you need them, and clean up the self-intersections before you attempt to model with them.

“Crab-claw” meshes are supported. These are meshes made up of connected sets of facets where facets also share a single common position at a single vertex with more than two laminar mfins.

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82.2 How to make use of Convergent Modeling in Parasolid

Note: There are additional royalty fees for distributing your Parasolid-based products with Convergent Modelling functionality enabled. If you have not licensed Convergent Modelling, your current maintenance fees cover the use of Convergent Modelling functionality in your development environment for evaluation and prototyping purposes. Your Parasolid Sales Representative can provide full information if you have any questions.

All calls to Parasolid functions must take place within a Parasolid session. By default, support for Convergent Modeling is disabled when you start a Parasolid session. Before you can enable this functionality and begin to perform Convergent Modeling in Parasolid, you should do the following:

Ensure your application’s frustrum can handle the facet file guise FFCXMM. We recommend you use keys instead of filenames when loading facet models into a Parasolid session. See Chapter 2, “File Handling” of the Downward Interfaces manual for more information on file guises and using key names.
Enable facet geometry using PK_SESSION_set_facet_geometry. See Section 82.2.1, “Enabling support for facet geometry” for more information.
Create facet bodies in Parasolid using one of the following methods:

PK_BODY_make_facet_body to convert classic bodies to facet bodies. See Section 84.3, “Creating facet bodies directly from existing bodies” for more information.
PK_MESH_create_from_facets to make Parasolid meshes from external facet data. See Chapter 85, “Creating PSM Data From Foreign Facet Data” for more information.
PK_MESH_make_bodies to create Parasolid bodies from mesh data. See Section 84.4, “Manipulating facet data” for more information.

It is also possible to convert a mesh to a trimmed classic surface using PK_MESH_make_surf_trimmed. You can subsequently use the trimmed surface import route to create a classic body from this trimmed surface data. See Section 84.5, “Creating classic geometry from a mesh” for more information.
Perform operations on facet models using the APIs you are familiar with calling on classic B-rep models and the additional APIs provided for dedicated Convergent Modeling operations.

Note: Not all Parasolid functions support Convergent Modeling.You should check the PK Interface Programming Reference Manual to see if the functions you wish to use support facet geometry. Interfaces that do not, are marked [NF] in the PK Interface Programming Reference Manual and need to be left at their default values. Those that offer partial support are marked [PF] and can be changed to values that support facet geometry. In general, Parasolid functions that are not supported will give the error PK_ERROR_facet_geometry to indicate that the model contains facet geometry but the function does not support facet geometry.

82.2.1 Enabling support for facet geometry

Parasolid provides a number of functions that let you manage Convergent Modeling within your application:

Function	Description
PK_SESSION_set_facet_geometry	Allows you to enable or disable facet geometry within a session using `facet_geometry` , which has the following values: PK_facet_geometry_no_c: facet geometry is disabled PK_facet_geometry_all_c: facet geometry is enabled Default: PK_facet_geometry_no_c
PK_SESSION_ask_facet_geometry	Allows you to enquire whether facet geometry is enabled within the session using `facet_geometry` as described in PK_SESSION_set_facet_geometry.
PK_PART_receive, PK_PART_receive_b and PK_PART_receive_u PK_PARTITION_receive, PK_PARTITION_receive_b and PK_PARTITION_receive_u PK_PARTITION_receive_deltas_2	The `receive_mixed` option within these functions allows you to control whether you want to receive mixed bodies in your application. On receiving any data into a Parasolid session, you can choose to either: Generate an error on encountering a mixture of facet and classic geometry. This is the default. Receive mixed parts into your application.

Function

Description

PK_SESSION_set_facet_geometry

Allows you to enable or disable facet geometry within a session using facet_geometry , which has the following values:

PK_facet_geometry_no_c: facet geometry is disabled
PK_facet_geometry_all_c: facet geometry is enabled

Default: PK_facet_geometry_no_c

PK_SESSION_ask_facet_geometry

Allows you to enquire whether facet geometry is enabled within the session using facet_geometry as described in PK_SESSION_set_facet_geometry.

PK_PART_receive, PK_PART_receive_b and PK_PART_receive_u
PK_PARTITION_receive, PK_PARTITION_receive_b and PK_PARTITION_receive_u
PK_PARTITION_receive_deltas_2

The receive_mixed option within these functions allows you to control whether you want to receive mixed bodies in your application. On receiving any data into a Parasolid session, you can choose to either:

Generate an error on encountering a mixture of facet and classic geometry. This is the default.
Receive mixed parts into your application.

By default, facet geometry is disabled when you start a Parasolid session. To use mixed geometry you must have the facet_geometry and the receive_mixed options enabled as follows:

To enable facet geometry, you must call PK_SESSION_set_facet_geometry once you are in the session and set facet_geometry to PK_facet_geometry_all_c.
To enable receiving mixed geometry, you need to set receive_mixed to a setting other than PK_receive_mixed_fail_c. At the present time, only PK_receive_mixed_allow_c is supported.

Figure 82-4 shows the typical workflow required to create and work in a Parasolid session.

Figure 82-4 Typical workflow in a session using Convergent Modeling

82.2.2 Enquiring if facet geometry is present

You can check to see if facet geometry is present in a partition using PK_PARTITION_ask_facet_geom. This function determines if facet geometry is present in a partition. In addition, it can optionally return those parts that have facet geometry, and their geometric category, and any orphan facet geometry in the partition.

If want_parts is PK_LOGICAL_true, all parts containing facet geometry are returned.
If want_geoms is PK_LOGICAL_true, all orphan meshes and polylines are returned.

See the PK Reference documentation for more information.

Parasolid also has two functions that provide information about the category of the geometric entities.

PK_GEOM_ask_geom_category returns the category of a given geometric entity.
PK_TOPOL_categorise_geom returns the geometric category of the topology, along with the category of any geometry directly attached to it. The geometric category of the topology is a combination of the category of the direct geometry and the categories of any related topology. If want_related_topols is PK_related_topols_top_c, these related topologies and their categories are returned. In cases where the given topology is an assembly, only the top level component parts are returned.

See the PK Reference documentation for more information on these functions.

82.2.3 Workflow for creating Parasolid meshes from foreign facet data

The following workflow describes the recommended best practice for creating Parasolid meshes from foreign facet data.

Create the mesh from foreign facet data by calling PK_MESH_create_from_facets. See Chapter 85, “Creating PSM Data From Foreign Facet Data” for more information.
After creating a Parasolid mesh you should attempt to repair any defects or invalidities present. See Chapter 86, “Checking and Repairing Mesh Data” for more information on detecting and repairing mesh defects. Any holes in the mesh can be filled using PK_MESH_fill_holes. See Chapter 87, “Filling Holes In Facet Bodies” for more information.
The mesh can be attached to topology to create a single-faced facet body, which can be split into individual faces to optimise downstream modelling operations. The simplest method is to call PK_MESH_make_bodies on the repaired mesh. See Section 84.4, “Manipulating facet data” for more information.

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