Checking and Repairing Mesh Data

Contents

• 86.1 Introduction
• 86.2 Finding and fixing defects in meshes
• 86.3 Repairing meshes attached to faces
• 86.4 Checking meshes

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86.1 Introduction

You can use the standard checking APIs (PK_BODY_check, PK_GEOM_check, etc.) to find invalidities in a mesh.

In addition, Parasolid provides APIs to find and fix defects in a mesh. Defects include invalidities and additionally other sub-optimal properties of a mesh whose removal may be desirable.

When creating a Parasolid mesh from foreign facet data, you should ensure that the resulting mesh is of good quality by attempting repair on any defects that may exist.

This chapter explains how you can use Parasolid functionality to manage defects and invalidities in mesh data.

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86.2 Finding and fixing defects in meshes

Parasolid provides two functions to let you look for defects on a mesh, and to attempt to fix defects found, as follows:

PK_MESH_find_defects returns the number of defects found, together with a PK_MESH_defect_details_t structure that provides details about the types of defects found, the entities affected and position and parameter information.

PK_MESH_fix_defects returns an array of new, repaired meshes, together with an array of PK_MESH_defect_details_t structures (one for each mesh returned), providing details about any defects that remain in that mesh.

These two functions can identify and attempt to fix a diverse range of mesh defects. Examples and a description of each mesh defect type are provided in the following table.

Defect type	Description
	PK_MESH_defect_disjoint_c The mesh has disjoint components.
	PK_MESH_defect_foldover_c Folds occur when the smaller of the two angles between two mfin-adjacent mfacets is less than or equal to the value in radians specified in max_foldover_angle . A foldover is a chain of one or more folds that bound a region of facets whose normals are opposed to those of their neighbours and are within tolerance of the neighbouring facets. The detection of foldovers is carried out either to: session linear precision (default), a linear precision specified using the have_tolerance and tolerance options, or a tolerance specified using the have_height_tolerance and height_tolerance options in the defect_tolerances sub-structure.
	PK_MESH_defect_flat_mfacet_c An mfacet of zero area through having a perpendicular height smaller than either: session linear precision (default), a linear precision specified using the have_tolerance and tolerance options, or a tolerance specified using the have_degen_tolerance and degen_tolerance options in the defect_tolerances sub-structure.
	PK_MESH_defect_degen_mfacet_c An mfacet with at least one mfin shorter than either: session linear precision (default), a linear precision specified using the have_tolerance and tolerance options, or a tolerance specified using the have_degen_tolerance and degen_tolerance options in the defect_tolerances sub-structure. A degenerate mfacet is also flat by definition.
	PK_MESH_defect_slit_c The mesh contains laminar mfins that the occupy the same position. The detection of coincident laminar mfins is carried out either: to session linear precision (default), to a linear precision specified using the have_tolerance and tolerance options, or to a tolerance specified using the have_slit_tolerance and slit_tolerance options in the defect_tolerances sub-structure.
	PK_MESH_defect_self_int_c The mesh contains intersecting mfacets.
	PK_MESH_defect_non_manifold_c The mesh is non-manifold.
	PK_MESH_defect_mvertex_normal_c The mesh has an mvertex with an incorrect normal direction.
Corrupted mesh	PK_MESH_defect_corrupt_c The mesh’s data structure is corrupt.

See PK_MESH_find_defects_o_t and PK_MESH_fix_defects_o_t for more detail about individual options.

If a mesh component containing slit defects (interior laminar mfins) is used to create a facet body, the resulting body will have non-boundary edges sat within faces, which are formed along the laminar mfins. These edges may be avoided if the mesh is repaired prior to creating bodies.

An mvertex has a bad normal when the normal at the mvertex points towards the mesh and not away from it as shown in Figure 86-1. In cases where an mvertex has multiple normals, the mesh is deemed to have a bad normal when at least one of the normals at the mvertex points into the mesh.

Figure 86-1 Examples of good and bad normals at an mvertex of a mesh (shown in cross-section)

Bad mvertex normals on a mesh can be addressed by discarding the existing mvertex normals and allowing Parasolid to assign new ones dynamically. This can be achieved using PK_MESH_store_normals, ensuring that the replace option is set to PK_MESH_replace_normal_mtopol_c, and the mvertices with bad normals are supplied in the mtopols array. See Chapter 88.3.2, “Managing mvertex normals”, for more information.

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86.3 Repairing meshes attached to faces

You can perform a limited range of repairs on meshes that are attached to faces using PK_FACE_fix_mesh_defects. The returned tracking records contain the original faces, the mesh attached to each face, and the repaired mesh for all faces whose meshes were edited.

The amount of modification that Parasolid can make when fixing mesh defects can be controlled by the following options:

have_max_deviation

max_deviation

Setting a max_deviation will allow Parasolid to carry out a greater level of repair, where possible, by altering the shape of the mesh, such that any point on the new mesh won’t be more than max_deviation distance away from the original mesh. The requirements of your application should therefore be taken into consideration when setting a max_deviation , to ensure that resulting changes to the shape of the mesh would be suitable for your application. Typically, if you specify a modelling tolerance in other operations, you should use the same value here.

See the API documentation for PK_FACE_fix_mesh_defects for more information.

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86.4 Checking meshes

In addition to finding and fixing mesh defects, you can check for invalidity in mesh data using the standard checking APIs.

You can use PK_BODY_check and PK_FACE_check to check meshes using the mesh option, which can be set as follows:

PK_GEOM_check can check meshes without specifying any additional options. Instead, it uses the geom option to determine how to examine the given geometry, whether it is classic B-rep or facet geometry.

PK_FACE_check_pair can also be used to check for face-face clashes between pairs of faces that have facet geometry attached.

Please see Chapter 30, “Checking”, for more information on those APIs.

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