Geometry Conversions

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Introduction

The previous lesson, Working with External Data in nTop, focused on bringing external data into nTop and converting it into geometry for modeling. A CSV file became a profile, while mesh and CAD files were converted into implicit bodies. This lesson focuses on the reverse process: preparing nTop geometry for use in downstream tools or FEA in nTop. An Implicit Body is nTop’s native geometry representation. While the number of tools that can work directly with implicit geometry through Implicit Interop, introduced earlier in this learning pathway, continues to grow, many downstream applications still require other formats, such as mesh or CAD files. In this lesson, you will learn how to prepare and export geometry from nTop. You will begin by meshing an implicit body, then explore four export paths: mesh, CAD, curves, and implicit geometry. Along the way, you will examine each approach’s strengths and limitations as geometric complexity increases.

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Meshing in nTop

Meshing discretizes a 3D model into a collection of smaller elements that approximate its geometry. Increasing the number and refinement of these elements generally produces a closer approximation of the original geometry. There are three primary mesh types used in nTop:

• Mesh (Surface Mesh): Represents the boundary of a 3D body using two-dimensional elements.
• Tetrahedral (Tet)/Volume Mesh: Discretizes a body’s interior volume using three-dimensional elements.
• Finite Element (FE) Mesh : A solid mesh prepared for use in finite element (FE) simulation. nTop supports linear and quadratic elements.

This lesson focuses on surface meshes and techniques for accurately capturing a geometry’s boundary. A surface mesh discretizes a surface with vertices, edges, and faces, typically composed of triangles or quad elements. Surface meshes are commonly used in manufacturing and rendering workflows, as well as in CAE applications that require a discretized representation of a model’s boundary. A surface mesh consists of two fundamental components:

• Mesh geometry defines each vertex’s position and geometric characteristics.
• Mesh connectivity defines the relationships between mesh elements, such as which vertices and edges form a face and which elements are adjacent.

Depending on the application, a surface mesh may also be called a mesh, surface, or domain.

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Mesh Properties and Terminology

Four fundamental mesh properties assess mesh quality: closed, manifold, oriented, and self-intersecting. These Boolean properties appear as True or False in the Properties section of the block details and indicate whether a mesh is clean and suitable for downstream use. Additional mesh-specific properties, including face count, vertex count, and vertices, are also available for inspecting the mesh geometry. The Properties panel of meshing blocks will contain mesh-specific properties If the mesh is not closed, manifold, or oriented, or if it is self-intersecting, you will receive a warning, which can cause issues when converting the mesh or exporting the part. A self-intersecting mesh will have this warning message Before continuing downstream operations, we recommend cleaning the mesh by identifying and removing defects like those described in this lesson. nTop provides tools for correcting simple mesh defects, which are covered in the Intro to Meshing course. For defective meshes imported into nTop, we recommend addressing these issues in the software that originally generated the mesh or in a dedicated mesh-cleaning tool before import. For meshes generated directly in nTop, you can address defects by refining the meshing parameters. This lesson will discuss these parameters and their effect on mesh quality later.

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Meshing an Implicit Body

The Mesh from Implicit Body block, found in the Utilities tab under Conversion, converts an implicit body to a mesh. An example of meshing using the Mesh from Implicit Body block

• Tolerance: Defines the maximum allowable deviation between the mesh and the implicit surface. Within the block, the tolerance determines the voxel size, which is set to half the specified tolerance. A voxel is a small, three-dimensional cell used to sample and represent the geometry during the meshing process. Smaller voxels capture finer geometric detail but require more mesh elements. As a result, halving the tolerance can approximately quadruple the triangle count.

Effect of increasing mesh tolerance on geometric fidelity and feature capture

• Min. feature size: An optional input that filters out features or holes smaller than the specified value. Use this input when the resulting mesh captures small geometric details that are unnecessary or below the limits of the intended downstream process.

Effect of applying the Minimum Feature Size input to filter small holes, resulting in a mesh with those features closed

• Sharpen: Reconstructs sharp edges and corners that may become rounded during voxelization. Enabling this option approximately doubles computation time.

Unsharpened mesh (left) and sharpened mesh (right) The block’s overload exposes the Sharpen Extents and Sharpen Iterations inputs, which limit sharpening to specific regions, such as mounting faces with GD&T requirements, reducing computational cost compared with sharpening the entire geometry. The transparent blue body defines the Sharpen Extents input and overlaps the mesh region to be sharpened

• Simplify: Reduces the number of mesh elements while maintaining the specified input tolerance. When enabled, it aggressively decimates and combines about 90% of the mesh elements, which can significantly reduce mesh size but may reduce geometric fidelity or introduce defects, such as self-intersections.

The Simplify input will reduce the number of mesh elements while maintaining the Tolerance For finer control over triangle count, two blocks in the Modeling > Utilities section can be applied to an existing mesh:

• Simplify Mesh by Amount reduces an input mesh’s face count by removing a specified proportion of its triangles. The Amount input must be greater than 0 and less than 1. For example, an input of 0.5 reduces the mesh face count by 50%.

Using the Simplify Mesh by Amount block to reduce mesh face count

• Simplify Mesh by Threshold removes as many mesh elements as possible while keeping the surface deviation between the input mesh and the simplified result below a specified threshold. Think of the Threshold input as Chord Height. This approach is useful for geometry that combines large, simple regions with fine features because it prioritizes geometric fidelity over achieving a fixed reduction percentage.

Simplify Mesh by Threshold removes mesh elements while keeping the surface deviation below the Threshold value After simplification, use the Mesh Quality block to evaluate the resulting mesh. The block reports several metrics that can help identify elements that may cause problems in downstream simulation workflows:

• Skewness measures how much an element deviates from its ideal shape. Values range from 0 to 1, with values closer to 0 indicating higher-quality elements and values closer to 1 indicating highly distorted or potentially degenerate elements.
• Orthogonality measures how closely an element conforms to an ideal orthogonal arrangement. Values range from 0 to 1, with values closer to 1 indicating better-quality elements and values closer to 0 indicating greater distortion.
• Intersecting triangles identify triangles that overlap or pass through one another. Ideally, no triangles should intersect.

The Mesh Quality block provides useful mesh metrics Review these metrics to identify mesh quality issues before continuing the workflow, particularly when the mesh will be used for simulation. The Mesh Quality block supports surface, volume, and FE meshes. For more information on visualizing mesh quality in the HUD and this block, see Mesh Quality Block.

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Remeshing

The Remesh Surface block improves mesh quality by redistributing elements across an existing surface mesh. It can reduce element count, improve element size and shape consistency, and vary mesh density spatially. Use this block to prepare high-quality surface meshes for downstream applications such as finite element analysis. The Remesh Surface block

• Edge Length: Specifies the approximate edge length of the mesh elements. It can be a constant value for uniform sizing or a scalar field to vary mesh density across the surface.

A mesh with a consistent Edge Length (left) versus a mesh with a field-driven Edge Length (right)

• Shape: defines the element shape of the output mesh. Triangular elements are the most common and computationally efficient option. Although quad and quad-dominant surface meshes cannot be converted directly into volume meshes in nTop, you can use them as inputs to the CAD Body from Quad Mesh block, which is discussed later in this lesson.
• Span Angle: controls refinement along curved regions by defining the maximum angle that a mesh element may span across a curve. Smaller values produce greater refinement, allowing the mesh to represent curved geometry more accurately. The following ranges provide general starting points:
• Coarse: 60°–91°
• Medium: 24°–75°
• Fine: 12°–36°
• nTop default: 30°

A mesh with a 30 degree Span Angle (left) versus a mesh with a 15 degree Span Angle (right)

• Growth Rate: Controls how quickly the element size may change between neighboring regions. The value must be greater than 1. Values closer to 1 produce a more gradual transition and a more uniform mesh, while larger values permit more rapid changes in element size.

A mesh with a Growth rate of 2 (left) versus a mesh with a Growth rate of 1.05 (right) The image above shows how reducing the growth rate from 2 to 1.05 creates a more uniform mesh. A growth rate of 1.2 is a useful general starting point.

• Feature Angle: Identifies edges to preserve during remeshing. It defines the maximum allowable angle between adjacent faces before it treats their shared boundary as a geometric feature. Lower values preserve more changes in surface direction and may create additional planar regions along curved geometry.

A mesh with a 10 degree Feature angle (left) versus a mesh with a 6 degree Feature angle (right)

• Min Edge Length: Defines the minimum allowable edge length.

A mesh with a Min Edge Length of 1 versus a mesh with a Min Edge Length of 5

• Chord Height: Defines the maximum allowable distance between the midpoint of a mesh element edge and the input surface. A value of zero disables chord-height-based sizing.

A mesh with a Chord Height of 0.001 (left) versus a mesh with a Chord Height of 0.1 (right)

• Min. Feature Size: Specifies the minimum feature size to preserve in the resulting mesh. It removes features smaller than this value. A recommended starting value is approximately 5% of the edge length. Set the value to zero to disable feature removal.

A small implicit body (left) is removed when meshing by using a small Min Feature Size (right)

• Preserved Nodes: Accepts a Point List specifying points to preserve as nodes in the resulting FE mesh. If left empty, node preservation is disabled. This input ensures that selected boundary points are retained during remeshing and is covered in greater detail in meshing for Simulation courses.

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Back to CAD Methods

As we have learned, an Implicit Body is defined by a function rather than discrete faces and edges. Converting it to a CAD Body requires fitting NURBS surfaces to approximate the geometry, with curvature changes and sharp transitions often creating additional faces. Fine features, such as thin trailing edges or dense lattices, can generate more faces than a CAD system can practically manage. The following back-to-CAD methods suit different geometries and applications. Select an approach based on the geometry’s complexity and the intended downstream use.

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Quadrangulate Mesh

The Quadrangulate Mesh block remeshes a triangular or quad mesh to produce a quad mesh. Common applications include:

• Preparing input for the Cell Map from Quad Mesh block
• Creating or remeshing a quad mesh for export
• Providing input to the CAD Body from Quad Mesh block

This lesson focuses on using the block to prepare a low-complexity model for conversion into a CAD part with CAD Body from Quad Mesh (more on this block later in this lesson). The Quadrangulate Mesh block

• Mesh: Accepts the mesh to quadrangulate.
• Target Count: Determines the target number of quad elements. If the output mesh is self-intersecting, try increasing the Target Count value or disable the Sharp features input.
• Adaptivity: Adjusts the target quad count to better capture the input mesh curvature; range [0, 1].
• Sharp features: Auto-detects and preserves sharp features in the input mesh.
• Feature angle: Defines the angle used to detect and preserve sharp feature edges in the input mesh. When you provide an input value and the angle between two faces of the mesh is greater than the provided angle, the edge between those faces is tagged as a sharp feature edge. If you don’t provide an input value, an internal algorithm detects sharp edges. This input is used only if the Sharp Features input is checked; otherwise, it is ignored.
• Symmetry: Is an optional input that accepts a frame defining the model’s symmetry plane.

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Refine Mesh

Sometimes you may get an error after using the Quadrangulate Mesh Block, and a warning appears on the block as shown in the image below. The solution is the Refine Mesh block. If your resulting mesh has both triangle and quad elements, you will encounter this warning message The Refine Mesh block uses subdivision to divide mesh elements into smaller elements, improving the representation of geometric details and surface curvature. The Refine Mesh block

• Mesh: Accepts the mesh to refine
• Refinement Steps: Specifies the number of subdivision iterations (1 to 5).
• Feature Angle: Optionally identifies edges to preserve as sharp. Edges with a dihedral angle greater than this value are treated as sharp during refinement.
• Sharpness Value: Controls the degree of sharpness applied to preserved edges, from 0 to 1. A value of 1 represents an infinitely sharp edge and is the default when a Feature Angle is specified.

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CAD Body from Quad Mesh

Now that we have learned about meshing and refinement techniques in nTop, we can move on to creating a CAD body. The CAD Body from Quad Mesh block converts a quad surface mesh into a CAD Body that you can export to other CAD applications as a Parasolid (.x_t, .x_b) or STEP (.step) file using the Export Part block. The CAD Body from Quad Mesh block Procedure:

1. Create a surface mesh: Use the Mesh from Implicit Body block to generate a triangular surface mesh.

A surface mesh created from the Mesh from Implicit Body block

2. Refine the mesh: Use Remesh Surface, Simplify by Threshold, Simplify by Amount, or another refinement method to prepare the mesh for quadrangulation. Before continuing, verify that the mesh contains no meshing errors. A clean, refined mesh enables a more reliable and efficient conversion to a quad mesh.

A mesh simplified using the Simplify Mesh by Threshold block

3. Create a quad mesh : Convert the refined mesh using Quadrangulate Mesh

Triangle elements converted to quads using the Quadrangulate Mesh block

4. Refine the quad mesh : If necessary, apply Refine Mesh to improve detail retention and reduce rounding near sharp edges.

Refine the quad mesh using the Refine Mesh block

5. Create a CAD Body: Use the CAD body from Quad Mesh block and input the refined quad mesh.

Convert the mesh to a CAD body using CAD Body from Quad Mesh This workflow is best suited to relatively simple geometry and most topology-optimized parts. It is not recommended for complex lattice structures. For small- to medium-sized lattices that this method cannot support, consider using CAD Body from Implicit Body instead.

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CAD Body from Implicit Body (BETA)

The CAD Body from Implicit Body block is a BETA block that converts an Implicit Body into a CAD Body for export to other CAD applications. You can export the resulting body as a Parasolid (.x_t, .x_b) or STEP (.step). This method is intended for medium-complexity textured models and lattices, including:

• Beam- or face-based lattices with up to 10,000 beams or faces
• TPMS lattices with up to 1,000 unit cells

A CAD Body generated using the CAD Body from Implicit Body (BETA) block Implicit models within these guidelines can typically be converted into CAD Bodies containing 50,000 or fewer faces, allowing traditional CAD systems to process the detailed geometry. The CAD Body from Implicit Body block

• Body: Input accepts the Implicit Body to convert.
• Tolerance: Defines the maximum permitted deviation between the CAD Body and the input Implicit Body. Increasing the tolerance generally reduces the face count and conversion time but produces a less accurate approximation. Begin with a value approximately half the size of the body’s thinnest feature.
• Preserved CAD: Accepts an optional list of CAD Bodies containing faces to incorporate into the output. The block attempts to stitch eligible faces into the resulting CAD Body. Faces that extend beyond the Implicit Body’s surface or cannot be stitched successfully are ignored.

A CAD Body generated without the Preserved CAD input (left) versus one generated using the Preserved CAD input (right)

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Simplified Body Representation

The Simplified Body Representation block creates a simplified boundary envelope of a complex Implicit Body while preserving its mass properties. Overall mass is always matched, with the option to also preserve the center of gravity and moments of inertia. This method is recommended for high-complexity geometry that would produce too many B-rep faces for traditional CAD systems, including:

• Fine surface textures
• Beam- or face-based lattices with more than 10,000 beams or faces
• TPMS lattices with more than 1,000 unit cells

The output combines a boundary envelope with density-adjusted bodies, allowing downstream CAD applications to support visualization, clearance checks, weight studies, assemblies, drawings, and other workflows without representing the complete internal geometry. The Simplified Body Representation block

• Envelope: A list of CAD Bodies defining the simplified external boundary.
• Body: The implicit body to simplify.
• Density: The material density assigned to the body.
• Match Inertia: Specifies whether the output should also preserve the center of gravity and moments of inertia.
• Uniform Density: Creates an output compatible with CAD systems that do not support variable-density parts. SolidWorks users should enable this option to preserve the correct center of gravity and moments of inertia.

Simplified Body Representation visibility is toggled on, showing the original CAD Body while preserving the mass properties of the final lattice part Mass Properties displayed in CAD tool

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Reference Geometry for CAD Reconstruction

As you learned in earlier lessons, nTop uses precise curves, including lines, arcs, conics, and splines, to construct implicit geometry. You can export these curves with relevant numerical parameters to support automated or manual surface reconstruction in CAD, especially for parts requiring highly tailored surfaces. An example of a curve sketch that has been created in nTop To export any Curve type or list of Curves, use the Export Part block. The Export Part block being used to export a set of Curves

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Convergent Body (BETA)

Some CAx software using Siemens Parasolid, such as NX or STAR-CCM+, supports a geometric representation known as Convergent Modeling. In this format, mesh face groups are treated as CAD faces within the topology, in addition to analytical and NURBS faces. This supports many downstream operations that mesh formats typically don’t support in the same software. nTop supports convergent body exports where each face group is defined by a mesh. Convergent Bodies are created using either the Convergent Part from Named Body or the Convergent Body from Multi-Region Mesh blocks. These workflows will be introduced in a separate course that explores meshing in nTop in greater depth. An example of a Convergent Body

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Exporting Parts

To export an Implicit Body from nTop, it must first be converted to a supported file type using one of the methods described above. The following steps outline how to export the converted part.

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Exporting Meshes

The Export Mesh and Export 3MF blocks, located under Utilities > Exchange, export mesh geometry in STL, OBJ, PLY, or 3MF format. STL offers the broadest compatibility, while 3MF files are typically about one-third the size for equivalent geometry and support color and multi-body assemblies. The Export Mesh and Export 3MF blocks

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Exporting CAD

The Export Part block exports CAD parts from nTop. Before exporting, convert an Implicit Body to a CAD Body using either the CAD Body from Quad Mesh block or the CAD Body from Implicit Body block, as described above. Parts can be exported as Parasolid (X_T, X_B), STEP, or IGES files. The Export Part block has several overloads for exporting geometry

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Export for Additive Manufacturing

All slicing tools are in the Manufacturing tab. You can export multiple slice file formats. To learn more about slicing for additive manufacturing, check out our course, 250: Slicing and Print Layout. There are several block options for exporting slices in nTop

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Exporting Implicits

The Export Implicit Body block, located under Utilities > Exchange, exports an Implicit Body as an .implicit file. This format preserves the underlying implicit function rather than an approximated or tessellated representation, maintaining full geometric fidelity while typically producing a file significantly smaller than an equivalent mesh. As introduced in Course 1, this workflow uses Implicit Interop and requires the receiving application to support the .implicit format. The Export Implicit Body block

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What to Take Away

• Surface meshing converts an Implicit Body into a discrete representation of its boundary for downstream manufacturing, simulation, and visualization workflows.
• A clean mesh should be closed, manifold, oriented, and free of self-intersections.
• Meshing and remeshing parameters balance geometric fidelity, element quality, file size, and computation time.
• The appropriate back-to-CAD method depends on geometric complexity and the intended downstream application.
• Complex geometry that is impractical to convert into a full CAD Body can be represented using a Simplified Body Representation block that preserves critical mass properties.
• nTop geometry can be exported as mesh, CAD, reference geometry, additive manufacturing, or .implicit files, depending on the capabilities of the receiving application.

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What’s Next

You have now learned the basics of exporting geometry from nTop. Next, test your understanding of geometry conversion in a knowledge check.