Converting a Mesh to BRep (Convert Mesh)

Overview#

A mesh cannot be filleted, dimensioned or used in most Solid features. Convert Mesh turns it into a BRep body. Fusion offers different conversion methods, and the right one depends on what the mesh represents:

Autodesk's Convert Mesh help describes these three methods: Faceted generates one face per mesh face, Prismatic is for geometry originally created with solid or surface tools, and Organic is for scan data or organic models. Face groups are used by Prismatic to infer features; the help says good face groups also help downstream modifications and Convert Mesh in general. The Faceted result is often used as a stepping stone.

Where to find it#

• Design workspace > Mesh tab > Modify > Convert Mesh.
• Generate Face Groups and Combine Face Groups are on the Mesh tab Prepare panel and are used before a Prismatic conversion.
• Output appears in the browser as a solid or surface body, or as a T-Spline Form in the timeline (Organic with Base Feature).

Step by step#

Faceted conversion#

1. Insert the mesh and run Repair so it is watertight.
2. Choose Mesh > Modify > Convert Mesh.
3. Select the mesh body. Set Operation to Parametric (creates a Convert Mesh feature that keeps upstream relationships) or Base Feature (no upstream parametric relationships).
4. Set Method to Faceted: each mesh face becomes one face on the new body.
5. Click OK. A watertight mesh becomes a solid body; a non-watertight mesh becomes a surface body, which you can turn into a solid with Stitch or Patch.

If the mesh has hundreds of thousands of triangles, the result will have just as many faces and may be unusable. Reduce first.

Prismatic conversion (with face groups)#

1. Reduce and repair the mesh. Remove noise, and keep the model clean.
2. Choose Mesh > Prepare > Generate Face Groups.
3. Select the mesh. Choose the Method:
   • Fast: prioritizes speed; uses an Angle Threshold (normal angle between faces) and a Minimum Face Group Size (percentage of the overall mesh area).
   • Accurate: prioritizes accuracy over speed; uses a Boundary Tolerance. Autodesk suggests running Fast first for an initial set, then selecting specific face groups and running Accurate on detailed areas.
4. Press Shift+F to display the face groups as colored patches.
5. Refine groups: Combine Face Groups merges two or more adjacent groups; in Direct Edit, Create Face Group groups selected faces (they need not be connected). Create groups that map as closely as possible to prismatic features.
6. Choose Convert Mesh, set Method: Prismatic, and click OK.

Organic conversion#

1. Run Repair and Reduce. For best results the mesh should be clean and moderate in density.
2. Choose Convert Mesh with Method: Organic.
3. Set the Resolution: By Accuracy (Low is fastest, then Medium, High; Precise is slowest) or By Face Number (a target number of faces). Optionally tick Preprocess Holes, which smooths the boundaries of open holes and improves the chance of success. You cannot edit the form before the conversion completes.
4. Click OK. With the Base Feature operation a T-Spline Form is created, which you can open in Form to push and refine the cage, then Finish Form to get BRep. If Organic fails with Parametric, switch to Base Feature. Organic requires the Fusion Design Extension.

Organic conversion is ideal for turning a scan of a person, a sculpture, or a rock into a smooth manufacturable shape. You trade accuracy for editability.

After conversion#

1. Run Inspect > Section Analysis to compare the BRep with the original (the Mesh help mentions Section Analysis for inspecting mesh interiors).
2. Use Combine/Cut to align with other solids.
3. Hide or remove the source mesh body when you are satisfied.

Options reference#

Tips and pitfalls#

Tip: Treat prismatic conversion as a "get me 80 percent there" tool. Get the main planes and cylinders, then add or correct features with sketches and Solid commands.

Warning: Prismatic conversion works best on clean, designed shapes such as housings, brackets and machined parts. It struggles on noisy scan data and on very complex free-form surfaces; use Organic or the reverse engineering workflow (model over the mesh) instead.

Warning: Large meshes can freeze Fusion during conversion. Reduce beforehand, and save your work first.

Tip: Before Faceted conversion of a printing mesh, remember that you might not need conversion at all: Fusion can print and export meshes directly. Convert only if you need Solid features.

Tip: Organic conversion followed by Finish Form gives you a BRep body with smooth surfaces, a good intermediate for thickening, shelling and filleting.

Tip: Keep the unconverted mesh in the file (hidden). Later you can compare deviation or redo the conversion with different settings.

Choosing a method with numbers#

The triangle counts in the second column are rules of thumb from practice, not documented limits. The principle is that Faceted preserves everything, so its cost scales directly with triangles, while Prismatic and Organic reduce the count by fitting surfaces.

Parametric or Base Feature?#

• Parametric adds a Convert Mesh feature to the timeline. If you later change the upstream mesh (for example by editing a Reduce feature), the conversion updates. It is the better choice in a history-based workflow.
• Base Feature discards upstream relationships and, for Organic, creates a T-Spline Form. Choose it when Parametric fails, when you want to edit the result freely, or when the mesh will not change.

Face groups: the quality gate for Prismatic#

Prismatic conversion infers features from face groups, so conversion quality equals face-group quality. Work in this order:

1. Repair and Reduce so the mesh is clean and moderate in size.
2. Generate Face Groups (Autodesk suggests Fast first, then Accurate on selected detailed groups).
3. Inspect the coloured patches. Look for a hole whose cylinder is split into three groups, a plane split by a scratch, or a fillet merged into a wall.
4. Merge groups with Combine Face Groups (or create groups with Direct Edit > Create Face Group) until each logical surface is one group.
5. Convert with Prismatic and check surface types of a few faces in Properties.

Worked example: a bracket scan with five 6 mm holes. After Generate Face Groups, three of the holes each show as two half-cylinder groups. Merge the pairs, and Prismatic should produce true cylindrical faces of about 6 mm. If you skip the merge, you get split cylinders that do not round-trip into a clean hole feature.

Result types and finishing#

• A watertight mesh converts to a solid body; a non-watertight mesh converts to a surface body. Stitch or Patch finishes a surface body into a solid.
• A Faceted solid is usually a stepping stone: its faces can be merged or combined, but many tools slow down on bodies with thousands of faces.
• After conversion, compare against the mesh with Section Analysis at three or four planes.

Common failures and fixes#

Practice exercise#

Convert two meshes by the three methods and compare.

1. Insert an STL of a simple mechanical part (a bracket with holes) and a figurine.
2. For the bracket: Repair, light Reduce (Tolerance 0.05 mm), Generate Face Groups (Fast, then Accurate on detailed groups), clean groups, then Convert Mesh with Prismatic and Parametric. Select a hole face and confirm in Properties that it is a cylinder.
3. For the figurine: Reduce to about 20,000 triangles and Convert Mesh using Organic, Base Feature, Resolution By Face Number set to 2,000. Open the Form and view the cage. Repeat with By Accuracy: Medium and compare the cage density.
4. Convert the bracket again with Faceted and compare face counts in the browser (expect orders of magnitude more faces).
5. Use Section Analysis on the Prismatic result to compare with the mesh outline at three planes.

Expected result: a clean solid with analytic faces for the bracket, an editable T-spline for the figurine, and a faceted version with far more faces. If Organic is unavailable, note that it needs the Design Extension and complete steps 2 and 4 only.

Related#

• Mesh repair, reduce and remesh
• Reverse engineering workflow
• Finish Form and converting between Form and BRep