Import 3MF as a solid body: extend mesh.convert beyond constant-section prisms

[tdslot]

Problem

SolidCraft imports .3mf files as mesh bodies. Mesh bodies don't work with most modelling operations. For example, #615 fails with a boolean needs a solid body.

mesh.convert (Convert Mesh) exists, but it only rebuilds closed meshes with a constant cross-section (prisms). Anything else fails with this mesh is not a supported constant-section prism; no geometry was converted (see #1200, #744, #346, #358).

Most 3MF files come from slicers, printer vendors and sharing sites, and they are plain triangle meshes. To modify one (add a hole, cut, fillet, shell, combine with other bodies), I currently have to rebuild it by hand.

Proposal

Let SolidCraft prepare an imported .3mf as a solid body (B-rep) that supports the normal operations, by extending mesh.convert beyond prisms and connecting it to the import.

• Import option: Import as: Mesh body / Solid body
• Right-click a mesh body → Convert to solid
• Same from the CLI, MCP and scripts, e.g. file.insert_mesh with as_solid: true

Proposed behaviour

• The mesh is welded and checked (closed, manifold, consistent normals). Small defects are repaired where possible.
• Faces are recovered from the triangles: coplanar triangles merge into planes, and cylinder, cone, sphere and torus regions are fitted where possible. Everything else stays as freeform or faceted faces.
• The result is a solid body, so boolean, fillet/chamfer, shell, hole, extrude-cut and sketch-on-face work on it.
• Keeps the component-per-file behaviour (Importing a STEP (or any model file) always creates a new component #1386) and the existing unit handling.
• Clear error when it can't be done (open edges, self-intersections, non-manifold geometry). No panic, and fall back to a mesh body.
• Bounded time and memory on large meshes, with progress and cancel (see 3MF build items re-copy every vertex and DXF nested INSERTs recurse exponentially (tiny files exhaust memory or hang) #144 and Uneven hostile-input coverage across the file importers (SVG and DXF have almost none) #469).

Related

• Parametric Mesh Conversion #63 Parametric Mesh Conversion
• Extrude Cut Error: "invalid input: a boolean needs a solid body" when intersecting with mesh #615 Boolean with a mesh: "a boolean needs a solid body"
• mesh.convert (constant cross-section only)
• Importing a STEP (or any model file) always creates a new component #1386 Importing a file creates a new component

Nice to have later

• Detect extrusions, revolves and holes and rebuild them as editable features in the timeline.
• Report which faces were fitted analytically and which stayed faceted.

[dwballance]

I support this idea! As a lightweight 3d print designer, I don't have a paid copy/subscription to any of the commercial CAD tools, and I have several times had to rebuild a 3mf/stl mesh in Fusion to make minor adjustments to it. I would really like to find a way to load a mesh as a solid.

[tdslot]

This is implemented in #3700: mesh.convert now rebuilds any closed, manifold mesh as a solid body — coplanar triangles fuse into planar faces (holes included), curved surfaces stay faceted (the proposal's fallback), and constant-section parts keep the exact planar/cylindrical reconstruction.

It is reached from a mesh body's right-click menu (Convert to Solid), from mesh.convert, and on import: file.insert_mesh / doc.open take as_solid, and Preferences → Design → Import meshes as solids has the app pass it for dropped and opened mesh files. A mesh it cannot convert stays a mesh body with a warning, and the conversion reports progress and stops on Cancel.

Limits, errors and what works afterwards are in docs/mesh-to-solid.md.

[dwballance]

This is working well!

Question - I imported an STL and it created a solid, but if I try to modify it I get the warning that "mesh bodies can only be moved, measured and exported". I don't have a paid Fusion license so I don't know if that is a similar constraint in Fusion, but what steps are needed to allow an imported STL to be subsequently modified?

[tdslot]

Thanks for trying it — the good news is that nothing is broken: the body you are editing is still a mesh body, and converting on import is opt-in.

Two ways to get a solid body:

• Right-click the body in the browser or the viewport → Convert to Solid. That opens Convert Mesh with that body and its tolerance already picked.
• Preferences → Design → Import meshes as solids, then import again. The app passes as_solid: true to the import, and every mesh in the file is converted where it can be.

If you already had that preference on (or used Convert to Solid) and still ended up with a mesh body, then the conversion refused that particular STL and deliberately left it as a mesh. The import card says why — an open or non-manifold mesh, more than 50 000 triangles, or more than 20 000 fused faces. If you can paste that warning, or share the STL, I will take a look.

Your confusion is fair, though, and worth saying out loud: nothing in the browser marks a body as mesh or solid yet, and there is no per-import "Import as" choice — only the global preference. That is a separate, UI-side gap and not something you did wrong.

One thing to be aware of: the conversion itself is on hold with the rest of the kernel work while a new geometry kernel is evaluated (#3992), so it will not land before that is decided. What you are testing is exactly what the branch does today.