Underneath every serious CAD application sits a geometry kernel — a library that represents shapes mathematically and performs the operations (extrude, boolean, fillet, offset) that modeling tools expose. When a CAD app says it produces "exact solids," it is talking about its kernel.

Boundary representation (B-rep)

A B-rep solid is described by its boundary: faces (pieces of analytic or NURBS surfaces), joined at edges (curves), meeting at vertices (points), with topology recording how everything connects. A cylinder in B-rep is a true cylinder — radius 10.000 mm, exactly — not a bundle of flat triangles pretending to be round.

That exactness is what makes B-rep the language of manufacturing. Toolpaths, tolerances, and measurements all depend on real curves and surfaces.

The major kernels

  • Parasolid (Siemens) — powers SolidWorks, NX, Onshape, and Shapr3D.
  • ACIS (Dassault/Spatial) — used across many mid-range tools.
  • Convergence/CGM — CATIA's kernel.
  • OpenCASCADE (OCCT) — the leading open-source kernel, used by FreeCAD, KiCad's 3D viewer, many CAM and simulation tools, and OpenShape3D on iPad and iPhone.

What a kernel actually does for you

  • Booleans: unioning two solids requires intersecting their surfaces exactly and stitching a watertight result.
  • Fillets and chamfers: rolling a ball of constant radius along an edge and trimming everything it touches — famously hard math.
  • Validity: the kernel guarantees the result is a closed, manufacturable solid, and refuses operations that would create impossible geometry.
  • Mass properties and measurement: exact areas, volumes, and centroids.

Kernel vs. mesh engines

Mesh-based tools (Blender, Nomad Sculpt, TinkerCAD's engine) approximate shapes with triangles. That is perfect for art and quick prints, but round faces are faceted, booleans accumulate error, and there are no exact dimensions to drive. See Mesh vs. Solid Modeling for the full comparison.

Why this matters on mobile

Running a full B-rep kernel on a tablet used to be unthinkable; today an iPad handles OpenCASCADE comfortably. OpenShape3D compiles OCCT natively for iPadOS and iOS, so a model made on an iPhone is the same class of exact geometry a desktop workstation would produce — and being open source, you can read exactly how it's done.