The plain English version. No equations. For design engineers who know CAD modelling and nothing else.

By Prasath | ScriptedCAD | https://scriptedcad.com

What B-Rep Actually Is

Your CAD file does not store the material inside the part. It stores only the outer skin.

• B-Rep means boundary representation. The boundary is the model.
• Think of a balloon. CAD stores the rubber. The air inside is just implied.
• Every solid you have ever modelled in SolidWorks or Solid Edge is stored this way.

The Two Halves

• Geometry is the shape maths. Where a surface sits and what type it is.
• Topology is the connection map. Which face touches which edge, which edge touches which vertex.
• Geometry without topology gives you loose surfaces floating in space. That is a failed STEP import.
• Topology without geometry gives you a connection list that describes nothing.

Example: geometry says "cylinder, radius 5". Topology says "that cylinder sits between the top face and the bottom face".

The Hierarchy

Top to bottom, biggest to smallest.

• Body is the whole solid. One file can hold several bodies.
• Shell is a closed bag of faces. The outer shell is the skin. An inner shell is a hollow cavity.
• Face is one bounded piece of a surface. The flat top of a plate is one face.
• Loop is the closed ring of edges that bounds a face. Outer loop is the border, inner loop is a hole.
• Edge is one bounded piece of a curve. It is where two faces meet.
• Vertex is a single point. It is where edges meet.

Each topology item sits on a matching geometry item.

The Bit That Confuses Everyone: Loops

• A plate with 4 drilled holes has one top face but five loops on it.
• One outer loop is the plate border. Four inner loops are the holes.
• That is how CAD knows the holes are holes and not islands of material.
• Without loops, a face is just an infinite plane with no boundary.

One extra term you will hit in the API: coedge, which Parasolid calls a fin. It sits between loop and edge and records which direction the edge runs for that particular face. Ignore it until you need it.

Geometry Kernels

• A kernel is the maths engine that builds and edits the B-Rep. Your CAD is the interface on top of it.
• When you hit Fillet, SolidWorks does not calculate the fillet. Parasolid does.
• A fillet or boolean failure is usually the kernel giving up, not you modelling wrong.

The ones actually in use:

Parasolid is the market leader by a wide margin, with over 350 compatible applications.

Why The Kernel Matters To You

• Same kernel means clean transfer. SolidWorks to Solid Edge to NX keeps the topology exactly as it was.
• Different kernel means the geometry gets rebuilt and rounded off. That is where gaps and failed knits come from.
• Version matters too. A newer Parasolid export cannot be read by an older Parasolid application.
• Revit sits on ACIS and SolidWorks sits on Parasolid, which is exactly why that exchange is never smooth.