CSG BREP NURBS

Hi guys, maybe this isn't the right place to post this topic (feel free to remove it if it's a problem): I know that 3D model can be divided in 2 categories: CSG (constructive solid geometry) and BREP (boundary representation). But where I can place NURBS? It's another method? Or a kind of BREP? So confused ... sorry in advance!!

Replies to This Discussion

Permalink Reply by Jon Mirtschin on June 9, 2013 at 3:38am

A Nurbs polysurface is an advanced brep (nurbs allows freeform faces), so my opinion it's categorized there. One of my requests for improvements to McNeel is for CSG objects to be saved. Note some of the shape representations in my IFC (Industry Foundation Class OpenBIM) plugin do save this state so it can be exchanged to other software (IFC is derived from STEP which can also convey CSG).

Permalink Reply by Mateusz Zwierzycki on June 9, 2013 at 3:53am

Not exactly... Let's take a look from the bottom-up :

NURBS (non-uniform rational b-spline/surface) is a type of curve/surface defined with control points. Their biggest advantage in terms of industry (their first application) is that you can redraw any NURBS entity (surface/curve) anywhere with high precision using only a tiny bit of data (control points coordinates, their weights, curve degree). Another great advantage is the ease of modelling.

BREP (boundary representation) is a type of geometry utilising NURBS surfaces. Practically one surface may be a BREP already, even solid (sphere). Usually BREP geometry consists of multiple NURBS surfaces (additionaly there may be more data about their topology to speed up some calculations like finding intersections). So BREP = set of NURBS Surfaces seen as solid.

CGS (constructive solid geometry) is a modelling technique which is contrary to the usual mesh modelling method (with vertices, edges, faces). In this case user defines only the primitives (cylinder, cone, box, sphere) and performs multiple boolean operations on them. It's easier to model that way with BREPs rather than mesh, because boolean operations work better with BREPs. If you search the internet for mesh boolean problem you can find the Autodesk guide how to do properly use booleans. There are also limits of booleans with BREPs, but these limits don't affect an usual workday, therefore making CGS more usable.

Permalink Reply by Landscape on June 10, 2013 at 4:33am