topic Re: How are models represented in data structures in Fusion Design, Validate & Document Forum

How are models represented in data structures

tommVS2MB — Sun, 30 Oct 2022 17:29:58 GMT

I am curious about the internal representation of my model. It was said in another post that I don't know much about that. And, that probably is true!

I want to understand how curves in my model are represented. My models are used for laser engraving/cutting and 3D printing.

This topic makes me recall the evolution of the data structures of fonts from bit mapped to TT.

I have read the following and it seems to imply that the representation is rectilinear in f360but that others like CATIA use nurbs:

"In rectilinear CAD systems, drawn curves are merely estimated; a view at a high resolution reveals the arcs as tiny stairsteps of right angles. In NURBS models, every point on the surface is calculated with pinpoint accuracy, regardless of resolution.

Although some 3D CAD applications may include a module to convert models to NURBS, this sideline function does not assist in solving curve-related design problems. Software wholly based on a NURBS engine, like those from Rhinoceros, CATIA, or Dassault, are able to parcel the surfaces into component parts, edit and enhance forms, and automatically perform analysis on curvilinear models – all of which require frequent NURBS re-calculation." source: https://www.designworldonline.com/nurbs-add-a-curve-to-cad-modeling/

Thanks, Tom

Re: How are models represented in data structures

TrippyLighting — Mon, 31 Oct 2022 02:15:36 GMT

All current main stream CAD software represents 3D geometry in form of NURBS of different (mathematical) degrees.

That includes Fusion 360!

The geometry is created by the geometric modeling kernels employed in these CAD systems. Fusion 360 and Autodesk Inventor use the ASM (Autodesk Shape Manager), which is a fork of an early ACIS kernel.

The two geometric modeling kernels used most are Parasolids and ACIS with Parasolids taking the lead.

Once you export that mathematically precise 3D geometry into a .stl file it is converted into a triangulated mesh, data/information is lost. NURBS are essentially of infinite resolution, whereas a mesh, particularly a triangulated mesh has a finite resolution. The second and equally important information lost in the conversion process is topology. As such .stl meshes are not great candidates as input to a precise design process.

Similarly curves created for .dxf files might require a spline to poly line conversion, because many laser cutter software use ancient algorithms that cannot deal with splines.

Re: How are models represented in data structures

tommVS2MB — Mon, 31 Oct 2022 14:25:02 GMT

Thank you very much.