Crash course on CAD data. Part 2.

How is 3D data presented in CAD?

There can be multiple approaches to describing 3D data. Depending on problem domain and required operations on such data, the choice can be among the following options:

  • Voxels;
  • Meshes;
  • CSG (Constructive Solid Geometry);
  • B-Rep (Boundary Representation).

Voxels

Voxel, or volume element, a 3D counterpart of a 2D pixel, can be found in medicine and scientific applications, and games to represent graphical objects on a scene via a set of fine-grained 3D elements. Voxel data model, for instance, can result from space scanning and attributing certain graphical property (such as color and transparency) to each small cell of a 3D grid.

A series of voxels in a stack, with a single voxel shaded (image from Wikipedia)

Meshes

Mesh is a tessellated (also known as facetted or polygonal) representation of a 3D model. Meshes can be:

  • 1D: free-form curve approximation with linear segments;
  • 2D: approximation of a surface with triangles, quadrangles or arbitrary polygons;
  • 3D: filling-in the internal body space with tetrahedrons, hexahedrons, prisms, pyramids and others.

CSG (Constructive Solid Geometry)

This technique describes a 3D body via a tree of (binary, Boolean) operations (such as fuse, common or cut) required to construct such a body.

CSG objects can be represented by binary trees (image from Wikipedia)

B-Rep (or Boundary Representation)

This technique describes a 3D body using its boundaries, namely:

  • surfaces defining the body boundary in 3D space;
  • curves defining intersections of above surfaces;
  • points defining intersections of above curves.

Wireframe representation

It is a particular case of B-Rep representation used to describe 1D bodies in 3D, i.e. bodies defined via curves without surfaces. (When I was studying computational geometry and computer graphics in the University this was taught as an independent mechanism but in my professional life it turned out to be just a subtype of B-Rep, so I won’t make a distinction below).

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CAD Exchanger

CAD Exchanger

CAD Exchanger is a technology that enables data exchange in the multi-CAD world.