Sustainability of Digital Formats: Planning for Library of Congress Collections

Scope
This diverse catagory includes formats for 2D and 3D computer-aided design (CAD) and computer-aided manufacturing (CAM), built environment, schematics, architectural drawings, photogrammetry scanning, point cloud data and more. The quality and functionality factors listed here are based on an analysis from 2008 in An Overview of 3D Data Content, File Formats and Viewers, a technical report (ISDA08-002) from the National Center for Supercomputing Applications (NCSA). According to this analysis, the key aspects of a 3D model are: geometry, appearance, scene, and animation.

3D Model Geometry
The geometry of a model describes its shape. There are several approaches to defining the geometry of a 3D model, including point clouds, line sets, meshes (often triangular), constructive solid geometry (built up by combining simple shapes), and free-form surfaces (often using non-uniform rational bi-splines, commonly known as NURBS). The use of NURBS is common in computer graphics for generating and representing curves and surfaces. It offers great flexibility and precision for handling both surfaces defined by common mathematical formulae and modeled shapes.

3D Model Appearance
Appearance incorporates colors, textures, material types, shading, etc. A common approach for modeling appearance is texture mapping, in which a 2D image is molded to the surface as defined by the geometry. Another approach is to assign appearance attributes (e.g., color, texture, material type) to each face of a surface defined by a mesh. Other aspects of surface appearance include reflectivity (aka specularity) and transparency.

3D Model Scene
The scene of a model includes the position of light sources, cameras, and the relative positions of objects.

3D Model Animation
Animation defines how a 3D model moves. For a discussion of different ways to represent the animation of the components of a "skeleton," see A Comparison of 3D File Formats by Marcus Lundgren.