Revenge of the NURBS: Glossary of Terms For CAD to Low Poly Conversion

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CAD, FBX, and LOD, all terms associated with CAD conversion and optimization, are showing up in a lot of technical journals these days. What do they mean? You may be familiar with a few or even all of them, but perhaps not how they pertain to digital content creation, 3D Web, Augmented Reality or Virtual Reality. Here’s a handy guide to give you a brief overview.


Real-time rendering: Creation of a 3D visual, scene or experience that is interactive. This is different than traditional still renders or video rendering, which are sequential experiences, lacking interactivity. Examples of real-time rendering would be a 3D web configurator created using WebGL, an AR experience on a Hololens, or a VR experience on an HTC ViveRead our Ultimate Guide to Your Products in 3D on the Web for more information.


CAD (Computer-aided design): Computer software that facilitates the creation, modification, or optimization of a design, streamlining the design process. Often times used by manufacturers to design consumer products. Examples of this type of software would be OnShape, Solidworks, AutoCAD, Autodesk Inventor, CATIA, PTC Creo, and Siemens NX.


NURBS (Non-uniform rational basis spline): Mathematical models used in CAD, CAM (manufacturing), and CAE (engineering) using control points to represent curves, surfaces, and shapes. Essentially the units used to represent CAD data, which is great for representing engineering data, but not real-time rendering data.


Assembly hierarchy: The term assembly is used to represent the entirety of a CAD model. The assembly has a hierarchical representation, known as the assembly hierarchy, made up of smaller sub-assemblies which contain individual parts with unique part names and/or numbers.


Polygon: A geometric primitive (simple shape) used in computer modeling and rendering. Much as the NURB is the basic unit in CAD data, the polygon is the base unit in 3D models.


Polygon mesh: A surface that is constructed out of a set of polygons that are joined together by common edges. In relation to CAD, one can think of a mesh as representing a specific part, such as a screw or bracket.


Tessellate: The mathematical definition is to cover (a plane surface) by repeated use of a single shape, without gaps or overlapping. For our purposes, it means to translate NURB data into solid polygonal meshes, normally while performing some level of optimization to reduce density of the original geometry.


Instancing: The practice of rendering multiple copies of the same mesh in a scene at once. Examples are screws or washers which may appear multiple times in the same model and are geometrically identical. Instancing reduces the number of draw calls, leading to improved performance.


Mesh merging: The act of combining two or more polygon meshes, usually those with the same material, to simplify the rendering process. For further explanation, see this handy tool for Unity developers.


LOD (levels of detail): Decreasing the complexity of a 3D model in a scene as it moves away from the viewer. This is common technique in gaming engines, as an object doesn't need to be as detailed as it gains distance from the player. Many game developers use Simplygon to automatically create a range of LODs for a 3D model.


Decimate: The dictionary definition is to destroy a great number or proportion of something. For our purposes, it means to reduce the density of a model by removing some number of polygons, which normally compromises the visual quality.


Defeature: The process of removing parts or features from a design or assembly. The purpose can be to either mask design details or to remove density from the model.


Retopology: The act of recreating an existing surface with more optimal geometry. Retopology is normally employed with performance as a consideration, when only a dense mesh is available.


FBX: A polygonal file format (.fbx), owned by AutoDesk, which has become standard within the digital content creation community. FBX files are commonly used in programs such as Blender, Maya, 3DS Max, Modo, Keyshot, Unity, Unreal, and ONU.


In an industry full of acronyms, sometimes it’s nice to spell things out. Complex CAD models often require a combination of tessellation, decimation, and defeaturing to reduce the number of polygons and create the low poly models that can be used in AR, VR, 3D web or mobile.  Using ONU 3DLite will help you automate these tasks and achieve up to 99% reduction in content creation time.