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NURBS and Polygon Mesh—why both models are needed.

As you may or may not know, when working in BIMscript you are required to create a Non-Uniform Rational Basis Spline (NURBS) and a duplicate Polygon Mesh of that model. What’s with the extra step? Explanations are in order. 

Both NURBS and Polygon Meshes are capable of producing high-quality, three-dimensional models, the difference is in the details. From mathematically precise objects with complex curves to connecting points in space for the creation of vertices, it starts with a clearer understanding of these two most commonly used techniques in developing 3D modeling. 

NURBS—get the smooth moves. 

Spline Modeling—or NURBS—generates accurate geometry with the smoothest surface. These geometries are mathematically defined by control points that result in complex curves to form surfaces around them. To adjust surfaces you manipulate control points. A lot of 3D scans will produce models using NURBS to create an exact replica. Therefore this method is ideal for CAD-based programs because its more accurate, takes up less data space and is easier to translate into different programs. 

Since NURBS consist of several small curved-surfaces, it can be difficult to attach all together. In order to bind them, one must define how they are connected by adjusting the geometry continuity. The three levels are G0—Positional, G1—Tangential and G2—Curvature. Some complex geometric surfaces may not be perfectly bound and this results in a visible seam. It’s extremely important that you carefully inspect your model for these lines, especially when exporting as a polygon mesh to generate an STL file for 3D printing. This medium requires watertight geometry. Other file formats include STP and RFA.

NURBS modeling.
Image taken from https://discourse.mcneel.com.

Polygon Mesh—shell out a surface.

Polygon Modeling creates a 3D object non-mathematically. It’s the preferred method of choice for STL (3D) printing. By connecting points in space you produce edges on a 2D surface, then you can combine and move the said surfaces to form a shell. So it’s not as accurate as NURBS, because it doesn’t average the curve between points. Instead it uses polygons—triangles or quadrilaterals—to fabricate the approximation of a smooth edge. This makes Polygon Modeling easy to manipulate, easy to work with in subdivisions and easy to attach. Object smoothness is achieved by increasing the number of polygons for full detail and density level control within a model. However, this control will impact file size. So note to self when converting NURBS into Polygon Modeling, when you set the resolution this will affect the number of polygons, which will result in the final file size. Other file formats include FBX and OBJ. 

Polygon Modeling
Image from Wikipedia.

Multiple application platforms. Multiple modeling techniques.  

Something else you may or may not know, BIMscript enables content developers to generate parametric BIM objects for multiple platforms—BIM systems, 2D CAD, 3D rendering, Augmented & Virtual Reality plus 3D printing. Some of these applications require NURBS. The others only support Polygon Modeling. Therefore all files created with BIMscript, need both techniques to satisfy all users of the BIMobject ecosystem. 

Here’s an BIMscript insider tip: Rhinoceros can convert NURBS into Polygon Modeling  through the Mesh command. However, it is unable to convert the opposite way natively. Good to keep in mind when choosing your modeling method. You can also source third party plugins for assistance, one example is Mesh2Surface. Once you’ve created both geometries, BIMscript will select the appropriate configuration that fits the correct file format. 

We hope you’ve learned a little to maximize your productivity. To find out more about BIMscript, becoming an Accredited BIMscript Content Developer (ABCD) or onsite training, simply log onto bimscript.bimobject.com

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