Patch Layout 101

This week we’ll be taking a brief look at “patch layout” for NURBS modeling, the fundamental concept underlying surface modeling in any surfacing package, be it Rhino, Alias, ICEM, SolidWorks, ProE/ISDX, UGNX, Catia, or Bob’s Pretty Good CAD. You already know how to “see” the building-block shapes that make up simple geometric designs, but a good surfacer needs to be able to see the building block *surfaces* that make up complex forms.

More videos here:

Surfacing is very similar to drawing, in that you should begin with loose, broad strokes, and then gradually work your way into finer and finer detail. The biggest (“primary”) surfaces are built first, followed by smaller (“secondary”) connecting surfaces, and lastly the tiny (“tertiary”) surfaces that close up the gaps.

Start by looking for the “Primary” surfaces, aka “slab” surfaces. These will be large, smooth surfaces. Try to imagine them as 4-sided; remember that *every* surface in NURBS is ultimately four-sided, and any surface with any number of sides other than 4 has been trimmed! Try to imagine the entire four-sided surface in your mind, and then look for the places where it will need to be trimmed! As a general rule, keep your surfaces as simple as possible. If you can build the entire model using only 4-sided surfaces, do it!

Primary surface tools in Rhino:

• Plane
• Extrude
• Revolve
• Loft
• Sweep 1 Rail
• Sweep 2 Rails

After you’ve built your primary surfaces, it’s time to start adding “secondary” surfaces. These are surfaces that connect primaries, usually smoothly. Your secondary surfaces rely on the primary surfaces as reference, so it is important that they be built AFTER the primary surfaces are complete. Once again, they should be four-sided wherever possible. Avoid large or complex secondary surfaces: it is better to have lots of small, simple surfaces than one great-big complex surface.

In Rhino, it is common to use “Match Surface” to connect two surfaces that may not meet up perfectly. This way you can use primary surface tools to build geometry that is ultimately secondary. This work flow is sub-optimal, but is unavoidable in Rhino.

Secondary surface tools in Rhino:

• Loft + Match Surface
• 2 Rail Sweep + Match Surface
• Curve Network

Last of all you’ll add what we call “Tertiary” surfaces. These are the most complex surfaces on your model, and should be as small and inconspicuous as possible. It is very difficult to control the surface curvature of tertiary surfaces with great accuracy, so it is best to avoid them where possible. Even though they are the smallest surfaces on your model, they will often take the most time to build.

Tertiary surface tools in Rhino:

• Curve Network
• Patch

We’ll look at all of these surface types in greater detail in the future. For now, practice by looking at products in every day life, and try to break them down visually into their individual four-sided surfaces. You may find you never look at a product the same way again!

about adam: Adam O'Hern is an industrial design consultant specializing in visual brand languages, and has designed products ranging from laptops to power tools, classroom toys to bathroom fixtures, and robots to lint rollers. He has published with 3DWorld Magazine, CGTuts+, and Luxology, and works with Josh Mings of SolidSmack.com on EngineerVsDesigner.com.