The "Res up" process, for new display resolutions

I build all of my 3D models using The Foundry's "MODO" application, which is a polygonal modeler. This process, and the models created with it, have served me well for many years. As my models have become more accurate and more detailed, some customers have started to request these in file formats that are more suitable to high-end CAD software, and/or machining.This brief article may help to explain the differences in file types, and why some prices will be affected. This can't be explained without some "tech talk", so bear with me here...

NOTE: The article below only affects customers who require the actual 3D model file. (as opposed to images and animation)

Can't you just "press a button" to save these models in different file formats?

In the case of polygonal models, (including native MODO format, (lxo) LightWave Object, McNeel Rhino 3DM, Autodesk DXF, Autodesk FBX, Wavefront OBJ, and 3D Studio 3DS) the answer is often "Yes". Sometimes a particular format has requirements that mean making a few changes, such as triangulating the polygons, etc., but it's still a fairly quick conversion. However.. To convert these same models to IGES, SAT, STEP, 3DM, and STL, the process is quite different, and in many cases, means that some or all of the model has to be completely rebuilt.

What is "Watertight", and what are "Solids"?

Imagine a cube made out of 6 pieces of square plastic, glued together. That sort of structure qualifies as watertight, and can be produced with polygonal modeling software, including MODO. If you were to make a hole in such a cube, you would obviously find that it was completely hollow. Watertight parts are not necessarily solids, and their wall thickness may be undefined. (infinitely-thin, based on one-sided polygons)

Now imagine the cube again, but this time, carved out of a single piece of wood. That is a "solid". If you're sending a part to a 3D printer, a part that is solid will use more material, and therefore cost more than a part that is "hollowed out". To solve this problem, parts can be constructed that are hollow in this way, but the walls that surround the hollow area would have a defined thickness, and still considered solid. Solids are inherently watertight.

It's not just about the file format...
For example, if you simply request a part "in STL format", I can take any polygonal model and quickly export it to STL. But, unless you've specified it, that doesn't necessarily mean that it's either watertight or solid,

The process:

MODO uses two different forms of modeling. One is standard "hard" polygons, and the other is subdivision surface modeling. The newest algorithm available to us is "Pixar Subdivision Surfaces", (Catmull-Clark algorithm) or "Psubs", which allows a sophisticated version of something called "Edge Weighting". With previous versions of sub-d models, we had to add additional geometry ("edge loops") to control the sharpness of edges, Now, edge weighting allows us to control the sharpness of edges via a vertex weight map, without adding more geometry. From a modeling point of view, this means a savings in both complexity and file size, and it means that once converted, these models are easier to edit. Models produced in this way are also inherently "smoother" than their polygonal counterparts, when viewed in extreme close-up.

In order to provide 3D models as true solids, all the parts have to be built in this "Psubs" format, which then allows their export in IGES, SAT, STEP, and 3DM format. Having done several major projects this way, I know that these generally will import into software such as Solidworks as true solids, with no problem. For those of you interested in the technical details, I'm using is the "Power SubD - NURBS" plug-in, created by IntegrityWare.

The "issue" with converting existing models is the result of a standard workflow in the polygonal modeling world. That is, we often prototype parts in "Psubs", because of their great ability to produce smooth surfaces, but unfortunately, at this time, Psubs (or any subdivision surfaces) don't lend themselves to using boolean operations very well. So, to create various doors, windows, hatches, etc., in something like an aircraft fuselage, we typically "freeze" the geometry to hard polygons, and then use boolean operations. At the end of a modeling project, this results in a model that is a mixture of subdivision surfaces and hard polygons. While this doesn't affect the appearance of a model in an image or animation, it does affect it's suitability to be used in a solids-based program, (i.e. NURBS) and/or for machining. So, while relatively-simple parts, like landing gear tires will probably remain in Psub form, and are therefore easy to convert, more complex parts, like the fuselage example above, have to be rebuilt in a Psub form. That's why this process takes much more time than a polygonal model, and is more expensive.

Click here for the follow-along article on the Aermacchi MB-326G Project, where a scale model was built for the purpose of machining the (IGES) parts.

- Mike James

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NOTE: I am not at liberty to redistribute any of the documentation used to build any of the 3D models on this site.