Artsy Rants

Sculpting in Virtual Clay

  • February 14, 2016

A while back I wrote a post here likening 3D modeling to creating a model in virtual clay. Today I’m going to extend that metaphor by talking about a modeling technique that makes it all but literal.

To set the stage, let’s recap the structure of your standard 3D model: in sum, they’re made of a bunch of points in 3D space that define the corners of a bunch of polygons, like this:

wires cage

This dude’s a bit rough around the edges

In calculus a curve is approximated by a bunch of line segments that, if you make those segments small enough, starts to look nice and smooth. That was Newton’s magic trick to make them computable and the same principle applies in 3D modeling. You can instantly smooth out a blocky mesh cage like the one above in essentially every 3D modeling application out there, so you end up with something that looks like this when it’s rendered:

wires subd

A smooth operator

That’s all well and good, but what if you want to add a very high level of detailing to your model, like scales or wrinkled skin—or even hair? You can use tricks like bump mapping to give the appearance of fine detailing without adding to the polygon count of a model but, since there’s no actual geometry in those details, the effect can fall apart under close-ups or in silhouette.

So, if you need to add actual geometry, just imagine how crazy it’s gonna get creating polygons to represent all that stuff!

Enter sculpting.

The key to many sculpting applications is what’s called a “voxel,” or a “pixel with volume.” The voxel takes the concept of curves with infinitely small (well, actually, just small enough to look smooth in renders) segments and expands it into three dimensions so, instead of working with a hollow polygon cage, you now have an actual (well, virtual) solid mass to poke and pull and mangle.

In a common sculpting workflow (for those of us who start building a model using our favored “traditional” modeling application) you import your working mesh into the sculpting app, then convert it to voxels. This new version of the model can now be sculpted using a variety of tools that allow you to (for example) push grooves, pull ridges, spread and pinch areas, smooth out other areas, and even press (or raise) pre-defined shapes on the “clay” surface such as patterns of scales or wrinkles.

Here are a couple of examples of the gargoyle model shown above after some sculpting work to add muscle definition:

upper body

Getting ripped the easy way

lower body

The nubs are where the wings and tail will be attached

As you can see, the detailing—while far from complete in these images—is starting to look pretty good. “Painting” details onto a surface with mouse or digital tablet is a skill that does require a little practice to get used to but, once you’ve settled into a technique to get the effects you want, can go quite quickly (especially with pre-defined stamps) and yield very nice results.

Once you’re done sculpting your new masterpiece you are, unsurprisingly, left with a model that’s not exactly amenable to animating—or even rendering except using software geared to voxel renders (popular in medical applications). As is, the model above would weigh in at over 15 million polygons—and it’s not even done yet. While Weta or Pixar might have the tools to animate such a poly load, we mere mortals are better served trying to mitigate that polygon count somewhat.

A common method for going from voxels back to polygons is called “retopo,” short for re-topology. The basic concept is you again “paint” on the voxel surface, but this time you’re painting polygons that adhere to the model and that you stitch together to build the new polygon cage you can export for use in your traditional animation and rendering software. This retopo process can be fraught with tedium but many sculpting programs offer tools to ease the burden (and even automate the chore to varying degrees).

This is a screen shot of the results of performing a fully-automated retopo on the above model in the 3D Coat application.



Without any operator intervention to guide the process the resulting topology shown above is, frankly, pretty crappy but you get the idea. In actual practice you’d never auto retopo a production model except for areas of lower detail. You don’t want to spend all that time sculpting David only to have the software wash out (or even mangle) your work.

And that’s how you do it. Please feel free to check out my post about the anatomy of creating a 3D character to see some of what comes next, such as rigging, surfacing, and posing.

Until next time!

Spinland Studios, LLC is a high-tech branding and marketing studio in the Mohawk Valley of Upstate New York. We leverage the power and magic of 3D modeling and animation to take your company’s image places you can only imagine. Defy conventional marketing and bring your brand to life! Visit for more information and examples—then hire us to boost your company’s marketing image into the 21st century!

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