## Subdivision Modeling Theory and Principles

*Subdivision Surfaces & Polygon Modeling*

*Subdivision Surfaces & Polygon Modeling*

Cartesian coordinates are a way of locating objects in 3d space. Virtually all 3d software uses this coordinate system. Coordinates are read out from the origin, or 0,0,0. When you create 3d models, vertices are positioned in space using X, Y, and Z. Lots of vertices = lots of points in RAM = slow performance!

When you are working, it is important to center your model around the world origin (0,0,0 which is the center of the workplane grid in 3DS Max). This is because many modifiers rely on the origin for symmetry, mirroring, and more.

Polygon modeling is the act of creating an object using the basic elements of a 3D model. It is also the foundation that subdivision modeling is based on. As you learned in Section 01, these elements are vertices, edges and faces.

Faces can be made of any number of vertices or edges. Faces with three vertices & three edges are called triangles/tris. Faces with 4 vertices and 4 edges are called quads. Faces with any more than 4 edges or tris are called n-gons.

Polygon modeling is easy to edit – just grab a mesh element and move it around.

However, the more detailed and/or smooth the model is, the higher the polycount and the more difficult it is to manipulate all those elements effectively.

*Image credits: **Math and Movies (Animation at Pixar) – Numberphile**@3:52, Youtube*

*Image credit: **Polycount Wiki*

*Image credit: **Polycount Wiki*

*Creating Subdivided Models in 3DS Max*

To create subdivision models in 3DS Max there are three modifier options; MeshSmooth, TurboSmooth, and OpenSubdiv.

We recommend that you use OpenSubdiv and never use MeshSmooth. TurboSmooth is OK to use but OpenSubdiv has more functionality and is a newer implementation of subdivision in 3DS Max. Given the choice, you should always choose OpenSubdiv.

Topology is used to describe the contours or shape of an object. In the 3D world, topology refers to the layout or “flow” of polygons as they describe a surface. Proper topology is important for three reasons:

- The model will be described correctly with consistent smoothing and highlights.
- Modeling shortcuts like selecting loops (connected chains of faces or edges) will work more efficiently and reliably.
- Subdivision surfaces will divide predictably, leaving no artifacts on the surface. For more details on these artifacts see
**this video by Guerilla CG**.

There are some general rules to topology that you should follow:

- Create a rough model first for the basic shape, then add detail once the proportions are correct.
- Model with a minimum amount of geometry, for easier work. More geo means more control (but more work). Only add detail when you need it.
- Model in quads as often as possible. Triangles and polygons with more than four sides can cause pinching and other smoothing errors.
- Edge loops should be preserved whenever possible. You will learn when and where to break edge loops as you get more experienced.
- A pole is a vertex that doesn’t connect four edges. Avoid when possible, but they are required for most models.
- Isolate direction changes to allow you finer control over them.

We recommend looking through some of the subdivision examples and tutorials on Polycount’s **Subdivision Surface Modeling** page for more detail.

*a. Detail-In (Box/Primitive Modeling)*

With Detail-In/Box Modeling, you start with a primitive, usually a box. By adding edge loops (rings of connected edges) you are adding extra detail that you can manipulate to create shapes.

*b. Detail-Out (Surface Modeling)*

This modeling methodology relies on creating strips of detail and joining them (or thickening, using a shell modifier) to create a model. When modeling with this method it is important to make sure your strips have the same number of faces to make sure only quads are generated when they are joined.

*a. Retaining Loops*

One thing you’ll notice pretty quickly is how subdivision will remove the hard edges in your model. For example, this cube was turned into a sphere by just adding an OpenSubdiv modifier on top of the stack.

*Image credit: * *Polycount Wiki*

*b. Crease Sets*

The other method for creating hard edges is by using edge creasing. This technique is only available when using Opensubdiv. Edge creases work by adding a “weight” to each edge, causing it to appear sharper when the object is subdivided. Edges can be creased arbitrarily, so it is not as reliant on topology as adding a retaining loop.

To make an edge crease, you have to add two modifiers – a CreaseSet and an OpenSubdiv. In the CreaseSet modifier, select the edges you want to crease, and then select “Create Set”. This will create a new “EdgeSet” entry. You can adjust the creasing of this EdgeSet with the number to the right. Here, these edges have been set to a crease level of .5.

Once you go to the OpenSubdiv modifier, you will see the effect of the crease set.

Properly setting up your modifier stack is important for the stability of your model and ease of editing. When working, your Edit Poly modifier should always be below the subdivision modifier. The only modifiers to use above subdivision are V-Ray modifiers, like VRayDisplacementMod.

This is because many modifiers will change the number of vertices in your mesh. If this happens, the vertices you have selected in Edit Poly will be incorrect and your model will display significant errors. In the example below, a change was made in the Edit Poly modifier above Turbosmooth. Then, the Turbosmooth amount was changed and the model broke. This is the **incorrect** way to organize your modifier stack.

This creates the results below:

From 3DS Max’s knowledge base entry:

“Smoothing groups are numbers assigned to the faces or patches of an object. Each face or patch can carry any number of smoothing groups up to the maximum of 32. If two faces or patches share an edge and share the same smoothing group, they will render as a smooth surface. If they don’t share the same smoothing group, the edge between them will render as a corner.”

*Image Credit: **Autodesk*

*a. Break it Down*

Before getting started with any model it really helps to break it down into its separate sub-objects so that you know exactly what it is that you have to model and what should be modeled separately vs. together.

When you do this, you should also identify any duplicate or similar pieces of geometry. These duplicates can be instanced instead of created from scratch, so any changes done to one will automatically affect the other. Items like the pillows on the seat and arms of this couch are similar and thus can be instanced.

*b. Guesstimating the Topology*

Once you’ve blocked out the different sections of the model, you should start to draw over it (in your mind or on paper) to get a general layout of the topology you want to use. It should not be complex – in fact the simpler, the better. The idea is to just get an idea of the topology that you will need to construct. Notice that the couch’s foot is just a simple cone without the pointy end on top. The topology for that cone is just a truncated pyramid – then once it’s subdivided it becomes a cone. So the point here is to keep your geometry very simple and think in broad strokes.

*c. Estimating Proportions*

Next, you will need to figure out how big things are. While you may have some product information that tells you how big the product is overall you may not know the specific dimensions of individual model components.

The first thing to do is to create a bounding box to match the dimensions of the product. This bounding box creates a limit to your model based on the maximum listed proportions of the product. If your model exceeds the size of this box in any direction then you know you probably did something wrong. When making your box, right click on it -> Display Settings -> Box to make it easily see-through so you can see what you’re working on. It also helps to Freeze/Lock the box as well so it can’t be selected or edited.

After creating the bounding box, there are two major ways of determining the proportions of a product They can be used together to produce an even more accurate result, or separately if one of them doesn’t work well.

i. The first method is to use perspective matching. We have a video tutorial which you can view **here**.

ii. The second is to use ratios between different points on the image as a comparison. So in the example below, we know that the leg height is 4” (roughly 10 cm). By drawing a line that matches the height of the leg in the image, we can create a ruler. By stacking these lines on top of each other, we know that the front of the couch is roughly 17-18 inches (42.5~45 cm) in height. We can compare different parts of the image together to get an idea of the proportions in the model.

*d. Guesstimating the Topology*

Once you’ve roughed out what you need to build in your head, the next thing to do is to block it in. Everything you create at this stage should be basically primitives or very simple shapes. The idea here is to get all the rough shapes and general proportions down before you move on to refining it. You can use either modeling technique to make this, but your first iteration should always be simple. Think of it like sketching an outline before you start the real drawing, using that outline as a guide.

**Polycount Wiki**.