Subdivision Surfaces

Table of Contents

Subdivision Surfaces

Working with Subdivs

MTOR Subdivision Surfaces

Subdiv Menus

Converting

Subdiv Attributes

Modeling

Creases

Corners

Managing Creases and Corners

Shading

Maya Subdivision Surfaces

Limitations

Creating, Modeling, and Texturing

Known Bugs and Limitations

Subdivision Surfaces

What are Subdivision Surfaces?
The subdivision surface is distinctly different from either NURBS or polygons. Subdivision surfaces combine many strengths of both surface types. For more info about the details of a subdivision surface, see the PRMan application note: Using the Subdivision Mesh Primitive.

Working with Subdivision Surfaces

You'll find that MTOR supports two implementations of subdivision surfaces: MTOR subdivisions and Maya subdivisions. Each implementation offers their own tools for working with subdivision surface, but regardless of which is used the final image will basically be the same, since both will be rendered by PRMan as high quality Catmull-Clark subdivision surfaces. So when modeling and texturing subdivision surfaces, you have two alternatives. 

First, MTOR provides its own subdivision tools, which are completely integrated with PRMan. Alternatively, MTOR provides direct support for Maya subdivision surfaces, as provided in Maya Unlimited, but there are some limitations imposed upon them (MTOR does not support hierarchical modeling). The best policy is to use MTOR subdivision surfaces when possible, unless Maya's subdivisions are required for a special purpose. The two implementations are not directly compatible, which means you should decide which is best for you before traveling to far down one workpath. 

 In any case, you should be familiar with the workflow for both MTOR Subdivision Surfaces and Maya Subdivision Surfaces.

Maya Subdivision Surfaces

Maya subdivision surfaces are the recommended way of working with subdivision surfaces in Maya. MTOR provides nearly full support for Maya hierarchical subdivision surfaces (see limitations below). Maya subdivs have a number of advantages over MTOR's subdivision tools, mainly because Maya's subdivisions can fully interact with a scene, which is especially helpful for dynamic effects and simulations. 

Maya Subdivision Surfaces: Creating, Modeling, and Texturing

Use Maya's subdivision tools for creating and modeling subdivisions. MTOR will translate the rest. 

MTOR Subdivision Surfaces

The MTOR subdivision tools can convert any polygonal mesh into a subdivision surface. The idea is to model with Maya's rich assortment of polygon modeling tools. MTOR provides the tools to allow you to convert polygons to subdivision surfaces and to tag polygon components with subdivision surface attributes which govern crease and corner hardness in your model. MTOR Subdivision surfaces are only understood by MTOR, not Maya.

The MTOR subdivision surface tools are available in the RenderMan menu…

The MTOR Subdivision Surface Menus

Mesh as Subdiv— Converts a polygon to a MTOR subdivision surface. Toggle Object X-ray— Causes the polygonal mesh defining the subdivision surface to become transparent (this occurs when a poly model is converted to a MTOR subdiv.), so the underlying subdivision surface can be seen. If the polygonal mesh is already in X-ray mode, it will toggle back. (When reopening Maya scenes you may need to toggle object x-ray to view the underlying subdiv mesh again.)  Add Crease— Edges of a polygonal mesh can be selected and then be given a crease value between 0 and 20. A setting of 0 produces no crease. 20 is a sharp crease.  Add Corners— CVs can also be picked and given a "pointy" value, between 0 and 20. Add Holes— Faces can be set as invisible "holes" via this setting, if for some reason you don't want to just delete them. Create Shading Set— This technique of attaching multiple shaders to a subdivision surface is far inferior to the new preferable method of attaching shaders to shading groups. Don't use this; use the shading group technique instead -- if you know what's good for you.

MTOR Subdivision Surfaces: Converting

Converting Polygons to Subdivs 

You can convert a polygon to a subdivision surface at any point during the modeling process. A MTOR subdivision surface can be just as easily edited as a polygon model. The choice of when to convert the model is up to you. Next we'll outline the workflow of converting a simple polygon model. We converted the following polygon with: RenderMan-> Pixar Subdivs->Mesh as Subdiv  
 

Converting a Polygon to a MTOR Subdivision Surface
Polygon model	Converted: in X-ray mode with subdiv mesh
Note that MTOR has converted the representation of the original poly mesh to X-Ray mode and has drawn an approximation of the subdivision surface as non-x-ray polygons. Now you can see both the polymesh and the resulting surface simultaneously. So you can continue to model by editing the original polygon mesh. This means that you can move edges or vertices and view the resulting surface interactively.

 
Subdiv Attributes

When you issue the Pixar Subdivs->Mesh as Subdiv command, special attributes are attached to the new MTOR subdiv. When you create a subdiv default settings are used, but you can control these attributes via a standard Maya option box, shown below. You can modify these attributes through either the standard Maya Channel box or via the Attribute Editor. We'll explore what these attributes do now.

MTOR Subdivision Attributes
These attributes can be set from the option box when creating a MTOR Subdiv
mtorSubdiv— Renders as a subdivision surface when enabled. mtorSubdivExtrap— Controls whether extrapolation occurs for disjoint subdivision meshes, when the subdivision surface is decomposed into several independent primitives for RenderMan. If mtorSubdivExtrap is enabled MTOR will analyze the topology of the mesh and for each shading group will output extra adjacent facets during this decomposition, in order to try to ensure proper continuity of the entire subdivision mesh across the disjoint primitives.  mtorSubdivInterp— Controls the interpolate boundary tag for the subdivision surface. mtorSubdivIgnoreEdges—  Ignores standard Maya edge hardness attributes, when enabled, as controlled by: Polygons->Edges->Soften/Harden. subdivSteps— Controls the display quality of the subdivision surface within Maya.. Typical useful range: 0-3. Lower subdiv settings will increase the interactive speed of Maya, but the subdiv will be displayed as a rougher mesh, which is the trade-off. You'll want to set this according to the demands of your model and workflow. This will not affect the quality of the final rendering, only interactive display.  drawingEnabled— Causes to MTOR subdivision surface to display in Maya. When the detail isn't needed, disabling the redraw will speed up interactive mode. This setting has no effect on rendering itself, just interactive display.
Decreasing the Subdiv Steps can increase interactive speed . . .
Subdiv Steps = 3 — the default	Subdiv Steps = 1 — rougher but faster

Attributes and the Dependency Graph
The one thing you must keep in mind, however, is where to find the attributes. The rule of thumb: attributes are located at the point in the dependency graph where they have the most relevance. In particular, attributes which control how the subdivision surface is approximated and drawn are located on the node responsible for drawing them: mtorSubdivNode. Attributes that are logically associated with the polygon mesh: how to render, which tags, etc. are associated with the polygon mesh shape node. Other tags, as we'll see below are controlled via the Maya set whose members are the individual polymesh components (edges, vertices or facets) which share the crease, corner or hole attribute.

Here's a fragment of the hypergraph for our scene:

Notice that the sole input into mtorSubdivNode1 comes from pTorusShape1. This connection represents the polygon mesh data. If you wish to modify visualization controls, you should select the mtorSubdivNode1 node and then either turn your attention to the Maya Channel box or invoke the Attribute Editor window.

MTOR Subdivision Surfaces: Modeling

Subdivision surfaces are very powerful geometric primitives because you can represent arbitrary topologies with relatively few points. All of Maya's polygon tools can be used to model MTOR subdivision surfaces, and that makes working with them pretty straight forward.

One of the nice features of subdivisions is the ability to crease corners and points, which allows you to shape the model without changing the underlying geometry. MTOR does have a few tools of it's own for creating creases and points, at varying degrees of intensity. Where NURBS demand additional geometry to create a crease, creases can simply by added to subdivision surfaces. We'll outline the use of MTOR's tools here . . .

 

Adding Corners
Corners can be added by selecting a few polygon CV's and applying corner hardness to them from MTOR's subdivision menu. On the right you can see the corner option box. It allows you to add degrees of corner hardness, from 0 to 20. Here are a few examples of corner hardness, applied to the four CVs on the outside rim.
Corner = 0	Corner = 1
Corner = 3	Corner = 20

The Crease and Corner Attribute
MTOR attaches an attribute to control the intensity of a crease or a corner. You can edit this attribute, mtorSubdivCrease by selecting the mtorCrease node in the hypergraph and, again, either referring to the Channel box or raising the Attribute Editor window.  

Managing Creases and Corners

With all of these creases and corners it can get pretty complicated trying to keep track of them. MTOR tries to help out the matter by creating a new Maya set with every Add Crease and Add Corner command (as well as Add Hole). You can you these sets to select and reselect groups of poly edges and CVs.
 

We've implemented corner, crease and hole support through the use of standard Maya sets in order to gain the benefits of the Maya Set Editor Window. Through the use of this window's functions, it's reasonably easy to modify the membership of a set. For example, let's say you've added a crease tag to your mesh. The edges that were selected at the time you invoked the Add Crease command are automatically added to the newly minted set whose name is some variant of mtorCrease. You can use the set editor window to re-select the edges in a single operation. You can also remove edges from the set or add the currently selected edges to any set. This leads us to suggest the following modeling strategy: create sets which are logical entities in your model. Even if you plan to associate the same crease strength to multiple logical entities in your model, there is added benefit to being able to select and manipulate the sets independently.

A Subdiv Example

This is a simple example a subdiv model. Here we've started with the little model from above. By using straight forward polygonal modeling operations, we add complexity to our model where it needs it. Mainly notice that it certain areas, like the "cups" of the object there isn't a need for a lot of geometry. The geometry is put where it's needed for details. In the best case, a well modeled subdiv is constructed with the underlying structure of the rigging required for animation, so the strategically placed geometry can function somewhat like a Maya lattice. 

Our Maya model from above, pushed, pulled, extruded and tweaked.	With the addition of creases and corners, a few CVs define a complex topology.

MTOR Subdivision Surfaces: Shading

Shading MTOR subdivision surfaces is really no different than working with their cousin, the polygonal mesh. The standard approaches from the polygonal world are valid. All of the shading and texturing techniques outlined in Shader Concepts apply equally well to subdivision surfaces.

UV Editing — This is the easiest way of texturing subdivs, by taking advantage of Maya's UV Texture Editor and the rest of Maya's polygonal texturing tools. MTOR subdivision surfaces provide full support for Maya's UV Texture Editor, including support for 3D paint (Here's the 3D Paint Tutorial). When working with textures on subdivs, you'll want to edit the UVs on the underlying polygonal mesh that defines the subdiv. You'll want to connect a ST Manifold to your Slim shading network to map textures and shaders to the UVs of your subdiv. If you create a Maya shader to help visualize the texture in Hardware Texturing mode, the texture will be displayed on the polygonal surface. 

Surface Parameterization — While generally not preferred, occasionally you'll want to apply textures to a subdiv using the surface parameterization, using the ST coordinates across the entire surface. Most subdivs have an arbitrary topology and the notion of ST coordinates stretched across the surface is irrelevant, but some subdivs, like a plane, are in fact good candidates. You'll notice, however, that "pinching" may result when using the subdiv defaults to a plane. If this is observed you can switch from the default subdiv mode, facevarying, by selecting the subdiv and attaching the smooth attribute on a per surface basis: RenderMan-> Attributes-> Maya Subdiv UVs-> Smooth. That will remedy the issue of pinching.

Projection techniques — Can be used to shade and texture subdivision surfaces. All types of projections can be used to  map textures and procedural shaders onto objects. MTOR Coordinate Systems can be used to accurately project shaders and textures onto objects. A MTOR Coordinate System can be used to take a picture of the object, and then that picture can be painted in a paint program and projected back onto the object from the same coordinate system.

Applying Shaders to Groups of Faces

Typically, you'll assign a single appearance to each subdivision surface and be on your way toward PhotoRealistic nirvana. However, it is occasionally useful attach shaders to groups of faces, assigning different shaders to different portions of a subdiv. Using this method, Maya shaders are assigned to groups of faces, and then Slim shaders are attached to the Shading Groups of these Maya shaders, and not the objects themselves. For optimal results, you'll generally want the attribute, mtorSubdivExtrap, enabled to preserve the mesh curvature. This method also works on normal polygons.  

This technique generally works well, but you might encounter some cases where the automatic approach fails.  Here you might try the older technique utilizing shading sets. This is a chance this baroque technique may prove more successful if the shading group method fails.

Known Bugs and Limitations for MTOR subdivs

• When a file is reopened, "Toggle Object X-ray" must be performed upon subdivs.
• Error reporting for invalid mesh topologies is weak. Invalid mesh topologies come about in many conditions such as:
  • coincident vertices or edges
  • facets have holes
• Currently there are some conditions where corners drawn by MTOR within Maya don't match the RenderMan image. Corners drawn within Maya always appear to have infinite hardness.
• Holes aren't yet supported in the visualization and result in undrawn subdivisions surfaces. They will, however, render correctly in RenderMan.
• There are some update problems when working with Shading Sets. You can force an update by changing one of the subdiv parameters associated with the mesh (eg: Subdiv Steps).
• Texture coordinates on polygon meshes can be generated though standard Maya projection commands. MTOR only outputs texture coordinates to RenderMan if there is exactly one UV for every vertex. This is usually not the case if you apply multiple projections onto a single polygon mesh by projecting onto different facet collections.
• There may be conditions in which drawing the subdivision surface in Maya has unintended visual effects on other objects in the scene.
• Every time a mesh changes, the dependency graph causes the subdivision surface to be recalculated. At this time the recalcs are far from optimal and, depending on the complexity of your polygon mesh, may impede productivity. You can work around this limitation by turning the mtorSubdivDraw attribute associated with the mtorSubdivNode off.
• If you turn up the number of subdivisions too high, it's easy to exhaust your memory supply and cause Maya to quit. We recommend keeping mtorSubdivSteps to a value less than 4, keep it at 2 or 1.