modeling tips.pdf
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Modeling Tips
TopologyUse Fewer SpansAvoid High Valence verticesEdgeLoop TransitionsPractical Topology PrimerTriangles and NGons
SemiSharp CreasesUse crease sets
Additional Resources
The following sections describe common techniques specific to modeling with subdivision surfaces.
Note
The following information contains techniques specific to the CatmullClark subdivision scheme. Someelements need to be adjusted for Loop surfaces.
TopologyWellconstructed subdivision meshes have several important properties:
They consist primarily of quad facesThey contain few extraordinary verticesThey efficiently describe the intended shapeThey are topologically manifold
Use Fewer SpansWhile polygon models need to use a large number of spans to approximate smooth curved surfaces, subdivision models requiresignificantly fewer control points.
In most situations, 6 spans are enough to create accurate circular shapes, and 4 is often enough to approximate background objects.
Avoid High Valence verticesA high valence vertex is a vertex connected to more than 4 adjacent edges.
High valence vertices cause several problems when subdivided:
The CatmullClark scheme can produce "wavy" surfaces when a revolution vertex is surrounded by triangles (seehere):
High valence vertices incur fairly large performance hits
Currently, OpenSubdiv has a hard constraint imposed by GPU shaders on the maximum valence of a vertex(~27 on current hardware)
Instead, here are some topological strategies to cap revolution shapes:
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Note that all these cylinders use only quad faces, and all the vertices in the caps have a valence of 4 (except the bottom left example)
EdgeLoop TransitionsIt is often necessary to vary the density of control vertices over the surface mesh: areas around the fingers of a hand require moreCVs than the comparatively simpler region around the palm. It is important to handle the topology around these transitions efficiently.
One strategy is to use extraordinary vertices, such as this example, using a valence 5 vertex to expand 3 edge loops into 5.
Practical Topology PrimerSome realworld examples showing how to produce detailed shapes with sparse topology, few extraordinary vertices, and no highvalence revolution poles.
Triangles and NGonsUsed sparsely, nonquads can be very useful to gather 3 or more diverging edgeloops. These are often encountered in highlydeforming areas with curvature saddlepoints (ex: armtorso connection). The strategic placement of a pentagon in one of these criticalspots ensures that the surface remains smooth, while allowing for complex topology to flow around.
SemiSharp CreasesSemisharp creases can be a very powerful tool for hardsurface modeling.
Both edges and vertices can be tagged with a sharpness value.Crease sharpness values range from 0 (smooth) to 10 (infinitely sharp)It is generally cheaper to use creases whenever possible, instead of adding extra edges/edgeloops. However...Creases introduce extra computation costs that are proportional to the sharpness value. So...Sharpness values above 5 should rarely be needed.
The following sections introduce some techniques to best leverage them.
Use crease setsComplex hardsurface models (giant robots, vehicles, buildings...) are likely to tag large number of edges : it is extremely useful toorganize these edges/edge loops into logical sets with descriptive names. Edges or vertices in a crease set group all share the samesharpness value.
If you are modeling with Maya, the CreaseSetEditor implements this type of workflow. Additionally, for debugging purposes, it is oftenvery helpful if the name of a set contains the sharpness value (ex: topDeck_2).
Besides authoring convenience, one of the benefits of having many edgeloops share identical sharpness values is that it enables verypowerful performance optimizations within the feature adaptive algorithm (faster renders & less memory).
Additional ResourcesAn excellent short tutorial from the Guerrilla CG Project that illustrates many of the common pitfalls of subdivision modeling, and thestrategies to overcome them:
User Docs API Docs Release Notes Forum
Subdivision Surfaces: Artifacts
Ivo Kos, Modelling Technical Director at Pixar Animation Studios, shows some of the modeling techniques he uses when modelingprops and architecture sets for feature films.
Why Model with Subdivs? By Ivofrom Pixar Graphics PLUS
07:23
Generated on: 20150814 17:39 UTC.