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<div class="document" id="modeling-tips">
<h1 class="title">Modeling Tips</h1>
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<h1 class="articleContentTitle">Modeling Tips</h1>
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<ul class="simple">
<li><a class="reference internal" href="#topology" id="toc-entry-1">Topology</a><ul>
<li><a class="reference internal" href="#use-fewer-spans" id="toc-entry-2">Use Fewer Spans</a></li>
<li><a class="reference internal" href="#avoid-high-valence-vertices" id="toc-entry-3">Avoid High Valence vertices</a></li>
<li><a class="reference internal" href="#edge-loop-transitions" id="toc-entry-4">Edge-Loop Transitions</a></li>
<li><a class="reference internal" href="#practical-topology-primer" id="toc-entry-5">Practical Topology Primer</a></li>
<li><a class="reference internal" href="#triangles-and-n-gons" id="toc-entry-6">Triangles and N-Gons</a></li>
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<li><a class="reference internal" href="#semi-sharp-creases" id="toc-entry-7">Semi-Sharp Creases</a><ul>
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<hr class="docutils" />
<p>The following sections describe common techniques specific to modeling with
subdivision surfaces.</p>
<div class="notebox docutils container">
<p><strong>Note</strong></p>
<p>The following information contains techniques specific to the Catmull-Clark
subdivision scheme. Some elements need to be adjusted for Loop surfaces.</p>
</div>
<hr class="docutils" />
<div class="section" id="topology">
<h3>Topology</h3>
<p>Well-constructed subdivision meshes have several important properties:</p>
<blockquote>
<ul class="simple">
<li>They consist primarily of regular faces (quads for Catmull-Clark, tris for Loop)</li>
<li>They contain few extraordinary vertices</li>
<li>They efficiently describe the intended shape</li>
<li>They are topologically <a class="reference external" href="subdivision_surfaces.html#non-manifold-topology">manifold</a></li>
</ul>
</blockquote>
<hr class="docutils" />
<div class="section" id="use-fewer-spans">
<h4>Use Fewer Spans</h4>
<p>While polygon models need to use a large number of spans to approximate smooth
curved surfaces, subdivision models require significantly fewer control points.</p>
<p>In most situations, 6 spans are enough to create accurate circular shapes, and
4 is often enough to approximate background objects.</p>
<a class="reference external image-reference" href="images/mod_notes.0.png"><img alt="images/mod_notes.0.png" class="align-center" src="images/mod_notes.0.png" /></a>
</div>
<hr class="docutils" />
<div class="section" id="avoid-high-valence-vertices">
<h4>Avoid High Valence vertices</h4>
<p>A high valence vertex is a vertex connected to more than 4 adjacent edges.</p>
<p>High valence vertices cause several problems when subdivided:</p>
<blockquote>
<ul>
<li><p class="first">The Catmull-Clark scheme can produce "wavy" surfaces when a revolution
vertex is surrounded by triangles (see <a class="reference external" href="subdivision_surfaces.html#triangle-subdivision-rule">here</a>):</p>
<a class="reference external image-reference" href="images/mod_notes.2.png"><img alt="images/mod_notes.2.png" class="align-center" src="images/mod_notes.2.png" style="width: 150px;" /></a>
</li>
<li><p class="first">High valence vertices incur fairly large performance hits</p>
</li>
<li><p class="first">Currently, OpenSubdiv has a hard constraint imposed by GPU shaders on the
maximum valence of a vertex (~27 on current hardware)</p>
</li>
</ul>
</blockquote>
<p>Instead, here are some topological strategies to cap revolution shapes:</p>
<a class="reference external image-reference" href="images/mod_notes.1.png"><img alt="images/mod_notes.1.png" class="align-center" src="images/mod_notes.1.png" /></a>
<p>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)</p>
</div>
<hr class="docutils" />
<div class="section" id="edge-loop-transitions">
<h4>Edge-Loop Transitions</h4>
<p>It is often necessary to vary the density of control vertices over the surface
mesh: areas around the fingers of a hand require more CVs than the comparatively
simpler region around the palm. It is important to handle the topology around
these transitions efficiently.</p>
<p>One strategy is to use extraordinary vertices, such as this example, using a
valence 5 vertex to expand 3 edge loops into 5.</p>
<a class="reference external image-reference" href="images/edge_loops.png"><img alt="images/edge_loops.png" class="align-center" src="images/edge_loops.png" style="width: 400px;" /></a>
</div>
<hr class="docutils" />
<div class="section" id="practical-topology-primer">
<h4>Practical Topology Primer</h4>
<p>Some real-world examples showing how to produce detailed shapes with sparse
topology, few extraordinary vertices, and no high-valence revolution poles.</p>
<a class="reference external image-reference" href="images/mod_notes.3.png"><img alt="images/mod_notes.3.png" class="align-center" src="images/mod_notes.3.png" style="width: 400px;" /></a>
<a class="reference external image-reference" href="images/mod_notes.4.jpg"><img alt="images/mod_notes.4.jpg" class="align-center" src="images/mod_notes.4.jpg" style="width: 400px;" /></a>
</div>
<hr class="docutils" />
<div class="section" id="triangles-and-n-gons">
<h4>Triangles and N-Gons</h4>
<p>Used sparsely, non-quads can be very useful to gather 3 or more diverging
edge-loops. These are often encountered in highly deforming areas with curvature
saddle-points (ex: arm-torso connection). The strategic placement of a pentagon
in one of these critical spots ensures that the surface remains smooth, while
allowing for complex topology to flow around.</p>
<a class="reference external image-reference" href="images/mod_notes.5.png"><img alt="images/mod_notes.5.png" class="align-center" src="images/mod_notes.5.png" /></a>
</div>
</div>
<hr class="docutils" />
<div class="section" id="semi-sharp-creases">
<h3>Semi-Sharp Creases</h3>
<p>Semi-sharp creases can be a very powerful tool for hard-surface modeling.</p>
<blockquote>
<ul class="simple">
<li>Both edges and vertices can be tagged with a sharpness value.</li>
<li>Crease sharpness values range from 0 (smooth) to 10 (infinitely sharp)</li>
<li>It is generally cheaper to use creases whenever possible, instead of adding
extra edges/edge-loops. However...</li>
<li>Creases introduce extra computation costs that are proportional to the
sharpness value. So...</li>
<li>Sharpness values above 5 should rarely be needed.</li>
</ul>
</blockquote>
<p>The following sections introduce some techniques to best leverage them.</p>
<div class="section" id="use-crease-sets">
<h4>Use crease sets</h4>
<p>Complex hard-surface models (giant robots, vehicles, buildings...) are likely to
tag large number of edges : it is extremely useful to organize these edges/edge
loops into logical sets with descriptive names. Edges or vertices in a crease
set group all share the same sharpness value.</p>
<p>If you are modeling with Maya, the CreaseSetEditor implements this type of
workflow. Additionally, for debugging purposes, it is often very helpful if the
name of a set contains the sharpness value (ex: topDeck_2).</p>
<a class="reference external image-reference" href="images/crease_editor.png"><img alt="images/crease_editor.png" class="align-center" src="images/crease_editor.png" /></a>
<p>Besides authoring convenience, one of the benefits of having many edge-loops
share identical sharpness values is that it enables very powerful performance
optimizations within the feature adaptive algorithm (faster renders & less
memory).</p>
</div>
</div>
<hr class="docutils" />
<div class="section" id="additional-resources">
<h3>Additional Resources</h3>
<p>An excellent short tutorial from the Guerrilla CG Project that illustrates many
of the common pitfalls of subdivision modeling, and the strategies to overcome
them:</p>
<a class="reference external image-reference" href="https://www.youtube.com/embed/k_S1INdEmdI"><img alt="https://graphics.pixar.com/opensubdiv/videothumbnails/subdiv_topology_guerrillacg.png" class="align-center" src="https://graphics.pixar.com/opensubdiv/videothumbnails/subdiv_topology_guerrillacg.png" style="width: 75%;" /></a>
<hr class="docutils" />
<p>Ivo Kos, Modelling Technical Director at Pixar Animation Studios, shows some of
the modeling techniques he uses when modeling props and architecture sets for
feature films.</p>
<a class="reference external image-reference" href="https://player.vimeo.com/video/70600180"><img alt="https://graphics.pixar.com/opensubdiv/videothumbnails/subdiv_modeling_pixar2013.png" class="align-center" src="https://graphics.pixar.com/opensubdiv/videothumbnails/subdiv_modeling_pixar2013.png" style="width: 75%;" /></a>
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Generated on: 2023-09-06 15:35 UTC.
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