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215 lines
6.9 KiB
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Copyright 2013 Pixar
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Licensed under the Apache License, Version 2.0 (the "Apache License")
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with the following modification; you may not use this file except in
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compliance with the Apache License and the following modification to it:
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Section 6. Trademarks. is deleted and replaced with:
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6. Trademarks. This License does not grant permission to use the trade
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names, trademarks, service marks, or product names of the Licensor
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and its affiliates, except as required to comply with Section 4(c) of
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the License and to reproduce the content of the NOTICE file.
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You may obtain a copy of the Apache License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the Apache License with the above modification is
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distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the Apache License for the specific
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language governing permissions and limitations under the Apache License.
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Modeling Tips
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-------------
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.. contents::
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:local:
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:backlinks: none
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----
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The following sections describe common techniques specific to modeling with
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subdivision surfaces.
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.. container:: notebox
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**Note**
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The following information contains techniques specific to the Catmull-Clark
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subdivision scheme. Some elements need to be adjusted for Loop surfaces.
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----
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Topology
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========
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Well-constructed subdivision meshes have several important properties:
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* They consist primarily of regular faces (quads for Catmull-Clark, tris for Loop)
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* They contain few extraordinary vertices
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* They efficiently describe the intended shape
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* They are topologically `manifold <subdivision_surfaces.html#non-manifold-topology>`__
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----
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Use Fewer Spans
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+++++++++++++++
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While polygon models need to use a large number of spans to approximate smooth
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curved surfaces, subdivision models require significantly fewer control points.
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In most situations, 6 spans are enough to create accurate circular shapes, and
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4 is often enough to approximate background objects.
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.. image:: images/mod_notes.0.png
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:align: center
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:target: images/mod_notes.0.png
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----
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Avoid High Valence vertices
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+++++++++++++++++++++++++++
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A high valence vertex is a vertex connected to more than 4 adjacent edges.
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High valence vertices cause several problems when subdivided:
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* The Catmull-Clark scheme can produce "wavy" surfaces when a revolution
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vertex is surrounded by triangles (see `here <subdivision_surfaces.html#triangle-subdivision-rule>`__):
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.. image:: images/mod_notes.2.png
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:width: 150px
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:align: center
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:target: images/mod_notes.2.png
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* High valence vertices incur fairly large performance hits
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* Currently, OpenSubdiv has a hard constraint imposed by GPU shaders on the
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maximum valence of a vertex (~27 on current hardware)
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Instead, here are some topological strategies to cap revolution shapes:
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.. image:: images/mod_notes.1.png
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:align: center
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:target: images/mod_notes.1.png
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Note that all these cylinders use only quad faces, and all the vertices in the
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caps have a valence of 4 (except the bottom left example)
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----
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Edge-Loop Transitions
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+++++++++++++++++++++
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It is often necessary to vary the density of control vertices over the surface
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mesh: areas around the fingers of a hand require more CVs than the comparatively
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simpler region around the palm. It is important to handle the topology around
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these transitions efficiently.
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One strategy is to use extraordinary vertices, such as this example, using a
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valence 5 vertex to expand 3 edge loops into 5.
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.. image:: images/edge_loops.png
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:align: center
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:width: 400px
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:target: images/edge_loops.png
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----
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Practical Topology Primer
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+++++++++++++++++++++++++
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Some real-world examples showing how to produce detailed shapes with sparse
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topology, few extraordinary vertices, and no high-valence revolution poles.
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.. image:: images/mod_notes.3.png
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:align: center
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:width: 400px
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:target: images/mod_notes.3.png
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.. image:: images/mod_notes.4.jpg
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:align: center
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:width: 400px
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:target: images/mod_notes.4.jpg
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----
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Triangles and N-Gons
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++++++++++++++++++++
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Used sparsely, non-quads can be very useful to gather 3 or more diverging
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edge-loops. These are often encountered in highly deforming areas with curvature
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saddle-points (ex: arm-torso connection). The strategic placement of a pentagon
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in one of these critical spots ensures that the surface remains smooth, while
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allowing for complex topology to flow around.
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.. image:: images/mod_notes.5.png
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:align: center
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:target: images/mod_notes.5.png
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----
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Semi-Sharp Creases
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==================
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Semi-sharp creases can be a very powerful tool for hard-surface modeling.
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* Both edges and vertices can be tagged with a sharpness value.
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* Crease sharpness values range from 0 (smooth) to 10 (infinitely sharp)
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* It is generally cheaper to use creases whenever possible, instead of adding
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extra edges/edge-loops. However...
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* Creases introduce extra computation costs that are proportional to the
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sharpness value. So...
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* Sharpness values above 5 should rarely be needed.
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The following sections introduce some techniques to best leverage them.
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Use crease sets
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+++++++++++++++
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Complex hard-surface models (giant robots, vehicles, buildings...) are likely to
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tag large number of edges : it is extremely useful to organize these edges/edge
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loops into logical sets with descriptive names. Edges or vertices in a crease
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set group all share the same sharpness value.
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If you are modeling with Maya, the CreaseSetEditor implements this type of
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workflow. Additionally, for debugging purposes, it is often very helpful if the
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name of a set contains the sharpness value (ex: topDeck_2).
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.. image:: images/crease_editor.png
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:align: center
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:target: images/crease_editor.png
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Besides authoring convenience, one of the benefits of having many edge-loops
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share identical sharpness values is that it enables very powerful performance
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optimizations within the feature adaptive algorithm (faster renders & less
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memory).
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----
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Additional Resources
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====================
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An excellent short tutorial from the Guerrilla CG Project that illustrates many
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of the common pitfalls of subdivision modeling, and the strategies to overcome
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them:
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.. image:: https://graphics.pixar.com/opensubdiv/videothumbnails/subdiv_topology_guerrillacg.png
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:align: center
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:width: 75%
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:target: https://www.youtube.com/embed/k_S1INdEmdI
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----
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Ivo Kos, Modelling Technical Director at Pixar Animation Studios, shows some of
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the modeling techniques he uses when modeling props and architecture sets for
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feature films.
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.. image:: https://graphics.pixar.com/opensubdiv/videothumbnails/subdiv_modeling_pixar2013.png
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:align: center
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:width: 75%
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:target: https://player.vimeo.com/video/70600180
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