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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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HBR Overview
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------------
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.. contents::
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:local:
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:backlinks: none
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Hierarchical Boundary Representation (Hbr)
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==========================================
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Hbr is an interconnected topological data representation. The high level of vertex
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connectivity information makes this representation well suited for creation and
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editing purposes. It is however inefficient for interactive refinement operations:
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Separate objects are allocated for each vertex and edge with pointers to neighboring
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vertices and edges.
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Hbr is also the lowest-level subdivision library in Pixar's `Photorealistic RenderMan`.
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.. container:: notebox
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**Note**
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As of OpenSubdiv 3.0, all **Hbr** dependencies have been removed from the
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core APIs (**Sdc**, **Vtr**, **Far**, **Osd**). The legacy source code of
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**Hbr** is provided purely for regression and legacy purposes. If your code
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is currently depending on Hbr functionality, we recommend migrating to the
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newer APIs as we cannot guarantee that this code will be maintained in
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future releases.
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For more information see the `3.0 release notes <release_notes.html>`_
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----
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Half-edge Data Structure
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========================
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The current implementation is based on a half-edge data structure.
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.. image:: images/half_edge.png
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:align: center
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----
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Half-edge cycles and Manifold Topology
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======================================
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Because half-edges only carry a reference to their opposite half-edge, a given
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edge can only access a single neighboring edge cycle.
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.. image:: images/half_edge_cycle.png
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:align: center
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This is a fundamental limitation of the half-edge data structure, in that it
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cannot represent non-manifold geometry, in particular fan-type topologies. A
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different approach to topology will probably be necessary in order to accommodate
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non-manifold geometry.
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----
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Templated Vertex Class
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======================
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The vertex class has been abstracted into a set of templated function accesses.
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Providing Hbr with a template vertex class that does not implement these functions
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allows client-code to use Hbr as a pure topological analysis tool without having
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to pay any costs for data interpolation. It also allows client-code to remain in
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complete control of the layout of the vertex data : interleaved or non-interleaved.
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----
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Boundary Interpolation Rules
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============================
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**Hbr** recognizes 4 rule-sets of boundary interpolation:
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+------------------------------------+
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| Interpolation Rule-Sets |
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+====================================+
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| k_InterpolateBoundaryNone |
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+------------------------------------+
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| k_InterpolateBoundaryEdgeOnly |
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+------------------------------------+
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| k_InterpolateBoundaryEdgeAndCorner |
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+------------------------------------+
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| k_InterpolateBoundaryAlwaysSharp |
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+------------------------------------+
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This enum is shared for both vertex and face-varying interpolation,
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with the following distinctions:
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- vertex boundaries:
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- the *BoundaryNone* rule skips all boundary vertices (results are ''undefined'')
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- the *AlwaysSharp* rule does not apply
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- face-varying boundaries:
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- the *BoundaryNone* rule selects bilinear face-varying interpolation
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