OpenSubdiv/documentation/hbr_overview.rst

..
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HBR Overview
------------

.. contents::
   :local:
   :backlinks: none


Hierarchical Boundary Representation (Hbr)
==========================================

Hbr is an interconnected topological data representation. The high level of vertex
connectivity information makes this representation well suited for creation and
editing purposes. It is however inefficient for interactive refinement operations:
Separate objects are allocated for each vertex and edge with pointers to neighboring
vertices and edges.

Hbr is also the lowest-level subdivision library in Pixar's `Photorealistic RenderMan`.

.. container:: notebox

   **Note**

       As of OpenSubdiv 3.0, all **Hbr** dependencies have been removed from the
       core APIs (**Sdc**, **Vtr**, **Far**, **Osd**). The legacy source code of
       **Hbr** is provided purely for regression and legacy purposes. If your code
       is currently depending on Hbr functionaliy, we recommend migrating to the
       newer APIs as we cannot guarantee that this code will be maintained in
       future releases.
       For more information see the `3.0 release notes <release_notes.html>`_


----

Half-edge Data Structure
========================

The current implementation is based on a half-edge data structure.

.. image:: images/half_edge.png
   :align: center

----

Half-edge cycles and Manifold Topology
======================================

Because half-edges only carry a reference to their opposite half-edge, a given
edge can only access a single neighboring edge cycle.

.. image:: images/half_edge_cycle.png
   :align: center

This is a fundamental limitation of the half-edge data structure, in that it
cannot represent non-manifold geometry, in particular fan-type topologies. A
different approach to topology will probably be necessary in order to accomodate
non-manifold geometry.

----

Templated Vertex Class
======================

The vertex class has been abstracted into a set of templated function accesses.
Providing Hbr with a template vertex class that does not implement these functions
allows client-code to use Hbr as a pure topological analysis tool without having
to pay any costs for data interpolation. It also allows client-code to remain in
complete control of the layout of the vertex data : interleaved or non-interleaved.

----

Boundary Interpolation Rules
============================

**Hbr** recognizes 4 rule-sets of boundary interpolation:

+------------------------------------+
| Interpolation Rule-Sets            |
+====================================+
| k_InterpolateBoundaryNone          |
+------------------------------------+
| k_InterpolateBoundaryEdgeOnly      |
+------------------------------------+
| k_InterpolateBoundaryEdgeAndCorner |
+------------------------------------+
| k_InterpolateBoundaryAlwaysSharp   |
+------------------------------------+

This enum is shared for both `vertex and face-varying interpolation
<subdivision_surfaces.html#boundary-interpolation-rules>`__, with the following
distinctions:

    - vertex boundaries:
        - the *BoundaryNone* rule skips all boundary vertices (results are ''undefined'')
        - the *AlwaysSharp* rule does not apply

    - face-varying boundaries:
        - the *BoundaryNone* rule selects bilinear face-varying interpolation