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With our second substantial release in the 3.x series we've reorganized the documentation links a bit. This establishes a more consistent arrangement for release specific documentation pages.
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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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Overview of Release 3.0
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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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Release 3.0
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===========
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OpenSubdiv 3.0 represents a landmark release, with profound changes to the core
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algorithms, simplified APIs, and streamlined GPU execution. Providing faster,
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more efficient, and more flexible subdivision code remains our principal goal.
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To achieve this, OpenSubdiv 3.0 introduces many improvements that constitute
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a fairly radical departure from previous versions.
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This document highlights some of the major changes that have gone in to the 3.0
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release.
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----
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Subdivision Core (Sdc)
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**********************
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In consideration of past, present and future topological representations,
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all low-level details fundamental to subdivision and the specific subdivision
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schemes have been factored into a new low-level layer called **Sdc**
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(SubDivision Core). This layer encapsulates the full set of applicable
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options, the formulae required to support semi-sharp creasing, the formulae
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for the refinement masks of each subdivision scheme, etc. As initially
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conceived, its goal was often expressed as "separating the math from the mesh".
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Sdc provides the low-level nuts and bolts to provide a subdivision
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implementation consistent with OpenSubdiv. It is used by OpenSubdiv's
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libraries and may also be useful in providing an existing client's
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implementation with the details necessary to make that implementation
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consistent with OpenSubdiv.
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----
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Topology and Refinement
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***********************
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OpenSubdiv 3.0 introduces a new *intermediate* internal topological
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representation named **Vtr** (Vectorized Topology Representation).
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Compared to the Hbr library used in previous versions, Vtr is much more
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efficient for the kinds of topological analysis required by Far and is more
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flexible. While Hbr is no longer used by OpenSubdiv, it will remain in
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the source distribution for legacy and regression purposes.
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**Faster Subdivision**
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A major focus of the 3.0 release is performance, and the improvement to
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the initial refinement of a mesh required for topological analysis is close
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to an order magnitude; often much more for uniform, but less for adaptive.
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**Supporting for Non-manifold Topology**
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With topology conversion no longer constrained by Hbr, OpenSubdiv is no
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longer restricted to meshes of manifold topology. With one exception
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(non-triangles with Loop subdivision), any set of faces and vertices that can
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be represented in common container formats such as Obj or Alembic can be
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represented and subdivided. With future efforts to bring the functionality
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for the Loop scheme up to par with Catmark, that last remaining topological
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restriction will be removed.
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**Simpler Conversion of Topology**
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Several entry-points are now available for client topology, rather than the
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single incremental assembly of an HbrMesh that previously existed. The new
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topological relationships can be populated using either a high-level interface
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where simplicity has been emphasized, or a more complex lower-level interface
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for enhanced efficiency.
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**Face Varying Topology**
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Previously, face-varying data was assigned by value to the vertex for each
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face, and whether or not the set of values around a vertex was continuous was
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determined by comparing these values later. In some cases this could result
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in two values that were not meant to be shared being "welded" together.
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Face-varying data is now specified topologically: just as the vertex topology
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is defined from a set of **vertices** and integer references (indices) to
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these **vertices** for the corner of each face, face-varying topology is
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defined from a set of **values** and integer references (indices) to these
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**values** for the corner of each face. So if values are to be considered
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distinct around a vertex, they are given distinct indices and no comparison
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of any data is ever performed. Note that the number of **vertices** and
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**values** will typically differ, but since indices are assigned to the
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corners of all faces for both, the total number of indices assigned to all
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faces will be the same.
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This ensures that OpenSubdiv's face-varying topology matches what is often
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specified in common geometry container formats like Obj, Alembic and USD.
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Multiple "channels" of face-varying data can be defined and each is
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topologically independent of the others.
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----
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Limit Properties and Patches
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****************************
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A fundamental goal of OpenSubdiv is to provide an accurate and reliable
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representation of the limit surface. Improvements have been made both to the
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properties (positions and tangents) at discrete points in the subdivision
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hierarchy, as well as to the representations of patches used for the
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continuous limit surface between them.
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**Removed Fixed Valence Tables**
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Limit properties of extra-ordinary vertices are computed for arbitrary
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valence and new patch types no longer rely on small table sizes. All tables
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that restricted the valence of a vertex to some relatively small table size
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have now been removed.
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The only restriction on valence that exists is within the new topology
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representation, which restricts it to the size of an unsigned 16-bit integer
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(65,535). This limit could also be removed, by recompiling with a certain
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size changed from 16- to 32-bits, but doing so would increase the memory cost
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for all common cases. We feel the 16-bit limit is a reasonable compromise.
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**Single Crease Patch**
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OpenSubdiv 3.0 newly implements efficient evaluation of semi-smooth
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creases(*) using single crease patches. With this optimization,
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high-order edge sharpness tags can be handled very efficiently for both
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computation time and memory consumption.
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(*) Niessner et al., Efficient Evaluation of Semi-Smooth Creases in
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Catmull-Clark Subdivision Surfaces. Eurographics (Short Papers). 2012.
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`<http://research.microsoft.com/en-us/um/people/cloop/EG2012.pdf>`_
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**New Irregular Patch Approximations**
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While "legacy" Gregory patch support is still available, we have introduced
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several new options for representing irregular patches: Legacy Gregory, fast
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Gregory Basis stencils, and BSpline patches. Gregory basis stencils provide
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the same high quality approximation of Legacy Gregory patches, but execute
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considerably faster with a simpler GPU representation. While BSpline patches
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are not as close an approximation as Gregory patches, they enable an entire
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adaptively refined mesh to be drawn with screen space tessellation via a
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single global shader configuration (Gregory Basis patches require one
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additional global shader configuration).
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The new implementations of the GregoryBasis and BSpline approximations relax
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the previous max valence limit. Legacy Gregory patch still has a limitation
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of max valence (typically 24, depending on the hardware capability of
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GL_MAX_VARYING_VECTORS).
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Users are still encouraged to use models with vertices of low valence for
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both improved model quality and performance.
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----
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Faster Evaluation and Display
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*****************************
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OpenSubdiv 3.0 also introduces new data structures and algorithms that greatly
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enhance performance for the common case of repeated evaluation both on the
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CPU and GPU.
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**Introducing Stencil Tables**
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OpenSubdiv 3.0 replaces the serialized subdivision tables with factorized
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stencil tables. The SubdivisionTables class of earlier releases contained
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a large number of data inter-dependencies, which incurred penalties from
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fences or force additional kernel launches. Most of these dependencies have now
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been factorized away in the pre-computation stage, yielding *stencil tables*
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(Far::StencilTable) instead.
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Stencils remove all data dependencies and simplify all the computations into a
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single trivial kernel. This simplification results in a faster pre-computation
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stage, faster execution on GPU, with less driver overhead. The new stencil
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tables Compute back-end is supported on all the same platforms as previous
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releases (except GCD).
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**Faster, Simpler GPU Kernels**
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On the GPU side, the replacement of subdivision tables with stencils greatly
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reduces bottlenecks in compute, yielding as much as a 4x interpolation speed-up.
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At the same time, stencils reduce the complexity of interpolation to a single
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kernel launch per primitive, a critical improvement for mobile platforms.
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As a result of these changes, compute batching is now trivial, which in turn
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enabled API simplifications in the Osd layer.
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**Unified Adaptive Shaders**
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Adaptive tessellation shader configurations have been greatly simplified. The
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number of shader configurations has been reduced from a combinatorial per-patch
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explosion down to a constant two global configurations. This massive improvement
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over the 2.x code base results in significantly faster load times and a reduced
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per-frame cost for adaptive drawing.
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Similar to compute kernel simplification, this shader simplification has
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resulted in additional simplifications in the Osd layer.
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----
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Updated Source-Code Style
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*************************
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OpenSubdiv 3.0 replaces naming prefixes with C++ namespaces for all API layers,
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bringing the source style more in line with contemporary specifications
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(mostly inspired from the `Google C++ Style Guide
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<http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml>`__).
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The large-scale changes introduced in this release generally break compatibility
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with existing client-code. However, this gives us the opportunity to effect
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some much needed updates to our code-style guidelines and general conventions,
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throughout the entire OpenSubdiv code-base. We are hoping to drastically
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improve the quality, consistency and readability of the source code.
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----
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Documentation and Tutorials
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***************************
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The documentation has been reorganized and fleshed out. This release
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introduces a number of new `tutorials <tutorials.html>`__. The tutorials
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provide an easier entry point for learning the API than do the programs
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provided in examples. The examples provide more fleshed out solutions and are
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a good next step after the tutorials are mastered.
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----
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Additional Resources
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====================
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Porting Guide
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*************
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Please see the `Porting Guide <porting.html>`__ for help on how to port
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existing code written for OpenSubdiv 2.x to the new 3.0 release.
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----
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Subdivision Compatibility
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*************************
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The 3.0 release has made some minor changes to the subdivision specification
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and rules. See `Subdivision Compatibility <compatibility.html>`__ for a
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complete list.
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