OpenSubdiv/documentation/release_notes.rst

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3.0.0.Beta Release Notes
------------------------

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

----

Release 3.0
===========

OpenSubdiv 3.0 represents a landmark release, with very profound changes to the
core algorithms. While providing faster, more efficient, and more flexible
subdivision code remains our principal goal, OpenSubdiv 3.0 introduces many
improvements that constitute a fairly radical departures from our previous
versions.

----

Improved performance
********************

Release 3.0.0 of OpenSubdiv introduces a new set of data structures and
algorithms that greatly enhance performance over previous versions.

This release focuses mostly on the CPU side, and  should provide
"out-of-the-box" speed-ups close to an order of magnitude for topology
refinement and analysis (both uniform and adaptive). Please note: a very large
portion of the 2.x code base has been completely replaced or deprecated.

On the GPU side, the replacement of subdivision tables with stencils allows
us to remove several bottlenecks in the Compute area that can yield as much as
4x faster interpolation on CUDA platforms. At the same time, stencils also
reduce the dozens of kernel launches required per primitive to a single one (this
was a known issue on certain mobile platforms). Compute calls batching is now
trivial.

We will continue releasing features and improvements throughout the release
cycle, both to match the feature set of previous releases, and to further the
general optimization strategy described above.

----

New topology entry-points
*************************

OpenSubdiv 3.0 introduces several new entry-points for client topology. Previous
releases forced client applications to define and populate instances of an Hbr
half-edge topology representation. For many applications, this representation
was both redundant and inefficient.

OpenSubdiv 3.0 introduces a new *intermediate* topological representation, named
**Vtr** (Vectorized Topology Representation). The topological relationships
held by Vtr can populated using either a high-level interface where simplicity
has been emphasized, or a lower-level interface for enhanced efficiency. Vtr is
much more efficient for the kinds of topological analysis required by Far and
additionally is more flexible in that it supports the specification of
non-manifold topology.

As a result, Hbr is no longer a core API of OpenSubdiv. While the code is marked
as deprecated, it will remain in the source distribution for legacy and
regression purposes.

The documentation for Vtr can be found `here <vtr_overview.html>`__

----

New treatment of face-varying data
**********************************

With Hbr no longer being the entry point for client-code, OpenSubdiv 3.0 has to
provide a new interface for face-varying data. Previous versions required
face-varying data to be assigned by value to the vertex for each face, and
whether or not the set of values around a vertex was continuous was determined
by comparing these values later. In some cases this could result in two values
that were not meant to be shared being "welded" together.

Face-varying data is now specified topologically. Just as the vertex topology
is defined from a set of vertices and integer references to these vertices for
the vertex of each face, face-varying topology is defined from a set of values
and integer references to these values for the vertex of each face. So if
values are to be considered distinct around a vertex, they are given distinct
indices and no comparison of values is ever performed.

This ensures that OpenSubdiv's face-varying topology matches what is specified
in common geometry container formats like Obj or Alembic. It also allows for
more efficient processing of face-varying values during refinement, and so the
cost of interpolating a set of face-varying data should now be little more than
the cost of interpolating a similar set of vertex data (depending on the number
of distinct face-varying values versus the number of vertices).

----

Subdivision Core (Sdc)
**********************

In consideration of the existing representations (Hbr and Vtr), all low-level
details fundamental to subdivision and the specific subdivision schemes have
been factored into a new low-level layer (the lowest) called Sdc. This layer
encapsulates the full set of applicable options, the formulae required to
support semi-sharp creasing, the formulae for the refinement masks of each
subdivision scheme, etc.

Sdc provides the low-level nuts and bolts to provide a subdivision
implementation consistent with OpenSubdiv. It is used internally by Vtr and
Far. but can also provide client-code with an existing implementation of their
own with the details to make that implementation consistent with OpenSubdiv.

The documentation for Sdc can be found `here <sdc_overview.html>`__

----

Introducing Stencil Tables
**************************

OpenSubdiv 3.0 replaces the serialized subdivision tables with factorized
stencil tables. Subdivision tables as implemented in 2.x releases still contain
a fairly large amount of data inter-dependencies, which incur penalties from
fences or force addition kernel launches. Most of these dependencies have now
been factorized away in the pre-computation stage, yielding *stencil tables*
instead.

Stencils remove all data dependencies and simplify all the computations into a
single trivial kernel. This simplification results in a faster pre-computation
stage, faster execution on GPU, and fewer driver overheads. The new stencil
tables Compute back-end is supported on all the same platforms as previous
releases (except GCD).

----

New Source-Code Style
*********************

OpenSubdiv 3.0 replaces naming prefixes with C++ namespaces for all API layers,
bringing the source style more in line with contemporary specifications
(mostly inspired from the `Google C++ Style Guide
<http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml>`__).

The large-scale changes introduced in this release generally break compatibility
with existing client-code. However, this gives us the opportunity to effect
some much needed updates to our code-style guidelines and general conventions,
throughout the entire OpenSubdiv code-base. We are hoping to drastically
improve the quality, consistency and readability of the source code.

While the bulk of code refactoring is mostly in place, we are still tweaking
some of the finer details. After this Beta release we are not anticipating any
further significant changes.

----

New Tutorials
*************

Documentation has been re-organized and fleshed out (although there is still a
lot of work remaining). Because the "code examples" have been generally overly
complicated, with this release we are introducing a number of new `tutorials
<tutorials.html>`__. We are trying to keep these tutorials as simple as
possible, with no external dependencies (although some of them generate OBJ or
Maya MEL scripts as a way of visualizing the output). We are planning on releasing
more tutorials as time and resources allow.

----

Changes to the Subdivision Specification
========================================

The refactoring of OpenSubdiv 3.0 data representations presents a unique
opportunity to revisit some corners of the subdivision specification and
remove or update some legacy features.

Vertex Interpolation Rules
**************************

Since the various options are now presented through a new API (Sdc rather than
Hbr), based on the history of some of these options and input from interested
parties, the following changes have been implemented:

    * The Sdc boundary interpolation enum has been renamed
      *"VtxBoundaryInterpolation"*, with the following values:

        * VTX_BOUNDARY_NONE - (default)
        * VTX_BOUNDARY_EDGE_ONLY
        * VTX_BOUNDARY_EDGE_AND_CORNER

      Funcationality remains unchanged.
    * Legacy modes of the *"smoothtriangle"* rule have been removed. Values for
      *"CreasingMethod"* are now:

        * TRI_SUB_CATMARK - Catmull-Clark weights (default)
        * TRI_SUB_SMOOTH - "smooth triangle" weights


    * The current implementation of the *"Chaikin"* rule shows small
      numerical differences with results obtained from Hbr in 2.x releases.
      Considering that the feature is rarely used and that the current
      implementation is likely the more correct one, we are considering
      declaring the current implementation as *the standard*. We will
      review input from the community on this matter during Alpha and Beta
      release cycles.

In these cases, features are not being removed but simply re-expressed in what
is hoped to be a clearer interface.

We will welcome feedback and constructive comments as we deploy these changes.
We hope to converge toward a general consensus and lock these APIs by the end
of Beta cycle.

Face-varying Interpolation Rules
********************************

Currently, all 5 of Hbr's legacy modes of face-varying interpolation
are supported (with minor modifications where Hbr has been found to
be incorrect). Additional modes have also been added to allow for
better control around T-junction face-varying topologies.

The new FVarLinearInterpolation enums are:

    * FVAR_LINEAR_NONE          - smooth everywhere ("edge only")
    * FVAR_LINEAR_CORNERS_ONLY  - sharpen corners only
    * FVAR_LINEAR_CORNERS_PLUS1 - ("edge corner")
    * FVAR_LINEAR_CORNERS_PLUS2 - ("edge and corner + propagate corner")
    * FVAR_LINEAR_BOUNDARIES    - sharpen all boundaries ("always sharp")
    * FVAR_LINEAR_ALL           - bilinear interpolation ("bilinear") (default)

Hierarchical Edits
******************

Currently Hierarchical Edits have been marked as "extended specification" and
support for hierarchical features has been removed from the 3.0 release. This
decision allows for great simplifications of many areas of the subdivision
algorithms. If we can identify legitimate use-cases for hierarchical tags, we
will consider re-implementing them in future releases, as time and resources
allow.

----

Beta Release Notes
==================

Our intentions as open-source developers is to give as much access to our code,
as early as possible, because we value and welcome the feedback from the
community.

The 'Alpha' release moniker means to us that our code is still far from being
finalized. Although we are now close from being feature complete, our
public-facing interfaces are still subject to change. Therefore, we do not
recommend this version of OpenSubdiv be used in client applications until both
features and interfaces have been locked and are ready for Beta release.

With the 'Beta' release cycle, we hope to give stake-holders a time-window to
provide feedback on decisions made and changes in the code that may impact
them. Our Beta code is likely not feature-complete yet, but the general
structure and architectures will be sufficiently locked in place for early
adopters to start building upon these releases.

Within 'Master' releases, we expect APIs to be backward compatible so that
existing client code can seamlessly build against newer releases. Changes
may include bug fixes as well as new features, 

.. container:: notebox

    **Beta Issues**

    The following is a short list of issues or details that we may still have
    to change before the master release:

        #. Refactor Far::TopologyRefiner interpolation functions:
           Templated interpolation methods such as Interpolate<T>(),
           InterpolateFaceVarying<T>(), Limit<T>() are not finalized yet. Both
           the methods prototypes as well the interface required for **T** are
           likely to change before release.

        #. Topology entry-point API:
           The *advanced* topology entry point interface in
           Far::TopologyRefinerFactory is not final yet. Some protected
           accessors are likely to be renamed, added or removed before release.


    **Beta Features**

    The following is a short list of features that hopefully will land before
    the master release:

        #. Limit Masks:
           Currently, Sdc generates weighted masks to interpolate *vertex* and
           *face-varying* primvar data between subdivision levels. We want to
           add functionality to evaluate closed-form evaluation of weight masks
           to interpolate primvar data at the limit.

        #. Implement arbitrary and discrete limit stencils:
           Subdivision tables have been replaced with discrete vertex stencils.
           We would like to add functionality for stencils to push these
           vertices to the limit. This work is contingent on the implementation
           of limit masks.


----

3.x Release Cycle RoadMap
=========================

Within the 3.x release cycle we would like to address many of the issues related
to scaling the application of subdivision surfaces to large amounts of
primitives within typical graphics pipelines.

Enabling workflows at larger scales will require improvements on several fronts:

* Handle more primitives, but with fewer overheads:

    * Reduce Compute kernel launches,which we will achieve using stencils instead
      of subdivision tables
    * Reduce Draw calls by addressing the combinatorial explosion of tessellation
      shaders
    * Provide back-ends for next-gen APIs (D3D12, Mantle, Metal, GL 5.x)

* Handle more semi-sharp creases: feature isolation needs to become much more
  efficient to allow for complete creative freedom in using the feature.
* Faster topology analysis


Release 2.x
===========

`Previous releases <release_notes_2x.html>`_