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https://github.com/PixarAnimationStudios/OpenSubdiv
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Copyright 2015 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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Porting Guide: 2.x to 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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Porting Guide: 2.x to 3.0
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=========================
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This document is a high-level description of how to port exiting OpenSubdiv 2.x
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code to use OpenSubdiv 3.0.
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**NOTE:** If your questions are not answered here, please contact us on the
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OpenSubdiv forum and we will be happy to help!
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Source Code Organization
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========================
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Given the scale of functional changes that were being made to the public
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interface, we took the opportunity in 3.0 to update the coding style and
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organization -- most notably making use of namespaces for each library.
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================= ==================== ===============================================
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Subdirectory Namespace Relevance
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================= ==================== ===============================================
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hbr/ N/A Historical, no longer used
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sdc/ Sdc New, low-level, public options, constants, etc.
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vtr/ Vtr New, internal use, topology representation
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far/ Far Revised, similar functionality with new API
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osd/ Osd Revised, similar functionality with new API
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================= ==================== ===============================================
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Hbr Layer Translation
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=====================
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Client mesh topology is now translated into an instance of Far::TopologyRefiner
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instead of HbrMesh.
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================= ====================
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OpenSubdiv 2.x OpenSubdiv 3.0
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================= ====================
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HbrMesh<VTX_TYPE> Far::TopologyRefiner
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================= ====================
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The Far::TopologyRefiner is now the core representation of topology from which
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all other major classes in Far and Osd are constructed. It was designed to
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support efficient refinement (uniform or sparse) of a base mesh of arbitrary
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topology (no manifold restrictions). Once constructed it can be directly
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refined to meet some need, or passed to other contexts that will refine it to
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meet their needs.
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In contrast to directly assembling an HbrMesh, the TopologyRefiner, like other
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classes in Far, requires a Factory class for its construction. One of the
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early goals of these factories was to allow a client to convert their existing
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boundary representation -- with its full topological traversal abilities --
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directly into the TopologyRefiners representation. While this is now possible,
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this also represents the most complex construction process and is only
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recommended for usage where this conversion process is critical.
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Details on how to construct a TopologyRefiner can be found in the
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`Far overview <far_overview.html#far-topologyrefinerfactory>`__ documentation.
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Additionally, documentation for Far::TopologyRefinerFactory<MESH> outlines the
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requirements, and Far tutorial 3.1 (tutorials/far/tutorial_3_1) provides an example
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of a factory for directly converting HbrMeshes to TopologyRefiners.
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Its worth a reminder here that Far::TopologyRefiner contains only topological
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information (which does include sharpness, since that is considered relating
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to subdivision topology) and not the positions or other data associated with
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a mesh. While HbrMesh<T> required some definition of a vertex type <T> and
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dimensions of face-varying data, TopologyRefiner is more clearly separated
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from the data. So the construction of the TopologyRefiner does not involve
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data specification at all.
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Subdivision Schemes and Options in Sdc
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++++++++++++++++++++++++++++++++++++++
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The creation of a new TopologyRefiner requires specification of a subdivision
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scheme and a set of options that are applicable to all schemes. With HbrMesh,
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the scheme was specified by declaring a static instance of a specific subclass
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of a subdivision object, and the options were specified with a number of
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methods on the different classes.
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Such general information about the schemes has now been encapsulated in the
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Sdc layer for use throughout OpenSubdiv. The subdivision scheme is now a
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simple enumerated type (Sdc::SchemeType) and the entire set of options that
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can be applied to a scheme is encapsulated in a single simple struct of
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flags and enumerated types (Sdc::Options).
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=============================================== ===========================================
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OpenSubdiv 2.x OpenSubdiv 3.0
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=============================================== ===========================================
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HbrMesh<T>::SetInterpolateBoundaryMethod() Sdc::Options::SetVtxBoundaryInterpolation()
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HbrMesh<T>::SetFVarInterpolateBoundaryMethod() Sdc::Options::SetFVarLinearInterpolation()
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HbrSubdivision<T>::SetCreaseSubdivisionMethod() Sdc::Options::SetCreasingMethod()
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=============================================== ===========================================
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Specifying Face Varying Topology and Options
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++++++++++++++++++++++++++++++++++++++++++++
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Both the way in which face varying data is associated with a mesh and the
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options used to control its interpolation have changed. The documentation on
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`Compatibility with OpenSubdiv 2.x <compatibility.html#compatibility-with-opensubdiv-2.x>`__
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details the equivalence of interpolation options between Hbr and the new
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*Sdc::Options::FVarLinearInterpolation* enum, while the section on
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`Face Varying Interpolation <subdivision_surfaces.html#face-varying-interpolation>`__
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illustrates their effects.
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Face varying data is now specified by index rather than by value, or as often
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stated, it is specified topologically. Just as vertices for faces are specified
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by indices into a potential buffer of positions, face varying values are
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specified by indices into a potential buffer of values. Both vertices and
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face varying values (frequently referred to as *FVarValues* in the API) are
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assigned and associated with the corners of all faces.
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In many cases this will simplify representation as many common geometry
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container formats such as Obj or Alembic specify texture coordinates the same
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way. For other cases, where a value per face-corner is provided with no
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indication of which values incident each vertex should be considered shared,
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it will be necessary to determine shared indices for values at each vertex if
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any non-linear interpolation is desired.
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Far Layer Translation
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=====================
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While TopologyRefiner was introduced into Far as the new intermediate
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topology representation, several other changes were made to classes in Far
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to provide more modular building blocks for use by the Osd layer or directly.
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===================== =====================
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OpenSubdiv 2.x OpenSubdiv 3.0
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===================== =====================
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FarMesh<U> N/A, no longer needed
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FarSubdivisionTables Far::StencilTable
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FarPatchTables Far::PatchTable
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===================== =====================
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Ordering of Refined Vertices
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++++++++++++++++++++++++++++
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The FarMesh was previously responsible for refining an HbrMesh -- generating
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new vertices and faces in successive levels of refinement in the
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FarSubdivisionTables. Vertices were grouped and reordered from the native
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ordering of HbrMesh so that vertices requiring similar processing were
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consecutive. Such grouping alleviated most of the idiosyncrasies of
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HbrMesh's native ordering but not all.
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Far::ToplogyRefiner is inherently a collection of refinement levels, and
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within each refined level (so excluding the base level), all components are
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still grouped for the same reasons. There are two issues here though:
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* the ordering of these groups has changed (though an option exists to
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preserve it)
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* the ordering of components within these groups is not guaranteed to have
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been preserved
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Vertices in a refined level are grouped according to the type of component in
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the parent level from which they originated, i.e. some vertices originate
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from the center of a face (face-vertices), some from an edge (edge-vertices)
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and some from a vertex (vertex-vertices). (Note that there is a conflict in
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terminology here -- face-vertices and edge-vertices most often refer to
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vertices incident a face or edge -- but for the sake of this discussion, we
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use them to refer to the component from which a child vertex originates.)
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The following table shows the ordering of these groups in 2.x and the two
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choices available in 3.0. The option is the *orderVerticesFromFacesFirst*
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flag that can be set in the Option structs passed to the uniform and adaptive
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refinement methods of TopologyRefiner:
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============================================ =============================================
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Version and option Vertex group ordering
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============================================ =============================================
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2.x face-vertices, edge-vertices, vertex-vertices
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3.0 default vertex-vertices, face-vertices, edge-vertices
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3.0 orderVerticesFromFacesFirst = true face-vertices, edge-vertices, vertex-vertices
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============================================ =============================================
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The decision to change the default ordering was based on common feedback;
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the rationale was to allow a trivial mapping from vertices in the cage to
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their descendants at all refinement levels. While the grouping is
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fundamental to the refinement process, the ordering of the groups is
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internally flexible, and the full set of possible orderings can be made
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publicly available in future if there is demand for such flexibility.
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The ordering of vertices within these groups was never clearly defined given
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the way that HbrMesh applied its refinement. For example, for the
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face-vertices in a level, it was never clear which face-vertices would be
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first as it depended on the order in which HbrMesh traversed the parent faces
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and generated them. Given one face, HbrMesh would often visit neighboring
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faces first before moving to the next intended face.
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The ordering with Far::TopologyRefiner is much clearer and predictable. Using
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the face-vertices as an example, the order of the face-vertices in level *N+1*
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is identical to the order of the parent faces in level *N* from which they
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originated. So if we have face-vertices *V'i*, *V'j* and *V'k* at some level,
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originating from faces *Fi*, *Fj* and *Fk* in the previous level, they will
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be ordered in increasing order of *i*, *j* and *k*. For uniform refinement
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the ordering of face vertices *V'i* will therefore exactly match the ordering
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of the parent faces *Fi*. For adaptive or otherwise sparse refinement, the
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subset of *Vi* will be ordered similarly, just with components missing from
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those not refined.
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The same is true of all vertices, i.e. edge-vertices and vertex-vertices,
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and also for other components in refined levels, i.e. the child faces and
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edges.
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For child faces and edges, more than one will originate from the same parent
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face or edge. In addition to the overall ordering based on the parent faces
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or edges, another ordering is imposed on multiple children originating from
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the same face or edge. They will be ordered based on the corner or
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end-vertex with which they are associated.
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In the case of refined faces, another way to view the ordering is to consider
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the way that faces are originally defined -- by specifying the set of vertices
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for the corners of each face, often aggregated into a single large array. The
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ordering of the set of refined faces for each level will correspond directly
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to such an array of vertices per face in the previous level.
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Osd Layer Translation
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=====================
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Two big changes in the 3.0 API have allowed the Osd layer to be significantly
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simpler, the first is the move to stencil tables from subdivision tables and the
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second is shader simplification. With this refactoring, the focus has been to
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use more meaningful names and to make the data contained within an object more
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apparent.
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Controller Objects
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++++++++++++++++++
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.. _Evaluator: doxy_html/a00024.html
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The API-specific ComputeController has been replaced with the Evaluator_. It
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reflects the fact that stencil compute batches are significantly simpler than
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subdivision table compute batches.
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The name "Evaluator" was chosen with the hope that is more meaningful than the
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generic "ComputeController" moniker: the Evaluator evaluates stencil and
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patch tables.
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In the 2.x code base, subdiv level buffers were always required to be allocated
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contiguously, however in 3.0 with the shift to stencil tables, this strict
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allocation scheme is no longer required. As a result, the EvalStencils() and
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EvalPatches() methods now accept both a source and a destination descriptor.
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======================================= ========================================
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OpenSubdiv 2.x OpenSubdiv 3.0
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======================================= ========================================
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ComputeController::Refine() Osd::...Evaluator::EvalStencils()
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ComputeController::Synchronize() Osd::...Evaluator::Synchronize()
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EvalStencilsController::UpdateValues() Osd::...Evaluator::EvalStencils()
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EvalStencilsController::UpdateDerivs() Osd::...Evaluator::EvalStencils()
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EvalLimitController::EvalLimitSample() Osd::...Evaluator::EvalPatches()
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======================================= ========================================
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Also note that OsdVertexDescriptor has been renamed, however it's data members
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and semantic purpose remains the same:
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======================================= ========================================
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OpenSubdiv 2.x OpenSubdiv 3.0
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======================================= ========================================
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OsdVertexBufferDescriptor Osd::BufferDescriptor
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======================================= ========================================
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ComputeContext, DrawContext
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+++++++++++++++++++++++++++
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ComputeContext and DrawContext have been replaced with API-specific StencilTable
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and PatchTable objects, for example Osd::GLStencilTableSSBO.
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======================================= ========================================
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OpenSubdiv 2.x OpenSubdiv 3.0
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======================================= ========================================
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ComputeContext Osd::...StencilTable (e.g. GLStencilTableTBO)
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EvalStencilsContext Osd::...StencilTable
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DrawContext Osd::...PatchTable (e.g. GLPatchTable)
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======================================= ========================================
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EvalLimitContext
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++++++++++++++++
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The data stored in EvalLimitContext has been merged into the Evaluator class as
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well.
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EvalCoords have been moved into their own type, Osd::PatchCoords. The primary
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change here is that the PTex face ID is no longer part of the data structure,
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rather the client can use a Far::PatchMap to convert from PTex face ID to a
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Far::PatchTable::PatchHandle.
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======================================= ========================================
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OpenSubdiv 2.x OpenSubdiv 3.0
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======================================= ========================================
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EvalLimitContext PatchTable
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EvalLimitContext::EvalCoords Osd::PatchCoords (types.h)
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======================================= ========================================
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OsdMesh
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+++++++
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While not strictly required, OsdMesh is still supported in 3.0 as convenience
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API for allocating buffers. OsdMesh serves as a simple way to allocate all
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required data, in the location required by the API (for example, GPU buffers for
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OpenGL).
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OsdKernelBatch
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++++++++++++++
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No translation, it is no longer part of the API.
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OsdVertex
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+++++++++
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No translation, it is no longer part of the API.
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Feature Adaptive Shader Changes
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===============================
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In 3.0, the feature adaptive screen-space tessellation shaders have been
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dramatically simplified, and the client-facing API has changed dramatically as
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well. The primary shift is to reduce the total number of shader combinations, and
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as a result, some of the complexity management mechanisms are no longer
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necessary.
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In the discussion below, some key changes are highlighted, but deep
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integrations may require additional discussion; please feel free to send
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follow up questions to the OpenSubdiv google group.
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* The number of feature adaptive shaders has been reduced from N to exactly 1
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or 2, depending on how end-caps are handled.
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* Osd layer no longer compiles shaders, rather it returns shader source for the
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client to compile. This source is obtained via
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Osd::[GLSL|HLSL]PatchShaderSource.
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* The API exposed in shaders to access patch-based data has been consolidated
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and formalized, see osd/glslPatchCommon.glsl and osd/hlslPatchCommon.hlsl for
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details.
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* Patches are no longer rotated and transition patches have been eliminated,
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simplifying PatchDescriptor to a 4 bits. Additionally, FarPatchTables::Descriptor
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has been moved into its own class in the Far namespace.
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The following table outlines the API translation between 2.x and 3.0:
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======================================= ========================================
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OpenSubdiv 2.x OpenSubdiv 3.0
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======================================= ========================================
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OsdDrawContext::PatchDescriptor N/A, no longer needed.
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OsdDrawContext::PatchArray OSd::PatchArray (types.h)
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FarPatchTables::PatchDescriptor Far::PatchDescriptor (patchDescriptor.h)
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FarPatchTables::PatchArray made private.
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======================================= ========================================
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End Cap Strategies
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++++++++++++++++++
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By default, OpenSubdiv uses Gregory patches to approximate the patches around
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extraordinary vertices at the maximum isolation level, this process is referred
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to as "end-capping".
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If ENDCAP_BSPLINE_BASIS is specified to PatchTableFactory::Options, BSpline
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patches are used, which gives less accuracy, but it makes possible to render an
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entire mesh in a single draw call. Both patches require additional control
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points that are not part of the mesh, we refer to these as "local points". In
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3.0, the local points of those patches are computed by applying a stencil table
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to refined vertices to construct a new stencil table for the local points.
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Since this new stencil table is topologically compatible with the primary
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stencil table for refinement, it is convenient and efficient to splice those
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stencil tables together. This splicing can be done in the following way::
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Far::StencilTable const *refineStencils =
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Far::StencilTableFactory::Create(topologyRefiner);
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Far::PatchTable cosnt *patchTable =
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Far::PatchTableFactory::Create(topologyRefiner);
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Far::StencilTable const *localPointStencils =
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patchTable->GetLocalPointStencilTable();
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Far::StencilTable const *splicedStencils =
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Far::StencilTableFactory::AppendLocalPointStencilTables(topologyRefiner,
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refineStencils,
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localPointStencils);
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**NOTE:** Once the spliced stencil table is created, the refined stencils can be
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released, but the local point stencils are owned by patchTable, it should not be
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released.
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OpenSubdiv 3.0 also supports 2.x style Gregory patches, if ENDCAP_LEGACY_GREGORY
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is specified to PatchTableFactory::Options. In this case, such an extra stencil
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splicing isn't needed, however clients must still bind additional buffers
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(VertexValence buffer and QuadOffsets buffer).
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See Osd::GLLegacyGregoryPatchTable for additional details.
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Changes to Subdivision
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======================
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The refactoring of OpenSubdiv 3.0 data representations presented a unique
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opportunity to revisit some corners of the subdivision specification and
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remove or update some legacy features -- none of which was taken lightly.
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More details are provided in
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`Subdivision Compatibility <compatibility.html>`__, while the
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following offers a quick overview:
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* All face-varying interpolation options have been combined into a single enum.
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* Vertex interpolation options have been renamed or removed:
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* The naming of the standard creasing method has changed from *Normal* to *Uniform*.
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* Unused legacy modes of the *"smoothtriangle"* option have been removed.
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* The averaging of Chaikin creasing with infinitely sharp edges has changed.
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* Support for Hierarchical Edits has been removed.
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Build Support for Combining 2.x and 3.0
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=======================================
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Running OpenSubdiv 2.0 and 3.0 in a single process is supported, however some
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special care must be taken to avoid namespace collisions, both in terms of
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run-time symbols (avoid "using OpenSubdiv::Osd", for example) and in terms of
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build-time search paths.
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To support both OpenSubdiv 2.0 and 3.0 in your build environment, you can
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prefix the header install directory of OpenSubdiv 3.0. Do this using the build
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flag "CMAKE_INCDIR_BASE" when configuring cmake (i.e.
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-DCMAKE_INCDIR_BASE=include/opensubdiv3) and then including files from
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"opensubdiv3/..." in client code.
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