# Porting Guide: 2.x to 3.0

# Porting Guide 2.x to 3.0

- Porting Guide: 2.x to 3.0
- Source Code Organization
- Hbr Layer Translation
  - Subdivision Schemes and Options in Sdc
  - Specifying Face Varying Topology and Options
- Far Layer Translation
  - Ordering of Refined Vertices
- Osd Layer Translation
  - Controller Objects
  - ComputeContext, DrawContext
  - EvalLimitContext
  - OsdMesh
  - OsdKernelBatch
  - OsdVertex
- Feature Adaptive Shader Changes
  - End Cap Strategies
- Changes to Subdivision
- Build Support for Combining 2.x and 3.0

### Porting Guide: 2.x to 3.0

This document is a high-level description of how to port exiting OpenSubdiv 2.x code to use OpenSubdiv 3.0.

NOTE: If your questions are not answered here, please contact us on the OpenSubdiv forum and we will be happy to help!

### Source Code Organization

Given the scale of functional changes that were being made to the public interface, we took the opportunity in 3.0 to update the coding style and organization -- most notably making use of namespaces for each library.

Subdirectory | Namespace | Relevance
--- | --- | ---
hbr/ | N/A | Historical, no longer used
sdc/ | Sdc | New, low-level, public options, constants, etc.
vtr/ | Vtr | New, internal use, topology representation
far/ | Far | Revised, similar functionality with new API
osd/ | Osd | Revised, similar functionality with new API

### Hbr Layer Translation

Client mesh topology is now translated into an instance of Far::TopologyRefiner instead of HbrMesh.

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
HbrMesh<VTX_TYPE> | Far::TopologyRefiner

The Far::TopologyRefiner is now the core representation of topology from which all other major classes in Far and Osd are constructed. It was designed to support efficient refinement (uniform or sparse) of a base mesh of arbitrary topology (no manifold restrictions). Once constructed it can be directly refined to meet some need, or passed to other contexts that will refine it to meet their needs.

In contrast to directly assembling an HbrMesh, the TopologyRefiner, like other classes in Far, requires a Factory class for its construction. One of the early goals of these factories was to allow a client to convert their existing boundary representation -- with its full topological traversal abilities -- directly into the TopologyRefiners representation. While this is now possible, this also represents the most complex construction process and is only recommended for usage where this conversion process is critical.

Details on how to construct a TopologyRefiner can be found in the Far overview documentation. Additionally, documentation for Far::TopologyRefinerFactory<MESH> outlines the requirements, and a Far tutorial (tutorials/far/tutorial_1) provides an example of a factory for directly converting HbrMeshes to TopologyRefiners.

Its worth a reminder here that Far::TopologyRefiner contains only topological information (which does include sharpness, since that is considered relating to subdivision topology) and not the positions or other data associated with a mesh. While HbrMesh<T> required some definition of a vertex type <T> and dimensions of face-varying data, TopologyRefiner is more clearly separated from the data. So the construction of the TopologyRefiner does not involve data specification at all.

#### Subdivision Schemes and Options in Sdc

The creation of a new TopologyRefiner requires specification of a subdivision scheme and a set of options that are applicable to all schemes. With HbrMesh, the scheme was specified by declaring a static instance of a specific subclass of a subdivision object, and the options were specified with a number of methods on the different classes.

Such general information about the schemes has now been encapsulated in the Sdc layer for use throughout OpenSubdiv. The subdivision scheme is now a simple enumerated type (Sdc::SchemeType) and the entire set of options that can be applied to a scheme is encapsulated in a single simple struct of flags and enumerated types (Sdc::Options).

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
HbrMesh<T>::SetInterpolateBoundaryMethod() | Sdc::Options::SetVtxBoundaryInterpolation()
HbrMesh<T>::SetFVarInterpolateBoundaryMethod() | Sdc::Options::SetFVarLinearInterpolation()
HbrSubdivision<T>::SetCreaseSubdivisionMethod() | Sdc::Options::SetCreasingMethod()

#### Specifying Face Varying Topology and Options

Both the way in which face varying data is associated with a mesh and the options used to control its interpolation have changed. The documentation on Compatibility with OpenSubdiv 2.x details the equivalence of interpolation options between Hbr and the new Sdc::Options::FVarLinearInterpolation enum, while the section on Face Varying Interpolation illustrates their effects.

Face varying data is now specified by index rather than by value, or as often stated, it is specified topologically. Just as vertices for faces are specified by indices into a potential buffer of positions, face varying values are specified by indices into a potential buffer of values. Both vertices and face varying values (frequently referred to as FVarValues in the API) are assigned and associated with the corners of all faces.

In many cases this will simplify representation as many common geometry container formats such as Obj or Alembic specify texture coordinates the same way. For other cases, where a value per face-corner is provided with no indication of which values incident each vertex should be considered shared, it will be necessary to determine shared indices for values at each vertex if any non-linear interpolation is desired.

### Far Layer Translation

While TopologyRefiner was introduced into Far as the new intermediate topology representation, several other changes were made to classes in Far to provide more modular building blocks for use by the Osd layer or directly.

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
FarMesh<U> | N/A, no longer needed
FarSubdivisionTables | Far::StencilTable
FarPatchTables | Far::PatchTable

#### Ordering of Refined Vertices

The FarMesh was previously responsible for refining an HbrMesh -- generating new vertices and faces in successive levels of refinement in the FarSubdivisionTables. Vertices were grouped and reordered from the native ordering of HbrMesh so that vertices requiring similar processing were consecutive. Such grouping alleviated most of the idiosyncrasies of HbrMesh's native ordering but not all.

Far::ToplogyRefiner is inherently a collection of refinement levels, and within each refined level (so excluding the base level), all components are still grouped for the same reasons. There are two issues here though:

- the ordering of these groups has changed (though an option exists to preserve it)
- the ordering of components within these groups is not guaranteed to have been preserved

Vertices in a refined level are grouped according to the type of component in the parent level from which they originated, i.e. some vertices originate from the center of a face (face-vertices), some from an edge (edge-vertices) and some from a vertex (vertex-vertices). (Note that there is a conflict in terminology here -- face-vertices and edge-vertices most often refer to vertices incident a face or edge -- but for the sake of this discussion, we use them to refer to the component from which a child vertex originates.)

The following table shows the ordering of these groups in 2.x and the two choices available in 3.0. The option is the orderVerticesFromFacesFirst flag that can be set in the Option structs passed to the uniform and adaptive refinement methods of TopologyRefiner:

Version and option | Vertex group ordering
--- | ---
2.x | face-vertices, edge-vertices, vertex-vertices
3.0 default | vertex-vertices, face-vertices, edge-vertices
3.0 orderVerticesFromFacesFirst = true | face-vertices, edge-vertices, vertex-vertices

The decision to change the default ordering was based on common feedback; the rationale was to allow a trivial mapping from vertices in the cage to their descendants at all refinement levels. While the grouping is fundamental to the refinement process, the ordering of the groups is internally flexible, and the full set of possible orderings can be made publicly available in future if there is demand for such flexibility.

The ordering of vertices within these groups was never clearly defined given the way that HbrMesh applied its refinement. For example, for the face-vertices in a level, it was never clear which face-vertices would be first as it depended on the order in which HbrMesh traversed the parent faces and generated them. Given one face, HbrMesh would often visit neighboring faces first before moving to the next intended face.

The ordering with Far::TopologyRefiner is much clearer and predictable. Using the face-vertices as an example, the order of the face-vertices in level N+1 is identical to the order of the parent faces in level N from which they originated. So if we have face-vertices V'i, V'j and V'k at some level, originating from faces Fi, Fj and Fk in the previous level, they will be ordered in increasing order of i, j and k. For uniform refinement the ordering of face vertices V'i will therefore exactly match the ordering of the parent faces Fi. For adaptive or otherwise sparse refinement, the subset of Vi will be ordered similarly, just with components missing from those not refined.

The same is true of all vertices, i.e. edge-vertices and vertex-vertices, and also for other components in refined levels, i.e. the child faces and edges.

For child faces and edges, more than one will originate from the same parent face or edge. In addition to the overall ordering based on the parent faces or edges, another ordering is imposed on multiple children originating from the same face or edge. They will be ordered based on the corner or end-vertex with which they are associated.

In the case of refined faces, another way to view the ordering is to consider the way that faces are originally defined -- by specifying the set of vertices for the corners of each face, often aggregated into a single large array. The ordering of the set of refined faces for each level will correspond directly to such an array of vertices per face in the previous level.

### Osd Layer Translation

Two big changes in the 3.0 API have allowed the Osd layer to be significantly simpler, the first is the move to stencil tables from subdivision tables and the second is shader simplification. With this refactoring, the focus has been to use more meaningful names and to make the data contained within an object more apparent.

#### Controller Objects

The API-specific ComputeController has been replaced with the Evaluator. It reflects the fact that stencil compute batches are significantly simpler than subdivision table compute batches.

The name "Evaluator" was chosen with the hope that is more meaningful than the generic "ComputeController" moniker: the Evaluator evaluates stencil and patch tables.

In the 2.x code base, subdiv level buffers were always required to be allocated contiguously, however in 3.0 with the shift to stencil tables, this strict allocation scheme is no longer required. As a result, the EvalStencils() and EvalPatches() methods now accept both a source and a destination descriptor.

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
ComputeController::Refine() | Osd::...Evaluator::EvalStencils()
ComputeController::Synchronize() | Osd::...Evaluator::Synchronize()
EvalStencilsController::UpdateValues() | Osd::...Evaluator::EvalStencils()
EvalStencilsController::UpdateDerivs() | Osd::...Evaluator::EvalStencils()
EvalLimitController::EvalLimitSample() | Osd::...Evaluator::EvalPatches()

Also note that OsdVertexDescriptor has been renamed, however it's data members and semantic purpose remains the same:

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
OsdVertexBufferDescriptor | Osd::BufferDescriptor

#### ComputeContext, DrawContext

ComputeContext and DrawContext have been replaced with API-specific StencilTable and PatchTable objects, for example Osd::GLStencilTableSSBO.

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
ComputeContext | Osd::...StencilTable (e.g. GLStencilTableTBO)
EvalStencilsContext | Osd::...StencilTable
DrawContext | Osd::...PatchTable (e.g. GLPatchTable)

#### EvalLimitContext

The data stored in EvalLimitContext has been merged into the Evaluator class as well.

EvalCoords have been moved into their own type, Osd::PatchCoords. The primary change here is that the PTex face ID is no longer part of the data structure, rather the client can use a Far::PatchMap to convert from PTex face ID to a Far::PatchTable::PatchHandle.

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
EvalLimitContext | PatchTable
EvalLimitContext::EvalCoords | Osd::PatchCoords (types.h)

#### OsdMesh

While not strictly required, OsdMesh is still supported in 3.0 as convenience API for allocating buffers. OsdMesh serves as a simple way to allocate all required data, in the location required by the API (for example, GPU buffers for OpenGL).

#### OsdKernelBatch

No translation, it is no longer part of the API.

#### OsdVertex

No translation, it is no longer part of the API.

### Feature Adaptive Shader Changes

In 3.0, the feature adaptive screen-space tessellation shaders have been dramatically simplified, and the client-facing API has changed dramatically as well. The primary shift is to reduce the total number of shader combinations, and as a result, some of the complexity management mechanisms are no longer necessary.

In the discussion below, some key changes are highlighted, but deep integrations may require additional discussion; please feel free to send follow up questions to the OpenSubdiv google group.

> - The number of feature adaptive shaders has been reduced from N to exactly 1 or 2, depending on how end-caps are handled.
> - Osd layer no longer compiles shaders, rather it returns shader source for the client to compile. This source is obtained via Osd::[GLSL|HLSL]PatchShaderSource.
> - The API exposed in shaders to access patch-based data has been consolidated and formalized, see osd/glslPatchCommon.glsl and osd/hlslPatchCommon.hlsl for details.
> - Patches are no longer rotated and transition patches have been eliminated, simplifying PatchDescriptor to a 4 bits. Additionally, FarPatchTables::Descriptor has been moved into its own class in the Far namespace.

The following table outlines the API translation between 2.x and 3.0:

OpenSubdiv 2.x | OpenSubdiv 3.0
--- | ---
OsdDrawContext::PatchDescriptor | N/A, no longer needed.
OsdDrawContext::PatchArray | OSd::PatchArray (types.h)
FarPatchTables::PatchDescriptor | Far::PatchDescriptor (patchDescriptor.h)
FarPatchTables::PatchArray | made private.

#### End Cap Strategies

By default, OpenSubdiv uses Gregory patches to approximate the patches around extraordinary vertices at the maximum isolation level, this process is referred to as "end-capping".

If ENDCAP_BSPLINE_BASIS is specified to PatchTableFactory::Options, BSpline patches are used, which gives less accuracy, but it makes possible to render an entire mesh in a single draw call. Both patches require additional control points that are not part of the mesh, we refer to these as "local points". In 3.0, the local points of those patches are computed by applying a stencil table to refined vertices to construct a new stencil table for the local points.

Since this new stencil table is topologically compatible with the primary stencil table for refinement, it is convenient and efficient to splice those stencil tables together. This splicing can be done in the following way:

    Far::StencilTable const *refineStencils =
                                  Far::StencilTableFactory::Create(topologyRefiner);

    Far::PatchTable cosnt *patchTable =
                                  Far::PatchTableFactory::Create(topologyRefiner);

    Far::StencilTable const *localPointStencils =
                                      patchTable->GetLocalPointStencilTable();

    Far::StencilTable const *splicedStencils =
            Far::StencilTableFactory::AppendLocalPointStencilTables(topologyRefiner,
                                                              refineStencils,
                                                              localPointStencils);

NOTE: Once the spliced stencil table is created, the refined stencils can be released, but the local point stencils are owned by patchTable, it should not be released.

OpenSubdiv 3.0 also supports 2.x style Gregory patches, if ENDCAP_LEGACY_GREGORY is specified to PatchTableFactory::Options. In this case, such an extra stencil splicing isn't needed, however clients must still bind additional buffers (VertexValence buffer and QuadOffsets buffer).

See Osd::GLLegacyGregoryPatchTable for additional details.

### Changes to Subdivision

The refactoring of OpenSubdiv 3.0 data representations presented a unique opportunity to revisit some corners of the subdivision specification and remove or update some legacy features -- none of which was taken lightly. More details are provided in Subdivision Compatibility, while the following offers a quick overview:

- All face-varying interpolation options have been combined into a single enum.
- Vertex interpolation options have been renamed or removed:
  - The naming of the standard creasing method has changed from Normal to Uniform.
  - Unused legacy modes of the "smoothtriangle" option have been removed.
- The averaging of Chaikin creasing with infinitely sharp edges has changed.
- Support for Hierarchical Edits has been removed.

### Build Support for Combining 2.x and 3.0

Running OpenSubdiv 2.0 and 3.0 in a single process is supported, however some special care must be taken to avoid namespace collisions, both in terms of run-time symbols (avoid "using OpenSubdiv::Osd", for example) and in terms of build-time search paths.

To support both OpenSubdiv 2.0 and 3.0 in your build environment, you can prefix the header install directory of OpenSubdiv 3.0. Do this using the build flag "CMAKE_INCDIR_BASE" when configuring cmake (i.e. -DCMAKE_INCDIR_BASE=include/opensubdiv3) and then including files from "opensubdiv3/..." in client code.

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