API Layers

OpenSubdiv is structured as a set of layered libraries. This structure facilitates operation on a variety of computing resources, and allows developers to only opt-in to the layers and feature sets that they require. From a top-down point of view, OpenSubdiv is comprised of several layers, some public, and some private.

Layers list:

Client mesh data enters the API through the Far layer. Typically, results will be collected from the Osd layer. However, it is possible to use functionality from Far without introducing any dependency on Osd.

Although there are several entry-points to provide topology and primitive variable data to OpenSubdiv, eventually everything must pass through the private Vtr and Sdc representations for topological analysis.

Using the Right Tools

OpenSubdiv's tiered interface offers a lot flexibility to make your application both fast and robust. Because navigating through the large collection of classes and features can be challenging, here are use cases that should help sketch the broad lines of going about using subdivisions in your application.

General client application requirements:

Use case 1: Simple refinement

The following example shows the most simple case to get your mesh refined uniformly.

1. Define a class for the primvar you want to refine. It's required to have Clear() and AddWithWeight() functions.

struct Vertex {
    void Clear() { x = y = z = 0; }
    void AddWithWeight(Vertex const &src, float weight) {
        x += weight * src.x;
        y += weight * src.y;
        z += weight * src.z;
    }
    float x, y, z;
};

2. Instantiate a Far::TopologyRefiner from the Far::TopologyDescriptor.

Far::TopologyDescriptor desc;
desc.numVertices         = <the number of vertices>
desc.numFaces            = <the number of faces>
desc.numVertsPerFace     = <array of the number of verts per face>
desc.vertIndicesPerFace  = <array of vert indices>

Far::TopologyRefiner * refiner = Far::TopologyRefinerFactory<Descriptor>::Create(desc);

3. Call RefineUniform() to refine the topology up to 'maxlevel'.

refiner->RefineUniform(Far::TopologyRefiner::UniformOptions(maxlevel));

4. Interpolate vertex primvar data at 'level' using Far::PrimvarRefiner

Far::PrimvarRefiner primvarRefiner(*refiner);

Vertex const *src = <coarse vertices>
Vertex *dst       = <refined vertices>

primvarRefiner.Interpolate(level, src, dst);

5. The topology at the refined level can be obtained from Far::TopologyLevel

Far::TopologyLevel const & refLastLevel = refiner->GetLevel(maxlevel);

int nverts = refLastLevel.GetNumVertices();
int nfaces = refLastLevel.GetNumFaces();

for (int face = 0; face < nfaces; ++face) {
    Far::ConstIndexArray fverts = refLastLevel.GetFaceVertices(face);

    // do something with dst and fverts
}

6. Done! See Far tutorial 1.1 for the complete code example.

Use case 2: GL adaptive tessellation drawing of animating mesh

The next example is showing how to draw adaptive tessellated patches in GL using OpenSubdiv. The osd layer helps you to interact with GL and other device specific APIs. Also for an efficient refinement of animating mesh on a static topology, we create a stencil table to refine the positions changing over time.

The following example code uses an Osd::GLMesh utility class which composites a stencil table, patch table, vertex buffer and evaluator in osd layer. You can also use those classes independently.

1. Instantiate a Far::TopologyRefiner from the Far::TopologyDescriptor, same as usecase 1.

2. Setup Osd::Mesh. In this example we use b-spline endcap.

int numVertexElements = 3; // x, y, z

Osd::MeshBitset bits;
bits.set(Osd::MeshAdaptive, true);           // set adaptive
bits.set(Osd::MeshEndCapBSplineBasis, true); // use b-spline basis patch for endcap.

Osd::GLMeshInterface *mesh = new Osd::Mesh<Osd::CpuGLVertexBuffer, Far::StencilTable,
                                           Osd::CpuEvaluator, Osd::GLPatchTable>
                                    (refiner, numVertexElements, 0, level, bits);

3. Update coarse vertices and refine (Osd::Mesh::Refine() calls Osd::CpuEvaluator::EvalStencils())

mesh->UpdateVertexBuffer(&vertex[0], 0, nverts);
mesh->Refine();

4. Bind index buffer, PatchParamBuffer and vertex buffer

// index buffer
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, mesh->GetPatchTable()->GetPatchIndexBuffer());

// vertex buffer
glBindBuffer(GL_ARRAY_BUFFER, mesh->BindVertexBuffer());
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, numVertexElements, GL_FLOAT, GL_FALSE,
                      numVertexElements*sizeof(float), 0);

// patch param buffer
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_BUFFER, mesh->GetPatchTable()->GetPatchParamTextureBuffer());

5. Draw. Since we use b-spline endcaps in this example, there is only one PatchArray in the patch table. You may need to iterate patch arrays as you use other type of endcap. To configure GLSL program for each patch type, see osd shader interface for more details.

Osd::PatchArray const & patch = mesh->GetPatchTable()->GetPatchArrays()[0];
Far::PatchDescriptor desc = patch.GetDescriptor();

int numVertsPerPatch = desc.GetNumControlVertices();  // 16 for B-spline patches
glUseProgram(BSplinePatchProgram);
glPatchParameteri(GL_PATCH_VERTICES, numVertsPerPatch);
glDrawElements(GL_PATCHES, patch.GetNumPatches() * numVertsPerPatch,
               GL_UNSIGNED_INT, 0);

6. As the mesh animates, repeat from step 3 to update positions, refine, and draw. See glViewer and other examples for more complete usage.

Tutorials and Examples

For more use cases, please see Tutorials and Examples