ModelMeshPart

DirectXTK	Model

This class is part of the Model hierarchy. The purpose of this class is to be container for a 'submesh' which is referenced by an instance of the ModelMesh class. A 'submesh' is a portion of a mesh that is all drawn using the same 'material' settings as reflected by an Effects instance. A submesh is drawn in a single Direct3D 11 Draw call using a single effect.

ModelMesh uses a collection of std::unique_ptr instances to ModelMeshPart since submeshes are not typically shared between mesh instances. Submeshes can easily share Direct3D 11 resources (index buffers, vertex buffers, effects, etc.) which are individually reference counted.

Header

#include <Model.h>

Initialization

ModelMeshPart instances are typically created by a Model loader along with the ModelMesh instances that contain the submeshes.

Type aliases

• ModelMeshPart::Collection is an alias for std::vector<std::unique_ptr<ModelMeshPart>>.

• ModelMeshPart::InputLayoutCollection is an alias for std::vector<D3D11_INPUT_ELEMENT_DESC>.

Data

All members of ModelMeshPart are public to facilitate writing custom model loaders and sophisticated rendering solutions. Care should be taken whenever modifying any of these elements as they have interdependencies, and can ultimately be referenced by numerous instances of Model via shared ModelMesh instances.

• indexCount - Number of indices in the index buffer associated with this submesh. This is used as the first parameter to Direct3D11's DrawIndexed method.

• startIndex - The index to start drawing with for this submesh. This facilitates sharing index buffers between multiple submeshes. This is used as the second parameter to Direct3D11's DrawIndexed method.

• vertexOffset - base vertex location which is added to each index in the index buffer before referencing the vertex buffer. This facilitates sharing of vertex buffers between multiple submeshes. This is used as the third parameter to Direct3D11's DrawIndexed method.

• vertexBuffer - COM smart pointer to the vertex buffer associated with this submesh. This is used when calling Direct3D 11's IASetVertexBuffers method. This vertex buffer can be shared by multiple submeshes or even other meshes.

• vertexStride - Stride in bytes for each vertex in the vertex buffer associated with this submesh. This is used when calling Direct3D 11's IASetVertexBuffers method.

• indexBuffer - COM smart pointer to the index buffer associated with this submesh. This is used when calling Direct3D11's IASetIndexBuffer method. This index buffer can be shared by multiple submeshes or even other meshes.

• indexFormat - Format of the index buffer. For 16-bit index buffers, this is set to DXGI_FORMAT_R16_UINT. For 32-bit index buffers, this is set to DXGI_FORMAT_R32_UINT. This is used when calling Direct3D11's IASetIndexBuffer method.

• primitiveType - Topology for the primitives in the submesh. It is usually set to D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST with each three indices in the index buffer forming a triangle. This is used when calling Direct3D11's IASetPrimitiveTopology method.

• inputLayout - COM smart pointer to the input layout associated with this submesh. This is used when calling Direct3D11's IASetInputLayout method. This must correctly match the signature of the effect and match the vertex data format in the vertex buffer.

• effect - An IEffect instance which is used to set up the proper constant buffer and shader states before drawing the submesh.

Metadata

The ModelMeshPart::isAlpha boolean indicates if the ModelMeshPart instance's effect makes use of alpha-blending. This information is used by ModelMesh::Draw to ensure that opaque effects are applied before transparent effects, and the depth buffer is used accordingly.

The ModelMeshPart::vbDecl contains the input layout element descriptor needed for creating input layouts that match this submeshes vertex buffer and effect instance. This information is used by ModelMeshPart::CreateInputLayout for setting up custom effect overrides and ModelMeshPart::ModifyEffect when changing the effect instance used by the submesh.

Simple drawing

Use the Model::Draw function which will call ModelMesh::Draw on all the meshes it contains, which in turn will call ModelMeshPart::Draw on each submesh part.

Advanced drawing

The ModelMeshPart::Draw method is used to draw each individual submesh. ModelMeshPart::Draw can be used to implement complex 'scene graph' policies (such as sorting all ModelMeshPart instance in a scene by Effects to minimize draw overhead or rough-sorting by bounding volume front-to-back for opaque parts and back-to-front for alpha blended parts), or can be used to temporarily override the effect instance being used to draw the mesh (for example when drawing shadows).

void Draw(ID3D11DeviceContext* deviceContext,
    IEffect* ieffect, ID3D11InputLayout* iinputLayout,
    std::function<void()> setCustomState = nullptr) const;

You can also perform instanced drawing, although this scenario requires custom shader state to get interesting results:

void DrawInstanced(ID3D11DeviceContext* deviceContext,
    IEffect* ieffect, ID3D11InputLayout* iinputLayout,
    uint32_t instanceCount, uint32_t startInstanceLocation = 0,
    std::function<void()> setCustomState = nullptr) const;

To support using custom effect instances, ModelMeshPart::CreateInputLayout provides the ability to create a new input layout with a signature that matches the submesh's vertex buffer and the new custom effect instance.

// An example of using a single custom effect when drawing all the parts of a Model
IEffect* newEffect = ... (device)

// Creating input layouts is expensive, so it shouldn't be done every frame
std::vector<Microsoft::WRL::ComPtr<ID3D11InputLayout>> newInputLayouts;
for(const auto& mit : model->meshes)
{
    auto mesh = it.get();
    assert( mesh != nullptr );

    mesh->PrepareForRendering(context, states);

    for(const auto& it : mesh->meshParts)
    {
        auto part = it.get();
        assert( part != nullptr );

        Microsoft::WRL::ComPtr<ID3D11InputLayout> il;
        part->CreateInputLayout( device.Get(), newEffect, il.GetAddressOf() );

        newInputLayouts.emplace_back(il);
    }
}

...

// Draw Model with custom effect override
auto imatrices = dynamic_cast<IEffectMatrices*>( newEffect );
if ( imatrices )
{
    imatrices->SetWorld( world );
    imatrices->SetView( view );
    imatrices->SetProjection( projection );
}

size_t count = 0;
for(const auto& mit : model->meshes)
{
    auto mesh = mit.get();
    assert( mesh != 0 );

    for(const auto& it : mesh->meshParts)
    {
        auto part = it.get();
        assert( part != 0 );

        // Could call if a custom transformation was desired for each part
        // if (imatrices) imatrices->SetWorld( local )

        part->Draw( deviceContext.Get(), newEffect,
            newInputLayouts[ count++ ].Get() );
     }
}

The ModelMesh::PrepareForRendering helper will only set up a texture sampler in slots 0 and 1. If you are using an Effects instance with more two textures, you need to set additional samplers manually.

Effects Control

A Model loader is responsible for creating Effects instances via an IEffectFactory so that the loaded Model is fully initialized. However, a developer may want to modify a Model's effect after it has been loaded. All the members of ModelMeshPart are public so this can be done directly, but must be done carefully.

If the effect instance is changed, the inputLayout must be updated as well. Furthermore, Model maintains a cache of unique effect instances in all ModelMesh instances it contains, so any change to the ModelMeshPart instances must invalidate this cache. This is done by calling Model::Modified. Keep in mind that since ModelMesh instances are shared, modification can impact multiple instances of a Model and all impacted Model instances need to have their effect cache invalidated to avoid crashes or lingering references to the wrong effect instances.

IEffect* newEffect = ... (device)

ModelMeshPart* part = ...

// This will regenerate the inputLayout and needs to indicate
// if newEffect is alpha-blended or not
part->ModifyEffect( device.Get(), newEffect, false );

// part comes from a Model instance 'model' and we assume it's
// not shared by another Model instance here
model->Modified();

Optimization

Each submesh is drawn as a single Direct3D 11 DrawIndexed call, so it constitutes a draw batch. For best performance, each ModelMesh should have the fewest number of ModelMeshPart instances as this one of the more direct impacts on runtime performance. Larger draw batches are generally more efficient than small ones, but keep the Feature Level limits on primitive count, index format, and maximum index buffer size in mind.

Feature Level Notes

If any ModelMeshPart makes use of 32-bit indices (i.e. ModelMeshPart:: indexFormat equals DXGI_FORMAT_R32_UINT) rather than 16-bit indices (DXGI_FORMAT_R16_UINT), then that model requires Feature Level 9.2 or greater.

If any ModelMeshPart uses adjacency (i.e. ModelMeshPart::primitiveType equals D3D_PRIMITIVE_TOPOLOGY_*_ADJ), then that model requires Feature Level 10.0 or greater. If using tessellation (i.e. D3D_PRIMITIVE_TOPOLOGY_?_CONTROL_POINT_PATCHLIST), then that model requires Feature Level 11.0 or greater.

Keep in mind that there are maximum primitive count limits per ModelMeshPart based on feature level as well (65535 for Feature Level 9.1, 1048575 or Feature Level 9.2 and 9.3, and 4294967295 for Feature Level 10.0 or greater).

Remember that instanced drawing require Feature Level 9.3 or later.