在DirectX 12中使用shadow map

shadow map是常见的实现阴影效果的手段，可以分为生成shadow map和采样shadow map两个阶段。生成shadow map首先需要准备好资源，和绑定的view：

	D3D12_RESOURCE_DESC texDesc;
	ZeroMemory(&texDesc, sizeof(D3D12_RESOURCE_DESC));
	texDesc.Dimension = D3D12_RESOURCE_DIMENSION_TEXTURE2D;
	texDesc.Alignment = 0;
	texDesc.Width = mWidth;
	texDesc.Height = mHeight;
	texDesc.DepthOrArraySize = 1;
	texDesc.MipLevels = 1;
	texDesc.Format = mFormat;
	texDesc.SampleDesc.Count = 1;
	texDesc.SampleDesc.Quality = 0;
	texDesc.Layout = D3D12_TEXTURE_LAYOUT_UNKNOWN;
	texDesc.Flags = D3D12_RESOURCE_FLAG_ALLOW_DEPTH_STENCIL;

    D3D12_CLEAR_VALUE optClear;
    optClear.Format = DXGI_FORMAT_D24_UNORM_S8_UINT;
    optClear.DepthStencil.Depth = 1.0f;
    optClear.DepthStencil.Stencil = 0;

	ThrowIfFailed(md3dDevice->CreateCommittedResource(
		&CD3DX12_HEAP_PROPERTIES(D3D12_HEAP_TYPE_DEFAULT),
		D3D12_HEAP_FLAG_NONE,
		&texDesc,
        D3D12_RESOURCE_STATE_GENERIC_READ,
		&optClear,
		IID_PPV_ARGS(&mShadowMap)));

shadow map本质上是一张光源空间的深度缓存，因此创建资源的方式类似depth buffer。由于shadow map在生成过程和采样过程都要用到，这里需要两个view，一个dsv，一个srv对其进行绑定：

    D3D12_DEPTH_STENCIL_VIEW_DESC dsvDesc; 
    dsvDesc.Flags = D3D12_DSV_FLAG_NONE;
    dsvDesc.ViewDimension = D3D12_DSV_DIMENSION_TEXTURE2D;
    dsvDesc.Format = DXGI_FORMAT_D24_UNORM_S8_UINT;
    dsvDesc.Texture2D.MipSlice = 0;
	md3dDevice->CreateDepthStencilView(mShadowMap.Get(), &dsvDesc, mhCpuDsv);

    
	D3D12_SHADER_RESOURCE_VIEW_DESC srvDesc = {};
	srvDesc.Shader4ComponentMapping = D3D12_DEFAULT_SHADER_4_COMPONENT_MAPPING;
    srvDesc.Format = DXGI_FORMAT_R24_UNORM_X8_TYPELESS; 
	srvDesc.ViewDimension = D3D12_SRV_DIMENSION_TEXTURE2D;
	srvDesc.Texture2D.MostDetailedMip = 0;
	srvDesc.Texture2D.MipLevels = 1;
	srvDesc.Texture2D.ResourceMinLODClamp = 0.0f;
    srvDesc.Texture2D.PlaneSlice = 0;
    md3dDevice->CreateShaderResourceView(mShadowMap.Get(), &srvDesc, mhCpuSrv);

在绘制shadow map的过程中，我们只需记录像素深度，并不需要实际进行像素写入，这里可以修改pixel shader，使其不输出color：

void PS(VertexOut pin) 
{
	...
}

相应地，也需要为shadow map添加控制绘制状态的pipeline state object：

    D3D12_GRAPHICS_PIPELINE_STATE_DESC smapPsoDesc = opaquePsoDesc;
    smapPsoDesc.RasterizerState.DepthBias = 100000;
    smapPsoDesc.RasterizerState.DepthBiasClamp = 0.0f;
    smapPsoDesc.RasterizerState.SlopeScaledDepthBias = 1.0f;
    smapPsoDesc.pRootSignature = mRootSignature.Get();
    smapPsoDesc.VS =
    {
        reinterpret_cast<BYTE*>(vs->GetBufferPointer()),
        vs->GetBufferSize()
    };
    smapPsoDesc.PS =
    {
        reinterpret_cast<BYTE*>(ps->GetBufferPointer()),
        ps->GetBufferSize()
    };
    

    smapPsoDesc.RTVFormats[0] = DXGI_FORMAT_UNKNOWN;
    smapPsoDesc.NumRenderTargets = 0;
    ThrowIfFailed(md3dDevice->CreateGraphicsPipelineState(&smapPsoDesc, IID_PPV_ARGS(&mShadowPso)));

注意这里NumRenderTargets设置为0。

在进行正式绘制之前，我们要把shader所需要的信息从CPU中传递过来。绘制shadow map需要将场景中的所有物体变换到光源空间，因此我们需要光源空间下的相机矩阵和投影矩阵：

    XMStoreFloat4x4(&mShadowPassCB.View, XMMatrixTranspose(view));
    XMStoreFloat4x4(&mShadowPassCB.InvView, XMMatrixTranspose(invView));
    XMStoreFloat4x4(&mShadowPassCB.Proj, XMMatrixTranspose(proj));
    XMStoreFloat4x4(&mShadowPassCB.InvProj, XMMatrixTranspose(invProj));
    XMStoreFloat4x4(&mShadowPassCB.ViewProj, XMMatrixTranspose(viewProj));
    XMStoreFloat4x4(&mShadowPassCB.InvViewProj, XMMatrixTranspose(invViewProj));
    currPassCB->CopyData(1, mShadowPassCB);

绘制时，要使用shadow map对应的const buffer，pso和dsv：

    mCommandList->RSSetViewports(1, &mShadowMap->Viewport());
    mCommandList->RSSetScissorRects(1, &mShadowMap->ScissorRect());

    mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(mShadowMap->Resource(),
        D3D12_RESOURCE_STATE_GENERIC_READ, D3D12_RESOURCE_STATE_DEPTH_WRITE));

    mCommandList->ClearDepthStencilView(mShadowMap->Dsv(), 
        D3D12_CLEAR_FLAG_DEPTH | D3D12_CLEAR_FLAG_STENCIL, 1.0f, 0, 0, nullptr);
    mCommandList->OMSetRenderTargets(0, nullptr, false, &mShadowMap->Dsv());
    mCommandList->SetGraphicsRootConstantBufferView(1, passCBAddress);
    mCommandList->SetPipelineState(mShadowPso);

    DrawScene();

    mCommandList->ResourceBarrier(1, &CD3DX12_RESOURCE_BARRIER::Transition(mShadowMap->Resource(),
        D3D12_RESOURCE_STATE_DEPTH_WRITE, D3D12_RESOURCE_STATE_GENERIC_READ));

得到shadow map之后，下一阶段就是对其采样。采样阶段相对简单，只需将光源空间的变换矩阵传给shader，然后设置好shadow map对应的srv：

    XMStoreFloat4x4(&mMainPassCB.ShadowTransform, XMMatrixTranspose(shadowTransform));

    mCommandList->SetGraphicsRootDescriptorTable(3, mShadowSrv);

然后就可以在shader中对shadow map进行采样，将采样值与深度值进行比较得到pixel是否在阴影中。很多现代硬件中已经原生支持采样比较操作，通过SampleCmpLevelZero这个API即可。相应地，要在CPU层额外传入一个sampler：

const CD3DX12_STATIC_SAMPLER_DESC shadow(
        1, // shaderRegister
        D3D12_FILTER_COMPARISON_MIN_MAG_LINEAR_MIP_POINT, // filter
        D3D12_TEXTURE_ADDRESS_MODE_BORDER,  // addressU
        D3D12_TEXTURE_ADDRESS_MODE_BORDER,  // addressV
        D3D12_TEXTURE_ADDRESS_MODE_BORDER,  // addressW
        0.0f,                               // mipLODBias
        16,                                 // maxAnisotropy
        D3D12_COMPARISON_FUNC_LESS_EQUAL,
        D3D12_STATIC_BORDER_COLOR_OPAQUE_BLACK);

最后实现的效果如下：

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