在DirectX 12中使用SSAO

SSAO是常用的实现环境光遮蔽，提高物体真实性的一种技术，它可以分为生成AO贴图和使用AO贴图两个阶段。在生成阶段，我们需要一张view space下的normal texture，和一张depth texture作为输入。depth texture是已有的，这里意味着需要新增normal texture资源：

    D3D12_RESOURCE_DESC texDesc;
    ZeroMemory(&texDesc, sizeof(D3D12_RESOURCE_DESC));
    texDesc.Dimension = D3D12_RESOURCE_DIMENSION_TEXTURE2D;
    texDesc.Alignment = 0;
    texDesc.Width = mRenderTargetWidth;
    texDesc.Height = mRenderTargetHeight;
    texDesc.DepthOrArraySize = 1;
    texDesc.MipLevels = 1;
    texDesc.Format = DXGI_FORMAT_R16G16B16A16_FLOAT;
    texDesc.SampleDesc.Count = 1;
    texDesc.SampleDesc.Quality = 0;
    texDesc.Layout = D3D12_TEXTURE_LAYOUT_UNKNOWN;
    texDesc.Flags = D3D12_RESOURCE_FLAG_ALLOW_RENDER_TARGET;


    float normalClearColor[] = { 0.0f, 0.0f, 1.0f, 0.0f };
    CD3DX12_CLEAR_VALUE optClear(DXGI_FORMAT_R16G16B16A16_FLOAT, normalClearColor);
    ThrowIfFailed(md3dDevice->CreateCommittedResource(
        &CD3DX12_HEAP_PROPERTIES(D3D12_HEAP_TYPE_DEFAULT),
        D3D12_HEAP_FLAG_NONE,
        &texDesc,
		D3D12_RESOURCE_STATE_GENERIC_READ,
        &optClear,
        IID_PPV_ARGS(&mNormalMap)));

这个normal texture有两个用处，首先它要作为render target，保存view space下的normal；其次它要作为绘制AO贴图的输入，因此它需要两个绑定的view：

    D3D12_RENDER_TARGET_VIEW_DESC rtvDesc = {};
    rtvDesc.ViewDimension = D3D12_RTV_DIMENSION_TEXTURE2D;
    rtvDesc.Format = DXGI_FORMAT_R16G16B16A16_FLOAT;
    rtvDesc.Texture2D.MipSlice = 0;
    rtvDesc.Texture2D.PlaneSlice = 0;
    md3dDevice->CreateRenderTargetView(mNormalMap.Get(), &rtvDesc, mhNormalMapCpuRtv);


    D3D12_SHADER_RESOURCE_VIEW_DESC srvDesc = {};
    srvDesc.Shader4ComponentMapping = D3D12_DEFAULT_SHADER_4_COMPONENT_MAPPING;
    srvDesc.ViewDimension = D3D12_SRV_DIMENSION_TEXTURE2D;
    srvDesc.Format = DXGI_FORMAT_R16G16B16A16_FLOAT;
    srvDesc.Texture2D.MostDetailedMip = 0;
    srvDesc.Texture2D.MipLevels = 1;
    md3dDevice->CreateShaderResourceView(mNormalMap.Get(), &srvDesc, mhNormalMapCpuSrv);

在绘制normal之前，还需要准备相应的pipeline state object：

    D3D12_GRAPHICS_PIPELINE_STATE_DESC drawNormalsPsoDesc = basePsoDesc;
    drawNormalsPsoDesc.VS =
    {
        reinterpret_cast<BYTE*>(vs->GetBufferPointer()),
        vs->GetBufferSize()
    };
    drawNormalsPsoDesc.PS =
    {
        reinterpret_cast<BYTE*>(ps->GetBufferPointer()),
        ps->GetBufferSize()
    };
    drawNormalsPsoDesc.RTVFormats[0] = mNormalMapFormat;
    drawNormalsPsoDesc.SampleDesc.Count = 1;
    drawNormalsPsoDesc.SampleDesc.Quality = 0;
    drawNormalsPsoDesc.DSVFormat = mDepthStencilFormat;
    ThrowIfFailed(md3dDevice->CreateGraphicsPipelineState(&drawNormalsPsoDesc, IID_PPV_ARGS(&mDrawNormalsPso)));

有了normal texture之后，就可以着手绘制AO贴图了。类似地，我们需要新增资源，同时该资源需要作为AO pass的输出和正常场景pass的输入，因此也需要两个绑定的view：

    texDesc.Width = mRenderTargetWidth;
    texDesc.Height = mRenderTargetHeight;
    texDesc.Format = DXGI_FORMAT_R16_UNORM;

    float ambientClearColor[] = { 1.0f, 1.0f, 1.0f, 1.0f };
    optClear = CD3DX12_CLEAR_VALUE(DXGI_FORMAT_R16_UNORM, ambientClearColor);

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

    md3dDevice->CreateShaderResourceView(mAmbientMap.Get(), &srvDesc, mhAmbientMapCpuSrv);
    md3dDevice->CreateRenderTargetView(mAmbientMap.Get(), &rtvDesc, mhAmbientMapCpuRtv);

类似地，在绘制AO之前，还需要准备相应的pipeline state object。因为上一步骤已经有了depth buffer了，所以这里不需要进行深度写入：

    D3D12_GRAPHICS_PIPELINE_STATE_DESC ssaoPsoDesc = basePsoDesc;
    ssaoPsoDesc.InputLayout = { nullptr, 0 };
    ssaoPsoDesc.pRootSignature = mSsaoRootSignature.Get();
    ssaoPsoDesc.VS =
    {
        reinterpret_cast<BYTE*>(vs->GetBufferPointer()),
        vs->GetBufferSize()
    };
    ssaoPsoDesc.PS =
    {
        reinterpret_cast<BYTE*>(ps->GetBufferPointer()),
        ps->GetBufferSize()
    };

    ssaoPsoDesc.DepthStencilState.DepthEnable = false;
    ssaoPsoDesc.DepthStencilState.DepthWriteMask = D3D12_DEPTH_WRITE_MASK_ZERO;
    ssaoPsoDesc.RTVFormats[0] = ambientMapFormat;
    ssaoPsoDesc.SampleDesc.Count = 1;
    ssaoPsoDesc.SampleDesc.Quality = 0;
    ssaoPsoDesc.DSVFormat = DXGI_FORMAT_UNKNOWN;
    ThrowIfFailed(md3dDevice->CreateGraphicsPipelineState(&ssaoPsoDesc, IID_PPV_ARGS(&mSsaoPso)));

SSAO的原理主要是根据采样点的坐标，随机采样若干个一定距离范围内的点，然后逐一判断每个点和采样点的遮挡关系，最后计算出一个遮挡的权重值。在shader代码中可以使用类似如下的方式判断两个点的遮挡关系：

float4 PS(VertexOut pin) : SV_Target
{
    ...
    // Test whether r occludes p.
    float distZ = p.z - r.z;
    float dp = max(dot(n, normalize(r - p)), 0.0f);
    float occlusion = dp*OcclusionFunction(distZ);
    ...
}

float OcclusionFunction(float distZ)
{
	float occlusion = 0.0f;
	if(distZ > gSurfaceEpsilon)
	{
		float fadeLength = gOcclusionFadeEnd - gOcclusionFadeStart;
		occlusion = saturate( (gOcclusionFadeEnd-distZ)/fadeLength );
	}
	
	return occlusion;
}

由于它实际上是一个screen space的pass，因此绘制时并不需要传入vertex buffer和index buffer：

    cmdList->IASetVertexBuffers(0, 0, nullptr);
    cmdList->IASetIndexBuffer(nullptr);
    cmdList->IASetPrimitiveTopology(D3D_PRIMITIVE_TOPOLOGY_TRIANGLELIST);
    cmdList->DrawInstanced(6, 1, 0, 0);

得到AO贴图之后，就可以进入正式的绘制物体阶段，根据pixel的坐标去采样AO贴图中的值即可。

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