前言

​ 本篇将展示如何使用常量缓冲区渲染一个移动的三角形,本篇基于绘制三角形篇展示新增步骤

流程图

shader file

//常量寄存器由tn指定，n为寄存器槽号
cbuffer SceneConstantBuffer : register(b0)
{
    float4 offset;	//用于实现移动的三角形
    float4 padding[15];
};

struct PSInput
{
    float4 position : SV_POSITION;
    float4 color : COLOR;
};

PSInput VSMain(float4 position : POSITION, float4 color : COLOR)
{
    PSInput result;

    result.position = position + offset;
    result.color = color;

    return result;
}

float4 PSMain(PSInput input) : SV_TARGET
{
    return input.color;
}

header file

struct SceneConstantBuffer
{
    XMFLOAT4 offset;
    float padding[60]; // 常量缓冲区的大小需要为最小分配空间256B的整数倍
};

static_assert((sizeof(SceneConstantBuffer) % 256) == 0, "Constant Buffer size must be 256-byte aligned");

ComPtr<ID3D12DescriptorHeap> m_cbvHeap;
ComPtr<ID3D12Resource> m_constantBuffer;
SceneConstantBuffer m_constantBufferData;
UINT8* m_pCbvDataBegin;

source file

初始化

加载管线

创建CBV描述符堆

// Describe and create a constant buffer view (CBV) descriptor heap.
// Flags indicate that this descriptor heap can be bound to the pipeline 
// and that descriptors contained in it can be referenced by a root table.
D3D12_DESCRIPTOR_HEAP_DESC cbvHeapDesc = {};
cbvHeapDesc.NumDescriptors = 1;
cbvHeapDesc.Flags = D3D12_DESCRIPTOR_HEAP_FLAG_SHADER_VISIBLE;
cbvHeapDesc.Type = D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV;
ThrowIfFailed(m_device->CreateDescriptorHeap(&cbvHeapDesc, IID_PPV_ARGS(&m_cbvHeap)));

加载资源

创建根签名

D3D12_FEATURE_DATA_ROOT_SIGNATURE featureData = {};

// This is the highest version the sample supports. If CheckFeatureSupport succeeds, the HighestVersion returned will not be greater than this.
featureData.HighestVersion = D3D_ROOT_SIGNATURE_VERSION_1_1;

if (FAILED(m_device->CheckFeatureSupport(D3D12_FEATURE_ROOT_SIGNATURE, &featureData, sizeof(featureData))))
{
    featureData.HighestVersion = D3D_ROOT_SIGNATURE_VERSION_1_0;
}

CD3DX12_DESCRIPTOR_RANGE1 ranges[1];
CD3DX12_ROOT_PARAMETER1 rootParameters[1];

ranges[0].Init(D3D12_DESCRIPTOR_RANGE_TYPE_CBV, 1, 0, 0, D3D12_DESCRIPTOR_RANGE_FLAG_DATA_STATIC);
rootParameters[0].InitAsDescriptorTable(1, &ranges[0], D3D12_SHADER_VISIBILITY_VERTEX);		//顶点着色器可以访问该根参数

// Allow input layout and deny uneccessary access to certain pipeline stages
D3D12_ROOT_SIGNATURE_FLAGS rootSignatureFlags =
    D3D12_ROOT_SIGNATURE_FLAG_ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT |
    D3D12_ROOT_SIGNATURE_FLAG_DENY_HULL_SHADER_ROOT_ACCESS |
    D3D12_ROOT_SIGNATURE_FLAG_DENY_DOMAIN_SHADER_ROOT_ACCESS |
    D3D12_ROOT_SIGNATURE_FLAG_DENY_GEOMETRY_SHADER_ROOT_ACCESS |
    D3D12_ROOT_SIGNATURE_FLAG_DENY_PIXEL_SHADER_ROOT_ACCESS;

CD3DX12_VERSIONED_ROOT_SIGNATURE_DESC rootSignatureDesc;
rootSignatureDesc.Init_1_1(_countof(rootParameters), rootParameters, 0, nullptr, rootSignatureFlags);

ComPtr<ID3DBlob> signature;
ComPtr<ID3DBlob> error;
ThrowIfFailed(D3DX12SerializeVersionedRootSignature(&rootSignatureDesc, featureData.HighestVersion, &signature, &error));
ThrowIfFailed(m_device->CreateRootSignature(0, signature->GetBufferPointer(), signature->GetBufferSize(), IID_PPV_ARGS(&m_rootSignature)));

创建常量缓冲区

const UINT constantBufferSize = sizeof(SceneConstantBuffer);    // CB size is required to be 256-byte aligned.

ThrowIfFailed(m_device->CreateCommittedResource(
    &CD3DX12_HEAP_PROPERTIES(D3D12_HEAP_TYPE_UPLOAD),
    D3D12_HEAP_FLAG_NONE,
    &CD3DX12_RESOURCE_DESC::Buffer(constantBufferSize),
    D3D12_RESOURCE_STATE_GENERIC_READ,
    nullptr,
    IID_PPV_ARGS(&m_constantBuffer)));

// Describe and create a constant buffer view.
D3D12_CONSTANT_BUFFER_VIEW_DESC cbvDesc = {};
cbvDesc.BufferLocation = m_constantBuffer->GetGPUVirtualAddress();
cbvDesc.SizeInBytes = constantBufferSize;
m_device->CreateConstantBufferView(&cbvDesc, m_cbvHeap->GetCPUDescriptorHandleForHeapStart());

//无需在此处进行Unmap()，因为后续在更新时会更改常量缓冲区的值
CD3DX12_RANGE readRange(0, 0);        // We do not intend to read from this resource on the CPU.
ThrowIfFailed(m_constantBuffer->Map(0, &readRange, reinterpret_cast<void**>(&m_pCbvDataBegin)));
memcpy(m_pCbvDataBegin, &m_constantBufferData, sizeof(m_constantBufferData));

更新

改变顶点坐标

const float translationSpeed = 0.005f;
const float offsetBounds = 1.25f;

m_constantBufferData.offset.x += translationSpeed;
if (m_constantBufferData.offset.x > offsetBounds)
{
    m_constantBufferData.offset.x = -offsetBounds;
}
memcpy(m_pCbvDataBegin, &m_constantBufferData, sizeof(m_constantBufferData));

渲染

设置描述符堆

ID3D12DescriptorHeap* ppHeaps[] = { m_cbvHeap.Get() };
m_commandList->SetDescriptorHeaps(_countof(ppHeaps), ppHeaps);

设置根描述符表

m_commandList->SetGraphicsRootDescriptorTable(0, m_cbvHeap->GetGPUDescriptorHandleForHeapStart());

reference

https://github.com/microsoft/DirectX-Graphics-Samples