DirectX 12 Resource binding - c++

Question

How can I bind resources to different shader stages in D3D12?
I wrote two shaders, one vertexshader and one pixelshader:
Here is the Vertex shader:
//VertexShader.vs
float4 main(float3 posL : POSITION, uniform float4x4 gWVP) : SV_POSITION
{
return mul(float4(posL, 1.0f), gWVP);
}
Here is the Pixelshader:
//PixelShader.ps
float4 main(float4 PosH : SV_POSITION, uniform float4 Color) : SV_Target
{
return Color;
}
If I compile those two shaders with the D3DCompile function and reflect it with the D3DReflect and examine the BoundResorces member in the shader description they both have a constant buffer called $Params with contains the uniform variable respectively. The Problem is that both of those buffers are bound to slot 0. When binding resources I have to use the ID3D12RootSignature interface, which can bind resources to the resource slots. How can I bind the $Params buffer of the vertex shader only to the vertex shader and the $Params buffer of the Pixel shader only to the pixel shader?
Thanks in advance,
Philinator

Answer 1

With DX12, the performant solution here is to create a root signature that meets your application needs, and ideally declare it in your HLSL as well. You don't want to change root signatures often, so you definitely want to make one that works for a large swath of your shaders.
Remember DX12 is Direct3D without training wheels, magic, or shader patching so you just have to do it all explicitly yourself. Ease-of-use is a non-goal for Direct3D 12. There are plenty of good reason to stick with Direct3D 11 unless your application and developer resources merit the extra control Direct3D 12 affords you. With great power comes great responsibility.
I believe you can achieve what you want with something like:
CD3DX12_DESCRIPTOR_RANGE descRange[1];
descRange[0].Init(D3D12_DESCRIPTOR_RANGE_TYPE_CBV, 1, 0);
CD3DX12_ROOT_PARAMETER rootParameters[2];
rootParameters[0].InitAsDescriptorTable(
1, &descRange[0], D3D12_SHADER_VISIBILITY_VERTEX); // b0
rootParameters[1].InitAsDescriptorTable(
1, &descRange[0], D3D12_SHADER_VISIBILITY_PIXEL); // b0
// Create the root signature.
CD3DX12_ROOT_SIGNATURE_DESC rootSignatureDesc(_countof(rootParameters),
rootParameters, 0, nullptr,
D3D12_ROOT_SIGNATURE_FLAG_ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT);
ComPtr<ID3DBlob> signature;
ComPtr<ID3DBlob> error;
DX::ThrowIfFailed(D3D12SerializeRootSignature(&rootSignatureDesc,
D3D_ROOT_SIGNATURE_VERSION_1, &signature, &error));
DX::ThrowIfFailed(
device->CreateRootSignature(0, signature->GetBufferPointer(),
signature->GetBufferSize(),
IID_PPV_ARGS(m_rootSignature.ReleaseAndGetAddressOf())));