DirectX 11: how to define input layout when using more than one vertex buffer?

Question

Using Direct3D 11, I would like to be able to use separate vertex buffers for positions and color. I want do do that when positions rarely change at runtime, whereas colors may change quite often. The subject is not new but I could not find a comprehensive answer to correctlty handle many vertex buffers/shaders. If I'm quite confident on how to create and set the vertex buffers, I'm not sure how to define the layout. To experiment, I started from a MSDN tutorial that just draw a single triangle with just one vertex shader. I splitted the shader into 2 parts, position and color.

Hereafter are the main lines of my HLSL and C++ code.

// ****** Shaders file *****

struct VS_OUTPUT
{
    float4 Pos : SV_POSITION;
    float4 Color : COLOR0;
};

// Position shader
VS_OUTPUT VPS( float4 Pos : POSITION )
{
    VS_OUTPUT output = (VS_OUTPUT)0;
    output.Pos = Pos;
    return output;
}

// Color shader
VS_OUTPUT VCS( float4 Color : COLOR )
{
    VS_OUTPUT output = (VS_OUTPUT)0;
    output.Color = Color;
    return output;
}

// Pixel shader
float4 PS( VS_OUTPUT input ) : SV_Target
{
    return input.Color;
}

// ********** C++ file abstract **********

// Position shader
ID3DBlob* pVPSBlob
ID3D11VertexShader* g_pVertexShader

// Color shader
ID3DBlob* pVCSBlob
ID3D11VertexShader* g_pVertexColorShader

// Pixel shader
ID3DBlob* pPSBlob
ID3D11InputLayout* g_pVertexLayout

// Vertex buffers
ID3D11Buffer* g_pVertexBuffer = nullptr;
ID3D11Buffer* g_pVertexColorBuffer = nullptr;

// I compile and create the shaders with:
D3DCompileFromFile(), CreateVertexShader()



// I define the input vertex position layout in slot 0
D3D11_INPUT_ELEMENT_DESC positionLayout[] =
{
    { "POSITION", 0, DXGI_FORMAT_R32G32B32_FLOAT, 0, 0, D3D11_INPUT_PER_VERTEX_DATA, 0 },
};
UINT numElements = ARRAYSIZE( positionLayout );

// Create the input vertex layout
hr = g_pd3dDevice->CreateInputLayout( positionLayout, numElements, pVPSBlob->GetBufferPointer(),
                                      pVPSBlob->GetBufferSize(), &g_pVertexLayout );

// Set the input vertex layout
g_pImmediateContext->IASetInputLayout( g_pVertexLayout );


 // I define the input color layout in slot 1
D3D11_INPUT_ELEMENT_DESC colorsLayout[] =
{
    { "COLOR", 0, DXGI_FORMAT_R32G32B32A32_FLOAT, 1, 0, D3D11_INPUT_PER_VERTEX_DATA, 0 },
};
numElements = ARRAYSIZE( colorsLayout );

// Create the input colors layout
hr = g_pd3dDevice->CreateInputLayout( colorsLayout, numElements, pVCSBlob->GetBufferPointer(),
                                      pVCSBlob->GetBufferSize(), &g_pVertexColorLayout );

// Set the input color layout
g_pImmediateContext->IASetInputLayout( g_pVertexColorLayout );

// I set the 3 shaders:
g_pImmediateContext->VSSetShader( g_pVertexShader, nullptr, 0 );
g_pImmediateContext->VSSetShader( g_pVertexColorShader, nullptr, 0 );
g_pImmediateContext->PSSetShader( g_pPixelShader, nullptr, 0 );


// I create 2 vertex buffers for positions and colors and set them

// Set the positions & colors vertex buffers
ID3D11Buffer *vertexBuffers[] = { g_pVertexBuffer, g_pVertexColorBuffer };
UINT strides[] = { sizeof( SimpleVertex ), sizeof( ColorVertex ) };
UINT offsets[] = { 0, 0};
g_pImmediateContext->IASetVertexBuffers( 0, 2, vertexBuffers, strides, offsets );

I also tried to define the layout element into a single array, in two different slots, but it didn't work either. In that case, what shader should I use?

All I could get was an empty screen, and I'm getting stuck... Thanks in advance if someone could help.

Answer 1

You can only have one input layout and one vertex shader set on the device context at one time. Each time you call IASetInputLayout and VSSetShader the previous layout/shader is replaced with the new one. Merge your two input layout structures into one structure, and your HLSL vertex shaders into one shader. The InputSlot field member of D3D11_INPUT_ELEMENT_DESC will determine which vertex buffer is used for that element.

Something like:

VS_OUTPUT VS( float4 Pos : POSITION, float4 Color : COLOR)
{
    VS_OUTPUT output = (VS_OUTPUT)0;
    output.Pos = Pos;
    output.Color = Color;
    return output;
}

D3D11_INPUT_ELEMENT_DESC positionLayout[] =
{
    { "POSITION", 0, DXGI_FORMAT_R32G32B32_FLOAT, 0, 0, D3D11_INPUT_PER_VERTEX_DATA, 0 },
    { "COLOR", 0, DXGI_FORMAT_R32G32B32A32_FLOAT, 1, 0, D3D11_INPUT_PER_VERTEX_DATA, 0 },
};