How to correctly pass multiple light structures to shader stage?

Question

I'm having some problems when passing multiple light structures to a shader stage. I'm using an array of point lights that is updated each frame.

This is my code for creating the buffer

D3D11_BUFFER_DESC lbDesc;
lbDesc.ByteWidth            = sizeof( PointLightData ) * NUM_LIGHTS;
lbDesc.Usage                = D3D11_USAGE_DYNAMIC;
lbDesc.BindFlags            = D3D11_BIND_CONSTANT_BUFFER;
lbDesc.CPUAccessFlags       = D3D11_CPU_ACCESS_WRITE;
lbDesc.MiscFlags            = 0;
lbDesc.StructureByteStride  = 0;

hr = mDevice->CreateBuffer( &lbDesc, nullptr, &mLightBuffer );

And this is how I update it

D3D11_MAPPED_SUBRESOURCE mappedResource;
hr = mDeviceContext->Map( mLightBuffer, 0, D3D11_MAP_WRITE_DISCARD, 0, &mappedResource );

if( SUCCEEDED( hr ) )
{
    memcpy( mappedResource.pData, &mPointLightData[0], sizeof(PointLightData) * NUM_LIGHTS );
    mDeviceContext->Unmap( mLightBuffer, 0 ); 
}

mPointLightData is declared like this

mPointLightData = new PointLightData[NUM_LIGHTS];

In my shader I declare the light

struct PointLight
{
    float4  positionAndRadius;
    float4  ambient;
    float4  diffuse;
    float4  specular;
    float3  attenuation;
};

cbuffer CB_LIGHT : register(b1)
{
    PointLight  pointLight[2];
};

I can only access the first light object in the buffer successfully. Rendering the second light results in a pitch black scene.

However, if I change the starting address at the mapping stage from &mPointLightData[0] to &mPointLightData[1] the second light works but obviously not the first.

So my guess is that I'm doing simething wrong at either the mapping stage or at the creation of the buffer..

Any suggestions? Thank you!

Answer 1

There are additional alignment and layout rules for constant buffers. The float3 probably needs padding on the CPU side to get the right stride.

Roughly paraphrased, things need to be on 16-byte boundaries.