Why does the Constant Buffer update partially in Direct11 - c++

Question

Working my way through the "Introduction to 3D Game Programming with Directx 11". I am reworking the samples so as to not use the Effects Framework, so far all good.
However, I have come up with a problem that one of the constant buffers only partially updates.
CPU side CB struct:
struct CBPerFrame
{
DirectionalLight DirLight[3];
DirectX::XMFLOAT3 EyePositionW;
DirectX::XMFLOAT4 FogColour;
float FogStart;
float FogRange;
int LightNumber;
double Padding;
};
Where I update the CB prior to any drawing.
CBPerFrame cbPerFrame { };
cbPerFrame.DirLight[ 0 ] = mDirectionalLights[ 0 ];
cbPerFrame.DirLight[ 1 ] = mDirectionalLights[ 1 ];
cbPerFrame.DirLight[ 2 ] = mDirectionalLights[ 2 ];
cbPerFrame.EyePositionW = mEyePosW;
cbPerFrame.FogColour = XMFLOAT4( Colors::Black );
cbPerFrame.FogRange = 1.0F;
cbPerFrame.FogStart = 0.0F;
cbPerFrame.LightNumber = mLightCount;
cbPerFrame.Padding = 0.0;
mD3DImmediateContext->UpdateSubresource( mCBPerFrame.Get( ), 0, nullptr, &cbPerFrame, 0, 0 );
Pixel Shader:
cbuffer CBPerFrame : register( b0 )
{
DirectionalLight gDirectionalLights[ 3 ];
float3 gEyePosW;
float4 gFogColor;
float gFogStart;
float gFogRange;
int gLightCount;
double gPadding;
}
cbuffer CBPerObject : register( b1 )
{
matrix gWorld;
matrix gWorldInverseTranspose;
matrix gWorldViewProjection;
float4x4 gTextureTransform;
Material gMaterial;
}
float4 main( VertexOut input ) : SV_TARGET
{
// Interpolating normal can unnormalize it, so normalize it.
input.NormalW = normalize( input.NormalW );
// The toEye vector is used in lighting.
float3 toEye = normalize( gEyePosW - input.PositionW );
// Cache the distance to the eye from this surface point.
float distToEye = length( toEye );
// Normalize.
toEye /= distToEye;
//
// Lighting.
//
// Start with a sum of zero.
float4 ambient = float4( 0.0F, 0.0F, 0.0F, 0.0F );
float4 diffuse = float4( 0.0F, 0.0F, 0.0F, 0.0F );
float4 spec = float4( 0.0F, 0.0F, 0.0F, 0.0F );
// Sum the light contribution from each light source.
/* [unroll]*/
for ( int i = 0; i < gLightCount; i++ )
{
float4 A, D, S;
ComputeDirectionalLight( gMaterial, gDirectionalLights[ i ], input.NormalW, toEye, A, D, S );
ambient += A;
diffuse += D;
spec += S;
}
float4 litColour = ambient + diffuse + spec;
// Common to take alpha from diffuse material.
litColour.a = gMaterial.Diffuse.a;
return litColour;
}
gLightCount is always set to 0, even though it should be set to 2 at the start of the application. If I change the condition of the loop to a hardcoded 1/2/3 the shader works as it should.
I realise that there are extra variables in the CB, but the sample code has this and I believe it is used in further examples.
I think the issue is to do with how the CBPerFrame struct is padded, so it isn't been copied over to the GPU correctly. Any thoughts?
Thanks for any help.

Answer 1

It seems to be an issue with packing. According to Packing Rules for Constant Variables data should be packed at 4-byte boundaries but also so data blocks won't cross 16-byte boundary. In this case there will be definitely a padding after EyePositionW:
struct CBPerFrame
{
DirectionalLight DirLight[3];
DirectX::XMFLOAT3 EyePositionW;
float padding1;
DirectX::XMFLOAT4 FogColour;
Also i'm not sure why is there double gPadding; at the end. It should probably be another int.