updating directx constant buffer from stack or heap variable

Question

hi i am learning directx11 recently. and i have problem in using constant buffer.

so what i am doing is, i create the constant buffer for directional light and update the value, so that i can use this in my shader.

constant buffer structure in application side

struct SHADER_DIRECTIONAL_LIGHT {

        XMFLOAT4 ambient;
        XMFLOAT4 diffuse;
        XMFLOAT4 specular;
        XMFLOAT4 dir;
        XMFLOAT4 enabled;
        XMFLOAT4 intensity;

    };

class DirectionalLight
{
    private:
        //1. member variable
        SHADER_DIRECTIONAL_LIGHT m_data;
        //2. static variable
        //static SHADER_DIRECTIONAL_LIGHT m_data;
        //3. allocated variable
        //SHADER_DIRECTIONAL_LIGHT* m_data;
}

constant buffer structure in shader side

cbuffer DIRECTIONAL_LIGHT : register(b0)
{
    float4 d_Ambient;
    float4 d_Diffuse;
    float4 d_Specular;
    float4 d_Dir;
    float4 d_Enabled;
    float4 d_intensity;
};

how i update constant buffer

    //of course, edit `m_data` before using map

    D3D11_MAPPED_SUBRESOURCE mappedData;
    ZeroMemory(&mappedData, sizeof(D3D11_MAPPED_SUBRESOURCE));

    HRESULT hr = dContext->Map(m_cb, 0, D3D11_MAP_WRITE_DISCARD, 0, &mappedData);
    r_assert(hr);
    CopyMemory(mappedData.pData, &m_data, sizeof(SHADER_DIRECTIONAL_LIGHT));
    dContext->Unmap(m_cb, 0);

    dContext->PSSetConstantBuffers(SHADER_REG_CB_DIRECTIONAL_LIGHT, 1, &m_cb);

and the problem is that

when creating m_data like SHADER_DIRECTIONAL_LIGHT m_data; or static SHADER_DIRECTIONAL_LIGHT m_data; ,

it works fine and the value i updated into the constant buffer using map properly applied into shader side too.

But when i create m_data like SHADER_DIRECTIONAL_LIGHT* m_data, the value i update doesn't really work. the value in shader side is just uninitialized random value.

by the debug, i am just guessing the problem is from the different memory space of variable that i use to update the constant buffer. if i use variable in stack, i successfully update the constant bufferm, and when using variable from heap, it doesn't.

hope somebody to clarify what actually problem here. thanks

mateeeeeee mateeeeeee · Accepted Answer · 2020-11-29T08:58:18

You are passing address of a member pointer m_data with the call

CopyMemory(mappedData.pData, &m_data, sizeof(SHADER_DIRECTIONAL_LIGHT));

Since pointer already holds the address, pass it without address operator like this:

CopyMemory(mappedData.pData, m_data, sizeof(SHADER_DIRECTIONAL_LIGHT));

updating directx constant buffer from stack or heap variable

1 Answers