How can I read the smallest mipmap back to the CPU after calling generateMips?

Question

I'm implementing an adaptive tonemapper based on the standard "scale down the luminance map to a 1x1 texture" approach.

This is my code following the rendering of the luminance map:

 //Average the luminance map by creating a 1x1 mipmap.
    m_pImmediateContext->GenerateMips(m_lumShaderResourceView);


    m_pImmediateContext->CopyResource(m_cpuLumTex, m_lumTexture);

    float* lumMap;
    D3D11_MAPPED_SUBRESOURCE texture;
    m_pImmediateContext->Map(m_cpuLumTex, 0, D3D11_MAP_READ, 0, &texture);
    lumMap = (float*) texture.pData;
    m_avgLuminance = lumMap[0]; // Should work? ^^;
    m_pImmediateContext->Unmap(m_cpuLumTex, 0);

    float lastLum = m_timedAvgLuminance;
    float currentLum = exp(m_avgLuminance);
    float adaptedLum = lastLum + (currentLum - lastLum) * (1 - exp(-.1 * 0.5));

    m_timedAvgLuminance = adaptedLum;

I'd like to read back the calculated average luminance to the CPU for reasons, but what I'm finding is that when I set m_avgLuminance to the mapped-back texture, it's grabbing the pixel at 0,0. If the map were a 1x1 texture, this wouldn't be an issue, but it seems I'm not getting the 1x1 texture, but instead the full-sized luminance texture (screen-sized), so what I wind up with is an image tonemapped by the 0,0 pixel instead of the overall average luminance of the scene.

What am I doing wrong here? How can I read the 1x1 generated mip map texture instead of the full-sized texture?

Answer 1

You need to map the sub-resource that corresponds to the mipmap level you want.

https://msdn.microsoft.com/en-us/library/windows/desktop/ff476901%28v=vs.85%29.aspx

https://msdn.microsoft.com/en-us/library/windows/desktop/ff476457%28v=vs.85%29.aspx

m_pImmediateContext->Map(m_cpuLumTex, (mipmap number here), D3D11_MAP_READ, 0, &texture);

if your texture is 256x256 that'd be resource 7.

(256 = 1 << 8, substract 1 for the last 1x1 mipmap => #7)