Texture readback showing all 0s for DirectX 11 - c++

Question

I am new to working with textures and DirectX and having issues reading back texture data from the GPU.
I am interested in reading back only a specific subset of my source texture. Also, I am trying to read it back at the least detailed miplevel (1x1 texture). Steps I follow:
Copy subregion of source texture into new texture
D3D11_TEXTURE2D_DESC desc = {0};
desc.Width = 1;
desc.Height = 1;
desc.MipLevels = 0;
desc.ArraySize = 1;
desc.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
desc.SampleDesc.Count = 1;
desc.SampleDesc.Quality = 0;
desc.Usage = D3D11_USAGE_DEFAULT;
desc.BindFlags = D3D11_BIND_SHADER_RESOURCE | D3D11_BIND_RENDER_TARGET;
desc.CPUAccessFlags = 0;
desc.MiscFlags = D3D11_RESOURCE_MISC_GENERATE_MIPS;
hr = pD3D11Device->CreateTexture2D(&desc, nullptr, &pSrcTexture);
D3D11_BOX srcRegion = {0};
srcRegion.left = 1000;
srcRegion.right = 1250;
srcRegion.top = 500;
srcRegion.bottom = 750;
srcRegion.front = 0;
srcRegion.back = 1;
pD3D11DeviceContext->CopySubresourceRegion(pSrcTexture, 0, 0, 0, 0, srcResource, 0, &srcRegion);
2. Create shader resource view and generate mipmaps for newly created texture
D3D11_SHADER_RESOURCE_VIEW_DESC srvDesc = {0};
srvDesc.Format = desc.Format;
srvDesc.ViewDimension = D3D11_SRV_DIMENSION_TEXTURE2D;
srvDesc.Texture2D.MipLevels = -1;
srvDesc.Texture2D.MostDetailedMip = 0;
ID3D11ShaderResourceView* pShaderResourceView = nullptr;
hr = pD3D11Device->CreateShaderResourceView(pSrcTexture, &srvDesc, &pShaderResourceView);
pD3D11DeviceContext->GenerateMips(pShaderResourceView);
3. Copy into staging texture to be read back by CPU
D3D11_TEXTURE2D_DESC desc2 = {0};
desc2.Width = 1;
desc2.Height = 1;
desc2.MipLevels = 1;
desc2.ArraySize = 1;
desc2.Format = DXGI_FORMAT_R8G8B8A8_UNORM;
desc2.SampleDesc.Count = 1;
desc2.SampleDesc.Quality = 0;
desc2.Usage = D3D11_USAGE_STAGING;
desc2.BindFlags = 0;
desc2.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
desc2.MiscFlags = 0;
ID3D11Texture2D* pStagingTexture = nullptr;
hr = pD3D11Device->CreateTexture2D(&desc2, nullptr, &pStagingTexture);
pD3D11DeviceContext->CopyResource(pStagingTexture, pSrcTexture);
4. Map the subresource to access the underlying data, unmapping when finished
D3D11_MAPPED_SUBRESOURCE mappedResource = {0};
hr = pD3D11DeviceContext->Map(pStagingTexture, 0, D3D11_MAP_READ, 0, &mappedResource);
FLOAT* pTexels = (FLOAT*)mappedResource.pData;
std::cout << pTextels[0] << pTextels[1] << pTextels[2] << pTextels[3] << std::endl; // all zeros here
pD3D11DeviceContext->Unmap(pStagingTexture, 0);
Please note that none of my hr results are failing. Why is my texture data showing as all zeros?
Any guidance on how to resolve?

Answer 1

CopyResource, CopySubresourceRegion, and GenerateMips do not return a HRESULT, but it may have failed in any of those functions. A good way to determine that is to enable the Direct3D Debug Device to look for debug output. See this blog post and Microsoft Docs.
I suspect the problem is that you when called GenerateMips it didn't do anything because you provided a 1x1 texture as the starting place so it doesn't have any mips. I also don't see how you set up srcResource, but you are trying to copy using CopySubresourceRegion from a 250x250 texture region to a 1x1 texture which is going to fail as well.
You should take a look at DirectXTK and the DDSTextureLoader / WICTextureLoader modules in particular which implement auto-mip generation, and ScreenGrab which does read-back.
One minor note: = {0}; was a way to zero-fill structs back in VS 2013 or earlier. With C++11 conformant compilers (VS 2015 or later), just use = {}; as that does the zero-fill.