Direct3D 11: How to access cube map faces in memory on the CPU side (should ID3D11DeviceContext::Map work with subresources?)?

Question

I'm currently trying to read the faces of a cube map in Direct3D 11, but it's not working as I'm expecting: I expected to use ID3D11DeviceContext::Map() to access each face as a subresource (as in D3D11CalcSubresource(), or (MipSlice + (ArraySlice * MipLevels)), but this only works for the first face and fails for all others.

My questions are:

1. SHOULD this be working, or are cube map faces fundamentally different from the array slices used to compute subresources? It's very possible there's some other bug causing the subresources to not be copied over correctly to the CPU-readable (Map-able) staging texture, and other things could be going wrong too. I'd just like to know for sure it's even possible to access cube map faces as subresources before spending ages trying to debug this under faulty assumptions.
2. Whether this should work or not, is there any better way to read cube map faces on the CPU side? For instance, are cube map faces guaranteed to be contiguous in memory? If so, is it possible (and defined behavior) to use pointer math to read all faces using a pointer to the first face? Otherwise, are there any other methods better-suited to reading cube map memory on the CPU side?

I can't imagine Direct3D 11 is designed to make reading cube maps impossible without re-rendering all 6 faces into a new render target (stitched together), so I'm hoping someone knowledgeable about D3D11 will know how to do this. I've checked out the MSDN documentation, but it's not a lot of help, because neither the page on subresources nor the page on Map mention cube maps: http://msdn.microsoft.com/en-us/library/windows/desktop/ff476901%28v=vs.85%29.aspx http://msdn.microsoft.com/en-us/library/windows/desktop/ff476457%28v=vs.85%29.aspx

Thanks!

Answer 1

Cube maps are no different from 2D texture arrays; in fact, under the hood they are exactly the same. Using Map is allowed, and should look similar to the following:

//pD3DContext - Initialized and valid ID3D11DeviceContext*
//pCubeMap - Initialized and valid ID3D11Texture2D*
//uiFace - Valid index between 0 and 5 referring to the cube map face
//Texel - C++ type representing the underlying pixel format of the cube map

int i=D3D11CalcSubresource(0,uiFace,1);
D3D11_MAPPED_SUBRESOURCE mappedResource={0};
pD3DContext->Map(pCubeMap,i,D3D11_MAP_DISCARD,0,&mappedResource);
Texel* data=(Texel*)mappedResource.pData;

//Do something with data

One thing to note is that the cube map should be created with the D3D11_RESOURCE_MISC_TEXTURECUBE option set in D3D11_TEXTURE2D_DESC::MiscFlags, and the ID3D11ShaderResourceView should also specify the D3D11_SRV_DIMENSION_TEXTURECUBE option. However, you may need to access the resource as a 2D array of textures if you decide to render to it (i.e. create an ID3D11RenderTargetView of the cube map).

One thing to ensure is that the data you read in from a file is provided in the proper format. Cube maps should be a contiguous chunk of memory, where the end of one face is the beginning of the next. Additionally, any custom mipmaps will need to be taken into account, as these all come before the next face of the cube map. If your data has been loaded in this manner, there's not much else I can think of that would be incorrect.

This contains a bit of information about cube maps in Direct3D 11, and there are small bits an pieces scattered among the pages of documentation about textures in D3D11.