Passing D3DFMT_UNKNOWN into IDirect3DDevice9::CreateTexture()

I'm kind of wondering about this, if you create a texture in memory in DirectX with the CreateTexture function:

HRESULT CreateTexture(
  UINT Width,
  UINT Height,
  UINT Levels,
  DWORD Usage,
  D3DFORMAT Format,
  D3DPOOL Pool,
  IDirect3DTexture9** ppTexture,
  HANDLE* pSharedHandle
);

...and pass in D3DFMT_UNKNOWN format what is supposed to happen exactly? If I try to get the surface of the first or second level will it cause an error? Can it fail? Will the graphics device just choose a random format of its choosing? Could this cause problems between different graphics card models/brands?

Solution

I just tried it out and it does not fail, mostly

When Usage is set to D3DUSAGE_RENDERTARGET or D3DUSAGE_DYNAMIC, it consistently came out as D3DFMT_A8R8G8B8, no matter what I did to the back buffer format or other settings. I don't know if that has to do with my graphics card or not. My guess is that specifying unknown means, "pick for me", and that the 32-bit format is easiest for my card.

When the usage was D3DUSAGE_DEPTHSTENCIL, it failed consistently.

So my best conclusion is that specifying D3DFMT_UNKNOWN as the format gives DirectX the choice of what it should be. Or perhaps it always just defaults to D3DFMT_A8R8G8B.

Sadly, I can't confirm any of this in any documentation anywhere. :|