How do I copy the depth buffer to CPU memory in Direct3D 9?

Question

I have code in OpenGL that uses glReadPixels to copy the depth buffer to a CPU memory buffer:

glReadPixels(0, 0, w, h, GL_DEPTH_COMPONENT, GL_FLOAT, dbuf);

How do I achieve the same in Direct3D?

I have looked at a similar question which gives a solution for copying the color buffer. I've tried to write similar code to copy the depth buffer:

IDirect3DSurface9* d3dSurface;
d3dDevice->GetDepthStencilSurface(&d3dSurface);

D3DSURFACE_DESC d3dSurfaceDesc;
d3dSurface->GetDesc(&d3dSurfaceDesc);

IDirect3DSurface9* d3dOffSurface;
  d3dDevice->CreateOffscreenPlainSurface(
  d3dSurfaceDesc.Width,
  d3dSurfaceDesc.Height,
  D3DFMT_D32F_LOCKABLE,
  D3DPOOL_SCRATCH,
  &d3dOffSurface,
  NULL
);

// FAILS: D3DERR_INVALIDCALL
D3DXLoadSurfaceFromSurface(
  d3dOffSurface,
  NULL,
  NULL,
  d3dSurface,
  NULL,
  NULL,
  D3DX_FILTER_NONE,
  0
);

// Copy from offscreen surface to CPU memory ...

The code fails on the call to D3DXLoadSurfaceFromSurface. It returns the error value D3DERR_INVALIDCALL.

So, is something wrong with my code? How can I copy the depth buffer contents?

Answer 1

Per the documentation, you can acquire a reference to the depth-stencil surface for the device (if one exists for that device) by calling GetDepthStencilSurface. If the method returns successfully, you can lock the bits of the resulting surface using the LockRect method of the surface interface.

This gives you a locked rectangle with the bits of the surface accessible via the pBits member. You will need to use the Pitch as well as surface format information obtained from GetDesc to interpret the bits correctly.

I don't know why you are trying to use the surface-copying routines as you are, but I don't believe they are necessary.

Answer 2

For the sake of completeness, the copy is necessary because in general you cannot lock the depth buffer. As far as I know you need your depth buffer to use a lockable format to be able to copy the contained data directly into your own containers. D24S8 for example is a format which prevents locking the depth buffer. (I am not allowed to add comments, so as an answer ...)