Problem: I need to retrieve a heightmap value, given a texel coordinate during runtime.

I'm trying to avoid having another copy of the texture on the CPU, as it's already sitting in the GPU and it's rather large.

So, what I've been trying to do is get a colour value from the pixel shader in D3DFMT_R32F format and then slap it into a 1x1 texture-surface in sysmemory.

Result so far: the returned height is always 1.0 and PIX claims the surface is empty.

Relevant code:

Vert/Pix shader (pastebin.com)
Initialization code (pastebin.com)
Render to surface code (pastebin.com)
Retrieval code (pastebin.com)

If anyone can provide a hint as to where I might have gone wrong, or maybe have a better idea of how I can solve my problem, or gotchas' I might not be conscious of, I would sure appreciate the help!

EDIT: This is Direct3D 9 in Windows 7, ATI HD4850

[Edited by - PSS on January 5, 2011 9:12:30 PM]

I had originally loaded my texture with D3DPOOL_DEFAULT, though this hasn't been as fruitful an effort as I had hoped. So I took your advice, and I'm simply locking the texture to extract the texels I need.

Thank you very much for the assistance MJP, it's appreciated.