Posted 16 January 2012 - 07:08 PM
If it's always a rectangle you could just transform your world coordinates to screen coordinates on the CPU, and use Clear() to clear that rectangle.
Otherwise you'll probably want to create a pixel shader that simply writes 1.0 out to the DEPTH semantic (it may need to write a colour too, but you can easily disable colour writes with D3DRS_COLORWRITEENABLE).
Depending on the result you're trying to achieve, you may be able to do the same thing with the stencil buffer, which doesn't need a special shader.