Clipping Quad ?

Hi guys,

I want to specify a rectangular portion of the screen to act as a clipping rect for 3D objects. I'm thinking that I can simply render a quad and set to 0 the z-values for the pixels in the quad.

I know how to render the quad of course but I don't know how to set the z-values. I'm using world coordinates for the quad.

Also, should I do this in a shader ? Please help.

Thanks.