unexpected depth value returned by gluUnProject

From :
>>>>
gluUnProject() does the opposite. It takes an XYZ window coordinate and returns the back-transformed XYZ object coordinate equivalent.

The concept of window space Z is often confusing. It's the depth buffer value expressed as a GLdouble in the range 0.0 to 1.0. Assuming a default glDepthRange(), a window coordinate with a Z value of 0.0 corresponds to an eye coordinate located on the zNear clipping plane. Similarly, a window space Z value of 1.0 corresponds to an eye space coordinate located on the zFar plane. You can obtain any window space Z value by reading the depth buffer with glReadPixels().
<<<<

So, basically depending upon the screen [z] value, you'll get back different world [z] values.  If you have a particular x,y on the screen that you want to get the world coordinates for the pixel, you need to do a readpixels and get that pixel's actual z-buffer value, and pass that into UnProject.

That help?

-d

Sorry, clicked submit and didn't mean to...

That was from http://www.opengl.org/resources/faq/technical/glu.htm 

Also look at http://www.gamedev.net/community/forums/topic.asp?topic_id=268798 for another discussion of the topic.

-d

I had found the description about how window Z is confusing and added a glReadPixels call to the application to get the depth value. The depth is coming back zero which causes the UnProject to generate a -1 for Z (if I understand the above). So does this mean that the Z value will always come back either zNear or zFar? Does the application need to use the depth value to scale between zNear and zFar?