from feedback buffer to gluUnProject

from feedback buffer i got the xMax, xMin, yMax
yMin,zMax, zMin values
I want to use gluUnproject to turn them into 3D coordinate, but i failed!the result is not right!

i use the glutLookAt function in my program!
is there any thing about it?

that gluUnproject function will ham-string you if youre not careful. mainly, watch the screen-z (non-linear) and viewport-y (reversed):

GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);
    screen_x,              // screen x  
    viewport[3]-1-screen_y,// flip screen y 
    screen_z,              // between 0.0 and 1.0
    dModelMatrix,          // model matrix
    dProjMatrix,           // projection matrix
    viewport,              // viewport
    &ObjectX,              // world x
    &ObjectY,              // world y
    &ObjectZ               // world z