why can't get pixel depth from proE model window

I am using pro/Toolkit to develope some code to read pixel depth of proE model from proE view window with openGL function glReadPixels(), but all I got was 1. Actually it should be different value for different pixel of the displaying model. The codes are as following,

const int width=1024;
const int height=768;
float z[width*height];

Any comment is welcome and appreciated.