One Issue In OpenGL Depth Texture Fetching

Problem: The color of fetched depth texture is with very poor contrast.

Reason and Solution:
    In depth texture fetching pass, projection transformation parameters DO effect the result.
    In my approach, here are two situations according to two different Z-Nears for the perspective projection transformation function:
         gluPerspective(GLfloat FOV, GLfloat aspect, GLdouble zNear, GLdouble zFar);
    
    By following results, it can be inferred that z-near DOES take effect on result of the fetched depth texture.
   
    Usually zNear = 2.0f would be enough for distinguishing different colors(actually the gray color represents depth).

First comes the original teapot scene:

/**********************************************************************************************************************************************/
Then zNear = 1.0f:

/**********************************************************************************************************************************************/
Finally Comes zNear = 6.0f: