We´ve searched in a OpenGL manual what gluPerspective() does, we also look something about glViewport():

Well, gluPerspective has 4 arguments:

fovy.- Specifies the field of view angle, in degrees, in the y direction.

aspect.- Specifies the aspect ratio that determines
the field of view in the x direction.
The aspect ratio is the ratio of x (width) to y (height).

zNear.- Specifies the distance from the viewer to the near clipping plane
(always positive).

zFar.- Specifies the distance from the viewer to the far clipping plane
(always positive).