gluUnproject in zooming

I’m doing a 2D paint program. I want to add a zoom feature. I think I must use glOrtho and glViewport. I read an example on the net that did what I want to do, but I can’t get it to work. It uses gluUnproject. What does this function do? I’m quite new to OpenGL so please explain in simple terms.

Thankful for help!


u can get the real cordinates of the window location through this function