why a 3D graphics engine

Ok I read the back of Quake 3 or something and it says ACTION PACKED and all this hoobla and NEW GRAPHICS ENGINE THAT MAKES GAMEPLAY…
Well alright you have opengl library and glut and all this stuff (i’m very new to oepngl by the way but i’m posting this in the advanced forum cause it seems advanced to me ) but what exactly does a graphics engine do, I mean how can the “graphics engine” from quake be so different than the “graphics engine” from half-life if they both are made in Opengl? What i’m asking is-what does a “graphics engine” consist of and what is it supposed to do?

an API, which opengl is, is simply a set functions that gives a layer between hardware and your program. For example, opengl has functions to help you place an object in a space and draw and set properties that will define its color characteristic

The graphic engine on the other side takes care of choosing what is going to be drawn with what characteristic and where, etc.

So once the engine has choose what to draw with what characteristic and where, it then calls opengl functions to display what it wanted.

This is a very simplistic overview, I hope it gave you a better understanding, but then if you don’t know much about graphic probably not. The best thing you can do is read, read and read some more. Grab graphics books and get an understanding of computer graphics. After that, the distinction will absolutly clear.