Not even gluOrtho2D is 2D only, it work’s perfectly well with 3D too, since it has a depth range of [-1, 1].

2D is just a special case of 3D (usually where you have everything on the same Z-plane), which means you can do 2D in both orthographic and perspective projection. And you can do 3D on orthographic projection too.

That said, it makes no sense to call orthographic projection gl2D, since you can do 3D aswell. And calling glFrustum/gluPerspective for gl3D makes no sense either, since it’s not clear what kind of 3D it is, perspective or orthographic?

It would sure make it easier for a handfull or newbies to understand what each function can be used for (note, I’m not saying it will be easier to actually USE them). But for the rest, who actually know how a frustum works, what orthographic projection really means, how to visualize it, it’s only confusing.

Your suggestion is only to help the handfull of newbies who not yet understand the basics of 3D graphics. I suggest those persons would take the time to understand what perspective and orthographic projection really is, and the current functions will start making sense.