Question

Let’s say that I have a perspective view in OpenGL and choose a point at a given depth. Let’s say that it’s at z=-10. How do I know which actual x- and y-coordinates this point have? It is easy when having an orth view because it then is the same for every depth. but for the perspective one: How do I find these x-y-values?

Answer 1

The coordinates you supply are basically “world” coordinates — i.e., where things exist in the virtual world you build. In other words, the coordinates you work with are always orthogonal.

Just for example, if I was going to do a 3D model of a house, I could set it up so I was working in actual feet, so when I could draw a line from 0,0 to 0,10 to represent something exactly 10 feet long. That would remain 10 feet whether I viewed it up close, so it filled my view, or from a long ways away so it was only a couple of pixels long.

Only when objects are being displayed is perspective transformation done. I don’t do it on the coordinates being fed into the system at all.

If you’re asking about computing the screen coordinates for an object, yes, you can. The usual way to do this is with gluUnProject. At least in my experience it’s relatively unusual that you end up needing to do this though.

The one time you sort of care is when you’re selecting something on-screen using the mouse. Though it’s possible to do with with gluUnProject, OpenGL has a selection mode that’s intended specifically for this kind of purpose, and it works pretty well.