Converting mouse position to world position OpenGL

Question

Hey, I'm working on a map editor for my game, and I'm trying to convert the mouse position to a position in the game world, the view is set up using gluPerspective

Answer 1

A good place to start would be the function gluUnProject, which takes mouse coordinates and calculates object space coordinates. Take a look at http://nehe.gamedev.net/data/articles/article.asp?article=13 for a basic tutorial.

UPDATE:

You must enable depth buffering for the code sample I provided to work. The Z value for mouse coordinates is determined based on the value in the depth buffer at that point.

In your initialization code, make sure you do the following:

glEnable(GL_DEPTH);

Answer 2

A point on the screen represents an entire line (an infinite set of points) in 3D space.

Most people with questions similar to yours are really trying to select an object by clicking on it. If that's what you're after, OpenGL offers a selection mode that's generally more effective than trying to convert the screen coordinate into real-world coordinates.

Using selection mode is (usually) pretty simple: you start with gluPickMatrix, which you use to specify a small box around the click point. You then draw your scene in selection mode. When you're done, instead of actually drawing anything, it gives you back records of what would have been drawn in the box you specified. If memory serves, those are arranged in Z order, so the first one in the list is what would have displayed front-most (i.e., the one you usually want).