Converting Coordinates

Heya, everyone! its been a while since I posted here (about a year). Anyhow, I’m currently programming an editor for my soon-to-come MMOFPS, and I need a little help!

You start off with a large height map, with small spheres on each vertex. There is an array of vertex structs, each having several values including x, y, and height. (yes, z is depth and not y.)

What I want: When the user clicks a sphere and drags the mouse up or down, the height of the vertex changes respectively.

The Problem: All of the Mouse Event callback functions return just window-relative coordinates. Of course, I need the x and y coordinates related to the vertecies in the world.