Not sure this will work since I have never really done something like this myself and it is a few years since my last math course... So follow this guide on your own risk... I give no guarantees that it will work.
I'll assume you got a global 3D coordinate system where you can get the location of stuff like cameras and location of characters in the 3D world...
You should have specified a clip plane somewhere in front of the camera. This should be a 2D surface and any object closer to the camera than this clip plane will not be rendered. This surface should be parallel to the screen surface.
Draw a line from the camera position in the viewing direction of the camera. This line should intersect the clip plane. Call the point of intersection P1. P1 should be the position of the camera translated in the viewing direction a distance equal to the specified clip plane distance. It should also be in the middle of your screen.
If you can get the position of a caracter you should be able to calculate a line or vector from the camera location to this location. When you got this line you should be able to calculate where it intersects the clip plane. Call this point of intersection P2.
Using the distance to the clip plane, and the specified viewing angle of the camera along the X-axis and Y-axis respectivly you should be able to calculate, with simple trigonometry, the maximum distances from P1 that any line drawn from the camera to an object in the 3D world will intersect the clip plane and still show up on the screen. Call these distances DX and DY.
Now, P1 and P2 should be in the same 2D plane parallel to the screen. It should be fairly easy to establish P2's distance from P1. Call this distance dX and dY for the respective directions.
Now, to calculate the position (x,y) for you 2D popup window derive x and y from:
x/[half screen width] = dX/DX
y/[half screen height] = dy/DY
Well, thats it... I just hope it is correct in case you decide to try it out...
/Jesper
