Get world coordinate under mouse (at a ground plane for Warcraft 3 like movement)

Question

I want to let a unit follow the mouse like in any RTS (Warcraft 3, Starcraft, ...). For that I have a "ground plane" which is defined through 2 vectors, the X and Z axis. Now I need to get the world coordinate of the mouse click on this plane in order to move the unit. I'm using OGRE & C++.

How can I get the coordinates of the mouse click on the ground plane?

Answer 1

This is known as 3D picking, and usually involves some kind of raycast from a point in screen space (the cursor position) in the camera's forward vector direction.

For some Ogre tutorials, check those links:

• http://www.ogre3d.org/tikiwiki/tiki-index.php?page=Intermediate+Tutorial+3
• http://www.ogre3d.org/tikiwiki/ThreeDeeObjectPicking
• http://www.ogre3d.org/forums/viewtopic.php?f=3&t=45041

Answer 2

Since you're using Ogre then you should probably read those ogre3d.org links, but if you want to know the math behind the intersect algorithm, here it is:

http://softsurfer.com/Archive/algorithm_0104/algorithm_0104B.htm#Line-Plane%20Intersection

When I do that I also use a bit from http://softsurfer.com/Archive/algorithm_0104/algorithm_0104.htm