I'm making a 3D game, and I'm stuck on a little experiment.
I currently have a flat plane with a free camera running around, and a sphere (really, a light approximated as a sphere) that rests above the surface of the plane at, say, 50 units up. I'm trying to move the sphere, and I can do this easily by simply adding to the position in the X and Z direction. However, I got a bit messed up because when I move the camera, pressing the left arrow might move the sphere to the right, because of the coordinates. So I'm trying to move the sphere along the X and Z dependant on where my camera is looking.
For example, if I was looking at it from behind, down on an angle, I expect the sphere to move to the camera's left along the plane, or move down towards the camera, still along the plane.
I have seemed to get left and right working with
a.Position -= Vector3.Cross(((FreeCamera)camera).Forward, ((FreeCamera)camera).Up); but trying to move the sphere using just the camera's forward causes the sphere to move into or out of the plane, depending on the angle of the camera (this makes sense, because the camera's forward may be pointing into the plane).
My question is, how do I move an object left or right along a plane (remaining 50 units above the Y axis) in accordance to the camera's direction? For example, viewing the plane from its left facing towards the center, moving left should make it move towards the top of the plane if viewed from the back or to the camera's local left.
Here's two screenshots showing what I mean:
When viewing the red light from here: Pressing right should give me this. Notice how the red sphere has moved (disregard the fact the camera angle changed slightly):