Question

I’m making a 3D game, and I need the player mesh always facing the back of the camera. I already figured out how to get a 2D speed vector (direction along the x-z plane), but now I need to rotate the mesh in the speed vector’s direction…

Basically, every mesh has a .rotation property, and that property is a 3D vector. I am only interested in rotation over the y-axis, that’s the one that is perpendicular to the surface (x-z) plane.

The rotation doesn’t use degrees, but radians, so I thought it would be something like this:

mesh.rotation.y = (mesh.direction.x - mesh.direction.z)*Math.PI*2;

But this doesn’t seem to cut it…

The direction/speed is a, as a said, 2D vector, and it consist of real numbers between -1 and 1. At all times sqrt(x*x + y*y) == 1, so it forms a “circle”, this is because speed needs to be equal in all directions, obviously.

The speed vector changes only when I drag the mouse over the screen, and so should the rotation, and it is calculated like this:

var c = Math.sqrt(cameraPos.x*cameraPos.x + cameraPos.z*cameraPos.z); //This is the distance from the camera to the mesh, which is at (0, 0) for simplicity of this presentation.
var rat = 1/c;

mesh.direction.x = cameraPos.x*rat; //Direction vector = the speed vector
mesh.direction.z = cameraPos.z*rat;

Answer 1

If I understand correctly, atan2 will do the trick:

mesh.rotation.y = Math.atan2(mesh.direction.z, mesh.direction.x)

Result is in radians. It basically calculates the angle between the vector and X axis. You might need to switch parameters or use minus operator here or there.