javascript - Walking on a sphere

I'm working on a game which involves walking your character on the surface of a sphere. Using the answer to Arbitrary Rotation about a Sphere, I've written my code as:

if (game.isKeyDown(37)) { // left
    this.quaternion.multiply(new THREE.Quaternion(0, Math.sin(-0.01), 0, Math.cos(-0.01)));
}

if (game.isKeyDown(39)) { // right
    this.quaternion.multiply(new THREE.Quaternion(0, Math.sin(0.01), 0, Math.cos(0.01)));
}


if (game.isKeyDown(38)) { // up
    this.quaternion.multiply(new THREE.Quaternion(Math.sin(-0.01), 0, 0, Math.cos(-0.01)));
}

if (game.isKeyDown(40)) { // down
    this.quaternion.multiply(new THREE.Quaternion(Math.sin(0.01), 0, 0, Math.cos(0.01)));
}

var qx = this.quaternion.x;
var qy = this.quaternion.y;

var qz = this.quaternion.z;
var qw = this.quaternion.w;
this.obj.position.x = 2 * (qy * qw + qz * qx) * radius;
this.obj.position.y = 2 * (qz * qy - qw * qx) * radius;
this.obj.position.z = ((qz * qz + qw * qw) - (qx * qx + qy * qy)) * radius;

Which works fine, however I would like to control the character in such a way that pressing up and down is equivelent to walking forwards and backwards (in the direction you are looking), whereas pressing left and right is the same as turning around on the spot.

I understand that I will have to store a forward vector, but I'm not clear on how that relates to the quaternion which allows the character to walk on the surface of the sphere.

Another problem I've got to overcome is making sure the character on the surface of the sphere is actually "looking" the way it is going, at the moment I'm modelling the player as another sphere, but in the futuer it will be a proper model which will need to orientate itself.

Answer

Store a direction vector as a tangent to the sphere. When you move, you can take this tangent, the normal vector (normalized position on the sphere) and cross them to get a general axis to rotate around. If you're limiting all movement to a single 2D plane (you're just using a single angle), for direction all you need is a sign (+1 or -1) to multiply your movement angle by. You can also use the sign to calculate a facing rotation about the sphere normal vector (or to just flip the character about its local Y axis) to orient your character properly.