function vec3(a,b,c){
	this.a = a
	this.b = b
	this.c = c
}
vec3.prototype.add = function(v){
	this.a += v.a
	this.b += v.b
	this.c += v.c
	return this
}
vec3.prototype.sub = function(v){
	this.a -= v.a
	this.b -= v.b
	this.c -= v.c
	return this
}
vec3.prototype.clone = function(){
  return new vec3(this.a, this.b, this.c)
}

// input: mat4 projection matrix
vec3.prototype.apply_projection = function (m) {
	var x = this.a, y = this.b, z = this.c;

	var e = m.elements;
	var d = 1 / ( e[3] * x + e[7] * y + e[11] * z + e[15] ); // perspective divide

	this.x = ( e[0] * x + e[4] * y + e[8]  * z + e[12] ) * d;
	this.y = ( e[1] * x + e[5] * y + e[9]  * z + e[13] ) * d;
	this.z = ( e[2] * x + e[6] * y + e[10] * z + e[14] ) * d;

	return this;
}