code: function(g, x, y, z, w) {
  		var n = g[0] * x + g[1] * y;
  		if(z){
  			n += g[2] * z;
  			if(w){
  				n += g[3] * w;
  			}
  		}
  		return n;
  	}
docs: Computes dot product in 2D.
  @param g 2-vector (grid offset)
  @param {Number} x
  @param {Number} y
  @param {Number} z
  @param {Number} w
  @return {Number} dot product
  @api private