'Return the approximate size of a screen pixel at the location pos
Pos may be a Vector or an (N,3) array of locations'

def pixelSize(self, pos):

    cam = self.cameraPosition()
    if isinstance(pos, np.ndarray):
        cam = np.array(cam).reshape((((1,) * (pos.ndim - 1)) + (3,)))
        dist = (((pos - cam) ** 2).sum(axis=(-1)) ** 0.5)
    else:
        dist = (pos - cam).length()
    xDist = ((dist * 2.0) * np.tan((((0.5 * self.opts['fov']) * np.pi) / 180.0)))
    return (xDist / self.width())
