def METHOD_NAME(self):
    """Emit one particle, its initial position and velocity are relative to the position and angle of the emitter"""
    p = self.particle_factory(self)
    p.center_x += self.center_x
    p.center_y += self.center_y
    # given the velocity, rotate it by emitter's current angle
    vel = _Vec2(p.change_x, p.change_y).rotated(self.angle)
    p.change_x = vel.x
    p.change_y = vel.y
    self._particles.append(p)