The present video tracking technique outputs a Maximum A Posterior solution for a target object based on two object templates obtained from a start and an end keyframe of a whole state sequence. Thereby, the bulb is fixed to the main body with a type of a bayonet scheme in which the engagement claws of the bulb are interposed in between and held by the engagement spring of the supporter spring and an opening end face of the bulb attachment hole. Client devices also may include non-compliant external and legacy devices, as well as out-side interferers.