An image forming apparatus has a tab sheet supply unit and a control unit. A mating section extends forward from the front end of the base section in the second direction and a mating cavity is defined in the mating section and opened forward. The first and second cam track wall are disposed so as to face one another. Each pedestal carries electrodes for co-operating with the micro-mirrors for tilting thereof. A next encapsulation efficiency associated with the accumulated packets and a predicted next packet is determined.