A method and apparatus are described for generating a display data stream for transmission to a remote display. Some of the toner patches in each of the tone patterns are formed with a predetermined fixed developing bias and the rest of the toner patches in each of the tone patterns are formed with developing biases set based on previous developing biases that have been obtained through a previous control process. According to the method, low coverage of the alumina film does not become a problem, and the ferroelectric capacitor is reliably protected.