1. An image data processing apparatus comprising: a memory for storing plural values indicating brightnesses when a predetermined display element is driven based on discrete plural drive values; and an operation circuit for converting a first conversion value converted from input image data, based on a value read out from the memory thereby generating the drive value; wherein the operation circuit executes an operation for evaluating a difference between the first conversion value and the value indicating brightness, and an operation for obtaining the drive value according to a result of the evaluation.