1. An image processor apparatus for processing image signals that contain discrete image data in a temporal direction and a spatial direction and for transferring the processed image signals to a display unit, the image processor apparatus comprising: a variation calculator that calculates, as a variation of grayscale, a difference in a grayscale value between adjacent pixels in a single dimension in a spatial direction, of an image signal; a variation limiter that limits a range of a possible value of the variation calculated by the variation calculator; a gradient determination unit that calculates a grayscale gradient of the image signal from the variation in which the possible value is limited to the range; an additional bit generator that generates an additional bit, based on the image signal grayscale gradient calculated by the gradient determination unit; and a bit adder that adds to the image signal the additional-bit-generator-generated additional bit, to output the additional bit added image signal.