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 outputs a value in a limited range between predetermined maximum and minimum values in response to the variation of grayscale being outside said limited range, and outputs a value of the variation of grayscale in response to the variation of grayscale being within said limited range, the predetermined maximum value being a positive value and the predetermined minimum value being a negative value; a gradient determination unit that calculates a grayscale gradient of the image signal from the outputs of the variation limiter; an additional bit generator that generates an additional bit, based on the grayscale gradient; and a bit adder that adds to the image signal the additional bit, wherein the gradient determination unit counts a number of consecutive pixels in the single dimension of the image signal in which the consecutive pixels have a same grayscale value, based on the outputs of the variation limiter, and calculates the grayscale gradient from both the outputs of the variation limiter and the counted number of consecutive pixels.