N is defined to be an integer of 2 or more, and K is a positive integer that satisfies K<N. An arithmetic operation unit performs 2.sup.K ways of processing, which are a sum of 2.sup.K1 ways of processing for obtaining converted gradation data based on addition of high-order gradation data composed of high-order (NK) bits of N-bit original gradation data and low-order K-bit data of the original gradation data, and processing directly using the high-order gradation data as the converted gradation data. A selector selects, in response to each of a plurality of heating resistors, any of the converted gradation data as gradation data for performing gradation recording by using a line thermal head including the plurality of heating resistors, the converted gradation data being obtained by the 2.sup.K ways of processing.

An electrophotographic image forming apparatus, which utilizes a line head in which a plurality of light-emitting elements is arranged as a light source, includes at least one controller having at least one processor and at least one memory. The controller is configured to obtain print data to perform printing, perform halftone processing for the obtained print data, generate a plurality of pieces of correction information for correcting variations of light amount on a photosensitive body by light radiated from the plurality of light-emitting elements, and correct the print data for which the halftone processing has been performed by using the correction information. The correction information includes various profiles for correcting the variations of light amount.