1. An image forming apparatus, comprising: input means for inputting lines of image data; formation means for forming second image data, of a plurality of lines, each equal in amount to data input via said input means, for each line input via said input means; selection means for selecting image data of a predetermined number of lines from among the image data of the plurality of lines formed by said formation means; and output means for outputting an image based on the image data of the predetermined number of lines selected by said selection means.