1. An image processing apparatus for adjusting an image that is formed by image data of a main scanning line, by either adding pixels to the image data or deleting pixels from the image data, the image processing apparatus comprising: a first image forming unit configured to segment an image area of the main scanning line into a plurality of regions and form a first image having a first number of pixels in each of the plurality of regions; a second image forming unit configured to form a second image having a second number of pixels that differs from the first number of pixels, in each of the plurality of regions; an arithmetic unit configured to derive a scaling factor of an image in each of the plurality of regions, in accordance with a difference in size between the first image and the second image in each of the plurality of regions, and a differential between the first number of pixels and the second number of pixels; a determination unit configured to determine the number of supplemental pixels to be either added to, or subtracted from, each of the plurality of regions, based on a total number of correction pixels that are either added to, or subtracted from, the image area of the main scanning line, and the scaling factor that is derived by said arithmetic unit; and an image correction unit configured to correct the image data that is formed in each of the plurality of regions, in accordance with the number of supplemental pixels determined by said determination unit.