1. An image forming apparatus comprising: a unit for determining a changing coordinate in a main scanning direction for correcting a color shift in a sub scanning direction on the basis of a shift amount of the color shift, the color shift expected to occur in image formation of a generated image; a unit for detecting a closed region from a drawing object based on which the image is formed, the closed region lying across the determined changing coordinate and formed of a plurality of neighboring color pixels; and a unit for performing a changing by using the determined changing coordinate without changing a shape of the detected closed region.