1. An image processing apparatus comprising: a reading unit configured to read a document on which a halftone-processed image is printed; a smoothing unit configured to perform smoothing processing on image data acquired by the reading unit reading the document; a calculation unit configured to set a target region of a predetermined size to the image data smoothed by the smoothing unit, and calculate a degree of variation in signal values of a plurality of pixels included in the target region; a determination unit configured to determine a brightness correction amount for the target region based on the degree of variation calculated by the calculation unit and the signal values of the plurality of pixels included in the target region; and a correction unit configured to perform brightness correction on the target region in the image data acquired by the reading unit reading the document, based on the brightness correction amount determined by the determination unit.