1. A method for processing image data read from a book document placed on a platen of an image reading apparatus, the method comprising: receiving, by an image processing apparatus, the image data read by the image reading apparatus and executing, by the image processing apparatus, a process below, wherein the image data comprises a plurality of lines arranged in a first direction and each of the plurality of lines comprises a plurality of pixels; determining background pixels in the image data, wherein a difference between the background pixel and a neighboring pixel is less than a threshold, the neighboring pixel being in an opposite direction to the first direction crossing a binding portion of the book document; calculating a magnification value for each of the lines, wherein the magnification value is a coefficient to substantially offset a change in the first direction in a brightness of the background pixels by multiplying it by the brightness of the pixels constituting each line; calculating a cumulative luminance correction value for each of the lines by multiplying the magnification value by line order in the first direction; extracting a shadow region of at least one of a shadow of a binding portion of the book document or a shadow of a front edge portion of the book document involved in the image data based on the cumulative luminance correction values; and performing a luminance correction on the shadow region based on the cumulative luminance correction values.