1. A method for processing an image data obtained by scanning opposing pages of a book that is laid-open facing-down against a scan plane, the method comprising: determining a background brightness value associated with a marginal area of the image data; determining a folding area of the image data representing a central part of the opposing pages of the book; determining a first brightness value associated with a first region of the folding area of the image data; determining a second brightness value associated with a second region of the folding area of the image data; and performing a correction on background pixels in the folding area of the image data by adjusting values of background pixels in the folding area of the image data on a pixel-by-pixel basis based on the background brightness value and an interpolation value obtained from an interpolation of the first brightness value and the second brightness value.