1. An image processing method for correcting, on a pixel-by-pixel basis, brightness of an original image which includes pixels that have greater pixel values when the pixels have higher brightness, the image processing method comprising: (a) generating coefficients corresponding to pixels of a reduced image of the original image by generating a greater coefficient for each of the pixels of the reduced image when a portion surrounding the pixel of the reduced image is darker; (b) generating coefficients corresponding to the pixels of the original image from the coefficients corresponding to the pixels of the reduced image, to expand the coefficients corresponding to the pixels of the reduced image to the coefficients corresponding to the pixels of the original image; and (c) multiplying, for each of the pixels of the original image, a pixel value of the pixel of the original image by one of the coefficients corresponding to the pixel of the original image, to correct the pixel values of the pixels of the original image, wherein in step (b), the coefficients corresponding to the pixels of the original image are generated such that a spatial change of the coefficients corresponding to the pixels of the original image is caused according to a boundary between a bright portion and a dark portion included in the original image, the spatial change occurring mainly on a dark side of the boundary.