1. An image processing apparatus for reading a document image and converting the read document image into image data, comprising: a reader constructed to read a document image to generate image data of the document image; a corrector constructed to perform shading correction on the generated image data using a shading coefficient which is based on a reflection density of a correction region to which a correction fluid is applied; a detector constructed to detect a background of the corrected image data; and a first extractor constructed to extract a lower density region having a density lower than a density of the detected background.