1. An image processing apparatus for correcting image distortion, comprising: a storage configured to store before-correction coordinates in a coordinate system of a before-correction image, corresponding to each pixel in a coordinate system of a corrected image; a coordinate output section configured, when coordinates to select each pixel of the corrected image are input, to read the before-correction coordinates corresponding to the input coordinates, from the storage and output the before-correction coordinates; a pixel data output section configured to output the data of the before-correction image, corresponding to the before-correction coordinates output from the coordinate output section, as the data of the pixel of the corrected image, corresponding to the before-correction coordinates; and a pixel data calculator configured, when the before-correction coordinates output from the coordinate output section include a value after the decimal point, to calculate a weighted average based on the data of a plurality of pixels close to the before-correction coordinates in the before-correction image to output the weighted average as the data of the pixel of the corrected image, corresponding to the before-correction coordinates: wherein the storage stores only before-correction coordinates corresponding to corners of each of rectangular blocks generated by uniformly dividing the coordinate system of the corrected image; and the coordinate output section includes, a coordinate reading section configured, when receiving an instruction to select one of the rectangular blocks in the corrected image, to read before-correction coordinates corresponding to the selected rectangular block, from the storage, and a coordinate calculator configured to calculate before-correction coordinates corresponding to each pixel in the rectangular block, based on the before-correction coordinates corresponding to the rectangular block, read by the coordinate reading section and a relative position of each pixel in the rectangular block in the corrected image with respect to each corner of the rectangular block, and to output the before-correction coordinates corresponding to each pixel to the pixel data output section and to the pixel data calculator.