1. A device configured to correct keystone distortion of a distorted image captured with misalignment of the axes of a sensor and a subject plane, comprising: a display configured to exhibit corners of a quadrilateral that represents a known rectangular feature in the subject plane; and digital circuitry configured: to locate intercept points that are the intersections of lines that are extensions of the top and bottom edges and the left and right sides of the quadrilateral; to locate vertical and horizontal reference points on sides of the quadrilateral by drawing a line through each intercept point and an image point of the distorted image; to compute distances from the respective intercept points to the image point of the distorted image and to the respective reference points; and to scale and correct the distances with an offset to locate a corresponding image point of a corrected image.