There has been increasing interest in document capture with digital cameras, since they are often more convenient to use than conventional devices such as flatbed scanners. Unlike flatbed scanners, cameras can acquire document images with arbitrary perspectives. Without correction, perspective distortions are unappealing to human readers. They also make subsequent image analysis slower, more complicated and less reliable.
The novel contribution of this paper is to view perspective estimation as a generalisation of the well-studied skew estimation problem. Rather than estimating one angle of rotation we must determine four angles describing the perspective. In our method, separate estimates are made for angles describing lines that are parallel and perpendicular to text lines. Each of these estimates is based on a twice-iterated projection profile computation. We give a probabilistic argument for the method and describe an efficient implementation. Our results illustrate its primary benefits: it is robust and accurate. The method is efficient compared with the time required to warp the image to correct for perspective.