DETECTING ASPECT RATIOS OF DOCUMENT PAGES ON SMARTPHONE PHOTOGRAPHS BY LEARNING CAMERA VIEW ANGLES

Detecting an aspect ratio of an image captured with a smartphone includes detecting at least one convex quadrangle of arbitrary shape on the image and generating a plurality of additional convex quadrangles having vertices in a pre-determined vicinity of vertices of the quadrangle on the image. A linear projective mapping matrix is generated for mapping each of the quadrangle and the plurality of additional quadrangles onto a unit square. A plurality of estimated focal lengths of the camera of the smartphone is determined according to matrixes corresponding to the linear projective mappings onto a unit square of the quadrangle and each of the plurality of additional quadrangles. The quadrangle is used to determine the aspect ratio of the image in response to a range of the plurality of estimated focal lengths including a true value of the focal length of the camera of the smartphone.