Coarse and fine skew measurement

Abstract

A processor measures skew of lines in an image by obtaining a first, coarse approximation to skew direction and then using the first approximation to obtain a second, fine approximation that is more precise than the first. A third, even more precise approximation can be obtained using the second approximation, and further successive approximations can be similarly obtained. In an image of text, the first approximation can be obtained by finding the direction in which characters are closest together, since intercharacter distances are typically smaller than interline distances. This direction can be determined by measuring distances from selected pixels at edges of connected components across white pixels to edges of other connected components. The measured distances in each direction can be combined to obtain a central value for the direction, such as an average. The central values form a profile indicating central value as a function of direction. The minima separated by 180° within the profile indicate the direction of intercharacter spacing, which provides a first approximation of the skew direction. Alternatively, the first approximation can be obtained by finding a direction that has the largest variance in black pixels per line, because the lines in the direction of skew either have many black pixels due to characters in a line or few black pixels due to spaces between lines. The second approximation can then be obtained by determining which one of a number of directions close to the first approximation has the largest variance in black pixels per line. The range of directions used to obtain the second approximation can be based on the angle separating directions used in obtaining the first approximation.