1. A method for processing a document image containing straight lines extending in a first direction, the method comprising: (a) identifying a straight line extending in the first direction; (b) calculating a run length profile for the line, the run length profile being a run length value as a function of a position in the first direction along the line, wherein each run length value at a given position is a number of consecutive non-white pixels extending in a second direction perpendicular to the first direction including non-white pixels within the line; (c) detecting a set of first type of intersection areas of the line by detecting a first type of characteristic features of the run length profile, each of the first type of characteristic feature including a valley and two peaks located on both sides of the valley within a predefined distance, a difference between run length values of each of the peaks and the valley being greater than a first threshold value; (d) detecting a set of second type of intersection areas of the line by detecting a second type of characteristic features of the run length profile, each of the second type of characteristic feature including a number of consecutive positions where the run length values are greater than a second threshold value; and (e) removing the line in areas outside of the first and second types of intersection areas while preserving the lines in the first and second types of intersection areas.