4. The skew detection apparatus of claim 1 , wherein in response to a determination that the first confidence value is below a threshold, the processor is to: detect, within each of one or more vertical slabs in the image and for each of a plurality of angles, a set of straight lines at each angle and containing no foreground pixels; calculate, for each angle, a measure of area of the respective set of straight lines within each of the vertical slabs; produce a second estimated skew angle corresponding to a largest calculated measure of area; and estimate a second confidence value for the second estimated skew angle.
4. The skew detection apparatus of claim 1 , wherein in response to a determination that the first confidence value is below a threshold, the processor is to: detect, within each of one or more vertical slabs in the image and for each of a plurality of angles, a set of straight lines at each angle and containing no foreground pixels; calculate, for each angle, a measure of area of the respective set of straight lines within each of the vertical slabs; produce a second estimated skew angle corresponding to a largest calculated measure of area; and estimate a second confidence value for the second estimated skew angle. 5. The skew detection apparatus of claim 4 , wherein in response to a determination that the second confidence value is below a threshold, the processor is to: compare the second estimated skew angle with the plurality of angles determined for each of the plurality of sides of the image; determine whether at least one of the plurality of angles is consistent with the second estimated skew angle; and in response to a determination that at least one of the plurality of angles is consistent with the second estimated skew angle, output the second estimated skew angle.