A displacement detection method for a portable processor-based device having an integrated image sensor, said method comprising: (a) receiving a current digital image from said image sensor, said current digital image depicting a background scene; (b) choosing the position of a first area within said current digital image; (c) receiving a later digital image from said image sensor, the later digital image overlapping at least a portion of said background scene; (d) identifying (403, 404) within said later digital image the position of a second area matching said first area, said matching being such that said first and second areas (101, 202) depict approximately the same portion of said background scene; and (e) calculating a current motion vector of said portable processor-based device according to a displacement between said first area and said second area, wherein said identifying (403, 404) of step (d) is performed according to the following steps: i) separately evaluating a distortion measure of each one of a plurality of area candidates within said later digital image according to differences in representational information of pixels of a respective one of the plurality of area candidates and pixels of said first area; ii) evaluating a quality level of said current motion vector, said quality level determined according to each of said distortion measures; and iii) choosing one of said area candidates as said second area matching said first area, said chosen area candidate having the lowest distortion measure; and wherein said calculating of step (e) is performed according to the following steps: (i) if said quality level exceeds a predefined threshold, calculating said current motion vector according to the displacement between said first area and said second area; and (ii) if said quality level does not exceed said predefined threshold, substituting a previously calculated and stored motion vector as said current motion vector. (a) receiving a current digital image from said image sensor, said current digital image depicting a background scene; (b) choosing the position of a first area within said current digital image; (c) receiving a later digital image from said image sensor, the later digital image overlapping at least a portion of said background scene; (d) identifying (403, 404) within said later digital image the position of a second area matching said first area, said matching being such that said first and second areas (101, 202) depict approximately the same portion of said background scene; and (e) calculating a current motion vector of said portable processor-based device according to a displacement between said first area and said second area, i) separately evaluating a distortion measure of each one of a plurality of area candidates within said later digital image according to differences in representational information of pixels of a respective one of the plurality of area candidates and pixels of said first area; ii) evaluating a quality level of said current motion vector, said quality level determined according to each of said distortion measures; and iii) choosing one of said area candidates as said second area matching said first area, said chosen area candidate having the lowest distortion measure; and (i) if said quality level exceeds a predefined threshold, calculating said current motion vector according to the displacement between said first area and said second area; and (ii) if said quality level does not exceed said predefined threshold, substituting a previously calculated and stored motion vector as said current motion vector.