Patent ID: 7326909

Claim:
A method for measuring relative motion between an illuminated portion of a surface (S) and an optical sensing device comprising a photodetector array ( 420 ), which includes a plurality of rows and columns of pixels respectively aligned along first and second axes (X, Y), said method comprising the steps of: a) detecting by means of said photodetector array a first light intensity pattern of said illuminated portion of the surface at a first point in time; b) comparing light intensity between neighbouring pixels of said photodetector array and extracting motion features along each of said first and second axes from said detected first light intensity pattern; c) storing said extracted motion features; d) detecting a second light intensity pattern of said illuminated portion of the surface at a second point in time; e) comparing light intensity between neighbouring pixels of said photodetector array and extracting motion features along each of said first and second axes from said detected second light intensity pattern; f) determining a measurement of the relative motion between said optical sensing device and said illuminated portion of the surface based on a comparison of motion features extracted at steps b) and e); wherein said method further includes for each motion feature extracted at step e) located in a determined location, the following steps of: g) checking if a same type of motion feature has been extracted at step b) in at least one of the neighbouring locations located on both sides along the same axis of said determined location including diagonals; h) checking if a same type of motion feature has not been extracted at step b) along the same axis in the same determined location; i) checking if a same type of motion feature has been extracted at step b) along the same axis in at least one of the neighbouring locations perpendicular to said determined location with respect to said axis; and j) discarding when determining motion at step f) each motion feature for which conditions of steps g), h) and i) are fulfilled, said method further comprising the preliminary steps of: counting the number of diagonals; comparing said diagonal number with a determined threshold; and enabling or disabling steps g) to j) if the diagonal number is greater or respectively lower than the determined threshold.