A method for measuring relative motion between an illuminated portion of a surface and an optical sensing device comprising a coherent light source and a photodetector device comprising an array of pixels and comparators for extracting motion features, said method comprising the steps of:a) illuminating by means of said coherent light source said surface portion at a determined flash rate;b) detecting by means of said array of pixels a speckled light intensity pattern of said illuminated portion of the surface for each flash;c) extracting edge direction data of two different types from said detected speckled light intensity patterns by comparing light intensity between pixels;d) determining a measurement of the relative motion between said optical sensing device and said illuminated portion of the surface based on extracted edge direction data;wherein the extracting edge direction data step comprises a preliminary step consisting of introducing a selecting factor which promotes detection of one type of edge direction data rather than the other type.
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