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. The method includes the steps (a)-(d). Step (a) includes illuminating with the coherent light source the surface portion at a determined flash rate. Step (b) includes detecting, using the array of pixels a speckled light intensity pattern of the illuminated portion of the surface for each flash. Step (c) includes extracting edge direction data of two different types from the detected speckled light intensity patterns. Step (d) includes measuring relative motion between the optical sensing device and the illuminated portion of the surface based on extracted edge direction data. The step of extracting edge direction data further includes a preliminary step of introducing a selecting factor, which promotes detection of one type of edge direction data rather than another type.