1. A method for defining a search window to search a region of interest for processing a road image, the search window having a longitudinal orientation directed according to the height of the image and being laterally delimited by two edges, wherein said method comprises the steps of: adapting a shape of the search window to the road configuration by defining at least one non-vertical portion on at least one of the edges; and positioning said non-vertical portion such that it is situated at least partially in a transitional area of the image between a road carriageway and the sky.