In raster-scan of a given electronically imaged field, a predetermined digital value is assigned to each raster element. A raster element scanned for the first time, within the boundaries of an object to be evaluated, is projected in at least one preselected direction upon raster elements of consecutive raster lines, thereby changing its digital value in each consecutive line scan by one digit position until said predetermined value has become displaced to a preselected guide value. All raster elements which after scanning are associated with said preselected guide value define an object which is to be evaluated.