In connection with the segmenting of a digital image divided into small elements (pixels) and acquired with a camera having several color channels, to discern an object from a background, the daylight locus and the color locus of the camera are, a priori, modeled after to the body reflectance characteristic of the subject. In this way a normal vector (nS) to the daylight locus is determined a priori and for each pixel, the scalar product between a color vector (cf) of the element in the form of the response from the color channels of the camera and the normal vector of the daylight locus is calculated, following which the element is characterized as representing object or background on basis of the calculation result, the elements characterized as objects (1) being assigned one value and the elements characterized as background being assigned a second value. On basis of the segmented image a control signal for an implement (4) is derived.