A dynamic reference range image generation method comprises providing a reference range image, to be dynamically updated, composed of pixels, each of which contains a reference range value. An acquired range image is provided, the pixels of which contain each a measured range value, the measured range values being updated at a predetermined rate. Pixels of the acquired range image containing an invalid measured range value are accordingly marked. The measured range value of each pixel of the acquired range image not marked as containing an invalid measured range value is compared with the reference range value of the corresponding pixel of the reference range image. The reference range value of that pixel of the reference range image is updated e.g. to the measured range value or to an average of the measured range value and one or more prior measured range values if a) the measured range value is considered less than the reference range value and has remained substantially constant for a first time period, or if b) the measured range value is considered greater than the reference range value and has remained substantially constant for a second time period smaller than the first time period. If neither of conditions a) and b) is fulfilled, the reference range value is kept substantially constant instead.