Products are arranged at positions along a fixture. Electronic labels associated with the products are arranged such that a product position for a product is obtainable once a position of the label is known. A camera is arranged to capture images of the fixture. A method includes controlling a label to change optical output; receiving an image containing the label and captured after the change of optical output has been initiated; detecting a position of the label in the image based on a characteristic associated with the change; and determining a position for the product associated with the label in an image. The determined position may be employed to determine when to restock a product. Further, a method for determining positions of labels in images is provided, as well a corresponding system and computer program product. The labels are controlled to include changing temporal patterns.