A method of watermarking video images of a particular instance of a virtual environment is provided. The method includes selecting a plurality of virtual objects for inclusion within the virtual environment and respectively altering a physical property of the selected virtual objects from a reference value to a respective different value. The respective different value is selected responsive to respective identification values of identification data associated with that particular instance of the virtual environment. The physical property has a visible effect on the appearance of the virtual objects. The method further includes rendering, for display, the virtual environment comprising the altered virtual objects.