1. A method for simulating the motion of particles in a computer generated image representing the appearance from a predetermined viewing point of a three dimensional worldspace through which the particles move, wherein a three dimensional model is defined which is made up from a regular array of abutting model portions of predetermined dimensions, an object is defined the dimensions of which correspond to the dimensions of each model portion, the object having characteristics such that the object represents the motion of particles through the volume which the object occupies, the three dimensional model is notionally positioned in worldspace such that the viewing point is located within the model, the field of view from the viewing point extends through the model, and a notional point which is in a predetermined location relative to the viewing point is within a selected region of the model, the worldspace model position is translated in steps, the magnitude of each step corresponding to the dimensions of the portions, and the model being translated in response to movements of the viewing point in worldspace so as to maintain the notional point within the selected region, and an image of the worldspace including the model is generated with the object representing the moving particles occupying each portion of the model.