1. A method that in use is performed by a computer animation and graphics generation system of the type including a processor, a 3D graphics processor coupled to said processor, and a memory coupled to at least the processor, said 3D graphics processor projecting 3D objects onto a 2D viewing plane for display, said method comprising: defining, in the memory, a first 3D model representing a first moving object in a 3D virtual space; defining, in the memory, a second 3D model representing a second moving object in the 3D virtual space; using the processor and the 3D graphics processor to animate and render the first 3D moving object onto the 2D viewing plane for display; using the processor and the 3D graphics processor to animate and render the second 3D moving object onto the 2D viewing plane for display; with the processor, determining whether said first and second 3D moving objects overlap in the virtual 3D space; if the processor determines said first and second 3D moving objects overlap in the virtual 3D space, the processor substituting use of a common 2D imposter model instead of the first and second 3D models, the common 2D imposter model simultaneously modelling both of said overlapping first and second 3D moving objects; and using the common 2D imposter model to generate and render onto the 2D viewing plane an animated image of said first and second moving objects wherein the first moving object moves in front of the second moving object.