1. A method of processing video signals, implemented on a video processing apparatus, captured by a camera, the video signals representing a scene as viewed by the camera, and the scene including a first object, the method comprising: combining, at the video processing apparatus, a computer generated object with the video signals with the effect that the computer generated object appears within the scene when the video signals are displayed; mapping a virtual model of the first object to a position of the first object within the scene so that the virtual model substantially corresponds with the first object within the scene, the virtual model having a degree of transparency such that the virtual model can be rendered as a substantially transparent virtual object within the scene; detecting occluded regions of the virtual model, the occluded regions corresponding to regions of the virtual model which are hidden from a virtual light source by the computer generated object; associating a first detected occluded region of the virtual model with a first alpha value and when a second detected occluded region is hidden from a second virtual light source, associating the second detected occluded region of the virtual model with a second alpha value and adding the first alpha value and the second alpha value for an occlusion overlap region where the first detected occluded region overlaps with the second detected occluded region, modifying the degree of transparency of the virtual model at the occluded regions to generate modified transparency regions based on the first alpha value and the second alpha value; and rendering the scene in combination with the computer generated object and the virtual model such that the modified transparency regions of the virtual model appear as shadows on the first object.