1. A method of processing a scene for display in a graphics processing system, the method comprising: identifying an object in the scene; generating a first bounding volume representing the object in the scene; processing some or all of the scene by performing a depth-only rendering pass in respect of the generated first bounding volume for the object to assess whether the first bounding volume representing the object is at least partially occluded by one or more other objects in the scene as it will be displayed; and determining whether or not to process the object for display on the basis of the assessment; wherein the graphics processing system is configured to operate in a plurality of rendering states, one of said rendering states being a depth-only rendering state, the method further comprising: tagging the first bounding volume and/or its corresponding primitive or primitives with a rendering state index or predefined fragment program address such that the graphics processing system, on recognizing said rendering state index or predefined fragment program address, will operate in said depth-only rendering state when processing the first bounding volume; the method further comprising, if it is determined by the depth-only rendering pass that at least part of the first bounding volume is present in the scene as it will be displayed, then: generating two or more second bounding volumes each representing different parts of the first bounding volume; processing some or all of the scene by performing a depth-test in respect of the generated two or more second bounding volumes for the object to assess whether the two or more second bounding volumes are at least partially occluded by one or more other objects in the scene as it will be displayed; and determining whether or not to process one or more parts of the object for display on the basis of the assessment.