1. A method for generating a computer graphics image from 3-D object data, comprising: inputting object data to a tiling unit, the object data describing positions of objects that may be visible in an image to be rendered; calculating, in the tiling unit, for each object, a respective set of tiles into which that object is to be rendered, wherein each tile corresponds to a set of screen space pixels in the image, and writing identifying data for the object into a display list that will be accessed when rendering pixel values in a tile of the set of tiles; before the tiling unit has finished calculating respective sets of tiles for all of the objects in the object data, beginning to render a part of the image, the rendering comprising calculating per-pixel depth values for pixels in that part of the image, based on the objects currently identified by the identifying data in the display list or lists for the tiles in that part of the image; producing a respective depth range for each tile in that part of the image, based on the per-pixel depth values in that tile; releasing memory used for the identifying data for objects currently identified by the identifying data, after parameter data for those objects has been read and before the tiling of all objects in the object data has completed; feeding back the respective depth ranges for each tile to the tiling unit; and deriving a depth range for an object for which the tiling unit has not yet determined a set of tiles, wherein the calculating of the respective set of tiles for the object comprises comparing the derived depth range of the object with a respective depth range of each tile that the object is determined to overlap, and culling the object for any tile for which the object fails the comparison and otherwise performing the writing of the identifying data for the object to a display list for that tile.