1. A method for shading 3-dimensional computer graphics images comprising the steps of: subdividing a display on which an image is to be viewed into a plurality of rectangular areas; for each rectangular area deriving a list of objects in the image which may be visible in that rectangular area; using the list of objects to determine how the rectangular area should be shaded for display; characterized in that the step of deriving a list of objects comprises the steps of: determining maximum and minimum values for each object in x and y directions; determining a set of sampling points from the maximum and minimum values; determining whether or not a bounding box surrounding the object covers any of the sampling points; and adding or rejecting the object from the list in dependence on the result of the determination.