1. A method for generating three dimensional computer graphics images using multisample antialiasing by sequentially processing a plurality of primitives, comprising: dividing at least a first pixel into a plurality of sample areas; determining that all the sample areas of the first pixel are located within a first primitive; storing a value for the first primitive in a multisample memory for a smaller number of the sample areas of the first pixel than the total number of the sample areas of the first pixel; and storing data indicating that all the sample areas of the first pixel are located within the first primitive; the method further comprising subsequently processing a second primitive, comprising the following steps: reading the stored data for the first pixel to determine that all the sample areas of the first pixel are located within the first primitive; determining that only some of the sample areas of the first pixel are located within the second primitive; determining whether the value for the first primitive is already stored in the multisample memory for each sample area of the first pixel which is not located within the second primitive; wherein, if it is determined that the value for the first primitive is not already stored in the multisample memory for each sample area of the first pixel which is not located within the second primitive, the value for the first primitive is read from the multisample memory, and then written to the multisample memory for each sample area of the first pixel which is not located within the second primitive and for which the value for the first primitive is not already stored; and storing a value for the second primitive in the multisample memory for each sample area of the first pixel which is located within the second primitive.