1. A method of operating a computer having a single framestore, a data input device and a display device responsive to the framestore, the method comprising receiving a plurality of component images and storing these component images separately within the computer memory, editing and combining the component images using a selected combining function and supplying the combined image to the framestore for display by the display device and wherein a component image comprises a fixed color image whose RGB values are fixed but whose alpha values vary between pixels, the editing step depending on said alpha value at the appropriate pixel being edited.