1. A digital data display system for presenting a graphical picture on an output device in which the area or screen from which the picture is to be viewed is notionally divided into a plurality of character cells each of which comprises a predetermined number of picture elements (pels), the system comprising first means for creating, in response to input information, a first level description of all the elements of a picture to be presented, second means for storing a screen definition table having an entry for each character cell of the display area, third means for calculating for each element of the picture, the pattern of pels in associated character cells required to display the elements, storing the calculated pattern in a table in a character cell store and associating the respective entry in the screen definition table with the required pattern in the character cell store, fourth means to determine when a particular character cell pattern has already been calculated as required for a picture and to associate the respective screen definition table entries with only one copy of the particular character cell pattern, and fifth means for transferring the screen definition table to a screen definition buffer store and the character cell pattern table to a character cell buffer store in the display device, whereby the contents of the screen definition buffer store and character cell buffer store control the construction of a picture presented by the display device.