1. Field of the Invention
The invention relates to a method of producing and modifying the contents of a picture memory in an apparatus for displaying synthetic pictures subdivided into elementary points or pixels whose color and/or luminance are defined by digital codes in said picture memory, in which a given position is assigned to each pixel, the picture comprising objects which can cover at least in part other objects, thus providing the visual effect of being arranged in front of the latter objects, the apparatus furthermore comprising an object memory or a magazine which contains the description, pixel per pixel, of a plurality of objects, among which those are selected which are needed to construct the picture in the picture memory, as well as a register in which data are stored which especially indicate the position of the objects in the picture.
2. Description of the Prior Art
Such a method is used in video games or in microcomputer display systems. The display is effected in general in these systems by means of a movable point describing a succession of lines sequentially scanned in response to horizontal and vertical synchronization signals. Methods are known of producing the picture, in which the coordinates X and Y of each object are compared with the contents of counters following the position of the point along the lines of the picture. The objects are introduced into the image "at random" during the displacement of the point when the latter passes the coordinates X, Y of the object. Such a method is described, for example, in British Patent GB-2,000,946 A. This method is not very suitable for pictures comprising a large number of objects because a quantity of calculations has to be carried out.
Another known method uses a picture memory which is filled beforehand with the elements of the picture and which is read in synchronization with the operation of scanning the lines of the picture. Methods of this type are described, for example, in U.S. Pat. No. 4,116,444, in which the picture memory contains only one line and has to be updated again at each line, or in U.S. Pat. No. 4,165,072, in which the memory contains a complete picture. In this case, the number of objects is not limited, but the method requires a high storage capacity.
The invention relates to a method of this type, in which a memory of a complete picture is used and in which a solution is provided for the problem involved in the control of the operation in which the pictures cover each other. It is known to handle this problem by defining several planes each containing one or several objects, in which event the objects of a so-called "front" plane have to cover the objects of a so-called "rear" plane. During the display of the picture, a choice is made between the planes in dependence upon the priority rules based on the front or rear position of the planes. Such a method is described, for example, in French Pat. No. 2,535,871.
This solution leads to the use of several memories of a complete picture and the storage capacity is extremely high.