Abstract:
A method and system for printing of gray scale images utilizes raster image data. A page is divided into tiles in a two dimensional grid. Tiles are checked for the presence of dither cells for generating gray scale values. Tiles without dither cells are compressed for transmission while tiles having dither cells are not compressed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for compressing and transmitting image raster data, whereby a data stream of image raster data is generated page-by-page from language elements of a graphics language, the data stream containing gray image areas in the form of dither cells whose gray scale values are determined by model dither cells. The invention also relates to a system for compressing and transmitting image raster data upon employment of the aforementioned method. The invention also relates to a computer system with whose assistance a computer can implement the method. 
   2. Description of the Related Art 
   It is not only black/white structures but also gray image areas that have a prescribed gray scale value that are being increasingly being employed when printing text and drawings. When, for example, an RIP module generates a data stream of image raster data page-by-page from language elements of a graphics language, for example the known printer language POSTSCRIPT, a noticeably larger dataset must be processed due to the gray image areas that are produced by dithering. The RIP module (RIP=raster image processing) is generally arranged outside the printer; this means that its image raster data must be compressed in order to be able to transmit the data in time-conforming fashion with a given data transmission rate. The following example makes this clear: A DIN A4 page contains approximately 4.3 megabytes of image raster data given a pixel density of 600 dpi (dots per inch). A high-performance printer has the capability of printing more than 400 DIN A4 pages per minute at 600 dpi. Accordingly, a data rate of more than 28 megabytes/s would have to be governed without compression. 
   Up to now, the image raster data were compressed with the assistance of a standardized compression method, for example the FAX G4 compression method, and were intermediately stored and/or directly transmitted to the printer in this compressed form. For example, International Patent Application No. PCT/DE95/01293, which claims the priority of German Patent Application No. 44 34 068.0, references compression methods in conjunction with printers. The PCT Application PCT/DE95/01293 is incorporated hereby by reference. When a printed page contains either no or only a few gray raster image areas, the compression time is relatively short and the efficiency of the compression is relatively high. When, however, a page contains a great proportion of gray raster image regions, then the compression time lengthens exponentially and the compression efficiency becomes low. 
   Picture elements in the form of gray rasters are often generated upon employment of the dithering method. In this dithering method, gray scales are generated by employing dot patterns (rasters). The dithering method exploits a unique property of the human eye: individual picture elements are no longer perceived beyond a specific viewer distance and a specific dot density, but blur to form a gray scale value. A dither cell, accordingly, contains a plurality of picture elements; of which only one picture element, several picture elements or all picture elements of a dither cell are inked dependent on the desired gray scale value. In order to achieve a good melding of the picture elements, the inked picture elements are scattered according to a predetermined algorithm. The gray scale values themselves are fixed by given model dither cells. When a dither cell contains 8×8 picture elements in the form of a matrix and a symmetrical arrangement of the inked picture elements is prescribed, then 32 or 64 gray scale values can be realized. Since dither cells and the distribution of the inked picture elements are relatively complex, standard compression methods for the reduction of the data volume often fail. 
   Section 6 in  Das Druckerbuch  of Océ Printing Systems GmbH, 3 rd  Edition, 1998, ISBN 3-00-001019-X describes raster technique, whereby a dither technique is also addressed. Among others, the dot pattern method is described as a dither technique. The above document is incorporated herein by reference. 
   The dithering method is also described in  Das groβe Data Becker Computer Lexikon , 1997 Edition, ISBN 3-8158-1575-4, and in  Computer Lexikon , Verlag C. H. Beck, Munich, ISBN 3-406-39696-8. 
   U.S. Pat. No. 5,073,953 discloses a method for the automatic segmenting of documents. The picture elements of the document are analyzed according to different types, for example black/white texts, graphics elements, continuous tone images, half-tone images, etc. The document to be analyzed is divided into sub-images and the type is assigned to these sub-images. 
   Image compression devices are known from German Patent Document DE-C2-38 24 717 and from the publication by W. Crocca et al., “Compression of grey digital images using grey separations”, Xerox Disclosure Journal, Vol. 15, No. 6, November/December 1990, pages 481-482. German Patent Document DE-C2-41 27 920 discloses an image processing device wherein image data are subdivided into blocks and the blocks are sequentially processed. German Patent Document DE-C2-29 53 109 and German Patent Document DE-A1-42 15 157 disclose image reception devices. Japanese Patent Document JP-A-11-65793 discloses a method with which data are compressed differently according to the object type (image or text). The contents of the above-cited documents are herewith likewise incorporated by reference into the present specification. 
   European Patent Document EP 0 774 858 A3 discloses a method for compressing and transmitting image raster data wherein picture elements (pixels) are combined into macro-cells in the fashion of tiles. These macro-cells are assigned to predetermined type classes, for example the type text, graphics, gray scale image, etc. The compression method to be applied is adapted dependent on the information about the type of macro-cell. For compression, the pixels of the respective tile are re-ordered (rescanning), whereby the respectively exposure value of the pixel is defined dependent on the position of the pixel with reference to the center of the macro-cell. 
   U.S. Pat. No. 5,465,173 is directed to an image processing method wherein half-tone image data are stored. The storing ensues block-by-block with a prescribed plurality of pixel data. A compression of the half-tone image data ensues on the basis of the block-by-block data, whereby the memory requirement is reduced. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to provide a method and a system for compressing and transmitting image raster data that also works with high efficiency when a page to be transmitted contains gray picture elements. 
   This object is achieved by a method for compressing and transmitting image raster data, whereby a data stream of image raster data is generated from language elements of a graphics language, the data stream containing gray image areas in the form of dither cells whose gray scale values are determined by model dither cells, the image raster data of each and every page are divided into tiles of a two-dimensional grid network, whereby each tile comprises a plurality of image raster data, the appertaining model dither cell and the gray scale value thereof being identified for each tile that contains only dither cells, and this tile being marked, and characteristic data of the marked tiles are transmitted for further processing of the image raster data, whereby these characteristic data contain information about the position of the respective tile and the respective gray scale value. 
   A further improvement provides that the dither cells contain rectangularly or quadratically arranged picture elements, and that the model dither cell with higher gray scale value at least contains inked picture elements at the same positions as the model dither cell with the next-lower gray scale value. Additionally, each tile is checked to see whether is contains dither cells of the type of the model dither cell with the lowest gray scale value. Furthermore, the check of the tiles ensues tile row by tile row, whereby the first row is investigated first per tile; and, given a lack of coincidence, the appertaining tile is investigated no further. In particular, the model dither cell with the highest gray scale value that is contained in all dither cells of a tile is determined for the tile that contains dither cells of the type of the model dither cell with the lowest gray scale value; and the gray scale value of this model dither cell is assigned to this tile. 
   In one improvement, the tiles have a uniform row length, preferably corresponding to the bit length of the register of a hardware module with which the method is implemented. The row length amounts to 8, 16, 32, 64 or 128 bits or an additive combination thereof. In a preferred development, for determining whether a tile contains dither cells at least with the lowest gray scale value corresponding to a model dither cell, a comparison cell is employed that contains only these model dither cells and whose length at least corresponds to the row length of a tile; and the comparison is implemented tile row by tile row. Furthermore, the length of the comparison row amounts to the smallest common multiple of row length of the tile and row length of the dither cell, which preferably has an 8×8 or 10×10 picture element matrix. An additional advantage is realized when a comparison row with appertaining model dither cells is employed for each gray scale value. 
   In one embodiment, neighboring tiles having a prescribed gray scale value corresponding to a model dither cell are combined to form a polygon; and the characteristic data of this polygon are transmitted, preferably compressed, for further processing of the image raster data. The polygon is either a square or a rectangle, for example. Specifically, the tiles combined to form a rectangle have a common minimal gray scale value; and the characteristic data of this rectangle are transmitted. The rectangle can contain a sub-rectangle whose tiles have a minimum gray scale value that is higher than the gray scale value of the tiles of the rectangle. In particular, a list of rectangles is produced; and characteristic data of this list are transmitted, preferably in compressed form. The list is organized such that rectangles with descending plurality of tiles assume a descending rank in the list; and only those rectangles from this list whose plurality of tiles exceeds a predetermined value are transmitted for further processing. The number of rectangles of the list is limited to a predetermined value. 
   In one further improvement, the dither cells of a row or of a sequence that adjoin a rectangle and have the same minimum gray scale value as the dither cells of the rectangle are incorporated into the expanded rectangle, whereby the boundaries of the rectangles are correspondingly expanded. Preferably, the position of the upper left corner, the height, the width and the gray scale value are determined for each rectangle with reference to a page, and these characteristic data are transmitted, preferably in compressed form. 
   The raster image data of the marked tiles or of the marked rectangles can be removed from the data stream by subtraction; and the remaining data stream compressed according to a standardized compression method and transmitted. For example, the FAX G4 compression method is employed as standardized compression method. The data of the marked tiles or of the rectangles may be transmitted according to the SPDS data format. As a variation, the transmitted image raster data as re-compiled upon employment of an OR function. 
   In the method, an RIP module, preferably a POSTSCRIPT converter module is employed for generating the data stream of image raster data from language elements of the graphics language. The present method may be employed for the transmission of print raster data to printers, preferably to high-performance printers. By way of definition, one example of the high-performance printer has a printing output greater than equal to 400 pages DIN A4 per minute at 600 dpi. 
   The present invention also provides a system for compressing and transmitting image raster data, including an RIP module that generates a data stream of image raster data page-by-page from language elements of a graphics language, the data stream containing gray image areas in the form of dither cells whose gray scale values are determined by model dither cells, the image raster data of each and every page being divided into tiles of a two-dimensional grid network, each tile including a plurality of image raster data, wherein the appertaining model dither cell and the gray scale value thereof are identified for each tile that contains only dither cells and this tile is marked; and characteristic data of the marked tiles are transmitted for further processing of the image raster data, whereby these characteristic data contain information about the position of the respective tile and the respective gray scale value. For example, the dither cells contain rectangularly or quadratically arranged picture elements; and the model dither cell with higher gray scale value at least contains inked picture elements at the same positions as the model dither cell with the next-lower gray scale value. In one embodiment, the system has neighboring tiles with predetermined gray scale value corresponding to a model dither cell are combined to form a polygon; and characteristic data of this polygon are transmitted, preferably compressed, for further processing of the image raster data. The invention provides that the polygon is a square or a rectangle. 
   In a further method for compressing and transmitting image raster data, a data stream of image raster data is generated page-by-page from language elements of a graphics language, the data stream containing gray picture elements in the form of dither cells whose gray scale values are defined by model dither cells, at least one area is determined that contains only dither cells, whereby the appertaining model dither cell and the gray scale value thereof is identified and this area is marked; and characteristic data of the marked area are transmitted for further processing of the image raster data, whereby these characteristic data contain information about the position of the respective tile and the respective gray scale value. A preferred development provides that the dither cells contain rectangularly or quadratically arranged picture elements; and the model dither cell with higher gray scale value at least contains inked picture elements at the same positions as the model dither cell with the next-lower gray scale value. The dither cells of a rectangular region have a common minimum gray scale value. A list of rectangles can be produced; and the characteristic data of this list are transmitted, preferably in compressed form. 
   According to the invention, a computer program product includes a computer-readable medium with which commands are offered in encoded form, these, after the loading of the computer program, causing the computer to implement the steps set forth above. A computer program element may also be provided comprising commands in encoded form that cause the computer to implement the foregoing steps. The computer program element is preferably present on a computer-readable medium. The invention also provides a computer-readable medium that contains a computer program which causes a computer to implement the above set-forth steps. 
   According to the invention, the image raster data of each and every page are divided into tiles, for example tiles of an identical size, of a two-dimensional grid network. A determination is made for every tile as to whether it exclusively contains dither cells or not. When the former applies, the appertaining model dither cell and the gray scale value thereof are determined and this tile is marked. When the tile does not exclusively contain dither cells, for example non-inked white parts, then this tile is not further-analyzed. The image raster data of such a tile is compressed according to traditional compression methods. Characteristic data, for example about the size and the gray scale value, are identified from the marked tiles, and these characteristic data are transmitted as compressed data. The image raster data of such marked tiles need not be compressed according to the traditional compression methods, i.e. they are bracketed out in the traditional compression method. In this way, the traditional compression method can compress a page faster and with higher efficiency since, on the one hand, the compression of marked tiles and, on the other hand, the considerable compression outlay for dither cells are eliminated. According to the inventive method, an added outlay is in fact required overall for the analysis of the tiles and the transmission of the characteristic data of marked tiles. This added outlay, however, is slight compared to the saving in compression outlay for the standardized compression method. 
   According to a further aspect of the invention, a system for compressing and transmitting image raster data has comprising an RIP module that generates a data stream of image raster data page-by-page from language elements of a graphics language, the data stream containing gray image areas in the form of dither cells whose gray scale values are determined by model dither cells, whereby the image raster data of each and every page are divided into tiles of a two-dimensional grid network, whereby each tile comprises a plurality of image raster data, the appertaining model dither cell and the gray scale value thereof are identified for each tile that contains only dither cells and this tile is marked; and characteristic data of the marked tiles are transmitted for further processing of the image raster data, whereby these characteristic data contain information about the position of the respective tile and the respective gray scale value. This system has the technical advantages already described above in conjunction with the method. 
   Further, a method for compressing and transmitting image raster data is provided whereby a data stream of image raster data is generated page-by-page from language elements of a graphics language, said data stream containing gray picture elements in the form of dither cells whose gray scale values are defined by model dither cells, characterized in that at least one area is determined that contains only dither cells, whereby the appertaining model dither cell and the gray scale value thereof is identified and this area is marked; and characteristic of data of the marked area are transmitted for further processing of the image raster data, whereby these characteristic data contain information about the position of the respective tile and the respective gray scale value. Given this method, at least one area is identified that contains only dither cells that agree with a pre-defined model dither cell and a gray scale value. The characteristic data of this area are transmitted for further-processing of the image raster data. 
   According to a further aspect of the invention, a computer program product, a computer program element and a computer-readable medium are provided. The computer program product and the computer program element contain commands and data for the control of a computer. After the loading of the computer program product or, respectively, of the computer program element that, for example, is loaded as a software module either individually or together with other software modules, the method steps defined in the claims are implemented when the commands are processed and the technical result is achieved. A diskette, a magnetic or optical storage disk (CD ROM), a data carrier tape or a remote memory can be employed as a computer-readable medium, a computer program being transmitted and loaded therefrom by remote data transmission, for example via the Internet. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the invention is explained below on the basis of the drawings. 
       FIGS. 1A and 1B  are illustrations of model dither cells with overlap of the inked picture elements. 
       FIG. 2  is a diagram showing the division of a page into tiles with the assistance of a grid network 
       FIG. 3  is a schematic illustration of a search algorithm upon employment of comparison lines composed of tiles. 
       FIGS. 4 ,  5  and  6  are schematic illustrations of a search algorithm for fixing the gray scale value of a tile. 
       FIG. 7  is an illustration of the combining of tiles to form rectangles. 
       FIG. 8  is an illustration of the deletion of a rectangle. 
       FIG. 9  is a schematic showing of an expansion possibility for a rectangle. 
       FIG. 10  is a schematic showing of the fixing of the bit length of a comparison line. 
       FIG. 11  is a flow diagram of the transmission of the image raster data upon employment of the inventive method. 
       FIG. 12  is a illustration of the inventive method in a block diagram. 
       FIG. 13  is a block diagram of a traditional method for compressing and transmitting image raster data according to the Prior Art. 
       FIG. 14  is a flowchart with method steps for allocating gray scale values to tiles. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A and 1B  show the structure of two model dither cells A and B. Both model dither cells A and B have 10 times 10 picture elements, i.e. the row length amounts to 10 picture elements and the column length likewise amounts to 10 picture elements. The model dither cell A contains black-colored picture elements in the middle in the fashion of a cross. The model dither cell B contains black-colored picture elements in the middle that together yield an L pointing to the left. The model dither cell B contains fewer black-colored picture elements than the model dither cell A. Accordingly, the gray of the model dither cell B is lighter than the gray of the model dither cell A. This means that the gray scale value of the model dither cell B is lower than the gray scale value of the model dither cell A. It is generally true for the arrangement of the black-colored picture elements of model dither cells that the model dither cell with the higher gray scale value contains inked picture elements at least at identical positions as the model dither cell with the next-lower gray scale value. This means that inked picture elements of model dither cells with a higher gray scale value overlap the inked picture elements of model dither cells with a lower gray scale value. In  FIG. 1A , the darker model dither cell A has black-colored picture elements at identical positions as the model dither cell B in  FIG. 1B . In addition, further picture elements are colored black in the model dither cell A. 
   Theoretically, a plurality of gray scale values can be realized with the assistance of a plurality of model dither cells in the fashion of the model dither cells A and B shown in  FIGS. 1A and 1B , whereby the lowest gray scale value is defined by a single black-colored picture element, and the highest gray scale value is present when all picture elements of the dither cell are colored black. In practice, a smaller plurality of gray scale values than the theoretically possible plurality and a correspondingly reduced plurality of model dither cells are employed, for example 16 or 32. In this example, the gray scale value then has a value range from 1 through 16 or, respectively, 1 through 32. An extreme gray scale value 0 means that none of the picture elements is colored black; in this respect, this is not a dither cell but a Q white, unprinted surface. Let it also be pointed out that two types of dither cell that have either 10 times 10 picture elements or 8 times 8 picture elements in a matrix arrangement dominate in practice. 
     FIG. 2  schematically shows the division of a page S into identically sized tiles K 1 , K 2 , Ki through Kn of a two-dimensional grid network GN Each tile K contains a plurality of image raster data in data lines of equal length. Within a tile row K 1  through Ki, the tiles are processed from left to right in the search algorithm that shall be subsequently described; the tile rows are processed from top to bottom. However, some other processing sequence is also conceivable. 
     FIG. 3  schematically shows the procedure for determining dither cells that are contained in a tile Ka. First, comparison rows Vz 1 , Vz 2 , Vz 3  and Vz 4  are offered that have at least the same row length as the tiles K 1  through Kn. For greater clarity, only four comparison rows Vz 1  through Vz 4  are employed. In practice, just as many comparison rows are offered as there are gray scale values defined by the model dither cells. Each comparison row Vz 1  through Vz 4  contains identical model dither cells with a specific gray scale value. The comparison row Vz 1  contains model dither cells with a gray scale value G=1, the comparison row Vz 2  contains model dither cells with a gray scale value G=2, the comparison row Vz 3  contains model dither cells with a gray scale value G=3, and the comparison row Vz 4  contains model dither cells with a gray scale value G=4. Comparison row Vz 1  thus contains the fewest black-colored picture elements. The comparison row Vz 2  with the next higher gray scale value G=2 contains black-colored picture elements of the model dither cell with the gray scale value G=1 at the same raster positions and additional black-colored picture elements. The model dither cells of the comparison row Vz 3  having the next-higher gray scale value G=3 contains the black-colored picture elements of the model dither cells of the comparison rows Vz 1  and Vz 2  at the same positions as well as additional black-colored picture elements. The similar case applies to the model dither cells of the comparison row Vz 4 . 
   Due to the overlap of the inked picture elements of the model dither cells of the comparison row Vz 1  through Vz 4 , it suffices for locating dither cells in the various tiles when a coincidence of the first row of each and every tile K with the comparison row Vz 1  is merely found in the first search step, as schematically shown in  FIG. 3  on the basis of the tile Ka. Tiles such as, for example, the tile Ki that already do not coincide with the comparison row Vz 1  in their first row are no longer taken into consideration in the rest of the process. Such tiles are assigned the extreme gray scale value G=0, i.e. they due not generally contain dither cells in the first row that coincide with the model dither cell having the gray scale value G=1. The search for tiles within a page that have dither cells with at least the gray scale value G=1 thus ensues very fast when the tile size is selected appropriately large. It can already be seen here that the method of the invention works very fast due to the division according to tiles. 
     FIG. 4  schematically shows the next step, whereby what actual gray scale value G the first row z 1  of the tile Ka is determined within the tile Ka by comparison to the comparison rows Vz 1  through Vz 4 . In the present case, it is found that there is coincidence between comparison row Vz 3  and the first row z 1  of the tile Ka, i.e. the gray scale value is G=3. 
     FIG. 5  schematically shows the next step wherein a finding is made within the tile Ka as to whether at least all rows z 1  through z 5  with dither cells contains the gray scale value G=3 that had been found in the preceding step ( FIG. 4 ). In the present case, all rows z 1  through z 5  of the tile Ka contains at least the gray sale value G=3 in conformity with the comparison row Vz 3 . Two rows z 2  and z 3  have a higher gray scale value G&gt;3, i.e. are darker. This is allowed since model dither cells with a higher gray scale value have colored the same picture elements black as model dither cells with a lower gray scale value. When, however, the gray scale value G is lower than found in the first row z 1 , then a branch is made to the next step, which is schematically shown in  FIG. 6 . 
   It is shown in  FIG. 6  that the row z 3  coincides with the comparison row Vz 1 , i.e. has the lowest gray scale value G=1. In this case, the entire tile Ka has the gray scale value G=1 allocated to it since this gray scale value G is at least also contained in all other rows z 1  through z 5  of the tile Ka. If, however, a gray scale value were found in a row that does not coincide with a model dither cell, for example the extreme gray scale value G=0 (which means that no comparison row fits), then the value G=0 is assigned to this tile and it is no longer taken into consideration in the continuing process. 
   It is to be pointed out that the comparison rows Vz 1  through Vz 4  that have dither cells arranged in them in row form in turn contain picture element rows. In practice, the comparison of the comparison rows Vz 1  through Vz 4  to dither cells of the tile Ka is implemented on the basis of such picture element rows. Instead of comparison rows Vz 1  through Vz 4 , comparison tiles can also be employed whose size corresponds to the tile Ka and that contain a plurality of rows of dither cells. 
     FIG. 7  schematically shows a part of a page with tiles K to which gray scale values G=0, 1, 2 or 3 have been assigned. This assigning of gray scale values corresponds to a marking. The value G=0 means that no standard gray scale value was capable of being found for this tile, for example because such a tile does not generally contain dither cells with the lowest gray scale value (gray scale value G=1). 
   The tiles shown in  FIG. 7  are investigated tile row by tile row from left to right. Tiles having the value G=0 are not taken into consideration. Tiles lying next to one another having the same value, in this case the gray scale value G=3, are combined to form a rectangle Ra. In this way, larger areas of the page S having the same gray picture elements are combined. 
   Another modification in the search for rectangles provides that tiles that contain the same gray scale value (for example, gray scale value G=1) or a higher gray scale value (for example, gray sale value G=2 or G=3) are combined to form rectangles. The rectangle Rb shows the combination of 6 tiles that contain the gray scale values G=1 or G=2. Such a combination is expedient since dither cells with gray scale value G=2 have black-colored picture elements at the same positions as dither cells with gray scale value G=1, as already explained above. Overall, the gray scale value G=1 is assigned to the rectangle Rb, i.e. the lowest gray scale value G within the rectangle Rb. An excess of black-colored picture elements thus remains for the tiles with gray scale value G=2. This excess derives when dither cells with gray scale value G=1 are subtracted from all dither cells of the rectangle Rb in a subtraction step. These excess picture elements are compressed as image raster data according to the traditional standard compression method and are transmitted. 
     FIG. 8  shows the further treatment of the rectangle Ra within the image raster data of a page S. The position of the upper left corner of the rectangle Ra is identified as characteristic data for the rectangle Ra. Further, the height and width as well as the gray scale value G, the gray scale value G=3 in this case, are identified. These characteristic data are entered into a list of rectangles that lists further rectangles of the type of rectangle Ra. Subsequently, the rectangle Ra within the page S is cleaned, i.e. the value G=0 is assigned to the tiles of the rectangle Ra, so that these tiles are no longer taken into consideration in the search for further rectangles. The analysis of the page S has been ended, when the entire page S has been searched for tiles that can be combined to form rectangles and the characteristic data of the rectangles have been entered into the list. 
   The list of rectangles can be sorted in a next step. Rectangles having a decreasing number of tiles are given a decreasing rank in the list. Only those rectangles whose number of tiles exceeds a predetermined value are then selected from this list and their characteristic data separately transmitted. In this way, only gray picture elements that are contained in large-area rectangles are separately transmitted. The efficiency of the compression method is further enhanced as a result thereof. A decision to discard a rectangle can already be made during the process of forming rectangles in this case and, thus, the discarded tiles can be made available in the formation of other rectangles. The efficiency of the compression method is further enhanced as a result thereof. 
   Another modification can be comprised therein that the number of rectangles of the list is limited to a predetermined value. Since the transmission of the characteristic data of rectangles means an additional compression outlay and this outlay increases given a great number of rectangles, it is expedient to limit this number. 
     FIG. 9  shows a further modification of the formation of rectangles whose boundaries need not coincide with tile boundaries. When, as shown in  FIG. 7 , tiles have been combined to form rectangles, then the boundaries of these rectangles coincide with tile boundaries. When, however, dither cells with the same gray scale value as those within a rectangle adjoin such a rectangle in terms of row or column and have the same minimum gray scale value G as the dither cells of the rectangle, then these dither cells can be co-incorporated into the rectangle. The height and width of the respective rectangle must then be correspondingly increased. The boundaries of the rectangles are thus enlarged to the same extent as the identical gray raster actually occurs as a rectangle in the original of a page S. The efficiency of the inventive compression method is further enhanced in this way. 
     FIG. 9  shows three rectangles Rc, Rd, Re with solid bold-face lines. The rectangles Rc and Re have a gray scale value G=2. The rectangle Rd has a gray scale value G=4. Dither cells whose gray scale values G agree with those of the respective rectangles adjoin the rectangles Rc, Rd, Re. The individual rectangles Rc, Rd and Re can be correspondingly enlarged in their respective height and length. Over and above this, it is possible to combine all enlarged rectangles to form a single rectangle Rf. This rectangle Rf then has the minimum gray scale value of all enlarged rectangles Rc, Rd and Re, namely the gray scale value G=2. In  FIG. 9 , Rd is a sub-rectangle with G=4 within the overall rectangle Rf with G=2. The overall gray scale value is G=2, this being transmitted as characteristic datum for the rectangle Rf. It is also possible to employ an inverse presentation for which the gray scale value G=4 is transmitted as a characteristic datum for the rectangle Rf. During further processing of the image data, the reduced gray scale value G=2 is then to be taken into consideration for the further rectangles Rc and Re. 
     FIG. 10  shows another modification wherein the size of the dither cells and the row length as well as the column length of a tile are taken into consideration. Usually, dither cells have an 8×8 or a 10×10 picture element matrix. The bit width of the registers of the computer hardware employed for realizing the inventive method is preferably suited as row length for a tile. Row lengths of 8, 16, 32, 64 or 128 are customary. An additive combination of the row lengths is likewise possible, for example an overall row length composed of the combination of 8 bits+32 bits=40 bits, etc. Since the row length of the dither cells need not coincide with the row length of the tiles, it is expedient that the aforementioned comparison rows Vz 1  through Vz 4  have a length corresponding to the smallest common multiple of the row length of the tiles and the row length of the dither cells D. In the example of  FIG. 10 , the dither cells D have a row and column length of 10. The tiles, for example the tile K 1 , have a row and column length of 32 bits, i.e. the tile K 1 —in terms of length and width—comprises 3 dither cells as well as 2 columns or, respectively, rows of the next, adjacent dither cells. The smallest common multiple of row length of the tile K 1  and row length of the dither cell D amounts to 160 bits. It is therefore expedient to equip the comparison rows Vz 1  through Vz 4  with a row length of 160 bits. Given a register width of 32 bits, a comparison within a tile row of 5 tiles, i.e. 16 dither cells, can be simultaneously implemented in this case with the assistance of 5 double word operations. Such double word operations can be executed very fast with the assistance of registers. 
     FIG. 11  shows the principle of the invention employed for compressing and transmitting the image raster data. The image raster data BD of a page S generated by the RIP module are analyzed according to the aforementioned method steps and rectangles R 1  and R 2  with identical dither cells are thereby identified. These two rectangles R 1  and R 2  on the page S are blanked out from the overall image raster data BD, for example with a subtraction method. The remaining image raster data E are transmitted according to a traditional standard compression method, for example according to the FAX G4 compression method. However, other compression methods are conceivable, for example those that work according to the run length encoding method. The characteristic data of the rectangles R 1  and R 2  are separately transmitted, whereby a list L of rectangles is preferably employed and only these list data are transmitted. At the receiver side, the data that have been transmitted according to the standard compression method as well as the image raster data BD of the rectangles R 1  and R 2  are re-compiled. In this way, not all image raster data BD of the page S need be compressed according to the standard compression method and transmitted, but only the data E, i.e. with the exception of the rectangles R 1  and R 2 . It is to be pointed out that not all gray picture elements with identical dither cells need be combined to form rectangles. For an improved reduction of the data stream to be transmitted and of the processing time in the compression, it suffices when approximately 80 to 90% of all identical dither cells are covered. 
     FIG. 12  shows a block diagram of a schematic illustration of the inventive method or, respectively, of the inventive system. In a computer  10 , an RIP module RIP generates image raster data from language elements of the printer language POSTSCRIPT PS that are investigated in the computer  10  with a corresponding computer program according to the above-described analysis steps, these corresponding to a filter function in a raster filter. Using a diskette  11 , the computer program can have been loaded into the computer  10  on a permanent storage (hard disk) or into a volatile main memory (RAM). The image raster data that still remain per printed page after subtraction of the identified rectangles are compressed in the computer  10  according to the FAX G4 compression method and are packed according to the data formats IOCA and/or SPDS. The data format IOCA is described in the IOCA reference manual, “Image Object Content Architecture” Reference, 4 th  Edition (August 1993) SC31-6805-03, International Business Machines Corporation. The data format SPDS is described in the SPDS reference manual “SPDS”, Edition of 11.94, U 9737-J-Z247-3, Océ Printing Systems GmbH. Both documents are herewith incorporated by reference into the disclosure of the present application. 
   The data with a high packing density are stored in a datafile Da in a volatile or permanent memory, particularly in the computer. The list of rectangles determined in the filtering steps is packed with high packing density according to the data format SPDS and is likewise stored in the datafile Da. The generation of the compressed data can be accelerated with the assistance of the filter function, i.e. an abbreviated time is needed for the editing of the data. The printer accesses the datafile Da, whereby the data volume to be transmitted to the printer is small due to the high packing density and the high informational content. An adequately great quantity of data can thus be transmitted with a given data transmission rate. This means that the printer, as a high-performance printer with a high printing output, is adequately supplied with image raster data even given printed pages with many identical, gray picture areas and can print interruption-free at a high printing speed. The printer likewise Q contains a computer or a controller with a decompression program with the assistance of the data of the datafile Da are converted into further-processable print data. Preferably, the decompression program contains an OR function with which the image raster data of the rectangles and the image raster data that are transmitted in a traditional way are combined to form common image raster data. 
     FIG. 13  shows the structure of a traditional system. The image raster data of the RIP module are compressed exclusively according to a standard compression method, for example the FAX G4 compression method and are then stored in a datafile Da as data with a high packing density, being stored with the assistance of the IOCA and/or SPDS data format. Since many data arise for each identical gray picture element, the memory requirement for the datafile Da is very high. Likewise, the compression algorithms for compressing the corresponding image raster data for identical gray picture elements are extensive and time-consuming. During printing, the printer accesses the data in the datafile Da. Since a considerable data volume is to be processed but the data transmission rate from the datafile Da to the printer is limited, it can occur that the printer can print faster than data can be offered via the datafile Da. A brief stoppage of the printing event occurs in this case. Such a start-stop operation, however, is extremely disruptive for a high-performance printer; it performance capability is only inadequately used. Typically, such a high-performance printer has a printing output greater than 400 pages DIN A4 per minute given a resolution of 600 dpi (dots per inch). 
   Using a flowchart,  FIG. 14  schematically shows method steps as were already explained in  FIGS. 3 through 6 . These method steps serve the purpose of determining a gray scale value G=0 or a higher gray scale value G for a tile K. As mentioned, a gray scale value G=0 is assigned when a row z of a tile K contains no gray picture element, i.e. no dither cell.  FIG. 14  only shows critical steps that serve the purpose of explaining the fundamentals of the method. Required intermediate steps, for example the definition of initial conditions for run variables, the modification of run variables, etc., are familiar to a person skilled in the art active in this field. 
   After the start (step S 1 ), a check is carried out in step S 2  whether all tiles K of a page have already been processed. When this applies, then the entire step sequence for a page is ended in step S 3 . When all tiles K have not yet been processed, then a check is carried out in the following step S 4  whether the appertaining tile K has its first row z 1  coinciding with the comparison row Vz 1  that, as mentioned, has the lowest gray scale value G=1. When no coincidence is found, then the extreme gray scale value G=0 is assigned to this tile K in step S 5  and a branch is made to step S 6 . In this step S 6 , the tile run variable k is incremented by 1 and a branch is made to step S 2 . 
   When it is found in step S 4  that the first row z 1  coincides at least with the comparison line Vz 1 , then a determination is made in the next step S 7  as to whether there are comparison rows Vz 2 , Vz 3  and Vz 4  with a higher gray scale value G that coincide with the first row z 1 . The comparison row Vzi (i is a run variable) with the highest gray scale value G is then further-employed. The following step S 8  is explained later. In the step S 9  following thereupon, a determination is made for every row zj (wherein j is a run variable for the row number) of the same tile K as to whether it coincides with the comparison row Vzi found in step S 7 . When this does not apply, then the run variable i is lowered by 1 in step  510 , i.e. a comparison row Vzi with the next-lower gray scale value G is employed for the further analysis. Whether the extreme gray scale value G=0 is reached is then found in step S 8 . When this applies, then a branch is made to step S 6  and the next tile K is analyzed. The gray scale value G=0 means that a white picture element to which no gray scale value can be assigned occurs within the tile K. 
   In step S 11 , the gray scale value G for the appertaining tile K is then determined. This gray scale value G states that dither cells of at least this gray scale value G are present in all rows zj of this tile. A higher gray scale value G is possible. 
   Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.