Abstract: A dispersed-dot stochastic dither array is provided for rendering halftone images having excellent visual quality and are created by a minimum density variance method. By minimizing the variance in the number of dots within each local region of the image, a smooth and dispersed distribution of dots may be obtained. For color printing, a separate threshold array is generated for each of the color planes, however, the stochastic screens are interlocked so that the threshold arrays are generated while considering the other color threshold arrays. In this manner, a blue noise distribution may be produced by the individual arrays as well as by any combination of the individual arrays. When generating a single threshold array for a color plane, a particular criterion is used to determine where the next threshold value should be located, and the selection of a threshold location in each array considers the criterion for all the threshold arrays being generated.

Abstract: A method for color error diffusion using semi-vector quantization is proposed that simplifies error diffusion calculations. When three color separations are used, the two separations with the most visually perceptive color output are compared with one another to determine their output colors, while the third, least visually perceptive separation, is treated separately. A correction term may be applied to the third separation based on the color output of the first two separations to insure uniformity of ink coverage. This method yields results close to vector quantization and also minimizes fluctuation in the two most visually perceptive output colors. Of course, this method may be used with more than three color separations. For example, when four color separations are used, the least visually perceptive color is processed differently than the three other color separations.

Abstract: An idealized clustered screen is characterized spot clustering which is optimized so that clusters are about the same size and shape as they grow and are approximately equally distanced though an image. Pixels are swapped within the screen to vary its appearance. An optimization process iteratively checks a quality metric based on the optimum conditions, as pixels are swapped until an optimum screen appearance is obtained.

Abstract: A technique for reducing image artifacts is described. The technique includes utilizing a clustered-dot dither matrix having a plurality of layers with each of the layers arranged in one of a plurality of complementary patterns within the dither matrix. By using a dither matrix which generates layers of dots in complementary patterns, image banding artifacts and ink bleeding characteristics resulting from a printer generating a halftone image using a clustered-dot dithering technique are reduced.

Abstract: A method is disclosed for rendering monochrome or colored continuous tone images by a system having restricted continuous tone rendering capabilities, such as electrographic printers capable of printing more than two density levels on each addressable micro dot. A method for preferred halftone dot growth is described, starting from isolated dots arranged along base lines and auxiliary lines, evolving to high density lines along the base lines and approximating full continuous tone rendition for high density regions. Preferred arrangements for the orientation, spacing and absolute location of the base lines and auxiliary lines are disclosed and a method to generate preferred arrangements. A moire free combination of three halftone images is described for the reproduction of color images. A method to select a restricted set of energy levels to obtain linear reflectance response is disclosed. Halftone cells are arranged in supercells to improve the density resolution of the system.

Abstract: There is a provided method for use with a printing system in which a second halftone pattern is selectively substituted for a first halftone pattern. The printing system preferably buffers one or more of print jobs and places each of the one or more print jobs in a print ready format. The printing system further includes a processing system in which the placing of the one or more print job into the print ready format is performed, as well as a halftone substitution subsystem for selectively operating on one or more halftone patterns which reside in one of the one or more print jobs.

Abstract: In a "hi-fi" color printing system, wherein colorants of hi-fi colors beyond the regular CMYK primary colorants are available, a main gamut obtainable with the CMYK colorants only is mutually exclusive with at least one extended gamut in which a hi-fi colorant is used and a complementary one of the CMY colorants is excluded. Because the main and extended gamuts are mutually exclusive, no more than four colorants are used in any part of the image, and no more than four halftone screens need be used to obtain any desired color.

Abstract: Method and apparatus for forming halftone images which enables image recording on a process film without reducing the size of the recording area beyond that which is necessary. The method and apparatus is preferred particularly in the preparation of a master printing plate such as a printing plate which includes a photosensitive member having an organic silver salt. According to the method and apparatus, an image recording apparatus propagates error data when there exists a restriction on the minimum recording size and thereby preserves halftone concentration and reproduces intermediate gradation of darkness with stability.

Abstract: A method of color halftoning using stochastic screening includes clustering, performing error diffusion and applying an adaptive criteria to change the cluster size according to the variation of the image color values. The method incorporates features from the traditional amplitude modulated (AM) digital halftoning methods, along with the advantages of frequency modulated (FM) techniques, recently introduced into the raster image processors of high resolution phototypesetters.

Abstract: Halftone correction systems are provided for producing dot overlap corrected halftone images at a digital printer. In the first embodiment, a system is provided for halftone correction on a black-and-white digital printer. The system first calibrates the black-and-white digital printer with seven test patterns to provide halftone correction information. Multi-level digital image signals representing a continuous-tone image are then received and stored. The system next halftones these multi-level digital image signals to provide an overlap corrected halftone image responsive to the halftone correction information at the digital printer. Halftoning is performed using an overlap corrected ordered dithering screen, which was generated based upon the correction of the partial overlap of adjacent dots in an uncorrected ordered dithering screen. In the second embodiment, a system is provided for producing dot overlap corrected color halftone images on a digital color printer.

Abstract: A bitmap-based digital color printing method and system is described which automatically detects when a black object is commanded to be printed, and changes the Bit Block Transfer (BitBlt) method so that the resultant composition of the black color at each point in the bitmap is responsive to the amount of non-black colorants already existing in the background color at that position on the page. For many printing systems the resulting prints exhibit fewer visual defects. Single-color black objects printed on top of a color field are printed as process black responsive to the background color. Therefore, they exhibit a greater density and gloss and show fewer problems caused by the misregistration of separations or other printer defects, such as white fringes around the black objects. They also avoid problems caused by excessive ink coverage differences between foreground black and background color.

Abstract: A method is disclosed for producing a colour image by printing on a sheet a plurality of monochrome images on top of each other. The monochrome image (37) is composed of microdots (36), having an address (x,y). Each microdot (36) is represented by a pixel (32). The pixel (32) carries information about the address (x,y) and an image signal .sup.I x,y that corresponds with the density to be printed on the microdot (36). This image signal is given as 8 bit signal data for example. The microdots (36) are partitioned by a screen (40) in identical screen cells (33), composed of M microdots (36). Each microdot is associated with a pixeltonecurve (34). The image signal .sup.I x,y is transformed to a bitmap signal B by the pixeltonecurve (34). The image signal .sup.I x,y is transformed to a bitmap signal B by the pixeltonecurve (34) corresponding to the microdot (36). The bitmap signal B is represented by 2 or 4 bit resulting in important memory savings for the bitmap signals.

Abstract: A method and apparatus for selectively depositing ink on a recording medium is described. The method and apparatus implement a technique in which portions of a clustered-dot dither matrix are identified and no ink is deposited under any circumstances in the so-identified portions. The portions are organized into a relatively large dispersed distribution matrix within the clustered-dot dither matrix.

Abstract: A halftone screen and method for generating halftones representing a continuous tone image, wherein the number of halftone dots per unit area varies non-proportionally with the tone values utilized as inputs to the screening process. The method incorporates an implicit gradation compensation as part of the frequency modulation halftoning itself to correct for the additional tone gain due to recording, plate processing and/or printing, as well as aesthetic corrections. This implicit compensation has the advantage that no external compensation is required, thereby maintaining the number of tone values which can be rendered on a digital system. Finally, the implicit gradation compensation enables the combination of page elements rendered according to either "conventional" or "frequency-modulated" halftoning techniques with appropriate tone-matching.

Abstract: A screening system comprises at least one screen of which the phase is tone or image level dependent. This tone dependent phase shift influences the rosette structure and can be used to improve the neutral balance and colour fidelity in multi-colour reproduction and to minimise the visibility of such rosette structures in highlight and shadow areas.

Abstract: A method for producing a half-tone image faithfully representing a continuous-tone original includes a step of forming in the image a pattern of substantially continuous, substantially parallel lines at a frequency of between about 100 and 400 lines per inch, such that the width of the lines at a given point in the image varies in relation to the optical density of at least one color component at a corresponding point in the continuous-tone original. The method may be used for color separations with each separation produced by scanning at a different angle. The method may be implemented using photomechanical or electronic scanning techniques.

Abstract: Method and apparatus for producing a grey-shade stencil in which a pattern which is bounded by edges is transferred, with a grey-shade grid superimposed on it, into a radiation-sensitive base. The grid is extended continuously over edges of the pattern which come to rest on one another, in order to avoid strips being formed in the area of mutually adjacent edges of pattern reproductions.

Abstract: An improved method of line screen printing. Instead of screening the image with only one set of lines, the image is also screened with a second set of lines, which are substantially orthogonal to those of the first set. (FIG. 3). The screening is accomplished with two types of cells, horizonal cells and vertical cells. The pixels are classified according to their distance from the nearest line in the horizontal cells. This process is repeated with respect to the vertical cells. (FIG. 5). Different tone curves, which map input pixel values to output pixel values, are associated with different pixel classifications. In general, pixels closer to lines have tone curves that map to higher output values. (FIGS. 6 and 7). Because each pixel is in both a horizontal cell and a vertical cell, each pixel has two values from the tone curves. For each pixel, its two tone curve values are combined to obtain a greyscale value for that pixel.

Abstract: An idealized clustered screen is characterized spot clustering which is optimized so that clusters are about the same size and shape as they grow and are approximately equally distanced though an image. Pixels are swapped within the screen to vary its appearance. An optimization process iteratively checks a quality metric based on the optimum conditions, as pixels are swapped until an optimum screen appearance is obtained.

Abstract: A system and method for halftoning multi-level pixels preferably uses a threshold array divided into two or more classes. The classes are ordered in a visitation order. Each class contains at least one element. A halftone cell comprised of a plurality of pixels corresponds to the threshold array. As the intensity level for the halftone cell increases, the pixels corresponding to the elements in the first class are halftoned by fractional values of pixels until all of the pixels in that class reach saturation. As the intensity of the halftone cell increases, none of the pixels corresponding to the elements in any other class change state. Once the pixels corresponding to the elements in the first class are saturated, the pixels corresponding to the elements in the second class are halftoned by fractional values of pixels. In more general terms, the next class in the class visitation order is not addressed until the pixels corresponding to all of the elements in the previous class are saturated.

Abstract: An image processing apparatus includes an input unit for inputting image information, and a conversion unit for comparing the input image information with a plurality of threshold values and converting the input image information into data of three or more levels, wherein the conversion unit converts the input image information so as to form a dot at the central area of a screen mask if the input image information is low in density, and converts the input image information so as to form dots at the whole area of a screen mask if the input image information is middle and high in density.

Abstract: Each image area in a total image is assigned with a screen angle mode Ms indicating whether screen angles of magenta and cyan separations are to be adjusted or not. Screen angle data representing screen angles of magenta and cyan are produced according to the screen angle modes Ms. A dot signal for recording a halftone image of each of the magenta and the cyan separations is generated by comparing screen pattern data of a certain screen angle specified by the screen angle data with image data for each color separation. Accordingly, the present invention effectively prevents a moire in any image area included in a printing image.

Abstract: A multi-level image output device performs correction of pixel values which have undergone halftone processing to compensate for non-linearities. This correction is carried out by means of a tonal reproduction transfer function, which linearizes the output values from a halftone processor before they are applied to an image generator, such as a laser modulator. By rectifying non-linear intensity of individual pixels, images can be generated with a smoother grayscale gradient. The function of the post-halftone correction can be combined with a modulator, to achieve an economy of components which make up the output device. As a further feature, a plurality of post-halftoning correction devices can be provided. One of these devices is selected to correct the data values produced by halftone processing, in dependence upon the values of neighboring pixels in an image.

Abstract: Apparatus and method for screening an image for half-tone reproduction by image reproduction apparatus which includes a marking device, preferably a laser beam, which is operative to mark a medium along a multiplicity of parallel marking lines. The method includes the steps of providing a digital representation of density values of an original image, determining whether to rotate dot shape information in accordance with the dot shape symmetry and density, rotating dot shape information if the symmetry and density criteria are met, storing said dot shape information in its rotated on nonrotated form, said dot shape information representative of a screen function; and extracting said dot shape information from storage for said density values in the form of information that designates on and off points for each marking line. The apparatus includes means for carrying the steps of the disclosed method.

Abstract: A method for generating watermarks in a digitally reproducible document which are substantially invisible when viewed including the steps of: (1) producing a first stochastic screen pattern suitable for reproducing a gray image on a document; (2) deriving at least one stochastic screen description that is related to said first pattern; (3) producing a document containing the first stochastic screen; (4) producing a second document containing one or more of the stochastic screens in combination, whereby upon placing the first and second document in superposition relationship to allow viewing of both documents together, correlation between the first stochastic pattern on each document occurs everywhere within the documents where the first screen is used, and correlation does not occur where the area where the derived stochastic screens occur and the image placed therein using the derived stochastic screens becomes visible.

Abstract: An image rendering system for generating discrete tone image data for representing continuous tone image data to be rendered by a rendering device. The continuous tone image data includes a plurality of pixel values each associated with a pixel. The rendering device renders a colorant in a plurality of colorant densities. The image rendering system comprises a threshold array, preferably stochastic, a colorant density array, and a colorant density supply arrangement. The threshold array comprising a plurality of threshold values. The colorant density array comprises a plurality of array elements, each associated with one of said pixel values and one of said threshold values. Each of the array elements identifies a particular one of a plurality of colorant density values each associated with one of the colorant densities rendered by said rendering device.

Abstract: A technique for screening an image for half-tone reproduction by image reproduction apparatus including a marking device which is operative to mark a medium along a multiplicity of parallel marking lines including the steps of providing a digital representation of density values of an original image, storing dot shape information representative of a screen function in a line-by-line, input density-by-input density arrangement and extracting the dot shape information from storage for the density values in the form of information that designates on and off points for each marking line. Apparatus for carrying out the technique is also disclosed.

Abstract: A spread spectrum digital screening mask and a method and system for digital screening a continuous tone image with the spread spectrum digital screening mask. The mask is characterized in the frequency domain by a function in magnitude independent of angle within a band of frequencies between a minimum frequency and a maximum frequency. The mask may be further characterized by its thresholded binary planes at each of a plurality of predetermined threshold values. The binary planes are characterized in frequency domain by magnitudes primarily distributed within the band of frequencies. For each binary plane, the number of pixels having one of the binary values divided by the total number of pixels in the binary plane equals a fraction determined by the predetermined threshold value of the binary plane divided by the maximum threshold value.

Abstract: In a method for creating at least five to seven color separations for input into a raster image processor for creation of at least five screen separations for use with at least a five color press to create at least a five color printed page, a scanned digitized image is created represented by at least three digitized color image signals. These at least three digitized color image signals are input into a digital color separation computer. The digital color separation computer is used to create at least five single color unscreened image separations in the form of computer files. The computer files are input into a raster image processor to create at least five single color screen separations in the form of electronic signals. The electronic signals are then fed to a film recorder to create at least five single color screen separation films used in conjunction with a five color press to create the at least five color printed page.

Abstract: The dot patterns which may be used to reduce dot gain and moire effects in halftone images involve printing areas which have inwardly curved edges and variable elongation. Colored images having screens which use these patterns may be printed with the screens at 45.degree. relative angular separations. Alternatively the dot patterns may be regarded as reversed variable oval patterns with varying elongation of the generally oval shaped non-printing areas.

Abstract: A method for applying FM screening, also known as stochastic screening, to a digital image in a computer. The method permitting manipulation and editing "on-screen" of a CMYK image having a random dot pattern (FM screening) applied. The method permitting the "on-screen" editing of each of the four color separations (cyan, magenta, yellow, and black) of the CMYK FM screened image while viewed as a single composite image. The "on-screen" editing of the CMYK image performed and viewed before the image is directed to an imaging device.

Abstract: In multi-gradation level processing performed by using an ordered dither scheme, offset information between one dither matrix value and dither matrices is set, and all multi-gradation level threshold values are calculated from the basic dither matrix values and the offset information. Multi-gradation level processing is performed by comparing an input image with the calculated threshold values. With this operation in the multi-gradation level processing performed by the ordered dither scheme, all the dither threshold values required for the multi-gradation level processing are generated from the basic dither matrix information and the offset information.

Abstract: A system and method usable in producing a display image limited to a lesser number of colors from an input image having a greater number of colors. The invention is particularly useful in a computer system utilizing a video monitor capable of simultaneously displaying a selected number of colors stored in a lookup table. The system and method receive data representative of respective colors at a multiplicity of individual pixels in the input image. This input image data is then temporarily stored in an input buffer. An appropriate processor receives the input image data and performs image quantization thereon. Specifically, the processor first determines the selected colors to be included in the display image. Next, the processor determines which of the selected colors respectively correspond to individual pixels of the display image.

Abstract: Halftone screens are generated for each separation in accordance with the goal of avoiding overlapping whenever possible. Initially, the black separation is halftoned, generating a dot pattern with a number of ON pixels and OFF pixels in accordance with the area density of the black separation. Next, a first color separation is halftoned, setting a number of the previous OFF pixels to ON. Then, if any white pixels remain, the second color separation is halftoned, setting a number of the previously OFF pixels to ON. If during the processing of the second and third separations, it is determined that no OFF pixels exist to be turned ON, a second layer of color is started, respectively superimposed over the first layer. The third color separation is halftoned in reverse fill-in order, setting a number of the previous OFF pixels to ON. Each layer is started and arranged so that the additional colors forming the dot pattern are not placed on any black areas.

Abstract: A method and apparatus perform vector error diffusion and result in reduced graininess of a color halftone image representing a continuous tone image. The method controls the output colors while reducing the visibility of the error diffusion pattern inherent in forming a bilevel image. Specifically, the method involves influencing the dot pattern correlation using vector error diffusion and takes into account the effects caused by the interactions of dot patters for all of the different color components.

Abstract: A digital halftoning method for creating a gray-scale monochrome reproduction of a source image includes subdividing the source image into regions based on a trace of the space filing curve, further subdividing each of the regions into equally sized pixels, scanning the source image along the trace of the space filling curve to visit all pixels of the source image, calculating an average image intensity for each region, establishing a range of pixel cluster intensity levels based on a number of pixels in a cluster, and generating a dot pattern for each of the pixel clusters by activating individual ones of the pixels to an 0N state in accordance with a sequential order along the trace of the space filling curve.

Abstract: A K separation screen comprises a grid of dots located at fixed positions, with the screen angle for each color, including black, maintained at 45.degree.. All of the primary color dots are printed at a predetermined screen frequency at predetermined locations. The black dots are also printed at a fixed screen frequency at predetermined locations. However, the black dots are printed in a different screen phase from the other colors, in particular, the phase offset is equal to one-half the screen frequency. Apparatus and a method are disclosed for generating the K separation screen using a cluster dot dithering array and offsetting the array pattern used to generate the colored halftone dots from the array pattern used to generate the black halftone dots.

Abstract: A method of halftoning over a region of output pixels by defining large halftone cells corresponding to regions of contiguous pixels and partitioning the larger halftone cells into smaller halftone cells is disclosed. The number of levels of gray determined by the number of pixels within the larger halftone cell while resolution is defined by the size of the smaller halftone cells within the larger halftone cells. A predetermined level of gray determines the number of pixels to darken within the larger halftone cell and the pixels are darkened within the smaller halftone cell according to a spot function. The larger halftone cells are partitioned into smaller halftone cells such that the larger halftone cell has exactly one center smaller halftone cell.

Abstract: An image signal processing apparatus for converting an image data signal input by an image input terminal into another image data signal suitable for an image output terminal. In the apparatus, a quantity of noise of a spatial frequency characteristic difficult to visually perceive is generated in accordance with a level of the input image data signal, and the noise generated is superposed on the input image data signal. In handling a color image data signal, the noise is superposed on the color image data signal in a regional color space difficult to visually perceive.

Abstract: A half-tone mask is characterized by an N.times.M array of mask value locations associated with at least a portion of the pixels of the digital image. The size of the mask array may contain a relatively small number of pixel locations, which is applied spatially to mutually contiguous subsets or sub-arrays of pixels of a relatively high resolution digital image. Although the mask may also be made large enough to encompass the entire image of interest, respective threshold values of the mask may be computed `on the fly`, so as to obviate the need to prestore all of the threshold values of the mask. Each of the locations of the mask array is assigned a respectively different grey scale/half-tone conversion threshold value. These threshold values are distributed among the locations of the mask array such that as one proceeds through the mask array, the next sequential location is specified in accordance with fractional parts of integral multiples of a two-dimensional analog of the Golden Mean.

Abstract: Halftoned frequency modulated color separations are produced from continuous-tone multicolored artefacts. The continuous tone values of the pixels of the scanned artefact are simulated by once and for all fixed halftone dot patterns within the pixels (tiles) of the halftoned reproduction. The dot patterns of the different tone levels are different for each individual color separation but are connected with each other through criteria specifying an overlapping dot ratio in conjunction with a translation prohibition, thereby substantially suppressing moire, disturbing effects of mis-register (e.g. color shifts) and graininess that is often seen in the highlights and midtones of reproductions with stochastic distribution of the halftone dots.

Abstract: Each image area in a total image is assigned with a screen angle mode Ms indicating whether screen angles of magenta and cyan separations are to be adjusted or not. Screen angle data representing screen angles of magenta and cyan are produced according to the screen angle modes Ms. A dot signal for recording a halftone image of each of the magenta and the cyan separations is generated by comparing screen pattern data of a certain screen angle specified by the screen angle data with image data for each color separation. Accordingly, the present invention effectively prevents a moire in any image area included in a printing image.

Abstract: An exposure signal representing a halftone image is generated by executing tone correction on at least either of an image signal and n pieces of dot pattern signals, and by comparing the image signal with the n pieces of dot pattern signals after the tone correction. The tone correction characteristics are set to compensate for a difference in dot gain characteristics between dot types. The halftone image is accordingly reproduced with the compensated dot gain characteristics.

Abstract: A color-image forming apparatus includes: an electrostatic latent image holder; electrostatic latent image forming unit for forming a plurality of electrostatic latent images on the image holder; a scanning unit for scanning the electrostatic latent image holder periodically at predetermined timings; developing devices for developing each of the plurality of electrostatic latent images by toners of each of different colors respectively, and forming thus developed toner image on the image holder; a simultaneously transferring device for simultaneously transferring the plurality of toner images onto a transfer medium; fixing device for fixing the toner image on the transfer medium; and timing-setting unit for setting a timing at which said scanning means starts scanning at each of slow scanning pitches in the direction of fast scanning for each of the plurality of colors; wherein the timings which are allotted to each of the colors are different each other so that the starting position of the scanning of each c

Abstract: A method of, and an apparatus for, recording halftone dots on a photosensitive film by selecting halftone dots having different minimum repeating units in response to respective original image components. SPMs (103a to 103d) previously store screen pattern (SP) signals having different minimum repeating units, while address generation controllers (AGCp to AGCs) previously store address signals corresponding to the respective SP signals. Pattern selection signals (PPS) are inputted in response to respective original image components and the address signals are selected in correspondence thereto and inputted in the SPMs (103a to 103d), thereby reading the SP signals. Comparators (104a to 104d) compare pixel signals (DSa to DSd) expressing density levels of respective pixels forming a total image with the selected SP signals, to output halftone dot signals (DOTa to DOTd) in response to the result of this comparison.

Abstract: A system and method for representing a multi-color, halftone image on a multi-color, continuous-tone device can be configured to employ an approximation technique for determining areas of overlap regions produced by adjacent device spots in a printed halftone image based on a halftone device model. The approximation technique enables consideration of the effects of a variety of halftone device characteristics such as addressability, device spot size, and color values on the appearance of a printed halftone image. The system and method can be configured to employ the approximation technique in a manner that also enables consideration of the effect of misregistration between color separations on the appearance of a printed halftone image. As an alternative, the system and method can be configured to employ a geometrical abstract that effectively centers device spots on the corners of addressable units.

Abstract: A method for generating a screened reproduction of an image including the steps of providing a representation of an original having input density values representing the grey levels of various locations of the original for a given color separation, defining a desired irregular screen function for the image and writing screen segments in a line by line fashion, wherein each screen segment is a line whose length and location is determined by employing the input density values of the original and the desired irregular screen function.

Abstract: A color space data conversion formula corresponding to the input and output conditions can be selected from basic profiles which consists of a plurality of the conversion formulas for color reproduction processes. A plurality of parameters or a plurality of variable conversion formulas can be selected from subprofiles, in accordance with the input and output conditions and on the ground of the color space data conversion formula selected from the basic profiles. Basic profiles or subprofiles can be added or modified. The color image data are processed highly accurately to obtain color images of high reproducibility, for thereby facilitating the setting of the input and output conditions.

Abstract: A color image processing apparatus for sequentially inputting image data which corresponds to each of a plurality of color components in units of pixels of a predetermined quantity, delaying, with respect to a reference phase, at least the first color component or the last color component and sequentially forming image in the units of pixels of a predetermined quantity in accordance with the delay.

Abstract: The object of the invention is a process for generating and storing digitized density thresholds for defining the grid of a half-tone image. The density thresholds of a section of a grid are stored in a data store as stored words depending on a spot function. In the grid section, several spots with a number of stored words form a multiple grid spot and are periodically arranged beside and above one another. A sorted sequence is found for the stored words of the section in accordance with the spot function. Density thresholds are allocated to the stored words depending on the position in the sorted sequence. The actual number of stored words is found individually for each spot in the section. The sorted sequence is generated according to the current spot function. Before position-dependent thresholds are allocated to the stored words, a center of concentration and possibly final point correction is performed for the spot concerned.