阅读说明：本技术 光栅图像处理器 (Raster image processor ) 是由 R·巴尔特尔斯 于 2020-04-29 设计创作，主要内容包括：一种光栅图像处理器(RIP),用于通过阈值图块(TT)将连续色调图像(CT)数字二进制半色调成半色调光栅图像(RT),用于能够在多个基板(S-(1)、S-(2)、…、S-(M))上印刷的印刷设备；其中,所述处理器包括输入字段,其中的信息关于要使用的基板；存储器,在其上存储有用于生成规则铺瓦的半色调网点的多个阈值图块(TT-(1)、TT-(2)、…、TT-(N))；以及阈值选择器,其能够基于提供给输入字段的信息来选择所述多个阈值图块(TT-(1)、TT-(2)、…、TT-(N))中的所述阈值图块(TT)。(A Raster Image Processor (RIP) for digital binary halftoning of a continuous tone image (CT) into a halftone raster image (RT) by Threshold Tiles (TT) for enabling multiple substrates (S) 1 、S 2 、…、S M ) Printing equipment for printing; wherein the processor includes an input field in which information regarding a substrate to be used; a memory having stored thereon a plurality of Threshold Tiles (TT) for generating regularly tiled halftone dots 1 、TT 2 、…、TT N ) (ii) a And a threshold selector capable of selecting the plurality of Threshold Tiles (TT) based on information provided to the input field 1 、TT 2 、…、TT N ) The Threshold Tile (TT) of (1).)

1. A Raster Image Processor (RIP) for digital binary halftoning of a continuous tone image (CT) into a halftone raster image (RT) by a single Threshold Tile (TT) for enabling multiple substrates (S)₁，S₂，…，S_M) Printing onA printing device of the brush;

wherein the processor comprises:

-an input field, wherein the information is about the substrate to be used;

-a memory on which a plurality of Threshold Tiles (TT) for generating regularly tiled halftone dots are stored₁、TT₂、…、TT_N）；

-a threshold selector capable of selecting the plurality of Threshold Tiles (TT) based on information about the substrate provided to the input field₁、TT₂、…、TT_N) The single Threshold Tile (TT) of (a).

2. Processor (RIP) according to claim 1, wherein the processor additionally comprises:

-an ink usage predictor; and

wherein the threshold selector is capable of selecting a single Threshold Tile (TT) additionally based on a predicted amount of ink from the ink usage predictor;

wherein the amount is by a plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) The threshold patches in (1) are computed from a halftone raster image of a continuous tone image (CT).

3. Processor (RIP) according to claim 1 or claim 2,

wherein the printing apparatus is capable of using a plurality of inks (I)₁、I₂、……、I_Q) Printing is performed, and

wherein the processor further comprises another input field in which information about the ink to be used; and

wherein the threshold selector is capable of determining a single Threshold Tile (TT) additionally based on the information about the ink provided to the further input field.

4. A processor (RIP) according to claim 3, wherein the regularly tiled halftone dots are arranged according to a screening frequency of more than 100 lines per inch.

5. Processor (RIP) according to claim 4, wherein regularly tiled halftone dots are arranged according to a screening angle selected from the group consisting of 0 °, 7.5 °, 15 °, 22.5 °, 75 °, 45 °, 67.5 ° and 82.5 °.

6. Processor (RIP) according to claim 4 or claim 5, wherein a plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) Including a threshold tile set for generating a halftone image, wherein at least a portion of all halftone cells are halftone dots comprising:

(i) image pixels arranged as a first cluster or a plurality of clusters together representing a first path; and wherein the image pixels have a feed point (1003, 2003); and

(ii) non-image pixels arranged as a second cluster or clusters together representing a second path.

7. The processor (RIP) of claim 6, wherein said first path is a space filling curve inside said halftone cell.

8. Processor (RIP) according to claim 6, wherein the first path is a spiral.

9. Processor (RIP) according to claim 4 or claim 5, wherein a plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) A threshold tile set for generating a halftone image is included, wherein at least a portion of all halftone cells include at least 2 image clusters defined as mutually separated groups of more than 4 adjacent image pixels.

10. Processor (RIP) according to claim 9, wherein the partial halftone cells have a quarter portion representing a relative image density which is at least twice the relative image density represented by the halftone cell as a whole.

11. Processor (RIP) according to claim 9 or claim 10, wherein said portion comprises at least 10% of all halftone cells having a relative image density of less than 50%.

12. Processor (RIP) according to claim 9 or claim 10, wherein the portion comprises at least 4 clusters of images.

13. A pre-press workflow system comprising a raster image processor according to any one of the preceding claims, wherein the pre-press workflow system further comprises:

-means for transferring the halftone raster image (RT) to a digital printing press or platemaking machine for obtaining a printing plate.

14. The prepress workflow system of claim 12, wherein the printing plate is a lithographic printing plate or a flexographic printing plate.

15. Use of a raster image processor according to any of claims 1 to 12 for converting an electronic document defined in a Page Description Language (PDL) into a continuous tone image (CT) and binary halftoning the image in a halftone raster image (RT) for printing.

Technical Field

The present invention relates to the field of Raster Image Processors (RIP) for digital halftoning of continuous tone images and printing images, in particular by means of lithographic or flexographic printing machines.

Background

Printers and digital printers are not capable of varying the amount of ink or toner applied to a particular image area, except through digital halftoning, also known as dithering (dithering). Digital halftoning is the process of rendering the illusion of a continuous tone image with multiple dots (also called halftone dots). The digital image produced by digital halftoning is referred to as a halftone raster image or screen (screen). Both multilevel and binary halftone methods are known. Halftone dots generated by the binary method are composed of pixels representing image data and pixels representing non-image data.

Binary Digital Halftoning is a well-known technique explained in detail by Robert Ulichney in his book "Digital halfning" (MIT press, 1987, ISBN 0-262-.

Another overview of digital halftoning is disclosed in the article "Recent trends in digital halftoning" (Proc. SPIE 2949, imaging science and display technology, (2.7.1997); doi: 10.1117/12.266335), wherein multi-level digital halftoning is also explained.

The digital halftoning method is carried out by a raster image processor (abbreviated as RIP). The processor (RIP) converts an electronic document defined mainly in a Page Description Language (PDL) into a continuous tone image (CT) and halftones the image in a halftone raster image (RT) for printing. A well-known Page Description Language (PDL) is PostScript^TM(PS) and "page description format" (PDF). The processor is primarily a pre-press workflow system (such as Apogee from the manufacturer AGFA NV)^TM) The kernel of (1).

AM (amplitude modulation) screening is a widely used centralized halftone dot-ordered dithering technique in which the size of halftone dots is modulated to represent different densities of an image. The halftone dots are tiled regularly according to screening frequency and screening angle. AM halftone dots are typically circular, oval, diamond, or square.

Other types of regularly tiled halftone dots, such as from Esko, have also been recently available^TMSuch as disclosed in US2007/0002384, wherein the halftone dots have a shape with one or more rings, or more recently a multi-centered halftone dot as disclosed in EP 3461116. Both involve complex mathematics and difficult calibration and therefore cannot be implemented on the basis of extensive installation of pre-press workflow systems with limited data storage and/or processing capabilities.

Another type of regular halftone dots is disclosed in application WO2019/081493 a1 (AGFA NV), i.e. spiral halftone dots, which may be implemented with a single threshold tile, such as in AM screening.

US2008123146 a1 (RICOH COMPANY LTD) discloses a dither matrix for use in halftone processing for converting input image data having M input halftone levels into output image data having N (M > N > 2) output halftone levels for using a multi-level halftone process. In fig. 10, there are three such halftone methods, which are implemented in the raster image processor according to the image object type in the literature: a character dithering step of characters; a step of figure dithering of lines and figures and a step of error diffusion of images.

Print quality is achieved by controlling the halftoning process in a Raster Image Processor (RIP). However, to achieve high quality printing on selected substrates, it is important to use appropriate threshold tiles to halftone the continuous tone image (CT). For multiple substrates (S)₁、S₂、……、S_M) Printing apparatus for printing, the operator selecting for halftoning according to experience and his own feeling, depending on the substrate to be printedThe threshold value tile. However, as discussed above, the number of halftone dot shapes and their variations are recently growing. When the processor can use the number of halftone dot shapes, it occasionally happens that an inappropriate halftone dot shape is selected for the selected substrate (S)_i) The continuous tone image (CT) above is halftoned. This results in a printed image that is not provided on the selected substrate as intended.

Disclosure of Invention

It is an object of the present invention to provide a Raster Image Processor (RIP) for digital halftoning of a continuous tone image (CT) into a halftone raster image (RT) for enabling application across a plurality of substrates (S)₁、S₂、...、S_M) Printing equipment for printing. The processor has several advantages, such as providing a threshold selection method for automatically selecting a threshold tile appropriate for a selected substrate. If threshold value tile (TT)₁、TT₂、…、TT_N) Is large, the selection makes it easier for the operator, especially when the threshold tile determines a large number of halftone dot shapes that include variations in the shape, and especially when the shape includes variations that severely affect print quality. Whereby N is greater than 2.

This problem is solved by a method as defined in claim 1, wherein the processor comprises an input field in which information about a substrate to be used, preferably a substrate of the plurality of substrates; a memory having stored thereon a plurality of Threshold Tiles (TT) for generating regularly tiled halftone dots₁、TT₂、…，TT_N) (ii) a And a threshold selector capable of selecting the plurality of Threshold Tiles (TT) based on information provided to the input field₁、TT₂、…，TT_N) The Threshold Tile (TT) of (1). For example, the selected threshold tile may be selected because it gives less pattern, less/more gloss, less ink usage, and/or less light transmission when used for halftone and representing a continuous tone image on the selected substrate.

The processor apparently manages the plurality of threshold tiles stored in memory and selects the most efficient threshold tile therefrom for the selected substrate. The processor further having means for receiving a continuous tone image (CT); and means for storing the continuous tone image (CT) in another memory.

The regularly tiled halftone dots are preferably arranged according to a screening frequency of greater than 100 lines per inch, and more preferably according to a screening angle selected from the group consisting of 0 °, 7.5 °, 15 °, 22.5 °, 75 °, 45 °, 67.5 °, and 82.5 °.

In a preferred embodiment, the processor additionally comprises: an ink usage predictor; and wherein the threshold selector is capable of selecting a Threshold Tile (TT) additionally based on the predicted amount of ink from the ink usage predictor; wherein the amount is by a plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) The threshold patches in (1) are computed from a halftone raster image of a continuous tone image (CT). The amount can be described in units of milliliters or ink layer thickness.

The ink usage predictor is capable of predicting the amount of ink from a selected halftone raster image that should be printed on the substrate. The calculations are based on complex mathematics, as further disclosed in the examples. The selection of the most effective threshold tile may be the one with the lowest ink amount prediction.

The type of ink used to render continuous tone images also affects print quality, e.g., the ink that must be cured or the ink having high penetration in the selected substrate. If the printing apparatus is capable of using a plurality of inks (I)₁、I₂、……、I_Q) Printing, the Raster Image Processor (RIP) preferably further comprises another input field, wherein the information relates to the ink to be used, preferably an ink of said plurality of inks; and wherein the threshold selector is capable of determining the Threshold Tile (TT) additionally based on information provided to the further input field.

The preferred processor manages a plurality of threshold tiles stored in memory and selects the most efficient threshold tile therefrom for the selected substrate combined with the selected ink.

It was found that the specific halftone dots may vary more than AM screening to optimize the print quality on the selected substrate, such as to affect the amount of ink. In a preferred embodiment, the plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) Comprises that

a) A set of threshold tiles for generating a halftone image, wherein at least a portion of all halftone cells are halftone dots comprising:

(i) image pixels arranged as a first cluster or a plurality of clusters together representing a first path; and wherein the image pixels have a feed point (1003, 2003); and

(ii) non-image pixels (= pixels) arranged as a second cluster or clusters together representing a second pathType AFIGS. 17-24); and/or

b) A threshold tile set for generating a halftone image, wherein at least a portion of all halftone cells comprise at least 2 image clusters, an image cluster being defined as a mutually separated group (= o) of more than 4 adjacent image pixelsType BFIGS. 1-16).

The first path is preferably a space-filling curve within the halftone cell, or the first path is a spiral for generating spiral halftone dots. The spiral halftone dots and possible variants are also disclosed in PCT/EP2018/079011 (AGFA NV).

In a preferred embodiment, the halftone cells of the portion have a quarter portion representing a relative image density that is at least twice the relative image density represented by the halftone cells as a whole, more preferably the portion comprises at least 10% of all halftone cells having a relative image density of less than 50%, or preferably the portion comprises at least 50% of all halftone cells having a relative image density of less than 50%. The portion may include at least 4 image clusters.

The raster image processor is preferably adapted to halftone the continuous tone image (CT) into a halftone raster image (RT) by said processor by means of a selected Threshold Tile (TT) from the threshold selector.

Drawings

Fig. 1 shows 6 halftone cells in the upper half of the figure, each cell consisting of a 16x16 grid of pixels. These halftone cells are all generated from the same threshold array using the raster image processor of the present invention. The threshold array is the matrix shown in the lower half of the figure. Each halftone cell represents another relative image density, as indicated by the dot percentage shown below each halftone cell. The image pixels are represented by black areas and the non-image pixels by white areas.

Fig. 2 to 11 differ from fig. 1 only in that another threshold array is used in each figure.

Fig. 12 shows 6 tiled halftone cells, each cell comprising 4 sets of 3x3 image pixels.

Fig. 13 and 14 show the same 6-tiled halftone cell as the 70% halftone cell of fig. 3 and 4, respectively.

FIG. 15 shows a halftone cell including 3 image clusters designated by dashed lines A-C; the image pixels designated d-h do not form groups of more than 4 neighboring pixels and therefore do not comply with the definition of image clustering according to the present invention.

Fig. 16 shows 6 halftone cells in the upper half of the figure, each cell consisting of a 16x16 pixel grid, and a threshold array for generating these cells in the lower half. The threshold values in the array range from 1 to 16, as opposed to the values ranging from 1 to 256 in fig. 1-11.

Fig. 17 is an enlargement of a raster image as a result of the present invention. The raster image comprises archimedean spiral dots which have a dot coverage of 50% and which are regularly tiled in a square grid.

FIG. 18 is an enlargement of a raster image as a result of the present invention. The raster image comprises archimedean spiral dots which have 90% dot coverage and which are regularly tiled in a square grid.

FIG. 19 shows the multiple dot coverage produced by the threshold tiles as a result of the present invention.

Fig. 20 and 21: FIG. 20 shows an example of a threshold tile including thresholds from 1 to 256, which may be included in the present invention. Fig. 21 shows spiral screen dots generated by the threshold tile of fig. 20 for a halftone screen dot having a threshold of 22, which corresponds to a screen dot coverage of 8.6% (= 22/256).

Fig. 22 shows four spiral dots comprising (i) image pixels arranged as a first arc (200) as an arc or as a plurality of arcs together representing a first clockwise rotated spiral (100) as a spiral, and (ii) non-image pixels arranged as a second arc (201) as an arc or as a plurality of arcs together representing a second clockwise rotated spiral (101) as another spiral. The first arc and the first spiral have feed points, also referred to as inner ends (2003, 1003) and outer ends (2005, 1005).

Fig. 23 and 24: an enlargement of a raster image generated by the present invention is shown, the raster image comprising archimedean spiral dots, and additionally comprising curved halftone dots having non-image pixels.

Fig. 25 and 26 show a raster image processor (650) of a preferred embodiment of the present invention in which an electronic document (630) is converted into a continuous tone image (750). The raster image processor (650) includes a halftone screen (850) for halftoning the contone image (850) into a halftone raster image (950) by 1 threshold tile selected from N threshold tiles stored in a memory (890). The single threshold tile is selected by a threshold selector (870). The selection is based on information provided in an input field (871) regarding the substrate to be used. The halftone raster image (950) is ready for printing by the printing apparatus (50). Fig. 26 is a raster image processor (650) in which the selection is additionally based on information provided in another input field (872) regarding the ink to be used. The printing device (50) is then able to use multiple inks.

The threshold selector may include an ink usage predictor, wherein a number of threshold tiles of the memory (890) are used to halftone the contone image (750) into a halftone raster image whose ink usage is predicted. One of the smallest ink uses is then selected.

Detailed Description

Ink usage predictor

The ink usage predictor is a tool for predicting the amount of ink on a selected printing device that will be used for a halftone raster image (RT). The tool is typically a data processing system adapted to predict the quantity, or a computer program comprising instructions for predicting the quantity. The computer program is preferably stored on a computer readable storage medium.

After halftoning a continuous tone image (CT) into a halftone raster image (RT), the pixels of the raster image are analyzed and a certain amount of ink per pixel is determined by the ink usage predictor. The sum of the determined ink amounts per pixel determines the amount of ink required to print a copy of the contone image on the selected substrate by the printing apparatus. Further, the amount of each ink is determined for each non-image pixel by the ink usage predictor. This does not auto-zero because during or after printing, ink can flow to the non-image pixels until the ink is fixed on the selected substrate. Thus, pixels from the raster image (RT), and also the adjacent pixels of pixels, are analyzed to predict the flow of ink during or after printing until the ink is fixed on the selected substrate. For the present invention, non-image pixels and ink flow behavior in image pixels on a selected substrate are modeled by using one or more printed targets comprising a plurality of patches (patch) with regularly tiled clusters having image pixels, wherein the clusters are intermixed with clusters having non-image pixels. The size and/or shape of the clusters differs between the patches.

The model can be calculated by measuring the print density (e.g. by an XRITE 500 series spectrodensitometer) or optical density, mottle and/or ink layer thickness of patches printed on a selected substrate for predicting the amount of ink per pixel in the halftone raster image. For each substrate, a model can be calculated, since the ink flow (e.g. by absorbance) depends on substrate parameters such as structure, composition and optical properties. For example, with an optical microscope (e.g., a Scanning Electron Microscope (SEM)), the ink flow in the non-image pixels and the image pixels can additionally be determined, and the presence of ink can be measured. The amount of pigment used for the printed patches may also be determined (e.g., by extraction) to recalculate the amount of ink used for the printed patches by knowing the percentage of pigment in the ink used. The model may use the ink amount for regular tiled image pixels in the patch for prediction. The ink usage predictor determines from a pixel whether it has some similarity between the pixels of the patches and returns a recalculation of whether the ink amounts of the patches, or the similarities, match. The pigment amount of ink used for a pixel can also be determined to recalculate the amount of ink used for that pixel (printed or unprinted) by knowing the percentage of pigment in the ink used.

The ink usage predictor may use an artificial neural network, which is a computing system inspired by biological neural networks that make up an animal brain, for its model. More specifically, the predictor is able to train itself through the artificial neural network to determine the amount of ink required to print a halftone raster image. By using previous measurements on printed targets, printed patches and/or printed pixels, the artificial neural network is trained to complete its task on any other halftone raster image (than the printed target), regardless of the threshold tiles and continuous tone image used, i.e. the task of predicting the amount of ink that should be used when printing on the selected substrate.

The use of artificial neural networks in the technical field of the present invention is known, for example, for color control and color management.

Raster image processor

Raster Image Processor (RIP) is a tool for digitally halftoning a continuous tone image (CT) or other file format, including PDF (portable document format), into a halftone raster image (RT) for use in a printing device. The means is typically a data processing system adapted to halftoning the continuous tone image (CT) or a computer program comprising instructions for digitally halftoning the continuous tone image (CT). The computer program is preferably stored on a computer readable storage medium. One example of such a raster image processor is the Harlequin Host render from Global Graphics.

A Raster Image Processor (RIP) may convert the halftone raster image (RT) into a format that can be understood by a printing device, such as an inkjet printer. In the present invention, the printing apparatus can be used for printing on a plurality of substrates (S)₁、S₂、…、S_M) Printing; and preferably, can use a plurality of inks (I)₁、I₂、…、I_Q) Printing, such as offset printing.

Halftone raster images (RT) are primarily well-known raster graphics file formats, such as TIFF^TM(tagged image File Format), RTL (raster transfer language) or BMP File Format: (Bitmap）。

Halftone raster image

In the present invention, a halftone raster image (RT) comprises regularly tiled halftone dots, which are generated by using threshold tiles (preferably, a single threshold tile). This generation is done by a halftone, also called halftone raster generator, which is part of the raster image processor.

The halftone raster image produced by the present invention is suitable for rendering a continuous tone image (CT), i.e., it creates the illusion of a continuous tone image (CT) on a printed copy. This requirement means that the screening frequency (i.e. the number of halftone cells arranged next to each other per length unit in the direction in which the maximum is generated) is higher than 40 lines per inch (LPI; 15.7 lines/cm), more preferably higher than 60 LPI (23.6 lines/cm), and most preferably higher than 100 LPI (39.4 lines/cm). If the screening frequency is below 40 LPI, the halftone dots become visible at a viewing distance (also called a reading distance), which is about 20 cm. Such low screening frequencies are typically used for artistic screening, which is used for decorative purposes, such as patterned illustrations, where individual dots are intended to be visible to the naked eye. Therefore, halftone raster images in which halftone dots are clearly visible at viewing distances are not suitable and should not be produced by the present invention.

The above-mentioned screening frequency defines the spatial frequency of the halftone cells included in the halftone raster image. As explained above, halftone cells are in turn composed of a grid of pixels, and their spatial frequency (referred to as resolution) is expressed as Dots Per Inch (DPI) or Pixels Per Inch (PPI). In case the halftone raster image is written by means of a scanning laser, e.g. on a film or a printing plate, the pixels are also called laser dots, and the resolution then refers to the number of laser lines per inch. The halftone raster image produced in the method of the present invention has a resolution preferably greater than 600 DPI, more preferably greater than 1200 DPI. Higher resolutions up to 9600 DPI may also be used, for example, for printing security features.

Since the plate can only transfer a single color, it is clear that the halftone raster image used in the method of the invention is a black and white image, which may represent a color choice for a multi-color printing process, e.g. one of the 4 basic colors in CMYK printing.

The quality of the printed image is preferably checked using color density measurements. The color density values can then be used as input parameters for the raster image processor of the present invention. The processor that generates the halftone raster image adjusts the image so that the quality of the printed image is improved in subsequent print runs and/or so that more ink is saved in subsequent print runs.

Half tone unit

The halftone raster image produced by the present invention includes regularly tiled halftone cells. The cells may be tiled along a triangular, rectangular or hexagonal grid, and more preferably along a square grid. Fig. 13 and 14 each show an example of a preferred embodiment in which 6 halftone cells are tiled into a square grid.

The halftone cells themselves are also comprised of a grid, more particularly a grid of pixels, which may be image pixels or non-image pixels. The pixels preferably have the shape of a regular polygon or a convex polygon, for example, a triangle, square, rectangle, diamond, or hexagon. The figure shows an example of a preferred embodiment in which the halftone cells are comprised of a grid of square pixels.

A halftone raster image that may be produced with one of the threshold tiles that the RIP of the present invention stores by way of it preferably comprises halftone cells having 2 or more image clusters, i.e., mutually separated groups (= 4 adjacent image pixels)Type B). In a more preferred embodiment, the halftone raster image comprises halftone cells having more than 2 image clusters, for example at least 3 or 4 image clusters, more preferably at least 5 and most preferably at least 6 image clusters.

In the definition of "image clustering" above, image pixels are considered to be adjacent if they share at least one edge of a polygon. Fig. 15 shows three such image clusters: cluster a consists of 7 adjacent image pixels; cluster B consists of 6 adjacent image pixels; and cluster C consists of 5 adjacent image pixels. Image pixels d, e, and f are in contact with another image pixel, but only through the corners of the square; since these pixels do not share edges, they are not considered to be adjacent, as defined above. According to the above definition, no group of 4 or fewer image pixels (such as group h) constitutes an image cluster.

FIG. 12 illustrates further refinement of the definition of image clusters as used herein. When tiling halftone cells, a group of image pixels in one halftone cell may be connected to another group of image pixels in another halftone cell with one or more adjacent edges. Fig. 12 shows an example in which the group of image pixels designated 3a is connected to the group designated 3 b. According to the invention, such groups should not be considered as separate clusters, but together represent a single cluster. As a result, the halftone cells represented in fig. 12 each contain only 3 image clusters.

In contrast to conventional AM dots, which represent the same dot percentage, image clustering as used in the present invention allows printed images of the same image density to be obtained with less ink. The large number of variations in the halftone dots makes it possible to optimize print quality and/or minimize ink usage. The reason for this advantage is not fully understood, but engineers have systematically measured that the printing runs of the method according to the invention consume significantly less ink than the printing runs of a conventional AM screen-exposed plate in which the same raw image is used. When compared to FM screens, it was observed that higher run lengths could be obtained because the clusters were larger than the FM microdots and therefore less prone to wear on the press. FM screens consist of groups of single image pixels or four (2 x 2) image pixels, which are more susceptible to degradation than the image clustering used in the present invention.

Without being limited by the underlying mechanism, it is presently assumed that having halftone cells that are clustered image pixels as described above, absorbs a thinner ink film on the ink-receiving areas of the plate and/or provides better spreading of the ink film into the blank (non-image) areas between image clusters when compared to conventional AM halftone dots representing the same relative image density. Ink saving effects have been observed for various images and various printing plates. Often an ink savings of about 10% is obtained. Ink and paper are the major cost factors for printers, so even a few percent reduction in ink consumption represents a high cost savings. Optimization of the halftone raster image in relation to the substrate to be printed, for example by adjusting the number, size, shape and/or distribution of image clusters in the halftone cells, may result in even more ink savings, ranging from 10% to 20% compared to conventional AM screening.

Less ink consumption provides the additional advantages resulting therefrom, e.g., faster drying and/or less energy consumption of drying equipment (such as curing units and ovens). Faster drying is particularly advantageous for printing on uncoated plastic foils and newspaper printing. Better spreading of the ink also reduces ink offset (setoff), i.e., the transfer of ink from one printed copy to the back of another copy that is on top of it, e.g., in a printer transport tray. The transmission of light (also known as strike-through) is also reduced, whereby the image becomes visible on the back side of the substrate, e.g. a thin ink-absorbing substrate for printing newspapers. For all these reasons, the method of the invention is particularly advantageous when carried out on a perfecting press, i.e. a press that allows simultaneous printing on both sides of a substrate in a single pass through the press.

In a preferred embodiment, the image clusters are not randomly distributed over the halftone cells, but are locally concentrated, such that the image clusters together mimic conventional AM dots, and the method maintains the advantages of an AM screen as much as possible. The image clusters may be concentrated in, for example, one quarter of a halftone cell. As a result, the quarter portion represents a higher relative image density than the other portions of the halftone cell. More preferably, one quarter of the halftone cell has a relative image density that is at least twice the relative image density represented by the halftone cell as a whole. Fig. 5 shows an example of such an arrangement: the 8x8 pixels around the center of the cell define a quarter, indicated by a thick line, which has a much higher image pixel density than the cell as a whole. The higher density image pixels should not necessarily be located near the center of the cell: fig. 3 and 4 show different local densities of image pixels, however when the cells are tiled next to each other they represent the same halftone image, as shown in fig. 13 and 14, respectively.

The halftone raster image generated by the raster image processor from the present invention with the selected threshold tiles may contain a combination of different types of halftone cells, e.g., halftone cells having multiple image clusters, as defined above, which are combined with conventional halftone cells, e.g., AM halftone cells, where all image pixels are grouped in a single cluster. One or more portions of the image may also be represented by an FM screen. In a highly preferred embodiment of the invention, the highlights and the intermediate tones of the image, i.e. the subset of all halftone cells in the image representing a relative image density of less than 50%, are entirely composed of halftone cells having 2 or more image clusters as defined above. In another embodiment, only a portion of the halftone cells representing the highlight and halftone of an image contain 2 or more clusters of images. The fraction may be as low as 5%. Preferably, said fraction is at least 10%, more preferably at least 25% and even more preferably at least 50%.

Substrate

The substrate on which the halftone raster image may be printed may be of any kind, e.g. plastic films or foils, release layers, textiles, metal, glass, leather, hides, cotton, and of course various paper substrates (light, heavy, coated, uncoated, cardboard, etc.). The substrate may be a rigid workpiece or a flexible sheet, web or sleeve. Preferred flexible materials include, for example, paper, transparent foil, bonded PVC sheets, etc., which may be less than 100 microns thick, and preferably less than 50 microns thick. Preferred rigid substrates include, for example, rigid board, PVC, cardboard, wood or ink receivers, which may be up to 2 centimeters in thickness, and more preferably up to 5 centimeters in thickness. The substrate may also be a flexible web material (e.g., paper, bonded vinyl, fabric, PVC, textile). A receiving layer (e.g., an ink receiving layer) may be applied to the substrate to provide good adhesion of the reproduced image to the substrate.

The quality of the printing on the substrate depends on several factors. With the selected threshold tiles from the present invention, print quality becomes better. The several factors are for example:

-the amount of paper sizing (paper sizing) additive in the substrate;

-smoothness, mass density and caliper variation of the substrate;

strength and dimensional stability of the substrate;

-porosity of the substrate;

moisture and curl of the substrate.

However, there are also some paper whose optics and appearance are factors that affect the print quality, such as brightness, whiteness, opacity, specular gloss.

Threshold tiles

According to the invention, the halftone raster image is generated by a threshold tile, preferably a single threshold tile. Digital halftoning by means of threshold tiles (sometimes also referred to as threshold matrices or threshold arrays) is known in the art. When used for multi-color printing, the number of threshold tiles is preferably the same as the number of color channels in the original contone image.

Digital halftoning using a threshold tile typically means that the original image is sampled into cells mapped on the threshold tile. The local density value of the original image is then compared to each value in the array (adjusting the density range of the original image so that it equals the range in the array, if necessary). The output pixel is set to 0 (= OFF (OFF)) if the original density value is below the threshold of the pixel. Otherwise, if the density value is equal to or exceeds the threshold value, the output pixel is set to 1 (= ON (ON)). These steps are repeated for all cells in the image.

The size of the array (i.e., the number of pixels per halftone cell) may depend on various factors, such as the resolution of the image composer and the desired quality of the printed image. The arrays are preferably arranged as squares (n x n size) or rectangles (m x n, where m > n). 1-16 show examples of square threshold tiles sized 16x16 locations, where each location contains a threshold within a particular range. In these particular examples, the threshold range (1-256) is equal to the number of locations in the array (16 x 16); in other embodiments, such as the example shown in the graph CL16, the range of values may be lower than the number of locations, and then each individual value may appear at multiple locations of the array.

To produce higher relative image densities, the threshold tiles are designed in such a way that the number and/or size of the image clusters increases in correspondence with the corresponding densities of the original images. For example, FIG. 1 shows a cell representing a relative image density of 25% with 7 image clusters whose size grows with increasing image density. Fig. 3 shows a 10% cell with 5 image clusters, where both the size and number of clusters grow at higher image densities. Although the image density of the halftone cell is increased by adding image pixels, it is preferable to keep non-image pixels together in a non-image cluster, the non-image pixels being defined as mutually separate groups of adjacent non-image pixels. By keeping the number of non-image clusters in the image shadow as low as possible, the ink can spread to a higher extent than when the non-image pixels are isolated or distributed over multiple clusters. In this way, ink may also be preserved in the shadow areas of the image.

In a preferred embodiment using spiral halftone dots, the relative image density may be increased by increasing the length and/or thickness of the first spiral (which includes the image pixels), as shown in fig. 6; or by inserting image pixels inside the second spiral; or by a combination of any of these methods. In the shadow of the image, more image pixels are added to the halftone cell in such a way that the ink channel formed by the second spiral (which includes non-image pixels) shrinks and/or becomes thinner, as shown in fig. 6 and 9.

In a preferred embodiment, a plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) Is that

a) A threshold tile for generating a halftone image, wherein at least a portion of all halftone cells are halftone dots comprising:

(i) image pixels arranged as a first cluster or a plurality of clusters together representing a first path; and wherein the image pixels have a feed point (1003, 2003); and

(ii) non-image pixels (= pixels) arranged as a second cluster or clusters together representing a second pathIn the case of the type A,FIGS. 17-24)(ii) a And/or

b) A threshold tile for generating a halftone image, wherein at least a portion of all halftone cells comprise at least 2 image clusters, an image cluster being defined as a mutually separated group (= n) of more than 4 adjacent image pixelsType BFIGS. 1-16). Preferred embodiments and advantages of type B are described in the heading "halftone cell".

For both types (type a and type B) many variations can be generated, but it was found that they have to be fine-tuned depending on the selected substrate and/or the selected ink. Both types have similar advantages, such as creating a thinner layer of ink on the selected substrate.

The substrate is also a substrate type in the present invention. The ink is also an ink type in the present invention.

For clusters having a plurality of clusters representing first paths togetherType AThe first path may be a portion of a space-filling curve (such as a Hilbert curve or a Peano curve) in the halftone cell.

All clusters that together constitute the first path are preferably connected to each other such that the first path represents a continuous line following the path. The first path may also contain isolated non-image pixels, or may include discontinuous clusters, such that the first path is interrupted by white space at one or more locations. In this embodiment, the white space separating adjacent clusters of the first path may be considered as a projection of the second path into the first path. Such a protrusion of the second path may completely cut the first path into discrete clusters, or not completely cut, so that the first path is not interrupted but locally reduced to a lower thickness.

In a preferred embodiment, the first cluster is an arc (200) and/or the first plurality of clusters is a plurality of arcs, which together represent a first spiral (100) as the first path. Hereby, the second cluster is another arc (201) and/or the second plurality of arcs is a plurality of arcs together representing a second spiral (101) as a second path, preferably along said first path. Such dots will be referred to herein as "spiral halftone dots". The image pixels are represented by the black areas in the figure. The non-image pixels define the non-printing areas and correspond to the empty spaces left in the dots as represented by the white areas in the figure. The two dots on the left hand side of fig. 22 have low dot coverage (low percentage of image pixels) and represent the highlights of the image, while the two dots on the right hand side of fig. 22 have high dot coverage and represent the shadows of the image. The first spiral as the first path is the preferred embodiment, but an intersecting path as the first or second path is also part of the invention. The first and second paths may also be other than spirals.

A first path, preferably a spiral, grows in length from the feed point (2003, 1003) (also referred to as the inner end) along the path, preferably a spiral direction, to the outer end (2005, 1005). Generally from the intermediate tone to the shadow, the path (preferably a spiral) may overlap with the adjacent halftone dots (fig. 19). If the first path is a spiral, the growth is also defined by the starting angle of its bending or winding. This is in contrast to AM halftone dots that extend a single cluster to a larger single cluster.

In the intense light of the image, the number of image pixels per dot is low, so that they cannot form a complete winding of the first spiral, but only a portion thereof, which is called "first arc". The empty space partially enclosed by the first arc may also be considered another arc, which is referred to herein as a "second arc". The number of image pixels per dot is high in the intermediate tones and shades of the image, so that they can form a convolution of one or more "first spirals", thereby also defining a "second spiral" of non-image pixels defined by the empty spaces between the convolutions of the first spiral (see, e.g., fig. 19).

Without being bound by theory, it can be observed that when the printed image is enlarged, the shape and size of the printed ink bolus is less affected by uncontrolled spreading of the ink, because excess ink printed by the first cluster or first path may flow into the empty space corresponding to the second cluster or second path when the ink bolus is pressed onto the substrate, e.g., by a printer. But the diffusion is dependent on the substrate and/or the ink. The white space defines an ink channel that can accept ink printed from the first cluster/path, thereby providing a means for controlling ink spread.

For type B, multiple clusters of images within a halftone cell are preferably grouped together within a virtual circle or virtual ellipse. Examples are illustrated in fig. 2 and 3, where the image clusters are positioned within a virtual circle that preferably grows depending on the local density values of successive images, such as in an AM halftone dot.

Threshold selector

A threshold selector is a tool for selecting threshold tiles based on one or more conditions. The tool is typically a data processing system adapted to select a threshold value from a plurality of threshold values, or a computer program comprising instructions for selecting the threshold value. The computer program is preferably stored on a computer readable storage medium.

In the present invention, the plurality of Threshold Tiles (TT)₁、TT₂、…、TT_N) Stored in a memory and the one or more conditions are based on information provided to an input field, the information in the input field being related to the substrate to be used. The number of threshold tiles is preferably greater than 5, more preferably greater than 10.

The input field from the present invention is preferably part of a Graphical User Interface (GUI) shown on a computer display such as disclosed in US9058105B2 (international business machines corporation). The input fields are preferably GUI features such as list boxes, combo boxes, and/or editable text fields that give the RIP user a number of ways to enter or select desired information. The input field may be part of a dialog box from the RIP.

Pre-press workflow system

The prepress workflow system is a tool for managing digital documents, preferably in a Page Description Language (PDL), such as Postscript^TM) Defining, wherein said document is ready to be printed on a printing device, the printingThe brush preparation is such as an offset printing machine. The tool is typically a data processing system adapted to manage and prepare the document for printing, or a computer program comprising instructions for managing and preparing the document for printing. The computer program is preferably stored on a computer readable storage medium. An example of such a prepress workflow system is Apogee from AGFA NV^TM。

In a preferred embodiment, the pre-press workflow system comprises the raster image processor from the present invention and further comprises means for transferring the halftone raster image (RT) to a digital printer or platemaker to obtain a printing plate. The printing plate is preferably a lithographic printing plate or a flexographic printing plate.

Printing plate

The printing plates of the present invention are obtained by exposing a halftone raster image onto a photosensitive or heat-sensitive material called a printing plate precursor. The plate precursor may be positive or negative. The positive plate precursor has a coating that accepts ink in the non-exposed areas and does not accept ink in the exposed areas after exposure and development. The negative plate accepts ink in the exposed areas and does not accept ink in the unexposed areas. The image pixels shown in the figures define the ink-accepting regions of the plate and thus correspond to the exposed or unexposed regions of the negative or positive plate precursor, respectively.

The plate of the present invention is preferably a lithographic printing plate or a flexographic printing plate. Lithographic printing plates are usually obtained by exposing a halftone raster image onto a photosensitive or heat-sensitive coating of a printing plate precursor by means of a scanning laser, preferably a violet or near-infrared laser, or another digitally modulated light source, such as a digital mirror device, LCD or LED display. After treatment of the exposed precursor with a suitable developer, a lithographic printing plate bearing a halftone raster image of the invention is obtained. The plate can then be mounted on a lithographic press, preferably an offset press, wherein ink is supplied to the plate which is then transferred to the substrate to be printed. Alternatively, the exposed precursor may be mounted directly on the press, i.e., without any prior liquid treatment or other development, and then development of the image may occur with the aid of the ink and/or fountain provided to the plate at the beginning of printing.

Flexographic printing plates are typically obtained by UV exposure of the photopolymer coating, typically using a UV lamp, through a mask, which may be a graphic film in contact with the photopolymer coating or an in situ mask present on top of the photopolymer coating. The mask is preferably obtained by exposing a halftone raster image on the film or on an in-situ mask layer by means of a scanning laser, preferably a near infrared layer.

Data processing system

The data processing system runs one or more computer programs, such as a raster image processor. The present invention includes such a data processing system capable of carrying out the halftoning method of the present invention and its preferred embodiments.

Part or all of the data processing system and/or its functional units or blocks may be implemented in one or more circuits or circuitry, such as integrated circuit(s) or as an LSI (large scale integration). Each functional unit or block of the data processing system may be individually fabricated as an integrated circuit chip. Alternatively, some or all of the functional units or blocks may be integrated and fabricated into an integrated circuit chip.

A software program, also called a computer program, which runs on a data processing system, is a program that controls a processor in order to implement the functions according to various preferred embodiments of the present invention. Thus, information processed by the data processing system is temporarily accumulated in the RAM at the time of processing. Thereafter, the information may be stored in various types of circuits in the form of ROMs and HDDs, and read out as needed, modified or written to by circuitry within or included in combination with the data processing system. As the recording medium storing the program, any of a semiconductor medium (e.g., ROM, nonvolatile memory card, and the like), an optical recording medium (e.g., DVD, MO, MD, CD, BD, and the like), and a magnetic recording medium (e.g., magnetic tape, floppy disk, and the like) may be used. In addition, by executing the loaded software program, not only the functions of the various preferred embodiments of the present invention are realized, but also the functions of the preferred embodiments of the present invention can be realized by processing the loaded software program in conjunction with an operating system or other application programs based on the instructions of the program.

Further, in the case of being a distributed pre-press workflow system or even a distributed raster image processor, the program may be distributed by being stored in a portable recording medium, or the program may be transmitted to a server computer connected through a network (such as the internet). Further, a part of the terminal device, the radio base station, the host system, or other devices, or all thereof may be implemented as an LSI, which is typically an integrated circuit. Each functional unit or block of the data processing system may be separately chiped, or a portion thereof or the entirety thereof may be chiped by being integrated. In the case where each functional block or unit is made into an integrated circuit, an integrated circuit controller that controls the integrated circuit is added.

Finally, it should be noted that descriptions referring to "circuitry" or "circuitry" are in no way limited to hardware-only implementations, and as one of ordinary skill in the relevant art will know and appreciate, such descriptions and representations of "circuitry" or "circuitry" include combined hardware and software implementations where circuitry or circuitry is operable to perform functions and operations based on any form of machine-readable program, software, or other instructions that may be used to operate the circuitry or circuitry.

There is no limitation on the location of the data processing system (18), which may be located at a pre-press office, at a printing device, or even at a third party location.

The digital connection to the data processing system may be made in any form. It may be a connection using optical fibers or a wireless connection, such as a wifi connection according to the IEEE 802.11 standard.