Abstract:
A method for enhancing appearance of a halftone image ( 204 ) for imaging on a flexographic plate ( 108 ) includes retrieving the halftone image from a data storage device ( 132 ); setting a minimal dot size value for printing; analyzing the halftone image with a computer ( 130 ); detecting areas ( 404 ) in the halftone image populated with a plurality of dots smaller than the minimal dot size value; replacing the plurality of dots with a reduced set of dots wherein each of the reduced set of dots are larger in size ( 504 ) than the plurality of dots and wherein each of the reduced set of dots maintains an original geometric characteristics of the plurality of dots; and saving the reduced set of dots.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned U.S. patent application Ser. No. 13/477,138 (now U.S. Publication No. 2013/0314750) filed May 22. 2012, entitled RESCREENING SELECTED PARTS OF A HALFTONE IMAGE, by Krol; the disclosure of which is incorporated herein. 
     FIELD OF THE INVENTION 
     The present invention relates to the field of graphic arts, printing and publishing technologies, specifically, to image reproduction systems characterized by limited numbers of levels of optical parameters. 
     BACKGROUND OF THE INVENTION 
     In graphic arts technology there is a need for reproduction of continuous tone images by an imaging device. The imaging device, for example, a laser based imager is capable of producing a limited number of levels representing optical parameter (in most common cases only two levels are used—black and white). This goal is achieved by applying a process called screening, wherein a gray tone value which represents a pixel to be screened in the original image is simulated by means of varying relative area covered by dark elements (pixels) as compared to light elements. 
     Different screening methods exist. In one of the most common, an area of an image is reproduced by subdividing it into equal, periodically repeated sub-areas called mesh cells, containing variable-size dark elements (printing dots, alternatively called halftone dots). Relative area coverage is defined as ratio of dark element area to a mesh cell area. Such screening method is commonly called Amplitude Modulation (AM) screening. Such screen, with a regular, usually square grid structure, is characterized by a screen period and a screen angle. The reciprocal of this period is called screen frequency or screen ruling. 
     Particular problems arise when reproducing characteristics of printing dots which are size dependant. Examples of such processes are flexographic, offset and xerographic printing processes. 
     In flexographic printing, the size of the smallest halftone printing dot on a printing plate that can be consistently reproduced on press is usually around 40 microns in size. Below this size, halftone dots tend to print unevenly, and may also include drastic increase in size and produce large blot artifacts, or not printing at all. For commonly used line ruling of 120 lpi and commonly used device resolutions of 2400 or 2540 dpi, 40 micron halftone dot corresponds to area coverage of 3%; for line ruling 1500 lpi, it corresponds to area coverage around 4%. This may result in discontinuity, with annoying artifacts in certain cases especially in the highlight parts of printed images. The high dot gain that is associated with flexographic printing process enhances the effect and exacerbates the problem. 
     In the offset printing process, the minimal halftone printing dot that can be reproduced consistently can often be as small as 10 micron. For lower quality paper and high-speed printing presses, for example, in newspaper printing, the same fundamental problem discussed above exists. A similar situation exists in electro-photographic printing, wherein minimal printing dot size is often defined by physical characteristics of toner particles. 
     One solution for the above problems is using Frequency Modulation (FM) screening techniques with controlled minimal dot size, so-called “green noise” frequency modulation, shown in U.S. Pat. No. 5,689,623 (Pinard), or with controlled midtone clustering, so-called “second order” frequency modulation, shown in U.S. Pat. No. 5,579,457 (Hall). While solving the problem of highlight region reproduction, frequency modulation introduces its own drawbacks. Relatively rough feature size needed for proper highlights reproduction, often leads to grainy appearance both in highlights and in the midtones areas. Additionally, high circumference-to-area ratio inherent for FM generated printing dots leads to significantly higher dot gain compared to AM halftone screening. Considering that flexographic printing process is already characterized by high dot gain, FM screening may lead to significant contraction of the dynamic range for printed images. 
     Another solution is known as the “double dot” technique or “Respi screen.” According to this technique, the extremes of tone scale, highlights and/or shadows, are rendered with halftone dots that are laid on a grid with the same angle, but with the frequency of the square root of the halftones in the rest of tone scale, thus halving the number of halftone dot and, consequently, doubling the size of each dot. This renders the transition area between extreme and main part of tone scale with halftone dots of two different sizes placed in checkerboard pattern. While moving the cutoff value for non reproducible part of the image farther to extreme parts of tone scale, it still does not completely solve the problem. Moreover, by introducing additional screen frequencies, such technique may produce highly undesirable moiré effects in multi-colored images in case of regions where part of color separations are in extreme parts of the tone scale and other separations are in non-extreme part of the color scale. 
     In order to overcome the deficiencies above stated, an approach was proposed by U.S. Pat. No. 5,766,807 (Delabastita et al.). This method is known as “hybrid screening.” In this approach, a “supercell” threshold matrix suitable for periodically tiling a plane is defined in such a way that it contains a plurality of locations for halftone dot centers and is filled with threshold values. When the matrix is used for screening a contone image, in the extreme parts of the tone scale halftone dot of predefined minimal size are produced, whereas in the remaining part of halftone dot centers no halftone dot are produced at all. This is performed in such a way that the area coverage for a whole supercell area corresponds to a tone value in the contone image. In other words, instead of modulating halftone dot size, below predefined dot percentage, dot size is kept constant but dot number is modulated as a function of tone value; accuracy of tone representation being defined by predefined minimal halftone dot size and count of halftone dot centers in supercell threshold matrix. 
     While free of most undesirable effects of previous solutions, this approach still has some problems. Notably, the supercell-based pattern is prone to grainy and “noisy” appearance; relatively rough quantization steps limited by number of halftone dot centers in a supercell threshold matrix may produce banding effects in vignette parts of image; “orphaned” and incomplete halftone dots still may produce undesirable “blot-like” artifacts; and supercell-based approach limits available number of screen angle/screen frequency combinations to those with rational tangent angles. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method for enhancing appearance of a halftone image for imaging on a flexographic plate includes retrieving the halftone image from a data storage device; setting a minimal dot size value for printing; analyzing the halftone image with a computer; detecting areas in the halftone image populated with a plurality of dots smaller than the minimal dot size value; replacing the plurality of dots with a reduced set of dots wherein each of the reduced set of dots are larger in size than the plurality of dots and wherein each of the reduced set of dots maintains an original geometric characteristics of the plurality of dots; and saving the reduced set of dots to a storage device. 
     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents in diagrammatic form a prior art imaging system adapted to expose flexographic plates; 
         FIG. 2  represents in diagrammatic form a prior art halftone image ready to be exposed; 
         FIG. 3  represents in diagrammatic form an enhanced halftone image, enhancing the image shown in  FIG. 2 ; 
         FIG. 4  represents in diagrammatic form a small dots area from a halftone image to be enhanced; and 
         FIG. 5  show in diagrammatic form enhanced area of small dots shown in  FIG. 4 , the small dots were replaced with a set of fewer dots wherein each new dot is larger in size. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, specific details are set forth in order to provide a thorough understanding of the disclosure. However, it will be understood by those skilled in the art that the teachings of the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the teachings of the present disclosure. 
     While the present invention is described in connection with one of the embodiments, it will be understood that it is not intended to limit the invention to this embodiment. On the contrary, it is intended to cover all alternatives, modifications and equivalents as covered by the appended claims. 
     In order to improve the reproduction characteristics of a halftoned image by of controlling halftone dot size and spatial distribution in extreme tone scale parts of said image, a halftone image already screened by means of traditional Amplitude Modulation (AM) screening is conceptually represented as rectangular array of black and white pixels. The present invention is not limited by any specific type, algorithm or method of AM screening and utilizes only two basic characteristics of AM screening, namely parameters of a periodic grid, its angle and its frequency represented by its line ruling, i.e. mesh, and derived parameter such as res/mesh ratio, equal to side length of single mesh cell in units of single pixel size. 
     In order to process the screened image  204  shown in  FIG. 2 . The screened image  204  is fetched from storage element  132  ( FIG. 1 ) by computer  130 . A “natural” system of coordinates is defined, where X axis runs along image lines and Y axis is perpendicular to Y axis, and unit of measure is single pixel size, and its “shifted” system of coordinates, where X′ axis and Y′ axis run along mesh grid directions that is, rotated by screen angle related to natural system of coordinates and unit of measure stated res/mesh ratio. 
     Additionally, DS 1  is defined as minimal reliably reproducible halftone dot size (in pixels) and DS 2 &gt;=DS 1  as halftone dot size (in pixels) corresponding to cutoff area coverage defining transfer from normal to “extreme” parts of tone scale. Separate halftone dots contained in the halftone image are identified and are processed in following the manner: if size of halftone dot is &gt;=DS 2 , this halftone dot is not modified. 
     If size of halftone dot DS is &lt;DS 2 , coordinates of its center of gravity Xc and Yc are calculated in natural system of coordinates, these coordinates are transformed into X′c and Y′c in shifted coordinates, and from these coordinates corresponding mesh cell indices Mx=floor(X′c) and My=floor(Y′c) are obtained. To these indices operator F(DS, mx, my) is applied, which returns 1 or 0 in such a way that average value (1/Nds)ΣF(DS, mx, my)*DS 2 , where sum is done for all halftone dots of size DS, is substantially equal to DS. Operator F can be analytical expression or simple lookup operation into predefined array. If operator F returns 0, we remove halftone dot by replacing all its pixels with pixels of an opposite color. If operator F returns 1, halftone dots are replaced with the halftone dots of the same shape and the same center of gravity, but of size DSn, where DSn is a value chosen out of array of values [DSn 1 , DSn 2 , . . . , DSnN] such as DSn 1 &gt;=DS 1 ,DSnX&lt;DSn(X+1) and average value (1/N)Σ(DSnX)==DS 2 . 
     This method ensures both consistent reproduction of halftone dots in regions with extreme values of tone scale via printing process and smooth, non-grainy, artifact-free appearance of said regions and transition regions from extreme values of tone scale to midtone ones. 
     In a preferred embodiment of the invention, an array of values [DSn 1 , DSn 2 , . . . , DSnN] contains four elements—DSn 1 , DSn 2 , DSN 3  and DSn 4  such that the size of element DSn 1  is equal to DS 1 , minimal reliably reproducible halftone dot size (in pixels); size of elements DSn 2 , DSn 3  and DSn 4  is DS 1 +1, DS 1 +2 and DS 1 +3 pixels, respectively. Cutoff size DS 2  in a preferred embodiment is defined as DS 1 +2 pixels. 
     Operator F(DS, mx, my) in the preferred embodiment is defined as a compare operation of a value derived from halftone dot size DS with a value from square lookup array B[N][N] where array size N is 2^n, n&gt;&gt;8. Lookup array B[N][N] is uniformly filled with integer values from 0 up to M=2^m−1, m&gt;=10 in such a way that a) array B[N][N] exhibits wraparound properties in both horizontal and vertical directions and b) when used as threshold array, results of threshold operation at any given level from 0 up to M exhibits blue-noise characteristics. 
     Screen angle is denoted as α, screen line ruling as mesh, image resolution as res and screen cell side length as r2 m=res/mesh. Given above these definitions, the preferred embodiment of the invention can be represented in pseudo code in following way: 
     
       
         
               
             
           
               
                   
               
             
             
               
                   While scanning a halftoned 1-bit image; 
               
               
                      identify halftone dots; 
               
               
                      assign each pixel to its respective dot. 
               
               
                      For each halftone dot 
               
               
                       If ( dot size in pixels DS &gt;= DS2) 
               
               
                         continue to next halftone dot without modification; 
               
               
                   else modify dot as described below: 
               
               
                   { 
               
               
                   // calculate center of gravity 
               
               
                   Xc = (1/DS)ΣXpix (summation for all pixels of halftone dot) 
               
               
                   Yc = (1/DS)ΣYpix (summation for all pixels of halftone dot) 
               
               
                   // transform coordinates into shifted system of coordinates 
               
               
                   X′c = Yc*sin(α)+Xc*cos(α); 
               
               
                   Y′c = Yc* cos(α)−Xc*sin(α); 
               
               
                   // Normalize to shifted coordinates units 
               
               
                   X′c = X′c/r2m; 
               
               
                   Y′c = Y′c/r2m; 
               
               
                   // Calculate mesh cell indices 
               
               
                   Mx = floor(X′c); 
               
               
                   My = floor(Y′c); 
               
               
                   // Calculate offset into lookup array 
               
               
                   Nx = Mx%N; 
               
               
                   Ny = My%N; 
               
               
                   // Calculate effective area coverage for current halftone dot 
               
               
                   Coverage = DS*100/(r2m*r2m); 
               
               
                   // Calculate compare value for thresholding operation with value // 
               
               
                   from lookup array B 
               
               
                   cutoffCoverage = DS2*100/( r2m*r2m); 
               
               
                   percentToKeep = 100.* Coverage / cutoffCoverage; 
               
               
                   arrayCompareVal = (100.-percentToKeep)*(M+1) /100; 
               
               
                 if (B[Nx][Ny]&gt; arrayCompareVal) 
               
               
                      then remove halftone dot; 
               
               
                      else 
               
               
                      { 
               
               
                      Stochastically choose dot size from array of dot size values 
               
               
                      Put halftone dot of chosen size with the same center of gravity 
               
               
                   as original halftone dot 
               
               
                      } 
               
               
                 } 
               
               
                   
               
             
          
         
       
     
       FIG. 3  shows the halftone image after enhancement. Numeral  304  shows an area of dots  304  which replaced the area of dots  204 . The replaced dots  304  include a smaller number of dots compared to dots  204  which originally populated the area in  FIG. 2 . Each of the newly created dots  304  are larger in size than the original dots  204  and maintain the geometric characteristics of the dots  304  in terms such as center of gravity and angle. 
       FIG. 4  shows a zoomed in area of small dots  404 , wherein  FIG. 5  shows dots  504  which are replacing dots  404 . It can seen much more clearly that the number of the replaced dots are fewer and each of the new dots  504  is larger than the original dots  404 , however the geometrical layout of the dots  504  is maintained to be similar to the those shown for dots  404 . 
       FIG. 1  shows an imaging system  100 . The imaging system  100  includes an imaging carriage  112  mounted on an imaging head  120 . The imaging head  120  is configured to image on a flexographic plate  108  mounted on a rotating cylinder  104 . The carriage  112  is adapted to move substantially in parallel to cylinder  104  guided by an advancement screw  116 . 
     The enhanced halftone  304  is delivered by controller  128  from computer  130  to imaging head  120  of imaging system  100 , and is further exposed on flexographic plate  108 , by imaging system  100  to form imaged flexographic plate. 
     While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     Parts List 
     
         
           100  imaging system 
           104  rotating cylinder 
           108  flexographic plate 
           112  carriage 
           116  screw 
           120  imaging head 
           128  controller 
           130  computer 
           132  data storage device 
           204  AM screened image before treatment 
           304  screened dots after treatment 
           404  area of small dots previously screened 
           504  area of screened dots after treatment