Abstract:
A method for forming an image on a flexographic media includes providing a screened image; locating transition points from data regions to non-data regions in said screened image; determining a distance between pixels in adjacent data regions for each transition point; if the distance is greater than a predetermined distance, modify said screened image to remove a shoulder of pixels in contact with the transition point; and forming the modified screened image on the flexographic media.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 13/765,755, filed Feb. 13, 2013, entitled SYSTEM FOR FORMING AN IMAGE ON FLEXOGRAPHIC MEDIA; by Krol; the disclosure of which is incorporated herein. 
     FIELD OF THE INVENTION 
     The present invention relates to methods and apparatus for image reproduction systems characterized by three-dimensional features imaged on a flexographic plate. 
     BACKGROUND OF THE INVENTION 
     In graphic arts technology, a number of well-established printing processes utilize image carriers with three-dimensional (3D) representation of data the most popular of them being flexographic printing, which uses flexible relief plates or sleeves. In a traditional flexographic prepress process with chemical etching there is no possibility of fine control of relief properties other than depth of relief. A flexographic prepress process, however, use direct laser engraving in place of chemical processes, which permits more detailed control. This enables a 3-D cross-section profile of relief elements to be used as controllable and regulated parameters that bear a direct relation to the quality of resulting image reproduction. 
     Specifically, the shape of cross-section profile directly influences quality of reproduction of small features such as highlight elements and/or file linework details, process tolerance to changes in pressure applied by plate and/or sleeve to substrate and other vital characteristics. A uniform 3D cross-section profile when applied uniformly on all image elements and features, however, results in sub-optimal performance. The reason for the sub-optimal performance is due to different behavior of the various image elements, such as halftone dots and/or linework elements which may differ in size. Several approaches were proposed to cope with this problem. 
     One approach is applying a cross-section profile of an imaged printing plate  500  including support layer  520  as shown in  FIG. 5 . Printing plate  500  shows imaged data elements of different sizes such as  512  and  504 . A linear slope cross-section to image elements is applied showing that slope angle is a function of image element size. A shallow angle slope  508  is applied on small printing area  504 , whereas a steep angle slope  516  is applied on large printing area  512 . 
       FIG. 6  shows another solution utilizing uniform, but more complex, 3D cross-section profile  600 . Profile  600  shows a printing area  604 , or a first engraved area situated on base  612  which is wider than printing area  604 , forming a two stage shoulders  616  resulting in a total relief size  608 . Another solution may be a combination of both of the above solutions. 
     While producing some improvement, all of the above approaches fail to decisively solve the problem because picture element size as a sole parameter is a suboptimal parameter for cross-section profile shape control. In fact, practical experience shows that local environment of specific feature and local gradient of ensuing relief pattern are more relevant parameters. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention a method for forming an image on a flexographic media includes providing a screened image; locating transition points from data regions to non-data regions in said screened image; determining a distance between pixels in adjacent data regions for each transition point; if the distance is greater than a predetermined distance, modify said screened image to remove a shoulder of pixels in contact with the transition point; and forming the modified screened image on the flexographic media. 
     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents in diagrammatic form of a digital front end driving an imaging device (prior art); 
         FIG. 2  represents in diagrammatic form the optical displacement sensor (ODS) together with the laser imaging head situated on the imaging carriage imaging on a plate mounted on an imaging cylinder (prior art); 
         FIG. 3  shows a halftone rendered image (prior art); 
         FIG. 4  shows a rendered image on flexographic plate (prior art); 
         FIG. 5  shows a cross-section of an imaged printing plate including a support layer (prior art); 
         FIG. 6  shows an engraved area situated on base which is wider than printing area forming a two stage shoulders (prior art); 
         FIG. 7  shows an engraved flexographic plate showing black and white areas; 
         FIG. 8  shows an engraved plate with two neighboring sections separated by a specified distance; 
         FIG. 9  shows an engraved plate with two neighboring sections separated by a specified distance wherein the neighboring shoulders are marked; and 
         FIG. 10  shows an engraved plate with two neighboring sections separated by a specified distance wherein the neighboring shoulders are cutoff. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous 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 alternatives, modifications, and equivalents as covered by the appended claims. 
       FIG. 1  shows a plate imaging device  108 . The imaging device is driven by a digital front end (DFE)  104 . The DFE receives printing jobs in a digital form from desktop publishing (DTP) systems (not shown), and renders the digital information for imaging. The rendered information and imaging device control data are communicated between DFE  104  and imaging device  108  over interface line  112 . 
       FIG. 2  shows an imaging system  200 . The imaging system  200  includes an imaging carriage  232  an imaging head  220  is mounted, imaging head  220  are controlled by controller  228 . The imaging head  220  is configured to image on a flexographic plate  208  mounted on a rotating cylinder  204 . The carriage  232  is adapted to move substantially in parallel to cylinder  204  guided by an advancement screw  216 . The flexographic plate  208  is imaged by imaging head  220  to form an imaged data on flexographic plate  212  on plate  208 . 
       FIG. 3  shows a halftone rendered image  300 . The rendered image  300  was prepared by DFE  104 , to be further imaged on the flexographic plate  208 .  FIG. 4  shows rendered image  300  imaged by imaging head  220  on flexographic plate  208  forming an imaged plate  400 . 
     In order to produce improved reproduction characteristics of image printed by means of relief plates or sleeves control relief of elements profile is suggested. The control relief will be achieved by means of relating to local environment of each addressable physical element (such as minimal physical pixel addressable on plate or sleeve by means of ablating laser). 
       FIG. 7  shows an engraved flexographic plate. Black areas (printed areas)  704  are shown on top surface of unengraved areas on the flexographic plates whereas non printed areas or white areas  708  are engraved on the flexographic plate. White areas at maximal depth are represented by numeral  712 . 
     Specifically, one can logically represent desired relief image carrier such as flexographic plate or sleeve by means of two-dimensional pixel array in such a way that value assigned to each element of said array represents a desired depth of a corresponding physical pixel on said relief image carrier. V0 is typically equal to value of zero as is shown on by numeral  704  which represents zero depth relative to unprocessed image carrier, which is an element holding ink during relief printing the process. Value Vmax (typically equal to 255 for convenience sake) represents maximum relief depth Dmax represented by numeral  712  and as such represents non-imaging blank area. Value V such that V0&lt;V&lt;Vmax represents a transition zone (“slope”) between imaging relief element and non-imaging blank area in such a way that corresponding intended relief depth is Dmax*(y−V0)/(Vmax−V0). 
     At least two different profile functions are defined. Fi(x,θ) is defined on region [0,Ximax], where Fi(0, θ)==V0 and Fi(Ximax, θ]==Vmax. The range of and 0&lt;Xi&lt;Ximax is equivalent to the range of V0&lt;Fi(Xi)&lt;=Vmax. Additionally value of XMax is defined as maximum of (X1max, . . . , XNmax), where N is number of defined profile functions. 
     A two-dimensional pixel array representing relief image carrier is constructed according to the following steps:
         a) For each pixel intended to be reproduced on substrate (black area  704 ) a zero value is assigned.   b) For each pixel intended not to be reproduced on substrate (white area  708 ,  712 ) such that its distance from closest black pixel DistB is not less than XMax, let us assign value Vmax,   c) Each remaining pixel (“slope” pixel) can be characterized by its distance from closest black pixel DistB, angle to nearest black pixel θ and distance from closest assigned white pixel DistW. For every such pixel let us choose relevant profile function Fi, where i=F(DistB,DistW), and assign to this pixel value V=Fi[DistB, θ].       

     For a preferred embodiment of the invention let us assume that there are two profile functions:
         A first function F1(x,θ) on region [0,X1max]   F1(0, θ)==V0   F1(X1max, θ]==Vmax   for 0&lt;X1&lt;X1max V0&lt;F1(X1, θ)&lt;=Vmax   for x&gt;X1max assume F1==Vmax.   In addition a second F2(x,θ) on region [0,X2max], F2(0, θ)==   V0; F2(X2max, θ]==Vmax   for 0&lt;X2&lt;X2max V0&lt;F2(X2, θ)&lt;=Vmax   for x&gt;X2max assume F2==Vmax, such that X2max&lt;X1max.       

     Constructing a two-dimensional pixel array in two passes, in first pass, use function F1 only. For construction of the array calculate for and associate with each pixel p[i,j] distance D[I,j] from nearest black pixel and angle θ [I,j] to said black pixel (in case that pixel p[I,j] is black, both these values are equal is zero). As a next step, assign to each pixel value V[I,j]=F1(D[I,j], θ[I,j]). 
     At second step, evaluate each pixel p[I,j] with assigned value 0&lt;V[I,j]&lt;Vmax. Calculate for each such pixel its “region of interest” size, namely, R[I,j]=X2max−D[I,j]. Pixels in a ROI (Region Of Interest) of pixel p[I,j] that is being evaluated are all pixels such that their distance from pixel p[I,j] is not more than ROI size R[I,j]. 
     Introducing bilevel evaluation function Feval[I,jθ] such that its value is 1 if pre-defined conditions are met and 0 otherwise. In simplest case such pre-defined condition is {value of pixel p[I,j]==Vmax}. For any one of the pixels in ROI of pixel p[I,j] evaluation function Feval returns 1, assign to pixel p[I,j] value Vnew[I,j]=F2(P[I,j], θ[I,j]), otherwise leave value of pixel p[I,j] unchanged. In such a way a relief profile with the desired characteristics is produced depending on local environment of each “slope” pixel. 
       FIG. 8  shows an engraved flexographic plate depicting two neighboring regions of engraved data, a first data region  804  and a second data region  808 . The two data regions  804  and  808  are separated by a maximal depth area  812 . Each of the neighboring data regions starts and ends with two step shoulder  616  profile. The two step shoulder  616  profiles on each side of data region create an area which may be not wide enough to accommodate ink quantities during printing. 
     This embodiment of the invention detects data area not distant enough.  FIG. 9  shows cutting off the bottom shoulders  904  on the neighboring data regions  804  and  808 . By cutting shoulders  904  a white area significantly distant from black area  1004  is created as is shown in  FIG. 10 . Practically a larger volume is formed between data regions  804  and  808  enabling more efficient accommodation of ink during printing, thus minimizing artifacts during printing. 
     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. 
     PARTS LIST 
     
         
         
           
               104  digital front end (DFE) 
               108  imaging device 
               112  interface line 
               200  imaging system 
               204  rotating cylinder 
               208  flexographic plate 
               212  imaged data on flexographic plate 
               216  screw 
               220  imaging head 
               228  controller 
               232  carriage 
               300  rendered halftone image to be imaged on a plate 
               400  rendered image imaged on a plate 
               500  relief area on a imaged printing plate 
               504  small printing area 
               508  shallow angle slope 
               512  large printing area 
               516  steep angle slope 
               520  support layer 
               600  profile of a basic 3D shape 
               604  printing area 
               608  relief height 
               612  shape base 
               616  two step shoulders 
               704  black area 
               708  white area 
               712  white area—maximal depth 
               804  first data region 
               808  second data region 
               812  maximal depth area 
               904  cutout shoulder 
               1004  white area significantly distant from black area