Screening method for overlapping sub-images

Abstract

For the reproduction of originals, images are generated on an image carrier, for example by printing. The imaging device that generates the image is usually not capable to cover at once the complete image area on the carrier. If the device is capable to cover the full width of the image area, the image may be generated line by line. Devices not having this capability will generate a first portion of an image line on the carrier. An adjacent second portion of the image line is then generated by another imaging device or after a period of time by the imaging device that generated the first portion. The region where the first and second portion meet on the carrier may cause visual artefacts on the final reproduction due to spatial misregistration of the adjacent line portions. This problem is solved by dividing the image in adjacent sub-images having an overlap zone on the carrier. Within this overlap zone two sub-images will be generated on top of each other for reproducing the original image in that zone, thereby reducing or avoiding the artefacts. According to one method, the resulting optical density of the first and second sub-image is reduced within the overlap zone as the outer edge of the sub-image in the overlap zone is approached. The density reduction may be achieved by reduction of the microscopic density of individual microdots or by reduction of the dot percentage or by a combination of these techniques.

1 . A method for reproducing an original image on an image carrier comprising the steps of: 
 generating a conjoined first and second sub-image, each representative for a portion of said original image;    defining an overlap region as a region where both sub-images give a contribution to the integral optical density of the image carrier;    establishing for each sub-image a peripheral edge in said overlap region;    increasing said contribution by said first sub-image from said peripheral edge of said first sub-image to said peripheral edge of said second sub-image.    
     
     
         2 . The method according to  claim 1 , comprising the steps of: 
 dividing said overlap region in a partition of microdots;    assigning to at least one microdot an intermediate microscopic density substantially different from a minimum and maximum microscopic density of said microdots.    
     
     
         3 . The method according to  claim 2 , wherein the step of increasing said contribution comprises increasing the microscopic density of said microdots by density steps being smaller than half the difference between said maximum and minimum microscopic difference.  
     
     
         4 . The method according to  claim 1 , comprising the steps of; 
 halftoning said first sub-image by a first frequency-modulated halftoning method; and,    halftoning said second sub-image by a second frequency-modulated halftoning method, substantially non-correlated to said first frequency-modulated halftoning method.    
     
     
         5 . The method according to  claim 4  comprising the steps of: 
 generating for a zone in said overlap region by said first sub-image a first per cent of blank microdots;  
 generating for said zone by said second sub-image a second per cent of blank microdots, said second per cent being equal to said first per cent.  
 
     
     
         6 . An imaging system for reproducing an original image by an imaging device on an image carrier comprising: 
 means for generating a conjoined first and second sub-image, each representative for a portion of said original image;    means for defining an overlap region as a region where both sub-images give a contribution to the integral optical density of the image carrier;    means for establishing for each sub-image a peripheral edge in said overlap region;    means for increasing said contribution by said first sub-image from said peripheral edge of said first sub-image to said peripheral edge of said second sub-image.