Abstract:
A method for enhancing detail information in rasterized, continuous tone images includes a function that enlarges the contrast of a pixel in a neighborhood of pixels when in that neighborhood one of the pixels has a value that is close to a white color. This is determined by selecting the whitest pixel in the neighborhood. The function shows a gradual transition between the situations in which the whitest pixel is close to the white color and in which the whitest pixel is far from the white color. The method results in a better rendering of fine print when the image is sent to a printing system that reproduces the image on receiving material. For small characters, the legibility is improved by application of this method.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of International Application No. PCT/EP2011/060292, filed on Jun. 21, 2001, and for which priority is claimed under 35 U.S.C. §120, and which claims priority under 35 U.S.C. §119 to Application No. 10167384.6, filed on Jun. 25, 2010. The entirety of each of the above-identified applications is expressly incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of transforming pixels of a first digital image into pixels of a second digital image, each pixel having a value that indicates a color of the pixel. The method includes the steps of defining a neighborhood of pixels for a pixel of the first digital image, determining a new value for the pixel based on the values of the pixels in the neighborhood, and placing the new value of the pixel in the second digital image. The present invention further relates to an electronic component, a computer program product embodied on at least one non-transitory computer-readable storage medium, and a printing system. 
     2. Background of the Invention 
     Digital images are a well-known representation of images. The image is divided in pixels, each pixel being associated with a position in the image, and each pixel being specified by a value of a finite set of quantized values. The set of quantized values refers to colors in a specific color space, usually having more than one dimension. Examples of color spaces are the RGB- and CMYK-color spaces, but also the XYZ-, Lab-, YCbCr-, and HSL-color spaces. A value of a color is formed by the components of a vector in these spaces. In each of these color spaces, a white color is defined. For printing purposes, this white color is often equivalent to the medium color on which the impression is made and therefore corresponds to the no ink color. This color is characterized by zero coverage for all process inks in the appropriate color space. Each color space has its own way of expressing distances between the various colors, either as a Euclidian distance between the points indicated by the vector components or as a multivariable function of the components of the two colors. The color difference between two neighboring pixels is also referred to as the contrast between two pixels. 
     Digital images stem from a variety of sources, including computer applications, scanners, digital cameras and other kinds of electronic image generators. In these sources, the number of pixels is sufficiently large to contain fine information, This is provided by pixels with a color that is clearly distinguishable from the colors of the pixels around them, when the pixels are rendered on a display or in a printed form. The information is conveyed by the contrast between a pixel belonging to the fine information and the pixels in its neighborhood, which includes pixels that are not more than a few pixel distances away. However, because of limitations of the physical processes at the time of generating the digital image, such as light scattering, or the limitations of the way the digital image is saved electronically, dependent on, e.g. the compression that is used, the contrast between a pixel and the pixels in the neighborhood may be less than necessary for good conveyance of the intended information. Several methods exist, such as discrete laplacian filtering and unsharp masking, that improve the legibility of fine information. Disadvantages of these methods include the enhancement of noise and the possibility of overshoot artifacts. This is especially inconvenient when the information is surrounded by a colored background. 
     A problem of the present state of the art is the occurrence of the artifacts around fine information when the legibility of fine information is increased. The goal of the present invention is to increase the legibility of the fine information in a digital image without causing artifacts. 
     SUMMARY OF THE INVENTION 
     According to the present invention, the goal is obtained by a method wherein the step of determining a new value for the pixel comprises the further steps of determining a value of a whitest pixel in the neighborhood that indicates a color of a pixel in the neighborhood that is most close to a white color, determining a contrast between the pixel and the whitest pixel in the neighborhood, and calculating a new value for the pixel, enlarging the contrast between the pixel and the whitest pixel in the neighborhood, based on the value of the pixel in the first digital image, based on a distance between the color of the whitest pixel in the neighborhood and the white color and based on the contrast between the pixel and the whitest pixel. The new value of the pixel is changed in the direction that the contrast between the pixel and the whitest pixel is increased to improve the legibility. This works remarkably well and no interference with other characteristics of the pixels around the fine information, which could be the source of artifacts, are observed. 
     In a further embodiment, the contrast between the new value of the pixel and the whitest pixel in the neighborhood, unless the contrast between the pixel and the whitest pixel in the neighborhood is smaller than a threshold value. This reduces the possibility of an unwanted contrast enhancement at the edges of low contrast information that is intended to be low contrast. 
     In a further embodiment, a gradual transition takes place from a situation in which the whitest pixel in the neighborhood is close to the white color and the contrast between the new pixel and the whitest pixel in the neighborhood is larger than the contrast between the pixel and the whitest pixel in the neighborhood to a situation in which the whitest pixel in the neighborhood is far from the white color and the contrast between the new pixel and the whitest pixel in the neighborhood is hardly larger than the contrast between the pixel and the whitest pixel in the neighborhood. This further reduces artifacts that occur due to noise in the values of pixels in the neighborhood of the pixel that is given a new value. 
     The present invention is also embodied in a electronic component configures as an application specific programming unit to apply one of the methods of the present invention. This embodiment has the advantage of a fast execution of the methods. 
     The present invention is further embodied in a printing system, including a CPU for controlling the behavior of the system and a printing process for marking receiving material, wherein of one of the methods of the present invention is used, giving the advantage of superior print quality. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a printing system in which the method of the present invention is used; 
         FIG. 2  is a schematic drawing of a neighborhood of pixels for a pixel for which a new value is determined; 
         FIG. 3  is a flow diagram for an embodiment of the image processing method; 
         FIG. 4  is a graphical representation of two transient factor functions; and 
         FIG. 5  is a printer controller adapted for performing the image processing method. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying drawings, wherein the same or similar elements are identified with the same reference numeral. 
     The printing system in  FIG. 1  comprises a number of workstations (2, 4, 6) that are connected to a printer controller  16  through a network N. On the workstations, print jobs are prepared involving documents in various formats and various sizes. The image data in these documents originate from different sources, such as scanners, digital cameras and computer applications. The print jobs are sent to the printer controller where the jobs are analyzed and the documents converted into signals that are appropriate to be accepted by one of the printers connected to the controller. In this case, a printer  14  for large size documents, such as CAD drawings and banners, and a printer  18  for office size documents are available. In the process of converting, the documents are rasterized and rendered employing the process colors of the printer to be used. In this process, also a number of methods are applied that improve the appearance of the image on the receiving material that is used in the printer. One of these methods is the method that is the subject of the present invention that improves the legibility of the documents. 
     In  FIG. 2 , an image  1  to be printed is shown in its rasterized form indicating a pixel  3  that is under consideration in its neighborhood  5  that is indicated by the shaded area. Also indicated are the indices i and j that are used to mark the position in the image with which a pixel is associated. 
       FIG. 3  shows a flowchart of the various steps in the method for improving legibility. Each pixel is subjected to a process in which a decision is taken whether to pass the color of the pixel unchanged or to adjust the color. The color is designated by C[i,j] that in this embodiments takes the form of the channels R[i,j], G[i,j], B[i,j] for the red, green and blue component of the color for the pixels in the image. All three channels may be adjusted, but in this embodiment only the G-channel is. For the designation of the color of the pixel, other color components may also be used. 
     After selecting a pixel in step S 1 , which is, e.g. pixel  3  in  FIG. 2 , a neighborhood of pixels, numbered by n is defined in step S 2 , each pixel of the neighborhood having its own color N[n]. For each pixel in this neighborhood, the distance to the color white, represented by CW, is calculated in step S 3 . CW is in RGB coordinates equivalent to (255,255,255) when 8-bit values for R, G, and B are used. In Lab coordinates, CW is given by (100,0,0), and in CMYK coordinates CW is (0,0,0,0). The distance is calculated as the Euclidean distance between the coordinates of the two colors. In step S 4 , the minimum of the distances between CW and the color of each of the pixels in the neighborhood is determined and the pixel n′ for which this minimum is attained. This pixel has a color that is most close to the color white and consequently it is the whitest pixel in the neighborhood. In step S 5 , a comparison between the minimum distance and a first threshold, TH 1 , is made. If the minimum distance is larger (branch No), which means the whitest pixel is far removed from the color white, a next pixel is selected in step S 6  and no adjustment takes place. If the minimum distance is the smaller (branch Yes), the whitest pixel is sufficiently close to the color white to start the determination of the contrast CT in steps S 7 . The value of TH 1  is typically 25 for RGB-signals, but other values in the range of 10 to 120 may also be used. 
     In this embodiment, the contrast is calculated by the distance between the color of the referred pixel and the color of the whitest pixel in the neighborhood. In the same way as in step S 3 , the distance between two colors may be calculated in various ways in step S 8 , but in this embodiment, the Euclidean distance between the color coordinates is used. If the contrast is not larger than a second threshold, TH 2 , (branch No), a next pixel is selected in step S 6  and no adjustment takes place. If the contrast is larger than the threshold TH 2  (branch Yes), a new value for the color of the pixel is calculated in step S 9 . Thereafter, a next pixel is referred to until all pixels in the image have been referred to. 
     The calculation of the adjusted value of the color of the pixel C′[i,j] in step S 9  starts with the original value C[i,j] from which a term of three factors is subtracted (in the case of RGB-colors). A first factor, depending on D[n′], the distance between the whitest pixel in the neighborhood and the color white is F(D[n′]), which is transient factor that is close to 1 when the distance D[n′] is close to 0. The transient factor F(D[n′]) is close to 0 when the distance D[n′] is large. This makes a gradual transition possible between the various values that the whitest pixel can have. A second factor, FACTOR, is a fixed value that is used to increase the contrast. This factor typically has a value of 5, but can be in the range from 2 to 10. The third factor is the difference between the contrast, CT, and the threshold, TH 2 . As is customary in this kind of image processing, the value of the colors that is outside of the available range is cut off at the lower or upper limit of the range. 
     In  FIG. 4  two possible transient factor curves are shown as a function of the distance D[n′]. These are generated by the relation:
 
 F ( D[n ′])=1/{exp[ f *( D[n′]−T )]+1}
 
     In a first curve, f=0.3 and T=25, giving a steep transient factor curve which has the value 0.5 when D[n′]=25. A second curves has f=0.2 and T=40, giving a somewhat less steep curve which has the value 0.5 at D[n′]=40. The use of these curves diminishes the effect of a varying value of the whitest pixel in the neighborhood. 
       FIG. 5  shows how the method can be incorporated as a special module  50  in a printer controller  16  that is connected to a network N by a network module  40 , which communicates with other modules in the printer controller through the central bus  44 . Further, a Central Processing Unit  41 , random access memory  42  and non-volatile memory  43  are provided. The special module  50  is equipped with a neighborhood module  51  that determines the neighborhood pixels of a pixel that is the input. The color of the neighborhood pixels is communicated to the color distance module  52 , that determines the whitest pixel in the neighborhood. The color of this pixel is sent to the new value calculation module  53  that determines the new value of the pixel referred to according to the method as described in  FIG. 3 . 
     Many different embodiments, including a Field Programmable Gate Array to execute the described method in a limited number of clock cycles and a computer program that executes the various steps on general purpose electronic hardware, can be made. These are representations of the invention as described by the following claims. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.