Patent Publication Number: US-2005141037-A1

Title: Method and apparatus to enhance printing quality of laser printer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims the benefit of Korean Patent Application No. 2003-99043, filed on Dec. 29, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present general inventive concept relates to a method of and apparatus to enhance the printing quality of a laser printer, and more particularly, to a method of and apparatus to enhance the printing quality of a laser printer, in which a window is generated using various values of Ipi and angles of a dithering mask to improve the quality of a non-edge image.  
      2. Description of the Related Art  
      Generally, a monochromic or color laser printer expresses images on a subpixel-based gray scale. A gray scale representation in which each pixel is expressed in various gray levels requires a large amount of data to be processed. Accordingly, a large amount of data is transmitted from a computer to a laser printer to process various gray scale pixels, and as such, data transmission time increases and a greater capacity memory is required for the laser printer. A general 1-bit gray scale representation involves halftone processing of simply turning on or off pixels or dots. However, halftone images look rough, especially, in lighter areas. To solve this problem, U.S. patent Publication No. 2003-0038853 discloses a method of determining the size of dots according to gray levels of pixels.  
      In a conventional 1-bit gray scale representation, an input halftone image of 9×4960 pixels is read in units of a window of 9×9 pixels to increase the number of pixels in the halftone image. The 1-bit gray process is performed while moving the window from the left to the right in units of a row, to a next line after the completion of the 1-bit gray process, and then from the left to the right of the next line in units of a row.  
      Before performing a 1-bit gray process, it is determined whether a window region is an edge region or not. This is because a boundary of the edge area becomes scattered when the edge is enhanced, and accordingly, the edge should not be enhanced.  
       FIG. 1  is a view of a window used in a conventional edge detection method.  
      Referring to  FIG. 1 , an image region surrounded by the window is detected to be an edge area if the following conditions are met: 
          1) a pixel marked with   has at least one dot.     2) the center pixel marked with ⊚ has no dot, only one of four pixels  110 , 120 , 130 , and  140  marked with ● has a dot, and three pixels marked with ♦, surrounding the pixel  110 ,  120 ,  130 , or  140  including the dot have one or more dots,     3) the center pixel marked with ⊚ has no dot, only two adjacent pixels of the four pixels  110 ,  120 ,  130 , and  140  marked with ● have a dot and the other two pixels have no dot.        

      After the edge detection as described above is performed, image quality enhancement is performed on non-edge areas.  
       FIG. 2  is a view of a window used in a conventional image enhancement method. The window has a size of 10×9 pixels and is used in a 1-bit gray process.  
      Referring to  FIG. 2 , the sum of the average value of the brightness of a pixel  200  to be enhanced and four pixels  201 ,  202 ,  203 , and  204 , which are diagonally separated from the pixel  200  by a distance of  342 , and the average value of the brightness of a pixel  210  on the left of the pixel  200  and four pixels  211 ,  212 ,  213 , and  214 , which are diagonally separated from the pixel  210  by a distance of  342 , is set as a target brightness value or size of the pixel  200  to be enhanced. The target brightness value of the pixel  200  may range from 0/10 to 10/10, resulting in enhanced, smoother images. Each pixel consists of 0/10 to 10/10 dots, and therefore the brightness can be set according to the number of dots in a pixel.  
      The above-described 1-bit gray method can be used with a dithering mask having an Ipi value of  141  and an angle of 45 degrees, in which the Ipi value indicates the number of lines per inch. A Ipi value is obtained by dividing the number of dots per inch (dpi) by the distance between printed dots. For example, when the number of dpi is 600 and the distance between printed dots is 342, the number of Ipi is 600/3{square root}2=141. The angle between printed dots is 45 degrees in  FIG. 2 . Referring to  FIG. 2 , since the pixel  202  is located three pixels to the right of and three pixels above the center pixel  200 , the distance between the pixels  202  and  200  is 3{square root}2 and the angle between the pixels  202  and  200  is 45 degrees.  
      As described above, the conventional 1-bit gray process can be used for quality enhancement only at a particular Ipi and angle, and cannot be used when dithering multi-channel images such as color images. When color images of cyan, magenta, yellow, and black are printed using a dithering mask having an Ipi of 141 and an angle of 45 degrees, there may be problems.  
     SUMMARY OF THE INVENTION  
      The present general inventive concept provides a method of enhancing the printing quality of a laser printer using in a 1-bit gray process a window generated based on a plurality of Ipi values and angles, not a particular value of Ipi and angle, of a dithering mask.  
      The present general inventive concept also provides an apparatus to enhance the printing quality of a laser printer using in a 1-bit gray process a window generated based on a plurality of Ipi values and angles, not a particular value of Ipi and angle, of a dithering mask.  
      Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
      The foregoing and/or other aspects and advantages of the present general inventive concept are achieved by providing a method of enhancing the printing quality of a laser printer using a 1-bit gray process, the method comprising: receiving a binary image, a dithering mask, and a plurality of values of Ipi and angles of the dithering mask, wherein Ipi indicates the number of lines per inch; generating a window having a size of N×N pixels considering the received values of Ipi and angles; determining whether a binary image of N×N pixels, which is defined by the window, is an edge region or not using the window and the dithering mask; and enhancing the quality of a binary image of N×N pixels determined to be a non-edge region.  
      The foregoing and/or other aspects and advantages of the present general inventive concept are also achieved by providing an apparatus to enhance the printing quality of a laser printer using a 1-bit gray process, the apparatus comprising: a reception unit receiving a binary image, a dithering mask, and a plurality of values of Ipi and angles of the dithering mask, wherein Ipi indicates the number of lines per inch; a window generation unit generating a window having a size of N×N pixels considering the received values of Ipi and angles; an edge detection unit determining whether a binary image of N×N pixels, which is defined by the window, is an edge region or not using the window and the dithering mask; and an enhancement unit enhancing the quality of a binary image of N×N pixels determined to be a non-edge region. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is a view of a window used in a conventional edge detection method;  
       FIG. 2  is a view of a window used in a conventional image enhancement method;  
       FIG. 3  is a block diagram of an image enhancement apparatus according to an embodiment of the present general inventive concept;  
       FIG. 4  is a flowchart illustrating an image enhancement method according to an embodiment of the present general inventive concept;  
       FIG. 5  is a flowchart of an edge detection method according to an embodiment of the present general inventive concept;  
       FIG. 6  is a flowchart of a method of calculating the size of dots in a center pixel according to an embodiment of the present general inventive concept;  
       FIG. 7  is a view of a window used to detect the positions of neighboring pixels around the center pixel;  
       FIG. 8A  is a view of a binary image window before quality enhancement; and  
       FIG. 8B  is a view of a binary image window after quality enhancement. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures.  
       FIG. 3  is a block diagram of an image enhancement apparatus  10  according to an embodiment of the present general inventive concept. The image enhancement apparatus  10  of a laser printer  1  includes a reception unit  12 , a window generation unit  14 , an edge detection unit  16 , and an image enhancement unit  18 .  
      The reception unit  12  receives a binary image, a dithering mask, and values of Ipi and angles of the dithering mask. Here, Ipi indicates the number of lines per inch. The window generation unit  14  generates a window having a size of N×N pixels in consideration of the received values of Ipi and angles. The edge detection unit  16  detects whether a binary image defined by the N×N window is an edge image using the binary image window and the dithering mask. The image enhancement unit  18  enhances the quality of a non-edge binary image having a size of N×N.  
      In particular, the edge detection unit  16  detects a maximum value among dithering mask values in non-dotted white pixel regions of the binary image and a minimum value among dithering mask values in dotted black pixel regions of the binary image, and compares the maximum value of the white pixel regions and the minimum value of the black pixel regions. If the maximum value of the white pixel regions is not greater than the minimum value of the black pixel regions or if a difference between the maximum value of the white pixel regions and the minimum value of the black pixel regions is not greater than a threshold value, the binary image defined by the window is determined to be a non-edge region. The threshold value may be a value from 0 to 255 and is used to eliminate unnecessary edge components and noise components. A larger threshold value eliminates more edge information. Therefore, in the present general inventive concept, the amount of edge information can be adjusted by varying the threshold value.  
      The image enhancement unit  18  detects the positions of neighboring pixels surrounding the center pixel in the window using the values of Ipi and angles, the number Pn of the neighboring pixels including the center pixel, and the number Pbn of black pixels among the neighboring pixels and the central pixel. The size of dots or the brightness of the center pixel is calculated using Pbn/Pn×255.  
      After the enhancement process described above, an enhanced image is transmitted to a LSU (laser scanning unit) interface unit  20 , and a pulse signal corresponding to the transmitted image is generated and transmitted to an LSU  22 . The LSU  22  performs laser scanning according to the received pulse signal to generate a printed image.  
      Hereinafter, an image enhancement method according to an embodiment of the present general inventive concept will be described with reference to  FIGS. 4 through 6 .  FIG. 4  is a flowchart of an image enhancement method according to an embodiment of the present general inventive concept,  FIG. 5  is a flowchart of an edge detection method according to an embodiment of the present general inventive concept, and  FIG. 6  is a flowchart of a method of calculating the size of dots in a center pixel.  
      Referring to  FIG. 4 , first, a binary image, a dithering mask, and the values of Ipi and angles of the dithering mask are received (S 10 ). Next, a binary image window having a size of N×N pixels is generated considering the received values of Ipi and angles of the dithering mask (S 12 ). Next, it is determined whether a binary image having a size of N×N pixels, which is defined by the N×N window, is an edge region using the N×N binary image window and the dithering mask (S 14 ). If the binary image of N×N pixels is a non-edge region, an image enhancement process is performed on the non-edge region (S 16 ).  
      An edge detection method will be described in detail with reference to  FIG. 5 . First, a window is generated prior to edge detection. The size of the window is determined considering the values of Ipi and angles of the dithering mask. In this embodiment, the size of the window is determined to include the center of a submask of the dithering mask around the center pixel of the window.  
      Second, a maximum value among dithering mask values in non-dotted white pixel regions of the binary image is determined (S 20 ). A minimum value among dithering mask values in dotted black pixel regions of the binary image is determined (S 22 ). Next, the maximum value of the white pixel regions and the minimum value of the black pixel regions are compared (S 24 ).  
      If the maximum value of the white pixel regions is not greater than the minimum value of the black pixel regions, the binary image is determined to be a non-edge region (S 30 ). If the maximum value of the white pixel regions is greater than the minimum value of the black pixel regions, a difference between the maximum value of the white pixel regions and the minimum value of the black pixel regions is compared with a threshold value (S 26 ). If the difference between the maximum and minimum values is not greater than the threshold value, the binary image is determined to be a non-edge region (S 30 ). If the difference between the maximum and minimum values is greater than the threshold value, the binary image is determined to be an edge region (S 28 ). The threshold value may be a value from 0 to 255 and is used to eliminate unnecessary edge components and noise components. As the threshold value is set to be larger, more edge information is eliminated. Accordingly, the amount of edge information can be adjusted by varying the threshold value.  
      A method of enhancing the quality of a non-edge binary image will be described with reference to  FIG. 6 . Referring to  FIG. 6 , the positions of neighboring pixels surrounding the center pixel in the window are detected using the values of Ipi and angles (S 40 ). Next, the number Pn of the neighboring pixels including the center pixel is detected (S 42 ), and the number Pbn of black pixels among the neighboring and center pixels is detected (S 44 ). Next, the size of dots or the brightness of the center pixel is calculated using Pbn/Pn×255 (S 46 ).  
      A technical goal of the present general inventive concept is to enhance the Ipi in a bright region to obtain smoother output images. Generally, in a halftoning process, the brightness of a gray-scale image is expressed using binary values, i.e., in black or white. A binary image can be expressed with more gray levels by using a dithering mask combined from a plurality of dithering submasks. However, in a bright area where no dot exits in a specific dithering submask, the Ipi is deteriorated. Thus, a method of spotting smaller dots in each submask is used. A smoother, higher-resolution output image can be obtained by spotting more bright dots in a region rather than spotting less dark dots in the region to express the same level of brightness.  
      A method of detecting the positions of neighboring pixels around the center pixel of a window using values of Ipi and angles of the dithering mask will be described with reference to  FIG. 7 .  FIG. 7  is a view of a window used to detect the positions of neighboring pixels around the center pixel.  
      Referring to  FIG. 7 , the position of the center pixel of a submask of a dithering mask having 134 Ipi and an angle of 63 degrees is shown. The value of Ipi and the angle are input values. The size of the window and the positions of neighboring pixels surrounding the center pixel are determined using the input value of Ipi and the angle.  
      The Ipi is a value obtained by dividing the number of dots per inch (dpi) by the distance between printed dots. For example, when dpi is 600 and the distance between dots is 2{square root}5, Ipi is 600/(2{square root}5)=134. The angle is an angle between printed dots. The positions of the neigiboring pixels are determined using the input Ipi and angles. In the case of  FIG. 7 , an input value of Ipi is  134 , an input angle is 63 degrees, and the distance between printed dots is (600 dpi)/(134 Ipi)=4.478≈{square root}20=2{square root}5. Using these values, the positions of the neighboring pixels  32 ,  34 ,  36 , and  38  with respect to the center pixel  30  in  FIG. 7  can be determined.  
      A conventional 1-bit gray process can be performed only when a dithering mask has 141 Ipi and 45 degrees. However, in the present embodiment, the value of Ipi and the angle of the dithering mask can be varied according to the values input by a user. Exemplary values of Ipi and angles of the dithering mask that can be used are as follows. 
          1) 141 Ipi and 45 degrees     2) 134 Ipi and 63 degrees     3) 150 Ipi and 90 degrees        

      In the conventional case (1), referring to  FIG. 2 , one neighboring pixel  202  among the pixels  201 ,  202 ,  203 , and  204  surrounding the center pixel  200  is located 3 pixels to the right of and 3 pixels above the center pixel  200  and has an angle of 45 degrees with respect to the center pixel  200 . In the present embodiment, case (2), referring to  FIG. 7 , one neighboring pixel  32  of the pixels  32 ,  34 ,  36 , and  38  surrounding the center pixel  30  is located 2 pixels to the right of and 4 pixels above the center pixel  30  and has an angle of 63 degrees with respect to the center pixel  30 . In the present embodiment case (3), although not shown in the drawings, one of neighboring pixels surrounding the center pixel is located 4 pixels above the center pixel and has an angle of 90 degrees with respect to the center pixel. In this case, the value of 150 Ipi is calculated by dividing 600 dpi by 4.  
      Unlike the conventional method, a plurality of Ipi values and angles can be used in the present general inventive concept. Therefore, the quality of color images, which require a plurality of channels for printing, also can be improved. In particular, when printing a color image, various values of Ipi and angles for different colors, cyan, magenta, yellow, and black, can be used.  
      A method of calculating the brightness or the size of dots of a center pixel will be described with reference to  FIGS. 8A and 8B .  FIGS. 8A and 8B  are views of a binary image window before and after quality enhancement, respectively.  
      As shown in  FIG. 8A , dots may be unevenly distributed in a bright gray scale image. A bright printed region has a deteriorated value of Ipi and looks rough. However, a smoother, higher-resolution image can be obtained by uniformly distributing dots in the image region, as shown in  FIG. 8B , to enhance quality. The brightness of the image can be adjusted based on the distribution of the center pixels in submasks of the window.  
      Size of dots or brightness of the center pixel is calculated as described in operation  46  of  FIG. 6 . Referring to  FIG. 8B , the number Pn of the neighboring pixels  52 ,  54 ,  56 , and  58  including the center pixel  50  is 5 (=4+1), and the number Pbn of block pixels among the neighboring pixels  52 ,  54 ,  56 , and  58  and the center pixel  50  is 4 (see  FIG. 8A ). Accordingly, the size of dots or brightness of the center pixel is Pbn/Pn×255 =4/5×255=204. The result of the quality enhancement is shown in  FIG. 8B . A pixel of the window that is enhanced by the quality enhancement process is the center pixel.  
      The enhanced image is transmitted to the LSU interface unit  20  of  FIG. 3  to generate a pulse signal corresponding thereto. The pulse signal is transmitted to the LSU  22 , and the LSU  22  performs laser scanning according to the received pulse signal to obtain a printed image.  
      As described above, according to the present general inventive concept, the printing quality of color images, which require several channels for printing, can be enhanced using a plurality of Ipi values and angles of a dithering mask.  
      In addition, a non-edge region of a dithered binary image is detected by an edge detection method using a plurality of Ipi values and angles of a dithering mask, and dots are uniformly distributed in the non-edge region to enhance the printing quality of the image.  
      Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.