Abstract:
A printing system comprises a memory configured to store image data representing an image. The printing system comprises a processor configured to perform a first digital halftone process on a first portion of the image and a second digital halftone process on a second portion of the image.

Description:
BACKGROUND 
     Color and gray value digital images are both composed of picture elements (pixels), where each pixel is represented by multiple binary bits that define either a color or a gray level. In order to represent such an image on a bi-level printer, the individual color or gray level pixels are typically converted to binary level pixels through use of a digital halftoning process. 
     Digital halftoning is the process of transforming a continuous-tone image into a lower bit-depth, typically binary, image that has the illusion of the original continuous-tone image, using a careful arrangement of lower bit-depth picture elements. The process is also referred to as spatial dithering. In the case of color images, the color continuous-tone image is typically separated into color channels first. Separate halftones are then formed for each of the color channels. 
     Typically, for laser printers, ordered cluster dot halftones using lower lines per inch (lpi), such as 100-150 lpi, are best for photographs, areas of constant gray scale, or gradient patterns. Halftones using a lower lpi reduce print artifacts, such as banding, but may result in jagged edges for the sharp edges found in text and line art. Halftones using a higher lpi, such as 200-300 lpi, are best for text and line art, but are not as good for photographs, areas of constant gray scale, and gradient patterns. Print artifacts, such as banding, become more pronounced as the lpi is increased. 
     SUMMARY 
     One aspect of the present invention provides a printing system comprising a memory and a processor. The memory is configured to store image data representing an image. The processor is configured to perform a first digital halftone process on a first portion of the image and a second digital halftone process on a second portion of the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram illustrating one embodiment of major components of a printing system. 
         FIG. 1B  is a block diagram illustrating another embodiment of major components of a printing system. 
         FIG. 2  is an image illustrating one embodiment of a 50% gray scale magnified letter “H.” 
         FIG. 3  is an image illustrating one embodiment of the magnified letter “H” after a digital halftone process has been applied to the image. 
         FIG. 4  is an image illustrating one embodiment of the magnified letter “H” after a dual digital halftone process has been applied to the image. 
         FIG. 5  is a flow diagram illustrating one embodiment of a method for applying a dual digital halftone process to an image. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
       FIG. 1A  is a block diagram illustrating one embodiment of major components of a printing system  100 A. Printing system  100 A includes host or computer  102  and printer  120 . In one embodiment, printer  120  is a laser printer or laser print apparatus. 
     Computer  102  includes processor  104 , memory  108 , and input/output (I/O) interface  116 , which are communicatively coupled together via bus  106 . Driver  110 , data  112  to be printed, and image data  114  are stored in memory  108 . In one embodiment, driver  110  is executed by processor  104  to render data  112  to be printed into image data  114 , including performing a dual halftone process as described in further detail below with reference to  FIGS. 2-5 . The data  112  to be printed may be any type of printable data, such as image files, word processing files, etc. In one embodiment, image data  114  comprises rows and columns, with one pixel defined at the intersection of each row and column. In one form of the invention, image data  114  includes a plurality of pixels, with each pixel being represented by a multi-bit value (i.e., each pixel is represented by an N-bit value, where N is greater than one). In one embodiment, each pixel in image data  114  is represented by a 2-bit value (e.g., black, white, and two gray levels). In another embodiment, each pixel in image data  114  is represented by a 4-bit value. In another embodiment, each pixel is represented by a 1-bit value (e.g., black and white). 
     Printer  120  includes processor  122 , I/O interface  126 , memory  128 , and laser print engine  130 , which are communicatively coupled together via bus  124 . I/O interface  126  of printer  120  is electrically coupled to I/O interface  116  of computer  102  through communication link  118 . In one embodiment, I/O interfaces  116  and  126  are serial interfaces, such as universal serial bus (USB) interfaces, and communication link  118  is a USB cable. In another embodiment, I/O interfaces  116  and  126  are network interfaces, and communication link  118  is a network, such as a local area network. In other embodiments, other types of interfaces and communication links may be used, including those for wireless communications. 
     After rendering data  112  into image data  114 , computer  102  outputs the image data  114  to printer  120  via communication link  118 . The received image data  114  is stored in memory  128  of printer  120 , where it is retrieved and processed by laser print engine  130  and printed to a medium. In one embodiment, image data  114  is compressed by computer  102  for transmission to printer  120  through communication link  118 . Image data  114  is then decompressed by printer  120  by firmware or dedicated hardware. 
       FIG. 1B  is a block diagram illustrating another embodiment of major components of a printing system  100 B. Printing system  100 B includes similar hardware as printing system  100 A. But in system  100 B, image data  114  is rendered by printer  120 , rather than by computer  102 . In one embodiment, driver  140  converts data  112  to be printed into a description file  142 . In one form of the invention, driver  140  is a printer command language (PCL) driver for converting data  112  into a description file  142  that includes data and high level commands (e.g., place a Helvetica 12 point letter “Q” at location x,y on the page). Computer  102  transfers description file  142  to printer  120  via communication link  118 , and printer  120  stores file  142  in memory  128 . 
     Processor  122  then renders description file  142  into image data  114 , including performing a dual halftone process as described in further detail below with reference to  FIGS. 2-5 . In one embodiment, printer  120  includes PCL firmware for rendering the description file  142  into image data  114 . Image data  114  is stored in memory  128  of printer  120 , where it is retrieved and processed by laser print engine  130  and printed to a medium. 
       FIG. 2  is an image illustrating one embodiment of a 50% gray scale magnified letter “H”  112   a . Magnified letter “H”  112   a  is a portion of data  112  to be printed. Magnified letter “H”  112   a  is rendered by processor  104  or processor  122  into image data  114 . 
       FIG. 3  is an image illustrating one embodiment of magnified letter “H”  112   a  after a digital halftone process has been applied to the image to generate a halftone image  114   a . Each square in halftone image  114   a  represents a pixel, as indicated, for example, at  150 . In this embodiment, halftone image  114   a  includes 2-bit per pixel data, which results in four possible pixel values. The four possible pixel values include 0 (white), as indicated for example at  152 , 1 (light gray), as indicated for example at  156 , 2 (dark gray), as indicated for example at  158 , and 3 (black), as indicated for example at  154 . The four pixel values indicate the amount of toner applied in each pixel, from white where no toner is applied to the pixel, to black where toner is applied to the entire pixel. The 2-bit per pixel halftone image  114   a  approximates the 50% gray scale letter “H”  112   a  when the letter “H” is printed at its true size. 
     The halftone process results in jagged edges, however, as indicated for example at  160 . When halftone image  114   a  is printed, the jagged edges make the image look less sharp. The jagged edges are due to the pixel edges having both black and white values and the spacing between the white (or black) pixels. A lower lpi pattern has larger spacing resulting in larger runs of adjacent white pixels and black pixels. The lower lpi pattern also has lower frequency content that the human visual system picks up on more easily than higher frequency content, such as a higher lpi pattern. 
       FIG. 4  is an image of one embodiment of the magnified letter “H” after a dual digital halftone process has been applied to the image to generate a dual halftone image  114   b . In this embodiment, the edges, indicated for example at  170 , of dual halftone image  114   b  do not have 0 (white) pixel values. The absence of 0 (white) pixel values on the edges of dual halftone image  114   b  results in sharp edges and prevents the jagged edges illustrated in halftone image  114   a.    
     The jagged edges are corrected by applying a different halftone to the edges of dual halftone image  114   b,  as described in further detail below with reference to  FIG. 5 . The interior of dual halftone image  114   b  is similar to halftone image  114   a  where a single halftone is applied. In this embodiment, halftone image  114   a  and the interior of dual halftone image  114   b  are halftoned with a 120 lines per inch (lpi) 45° black screen. The edge of dual halftone image  114   b  is halftoned with two possible pixel values per edge portion to approximate the edge value of each edge portion. 
     In one embodiment, the halftone algorithm darkens the edge input slightly and then semi-randomly outputs the two output levels closest to the input percentage. For example, if for the 2-bit per pixel output: 0=0/3 pulse of the laser, 1=1/3 pulse of the laser, 2=2/3 pulse of the laser, and 3=3/3 pulse (or full pulse) of the laser, then for 8-bit per pixel input, an edge value of 128 may be biased to 153. This is approximately 60% of the full pulse value of 255. Therefore, the halftone algorithm attempts to cover on average approximately 60% of the edge. This coverage is obtained by semi-randomly varying the output levels between 1 and 2 (1/3 and 2/3 ), while biasing toward 2&#39;s to get closer to 60%, instead of the 50% that would result if the halftone algorithm evenly alternated between 1 and 2. 
     By using a gray scale to prevent varying between black and white, the amplitude of modulation is lowered. By semi-randomly varying the output levels, the pattern has significant high frequency content. The combination of the gray scale and the semi-random variation of the output levels results in sharp edges when viewed by the human visual system. 
       FIG. 5  is a flow diagram illustrating one embodiment of a method  200  for rendering data  112  into image data  114  having dual halftones. Method  200  is performed by processor  104  or processor  122 . At  202 , image processing is started. At  204 , the row (Row) of image data  114  is set equal to one and the column (Col) of image data  114  is set equal to one. At  206 , a window of data is generated around the selected pixel. At  208 , the processor determines whether the selected pixel is an edge pixel. If the selected pixel is not an edge pixel, then at  216 , the output is based on halftone method one (normal halftone). If the selected pixel is an edge pixel, then at  210 , the processor determines if the intensity difference from neighbor pixels outside the edge is greater than 25%, or other suitable value. If the intensity difference from neighbor pixels outside the edge is greater than 25%, or other suitable value, then at  212 , the intensity of the edge pixel is adjusted. If the intensity difference from neighboring pixels from outside the edge is less than 25%, or other suitable value, then at  216 , the output is based on halftone method one (normal halftone). 
     At  214 , the output is based on halftone method two (alternate halftone) for the edge pixel. At  218 , Col is incremented by one. At  220 , the processor determines if the last column of image data  114  has been processed. If the last column of image data  114  has not been processed, then control returns to block  206  where the next column of image data  114  begins processing. If the last column of image data  114  has been processed, then at  222 , Col is set equal to one and Row is incremented by one. At  224 , the processor determines whether the last row of image data  114  has been processed. If the last row of image data  114  has been processed, then at  226 , processing of image data  114  is completed. If the last row of image data  114  has not been processed, then control returns back to block  206  where the next row of image data  114  begins processing. 
     In one embodiment, halftone method two (alternate halftone) is any suitable halftone capable of recreating edges that look sharp rather than jagged when printed. Halftone method one (normal halftone), in one embodiment, is any suitable halftone capable of rendering smooth intensity ramps and substantially eliminating banding. In one embodiment, halftone method two (alternate halftone) uses a higher lpi than halftone method one (normal halftone) used for the portions of the image that are not edges. For example, in one embodiment, halftone method one (normal halftone) is a halftone in the 100-150 lpi range, and halftone method two (alternate halftone) is a halftone in the 200-300 lpi range, such as 212 lpi. In other embodiments, other halftones can be used for halftone method one (normal halftone) and halftone method two (alternate halftone). 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. This application is intended to cover any adaptations or variations of the specific embodiments discussed herein. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.