WO1992006554A1 - Procede et appareil servant a effectuer la juxtaposition de points de couleur differente en points groupes pour des imprimantes a laser - Google Patents

Procede et appareil servant a effectuer la juxtaposition de points de couleur differente en points groupes pour des imprimantes a laser Download PDF

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Publication number
WO1992006554A1
WO1992006554A1 PCT/US1991/007371 US9107371W WO9206554A1 WO 1992006554 A1 WO1992006554 A1 WO 1992006554A1 US 9107371 W US9107371 W US 9107371W WO 9206554 A1 WO9206554 A1 WO 9206554A1
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WIPO (PCT)
Prior art keywords
clustered
gray scale
dot
pixels
image
Prior art date
Application number
PCT/US1991/007371
Other languages
English (en)
Inventor
John Worthington
John Mark Gilbert
Lawrence J. Lukis
Leonard R. Steidel
Original Assignee
Lasermaster Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US07/591,537 external-priority patent/US5212559A/en
Application filed by Lasermaster Corporation filed Critical Lasermaster Corporation
Priority to JP3518554A priority Critical patent/JPH06501825A/ja
Publication of WO1992006554A1 publication Critical patent/WO1992006554A1/fr

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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K15/00—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers
    • G06K15/02—Arrangements for producing a permanent visual presentation of the output data, e.g. computer output printers using printers
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
    • G06T11/00—2D [Two Dimensional] image generation
    • G06T11/20—Drawing from basic elements, e.g. lines or circles
    • G06T11/203—Drawing of straight lines or curves
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06K—GRAPHICAL DATA READING; PRESENTATION OF DATA; RECORD CARRIERS; HANDLING RECORD CARRIERS
    • G06K2215/00—Arrangements for producing a permanent visual presentation of the output data
    • G06K2215/0002—Handling the output data
    • G06K2215/0062—Handling the output data combining generic and host data, e.g. filling a raster
    • G06K2215/0071—Post-treatment of the composed image, e.g. compression, rotation

Definitions

  • This invention pertains to methods and apparatus for producing gray scale or halftone values on a bi-level graphic device, such as a laser printer. More particularly, this invention pertains to a method and apparatus for performing cluster-dot dithering for laser printers and the like, especially laser printers which cannot print individual discrete output pixels or which have output pixels with unequal horizontal and vertical dimensions.
  • Spatial dithering techniques are generally divided into two classes based upon the nature of the "dots" produced by the dithering technique, dispersed-dot dithering and clustered-dot dithering.
  • Dispersed-dot dithe ⁇ ng methods depict a gray level by covering a small area with evenly distributed output pixels, each output pixel representing a discrete, uniform size dot.
  • Clustered-dot dithering methods depict a gray level by concentrating the output pixels in small groups or cells, each cell representing a variable size dot.
  • clustered-dot dithering in connection with digital graphic display devices generally has been reserved for those bi-level display devices whose physical reconstruction functions are not capable of properly displaying discrete and isolated output pixels, e.g., many types of lower resolution laser printers and thermal printers.
  • clustered-dot dithering technique used for laser printers and the like is ordered dithering.
  • ordered dithering is the type of dithering which is generally assumed when referring to digital halftoning, and the most commonly used page description language for laser printers only has an ordered dithering technique for performing digital halftoning.
  • Ordered dithering techniques replicate the more traditional analog clustered-dot dithering techniques by filling the output cell with a periodic or ordered pattern representing the various gray scales to be simulated (e.g., classical screen, spiral dot, line screen, etc.).
  • An alternative to the ordered dithering techniques that was recently proposed in Velho, L. and Miranda Gomes, J. in "Digital Halftoning with Space Filling Curves", ACM Computer Graphics. Vol. 25, No. 4, July 1991, pp. 81-90, uses an aperiodic space filling curve, instead of an ordered pattern, to fill the individual pixels in the cluster-dot cells.
  • clustered-dot dithering technique Regardless of the type of clustered-dot dithering technique which is used, the primary problem with present clustered-dot dithering techniques for digital graphic devices is that there is no good solution to the tradeoff between the desire to increase the resolution of the image, i.e., the number of lines per inch (lpi) or dots per inch
  • Another problem with present clustered-dot dithering techniques is the difficulty in dispersing error among adjacent cells.
  • the ability to successfully implement error dispersion or error diffusion techniques is partly responsible for the preference of dispersed-dot dithering over clustered-dot dithering for digital graphic devices.
  • error diffusion techniques which work well for distributed-dot dithering are not as successful when applied to the larger output cells associated with clustered- dot dithering.
  • U.S. Patent No. 4,654,721 describes one technique for implementing error diffusion for clustered-dot dithering wherein the entire error is dispersed to at least two adjacent cells in a random proportion.
  • error diffusion is accomplished by propagating the entire error along the space filling curve to the first pixel in the next output cell being processed. In both of these cases, because the entire error is dispersed, the tendency of the error dispersion to blur the resulting image remains.
  • printer controller cards which have been introduced as a way to increase the speed and quality of the images produced with the smaller, less expensive laser printers have pixel resolutions which are non-square.
  • printer controller cards are installed directly in the personal computer and increase the quality and speed of the laser printers by using a separate co-processor and page frame buffer to create a pixel representation of the image that is then printed from the page frame buffer by directly controlling the modulation of the laser in the print-engine of the laser printer.
  • printer controller cards it is possible to increase the horizontal resolution of the laser printer by controlling the modulation of the laser in the print- engine of the laser printer at a rate faster than the vertical resolution rate of the print engine.
  • An example of such a printer control card is the LX6 Professional printer controller card available from LaserMaster Corporation, Eden Prairie, Minnesota, the assignee of the present invention, which can increase the horizontal component of the pixel resolution of the laser printer to 600 X 300 dpi or 1000 X 400 dpi, depending upon the type of laser printer.
  • the increase in horizontal resolution generated by a printer controller card which produces non-square pixels significantly improves the quality of the resulting printed image; however, the output pixels which are defined in the page frame buffer of this system have unequal horizontal and vertical dimension.
  • distributed-dot dithering techniques are preferred because they provide a good solution to the tradeoff between the desire to increase the resolution of the image and the desire to increase the number of gray levels which are reproduced.
  • such techniques fail in the classical sense for print engines which cannot produce individually discrete output pixels. Consequently, there is a need for a clustered-dot dithering technique which can achieve the advantages of distributed-dot dithering techniques when used with laser printers and the like which cannot print individually discrete output pixels, especially laser printers which have output pixels with unequal horizontal and vertical dimensions.
  • the present invention is a method and apparatus for clustered-dot dithering which is capable of rendering a halftone image on a laser printer or the like. Unlike classical ordered dithering techniques which use a series of ordered gray scale patterns, the present invention uses a series of interactive gray scale patterns which result in more output gray scale values than there are output pixels in the clustered-dot cell.
  • the interactive gray scale patterns of the present invention utilize the interaction among output pixels in order to generate gray scale patterns which have the same number of output pixels turned on, but which produce different gray scale values when printed.
  • the present invention renders the digital halftone output image from a continuous tone input image having a plurality of input pixels which have been digitized as gray scale values, each gray scale value having a certain number of bits of resolution.
  • a page frame buffer is used for storing a pixel representation of the halftone image.
  • the page frame buffer is defined in terms of a plurality of clustered-dot cells, each clustered-dot cell encompassing a unique set of output pixels in the page frame buffer.
  • a number of interactive gray scale patterns are defined for use in the clustered-dot dithering process in terms of the same number of output pixels that define a single clustered-dot cell.
  • Each interactive gray scale pattern is capable of producing a unique gray scale value when applied to the clustered-dot cell.
  • a computer processor produces the halftone output image by dithering each input pixel of the continuous tone input image to produce a unique clustered-dot cell in the page frame buffer which is filled with one of the interactive gray scale patterns closest to the gray scale value of that input pixel.
  • the interactive gray scale patterns used with the present invention include a certain number of the output gray scale values are created by different interactive gray scale patterns that have the same number of output pixels which are turned on in the clustered-dot cell. These different interactive gray scale patterns having the same number of output pixels which are turned “on” produce different output gray scale values due to differences in the selective positioning of the output pixels within the clustered-dot cell. As discussed in greater detail hereinafter, the effectiveness of selectively positioning the output pixels within the clustered-dot cell to create the interactive gray scale patterns of the present invention is most noticeable in those type of laser printers and the like which cannot print individually discrete output pixels, especially those which have output pixels with unequal horizontal and vertical dimensions.
  • a unique error diffusion technique is utilized. Li performing the clustered-dot dithering, the computer processor determines an e ⁇ ror value representative of the mismatch between the gray scale value of the input pixel and the gray scale value of the interactive gray scale pattern selected to fill the clustered-dot cell for that input pixel. This error value is then diffused to a randomly selected one of a plurality of adjacent input pixels.
  • the dithering of the input pixels to produce the clustered-dot cells in the page frame buffer is accomplished in a serpentine raster fashion and the error value is diffused to a randomly selected one of four adjacent input pixels which have yet to be processed and which are forward and inward relative to the input pixel being processed. Because the use of any error diffusion technique can result in the blurring or smearing of the resulting halftone image, particularly along image edges and areas of high tonal variation, a unique error diffusion scaling technique is applied to the error diffusion process to minimize the effects of this problem.
  • the computer processor determines an absolute difference between the gray scale value of the input pixel being processed and the gray scale value of the randomly selected input pixel to which the error value will be diffused. This absolute difference is then used to scale the error value which will be diffused.
  • the majority of the error will be dispersed, but in high tonal contrast areas, and especially image edges, the error will not be effectively dispersed because it is scaled by the difference in intensity between the two input pixels.
  • the present invention may optionally use a sub-cell analysis to preserve the integrity of the oversampled input data.
  • the sub-cell analysis divides the clustered-dot cell of the present invention into sub-cells and analyzes the gray scale value of each sub-cell using the same technique of matching the interactive gray scale pattern as would be used for a full-sized clustered-dot cell.
  • the present invention is implemented using a printer controller card which can modulate the print engine of the laser printer to produce output pixels having unequal horizontal and vertical resolutions.
  • the output pixels in the preferred embodiment have an aspect ratio of 2.5 (i.e., 2.5 times as high as they are wide), and the clustered-dot cells are 2 pixels high by 10 output pixels wide for a 400x1000 dpi resolution system.
  • the clustered-dot cells are positioned at equal offsets to each other on adjacent raster lines in the page frame buffer so as to form a diagonal pattern that will produce the halftone output image oriented at a 45° angle with respect to horizontal.
  • the effectiveness of selectively positioning the output pixels within the clustered-dot cell to create the interactive gray scale patterns of the present invention is most noticeable in those type of laser printers and the like which cannot print individually discrete output pixels, especially those which have output pixels with unequal horizontal and vertical dimensions.
  • the present invention utilizes this aspect of the physical reconstruction function of the print engine to enhance the dithering process. It will be recognized that the physical reconstruction function of a laser printer is neither truly binary nor truly gray scale.
  • the physical reconstruction function of the print engine of a laser printer is binary in the sense that toner either sticks to the imaging drum or it does not.
  • the physical reconstruction function is also not binary in the sense that the formation of neighboring pixels can and do contribute to the formation of the current pixel in the form of energy overlap of the spot image, especially for less expensive laser printers which do not have precise optical and mechanical control over the spot image.
  • Those prior art dithering techniques which do attempt to address the issue of inability to print individual discrete output pixels because of spot image overlap in the print engine of the laser printer attempt to solve this phenomenon by the use of tone scale adjustments.
  • the tone sale adjustment is used to compensate for a non-linear intensity reconstruction function caused, for example, by the apparent overlapping of contiguous pixels.
  • the present invention recognizes that the overlap in the spot images of contiguous pixels can also be used as a means to control the interaction of adjacent pixels in order to produce "modified" output pixels which vary in the amount of toner that will be attracted to these pixels.
  • the ability to control the formation of such "modified” output pixels is particularly suited to lower resolution laser printers which are modulated by a printer controlled card at a higher speed to produce output pixels having unequal horizontal and vertical dimensions. Because the laser in the print engine is modulated at a higher speed than the speed for the square pixels for which the print engine is designed, the threshold for toner attraction is altered and the ability to produce "modified" output pixels at a higher pixel resolution is created.
  • the pixel representation created in the frame buffer for each horizontal raster line is used to control the laser in the laser printer.
  • the laser is turned on in response to an "on" pixel, a generally circular laser beam image is reflected on the print drum of the print engine to transfer charge elements to the drum.
  • the charge elements attract and pick up toner on the drum, or repel and erase toner from the drum in the case of an erase-white print engine.
  • the toner remaining on the drum is then transferred to a sheet of paper.
  • the pixel elements are thought of as square or rectangular shapes, in actuality, the edges of the pixels typically bleed into one another.
  • the parent application of the present invention taught that the method of selectively modifying pixels around a vertical transition point was effective at providing a non-gray scale anti-aliasing method for laser printers having unequal horizontal and vertical resolutions. It was thought that this method was effective due to the fact that the laser power function at the surface of the drum typically decreases with the distance from the center of the focal point over a generally circular or elliptical area covering a space larger than the theoretical rectangular pixel.
  • the total laser power impinging on a given point determines the resulting amount of charge on the drum at that point which in turn determines whether toner will stick to that point or not, and because the laser beam spot image is larger than the theoretical pixel, the total laser energy impinging upon a given point on the drum may have components contributed by more than a single sweep of the laser beam.
  • the present invention takes advantage of the physics of this process at the higher laser modulation rates to create the desired interactive gray scale patterns.
  • For pixels in a first horizontal raster line that makes up the clustered-dot cell some of those charge elements overlap onto the next horizontal raster line clustered-dot cell.
  • the application of the laser in response to the modified pixel representation briefly turns the laser on, for example, for a selectively modified "on" pixel.
  • the duration of this pulse of laser energy is insufficient to charge this pixel location enough to attract toner, however, in the overlap region, sufficient charge may be added to the charge already deposited by pixels the lower raster line to alter the boundary of the region in which toner sticks to the drum.
  • certain pixels within the clustered-dot cell may be controlled to within less than the height of a full pixel.
  • the present invention also recognizes that for laser printers where the buildup of toner from the bottom to the top of a raster line behaves in a non-linear fashion in response to the duty cycle of the control signal, it may be desirable to selectively modify the control signal by the application of a pseudo-duty cycle signal based upon the selective modification of pixels within the clustered-dot cell. It is believed for these types of laser printers that behave in a generally non-linear fashion, the physics of the overlapping of vertically adjacent pixels is such that there are an upper and lower limit to the duty cycle of the control signal that practically limit the effectiveness of the present invention.
  • clustered-dot dithering can be more effectively performed on laser printers, particularly less expensive, lower resolution laser printers and the like which cannot print individual discrete output pixels or which have output pixels with unequal horizontal and vertical dimensions.
  • the present invention allows the advantages of distributed-dot dithering techniques to be realized by a clustered-dot approach.
  • the clustered-dot dithering technique of the present invention allows images to be prepared for traditional offset and photo type setting printing press which require clustered-dot output images in order to produce gray scale patterns.
  • a primary objective of the present invention is to provide a simple and cost- effective solution that allows a clustered-dot dithering technique to achieve the advantages of distributed-dot dithering techniques when used with laser printers and the like which cannot print individually discrete output pixels or which have output pixels with unequal horizontal and vertical dimensions.
  • Another primary objective of the present invention is to provide a clustered-dot dithering technique with gray scale patterns which result in more output gray scale values than there are output pixels in the clustered-dot cell.
  • Another objective of the present invention is to provide a unique error diffusion technique, including an error scaling technique for use with the error diffusion technique, which is applicable to clustered-dot dithering.
  • a further objective of the present invention is to provide a clustered-dot dithering technique which can print high quality halftone images using a printer controller card without increasing the amount of frame buffer memory required to store the pixel representation of that image.
  • a still further objective of the present invention is to provide a clustered-dot dithering technique which can print high quality halftone images using a printer controller card with a less expensive, lower resolution laser printer.
  • Still another objective of the present invention is to provide a clustered-dot dithering technique which can produce input images for traditional offset and photo type setting printing processes.
  • FIG. 1 is a block diagram of a laser printing system having a printer controller card for generating an image to be printed by the laser printer.
  • Fig. 2 is a block diagram of the various functional components performed by the printer controller card of the preferred embodiment of the present invention.
  • Figs. 3a and 3b are graphical representations of the clustered-dot cell of the preferred embodiment of the present invention.
  • Fig. 4 is a graphic representation of the set of interactive gray scale patterns of the preferred embodiment of the present invention in the format for the clustered-dot cells shown in Fig. 3.
  • Fig. 5 is a graphical representation of the toner deposit functions of two different laser printers.
  • Figs. 6a and 6b are graphical representations of the spot images of the two different laser printers.
  • Fig. 7 is a flowchart showing the steps of the preferred embodiment of the dithering technique of the present invention.
  • Fig. 8 is a graphic representation of a prior art set of classical ordered gray patterns for a spiral dot clustered-dot
  • a typical personal computer system such as a desktop publishing system 10, which includes a printer controller card that executes the software that comprises the preferred embodiment of the present invention will be described.
  • the hardware for the desktop publishing system 10 is comprised of a personal computer 12 connected with a small, inexpensive laser printer 14.
  • the personal computer 12 may be any one of a variety of personal computers such as an
  • the personal computer 12 may be provided with a desktop publishing software package, such as Ventura Publisher, available from Xerox Corporation, PageMaker, available from Aldus Corporation, or GEM Desktop Publisher, available from Digital Research, Inc.
  • a desktop publishing software package such as Ventura Publisher, available from Xerox Corporation, PageMaker, available from Aldus Corporation, or GEM Desktop Publisher, available from Digital Research, Inc.
  • the personal computer 12 may also be provided with a graphics software package, such as Adobe Photo Shop, available from Adobe Systems, Inc.
  • the laser printer 14 may be any type of Canon CX or SX laser printer such as the HP LaserJet Series II available from Hewlett Packard, the LaserWriter, available from Apple Computer, Inc., or the Canon LBP8-III, available from Canon Inc., although it will be recognized that other type of laser printer print engines can also work with the present invention.
  • the personal computer 12 In normal operation without a printer controller card, the personal computer 12 typically includes a main PC mother board 16 and a serial port 18 through which the personal computer 12 communicates with the internal controller 20 of the laser printer 14.
  • the computer 12 sends a serial bit stream to the laser printer 14 over an RS-232-C serial interface 22 in the form of various commands and data that represent the image to be printed by the laser printer 14.
  • the internal controller 20 decodes the serial bit stream and generates the control signals that will be supplied to the laser 24 to drive the print engine 26 that creates the printed images corresponding to the various commands and data received by the internal controller.
  • the LaserJet II Printer User's Manual, Edition 1 available from Hewlett-Packard.
  • the page throughput for the laser printer 14 is typically very low, 6 pages per minute or less, depending upon the formatting and type of information being printed.
  • various printer controller cards 30 have been introduced, such as the LX6 Professional, available from LaserMaster Corporation, Eden Prairie,
  • the printer controller card 30 of the preferred embodiment is located in the personal computer 12 and communicates directly with the PC mother board 16 via an internal bus 28. It will be recognized, however, that the printer controller card 30 may be located in the laser printer 14 or that the present invention may be implemented within the internal controller 20 of the laser printer 14.
  • the printer controller card 30 is an LX6 Professional printer controller card selected to match the particular type of laser printer 14 being and personal computer 12 used in the desktop publishing system 10.
  • the LX6 Professional controller consists of a frame buffer 32, a co-processor 34 and a video interface 36.
  • the co-processor 34 receives the various commands and data representing the image to be printed from the PC mother board 16 via the bus 28. Because the bus 28 is a parallel bus, the transfer rate is much faster than for the serial port 18.
  • the co-processor 34 is responsible for receiving the continuous tone digital image data and converting it to halftone digital image data to be stored in the frame buffer 32.
  • the pixel representation in the frame buffer 32 is transferred to the laser printer 14 via the video interface 36 in the printer controller 30 that communicates directly with the laser 24 via a video cable 38.
  • An internal switch 40 in the laser printer 14 allows the printer controller 30 to override the internal controller 20 of the laser printer and directly modulate the laser 24. It is this direct modulation of the laser 24 that allows the printer controller 30 to increase the horizontal resolution of the laser printer 14.
  • the printer controller card 30 When printing a gray scale image, the printer controller card 30 will receive a Continuous-tone Digital Image, J[n], from the computer 12.
  • the Continuous-tone Digital Image J[n] typically includes a set of data points representative of the image to be printed, each point having a gray scale value represented in terms of N bits of resolution, where N is a number of bits greater than 1 and is most typically 8 bits.
  • N is a number of bits greater than 1 and is most typically 8 bits.
  • the Continuous-tone Digital Image J[n] represents data points from a physical image which has been scanned.
  • the Continuous-tone Digital Image J[n] represents data points from a physical image which has been scanned.
  • the Continuous-tone Digital Image J[n] represents data points from a physical image which has been scanned.
  • Image J[n] represents data points which have been generated by a computer graphics program.
  • graphic file formats for representing the data points which comprise the Continuous-tone Digital Image J[n], such as lF,PICT, EPS, GIF, etc.
  • the input data from the graphic file format is converted into PostScript® format before being transferred to the printer controller card 30.
  • the Continuous-tone Digital Image J[n] is processed by the printer controller card 30 to generate the Halftone Digital Image I[n'].
  • the Halftone Digital Image I[n'] is then supplied to the laser printer 14 in order to generate the desired Halftone Image, I(x).
  • the Physical Reconstruction Function represents a system model for the given binary display device, in this case the laser printer 14, which takes as its input the binary discrete-space image represented by the Halftone Digital Image I[n'] and creates the continuous-space visual image represented by the Halftone Image I(x). It will be recognized that the Physical Reconstruction Function will vary widely with the particular binary display device and that the interactive gray scale values of the present invention can be modified to accommodate these variances in the Physical Reconstruction Function of the binary display device.
  • the various functional elements within the printer controller card 30 which accomplish the transformation of the Continuous-tone Digital Image J[n] into the Halftone Digital Image I[n'] include a Retrospective Resampler means 40, a Tone Scale
  • the Retrospective Resampler means 40 is typically a digital sealer which adjusts the resolution of the Continuous-tone Digital Image J[n] to the resolution of the Halftone Digital Image I[n']. Because of the lower resolutions of the laser printers 14 used in connection with the present invention, the scaling performed by the Retrospective Resampler means 30 may reduce the resolution of the Continuous-tone Digital Image J[n]. In the event that the input data is oversampled (i.e., the sampling density is greater than the clustered-dot cell density), the present invention may optionally use a sub-cell analysis to preserve the integrity of the oversampled input data.
  • the Retrospective Resampler means 30 reconstructs the original continuous image from the given samples for J[n] by a process of interpolation, and then resamples this reconstruction at the resolution of the clustered-dot cells to produce a new Continuous- tone Digital Image J[n'] having a resolution that is equal to that of the Halftone Digital Image I[n'].
  • the resolution of the Halftone Digital Image I[n'] of the preferred embodiment is either 141 lpi with a dot or cell size of 2 pixels high by 10 pixels wide, or 90 lpi with a dot or cell size of 3 pixels high by 16 pixels wide, depending upon the degree of resolution which is desired for the image.
  • Tone Scale Adjust means 42 and Sharpen means 44 applies a non-linear transformation to the Continuous-tone Digital Image J[n'] to compensate for the particular tone scale modifications peculiar to the Physical Reconstruction Function of the particular type of laser printer 14 in combination with the halftoning method which is used.
  • this is the point at which most systems would attempt to compensate for the spot image overlap in the print engine of the laser printer by clipping the light and dark ranges of the Continuous-tone Digital Image J[n'] which are to be mapped as either completely black or completely white. Accordingly, it will be recognized that the prior art solution to the problems of dot overlap in the print engines was to effectively reduce the number of gray scale levels which would be produced by the system.
  • the Sharpen means 44 can also operate on the data which is closest to the black or white levels so as to precompensate for the blurring of fine details in the image which tend to occur during the halftoning process. Again, however, the prior art has primarily used the Sharpen means 44 as a preparatory step to the halftoning process, not as part of the halftoning process.
  • each clustered-dot cell 50 is defined within the frame buffer 32 as comprising twenty output pixels 52 in a 2x10 rectangular grid. Because the output pixels 52 have an aspect ratio of 2.5 (400x1000 dpi resolution), the aspect ratio of each clustered-dot cell 50 is also 2.5. To orient the halftone grid at 45° with respect to horizontal, the clustered-dot cells 50 are staggered at half cell increments along each pair of raster lines.
  • the Halftone means 46 of the present invention uses a set of interactive gray scale patterns 60 to create more gray scale levels than there are pixels in the clustered-dot cell 50.
  • the cross-hatched output pixels 52 in Fig. 4 indicate those pixels in the frame buffer 32 which would be turned “on” and the blank output pixels 52 indicate those pixels in the frame buffer which would be turned “off” to create the gray scale value for that particular gray scale pattern 60.
  • the dark lines 62 indicate which pixels in the clustered-dot cell 50 will generally contribute to which theoretical cluster-dots 54 as shown in Fig. 3b.
  • each gray scale pattern 60 is the range of gray scale values shown in brackets ⁇ for an 8 bit resolution continuous-tone digital input expressed from in terms of the values from 0 - 255 which are associated with that gray scale pattern.
  • the gray scale pattern for the range [69-76] has the same number of output pixels 52 which are turned on (5 out of 20) as the gray scale pattern for the range from [77-84], The difference between these two patterns stems from the selective location of two of the "on" pixels as shown at 64 and 66.
  • the pixels shown at 64 are separated by one pixel from each other, whereas for the gray scale pattern for the range from [77-84], the pixels shown at 66 are located adjacent each other.
  • the spot images overlap and each pixel is fully charged to attract toner.
  • the pixels 64 are separated, neither pixel is fully charged to attract toner and, as a result, the total toner deposited for the clustered-dot cell 50 is less than when the pixels 66 are adjacent each other, even though the same total number of pixels are turned "on" in the clustered-dot cell 50.
  • Fig. 8 shows a typical prior art spiral dot gay pattern for a 2x5 square pixel clustered-dot cell covering the same total surface area as the 2x10 cell of the preferred embodiment It will be seen that the present invention has almost three times as many gray levels as can be produced in the prior art scheme shown in Fig. 8. Even if a 2x10 cell with rectangular pixels is used as the clustered-dot cell 50, the present invention still provides 50% more gray levels than the theoretical maximum of twenty one gray scale values (4x5 pixels plus one for all white) which could be represented using the classical spiral dot ordered dithering technique.
  • the size and shape of the clustered-dot cell 50 can be varied in accordance with the output resolution and number of gray scale levels which are desired.
  • the manner in which adjacent output pixels 52 will interact may also vary, depending upon the physical and optical characteristics of the print engine of the laser printer 14. It will also be apparent that different fill techniques for dithering the clustered-dot cell 50 can be utilized other than filling the clustered-dot cell 50 according to the theoretical cluster-dot 54.
  • the gray scale pattern 60 is set equal to a gray scale value range of [252-253] , even though there are 4 of the 20 pixels which are turned "off in this pattern.
  • the gray scale pattern 68 is set equal to a gray scale value range of [252-253] , even though there are 4 of the 20 pixels which are turned "off in this pattern.
  • the interaction of the pixels in the SX print engine in the preferred embodiment has been found to be such that less than 4 pixels turned "off” in this particular pattern effectively produces an entirely black clustered-dot cell 50 as the output result. Consequently, the gray scale patterns 60 are arranged to take advantage of the more significant differences in the interaction of pixels near the middle of the gray scale range.
  • the ability to create the interactive gray scale patterns 60 is based on obtaining a more accurate understanding of the Physical Reconstruction Function of the particular print engines of the laser printer 14 and applying that knowledge to create a better dithering process for the particular print engine.
  • Figs. 5 and 6a and 6b a background explanation will be presented of why it is believed that the present invention is effective in creating multiple gray scale values for clustered-dot cells having the same number of output pixel which are to be printed by a binary imaging printer.
  • Different types of print engines having uniquely different sizes, shapes and power densities associated with their laser beam spot images can affect the way in which toner will be deposited on the drum in response to a particular video control signal.
  • FIG. 5 shows the response function of two different laser printers to a control signal having a varying duty cycle.
  • the response function is plotted in terms of the buildup of toner from the bottom to the top of a raster line for the vertical height of a single pixel (vertical axis) versus the percentage duty cycle of the control signal (horizontal axis)
  • the buildup of toner from the bottom to the top of a raster line for this print engine in response to the control signal behaves in a generally linear fashion as the duty cycle of the control signal is increased.
  • the buildup of toner from the bottom to the top of a raster line will behave in a non-linear fashion in response to the duty cycle of the control signal.
  • the differences in the toner buildup function in response to the duty cycle of a control signal for a particular print engine are due, in part, to the different type of spot images produced by different print engines.
  • the energy distribution function is a smooth gaussian function
  • the toner buildup function will be linear and the best approach is to accumulate the "area error" as described in the parent application to the present invention.
  • the energy distribution function is like that shown in Fig. 6b (more exponential with a sharper center band most likely due to focusing of the laser beam by more expensive optics)
  • the toner buildup function in response to the duty cycle of a control signal will be non-linear. In this case, the technique of accumulating the area error may not result in the desired smoothing because of the smaller degree of overlap among vertically adjacent pixels.
  • the present invention also may allow multiple gray scale values for clustered-dot cells having the same number of output pixel to be created with binary imaging printers having a non-linear toner buildup function in response to the duty cycle of a control signal.
  • there are an upper and lower limit to the duty cycle of the signal above and below which there is no practical effect on the height of the toner deposit in response to the control signal.
  • the selective modification of pixels at these lower duty cycles does not result in any appreciable modification upward to the edge of the image.
  • the selective modification of pixels at these higher duty cycles does not result in any appreciable modification downward to the edge of the image.
  • the inventors of the present invention have discovered that a control signal having a duty cycle within these upper and lower limits can be effective in controlling the buildup of toner, even for those print engines having non-linear toner buildup in response to the control signal.
  • the Dither means 46 of the present invention is accomplished by the co-processor 34 as it is generating each horizontal raster line 50 to be stored in the frame buffer 32.
  • the horizontal raster lines are processed from top to bottom in the frame buffer 32 and the output pixels 52 in each horizontal raster line are processed from left to right and then right to left in a serpentine manner, although it will be recognized that the raster lines and output pixels 52 may be processed in any direction so long as the processing is consistent throughout an entire page in the frame buffer 32.
  • two tables relating to the interactive gray scale patterns 60 are created.
  • the first table contains the assigned interactive gray scale pattern 60 for each of the 256 gray scale values represented by a gray scale input value having 8 bits of resolution.
  • the second table contains the error in terms of gray scale value between the assigned interactive gray scale pattern and the actual gray scale value corresponding to the gray scale input value.
  • the co-processor 34 determines an Error Dispersion Pixel, J'[P'], which is downward and inward from the pixel presently being processed J'[P].
  • J'[P'] an Error Dispersion Pixel
  • a random direction number from 0-3 will select one of the four pixels which are adjacent pixel J[P] and which have yet to be processed.
  • the value of the difference between the pixel J'JP] and the Error Dispersion Pixel, J'[P'], divided by the gray scale range (i.e., 256) is calculated. This value will be used to scale the amount of error to be dispersed to the Error Dispersion Pixel, J'[P'].
  • the Dispersion Error for J'[P] is determined by lookup from the second table.
  • the Dispersion Error is scaled by the result of step 72 and added to (or subtracted from) the Error Dispersion Pixel, J'[P'].
  • the interactive gray scale pattern from the first table corresponding to J'[P] is moved into the corresponding distributed-dot cell 50 in the frame buffer 32.
  • the co-processor may further sub-divide the dithering process in the event that the Continuous-tone Digital Image is presented with a higher degree of resolution than the output resolution as defined by the clustered-dot cells 50 (i.e., 141 lpi or 90 lpi).
  • the analysis of each clustered-dot cell 50 for purposes of matching it to a corresponding gray scale pattern 60 could be divided into an analysis of sub-cells of the clustered-dot cell 50, e.g., four 1x5 sub-cells. Each sub-cell is then matched to that portion of the gray scale pattern which corresponds to the gray scale value of the Continuous-tone

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Abstract

Une image de sortie numérique en demi-tons est produite à partir d'une image d'entrée à tons continus comprenant une multiplicité de pixels d'entrée qui ont été numérisés sous forme de valeurs d'échelle de gris. Un tampon de cadre de page (32) servant à stocker une représentation en pixels de l'image de sortie en demi-tons est défini en fonction d'une multiplicité de cellules (50) de points groupés, chaque cellule (50) de points groupés recouvrant un ensemble unique de pixels de sortie (52) dans le tampon de cadre de page (32). Un certain nombre de configurations interactives (60) d'échelle de gris sont définies pour être utilisées dans le procédé de juxtaposition de points groupés en fonction du même nombre de pixels de sortie (5) définissant une cellule (50) de poits groupés unique. Chaque configuration interactive (60) d'échelle de gris peut produire une valeur d'échelle de gris unique lorsqu'elle est appliquée à la cellule (50) de points groupés. Un processeur d'ordinateur (34) produit l'image de sortie en demi-tons en juxtaposant chaque pixel d'entrée de l'image d'entrée à tons continus pour produire une cellule (50) de points groupés unique dans le tampon de cadre de page (32) qui est rempli par la configuration interactive (60) d'échelle de gris la plus proche de la valeur d'échelle de gris de ce pixel d'entrée. Contrairement aux techniques de juxtaposition ordonnée classiques qui utilisent une série de configurations ordonnées d'échelle de gris, l'utilisation de la série de configurations interactives (60) d'échelle de gris résulte en un nombre de valeurs d'échelle de gris de sortie supérieur au nombre de pixels de sortie (52) dans la cellule (50) de points groupés. Les configurations interactives (60) d'échelle de gris utilisent l'interaction ayant lieu entre les pixels de sortie (52) pour générer des configurations d'échelle de gris qui possèdent le même nombre de pixels de sortie (52) lorsqu'elles sont allumées, mais produisent des valeurs d'échelle de gris différentes lorsqu'elles sont imprimées. Comme l'interaction entre les pixels de sortie (52) est plus prononcée dans les types d'imprimantes à laser et analogue qui ne peuvent pas produire des pixels de sortie individuels et séparés ou qui produisent des pixels de sortie (52) de dimensions horizontales et verticales différentes, l'efficacité de la présente invention est supérieure dans ces types de dispositifs d'affichage graphique.
PCT/US1991/007371 1990-10-02 1991-10-02 Procede et appareil servant a effectuer la juxtaposition de points de couleur differente en points groupes pour des imprimantes a laser WO1992006554A1 (fr)

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JP3518554A JPH06501825A (ja) 1990-10-02 1991-10-02 レーザプリンタ用集合ドットディザ方法及び装置

Applications Claiming Priority (2)

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US07/591,537 US5212559A (en) 1989-11-13 1990-10-02 Duty cycle technique for a non-gray scale anti-aliasing method for laser printers
US591,537 1990-10-02

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PCT/US1991/007372 WO1992006555A1 (fr) 1990-10-02 1991-10-02 Technique de cycle operatoire pour un procede d'anti-crenelage n'utilisant pas l'echelle des gris et destine a des imprimantes a laser

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US5519426A (en) * 1993-11-01 1996-05-21 Lasermaster Corporation Method for controlling a thermal printer to increase resolution
EP0924651A2 (fr) * 1997-12-22 1999-06-23 Adobe Systems Incorporated Mélange de données d'images dans un tampon d'image
US11303779B2 (en) 2018-01-26 2022-04-12 Hewlett-Packard Development Company, L.P. Halftone image creation

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US4232343A (en) * 1977-11-17 1980-11-04 Dr. Ing. Rudolf Hell Gmbh Method and apparatus for recording graphic or image information by means of punctiform recording spots
US4630947A (en) * 1984-02-16 1986-12-23 Sharp Kabushiki Kaisha Enlarged pattern generator
US4679039A (en) * 1983-11-14 1987-07-07 Hewlett-Packard Company Smoothing discontinuities in the display of serial parallel line segments
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Publication number Priority date Publication date Assignee Title
US4232343A (en) * 1977-11-17 1980-11-04 Dr. Ing. Rudolf Hell Gmbh Method and apparatus for recording graphic or image information by means of punctiform recording spots
US4679039A (en) * 1983-11-14 1987-07-07 Hewlett-Packard Company Smoothing discontinuities in the display of serial parallel line segments
US4630947A (en) * 1984-02-16 1986-12-23 Sharp Kabushiki Kaisha Enlarged pattern generator
US4680645A (en) * 1986-08-25 1987-07-14 Hewlett-Packard Company Method for rendering gray scale images with variable dot sizes
US5005139A (en) * 1988-08-16 1991-04-02 Hewlett-Packard Company Piece-wise print image enhancement for dot matrix printers

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5519426A (en) * 1993-11-01 1996-05-21 Lasermaster Corporation Method for controlling a thermal printer to increase resolution
EP0924651A2 (fr) * 1997-12-22 1999-06-23 Adobe Systems Incorporated Mélange de données d'images dans un tampon d'image
EP0924651A3 (fr) * 1997-12-22 2001-01-31 Adobe Systems Incorporated Mélange de données d'images dans un tampon d'image
US6313847B1 (en) 1997-12-22 2001-11-06 Adobe Systems Incorporated Blending graphics objects in a frame buffer
US11303779B2 (en) 2018-01-26 2022-04-12 Hewlett-Packard Development Company, L.P. Halftone image creation

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AU8949191A (en) 1992-04-28
AU8949491A (en) 1992-04-28
JPH06501825A (ja) 1994-02-24
WO1992006555A1 (fr) 1992-04-16

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