WO1990004234A1 - Procede numerique de tramage a diffusion d'erreurs - Google Patents

Procede numerique de tramage a diffusion d'erreurs Download PDF

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Publication number
WO1990004234A1
WO1990004234A1 PCT/US1989/004581 US8904581W WO9004234A1 WO 1990004234 A1 WO1990004234 A1 WO 1990004234A1 US 8904581 W US8904581 W US 8904581W WO 9004234 A1 WO9004234 A1 WO 9004234A1
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WO
WIPO (PCT)
Prior art keywords
values
gray scale
pixel location
preselected pixel
range function
Prior art date
Application number
PCT/US1989/004581
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English (en)
Inventor
John S. Bowers
Harry Bowers
Original Assignee
Bowers Imaging Technologies, Inc.
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
Application filed by Bowers Imaging Technologies, Inc. filed Critical Bowers Imaging Technologies, Inc.
Publication of WO1990004234A1 publication Critical patent/WO1990004234A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/40—Picture signal circuits
    • H04N1/405—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels
    • H04N1/4051—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a dispersed dots halftone pattern, the dots having substantially the same size
    • H04N1/4052—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a dispersed dots halftone pattern, the dots having substantially the same size by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions

Definitions

  • the present invention relates generally to a method for reproducing continuous tone images and more particularly to a reproduction process based upon digital half-toning with error diffusion.
  • Continuous-tone images are reproduced by screening processes for conventional half-tone printing.
  • picture elements of original images are converted into dots whose diameters vary in size according to the amplitude of light penetrating a screen. More particularly, the dots are larger where more light penetrates a screen and are smaller where less light is present.
  • Such processes can be described as being of the analog type since dot size is the analog of light intensity for individual picture elements, or pixels, of an image.
  • a color-filtered screen is prepared for each of the basic colors; then the segregated colors are printed sequentially with careful mechanical registration of the screens to avoid shadows.
  • an image is scanned with an optical scanner which detects and quantifies the light intensity values at preselected pixel locations on the image.
  • the detected data typically comprises quantified representations of gray scale values ranging from about 0 to 255 for each pixel location.
  • dots of fixed size are printed at locations corresponding in scale to the original pixel locations. It is well known, for instance, to print about forty thousand spaced-apart dots per square inch.
  • the placement and density of dots printed within a given range determines the spatial resolution and perceived grayness of the area to an observer. For example, if an area is filled with the maximum number of dots, an observer will perceive the area as a dark, solid color. As dot density within a given area decreases, the area will have a successively lighter appearance. Also, the quality of detail in a reproduced image depends upon the number of picture elements that are reproduced.
  • the gray scale values at the pixel locations are binary encoded.
  • the typical reference point or "threshold" for binary encoding gray scale values is 126, i.e., half way between the minimum and maximum gray scale values.
  • the binary number 0 would be assigned to every pixel which has a detected gray scale value less than 125, and a binary 1 would be assigned to every pixel whose detected gray scale value is 126 or
  • a pixel location which has a detected gray scale value of 100 would be binary
  • the magnitude of the encoding error would be 100 units as measured by gray scale values.
  • the magnitude of the encoding error would be 110 units as measured by gray scale values.
  • the present invention provides a digital half-toning process with error diffusion comprising the steps of:
  • preselected pixel location equal to its detected gray scale value multiplied by the first of the selected randomly located values and assigning a second error value for the preselected pixel location equal to its detected gray scale value multiplied by the second of the selected randomly located values;
  • the lower limit of the range function is between about 0 and 0.2, and varies linearly with the detected gray scale value of a preselected pixel location. More particularly, the lower limit, w min , of the range function preferably is determined by the following equation:
  • Z is the detected gray scale value at a preselected pixel location.
  • the upper and lower limits of the range function can be determined from the graph of Figure 3.
  • a digital half-toning process with error diffusion comprising the steps of:
  • the detected gray scale value at a preselected pixel location is less than a predetermined threshold value, encoding the pixel location as a binary 0, assigning a first error value for the preselected pixel location equal to its detected gray scale value multiplied by the first of the selected randomly located values and assigning a second error value for the preselected pixel location equal to its detected gray scale value multiplied by the second of the selected randomly located values;
  • preselected pixel location equal to its detected gray scale value multiplied by the first of the selected randomly located values and assigning a second error value for the preselected pixel location equal to its detected gray scale value multiplied by the second of the selected randomly located values;
  • Figure 1 is a functional block diagram of a digital half-toning system which operates according to the process of the present invention
  • Figure 2 schematically shows a field of picture elements or pixels
  • Figure 3 shows a function for use in the process of the present invention.
  • Figure 1 shows generally a system for digital printing.
  • the components of the system of Figure 1 are an image scanner 5 of the kind conventionally used for making color separations, a digital computer 7, and a digital printer 9.
  • the components of the system are conventional but, as will be described below, can be advantageously operated to provide digital half-toning with error diffusion.
  • the system of Figure 1 operates on a photographic transparency, although it can operate on a photographic print or a high-resolution printed original.
  • scanner 5 digitizes picture elements of an image according to gray scale values and provides the resulting digital information to computer 7.
  • an image can be retouched, adjusted, recomposed, and modified using conventional graphics software.
  • the digital image information is binary encoded and operated upon by the half-toning algorithm with error diffusion as described below.
  • the error- diffused information is provided to digital printer 9 which reproduces the encoded information on a suitable medium.
  • the error-diffused half-toning information can also be presented for display on a CRT (cathode ray tube) screen.
  • Figure 2 The purpose of the process is to adjust binary encoded values at selected pixel locations so that a reproduced image can provide the illusion of a continuous tone image without discernible artifacts.
  • each dot represents one pixel location on a continuous tone image which is presented for scanning.
  • scanning proceeds from left to right across each of the pixel rows and sequentially from the top to the bottom of the field.
  • variations on the scanning pattern are possible. For example, scanning could proceed from left to right across one row of pixels, and then from right to left across the next lower row, and so forth in a serpentine pattern.
  • each field is usually scanned several times, each time with a different filter.
  • a field can be scanned once with each picture element having multiple exposures, each exposure being to a different filter.
  • Color filtering does not, however, affect the essential steps in the error diffusion process.
  • the location of each pixel can be described by its cartesian coordinates.
  • an arbitrary pixel "P" in Figure 2 can be identified at location (x,y). It should be noted that pixel P is in the interior of the field and therefore has eight immediately adjacent neighbors. It should also be noted that four of the pixels neighboring pixel P will be scanned before it and four of the neighboring pixels will be scanned after it.
  • the neighboring pixel to the immediate right of pixel P is designated by arrow d 1
  • the neighboring pixel immediately below pixel P is designated by arrow d 3
  • the pixel diagonally below pixel P in the direction of scanning is designated by arrow d 2 .
  • the pixel designated by arrow d 1 has coordinates (x+1,y)
  • the pixel designated by arrow d 3 has coordinates (x,y+1)
  • the pixel identified by arrow d 2 has coordinates (x+1,y+1).
  • the digital half- toning and error diffusion process can be assumed to begin at pixel location (x,y) in Figure 2, and the magnitude of the detected gray scale value of pixel P at this location can be assumed to be a number Z.
  • Z is between the threshold value and the maximum gray scale value (255)
  • the magnitude of the encoding error will be a negative number equal to Z minus 255 gray scale units.
  • Z is a number between 0 and the threshold value
  • the magnitude of the encoding error is simply equal to the value Z in gray scale units.
  • the encoding error will be referred to by the symbol e x,y , where the subscript identifies a pixel location by its cartesian coordinates.
  • the threshold value is normally constant. However, it is possible that the threshold value could be randomly varied, or dithered, in a relatively narrow range about a fixed central value (e.g., 128). In that case, another degree of randomness would be introduced into the error diffusion process.
  • the gray scale values, Z, of pixels are used as the arguments of range functions.
  • range functions One example of range functions, w min and w max , is shown in Figure 3.
  • the range functions provide variable minimum and maximum limits which are linearly dependent upon the detected gray scale values, Z, of pixels. For pixel values ranging between 0 and 128, the smallest minimum value of the range function w min is 0, and the largest minimum value is 0.2. Also, for pixel values Z between 128 and 255, the smallest minimum value of the range function w min is 0, and the largest minimum value is 0.2. Algebraically, the range function w min can be approximated as follows:
  • the range function w max can be approximated as follows:
  • the range functions w min and w max are employed to calculate pseudo-random weights r 1 , r 2 , and r 3 .
  • pseudo-ransom weight r 1 a random number is
  • the random selection process does not employ a random number generator but (to minimize processing time) uses a look-up table of uniformly distributed random numbers expressed as decimals.
  • An identical procedure is followed to calculate weight r 3 .
  • the propagated errors err 1 , err 2 , and err 3 can be understood to be pseudo-randomized error values which are diffused, or distributed, to pixels neighboring pixel P in the directions of the arrows d 1 , d 2 , and d3 , respectively, of Figure 2.
  • the propagated errors can be either positive or negative.
  • pixels with detected gray scale values of either 0 or 255 do not generate error values. That is, error values are not diffused from pixels having error values of 0 and 255, but error values can be diffused to such pixels.
  • the gray scale value of the pixel located at (x+1,y) becomes the detected gray scale value for that pixel location plus the propagated error err 1 .
  • the gray scale value of the pixel located at (x+1,y+1) becomes the detected gray scatle value for that pixel plus the propagated error err 2 .
  • the gray scale value of the pixel located at (x,y+1) becomes the detected gray scale value at that pixel plus the propagated error err 3 .
  • the binary encoding error for the pixel located at (x+1,y) will reflect the propagated error err 1 , as well as the error propagated from other previously scanned pixel locations.
  • the error value for pixel location (x+1,y) is weighted by the pseudo-random process and then diffused to the pixels located at (x+2,y), (x+1,y+1), and (x+2,y+1). The process continues for each pixel in a row and then begins again at the first pixel on the next lower row.
  • r 1 RANDOM [w min (pixel (x,y) ), w max (pixel (x,y) )]
  • pixel (x,y) is black
  • pixel (x,y) is white
  • pixel (x+1,y) pixel (x+1,y) + err 1
  • pixel (x+1,y+1) pixel (x+1,y+1) + err 2
  • pixel (x,y+1) pixel (x,y+1) + err 3 .
  • the above- described error diffusion process could be operated such that encoding errors are propagated only to two pixels adjacent each sampled pixel.
  • the error diffusion process could be operated such that error assignments are interchanged (e.g., arrow d 3 in Figure 2 might be interchanged with arrow d 2 ). Also, modifications to the range functions can be made, especially in terms of slope and intercept values.

Abstract

Procédé numérique de tramage à diffusion d'erreurs réduisant les imperfections de reproduction visibles. On détecte à l'aide d'un analyseur optique (5) des valeurs d'échelle de gris des pixels d'une image. Dans le procédé de diffusion d'erreurs on utilise une fonction de plage linéaire et une valeur seuil déterminée. La fonction de plage donne des valeurs aléatoires qui sont utilisées pour propager les valeurs d'erreur, pour chaque pixel, à au moins trois pixels adjacents, dont un est situé sur la même ligne et les deux autres sur une ligne adjacente. On imprime les images en fonction des valeurs d'erreur propagées, en utilisant une imprimante numérique (9).
PCT/US1989/004581 1988-10-14 1989-10-13 Procede numerique de tramage a diffusion d'erreurs WO1990004234A1 (fr)

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US25784388A 1988-10-14 1988-10-14
US257,843 1988-10-14

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Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992008314A1 (fr) * 1990-10-24 1992-05-14 Eastman Kodak Company Procede de transmission d'erreurs numerique a grande vitesse servant a convertir une image a tonalite continue en image binaire
DE19511417A1 (de) * 1994-04-15 1995-12-07 Fuji Photo Film Co Ltd Verfahren und Vorrichtung zur Binärumwandlung eines Bildsignals

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA2063879C (fr) * 1991-05-28 1998-05-05 Albert D. Edgar Methode a reaction positive pour diffuser les erreurs contenues dans les signaux

Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4339774A (en) * 1979-12-20 1982-07-13 Cambridge Consultants Limited Apparatus and method for generating a dispersed dot half tone picture from a continuous tone picture
US4345313A (en) * 1980-04-28 1982-08-17 Xerox Corporation Image processing method and apparatus having a digital airbrush for touch up
US4578713A (en) * 1984-07-20 1986-03-25 The Mead Corporation Multiple mode binary image processing
US4651287A (en) * 1984-06-14 1987-03-17 Tsao Sherman H Digital image processing algorithm for output devices with discrete halftone gray scale capability
US4654721A (en) * 1985-04-12 1987-03-31 International Business Machines Corporation System for reproducing multi-level digital images on a bi-level printer of fixed dot size
US4680645A (en) * 1986-08-25 1987-07-14 Hewlett-Packard Company Method for rendering gray scale images with variable dot sizes
US4733230A (en) * 1984-09-06 1988-03-22 Hitachi, Ltd. Method of displaying continuous tone picture using limited number of different colors or black-and-white levels, and display system therefor

Patent Citations (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4339774A (en) * 1979-12-20 1982-07-13 Cambridge Consultants Limited Apparatus and method for generating a dispersed dot half tone picture from a continuous tone picture
US4345313A (en) * 1980-04-28 1982-08-17 Xerox Corporation Image processing method and apparatus having a digital airbrush for touch up
US4651287A (en) * 1984-06-14 1987-03-17 Tsao Sherman H Digital image processing algorithm for output devices with discrete halftone gray scale capability
US4578713A (en) * 1984-07-20 1986-03-25 The Mead Corporation Multiple mode binary image processing
US4733230A (en) * 1984-09-06 1988-03-22 Hitachi, Ltd. Method of displaying continuous tone picture using limited number of different colors or black-and-white levels, and display system therefor
US4654721A (en) * 1985-04-12 1987-03-31 International Business Machines Corporation System for reproducing multi-level digital images on a bi-level printer of fixed dot size
US4680645A (en) * 1986-08-25 1987-07-14 Hewlett-Packard Company Method for rendering gray scale images with variable dot sizes

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992008314A1 (fr) * 1990-10-24 1992-05-14 Eastman Kodak Company Procede de transmission d'erreurs numerique a grande vitesse servant a convertir une image a tonalite continue en image binaire
DE19511417A1 (de) * 1994-04-15 1995-12-07 Fuji Photo Film Co Ltd Verfahren und Vorrichtung zur Binärumwandlung eines Bildsignals
DE19511417C2 (de) * 1994-04-15 2000-11-02 Fuji Photo Film Co Ltd Verfahren und Vorrichtung zur Binärumwandlung eines Bildsignals

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CA2000705C (fr) 1999-09-14
CA2000705A1 (fr) 1990-04-14

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