WO1991002426A2 - Visualisation numerique sur ecran d'un angle variable rapide - Google Patents

Visualisation numerique sur ecran d'un angle variable rapide Download PDF

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Publication number
WO1991002426A2
WO1991002426A2 PCT/US1990/004185 US9004185W WO9102426A2 WO 1991002426 A2 WO1991002426 A2 WO 1991002426A2 US 9004185 W US9004185 W US 9004185W WO 9102426 A2 WO9102426 A2 WO 9102426A2
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WIPO (PCT)
Prior art keywords
screen
optical density
noise
value
address
Prior art date
Application number
PCT/US1990/004185
Other languages
English (en)
Other versions
WO1991002426A3 (fr
Inventor
Robert C. Kidd
J. Brett Lefebvre
Original Assignee
Intergraph 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
Application filed by Intergraph Corporation filed Critical Intergraph Corporation
Publication of WO1991002426A2 publication Critical patent/WO1991002426A2/fr
Publication of WO1991002426A3 publication Critical patent/WO1991002426A3/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
    • H04N1/4055—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern
    • H04N1/4058—Halftoning, i.e. converting the picture signal of a continuous-tone original into a corresponding signal showing only two levels producing a clustered dots or a size modulated halftone pattern with details for producing a halftone screen at an oblique angle

Definitions

  • the present invention relates to image reproduction, and, more particularly, to the use of digital techniques to achieve rapid production of screen images at variable screen angles.
  • Digital screening typically involves superimposing onto digital image data a collection of data representing a screen to produce a digital output that is representative of a screened image.
  • United States patent 4,413,286, issued for an invention of Boston which is hereby incorporated herein by reference, discloses a digital technique for obtaining a screen image at variable screen angles.
  • the Boston approach in one embodiment involves storing in an array (which there and herein is called a "screen cell" matrix) a series of optical density values representative in each case of the optical density in a specific location within a single screen cell.
  • a given storage address in the array corresponds to a specific location within the screen cell.
  • Boston In order to reproduce in raster form an image that has been subjected to the screen, Boston begins with the optical density values of the image at address locations identified by reference to a coordinate system, for example, the line number and pixel number of each pixel. The problem is then how to compare each pixel of image data with the optical density in the applicable screen cell location. The problem is nontrivial because the principal axis of the screen does not necessarily coincide with either the line number (i.e., y) axis or the pixel number (i.e., x) axis. Furthermore, the physical size of the screen cell or of the spacing between address locations within the cell for its density data does not generally match the spacing between adjacent pixels or adjacent lines.
  • the optical density of the screen at a given set of coordinates of the image data can be determined by computing the pertinent address in the screen cell and accessing the optical density value stored there. Once the screen density at a given set of coordinates of the image data is known, it is a straightforward matter to compare the image pixel density at the same coordinates. Boston further teaches that a method for computing the pertinent screen cell address for each image coordinate is to determine an initial screen cell address for a given image pixel address and thereafter for each increment in pixel number or line number to determine the corresponding increments in the screen cell address coordinates. See column 4, especially lines 35-45.
  • One embodiment of Boston's invention takes advantage of these teachings by processing image data one pixel at a time, performing the calculation to identify for such pixel the corresponding screen cell address, accessing from the screen cell array the pertinent optical density of the screen, comparing the screen density with the image pixel density, and generating an appropriate output based on the comparison. See column 4, line 45, through column 5, line 34.
  • the present invention provides an improvement to the approach taught by Boston by eliminating the need for calculation of the screen cell addresses during the screening processing. Instead, in a preferred embodiment of the invention, all screen cell address calculations are performed in advance and the necessary data is stored in what is called herein a "screen tile" matrix.
  • the screen tile matrix is based on a seed screen cell matrix that is predefined just as in the Boston patent. As in the Boston patent, the screen itself is formed simply of contiguous repetitions of the screen cell disposed at some desired angle. However, in the present invention the screen tile is employed. In a preferred embodiment, the screen tile matrix is calculated with respect to the same coordinate system (for example, line number and pixel number) as that identifying addresses for the image data being screened.
  • the same coordinate system for example, line number and pixel number
  • a screen tile is defined as contiguous replications of the seed screen cell disposed at a predetermined angle; the number of replications occurring along each axis of the coordinate system is the minimum (which may be 1) necessary to cause a corner of the last replication of the seed screen cell to coincide (within a prescribed degree of accuracy) with a relative address on the axis. This relative address on such axis defines the tile's address period for such axis.
  • the screen tile matrix contains, for each address therein given with respect to the image data's coordinate system, the optical density value of the screen at such address. Because typically the parameters defining the screen tile can be determined prior to the actual screening operation, the screen tile can be calculated in advance and loaded into screen memory prior to commencement of screening.
  • the screening operation is then simplified (in relation to the approach taught by Boston) to systematically accessing screen density and image density values for each pixel in a given address period of each axis and comparing the two values to produce an output value. This process is repeated for all address periods of each axis. Further embodiments of the invention provide for adding carefully controlled amounts and types of noise to the screen density values so as to minimize Moire patterns.
  • Fig. 1 presents a simplified representation of a portion of screen tile in accordance with a preferred embodiment of the present invention
  • Fig. 2 shows boundaries of the screen tile of Fig. 1
  • Fig. 3 is a schematic representation of logical flow in calculation of a screen tile
  • Fig. 4 is a schematic representation of logical flow in screening processing after a screen tile has been calculated
  • Fig. 5 is a block diagram of a screen processing system in accordance with a preferred embodiment of the invention and showing the arrangement for generation, control, and use of noise data
  • Fig. 6 illustrates maximum and minimum screen density values triggering the mask of noise values
  • Fig. 7 illustrates a portion of a sample look-up table referenced in Fig. 5.
  • Fig. 8 is a block diagram of a screen processing system in accordance with an alternative preferred embodiment of the invention.
  • Fig. 1 there is shown in somewhat exaggerated detail the structure of a portion of a screen tile in accordance with a preferred embodiment of the invention.
  • the X and Y axes are used to describe coordinates of the pixel number and line number respectively of optical density values of pixels of the graphic image to be reproduced.
  • graphic image pixel value at the origin (0,0), another at (0,1), another at (0,2), and so on down the 0th pixel number coordinate.
  • graphic image pixel values exist for each intersection of line number coordinate Y and pixel number coordinate X.
  • the spacing of pixels produces a typical parallelogram 12 (which here is square) defined by pixel pairs on adjacent lines and which exhibits the spacing a between pixels in the direction of line number and b in the direction of pixel number.
  • the tile of Fig. 1 consists of contiguous replications of a seed screen cell 11 disposed at an arbitrarily specified angle theta to the graphic image coordinates.
  • the seed screen cell has a screen optical density value associated with each intersection of its coordinates 15 and 16.
  • the seed screen cell optical density values are referred to by coordinates 15 spaced apart by a distance p defining the width and coordinates 16 spaced apart by a distance q defining the height.
  • the screen tile of Fig. 1 includes contiguous replications of the seed screen cell 11, exemplified, for example, by item 17.
  • the screen density values in the screen tile are addressed by the X-Y coordinates of the image that is to be reproduced rather than the coordinates 15 and 16 of the seed screen cell itself.
  • the screen tile may be filled with screen optical density values accessed by X-Y coordinates over the period of the screen tile.
  • the boundaries of the screen tile of Fig. 1 are defined by points ABCD in Fig. 2. It can be seen in Fig.
  • a border of a screen cell will exactly coincide with the X axis at coordinate (N,0) and with the Y axis at coordinate (0,M) .
  • Screen angles having rational tangents are simply individual instances for handling in accordance with the invention and require no special processing.
  • the screen tile matrix is at least 512 x 512, it has been found that the effect of "rounding" to the nearest period on each axis to enable the screen tile to fit the matrix produces a small error manifested only in a very slight change in the average dot frequency with respect to that desired. It will be appreciated that even when theta does not have a rational tangent, the angle of screen actually produced by the present invention will have a rational tangent.
  • the invention effectively rotates the screen to the angle having a rational tangent that is closest to the desired angle theta.
  • Fig. 3 sets forth the logic flow for calculating screen optical density values for a screen tile matrix in accordance with a preferred embodiment of the invention.
  • the coordinate system for the screen optical density values is the same as that for the image pixels.
  • the coordinate system used has axes X and Y.
  • the optical density value at the address is accessed in step 34 and stored in step 35 as the value for the screen density at the particular image pixel .address involved.
  • FIG. 4 shows the logical flow in a preferred embodiment of the invention pursuant to which a screen tile may be used in screening.
  • a pixel address is selected according to step 42, and the optical density values are accessed in step 43 from the image data and from screen memory.
  • These densities are processed in step 44 in accordance with the prior art, as for example, comparing them and generating an output signal. See the Boston patent at column 5, lines 16-34.
  • the output value is typically set to a maximum or black pixel.
  • a screen memory address has exceeded the address period of its applicable pixel number address or line address, it is reset in step 47, until all address periods of each axis have been processed. It can be seen that this approach to screening can be very fast, because virtually all calculations are performed in advance of screening, and the limiting factors for speed of the screen are in accessing the data and performing the comparison.
  • Fig. 5 shows a preferred embodiment of the invention for dramatically reducing the Moire patterns while avoiding undue degradation of the output.
  • a noise value is added in summer 55 to each screen density value obtained from the screening tile 53, before the screen density value is provided to the comparator 57 along with the image data 56.
  • the noise value is obtained from a look ⁇ up table 54, that is addressed by a pseudorandom number generator 51.
  • the numbers in the look-up table may be chosen to have any desired distribution, for example, uniform (which has been found to produce generally good results) or gaussian, and preferably have an average value of approximately zero to avoid adding a dc density component to the output. They are stored as shown in Fig. 7 in the look-up table in order of increasing absolute value, and are as a matter of convenience limited to signed integers.
  • the noise values summed with the screen values are reduced when the screen values lie outside predetermined limits. That is, as shown in Fig. 6, when the value of a screen density exceeds the value shown as Noise Max, the address of the value is near a border of the screen cell, and in halftone printing it is desirable to avoid placing a dot there. Similarly, when the value of a screen density is below the value shown as Noise Min, the address of the value is near the center of the cell, and if a dot is located there it is desirable to avoid creating a hole in its center.
  • the manner in which the Fig. 5 embodiment reduces the noise value outside these limits is to use the masking logic 52 to sense when the screen value from the screening tile 53 is outside the predetermined limits.
  • the 11-bit binary address used to access the look-up table is masked so that only the rightmost 8 bits are used to access the look-up table. Because the look-up table entries are stored in order of increasing absolute value, only the smaller noise values are provided when the screen value is outside Noise Max and Noise Min.
  • Fig. 5 is illustrated to show the addition of the noise value to the screen density value obtained from the screen tile, it is equally possible to incorporate the addition of noise directly into the screen tile matrix itself.
  • the same basic system as in Fig. 5 can be used to produce a noise value, but the value can be produced at the time of creation of the screen tile matrix, and instead of storing the accessed screen density value from step 34 of Fig. 3. at a given location in the screen tile matrix, there can be stored a modified value equal to the accessed value plus the noise value.
  • the noise input to the summer 55 a function of the magnitude of the image pixel value, as illustrated in Fig. 8.
  • the image pixel data is used to access an appropriate entry from a look-up table constituting the mask function 82.
  • the mask function is thus a table of scale factors, and the accessed scale factor from the look-up table 82 is multiplied by multiplier 81 with the noise value from look-up table 54 to produce noise that is scaled in accordance with the fixed brightness. This approach can reduce artifacts caused by noise in highlight and shadow regions of the image.
  • a dashed line from screening tile 53 indicates that the mask function 82 may also be made a function of the magnitude of the screen density value.
  • the function can be the simple masking as described in connection with Fig. 6 and item 52 of Fig. 5, or it may be a look-up table as just described in connection with the image data input to the mask function 82.
  • screen density values may be associated with such coordinates using any suitable method. For example, it is within the scope of the invention to use a physical screen oriented at a desired angle and having a desired mesh and an optical system and digitizer to provide a direct readout of screen density values to be stored in the tile.
  • this byte map can be used for either general c rational tangent/magic number screening or for V.A.R. c screening.
  • C Input variables are the screen cell array, the size of
  • ⁇ bmptr fopen(* r bm.dat",'w*) ;

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Image Processing (AREA)
  • Facsimile Image Signal Circuits (AREA)

Abstract

Méthode de reproduction d'images utilisant des écrans tournants, dans laquelle les écrans tournants sont formés par la reproduction d'un carré d'écran (ABCD). Le carré d'écran (ABCD) a pour base un point de cellule d'écran (11, 17) qui se situe à l'angle voulu de rotation de l'écran, thêta, et qui est reproduit afin de remplir le carré d'écran. Ainsi, l'écran peut être généré par la reproduction du carré d'écran en fonction du même système de coordonnés que celui qui identifie les adresses pour les données de l'image à visualiser, c'est-à-dire, sans tourner le carré d'écran. Le carré d'écran ainsi que le point de cellule d'écran (11, 17) sont représentés par des matrices. Le bruit (51) peut être introduit dans le processus de formation d'écran afin d'atténuer ensuite le moirage.
PCT/US1990/004185 1989-08-04 1990-07-20 Visualisation numerique sur ecran d'un angle variable rapide WO1991002426A2 (fr)

Applications Claiming Priority (2)

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US38983589A 1989-08-04 1989-08-04
US389,835 1989-08-04

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WO1991002426A3 WO1991002426A3 (fr) 1991-03-21

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992016070A1 (fr) * 1991-03-09 1992-09-17 Linotype-Hell Ag Procede et dispositif pour l'enregistrement d'images a trame
WO1992017979A1 (fr) * 1991-04-01 1992-10-15 Eastman Kodak Company Procede permettant de generer des donnees d'images demi-ton avec des matrices de seuil selectionnees au hasard
EP0525520A2 (fr) * 1991-07-25 1993-02-03 Bayer Corporation Système de tramage et de reproduction en couleurs pour l'impression offset
WO1993007709A1 (fr) * 1991-10-02 1993-04-15 Siemens Nixdorf Informationssysteme Ag Procede et dispositif pour la production d'angles de trame en technique numerique d'impression
US5315406A (en) * 1991-09-03 1994-05-24 Levien Raphael L Screen generation for halftone screening of images using arbitrary distribution of transformed screen coordinates
EP0808056A2 (fr) * 1996-05-17 1997-11-19 Dainippon Screen Mfg. Co., Ltd. Procédé et appareil pour la génération de points de demi-teintes
EP1865706A2 (fr) 2006-06-09 2007-12-12 Xerox Corporation Trames en demi-teinte texturées pour un aspect d'uniformité améliorée

Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4084183A (en) * 1970-03-18 1978-04-11 Dr. Ing. Rudolf Hell Gmbh. Method for the electro-optical reproduction of half-tone pictures
FR2535477A1 (fr) * 1982-10-28 1984-05-04 Dainippon Screen Mfg Procede pour produire des points de demi-teinte dans un dispositif d'enregistrement de plaques de demi-teinte
US4578714A (en) * 1981-03-30 1986-03-25 Canon Kabushiki Kaisha Halftone image recording apparatus
EP0141869B1 (fr) * 1983-11-14 1987-09-16 DR.-ING. RUDOLF HELL GmbH Procédé et appareil pour la fabrication de formes d'impression tramées
EP0274196A1 (fr) * 1986-11-26 1988-07-13 Crosfield Electronics Limited Génération de reproductions en demi-teintes
EP0293214A2 (fr) * 1987-05-29 1988-11-30 Digital Equipment Corporation Système produisant des images en demi-teintes selon des grilles asymétriques

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4084183A (en) * 1970-03-18 1978-04-11 Dr. Ing. Rudolf Hell Gmbh. Method for the electro-optical reproduction of half-tone pictures
US4578714A (en) * 1981-03-30 1986-03-25 Canon Kabushiki Kaisha Halftone image recording apparatus
FR2535477A1 (fr) * 1982-10-28 1984-05-04 Dainippon Screen Mfg Procede pour produire des points de demi-teinte dans un dispositif d'enregistrement de plaques de demi-teinte
EP0141869B1 (fr) * 1983-11-14 1987-09-16 DR.-ING. RUDOLF HELL GmbH Procédé et appareil pour la fabrication de formes d'impression tramées
EP0274196A1 (fr) * 1986-11-26 1988-07-13 Crosfield Electronics Limited Génération de reproductions en demi-teintes
EP0293214A2 (fr) * 1987-05-29 1988-11-30 Digital Equipment Corporation Système produisant des images en demi-teintes selon des grilles asymétriques

Cited By (11)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1992016070A1 (fr) * 1991-03-09 1992-09-17 Linotype-Hell Ag Procede et dispositif pour l'enregistrement d'images a trame
WO1992017979A1 (fr) * 1991-04-01 1992-10-15 Eastman Kodak Company Procede permettant de generer des donnees d'images demi-ton avec des matrices de seuil selectionnees au hasard
EP0525520A2 (fr) * 1991-07-25 1993-02-03 Bayer Corporation Système de tramage et de reproduction en couleurs pour l'impression offset
EP0525520A3 (en) * 1991-07-25 1993-04-07 Miles Inc. Screening system and method for color reproduction in offset printing
US5315406A (en) * 1991-09-03 1994-05-24 Levien Raphael L Screen generation for halftone screening of images using arbitrary distribution of transformed screen coordinates
WO1993007709A1 (fr) * 1991-10-02 1993-04-15 Siemens Nixdorf Informationssysteme Ag Procede et dispositif pour la production d'angles de trame en technique numerique d'impression
EP0808056A2 (fr) * 1996-05-17 1997-11-19 Dainippon Screen Mfg. Co., Ltd. Procédé et appareil pour la génération de points de demi-teintes
EP0808056A3 (fr) * 1996-05-17 2003-05-14 Dainippon Screen Mfg. Co., Ltd. Procédé et appareil pour la génération de points de demi-teintes
EP1865706A2 (fr) 2006-06-09 2007-12-12 Xerox Corporation Trames en demi-teinte texturées pour un aspect d'uniformité améliorée
EP1865706A3 (fr) * 2006-06-09 2008-12-31 Xerox Corporation Trames en demi-teinte texturées pour un aspect d'uniformité améliorée
US7667875B2 (en) 2006-06-09 2010-02-23 Xerox Corporation Textured HT screens for improved uniformity appearance

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