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
  • H04N1/46—Colour picture communication systems
  • H04N1/52—Circuits or arrangements for halftone screening

Definitions

• the present invention is directed to a method and apparatus for making color prints and. more particularly for making half-tone screens for use in color printing.
• an original color print or transparency is scanned by a photosensitive device that senses the variations in light intensity at each of the three primary color frequencies, namely, red, blue, and green.
• the average light intensity level for each primary color for each incremental area (pixel) of the scanned original is quantized and stored digitally.
• the digital values for red, green, and blue could be from other sources, such as by a programmed computer.
• This data is then processed to convert the values to the equivalent light intensity levels required to produce the same color from the three primary pigment colors, cyan, magenta, and yellow. It is also desirable to introduce a "black" component in addition to the three basic colors in the processed output data.
• a set of four screens are produced, using a suitable plotter, from these four sets of values.
• Each screen called a half-tone screen, is in the form of a grid.
• the grid may be in the form of physical openings or in the form of clear areas of a photographic negative.
• the ratio of the area of each opening or clear area to the surrounding opaque area is determined by the required color intensity for the particular pixel of the reproduced image.
• Each screen is then used to lay down a grid pattern of dots of the associated one of the three primary colors on the color print reproduction.
• the resulting color print is a reproduction of the original but composed of certain patterns of four dots of varying size.
• the human eye integrates these dot patterns into the various color tones and detail of the original.
• the present invention is directed to an improved method of producing half-tone screens for color reproductions.
• the screens do not require relative angular rotation to avoid the Moire effect. Instead, a rectilinear transposition of successive halftone screens is used to produce the color reproduction.
• the shape and relative position of dots in the different separations are chosen to minimize color shifts due to variations in registration at the printing stage. More accurate conversion from the RGB colors to the cyan, magenta, yellow, and black is possible due to a more regular pattern of superposition. In the past this conversion has required a geometrical analysis that is complex and uses some simplifying assumptions and approximations that introduce some deterioration of color quality.
• the present invention reduces the computational steps required to process the color data.
• each screen is created by dividing the screen area into a plurality of cells, each cell corresponding to one half ⁇ tone period.
• Each cell is formed by the plotter as a binary matrix of elemental areas, the plotter creating each elemental area in one of two states, e.g., clear or opaque.
• the screen function matrices have the same dimensions as the cell matrices, so that each elemental area in a cell has a corresponding intensity level value in the screen function matrix.
• Each cell of a picture screen is mapped to one or more particular pixel areas of the picture being reproduced.
• the binary state for each elemental area within a cell is determined by comparing the corresponding intensity level value in the screen function matrix with the stored intensity level values derived from the associated pixel areas.
• FIG. 1 is a block diagram of an embodiment of the invention
• FIG. 2 is a diagram showing the form of the data generated by the scanner from an original color picture being reproduced
• FIG. 3 is a diagram showing the form of the data after conversion for printing
• FIG. 4 is a diagram of screen cell as form by the plotter
• FIGS. 5A-D are sets of diagrams showing examples of the four screen function matrices
• FIGS. 6-9 are diagrams of screen cell patterns for each basic color at four different color intensities.
• FIG. 10 is a diagram showing the superimposed color positions.
• the numeral 10 indicates generally a scanner for receiving the original color picture that is to be reproduced.
• the original may be a color photograph, for example, either a print or transparency.
• the scanner may be of any well-known optical scanning device in which the picture is traversed in a raster type pattern by a photosensitive element that detects the level of reflected or transmitted light for each of the primary colors, red, green, and blue. These detected light intensity levels are converted to three electrical analog signals which are connected to an analog-to-digital (A/D) converter 12.
• A/D analog-to-digital
• the scanner 10 generates two signals x and y which define the pixel position and the scanning line respectively of the scanning element as it moves relative to the picture.
• the A/D converter 12 is synchronized with the pixel position signal x so that a digital output is generated with each predetermined incremental advance of the scanning element.
• the picture is converted by the scanner 10 and A/D converter 12 into a series of picture elements (pixels) , the average light intensity in each pixel for each of the three basic colors being a digital value, preferably on a scale of O to 1.
• pixels picture elements
• the invention is not limited to any particular method of generating the digital color image data.
• graphical information generated by a computer may be the source of the color image data.
• the three digitized red, green, and blue output signals R, G and B, together with x and y position signals, are inputted to a digital computer 14.
• the computer stores the data in memory as a picture matrix, shown by the diagram of FIG. 2.
• the positions X m ,Y n in the stored picture matrix correspond to the pixel positions of the scanned original.
• the three intensity level values R, G, and B for each pixel are stored in the corresponding position in the picture matrix.
• the computer processes the data to convert it to an equivalent set of intensity values for the primary printing colors cyan, magenta, and yellow.
• the mathematics for this conversion process is well known. See for example "Principles of Color Reproduction” by J.A.C. Yule, Wiley & Sons, 1967, Chapters 10 and 11.
• a fourth set of values for black are also preferably computed.
• the resulting data is stored in a print matrix, as shown in FIG.3, the four intensity level values C, M, Y, and K for each pixel being stored in the corresponding position x, y in the print matrix.
• the print matrix data is used by the computer to provide control information to a plotter 16 which creates the four half-tone screens 18 required to make the color prints.
• the plotter is a conventional high resolution x-y plotter.
• the plotter printing element or stylus may be a laser beam, an ink jet, or other device capable of producing, on command, a contrasting dot in an elemental area on whatever medium the screen is being formed.
• the stylus can be positioned by x-y coordinate digital input signals at any selected incremental area within the plotting range.
• the screens may take a variety of forms depending on the particular printing process employed, such as a photographic negative. Once created, the screens are used in a conventional printer 20 to produce color prints.
• the command signal for the stylus and the position control signals for the plotter are produced by the computer in the following manner.
• the screen 18 being created is divided logically into cells 22, each cell corresponding to one half-tone period of the screen.
• the cell size depends on the definition of the printing process. For example, printing on newsprint provides relatively poor definition and therefore the size of a cell can be relatively large.
• the cell size is also limited by the elemental size (ES) of the plotter stylus. The more plotter dots within a cell, the better the color range of the reproduced prints. If the cell size is the same as the size of an elemental area, for example, no half ⁇ tones can be reproduced.
• the computer stores a different value at each position in a screen function matrix, taken from a set of values representing all the levels of color intensity on a scale of 0 to 1.
• the number of increments into which the intensity scale is divided is equal to the number of positions in the matrix, namely, n x n.
• four screen function matrices 26 are shown by way of example, one for each of the four basic colors, cyan, magenta, yellow, and black. In the figures, the numbers of the positions of each increment on the intensity scale are shown in place of the actual intensity level values.
• the computer controls the plotter to advance the stylus from one elemental area to the next in a predetermined sequence.
• the computer issues a binary control signal to the stylus to either activate the stylus or not.
• the binary control signal is set by comparing the required color intensity level value for the particular half-tone period or cell as derived from the print matrix (see FIG. 3) , with the intensity level value stored in the screen function matrix for the particular stylus position (elemental area) within the cell. For example, only if the required color intensity level is greater than the value derived from the screen function matrix will the computer activate the stylus (or not activate the stylus depending on the particular printing process) .
• Figures 6-9 show the plotted cells for each color at four different levels of color intensity.
• the number of pixels generated by the scanning process is to a degree independent of the number of cells in the printing process, the number of pixels being determined by the size (SS) of each pixel relative to the size of the original being scanned.
• the cell boundaries do not necessarily coincide with the pixel boundaries, so that one cell may involve the intensity values from more than one of a group of adjacent pixels.
• the intensity value of a single pixel may be used to control the plotter in more than one cell.
• a set of screen function matrices for the four colors, cyan, magenta, yellow, and black is shown for a cell formed, for example, as an 8 8 binary matrix of plotter elemental areas.
• the dimensions of the binary matrix are fixed by the ratio of the required cell size to the size of the elemental area produced by the particular plotter.
• the intensity level numbers stored in each matrix are arranged in a predetermined pattern of positions which produce unique color patterns in the screen cells.
• Figures 6-9 illustrate the color patterns generated for each of the four colors at each of four color intensity levels, namely, at 12.5%, 37.5%, 50% and 81.8% color intensity, respectively.
• V 4 (i,j) V x (i, (j+n/2) mod n)
• V 2 (i,j) V ⁇ ((i+n/4) mod n, (j+n/4) mod n)
• the numbers in the function screen matrices for magenta and yellow respectively approximately center two color areas respectively at the centers of two adjacent boundaries of the cell, and at the center and one corner of the cell.
• the centers of the color areas for all four basic colors are arranged in the pattern shown in FIG. 10. It will be noted that, with increasing color intensity, the pattern of numbers in the screen function matrices produces expanding areas of color in the cells by causing additional contiguous elemental areas to be added to each of the two color areas. In the case of magenta and yellow, because some of the color areas are centered on the boundary of the cell, the areas expand into the adjacent cells.
• each cell has only two areas centered in the cell, some cells, at higher than the minimum color intensity level, may, in effect, have more than two color areas within the boundaries of the cell.
• the color areas for cyan and yellow are located at equally- spaced interspersed positions along a first diagonal A, while the color areas for magenta and black are located at equally-spaced interspersed positions along a second parallel diagonal B.
• This arrangement insures the maximum spacing and minimum overlap between the colors in the pattern of color dots produced by the four screens. While a diagonal orientation of the color dots is preferred for best visual results, arrangement for the lines A and B at an arbitrary angle is equally possible.
• the primary printing colors have been assigned to specific screen matrices by way of example. In practice, this assignment can be arbitrarily changed.

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

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• 1990
  • 1990-03-07 CA CA002050289A patent/CA2050289A1/en not_active Abandoned
  • 1990-03-07 WO PCT/US1990/001248 patent/WO1990010991A1/en not_active Ceased
  • 1990-03-07 EP EP19900905852 patent/EP0465558A4/en not_active Withdrawn

Patent Citations (2)

Non-Patent Citations (1)

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