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WO2007133192A2 - Correction de luminance à déphasage de teinte réduit - Google Patents

Correction de luminance à déphasage de teinte réduit Download PDF

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Publication number
WO2007133192A2
WO2007133192A2 PCT/US2006/017587 US2006017587W WO2007133192A2 WO 2007133192 A2 WO2007133192 A2 WO 2007133192A2 US 2006017587 W US2006017587 W US 2006017587W WO 2007133192 A2 WO2007133192 A2 WO 2007133192A2
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WO
WIPO (PCT)
Prior art keywords
color
brightness
primary
gray
color space
Prior art date
Application number
PCT/US2006/017587
Other languages
English (en)
Other versions
WO2007133192A3 (fr
Inventor
Gregory Braverman
Gennady Karvitsky
Shaul Costis
Shlomo Harush
Original Assignee
Hewlett-Packard Development Company, L.P.
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 Hewlett-Packard Development Company, L.P. filed Critical Hewlett-Packard Development Company, L.P.
Priority to PCT/US2006/017587 priority Critical patent/WO2007133192A2/fr
Publication of WO2007133192A2 publication Critical patent/WO2007133192A2/fr
Publication of WO2007133192A3 publication Critical patent/WO2007133192A3/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
    • H04N1/46—Colour picture communication systems
    • H04N1/56—Processing of colour picture signals
    • H04N1/60—Colour correction or control
    • H04N1/6027—Correction or control of colour gradation or colour contrast

Definitions

  • the present invention generally relates to methods, systems and computer program products for brightness correction, and for example, to methods, systems and computer program products for brightness corrections directly performed on a printing press.
  • An image to be printed on a digital printing press is usually provided by a designer who prepares and processes the image for printing, usually in the RGB color space of the device.
  • the color values are translated into the device-dependent color space of the printing press, such as a CMY (cyan, magenta and yellow) color space or the CMYK color space, which further includes black (K) as a fourth component.
  • CMY cyan, magenta and yellow
  • K black
  • the printer Before making a production run of the printing job and printing the image in quantity, generally the printer is controlled to execute a proof run and provide a sample of the printed image which is vetted to determine whether the image is satisfactory. If the proof is satisfactory, a production run of the printing job is executed. If not, brightness corrections are made to the printed image and another proof image is printed and vetted. The process is repeated until a satisfactory proof is obtained and production printing of the image is performed.
  • Some brightness corrections to the printed image can be satisfactorily made "on-press” by adjusting the raster image in the printer or the printer's response to the raster image.
  • Such on-press corrections are generally one-dimensional corrections such as enhancing a single color, e.g. increasing a blue tone in the image.
  • On-press brightness corrections are relatively conservative of labor and materiel and can generally be made quickly and easily using a suitable "brightness correction wizard".
  • More complicated brightness corrections such as brightness or contrast corrections are "multidimensional" corrections that involve interplay between a plurality of color components in the image.
  • brightness and/or contrast adjustments usually require adjusting all three or four color components in a CMY or CMYK printer.
  • Multidimensional brightness corrections are therefore generally made by returning the raster image, with any corrections made thereto, off-press by the designer.
  • the image is reconverted to an image in RGB and the designer works on the RGB image to make desired changes.
  • the vector image is "re-ripped" (i.e. re-processed by the RIP into a raster image) and proofs run of the image again.
  • a method is provided of correcting brightness without, or with reduced, hue- shift of a pixel color represented as a set of primary-color values in a reproducing device's color space, by means of a composite transformation that has the property that it preserves the hueless character of gray pixels and is made up of independent primary-color related transformations.
  • At least some of the primary-color related transformations include applying a virtual color transformation to a primary-color value of the pixel color to be brightness-corrected, thereby transforming the primary-color value into a primary-color value of a virtual gray-idealized color space, applying a brightness change to the primary-color value in the virtual color space, and applying the inverse of the virtual color transformation to the brightness- changed primary-color value, thereby transforming the brightness-changed primary- color value back to a primary-color value of the reproducing device's color space.
  • a printing device including a controller for correcting brightness without, or with reduced, hue-shift of a pixel color represented as a set of primary-color values in a reproducing device's color space, by means of a composite transformation that has the property that it preserves the hueless character of gray pixels and is made up of independent primary-color related transformations.
  • At least some of the primary-color related transformations include applying a virtual color transformation to a primary-color value of the pixel color to be brightness-corrected, thereby transforming the primary-color value into a primary-color value of a virtual gray-idealized color space, applying a brightness change to the primary-color value in the virtual color space, and applying the inverse of the virtual color transformation to the brightness- changed primary-color value, thereby transforming the brightness-changed primary- color value back to a primary-color, value of the reproducing device's color space.
  • a computer program product which is either in the form of a machine-readable medium with program code stored on it, or in the form of a propagated signal comprising a representation of program code, wherein the program code is arranged to carry out a method, when executed on a computer system of correcting brightness without, or with reduced, hue-shift of a pixel color represented as a set of primary-color values in a reproducing device's color space, by means of a composite transformation that has the property that it preserves the hueless character of gray pixels and is made up of independent primary-color related transformations.
  • At least some of the primary- color related transformations include applying a virtual color transformation to a primary-color value of the pixel color to be brightness-corrected, thereby transforming the primary-color value into a primary-color value of a virtual gray- idealized color space, applying a brightness change to the primary-color value in the virtual color space, and applying the inverse of the virtual color transformation to the brightness-changed primary-color value, thereby transforming the brightness- changed primary- color value back to a primary-color value of the reproducing device's color space.
  • Fig. 1 illustrates a printing press which is equipped with a brightness correction unit enabling on-press brightness correction without, or with reduced hue-shift, according to embodiments of the invention
  • Fig. 2a shows the device-independent LAB color space in which planes perpendicular to the brightness axe refer to color sets having different hues but the same brightness;
  • Fig. 2b illustrates a CMY color space of a printing device with gray values located on a curved line connecting the white and black pixel color line, according to embodiments of the invention
  • Fig. 2c shows two brightness correction curves, according to embodiments of the invention.
  • Fig. 3 a shows the workflow of brightness correction, according to embodiments of the invention.
  • Fig. 3b schematically shows a magnified view of an extract of an image (represented in a halftoning raster) before and after the brightness correction, according to embodiments of the invention
  • Fig. 4 shows gray-balance curves M(C), Y(C) as functions of a reference color cyan C, according to embodiments of the invention
  • Fig. 5a shows an example of transforming a non-gray color pixel in an image, according to embodiments of the invention
  • Fig. 5b shows a brightness correction curve transforming color values to brighter color values, according to embodiments of the invention
  • Fig. 6 shows the transformation of a non-gray color pixel in an image in a three-dimensional view, according to embodiments of the invention
  • Fig. 7a illustrates gray-balance curves as functions of a reference color cyan and the transformation of a gray color pixel, according to embodiments of the invention
  • Fig. 7b shows the brightness correction curve of Fig. 5b indicating the transformation of the gray color values of Fig. 7a, according to embodiments of the invention
  • Fig. 8 shows the transformation of a gray color pixel in an image in a three- dimensional view, according to embodiments of the invention
  • Fig. 9 shows a transformation combining the transformations shown in Figs. 4, 5a and 7a with the brightness correction graph shown in Figs. 5b and 7b;
  • Fig. 10 shows a contrast correction function that enhances the contrast of an image, according to embodiments of the invention.
  • Fig. 11 illustrates a first menu of a wizard in which a user enters how many reproduced images she/he wants to be presented, according to embodiments of the invention
  • Fig. 12 illustrates a second menu of the wizard in which a user is requested to change the press status to ready
  • Fig. 13 illustrates a third menu of the wizard in which a user is requested to print a page, according to embodiments of the invention
  • Fig. 14 illustrates a fourth menu of the wizard in which a user is requested to select her/his preferred print-out, according to embodiments of the invention
  • Fig. 15 shows a preview in which three reproductions and the corresponding contrast correction curves are shown, according to embodiments of the invention.
  • Fig. 16 shows a preview in which five reproductions and the corresponding contrast correction curves are shown, according to embodiments of the invention.
  • Fig. 17 shows a preview in which nine reproductions and the corresponding contrast correction curves are shown, according to embodiments of the invention
  • Fig. 18 shows a flowchart indicating the course of actions of determining a preferred color reproduction of an image to be reproduced according to embodiments of the invention.
  • Fig. 19 illustrates a brightness correction computer system which provides the functionality of a brightness correction unit, according to embodiments of the invention.
  • Fig. 1 shows a printing press including a brightness correction unit. However, before proceeding with the description of Fig. 1 a few items will be discussed.
  • a pixel color represented as a set of primary- color values in a reproducing device's color space, is brightness-corrected without, or with reduced hue shift by means of a composite transformation that has the property that it preserves the hueless character of gray pixels and is made up of independent primary-color related transformations.
  • the primary- color related transformations include applying a virtual color transformation to a primary-color of the pixel color to be brightness-corrected. Thereby, the primary- color value is transformed into a primary-color value of a virtual gray-idealized color space.
  • a brightness change is applied to the primary-color in the virtual color space, and the inverse of the virtual color transformation is applied to the brightness-changed primary-color value. Thereby, the brightness-changed primary- color value is transformed back to a primary-color value of the reproducing device's color space.
  • the term "reproducing device” refers to a printing device, such as an inkjet printer or a printing press on which a brightness correction is performed, whereas in other embodiments, the term “reproducing device” refers to a monitor, such as a cathode ray tube display operating in an RGB color space, having brightness correction capabilities.
  • a printing device such as an inkjet printer or a printing press on which a brightness correction is performed
  • the term “reproducing device” refers to a monitor, such as a cathode ray tube display operating in an RGB color space, having brightness correction capabilities.
  • pixel refers to a representation of a color by means of primary-colors.
  • the primary-colors are red, green and blue
  • subtractive color mixing as used in inkjet-printers or printing presses
  • the primary-colors are cyan, magenta and yellow.
  • any color may be obtained.
  • a pixel may therefore be represented as three dots of phosphors on a lighting layer.
  • the three color values of a pixel e.g. (40%;60%;75%), indicate how intensely the three different phosphors of the color monitor have to be fired at with electrons to obtain a certain color.
  • gray-idealized color space refers to a color space, in which, gray- colors are obtained, if, for example, the same coverage of primary-colors are provided. This implies, that gray colors lie on a space diagonal within the color space.
  • virtual as used in combination with “gray-idealized color space” indicates that this color space is not related to a specific reproducing device, but rather refers to an imaginary reproducing device in which the same coverage of the primary-colors refers to a gray color.
  • the inkjet printer is actually unable to print a pixel color as such, since an inkjet printer is only aware of dots printed in one of its primary-colors.
  • profiles are applied which define a mapping from the RGB color space of the monitor into a device-independent color space, such as the LAB color space, and from the LAB color space into the device- dependent CMY color space of the inkjet printer.
  • a pixel is resolved into a multitude of dots within a certain area (herein called "pixel area"), which may be printed by the inkjet printer, whereby each dot is only of one of the primary- colors. This technique is also referred to as "halftoning". To obtain a certain color impression in the eye of the observer, a certain area within a pixel area needs to be covered with dots of each of the three primary-colors.
  • each pixel has a fixed number of dots of each primary-color with fixed distances between the dots, but the dots vary in size.
  • the dots do not have sizes from a continuous set of possible sizes, but may only have sizes from a discrete set of possible sizes. It should be mentioned that the dots are arranged in a raster on the paper. In fact, a raster of dots is calculated for each of the primary-colors and each raster of dots is sprayed one after another onto the paper.
  • the dots are printed at least partially on top of each other, whereas in other halftoning techniques the dots do not overlap, but lie next to each other.
  • the halftoning is performed in a raster image processor (RIP) which calculates a raster of dots for each of the primary-colors of the printing device. Normally, these colors are cyan, magenta, yellow and black.
  • the raster image processor is also able to transform a vector image, in which image elements are composed of graphical primitives such as lines, circles and polygons, in a raster image which can be displayed on a computer monitor in pixel mode.
  • the pixel raster For printing the image by means of an inkjet printer or a printing press, the pixel raster has to be further resolved to dots, as mentioned above.
  • Halftoning means that an image consists of nothing more than individual dots of the primary-colors of the printing device, so that an error is introduced since the dots may only have a discrete size so that not all colors may be obtained. This error can be dispersed on neighboring dots so that the error is diffused over the entire image.
  • a well-known error-diffusion algorithms is for example the Floyd-Steinberg algorithm.
  • the user is enabled to perform brightness corrections directly on the dot raster and does not have to repeat the raster image processing for brightness/contrast - corrected color reproductions represented in the LAB color space, until one has obtained a satisfactory printout.
  • a brightness correction function which indicates the brightness change for a multitude of primary-color values. It should be mentioned that in some of the embodiments, the brightness correction function brightens or darkens all primary-color values (whereby the individual offset may be different for different input values), whereas in other embodiments, the brightness correction function renders some primary-color values brighter and others darker, so that the contrast of the image increases.
  • the term "brightness correction function” may therefore also refer to a contrast correction function.
  • the different pixels with their corresponding primary-color values are brightness-changed one after another
  • the different primary-colors of the image are brightness-changed one after another, i.e. not the pixels are brightness-changed one after another, but first, for example the cyan color values for all pixels, then the magenta color values of all pixels, etc.
  • the virtual color transformation, the brightness change and the inverse of the virtual color transformation are stored in three individual look-up tables.
  • the LUTs are applied one after the other which is a rather time-consuming approach. Therefore, in some of the embodiments, the mappings defined in the three LUTs are integrated into one look-up table that defines a mapping within the printing device's color space. In these embodiments, only one LUT is applied to perform the brightness correction of a pixel color.
  • the brightness correction is performed in a controller of the reproducing device.
  • a primary-color value indicates percentages of a pixel area that is covered by dots of the primary-color according to a halftoning technique.
  • the halftoning technique is implemented either in a frequency modulation mode in which the pixel area is covered by dots having the same size but are different in number or in an amplitude modulation mode in which the pixel area is covered by dots of the same number but with different sizes.
  • the reproducing device's color space is a CMY-color space, in which the color black is obtained by printing maximum areas within a pixel areas with each of the primary-colors cyan, magenta and yellow.
  • black ink For several reasons, the "black” obtained like this is not ideal and so four-color printing uses black ink additionally.
  • the reasons for using black ink include: Maximum areas printed with cyan, magenta and yellow does not yield a pure black, but a dark murky color. Printing maximum areas with dots of each of the primary inks to obtain black, can make the paper rather wet.
  • text is typically printed in black and includes fine details (such as serifs); so to reproduce text using three inks without slight blurring would require impractically accurate registration (i.e. all three raster images would need to be positioned extremely precisely).
  • the reproducing device's CMY-color space is extended by a black component which results in a CMYK color space.
  • the black is referred to as K for key.
  • the CMYK color space is still extended by a light cyan and a light magenta component. This leads to a six-dimensional color space which is also referred to as a CMYKcm color space and improves printing quality.
  • a lighter cyan and magenta allows to reduce the visibility of dots in lighter zones of an image.
  • the color space has even more primary-colors, such as in seven-color printing (red, green, blue, cyan, magenta, yellow and black) invented by Harald Kueppers.
  • the virtual gray-idealized color space is a color space in which hueless gray colors lie on the space diagonal between the white and black color pixel.
  • the gray-idealized virtual color space is a color space in which brightness of a gray color is changed by means of a shift along the space diagonal between the white and black pixel color.
  • the composite transformation that has the property that it preserves the hueless character of gray pixels is determined empirically by varying the dot-percent coverage of a reference color (such as cyan in the example below) and verifying how much of the other colors has to added to obtain a hueless gray color. Normally, one would be inclined to think that the same dot-percent coverage of the other colors has to be added to obtain a gray color, but due to the physico-chemical properties of the inks, the paper and the reproducing device, these dot-percent coverages are not identical and have to be determined by measures.
  • the compositions of the ink colors to obtain a hueless gray color, given a certain reference color are characteristic of a printing device.
  • the composite transformation that has the property that it preserves the hueless character of gray pixels can be conceived as a mapping from a curved line connecting the white and black pixel color representing hueless gray colors in the reproducing device's color space to the space diagonal between the white and black pixel color representing corresponding gray colors in the virtual gray-idealized color space.
  • the transformation is only accurate for hueless gray colors.
  • the brightness correction according to embodiments of the invention is the worse, the farther the pixel colors to be brightness-corrected are away from a gray color value.
  • a user is enabled to select a preferred image reproduction from a set of image reproductions, pertaining to different brightness correction functions, of an image to be reproduced by a reproducing device.
  • the term "brightness correction function" may either relate to the brightness or to the contrast of an image.
  • contrast correction function For changing the contrast of an image, the brightness of individual pixels is altered in that some pixels are darkened and some pixels are brightened. In total, however, the overall brightness of the image remains constant, at least more or less.
  • the brightness of all pixels is either increased or decreased by some offset, so that the overall brightness of an image is changed.
  • the composite transformation that has the property that it preserves the hueless character of gray pixels, the brightness correction function and the inverse of the composite transformation are integrated into one look-up table that can be applied to other images to be reproduced by the reproducing device. Thereby, the brightness reproduction of an image is adapted to a user's preference.
  • Some of the embodiments of the invention enable a user to select a preferred image reproduction from a set of image reproductions. This is performed, in some of the embodiments, by means of a wizard-guided menu.
  • a user-selected number of image reproductions, pertaining to different brightness correction functions are shown on one page within the wizard-guided menu. Accordingly, the user-selected number of image reproductions, pertaining to different brightness correction functions, are printed on one page by the printing device.
  • the user prints the one or more pages containing the selected number of image reproductions and assesses them in terms of their brightness. Then, s/he selects in the wizard-guided menu the image reproduction s/he prefers.
  • the brightness correction functions which correspond to the image reproductions are also shown next to the image reproductions in the wizard-guided menu.
  • Some of the embodiments refer to a reproducing device that includes a controller for correcting brightness without, or with reduced hue shift of a pixel color, represented as a set of color values represented in the reproducing device's color space.
  • the brightness correction is performed in accordance with the embodiments described above.
  • Some of the embodiments of the computer program product with program code for performing the described methods include any machine-readable medium that is capable of storing or encoding the program code.
  • the term "machine- readable medium” shall accordingly be taken to include, for example, solid state memories and, removable and non removable, optical and magnetic storage media.
  • the computer program product is in the form of a propagated signal comprising a representation of the program code, which is increasingly becoming the usual way to distribute software.
  • the signal is, for example, carried on an electromagnetic wave, e.g. transmitted over a copper cable or through the air, or a light wave transmitted through an optical fiber.
  • the program code may be machine code or another code which can be converted into machine code, such as source code in a multi-purpose programming language, such e.g. C, C++, Java, C#, etc.
  • the embodiments of a computer system may be commercially available general-purpose computers programmed with the program code.
  • Fig. 1 which shows a printing press 1 with a brightness correction unit 2 which is able to correct colors in brightness directly on the printing press 1, without having to return the image to be printed to the designer who is able to make corrections in a device-independent color space appropriate for performing brightness or contrast corrections.
  • the printing press 1 enables a user to make brightness or contrast corrections after a raster image processor (RIP) has converted an image source into a raster image which serves as an input for the printing press 1 by providing instructions how to implement the colors of the image in terms of sizes and positions of ink dots.
  • the inks refer to primary-colors of the printing press 1, in the example cyan, magenta and yellow, and span the printing press's 1 color space.
  • the printing press 1 is equipped with a video display 3 via which a wizard guided menu is presented to a user who may select color reproductions according to her/his preferences.
  • the user's decision of a preferred color reproduction is based upon printouts of the image on the printing press 1 and not on color reproductions as shown on the video display 3.
  • the preferred color reproduction is stored by means of a lookup table 4 and may be applied to further images to be printed.
  • the calculation of the lookup table 4 will be explained by means of Figs. 2 to 10, whereas the wizard-guided menu will be explained in Figs. 11 to 18.
  • Fig. 2a shows the LAB color space 10 in form of a color cylinder. In contrast to the RGB or CMY(K) color model, the brightness information is separated from hue information.
  • brightness axe 11 which is perpendicular to both color axes 12 and 13.
  • Axe 12 (labeled with “a”) refers to green colors in its negative domain and to red colors in its positive domain.
  • Axe 13 (labeled with “b”) refers to blue colors in its negative domain and to yellow colors in its positive domain.
  • Hueless gray values are located on the brightness axe 11 ranging from black to white.
  • Each plane within the LAB color space 10 which is perpendicular to the brightness axe 11 (and parallel to the a-b plane) contains colors of different hues but the same brightness.
  • the LAB color space enables changing of the brightness of a color by simply changing the L- coordinate of the color.
  • the L- coordinate of all pixels is shifted by an offset, whereas the a and b coordinates remain unaltered.
  • Fig. 2b illustrates the CMY color space 17 of the printing press 1.
  • hueless gray values lie on a space diagonal 18.
  • the printing press 1 is a physical device using inks with physico-chemical properties that do not obey to the rules of a theoretical color space, all hueless gray values are located on a curved line 19 connecting the white and black pixel color which is determined empirically.
  • the idea is to define a transformation which maps hueless gray colors located on the curved line connecting the white and black pixel color 19 of the CMY color space of the printing press 1 onto the space diagonal 18 of a virtual CMY color space. By means of this mapping, non-gray colors of the color space are transformed accordingly, as will be explained in Figs. 5a and b.
  • Fig. 2c shows a brightness correction curve Bi(x) 20 which adds 20% to the brightness coordinate of a color point and therefore corresponds to the shift of color point 14 of Fig. 2a.
  • a shift by 20% in positive direction refers to brightening an image in the LAB color space.
  • all color values being above 80%, for example color point 16 are mapped to 100% since more than 100% is not possible. Therefore, the image gets brighter, but all color points of the cylinder volume element L G [80..100] are mapped to white. This behavior is undesirable because it removes dynamic from the image in that it maps colors from a brightness between 0 and 100 to a brightness between 20 and 100.
  • B 2 (x) 21 is preferably used.
  • This brightness correction curve 21 has the property that the colors white and black remain invariant, and that color values from a mid range are brightened most, whereas colors in the neighborhood of white and black are less brightened.
  • the brightness correction curve 21 does not alter the dynamic of the image since all brightnesses, from 0% to 100%, may still appear after the transformation. This difference in change of brightness for different percentage ranges may be compared with the brightness correction curve Id(x) 22 which is the identity function leaving all colors unchanged.
  • Fig. 3 a illustrates a brightness correction workflow in accordance with embodiments of the invention.
  • a raster image processor (RIP) 23 color values of an image pertaining to a RGB device-dependent color space 24 are changed into information (or instructions) which is understood by the printing press 1. Since the printing press 1 operates in its own CMY color space, the color values of the RGB color space are transformed by means of profiles.
  • a first ICC-profile 25 maps color values from the RGB color space into the device- independent LAB color space 10. By means of a second ICC-profile 27, these LAB color values are transformed into
  • CMY color space 28 of the printing press 1 Up to here, only a color space transformation has taken place. However, the task of the RIP 23 is to tell the printing press 1 how to realize the CMY color points in terms of typography. To this end, a halftoning procedure 29 transforms colors defined in the CMY color space 17 of the printing press 1 into individual dots to be printed by the printing press 1. In terms of implementing the colors, the printing press 1 is only able to print, at a certain position, a dot of one of its primary-colors or not. It does not mix the inks of the individual primary-colors to obtain any color of the color space.
  • a color impression comes into being in the eye of the beholder.
  • the color dots normally do not mix when being sprayed on the paper, but may overlap partially.
  • a color impression may be implemented by three ink (cyan, magenta and yellow) dots of different sizes located close to each other.
  • a raster of dots wherein a small area is covered with dots of the primary-colors according to percentages for the individual primary-colors. More precisely, there is a raster for each of the inks and the rasters are sprayed one after the other.
  • a raster of dots may also be referred to as a bitmap.
  • the halftoning 29 also introduces some fault since only a discrete set of colors may be implemented, since the primary- color dots are only provided in discrete sizes. Therefore, an error diffusion procedure 30 is used to distribute local errors across larger parts of an image.
  • the bitmaps obtained after the error diffusion 30 are then transmitted to the printing press 1 and are stored in the storage of the printing press 1.
  • the color values are directly provided by the designer in a CMY raster format 31, no conversion needs to be performed on the color values, and the CMY color values are directly stored in the storage of the printing press 1 at 32.
  • the color values are subjected to a transformation T "1 from the CMY color space of the printing press 1 into a virtual gray-idealized color space 33.
  • a brightness correction function in the example brightness correction function B(x) 34, is applied to the color values in the virtual color space 33.
  • a transformation T transforms the color values of the virtual gray-idealized CMY color space 33 back into the CMY color space of the printing press 1, so that the printing press 1 has the colors coded with respect to its own color space.
  • the image is printed and the user may assess different print-outs obtained by applying different brightness correction functions and may then opt for her/his preferred color reproduction of the image.
  • the transformation T is defined as a function R 3 -> R 3 which transforms color values from the virtual gray-idealized CMY color space 35 into the CMY color space 32 of the printing press 1.
  • the transformation T "1 is a function which transforms color values from the CMY color space of the printing press 1 back into the virtual gray-idealized CMY color space.
  • the magenta component is first transformed from the CMY color space of the printing press 1 back into the virtual CMY color space, the brightness correction curve is applied, and then the color value is retransformed into the CMY color space of the printing press 1. This is expressed by means of the following formula:
  • Fig. 3b schematically illustrates the brightness correction as shown in Fig. 3 a.
  • An extract of a dot raster 38 representing an image before correction is indicated, whereas the same extract of the dot raster 39 is shown after correction below.
  • the dot raster at 38 has been calculated by the raster imaging processor 23 (steps 24 to 30 in Fig. 3a), whereas the dot raster 39 has further been subjected to the brightness correction (steps 32 to 36 in Fig. 3 a) without any further processing by the raster imaging processor 23.
  • a raster is calculated for each color individually and is printed one after the other.
  • the rasters for the individual colors are shifted by a certain angle.
  • each set of three primary-color dots defines a color which is perceived by an observer.
  • a color is determined by the percentage area covered by each dot.
  • the distances between the centers of the dots remain constant, whereas their radiuses depend on the area required for obtaining a certain color.
  • the brightness correction effects that in the first cell of the upper dot raster in comparison to the first cell of the dot raster below, the radius of the cyan dot has been reduced, the magenta dot has been increased and the yellow dot has not been altered.
  • the dots are printed next to each other, whereas some halftoning techniques also allow the dots to be printed on top of each other.
  • Fig. 4 shows a schematic graph of values or amounts of magenta and yellow that are required to be combined with amounts of cyan in a printing press to print hueless gray pixels in an image at different brightnesses.
  • Cyan by way of example, is the designated reference color component in graph 40 and the abscissa of the graph is graduated in values of C in dot-percent coverage. Dot-percent coverage for M and Y that provide a hueless gray for a given dot-percent coverage of C along the abscissa are indicated by the ordinates of gray balance curves M(C) 42 and Y(C) 43 respectively for the given dot-percent coverage of C. A gray balance curve 41 for cyan, which has a slope of one because cyan functions as the reference color component, is also given for comparison. For convenience of presentation, the color component with which a gray balance curve is associated is given in parentheses next to the numeral labeling the curve.
  • graph 40 does not show a correspondence between a given C value and a particular brightness, (the abscissa is graduated in values of C not brightness) values of C equal to 0 and 100 substantially correspond to white (no print) and black (darkest gray printable) respectively.
  • magenta and yellow gray balance curves 42 and 43 shown in graph 40 are not universal.
  • the gray balance curves will in general vary with the printing press, the particular CMY inks used to print an image using the press and the paper on which the image is printed.
  • the C, M(C) and Y(C) gray balance curves are represented in LUTs 45, 46 and 47 below.
  • Figs. 5a and b schematically illustrate performing a brightness correction, which by way of example is a brightness correction, on a non-gray color of a pixel in an image, in accordance with an embodiment of the present invention.
  • Fig. 5a shows graph 40 shown in Fig. 4 in which, additionally, the original values (dot percent coverage) of the cyan, magenta and yellow color components of the pixel color are indicated along the ordinate of the graph by witness lines 51, 52 and 53 respectively.
  • the ordinate corresponds to the CMY color space of the printing press 1 from which the color values are taken.
  • the color values are transformed into the virtual CMY color space, which corresponds to the abscissa.
  • the values for the C, M and Y color components are assumed to be equal to 30, 36 and 22. Each of these values, together with the letter indicating the color component with which it is associated, is given in parentheses next to its corresponding witness line.
  • C, M and Y points 61, 62 and 63 are determined that are located respectively on C, M(C) and Y(C) gray balance curves 41, 42 and 43 and have ordinates equal respectively to the ordinates 51, 52 and 53.
  • C, M and Y points 61, 62 and 63 are graphically represented by circle inscribed plus signs and as a visualization aid, an arrow leads from each ordinate 51, 52 and 53 towards its associated C, M or Y point.
  • the abscissas of C, M and Y points 61, 62 and 63 are then determined.
  • the abscissas are respectively equal to 30, 47 and 34 and are indicated on graph 40 by abscissa witness lines 71, 72 and 73.
  • the color component and value of the abscissa associated with a given abscissa witness line 71, 72 and 73 is given in parentheses next to the witness line. It is noted that as a result of the pixel color being non-gray, at least two of the abscissas of the C, M and Y points corresponding to the color component values of the pixel color have different values. Were the pixel color to be gray, all the abscissas would have a same value, as is shown in Figs. 7a and b.
  • the color values 71, 72 and 73 are now in the virtual color space, in which gray colors lie on the space diagonal and in which a brightness correction is performed according to the brightness correction curve shown in Fig. 5b.
  • the brightness correction to be applied to the original pixel color having C, M and Y values is, for example, defined by a schematic graph 78 shown in Fig. 5b.
  • a "brightness correction curve" B(x) 80 provides a relationship between an "input” dot percent coverage of a pixel color in the image that is to be brightness corrected and a brightness corrected "output" dot percent coverage for the pixel color.
  • the abscissas of C, M and Y points 61, 62 and 63 are transformed responsive to brightness correction curve 80 to provide a new "brightness corrected" value for the abscissa of each color component C, M and Y of the pixel color.
  • the new brightness corrected abscissas for C, M and Y are equal to the ordinates of brightness correction curve B(x) 80 at respectively the "old" abscissas 71, 72 and 73 determined from the gray balance curves 41, 42 and 43 shown in Fig. 5a. It should be mentioned that the brightness correction curve B(x) 80 maps color values from the virtual color space into the virtual color space, i.e. a change of the color space does not take place.
  • ordinates in Fig. 5b corresponding to abscissas 71, 72 and 73 are equal respectively to 65.5, 84 and 71.5 and are indicated in Fig. 5b by ordinate witness lines 81, 82 and 83.
  • dashed vertical and horizontal lines connect corresponding abscissas and ordinate witness lines.
  • new brightness corrected C, M and Y points 91, 92 and 93 on gray balance curves 41, 42 and 43 respectively in Fig. 5 a are located.
  • the new brightness corrected points graphically represented as "diamond inscribed" plus signs, have abscissas equal respectively to the brightness corrected abscissas 81, 82 and 83 determined from brightness correction curve 80 (Fig. 5b).
  • the brightness corrected values for the C, M and Y color components of the brightness corrected pixel color are equal to the ordinates of new, brightness corrected C, M and Y points 91, 92 and 93.
  • the ordinates and thereby the new brightness corrected C, M and Y values are equal respectively to 68, 62.5 and 55.5, which are indicated by ordinate witness lines 101, 102 and 103.
  • an arrow leads from each brightness corrected C, M and Y point 91, 92 and 93 to its associated ordinate witness line 101, 102 or 103.
  • Dashed horizontal and/or vertical lines connect each old C, M and Y point 61, 62 and 63 with its corresponding brightness corrected C, M and Y point 91, 92 and 93.
  • Fig. 6 illustrates the transformation T "1 (mentioned in Fig. 3 a) of a non-gray color from the color space of the printing press into the virtual gray-idealized CMY color space in a three-dimensional view.
  • the space diagonal 110 refers to the color space in which all hueless gray colors lie on the space diagonal
  • the curved line 111 refers to the CMY color space of the printing press 1.
  • the curved line may be represented in a parameterization as follows:
  • C, M(C) and Y(C) are the gray balance curve 41, 42 and 43, shown in Fig. 4.
  • mapping T By means of the mapping T, all points of the space diagonal 110 (the gray values of the virtual gray-idealized color space) are mapped onto the curved line connecting the white and black pixel color 111 (the gray values of the color space of the printing press 1).
  • a mapping with this property may also be used to map non- gray colors accordingly.
  • non-gray color 112 pertaining to the color space of the printing press 1 is mapped on its corresponding color in the virtual gray-idealized CMY color space.
  • a plane 113 is regarded which is parallel to the C-M plane and which includes the non-gray color 1.12.
  • the two points of intersection 114 and 115 of the space diagonal 110 and the curved line 111 are regarded and the difference in direction to C is calculated, which is referred to as ⁇ Y 116.
  • a plane 117 is regarded which is parallel to the C-Y plane and all points of the plane 117 have the same M values as point 112.
  • the two intersection points 118 and 119 of the space diagonal 110 and the curved line 111 are regarded and the difference in direction to C is calculated, which is referred to as ⁇ M 120. Consequently, the point 112 to be transformed is shifted by ⁇ Y 116 in positive direction of the Y-axis, and by ⁇ M 120 in positive direction of the M-axis.
  • the point obtained 121 corresponds to the point 112 (pertaining to the color space of the printing press 1) in the virtual color space. This point 121 may then be transformed by means of the brightness correction curve and may then be retransformed to the color space of the printing press 1. (The last two steps are not depicted in Fig. 6.)
  • Figs. 7a and b are similar to Figs. 5a and 5b respectively.
  • Fig. 7a shows the graph 40 in which values of C, M and Y color components of a gray pixel in an image are indicated along the ordinate of the graph by witness lines 131, 132 and 133.
  • the C, M and Y values of the gray pixel color are assumed, by way of example, to be equal to 38.5, 26 and 22.8 respectively.
  • Corresponding C, M and Y points 141, 142 and 143 on gray balance curves 41, 42 and 43 respectively are indicated graphically by plus signs inscribed in circles.
  • Fig. 7b shows graph 78 and brightness correction curve 80 shown in Fig. 5b, which is used, as in the case of non-gray pixel color, to perform the brightness correction of the gray pixel color.
  • the C, M and Y points on the gray balance curves in graph 40 corresponding to a gray color have a same abscissa in the graph, as noted in the discussion of Fig. 5a.
  • C, M and Y points 141, 142 and 143 respectively have a same abscissa, which has a value equal to 36 and is indicated by abscissa witness line 150.
  • the new brightness corrected abscissa has a same value for all three components.
  • the brightness corrected abscissa is equal to 73 and is indicated by ordinate witness line 150.
  • New brightness corrected C, M and Y points 151, 152 and 153 corresponding to the brightness corrected abscissa determined from graph 78 in Fig. 7b are represented graphically in graph 7a by diamond inscribed plus signs.
  • the brightness corrected values for the C 5 M and Y components 131, 132 and 133 of the gray pixel are equal to 74, 54 and 57.9 respectively and are indicated by witness lines 161, 162 and 163 respectively. Since all the brightness corrected C, M and Y points have a same abscissa the brightness corrected gray pixel color is also a gray color and the correction preserves the non-hue gray quality of the original pixel color.
  • the combinations of color component values that provide gray colors are referenced to one of the color component values, which in the instant case is the C color component.
  • the color components may be referenced to any suitable reference variable for which, for a given value of the reference variable, the associated values of the color components provide a gray color.
  • a suitable reference variable may be brightness.
  • Fig. 8 shows in a tree-dimensional view how a gray color point 170 located on the curved line connecting the white and black pixel color 111 is going to be transformed into the virtual color space.
  • a plane 171 is considered which is parallel to the C-M plane and has the same Y-value as point 170.
  • the intersection between the diagonal 110 and the plane 171 is point 172, and the distance between point 170 and point 172 is ⁇ Y 173.
  • Another plane 174 is considered which is parallel to the C-Y plane and has the same M- value as the point to be transformed 170.
  • the distance between the point 175 and the point 170 direction to C is considered as ⁇ M 176.
  • the point 170 can now be shifted by ⁇ M 176 in direction to M, and by ⁇ Y in direction to Y. Thereby, the point 170 is transformed into point 176 located on the diagonal 110 and represents a gray value in the virtual color space.
  • Fig. 9 shows a combined brightness correction graph 180 corresponding to gray balance graph 40 (Figs. 4, 5a or 7a) and brightness correction graph 78 (Figs. 5b, 7b) which converts a dot-percent coverage input value for a color component into a brightness corrected dot-percent coverage output value.
  • Graph 180 includes
  • Graph 180 is useable to perform the same function as graphs 40 and 78 and provides the same results. For example, in performing the brightness correction illustrated using Figs. 5a and b, original C, M and Y values 30, 36 and 22 were brightness corrected to corresponding brightness corrected values 68, 62.5 and 55.5 respectively. The same results are obtained using graph 180 by determining the ordinates of C, M and Y brightness correction curves 80, 186 and 188 respectively for values along the abscissa of the graph equal to the original C, M and Y values 68, 62.5 and 55.5 respectively.
  • abscissa witness lines 191, 192 and 193 indicate the original C, M and Y input values.
  • Corresponding output brightness corrected C, M and Y values are indicated by ordinate witness lines 201, 202 and 203 respectively. Dashed vertical and horizontal lines connect corresponding input and output C, M and Y values.
  • Fig. 10 shows a schematic graph 208 having a contrast correction curve 210 that defines a contrast correction.
  • contrast correction curve 210 provides a relationship between an input dot-percent coverage and an output dot percent coverage.
  • Curve 210 is used similarly to the manner in which curve 80 is used, i.e. to provide a new abscissa for an old abscissa of color component values of a pixel color.
  • the curve 210 has a S-shape which is typical of contrast correction curves. In a first interval, i.e. interval [0%..25%] the contrast between two pixels lying within this interval is decreased.
  • the contrast correction curve does a compression in this interval, since the slope s of the contrast correction curve is between zero and one.
  • a second interval i.e. interval [25%...75%] the contrast between two pixels lying within this interval is increased which corresponds to a dilatation, since the slope s of the contrast correction curve is above one in this interval.
  • a third interval [75%..100%] the contrast between two pixels lying in this interval is decreased.
  • the contrast correction curve performs a compression in this interval, and the slope s of the contrast correction curve is between zero and one.
  • a brightness correction curve such as curves 80 and 210
  • Brightness correction that preserves gray balance and at least approximately color balance, may be implemented in accordance with the present invention using substantially any single valued brightness correction curve.
  • Figs. 11 to 17 pertain to a graphical user interface by means of which a user may select her/his preferred brightness correction.
  • a first page "Settings" of an interactive brightness correction wizard 220 is shown, which enables the user to select in a print settings menu 221, whether s/he wishes to have a single preview, three preview, five previews or nine previews printed on one page. If the user decides to print each preview on a single page, s/he is enabled to select in box "Steps" how many previews she/he wants to print. The steps number has to be odd, larger than 3 and less than 11.
  • a color settings menu 222 the user is enabled to choose whether s/he wants to make correction for a single separation or for all of them together. In the first case, s/he will choose the separation name from the combo - box together with the ink.
  • This combo-box is enabled, if single separation correction mode check box is ticked. If the job is not of the CMYK or CMY color set, the single separation mode is forced: the check box is disabled and ticked and the combo-box is always enabled.
  • a press operator defines the type of corrections, which are to be done in the bottom part of the wizard dialogue.
  • the contrast range is bounded from -15 to 15, and the brightness range is limited to -20 and 20.
  • the brightness and contrast check boxes cannot be ticked simultaneously.
  • the "Next" button is enabled only if one of the brightness or contrast check boxes are ticked. When "Cancel” button is pressed, the whole process is canceled without any job modification. At this stage, there is no need in confirmation.
  • Fig. 12 shows a second brightness correction wizard menu page, referred to as "Go to ready". This page is activated only, if the press is not in the ready state, when the "Next" button of the "Settings” (Fig. 11) or “Results” page (Fig. 13) is pressed.
  • Fig. 13 illustrates a third brightness correction wizard menu page, referred to as "Print”. It is activated, if the press is in the ready state after the "Settings", “Results” and “Go to Ready” pages. The sentence “Click the Print button to proof the page” is changed to “Click the print button to proof the element” in the case when element correction was chosen.
  • Fig. 14 illustrates a fourth wizard menu page, named "Results”. This page appears each time, when a print is stopped. It is similar to the first wizard menu page “Settings” shown in Fig. 11. In addition to this page, it has a graphical user interface for choosing the best printed configuration. When the "Finish” button is pressed, the following pop-up will appear: “Your look-up tables are going to be changed.” with two options: OK and Cancel.
  • Fig. 15 illustrates on the left side the placement of three preview images on a single page, and the corresponding brightness correction curves are shown on the right side.
  • the new element maximal height is given by the following formula
  • Fig. 16 shows on its left side the placement of five preview images on a single page, and the corresponding brightness correction curves are shown on the right side.
  • new element maximal width w new and height h new are calculated as follows:
  • Fig. 17 shows on its left side the placement of nine preview images on a single page, and the corresponding brightness correction curves are shown on the right side.
  • the maximal width and height of the image element is given by the following expression:
  • CMY set of printing inks i.e. color components
  • other sets of printing inks may be used in the practice of the present invention.
  • printing ink sets including RGB inks or printing
  • Fig. 18 illustrates a flowchart of the course of actions of on-press brightness corrections.
  • the user is enabled to set print settings which means that he selects how many previews he wishes to print on a single page.
  • the print pages are printed on the press, and at 244, the user is prompted to assess the print-outs. In a menu, similar to the settings page, he chooses the best configuration of the printed previews at 245.
  • Fig. 19 is a diagrammatic representation of a computer system which provides the functionality of the brightness correction unit 2 of Fig. 1, and is therefore denoted as "brightness correction computer system 2".
  • the brightness correction computer system 2 includes a processor 250, a main memory 251 and a network interface device 252, which communicate with each other via a bus 253.
  • it may further include a static memory 254 and a disk drive unit 255.
  • a video display 3, an alpha-numeric input device 256 and a cursor control device 257 may form a brightness correction user interface.
  • the network interface device 252 connects the brightness correction computer system 2 to the Internet.
  • a machine-readable medium on which the software 258 resides may also be a data carrier 259 (e.g. a non-removable magnetic hard disk or an optical or magnetic removable disk) which is part of disk drive unit 255.
  • the software 258 may further be transmitted or received as a propagated signal 260 via the Internet through the network interface device 252.
  • the embodiments of the invention described above allow for an on- press contrast or brightness correction without any transformations into a LAB color space, whereby hueless gray values are only changed in brightness or contrast and non-hueless gray values are mapped approximately.

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Image Processing (AREA)
  • Facsimile Image Signal Circuits (AREA)
  • Processing Of Color Television Signals (AREA)

Abstract

L'invention concerne un procédé de correction de luminance sans déphasage de teinte ou avec un déphasage de teinte réduit d'une couleur de pixel représentée sous forme d'ensemble de valeurs de couleurs primaires dans un espace de couleur d'un dispositif de reproduction, par l'intermédiaire d'une transformation composite qui a la propriété de préserver le caractère sans teinte de pixels gris et qui est constituée de transformations associées à des couleurs primaires indépendantes. Au moins certaines des transformations associées à des couleurs primaires consistent à appliquer une transformation de couleur virtuelle à une valeur de couleur primaire de la couleur du pixel dont la luminance doit être corrigée, ce qui permet de transformer la valeur de couleur primaire en valeur de couleur primaire d'un espace de couleur virtuel idéalisé en gris. Un changement de luminosité est appliqué à la valeur de couleur primaire dans l'espace de couleur virtuel, et l'inverse de la transformation de couleur virtuelle est appliquée à la valeur de couleur primaire dont la luminosité a changé. La valeur de couleur primaire dont la luminosité a changé est ainsi retransformée en valeur de couleur primaire de l'espace de couleur du dispositif de reproduction.
PCT/US2006/017587 2006-05-08 2006-05-08 Correction de luminance à déphasage de teinte réduit WO2007133192A2 (fr)

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Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE4309878A1 (de) * 1992-04-06 1993-11-04 Hell Ag Linotype Verfahren und einrichtung zur analyse und korrektur der bildgradation in bildvorlagen
EP0732844A2 (fr) * 1995-03-16 1996-09-18 Dainippon Screen Mfg. Co., Ltd. Dispositif de correction du contraste
DE19701527A1 (de) * 1996-01-18 1999-05-12 Ricoh Kk Bildakzentuierungseinrichtung
US20030053690A1 (en) * 2001-07-06 2003-03-20 Jasc Software, Inc. Automatic contrast enhancement
US6917444B1 (en) * 1999-07-29 2005-07-12 Minolta Co., Ltd. Image correction for color image
EP1569438A2 (fr) * 2004-02-26 2005-08-31 Fuji Photo Film Co. Ltd. Système, appareil et support de stockage de programme pour la conversion de couleurs

Patent Citations (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE4309878A1 (de) * 1992-04-06 1993-11-04 Hell Ag Linotype Verfahren und einrichtung zur analyse und korrektur der bildgradation in bildvorlagen
EP0732844A2 (fr) * 1995-03-16 1996-09-18 Dainippon Screen Mfg. Co., Ltd. Dispositif de correction du contraste
DE19701527A1 (de) * 1996-01-18 1999-05-12 Ricoh Kk Bildakzentuierungseinrichtung
US6917444B1 (en) * 1999-07-29 2005-07-12 Minolta Co., Ltd. Image correction for color image
US20030053690A1 (en) * 2001-07-06 2003-03-20 Jasc Software, Inc. Automatic contrast enhancement
EP1569438A2 (fr) * 2004-02-26 2005-08-31 Fuji Photo Film Co. Ltd. Système, appareil et support de stockage de programme pour la conversion de couleurs

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