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
  • H04N1/40087—Multi-toning, i.e. converting a continuous-tone signal for reproduction with more than two discrete brightnesses or optical densities, e.g. dots of grey and black inks on white paper
• • 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
• • 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
  • H04N1/6016—Conversion to subtractive colour signals
  • H04N1/6019—Conversion to subtractive colour signals using look-up tables
• • 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
  • H04N1/603—Colour correction or control controlled by characteristics of the picture signal generator or the picture reproducer
  • H04N1/6033—Colour correction or control controlled by characteristics of the picture signal generator or the picture reproducer using test pattern analysis

Definitions

• the present invention relates to the field of image rendering by means of printing devices, particularly multicolor output devices; the invention especially concerns characterization of these devices .
• CMYK cyan, magenta, yellow and black
• the colorant values range from 0% (no colorant laid down on the receiving substrate such as paper) to 100% (maximum amount of colorant laid down on the receiving substrate) .
• colorant space is meant an n-dimensional space with n the number of independent variables with which the printer can be addressed.
• the dimension of the space corresponds to the number of inks of the printer.
• the dimension of the colorant space is normally four .
• the colorant gamut is defined by the possible combinations of the colorant values, normally ranging from 0% to 100%. If there are no colorant limitations, the colorant gamut is a n-dimensional cube.
• color space is meant a space that represents a number of quantities of an object that characterize its color. In most practical situations, colors will be represented in a 3-d ⁇ mens ⁇ onal space such as the CIE XYZ space. However, also other characteristics can be used such as multi-spectral values based on filters that are not necessarily based on a linear transformation of the color matching functions to represent color.
• a printer model is a mathematical relation that expresses color values as a function of colorants for a given printer.
• the variables for the colorants are denoted as c,,c 2 ,...,c__ with n the dimension of the colorant space.
• An n-ink process is completely characterized by its colorant gamut with a number of colorant limitations and the printer model. Because of this close relationship between an n-ink process and the printer model, the operations typical for a printer model are also defined for the n-ink process.
• the printer model is often based on a printer target.
• a printer target consists of a number of uniform color patches, defined in the colorant space of the printing device.
• the printer target is printed and measured, and based on the values of the patches in colorant space and the measured values, the printer model is made.
• a printer target is normally based on a number of sampling points along the different colorant axes. Based on the sampling points a regular grid can be constructed in colorant space of which a number of grid points are contained by the printer target. Hence a target can be said to be complete or incomplete (see EP-A-1 146 726 for complete and incomplete printer targets) .
• Creating the printer model is also called characterizing the printer; this is an important step in the consistent reproduction of images.
• a printer Before a printer is characterized, it is first calibrated, i.e. put in a standard state.
• the printer model When the printer model is created, it can be inverted in order to obtain a so-called characterization transformation (or inverse printer model) .
• the characterization transformation transforms given colors from color space (typically CIELAB) to the colorant space of the printing device, whereas the printer model transforms given colorant values in the colorant space of the printer to color values in color space.
• the calculation of the correct amounts of colorant for the rendering of color images on a printer is also called the color separation problem.
• Most of the color separation strategies known in the art comprise the following steps.
• a first step the relation between the amounts of colorants and the resulting colors on a printer is characterized. This is done by first printing a set of colorant combinations that spans the dynamic range of the printer and measuring the resulting colors.
• An example of such a set is the ANSI IT8.7/3 reference target.
• printer models can be used, ranging from analytical models simulating the printing process, over polynomials approximating the global behavior of the printer, to localized approximations of the printer in the colorant domain.
• the inverse relation embodied in the inverse LUT, gives the colorant values required to obtain given color values, i.e. it represents the characterization transformation of the printer.
• a LUT is often characterized by a number of sampling points (or sampling values) per axis. Based on these sampling points, usually a regular grid is constructed. However, it is also possible to construct LUT' s with non-regular grids. Also in this case the LUT's can be characterized by sampling points per axis but not all combinations of the sampling points of the different axes result in grid points.
• EP-A-1 146 726 for more information on grids, printer models, complete and incomplete printer targets, and related terms
• patent application EP-A-1 083 739 for more information on calibration, characterization, and other relevant terms.
• printers have, for one or more of the colorants, multi- density inks, i.e. two or more inks that have a different density and a similar hue, e.g. light cyan and heavy cyan.
• multi-density inks the apparent visual resolution of the printed images can be increased.
• Multi-density inks can be used in several ways; however, if the calibration is based on 1-ink processes, the relation between the multi-density inks is fixed.
• a relation is given that converts a global cyan value to a light and a heavy cyan value.
• the printer is still considered as a CMYK device, but internally the global ink values can be converted to multi-density ink values .
• the relation between a global ink value for a particular colorant and the multi-density ink values is given by an ink splitting table, also called ink mixing table.
• the present invention is a method for characterizing a printing device as claimed in independent claim 1, and a system therefore as claimed in claim 13. Preferred embodiments of the invention are set out in the dependent claims. Preferably, a method in accordance with the invention is implemented by a computer program as claimed in claim 15.
• the present invention concerns the selection of sampling points for an inverse look up table for the characterization of a printing device.
• color separation values for a plurality of points on a path in color space are analyzed and, based on the analysis, a particular point on the path is determined. A projection of this particular point on an axis in color space is taken as a sampling point of the inverse look up table.
• the axis in color space may be L* in CIELAB space.
• the path in color space coincides with the axis, i.e. the particular point is directly determined on the axis in color space.
• Fig. 1 shows an embodiment of the invention
• Fig. 2 shows color separation curves for the color values of the path of Fig. 1;
• Fig. 3 shows a color separation curve for the L* axis in CIELAB space
• Fig. 4 shows an ink mixing table
• Fig. 5 represents a graph of CIE lightness L* for the ink mixing table of Fig. 4.
• Fig. 1 shows a path 55 in a color space 50.
• the color space is CIELAB.
• path 55 represents a specific Pantone color; in that case, point 57 is the location in CIELAB of the point
• Fig. 3 shows a color separation curve 79 for the L* axis in CIELAB space.
• Curve 79 gives the relation between a single colorant value, c, in ordinate, and L* in abscissa. If the colorant space is

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Physics & Mathematics (AREA)
• Discrete Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Facsimile Image Signal Circuits (AREA)
• Color Image Communication Systems (AREA)
• Color, Gradation (AREA)
• Fax Reproducing Arrangements (AREA)
• Ink Jet (AREA)
• Image Processing (AREA)

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• 2003
  • 2003-04-04 WO PCT/EP2003/050090 patent/WO2003085955A1/en not_active Ceased
  • 2003-04-04 EP EP03730174A patent/EP1495632A1/en not_active Withdrawn
  • 2003-04-04 DE DE60307256T patent/DE60307256T2/de not_active Expired - Lifetime
  • 2003-04-04 EP EP03730173A patent/EP1495631B1/en not_active Expired - Lifetime
  • 2003-04-04 WO PCT/EP2003/050091 patent/WO2003085956A1/en not_active Ceased
  • 2003-04-04 JP JP2003583013A patent/JP2005526640A/ja active Pending
  • 2003-04-04 JP JP2003583014A patent/JP2005527142A/ja active Pending
  • 2003-04-07 JP JP2003102678A patent/JP2004001462A/ja active Pending

Non-Patent Citations (1)

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