Facsimile and static presentation processing – Natural color facsimile – Measuring – testing – and calibrating
Reexamination Certificate
1998-03-31
2002-06-11
Williams, Kimberly A. (Department: 2622)
Facsimile and static presentation processing
Natural color facsimile
Measuring, testing, and calibrating
C358S406000, C399S009000
Reexamination Certificate
active
06404517
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to color image reproduction systems, and more particularly to systems that improve color matching between original color images and reproductions of those images.
2. Description of the Related Art
Color image reproduction systems typically include an input device for providing a representation of an original image, an output device for generating a replica of the image, and a controlling device that processes signals received from the input device to generate new signals sent to the output device to produce the replica, which preferably is a high-fidelity reproduction of the original image. The controlling device may be implemented by a general-purpose computer with appropriate software and/or hardware for peripheral control and signal processing or may be a dedicated image processing unit associated with a particular input device/output device pair. Examples of an input device include hand held, flatbed and sheet-fed optical scanners, digital and video cameras, and software applications. In other words, an original image may be sensed or it may be created by a process. Examples of an output device include ink jet, laser, and photolithography printers, electrostatic, flatbed and drum plotters, and video displays such as cathode ray tubes, thin-film-transistor and liquid crystal display panels.
Generally, input and output devices use a device-dependent or device-specific color space or coordinate system to specify colors. These coordinate systems map the color coordinates to the color-sensing or color-generating process of the particular device. The term “color space” refers to an N-dimensional space in which each point corresponds to a particular color.
One example of a three-dimensional color space is an RGB space in which point coordinates specify particular amounts of additive primaries red (R), green (G) and blue (B), represented by the three axes of the cube. All other colors within the cube can be represented by the R, G, B triplet, where the values for R, G, and B fall within a range from minimum intensity (e.g. 0) to maximum intensity (e.g. 1). White is represented when all three primaries are added at their maximum intensity (1,1,1) and black is represented by the complete absence (0,0,0) of all three primaries. Shades of gray are represented along the diagonal of the cube connecting the black and white coordinates. In an additive system light emitters are controlled to obtain color. The operation of many scanners and color display devices may be conveniently controlled by signals that are specified in RGB space. Another example of a three-dimensional color space is a CMY color space in which point coordinates specify particular amounts of subtractive primaries cyan (C), magenta (M), and yellow (Y) which combine to represent a specific color. In a subtractive system light is absorbed rather than emitted. Typically used in printing, the subtractive colors are printed on a white surface (e.g. paper) and the inks selectively absorb certain ranges of wavelengths of light and the remaining spectral radiant power is reflected. The subtractive system is the complement of the additive system and ideally the mixture of two additive primaries produces a subtractive primary. For example, the mixture of red and green is yellow. Similarly the mixture of two subtractive primaries produces a additive primary. When cyan and magenta are mixed, the cyan absorbs the red wavelengths of the magenta and the magenta absorbs the green wavelengths of the cyan, leaving only blue. Since complete absorption is difficult to achieve in printing inks, a fourth ink, black, is used as well in many devices. This system is specified as CMYK with the K representing black. The operation of many ink jet and laser printers may be conveniently controlled by signals that are specified in CMYK space or CMY space. Other color spaces that are related to particular devices are also known.
Most devices are capable of sensing or reproducing only a portion of the full range of colors that can be discerned by a human observer. A device “gamut” refers to the range of colors that can be sensed or reproduced by a particular device. For example, the gamut of a particular scanner refers to the range of colors that can be sensed by that scanner and the gamut of a particular printer refers to the range of colors that can be reproduced or printed by that printer.
A scanner gamut is determined by a variety of factors including the spectral response of the optical sensors, the spectral characteristics of color filters, spectral characteristics of the illumination source and the resolution and linearity of analogto-digital converters.
A printer gamut is determined by a variety of factors including spectral characteristics of colorants such as ink, spectral and porosity characteristics of media such as paper, resolution or dots-per-inch of the printed image, and half-toning methods applied.
A video display gamut is determined by a variety of factors including spectral characteristics of the light emitting material, type of display device, resolution of pixels or video lines, and excitation voltage.
Although it is possible in principle to construct a color image reproduction system by merely connecting an output device directly to an input device, the results generally would not be satisfactory because the device-dependent coordinate systems and color spaces for the input and output devices are generally not the same. Even if the two sets of coordinate systems and color spaces are the same, the fidelity of the reproduced image as compared to an original image would probably be very poor because the gamut of the input device generally is not coextensive with the gamut of the output device. Values representing “out-of-gamut” colors that are not in the output device gamut cannot be reproduced exactly. Instead, some “in-gamut” color that is in the gamut of the output device must be substituted for each out-of-gamut color.
Color image reproduction systems can achieve high-fidelity reproductions of original images by applying one or more transformations or mapping functions to convert point coordinates in one color space into appropriate point coordinates in another color space. These transformations may be conveniently performed by the controlling device. In particular, with respect to the output device gamut, transformations are used to convert values representing in-gamut and out-of-gamut colors in an input device-dependent color space into values representing in-gamut colors in an output device-dependent color space. The mapping of in-gamut colors and out-of-gamut colors is discussed separately.
Mapping In-Gamut Colors
The transformation of output device in-gamut colors for many devices are non-linear and cannot be easily expressed in some analytical or closed form; therefore, practical considerations make accurate implementations difficult to achieve. Many known methods implement these transformations as an interpolation of entries in a look-up table (LUT) derived by a process that essentially inverts relationships between measured responses to known input values. For example, a transformation for an input device may be derived by using a medium conveying patches of known color values in some device-independent color space such as the Commission International de L'Eclairage (CIE) 1931 XYZ space, scanning the medium with the input device to generate a set of corresponding values in some input device-dependent color space such as RGB color space, and constructing an input LUT comprising table entries that associate the known color XYZ values with the scanned RGB values. In subsequent scans of other images, scanned RGB values can be converted into device-independent XYZ values by finding entries in the input LUT having RGB values that are close to the scanned values and then interpolating between the associated XYZ values in those table entries. Various interpolation techniques such as trilinear, prism, pyramidal and tetrahedral inter
Seiko Epson Corporation
Watson Mark P.
Williams Kimberly A.
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