Image analysis – Color image processing – Color correction
Reexamination Certificate
1997-07-31
2001-10-23
Boudreau, Leo (Department: 2621)
Image analysis
Color image processing
Color correction
C358S518000, C345S156000
Reexamination Certificate
active
06307961
ABSTRACT:
BACKGROUND OF THE INVENTION
All image rendering devices impose distortions on the color characteristics of an image. Thus, the matching of color appearance between images and documents transferred among any combination of digital image rendering devices such as monitors, scanners, and printers requires the imposition of specialized color image processing knowledgeable about the color rendering characteristics of the different imaging devices. Generalized open systems for applying such color processing are known as Color Management Systems (CMS) and are proliferating in the arena of desktop computing. These systems depend upon standard data structures known as color profiles (hereinafter “profiles”) to determine how to process image pixels. Due to a large number of factors, a CMS in combination with available profiles may deliver colors not all of which a user would consider optimal for a color appearance match. In the current CMS arena when a combination of profiles are found to produce very acceptable matches for most colors but have a limited number of known deficits, there are practically no useful tools available to even the most knowledgeable user and none at all to the naive user for making fine adjustments to the system results.
Color Management Systems are necessary because different imaging devices have different color capabilities, describe color characteristics in different terms, and operate in different color spaces. For example, a color display monitor in a computer system may operate in RGB color space by creating and describing colors in terms of red, green and blue (RGB) values. The RGB values associated with particular colors for the display monitor are device-dependent in that the RGB values associated with specific colors are particular for the given monitor. Since the RGB values are device-dependent, colors displayed on different monitors will probably not be visually identical even for the same RGB input values.
Most printers create and describe colors in device-dependent terms differing from those used by monitors. Printers typically use cyan, magenta, yellow and black (CMYK) values to describe colors, and are said to operate in the CMYK color space. Since the CMYK values are device-dependent, colors printed on any given printer will probably not match colors printed on a different printer for the same CMYK values.
Further complicating color matching between devices is the fact that different devices have different color capabilities. Every rendering device, such as a printer or monitor, has a limited range of colors, i.e. gamut, that it can reproduce. Those skilled in the art will recognize that color display monitors tend to be able to produce a wider range of lighter colors whereas color printers tend to be able to produce a wider range of darker colors. Consequently, the gamut for a color display monitor is different from the gamut for a color printer. As a result, some colors displayed on monitors cannot be reproduced on color printers and vice versa.
Parameterized color transform models are used by a CMS to translate between devices with the goal of maintaining color appearance. For example, suppose that the user displays an image on a monitor. If he prints the image without any color correction, the color appearance of the printed image will differ significantly from that of the original. Using a color transformation model with parameters which take into account the idiosyncratic color rendering characteristics of the user's monitor and the user's printer, a CMS will ideally reduce the color appearance difference to a perceptively acceptable level. The data from which the CMS derives the parameters to the color transform model are stored in the color profiles for the two devices. For the purposes of this example, where the image displayed on the monitor is to be matched by a printed sample, the monitor is considered the source device and its associated profile the source profile; whereas, the printer is considered the destination device and its associated profile the destination profile.
The International Color Consortium (ICC) has in recent years described a standard file format for color profiles. The draft ICC Profile Format Specification, Version 3.3, Nov. 11, 1996 is incorporated herein in its entirety for supplemental background information which is non-essential but helpful in appreciating the applications of the present invention. The ICC defines five major classes of color profile: device profile, device-link profile, color space conversion profile, abstract profile and named color profile. Device profiles are further broken down into subclasses of input profiles, output profiles and display profiles. The ICC profile is a tagged file structure which includes three main sections: a header, a tag table and tagged element data. A CMS determines from the profile header general device, file and profile information such as the profile class. Among the most essential fields found in a device profile's header is the profile connection space (PCS) field which indicates which device-independent units are used within tags which are utilized by the CMS when deriving parameters for its color transform model. The tag table acts as a table of contents for the tag element data which is where the profile tags are stored. Tags fall into the categories of required, optional and private. Each profile class has a different set of required tags. The intent of requiring tags for specific profile classes is to ensure a common base level of functionality which all CMS's can perform using conforming profiles. Profiles may also contain optional tags defined by the ICC or private tags defined by others. These additional tags may be ignored by the CMS or a knowledgeable CMS can take advantage of them.
The two most relevant ICC profile classifications with respect to the present invention are the device profile and the device-link profile. A device profile's tags describe the relationship between device digits and the device-independent units of the profile's PCS. For output device profiles and CMYK input device profiles the AtoB tags are required by the ICC to describe the relationship from device digit to PCS and the BtoA tags are required for describing the inverse relationship. For other input device profiles and all display device profiles the required tags for describing color characteristics are the TRC and Colorant tags. A device-link profile contains tags which describe the relationship between the digits for a first device and the digits of a second device. The AtoB0 tag describes the relationship between source digits and destination digits as required by the ICC for device-link profiles.
In an end-user's color managed environment, device profiles are usually only exercised in pairs where one is designated as source and the other as destination. This architecture allows the CMS to process images which originate at the source device with the goal of matching the original color appearance when rendered at the destination device. Device-link profiles are used alone and already have been conditioned to describe the end-to-end relationship between a specific source and a specific destination device. An advantage which device profiles have over device-link profiles is that they can be used in series with any other device profile allowing the CMS complete flexibility to manage any configuration of profiled source and destination devices. Device-link profiles while more limited in their application are associated with computational efficiencies and quality improvements which make them more desirable to use in those CMS situations where source and destination devices are fixed and known at profile making time, such as for a proofing system.
The original profile creator, hereafter the “craftsman,” may be associated with the device manufacturer, or with a third party service bureau or may be the end-user himself. Any number of commercially available software packages or the craftsman's own custom software may be used to take ra
Balonon-Rosen Mitchell R.
Thornton Jay E.
Boudreau Leo
Brown Rudnick Freed & Gesmer
Michaelis Brian L.
PGI Graphics Imaging LLC
Quinn Joseph P.
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