Computer graphics processing and selective visual display system – Computer graphics processing – Attributes
Reexamination Certificate
1999-11-15
2003-08-05
Luu, Matthew (Department: 2672)
Computer graphics processing and selective visual display system
Computer graphics processing
Attributes
C345S594000
Reexamination Certificate
active
06603483
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention concerns a color management system whereby an efficient sequence of transform steps is generated for transforming color image data through one or more color spaces. The sequence of transform steps is generated based upon pre-selected color profiles and gamut mapping algorithms. In this manner, complex transform sequences for performing color management of color image data, such as proofing, creative color modeling, and gamut boundary determinations, can be quickly created, applied and evaluated by a developer.
2. Description of the Related Art
Traditionally, proofing is performed in the graphics art industry to simulate the output of a printing press without having to invest the time and cost of actually printing a sample for review. Proofing is commonly done with a proofing machine that uses the same input medium as that used by the printing press, such as film or digital color image data, in order to create a simulation of the printed image. More recently, proofing systems have been developed which allow a graphic artist to simulate a printed image by rendering digital color image data on a CRT display. Proofing systems which utilize digital color image data attempt to provide an accurate rendering of a color image as it would appear on a printing press, or other output device.
Traditional digital color management systems attempt to account for limitations in the range of colors that can be produced by a given output device, such as a printing press, on a given medium, such as coated paper. The color management system therefore attempts to adjust the color data of the input image to account for those colors that are outside the color gamut boundary of the output device. This adjustment is known as gamut mapping, and is performed by application of a gamut mapping algorithm to the color image data. There are several different types of gamut mapping algorithms in use by various color management systems. In addition to gamut mapping, color management systems also attempt to obtain an accurate mapping between a combination of certain device colorants and the appearance that the combination will make on a particular medium by a particular output device under particular viewing conditions. The transformation of color image data from one colorant space to another colorant space is known as appearance modeling. Digital color management systems therefore attempt to achieve accurate gamut mapping and accurate appearance modeling in an efficient manner. This can be difficult in unique situations such as proofing where it is desirable to simulate how color image data from an input device will appear on a given output device by viewing the simulated appearance on a different output device.
The original International Color Consortium (ICC) architecture provides a color management system in which device profiles of an input device and an output device are utilized in order to transform color image data from the input device for rendering on the output device. In this scheme, the device profile of a given device contains several data object tags, some of which comprise multi-dimensional look-up table (LUT) tags. These LUTs are used in the original ICC architecture for appearance modeling to map color image data from a device-dependent color space, such as RGB, into a device-independent color space. On the other hand, the LUTs may be used to map from a device-independent color space to a device-dependent color space. One drawback of this architecture is that it requires mapping to and from the device-independent color space for every transformation between two color profiles.
In the current ICC architecture, device profiles combine appearance modeling and gamut mapping together into LUTs that are contained in the color profiles. The LUTs perform these functions simultaneously by mapping color image data in a color space corresponding to a given combination of output device and viewing conditions into a fictitious color space known as the Profile Connection Space (PCS). The PCS is a standardized color space based upon a fictitious output device, recording medium and set of viewing conditions. Thus, in the current ICC architecture, the application of a LUT to color image data performs both appearance modeling and gamut boundary mapping in one step. A color profile may contain several different sets of LUTs, each of which represents a specific combination of gamut mapping algorithm and viewing condition. The color profile format contains an “intent” flag which indicates to the color management system the particular type of gamut mapping algorithm to apply, upon which the color management system selects the appropriate LUT that corresponds to the particular type of gamut mapping algorithm.
The current ICC architecture supports alternate methods that can be utilized for proofing in order to simulate the appearance of color image data from an input device as it would appear on a given output device by viewing the simulated appearance on another output device. Under the current ICC scheme, special preview tags are provided in the color profile for accessing special preview LUTs which are used to simulate a proof image as it would appear on a given output device. The preview LUTs achieve this simulation by mapping the color image data to a different gamut boundary by using a particular gamut mapping algorithm. The Preview LUTs perform this gamut mapping function by transforming the color image data between device-dependent color space and the fictitious PCS color space.
Like the original ICC scheme, the current ICC scheme for proofing has several drawbacks. First, the preview LUTs of the current ICC scheme require transformation from the PCS color space for every device color profile that is in the color transformation scenario. Every such mapping may introduce some errors due to interpolation errors, round-off errors, and the like. In addition, a preview LUT which is used for simulated proofing does not contain the same data as the LUT which is used to transform the color image data for actual rendering on the output device. Therefore, preview LUTs must be maintained in a coordinated fashion with the actual transformation LUTs for each combination of gamut mapping algorithm and viewing conditions in order to avoid inaccuracies in appearance between the simulated output of color image data in a proofing context and the actual output of the color image data by the output device. Such coordinated maintenance of predetermined LUTs is logistically cumbersome.
An alternative method for proofing under the current ICC architecture is to perform separate consecutive transformations of the color image data by using the actual transformation LUTs for the simulated output device instead of the preview LUTs. For example, the color image data is first transformed from the color space of the input device to the PCS color space, then from the PCS color space to the color space of the simulated output device, then from the color space of the simulated output device to the PCS color space, and lastly from the PCS color space to the color space of the actual output device. Although this alternative method avoids appearance inconsistencies that might be introduced through the use of preview LUTs, the alternative method requires the unnecessary computational overhead associated with repeatedly mapping between device-dependent color spaces and the fictitious PCS color space.
Another ICC architecture has been proposed in the ICC Reference Implementation Working Group (RIWG) which separates the function of appearance modeling from the function of gamut mapping by introducing a new pair of LUT tags in the device color profile format which contain special LUTs for performing appearance modeling only. Special tags called gamut boundary descriptors are also provided in the color profile for containing descriptions of the device's color gamut boundary. The gamut boundary descriptors are used by gamut mapping algorithms when performing gamut mapping on col
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Havan Thu Thao
Luu Matthew
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