Robust colorimetry transform

Details Robust colorimetry transform Robust colorimetry transform

2000-05-02

2004-06-01

Williams, Kimberly (Department: 2626)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C345S590000, C345S589000, C358S001900, C358S518000, C358S530000, C358S001150

Reexamination Certificate

active

06744534

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the art of color image rendering. It finds particular application where an image created on or prepared for rendering on a first or source device is rendered on a second or destination device.
2. Description of Related Art
When an image is prepared for rendering on an electronic device the image is represented as a set of pixels. Each pixel describes a small portion of the image in terms of colorant pixel values for the colorants available on the rendering device. For example, typically a cathode ray tube (CRT) based computer display screen is comprised of red (R), green (G) and blue (B) phosphors. An image prepared for display on a CRT is described with a set of pixels. Each pixel describes the intensity with which the red, green and blue phosphors are to be illuminated on a small portion of the CRT. A similar procedure is followed when an image is prepared for rendering on a printing device. Currently, at least some color printing devices apply cyan (C), magenta (M), yellow (Y), and sometimes black (K) colorants to a print medium, such as paper or velum, in order to render an image. Such printing devices are said to operate in a CMY or CMYK color space. When an image is prepared for rendering on a color-printing device, the image is represented as a set of pixels. Each pixel describes a small portion of the image by calling for an appropriate mixture of the available colorants. Typically, the pixel value for each colorant can range from 0 to 255. The higher a colorant's pixel value is, the more of that colorant the color image processor applies to the print medium. In a system employing 8-bit precision for the colorant signals, the number 255 represents the maximum or fully saturated amount of colorant. The number 0 is used when none of a particular colorant is required. It should be noted that sometimes, for the purposes of analysis or discussion this range is normalized to a range of 0 to 1.
In a CRT operating in RGB (red, green, blue) space, fully saturated red is described by pixel calling for R=255, G=0, B=0. In a printer operating in CMYK (cyan, magenta, yellow, black) space, fully saturated red is described by a pixel calling for C=0, M=255, Y=255, K=0. Magenta and yellow colorants combine through simple subtractive mixing and are perceived as red. There is no guarantee that the red described in RGB space and displayed on the CRT is the same red described in CMYK space and printed on a page. In fact, it is quite likely that the spectral properties of the red phosphor used in the CRT will be different than the spectral properties of the subtractively mixed magenta and yellow colorants of a particular printer.
As mentioned above, the CRT and the CMYK printer use different materials to generate the perception of color. The materials used impact a set of colors that each device can reproduce.
The set of colors a device can produce is referred to as the color gamut of the device. There is no guarantee that a color that can be produced by a first device can also be produced by second device. This is even true when both devices are CMYK printers.
Where color matching is required between two devices such as the CRT operating in RGB space and the printer operating in CMYK space, transforms based on careful calibration and measurement are required. In such a situation it is possible, for example, that the pure red RGB CRT pixel mentioned above, is mapped to a CMYK printer pixel calling for a less than fully saturated magenta component and a small amount of a cyan component. For example, the CMYK version of the original RGB red pixel referred to above might call for C=27, M=247, Y=255, K=0. Furthermore, if one wants to print a copy of the original pure red RGB CRT pixel on a second printer it is quite likely that a second transform will have to be used. That transform may translate the original RGB CRT pixel to a second CMYK pixel. For example, the second transform may map the original RGB CRT pixel to a second CMYK pixel calling for C=20, M=234, Y=240, K=35. One reason two different CMYK printers may require different transforms is that different printers use different colorants. For example, a first magenta colorant used in a first printer may have a different spectral content than a second magenta colorant used in a second printer. Likewise, a first yellow colorant used in a first printer may have a different spectral content than a second yellow colorant used in a second printer.
From the foregoing discussion it can be seen that an image prepared for rendering on a first device may need to be transformed if it is to be properly rendered on a second device. Such a transformation is an attempt to emulate the first or source device onto the second or destination device. In order to achieve spectral content matching, the emulation of the color gamut of the CRT on the first CMYK printer caused the red CRT pixel to be mapped to a first CMYK pixel calling for C=27, M=247, Y=255, K=0. The emulation of the color gamut of the CRT on the second CMYK printer caused the red CRT pixel to be mapped to the second CMYK pixel calling for C=20, M=234, Y=240, K=35. Obviously, therefore, even where there is no RGB CRT image involved, an image prepared for printing on the first printer may have to be transformed before its spectral content can be matched on the second printer. In such a situation the first printer is said to be emulated on the second printer.
For example, when, a photographic image has been prepared for rendering on a first CMYK device, for example a Standard Web Offset Printing (SWOP) device, but must then be rendered on a second CMYK device, for example, a xerographic printer, a “4 to 4” transform is typically used to emulate the first device on the second device.
In order to generate the 4 to 4 transform, a color characterization profile is needed for both devices. Each color characterization profile maps a calorimetric space, such as CIELAB to the device's color gamut. The mapping is bi-directional, so that each device's color gamut can also be mapped to the calorimetric space. The source image, the image prepared for printing on the first device, is transformed from the first device's CMYK space, via the first device's color characterization profile, into calorimetric space e.g. CIELAB. The calorimetric version of the image is then transformed via the second device's color characterization profile, into the second device's CMYK space.
Spectral matching, however, is not always the desired goal when rendering color images. For example, when rendering business graphics, such as pie charts and bar charts, a user is concerned with how vivid and pure the colors in the chart are and not with how well the rendered colors match a set of original colors.
Business graphics are most often composed of primary colors. For the purposes of this discussion the primary colors include red, green, blue, cyan, magenta, yellow, black and white. Red, green and blue are considered primary colors because they can be additively mixed to produce the perception of other colors in the human eye. Cyan, magenta, and yellow are considered primary colors because the human eye also perceives their subtractive mixture as other colors. White is perceived when red, green and blue are mixed in a well-balanced manner. Likewise, black is perceived when cyan, magenta and yellow are mixed in a well-balanced manner. Additive mixing of any two of red, blue and green produces one of cyan, magenta and yellow. Subtractive mixing of any two of cyan, magenta and yellow produces one of red, blue and green. For example, as indicated above, a balanced mixture of magenta and yellow is perceived as red.
In business graphics applications, the exact shade of color, for example, red, produced is not an issue. What is required is that the red produced appears p

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