Facsimile and static presentation processing – Static presentation processing – Attribute control
Utility Patent
1998-06-09
2001-01-02
Lee, Thomas D. (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S518000, C358S523000, C358S529000, C358S534000
Utility Patent
active
06169609
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to color imaging and, more particularly, to systems and methods for constructing color interpolation tables used in color image reproduction systems, such as printers, photocopiers, scanners, and the like.
BACKGROUND OF THE INVENTION
Laser driven color printers and copiers employ transparent toners which enable light to reflect off the page and to be directed back towards the eye. In general, such devices employ Cyan (C), Magenta (M) and Yellow (Y) toners as the principal component colors, from which other colors are created. Light passing through CMY toners has part of its color filtered out or absorbed by the toner such that the reflected light takes on the color of the toners that it passes through. In laser printers (and some copiers), a black (K) toner is used which is opaque to light. If a K toner is overprinted onto CMY to achieve darker colors, such as found in shadows, much of the colorfulness of the darker colors is lost because less surrounding colorant is used. Thus, to increase the range of colors available from a printer/copier, it is necessary to find a correct balance of K and CMY toners to produce dark colors, while allowing the dark colors to remain as colorful as possible.
While the prior art has employed combinations of the three primary colors cyan, magenta and yellow (CMY) to produce darker colors ranging to black, the need to deposit 100% density toner layers for each color causes an excessive amount of toner to be deposited on the media sheet (e.g. up to 300%). Such a high level of toner deposition does not fuse well and, in general, creates unsatisfactory images. Accordingly, conventional printing procedures have utilized combinations of K and CMY toners to achieve dark or shadowed color images.
When a printer receives image data from a host processor, the data is typically received in the form of either Red, Green, Blue (RGB) or CMY values. In either case, the received values are converted to CMYK values in order to achieve desired levels of color representation on the final printed document. Such conversion, depending upon the color value parameters that are set by the printer manufacturer, can result in variable color representations when printers of different manufacturers are coupled to a host processor.
Accordingly, a device color map or maps (also referred to as a table or tables) is conventionally used to convert the RGB or CMY values to CMYK values. In other words, the color map produces color separation information. The color map holds entries that specify relative quantities (or blends) of CMYK color values, resulting in various color shades, indexed by (or accessed from) the three primary input colors (e.g., RGB or CMY) received. The color map can be conceptualized as a three-dimensional color space defined by the three primary colors. The map entries define equal spaced color points in this space along the three color axes. As an example, a color map might have seventeen (17) color points along each of the three color axes. Movement along each axis results in a different color output.
The initial (input) RGB or CMY values received are typically three 8-bit values that identify one color point on each color axis. However, only four bits of each eight bit value typically identify one of the first sixteen (16) of the seventeen (17) color points along its respective axis. The other four bits of each eight bit value are then used to interpolate to a further point along each axis (relative to the point identified with the first four bits) to obtain a more accurate and truer resultant blended color quantity output. All in all, the image reproduction system uses the initial RGB or CMY color value received to lookup in the color map(s) and retrieve the appropriate CMYK color blending quantities to form the desired color. The parameters retrieved from the color map(s) are passed to the imaging subsystem to produce a colored dot. As an example, in an interleaved color map the entries (or outputs) include four 8-bit values, one for each of the three CMY colors and a fourth value for K-black. Alternatively, in a planar color map (where a separate color map is used for each individual CMY or K color value), a respective 8-bit value is output from each individual CMYK color map.
In essence, device color space can be represented mathematically by a three dimensional to four dimensional color table. The 3D input (RGB) is used as input coordinates to lookup or interpolate to the 4D output (CMYK). In the context of the present invention, the RGB input values are monitor (or display apparatus) color space values, and the CMYK entry (or output) values are printer toner values (hereafter, the term, toner, will be used to encompass both ink and toner). The four dimensional output (i.e., CMYK) represents possible combinations of the three input colors (i.e., RGB or CMY). In a device color space, each axis or basis vector represents an amount of input (RGB or CMY), starting at the origin with zero as its value and moving outward until a maximum value is reached, i.e., either 1.0 or 255, depending upon the current usage. Each axis represents a primary input color and combinations of toner amounts (or values) are represented as entries in the device color space.
Theoretically, equal amounts of input color values should produce output colors that are perceived by an observer to be neutral (gray) in color or to have no colorfulness. However, in reality, although equal RGB values in monitor color space are neutral, equal amounts of output color values (for toner deposit) in printer color space are virtually never neutral. The loci of points that have coordinates of equal values of component colors is defined to be the neutral axis of the device color space. Perceptually, the neutral axis of the device color space is expected to go from one extreme of lightness to the other (i.e., white to black or vice versa) without any perceivable colorfulness.
“Process black” is where equal amounts of CMY toners are used to obtain a black color. Theoretically, if the darkest most colorful black is required, and the printer uses CMY to create the black color, then 300% toner will need to be deposited on the paper (i.e., 100% density quantities of each of the three CMY primary colors). However, under such circumstance toner usage is excessive and it can run off the page or may not fuse properly to the page. Further, since CMY toners are transparent, the blackest black may not be obtainable using only CMY toners.
When K toner is used to create black, little, if any, CMY needs to be used, resulting in a savings in toner. However, a K-only black doesn't have as smooth of appearance as a black produced using some combined amounts of CMY and K. Determining the amount of K toner to use and how much CMY not to use when creating black, is termed “color separation”. Further, determining how much K toner to use to replace equal amounts of CMY toner is called “gray component replacement” or GCR. For any CMY color, the portion of the component colors that equal the value of the minimum color component (i.e., of C, M or Y) is called the neutral or “gray component”. The gray component is also defined as the “undercolor” and, again, is the minimum amount of C, M and Y found in any CMY color. The percentage of undercolor that is removed when K toner is used (instead of the CMY undercolor gray component) is called undercolor removal (UCR) and is usually a number from 0-1 or a percentage from 0 to 100%.
Now, when building color maps, it is desirable to have process black on the neutral axis to maximize image quality. On the other hand, it is desirable to have K-only on the neutral axis to minimize line art and text artifacts (i.e., registration errors, halftone jaggies, etc. that tend to occur with process black). However, combining both process black and K-only in a single color map is a problem.
One conventional compromise is to retain process black on the neutral axis while designating K-only at the black point in the color map. This all
Jacob Steve A.
Johnson David A.
Trask Jeffrey L.
Hewlett--Packard Company
Lee Thomas D.
Simmons Lane R.
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