Patent Application
20090153887
This disclosure generally relates to color imaging systems, such as printers, copiers and other color imaging systems and specifically relates to printing spot colors consistently in a color imaging system.
In today's business and scientific world, color has become essential as a component of communication. Color facilitates the sharing of knowledge and ideas. Inventors, who develop color imaging systems, find innovative ways to improve the technology, for machines and processes for printing color images, which may be used by businesses and people to share knowledge and ideas.
Color imaging systems, such as printers commonly provide a limited number of output possibilities and the output possibilities are typically binary. For example, a printer produces either a dot (1) or no dot (0) at a given pixel location. Given a color separation with 256 shades of each additive primary color, a set (or array) of binary printer signals may be produced, representing a contone effect. This process is referred to as halftone image processing or halftoning.
In halftoning, each pixel value of an array of contone pixels within a given area of a color image is compared to one of a set of pre-selected thresholds. These thresholds may be stored as a dither matrix and the repetitive pattern generated by this matrix is considered a halftone cell as taught, for example, in U.S. Pat. No. 4,149,194 to Holladay. In practice, some of the thresholds in the matrix will be exceeded for an area where the image is a contone. In other words, the image value at that specific location will be larger than the value stored in the dither matrix for that same location, while at other locations, the values will not exceed the thresholds in the matrix. In the binary case, the pixels or cell elements for which the thresholds are exceeded might be printed as black or some other color, while the remaining elements are allowed to remain white or uncolored, dependent on the actual physical quantity described by the data. Because the human visual system tends to average out rapidly varying spatial patterns and perceives only a spatial average of the micro-variations in spot color produced by a printer, the halftone process may be used to produce a close approximation to the desired color in the contone input. Each color separation of an image may be halftoned sequentially to render or form the full color image.
In addition to process color images, spot color and/or high-fidelity images may be produced by color imaging systems. Process color images may be augmented with additional primary colors beyond the usual four primaries colors (i.e., CMYK) typically used to produce the process color output. These additional colors extend the color gamut of the process color output to produce high fidelity color for a color imaging system and thereby more closely emulate standardized spot colors, such as those defined by Pantone.
A color imaging system, such as a printer typically includes an output device called an image output terminal (IOT) that is generally capable of producing color and back-and-white tones to produce images. Some examples of IOTs include xerographic print engines, thermal inkjet devices and the like. The color imaging system typically accepts color level specifications for each of four or more colors. such as CMYK as input and produces color images as output using halftoning to print a fine pattern of color spots. The spots are grouped together to form dots, which appear as varying color tones in accordance with the number of color spots used when viewed from a distance.
A color imaging system, such as a printer may be calibrated so that it operates to produce relatively consistent output in an operating environment having parameters such as, for example, specific humidity, temperature, dust count, etc. If one or more of the desired operating parameters deviate from the desired operating parameters, the printer may drift away from ideal conditions. One way to compensate for the drift within the printer includes modifying data within a digital front end (DFE) in the color imaging system. The digital front end converts image data (e.g., PostScript and PDF) into raster data, which is transmitted to the image output terminal (IOT) that produces the color image. A tone reproduction curve (TRC) may modify the raster data before it is sent to the image output terminal. A TRC is an electronic map or a graphic representation of a relationship of input image data to output image data. The TRC is used to convert input image data into image output data. The TRC may be calibrated as part of calibrating the whole color imaging system. During calibration, a test suite of grey patches may be printed from known image values and then the resulting test patches may be measured (for example, by using a calorimeter or spectrophotometer) to determine the actual printed image values. A calibration TRC is generated by determining the difference between the known and the actual image values. The calibration TRC may therefore be used to correct for drift and/or other factors.
During image processing, input image data is typically described in a page description language (PDL), such as PostScript or PDF. Nearly all page description languages include a set of programming language commands for processing the image data. The page description language is processed (or interpreted) by a color imaging system so that the proper type and sequence of color images are printed. The process of converting input image data into pixel display values arranged in a frame buffer for ready use by a print engine is called rendering an image.
Pixel display values may be associated with tag information that classifies portions of an image according to object types. U.S. Pat. No. 6,429,950 by Ebner discloses a method and apparatus for registering object characterization information in the form of tags appended to pixel display values stored in the frame buffer of an imaging system using page description language commands. The page description language commands are used to generate and register the tag information on a per-pixel level in the frame buffer based on the object type of imaging data derived from the page description language data received into the imaging system. The pixel data image tagging is useful to differentiate between bit maps, text, line art, graphics (including fills and sweeps), and images in gray scale, RGB, CMYK and other color space frame buffers to enable per-pixel segmentation tags.
When printing spot colors, color fidelity is important. Color fidelity means that colors consistently look the same (e.g., match a standard color) regardless of how they are printed. In principle, user TRC should not affect spot color output, and the fidelity of the spot colors should be solely maintained by the calibration TRC, but existing image processing systems do not maintain the color fidelity of spot colors due to the involvement of the user TRC. To date, all known attempts to print consistent spot colors have been inefficient. For example, one known method includes pre-compensating for the user TRC by embedding an inverse user TRC mapping in the contone values for the spot colors and sending pre-compensated spot colors to the printer. First, the concatenation of the inverse TRC and the user TRC create a loss of output levels. That is, the spot colors might not be the same as that in the original recipe. Second, because a user TRC is usually iteratively designed, the spot colors have to be iteratively re-compensated. Finally, if a print job was raster image processed (RIPped) and saved with one user tone reproduction curve and ifs at the reprint time, that user TRC is not needed or a new TRC is demanded, then it may be necessary to re-raster image process the job and redo the pre-compensation process for the spot colors all over again. Such a workflow is inefficient.
Exemplary embodiments of the disclosure provide an apparatus and method for printing consistent spot colors. The apparatus may include a remap mechanism in a digital front end and an image output terminal. The remap mechanism bypasses application of a user TRC for at least one spot color in a color image by using tags associated with the spot color in image data. The digital front end processes the image data and generates printer data. The printer data effects the bypass of the user TRC for the spot color. The image output terminal receive the printer data from the digital front end and generates the color image, resulting in the spot color in the color image not being effected by the user TRC.
The apparatus may include a storage device associated with the remap mechanism for storing the image data and/or printer data. The apparatus may also include circuitry in the remap mechanism for loading a user TRC, a user tone reproduction remap table, a rendering hint remap table and a calibration TRC into the storage device. The rendering hint remap table may be generated by the digital front end based on a halftone selection. A tag and contone value may be associated with each pixel in a data structure for the color image. The remap mechanism may processes a pixel having a tag that is allocated for a spot color by indexing into the user tone reproduction remap table and instructing the circuitry whether to apply the user TRC on the contone value. The circuitry bypasses applying the user TRC for the pixel for the spot color. A xerographic device may include such an apparatus.
The color imaging system 100 processes the input information 102 in a way that enables consistent spot color printing, regardless of whether a user TRC (e.g., “no cyan”) is applied. In
During the decomposition process, the digital front end 104 may recognize spot color objects and assign a special spot color intent to pixels that are associated with the spot color objects. Later on, when the digital front end 104 generates the rasterized data, the digital front end 104 assigns appropriate tags for each pixel of the color image based on whether the special spot color intent was assigned for that pixel.
The data 118, 130 generated by the digital front end 104 may include contone planes 126, 132, tag planes 128, 134, user TRCs 120, rendering hint remap tables 122 and user TRC remap tables 124. The data 118, 130 maybe stored in a storage device for future printing or reprinting. After processing by the remap mechanism 116 in the digital front end 104, the data 130 includes tone corrected contone values and remapped tags, which are sent to the image output terminal 106 for printing. The image output terminal 106 halftones the final contone values using halftone screens specified in the tags.
The rendering hint remap table 122 is generated by the digital front end 104 based on the halftone selection. The rendering hint remap table 122 is used to instruct the hardware what calibration TRCs to load for each available calibration TRC slot. The rendering hint remap table 122 is also used to remap the incoming tag (or hint) to the real image output terminal tag that represents the halftone on the printer. For example, the rendering hint remap table may have “t” at multiple indices, which means the incoming tags with those index values will be mapped to “t” (i.e., using the same halftone) in hardware before the remapped data is sent to the printer. in one exemplary embodiment of the color image system 100, the rendering hint remap table is a length sixteen array to accommodate the incoming tag 134 (or hint) values that range from zero to fifteen.
On some image output terminals 106, there are more hints than the simplified example in
The user TRC remap table 124 is first created by the digital front end 104 and then passed and loaded onto the user TRC remap table slot in the remap mechanism 116. When the remap mechanism 116 is processing a pixel with a tag that is allocated for spot colors, the remap mechanism 116 indexes into the user tone reproduction remap table 124 to find a value of 1 and therefore instructs the circuit 200 to bypass the user TRC 120. When the remap mechanism 116 is processing a pixel with a tag that is allocated for non-spot colors, the remap mechanism 116 indexes into the user TRC remap table to find a value of 0 and therefore instructs the circuit 200 to apply the user TRC 120 on the input contone values.
In the color imaging system 100 of
The remap mechanism 116 may be implemented on an image output terminal board hardware and in a driver on the digital front end 104. The compression/decompression may be implemented on a separate board. The remap mechanism 116 may be implemented in hardware, software, ant/or firmware.
The existence of the user TRC remap table 124 is optional, since it can be generated by examining the rendering hint remap table 122. For example, the user TRC remap table value would be one (1) if the index value and the rendering hint remap table value are different (see
It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also, various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art, and are also intended to be encompassed by the following claims.
