Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-10-05
2003-12-30
Lee, Thomas D (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C382S256000, C382S258000
Reexamination Certificate
active
06671070
ABSTRACT:
FIELD OF INVENTION
This invention relates to the field of graphic arts, and in particular to a method and system that compensates for coverage-area spread distortions that occur during printing.
BACKGROUND
Image Reproduction
The field of graphic arts deals with the reproduction, for example the printing of images, and with the preparation of images for such reproduction. In recent years, the process of preparing data for printing has been computerized, so that image data is typically prepared on a computer by combining the image elements, which might include scanned or digitally acquired photographs and digitally designed graphic elements. Image data thus may include linework and continuous tone images, and therefore may include areas that have various tones. The color at any point in an image is represented by a set of color components, may be the quantities of inks or toners that will be used for printing (e.g., cyan, magenta, yellow, and black) or in some other component system which eventually will need to be converted (“separated”) into the ink values for printing. Such other components include red, green and blue components (RGB) and other color co-ordinate schemes (e.g., CIE-Lab). Thus, color images are represented by a set of monochrome component images, each representing the amount of one of the color component. When these components are the quantities of inks or toners, the monochrome component images are called separations.
Most printing processes (e.g., offset press, gravure, flexography) are usually capable of printing only two tones of any ink or toner, i.e., either deposit or not deposit ink or toner onto a substrate or carrier, which is usually but not necessarily a sheet of paper. Halftoning is used to reproduce images having continuous tones. Halftoning converts varying values of tints and tones into a geometric distribution of small objects (e.g., dots) that can be printed. The human eye “spatially integrates” these small objects over a larger area so that one perceives continuous tones when viewing the image from a distance. The most common form of halftoning uses small dots, with either the size of the dots or the frequency of dots per unit area varying to provide for different grey tones. Halftoning using dots is called traditional halftoning herein.
The steps in the reproduction of an image may include recording the separations of the image on film using a recorder such as an imagesetter, then making a set of plates from the film. Alternatively, a plate may be directly exposed. Other forms of reproduction include flexography, gravure, and direct imaging onto the substrate using electro-photography, for example, xerography. In all these cases, the image is usually converted to picture elements (pixels) which are used to modulate a light source (e.g., a laser) to expose some photosensitive medium. The pixels are usually organized in a raster.
The image data (“artwork”) may be available directly in pixel form, or may be provided in some other format, for example, as a page description language (PDL) file such as a PostScript® file (Adobe Systems Incorporated, San Jose, Calif.), or in some other object-oriented format. Any of these non-pixel formats needs to be converted into a raster stream of pixels prior to exposure, and this conversion process is called raster image processing (RIPping). The device for performing RIPping is called a raster image processor (RIP), and typically comprises a computer running RIP software. Halftoning may be carried out during RIPing, prior to RIPping, or even after RIPing.
Some forms of printing require high accuracy. One example is the printing of security documents such as banknotes, travelers checks, and share certificates, in which the artwork is extremely precise.
Coverage Area Spread Distortions
There typically are deviations in the appearance of a print of reproduced artwork from what is expected. One of these deviations is due to a spread in the area of ink coverage of solid objects. This distortion occurs as a result of making a printing plate (if used) and as a result of the printing process itself. It is called “coverage-area gain,” “coverage area spread,” or simply “coverage-gain” herein, and is present in most printing processes, including lithography, flexography, gravure printing, and even in electro-photography (xerography).
Explanations for the effect include the spread that occurs when an amount of ink is applied to a reasonably large area and then pressed onto the substrate or carrier during printing. The thickness of the ink is then forced over the edges of the area over a small distance. Depending on the printing process, other elements such as ink transfer efficiency, ink absorption in the paper, and optical characteristics of the ink/paper combination can also influence the amount of coverage gain. Sharp inward or outward image edges will tend to fill in or to disappear, so the shape of the ink coverage area is another factor.
The effect of coverage gain is to reduce accuracy of printing, and this is especially noticeable when printing small elements. For example, this is commonly noticeable in halftone printing, where the effect is called “dot gain.” When printing a single small dot, for example, a small circular area, the radius of the area will increase by some small distance. Therefore, if one attempts to reproduce a halftone region of a certain coverage percentage, the effect will be to increase the perceived coverage percentage. This causes a shift in the grey scale, and in the case of color images, a shift in the tone.
Dot gain compensation is a technique commonly used to compensate for the dot gain by reducing the input area coverage prior to printing. This typically is done electronically using a lookup table that maps desired dot percentage to what dot percentage to apply in order for the desired dot percentage to be achieved. The lookup table may be generated by printing step wedges of desired coverage percentages and measuring the resulting coverage percentages after imagesetting and printing.
While traditional halftoning, the most common form of halftoning, uses small dots, with either the size of the dots or the frequency of dots per unit area varying to provide for different grey tones, other forms of halftoning also are known that use small graphic elements of varying size or frequency that are not dot-like. Non-traditional halftoning schemes include:
halftoning using thin lines segments that have different thicknesses. Such techniques are common in the design of security documents such as banknotes. See, for example, the image of George Washington in the common United States one dollar bill;
halftoning using character fonts; and
Halftoning using any other small elements arranged so that the average ink coverage per unit area is modulated according the grey scale.
Lookup table coverage gain compensation techniques such as commonly used for dot gain compensation are not very effective for compensating for the coverage gain in most non-traditional halftoning techniques.
Thus there is a need in the art for applying coverage gain compensation to the graphic elements used for non-traditional halftoning to account for the coverage gain that occurs on such elements in the reproduction process.
Coverage gain effects are known also to cause problems when printing a barcode comprising lines of varying width. Barcodes are used, for example, to print universal product codes (UPCs). Thus, artwork, for packaging for example, may comprise a is barcode. Coverage gain effects may cause bars that are too closely spaced to merge, leading to error in reading the barcode. The “Film Master Method” of verification of barcodes is known which includes exposing a film master positive or negative image of a test symbol having several lines of varying widths with smaller and smaller gaps between the lines, followed by precise measurement of the film master positive or negative image using light transmitted through the film. The “bar-width-reduction” parameter is determined as half the width of the gap that first g
De Stecker Hans
Schildermans Luc H. M. L.
Van Laethem Jan
Dotrix N.V.
Inventek
Lee Thomas D
Rosenfeld Dov
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