Incremental printing of symbolic information – Marking devices
Reexamination Certificate
1998-12-28
2001-10-16
Barlow, John (Department: 2853)
Incremental printing of symbolic information
Marking devices
Reexamination Certificate
active
06304278
ABSTRACT:
BACKGROUND OF TIRE INVENTION
1. Field of the Invention
The present invention relates to highquality digital printing in which objects are intermixedly printed, and more particularly, in which a printing process exhibits characteristic, statistically predictable defects which degrade quality and whose characteristics vary sufficiently slowly over time. For systems which pre-compensate for such predictable defects by modifying the digital data in advance, it is possible to measure known sample prints at appropriate intervals and use the results of such measurement to modify the pre-compensation functions so as to more closely track variations in each defect's signature.
2. Description of the Related Art
Digital color printers form a digital image for each of several separations, such as cyan, magenta, yellow, and black. The digital image instructs the printing mechanism in the amount of each color ink to deposit and the method of deposition at each addressable point on the page.
A digitally imaged page can consist of graphical objects such as text, lines, fills, pictures, etc, all imaged in ways which can be isolated from each other, can abut one another at one or more points, can partially overlap, one another, or can completely overlap one another. The resulting printed page or graphic image is therefore made up of a patchwork of shapes representing the graphic objects, some of which are “clipped” by objects imaged later in the succession.
In practice, every color printing system has characteristic defects which can cause subtle problems that detract from achieving the highest possible quality color printing. For example, ink jet printing must handle excessive ink coverage which can cause bleeding or spreading of colors and paper distortion. Xerographic printing contends with a different set of problems which can detract from print quality. Examples are “haloing”, in which toner in one separation interferes with toner transfer at the same location in another separation, “tenting”, which is toner deletion caused by high toner pile casting a mechanical or electrostatic “shadow” which prevents correct development of abutting toner, trail-edge deletion and starvation, which cause toner deletion at certain edges, or misregistration between two colors. Many of these characteristic problems in printing systems can be traced to undesirable interactions between abutting colors on the page.
Despite known problems, the digital image sent to the printer has in the past assumed a perfect printing mechanism, and provided an ideal image to print. While increasingly sophisticated controls have been added to printing mechanisms to reduce defects and come closer to the perfect printer expected by the digital image, electro-mechanical defects in any printing system are still common and are to be expected at both the low end where system cost restraints preclude use of expensive controls and the high-end where production speeds challenge existing control systems.
Recent work has begun to look at modifying a digital image in advance in order to pre-compensate for expected problems in a printer. The work may be divided into three groupings. A first grouping of prior art does “object-based compensation”, which predicts and pre-compensates for printing problems unique to each object type (text, fill, image, etc.). A second grouping of art does “tapping compensation”, which predicts and pre-compensates for only one printing problem: misregistration between two abutting colors. A third grouping of art does a more generalized, object-optimized “edge-defect compensation.”
The first (object-based) grouping deals with individual printing objects such as text, fill, or picture, without reference to other adjacent objects. Different object types have different predictable printing problems. For example, a large uniform color fill can contain visible mottle in what should be smooth color, because the random noise of the print mechanism causes tiny variations in the amount of color put on the page; a “quiet” halftone that masks engine noise can be used in printing such a fin. Text can show fuzzy edges if the normal halftone resolution is too large; a small halftone can be chosen to print text with sharper edges. Graphics can have dull colors while images can have unreal colors; the solution is to pre-compensate by choosing a different color transform for graphics than for images.
The second (trapping) grouping of prior art is more limited in scope in that it attempts only to pre-compensate for a single painter defect caused by adjacent colors: misregistration. If a printer misregisters between separations, an thin unwanted white or color line occurs when certain adjoining colors don't abut perfectly. This second group of inventions doesn't care about individual object types or a large range of printer defects as the first (object-based) grouping does. Instead, this group of prior art simply looks at the edge between two color areas, attempting to predict when two abutting colors could cause a thin line problem if the printing system misregisters. The solution used is to generate a fixed-width, constant color fill (a “frame” or “trap”) whose color and position is calculated with various methods from the two abutting colors, and to superimpose that new digital signal with the original signal to produce prints that show the misregistration problem less.
The third (generalized edge-defects) approach combines both object and color information to predict and correct a wider range of adjacency problems in a novel way. Unlike the group one (object-based) inventions above which use object information to predict individual object printing problems, the third approach uses object information to help predict and solve printing problems caused by object adjacencies. Unlike group two (trapping) inventions above which only correct for misregistration, the third approach significantly extends the range of adjacency problems that can be detected and corrected. Detection of a larger number of adjacency problems is made possible by including not just color information in predicting adjacency problems but also object information such as object type, object size in the scan and process directions, rendering intent, and other relevant object parameters. Pre-compensation/correction of a larger number of adjacency printing problems (beyond simply misregistration) is made possible by using a novel approach different from the simple trapping solution of adding a uniform-width, constant-color frame between two adjacent colors. Instead, a function is applied to an object edge that can change both its color and rendering hint anamorphically (that is, differently in the process or scan directions) as a function of the distance from the edge.
All three of the approaches to digital data pre-compensation are described in the patent application Rumph et al., application Ser. No. 09/222,486 filed Dec. 28, 1998, titled “Anamorphic Object Optimized Function Application for Printer Defect Pre-Compensation”, which is incorporated herein by reference.
The focus of this invention is not on the method used to pre-compensate digital data to reduce printer defects, but rather on achieving long term stability in the results. If the extent and severity (the “signature”) of a particular defect changes over time, it will not be optimally effective to apply a fixed pre-compensation to the defect based on one or a few measurements made at only one particular point in time.
It is well known in the art to use color measurements to maintain color fidelity in color printers. Printer characterization is done to map requested colors to c,m,y, k values used by the actual printer. Then, printer calibration is done to prevent the printer from drifting in its color representations. To do this, color patches with known values are printed at periodic intervals. The actual printed color values are measured, and by comparing the actual measurements with the expected values, color transform data can be modified to correct for the current state of the printer
Barlow John
Stewart Jr. Charles W.
Xerox Corporation
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