Gradient-based trapping using patterned trap zones

Details Gradient-based trapping using patterned trap zones Gradient-based trapping using patterned trap zones

1999-11-30

2004-08-24

Coles, Edward (Department: 2622)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C358S001900, C358S003230, C358S003260, C382S275000

Reexamination Certificate

active

06781720

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to electronic processing of graphic images to produce multi-color prints using multiple separations. Typically, four (4) process color inks (i.e., cyan, magenta, yellow, and black) are used to print multiple separations, which tend to have minor misregistration problems. Trapping is a process for adjusting images to correct for misregistration. The present invention is directed to a process for controlling trapping, based on the appearance of any misregistration artifacts against the remainder of the image.
The layout of a page or graphic image depends upon combining “structured graphics” according to a pre-established graphic design. The structured graphics are contiguous regions of color, usually represented in a plurality of separation images, which in turn represent a succession of graphic objects imaged on the printing medium (e.g. the paper). The objects so imaged are shapes 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” (or hidden) by objects imaged later in the succession.
The result of abutting or overlapping shapes is a boundary between adjacent regions of color which, under ideal printing conditions, should have zero (0) width. That is, the one color should stop exactly where the other begins, with no new colors introduced along the boundary by the printing process itself. The “colors” which fill the shapes can be solid colors, tints, degrades, contone images, or “no fill” (i.e., the paper with no ink applied). In general, the “colors” represented in these adjacent regions are printed using more than one colorant. In practice, therefore, the realization of a zero (0) width boundary between regions of different color is impossible as a result of small but visible misregistration problems from one printed separation to another. The error is manifested as a “light leak” or as a visible boundary region of an undesired color.
As an example,
FIG. 1A
shows an ideal boundary between a red region on the right and a cyan region on the left, while
FIG. 1B
shows a non-ideal boundary, resulting from a slight misregistration of the magenta separation to the left on the page. Between the red and cyan regions is formed a blue line, from the unintended combination of cyan and magenta. On the right-hand side of the red region a yellow line is formed, again resulting from a slight misregistration of the magenta separation to the left on the page.
The problem of misregistration is a mechanical problem, almost always existing in printing systems. The problem arises because color separations are not laid exactly where intended, due to inherent imperfections in any separation registration process. It is somewhat correctable by mechanical registration methods; however it is rarely completely correctable. In expensive, high-end printing processes, customers have high expectations that misregistration artifacts will not be visible. In inexpensive, low-end printers, mechanical registration techniques are so expensive as to make correction or trapping essential.
As will become apparent, different printing technologies have distinct misregistration artifacts. Offset printing tends to have uniform misregistration in all directions. However, xerographic printing tends to have more misregistration in a single direction.
Methods for correcting such misregistration are known. The general approach is to expand one of the abutting regions' separations to fill the gap or misregistration border region with a color determined to minimize the visual effect when printed. Borders or edges expanded from a region of one color to another in this manner are said to be “spread.” A border which has been expanded is referred to as a “trap,” and the zone within which color is added is called the “trap zone.”
Commonly used methods for automatic trapping of digital images fall into the categories of vector-based and raster-based methods. Vector-based methods rely on images that have been converted from a page-description language (“PDL”) form, which describe objects as characters, polygonal shapes, etc., into an internal data structure. The internal data structure contains not only object information, but also a list of all the edges between regions of different color. Raster-based methods rely on images that have been first scanned or converted from page-description based form and are stored internally as a sequence of (high resolution) scan lines each containing individual scan elements or pixels. These methods process each raster line in sequence and compare one or more adjacent pixels to determine color boundaries. After some initial processing to find edges, both vector-based and raster-based methods apply rules for determining whether or not to create a trap at such boundaries, and finally apply a second set of rules to determine the nature of the trap if one is to be created.
Thus, it can be seen at
FIG. 2
that most trapping processes take the following format, which shall be referenced throughout this discussion:
A. Find edges in the image, no matter how described (step
101
);
B. For each pair of colors on each side of the found edge, determine:
1) Whether trapping should be used (step
102
);
2) If so, what color should be used (step
103
); and
3) Where should that color be located or placed (step
104
); and
C. Modify the image accordingly (Step
105
).
The present invention focuses on several elements of step C.
For example, the method of Taniguchi, described in U.S. Pat. No. 4,931,861, uses two rasterized images representing abutting or overlapping objects within an image field to define a third binary image, which represents the map of the pixels which make up the borders between the first and second images. These three (3) images are superimposed, pixel-by-pixel, to create a fourth and final binary image.
The method of Darby et al., described in U.S. Pat. No. 4,725,966, again defined on a pixel basis, uses a mask which is moved, one resolution element at a time, to evaluate the presence or absence of (pixel) colors upon which a positive or negative spread decision is based.
The method of Yosefi, described in U.S. Pat. No. 5,113,249, uses a set of automated rules as the basis for deciding, for each pair of abutting or overlapping shapes, whether or not to create a trap (an overlap region referred to as a “frame”), and, if so, the nature of the trap to create. The embodiment described by Yosefi makes use of scanned data, and processes each line of pixels in order. Each pixel is compared with three (3) pixels from the previous scan line and two (2) pixels from the same line to determine if a color change has occurred. The decisions regarding whether or not to create a trap, and the nature of such a trap if created, are imbedded within the processing sequence, making use of criteria established prior to the onset of processing. Yosefi describes rules to follow after finding an edge and knowing the two (2) colors. There are 24 rules based on whether the colors are tints, special colors (like gold leaf), black, yellow, “window” (meaning scanned image) and various combinations.
A commercially available product, “TrapWise”, from Aldus Corporation, Seattle, Wash., also makes use of a raster approach to trapping. In this product, the processing time is proportional to the number of resolution elements, thereby increasing quadratically with resolution, and leading to greater computation times for high device resolution, e.g., 3,600 dots per inch (d.p.i.). Furthermore, traps are created with this package using pre-set rules, and are not editable by a user without the requirement for repeating the computation.
U.S. Pat. No. 4,583,116 to Hennig et al. describes a trapping process that evaluates the darkness on both sides of an edge in order to determine which object determines the contour. The ob

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