Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
2001-06-29
2004-09-28
Lee, Thomas D. (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S001900, C358S003010, C358S003070, C382S299000
Reexamination Certificate
active
06798541
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to systems and methods for generating binary irrational halftone dots based on a spatially modulated stimulus.
2. Description of Related Art
When creating image regions using halftoning, binary clustered halftone dots are desirable. In particular, binary clustered halftone dots often produce the least amount of noise and the best highlights. Conventional halftoning adds a two-dimensional, spatially periodic, dot screen or line screen structure to the images to be halftoned. Typically, the same screen, or at least a number of essentially identical screens, are used to halftone each of the color image separation layers of a polychromatic, i.e., color, image. However, the halftone screens are oriented at different angles for printing the respective halftone color image separation layers.
Digital halftoning has evolved as a method of rendering the illusion of continuous tone, or “contone”, images using devices that are capable of producing only binary picture elements. However, digital halftoning can suffer from misregistration between the various color image separation layers used in color image, for example, cyan, magenta, yellow and black (CMYK) color image separation layers. This misregistration can be caused by misalignment among the various halftone screens and also by misalignment between the halftone screens and an image forming apparatus grid structure, i.e., an output grid structure, used to generate electronic image data from an image, on an image forming member. This misregistration can also include errors in rotation of the screen angle. This misregistration can cause moir é patterns.
Moir é patterns can also be generated based on the screen angles used for each of the color separation layers, even without any misregistration. Regardless of how the moir é patterns are formed, moir é patterns are detrimental to the accurate rendering of the color image. Moir é patterns arise due to “beating”, i.e., periodically mismatching patterns of interference that degrade the resulting rendered images. When the various color separation layers are combined during rendering of a multicolor image, where each color separation layer uses a different halftone screen or the same screen at a different angle, a moir é pattern can result. The resulting moir é pattern can cause a color shift or variation in tone.
Substantial effort and expense have been invested in minimizing the moir é patterns caused by halftoning techniques for producing binary renderings of contone images. Misregistration, improper screen angle, and improper screen frequency can increase the halftone screens' susceptibility to moir é patterns. Additionally, because the moir é patterns can be caused by halftone screens beating with the output grid structure, the moir é pattern may be caused by a difference between the halftone screen, pitch frequencies and the re-sampling rate frequency within the image forming apparatus. Even minor variations in the dot position caused by systematic errors, such as quantization round off errors, can produce moir é patterns resulting from beat frequencies between the periodic screens.
In general, increasing the angle differences between the halftone screens reduces the prominence of moir é patterns because the interference between the image separation layers is more frequent but the amplitude of the interference is lessened. In addition to errors in frequency or in angle, the grid structure of the stimulus applied by the image output apparatus used to create the color separation layers can also contain imperfections. If the respective grid structures for all of the color separation layers do not exactly align, the halftones can be misregistered, becoming another source of moir é patterns.
Thus, the perceived quality of the resulting color image is strongly dependent on the precision with which the color image separations are spatially registered with each other, as well as the precision with which the halftone screens are oriented in relationship to each other and/or to the output grid used by the image forming apparatus. Conventional halftoning methods, such as those disclosed in U.S. Pat. No. 5,410,414 to Curry, incorporated herein by reference in its entirety, and U.S. Pat. No. 4,537,470 to Schoppmeyer, warp, i.e., adjust or move, the image data produced by an image data generator to improve registration. Such image data generators include gray scale image generators and binary image generators. However, merely warping the image data to improve registration results in offsets with the image data which have no corresponding adjustment or warp in the halftone screens used to render the color image separation layers.
Therefore, minimizing the moir é patterns conventionally includes also warping one or more of the halftone screens in a halftone screen system to correspond to the warping of the image data. This is disclosed in greater detail in U.S. Pat. No. 5,732,162 to Curry, incorporated herein by reference in its entirety. The 162 patent provides a detailed discussion of warping both image data and halftone screens.
High addressability or hyperacuity refers to the ability to locate an edge, occurring between one portion of an image and another portion of an image, at a resolution that is greater than the resolution of the stimulus used to form the image. Such edges often occur between halftone dots and the non-image background regions of each of the color separation layers.
One common stimulus used by various image forming apparatus to form images is a light beam scanned by a raster output scanner (ROS). A raster output scanner scans one or more such light beams across a photoreceptor drum or belt. In general, the raster output scanner scans each of the light beams across the photoreceptor drum or belt in a fast scan direction while the photoreceptor drum or belt simultaneously moves relative to the scanned light beam in a slow scan direction. As the one or more light beams are scanned across the photoreceptor drum or belt in the fast scan direction, the one or more light beams are individually modulated between off and on at a high rate. In particular, in various known high addressability systems, each light beam is modulated at a rate that is four times the period it takes the raster output scanner to move the one or more light beams a distance along the fast scan direction that is equal to the diameter of the light beams. This is known as 4× high addressability. As shown in
FIGS. 1 and 2
, 4× high addressability allows the location at which the one or more light beams are turned on to be spatially controlled to one-quarter of the diameter of the light beam along the fast scan direction.
However, as also shown in
FIGS. 1 and 2
, the center-to-center spacing of two adjacent light beams or of two adjacent scans of a single light beam are offset by the diameter of the one or more light beams. Therefore, as shown in
FIG. 3
, when the edges of an image structure, such as a halftone dot, extend across the laser beam in directions that are not substantially aligned across the fast scan direction, the light beam cannot merely be turned on when the current scan of the light beam intersects with the image structure, such as a halftone dot, and left on until the light beam no longer intersects the image structure. Doing so would result in significantly more toner being applied to the resulting developed image at that area. This would itself result in that portion of the image having an image density that significantly departs from the desired image density represented by the image structure, such as the halftone dot. Conventionally, as shown in
FIG. 16
, to avoid this change in image density, the edge of the image structure, such as the halftone dot, that extends along the fast scan direction, and therefore, across the slow scan direction, is “dithered”, i.e., modulated, at a very high rates, so that the actual amount of image density of the developed image more closely corresponds to the image density of
Lee Thomas D.
Thompson James
Xerox Corporation
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