Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2001-05-14
2003-03-25
Pendegrass, Joan (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S140000, C358S001900, C358S003100
Reexamination Certificate
active
06538677
ABSTRACT:
FIELD
This invention relates to electrography, and in particular, to the generation of halftone images with reduced image artifacts and increased levels of gray by the use of a rotating magnetic brush with a hard magnetic carrier, in conjunction with a digital, multi-bit printhead and halftone rendering system capable of printing variable dot sizes.
BACKGROUND OF THE INVENTION
Electrographic print engines are used in printers and copiers to provide one or more copies of documents. Analog print engines rely upon a light lens to focus an image onto a charged image carrying member. Light strikes the charged image carrying member, discharges it and leaves a latent image on the member. Such print engines produce acceptable continuous tone images when the latent image on the image member is developed with developer comprised of a toner and a hard magnetic carrier. See for example U.S. Pat. Nos. 4,473,029; 4,531,832; 4,546,060; and 5,376,492, whose disclosures are incorporated by reference. Such copiers can reproduce images of photographs that are acceptable because they provide multiple levels of gray.
With the advent of digital technology, many images are captured with charge coupled arrays or other digital apparatus that converts the image into a set of pixels. In pure binary machines, the pixel is either on (black) or off (white). Such techniques are well suited to reproducing text because the sizes of the individual pixels that make up text symbols are much smaller than the symbols and the symbols are best seen with high contrast edges. Thus, the human eye sees the text as a continuous image even though it is a collection of closely spaced dots.
However, binary electrographic print engines do not provide acceptable levels of gray for other images, such as photographs. Those skilled in the art have used halftone dots to emulate gray scale for reproducing images with continuous tones. Newspapers and magazines are common examples of halftone printing. The reader does not see the halftone dots because they may be as small as ½,500
th
to ⅕,000
th
of an inch. Such small sizes are possible with ink and with newsprint and magazine media. However, such small sizes are virtually impossible with electrographic toner. Indeed, the toner particles themselves are larger than the size of halftone dots used by newspapers and magazines.
Conventional binary electrographic halftone print engines try to make the dots as small as possible. Conventional toner stations provide binary dots that are too large for acceptable halftone imaging. Hard dots, ideally having sharp edges, are also deficient when made with conventional binary arrays or rendering techniques using developer comprised of a toner and a hard magnetic carrier. The hard dots break up and do not provide the desired sharp edges. Accordingly, there is a need for a new electrographic print engine that provides better halftone imaging. Conventional binary electrographic print engines do not meet this need.
In the area of digital printing, all colors including black or gray are represented on paper as one or more gray levels where gray refers to a color density between no color and saturation. There are a number of algorithms for rendering halftone images. Digital printers commonly make a mark, usually in the form of a dot pixel, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch (dpi), on paper. A digital printer emulates color intensity by placing marks, or dots, on the paper in a geometrical pattern. The effect is such that a group of dots and dot-less blank spots, when seen by the eye, gives a rendition of an intermediate color tone or density between the color of the initial paper stock, usually white, and total ink coverage, or a solid density halftone dot. It is conventional to arrange the dots in rows, where the distance between rows is known as line spacing, and determines the number of lines per inch (lpi). In the ensuing paragraphs, discussions will be made in terms of white paper stock; it is understood that white paper stock is used as an illustration and not as a limitation of any invention.
Continuous tone images contain an apparent continuum of gray levels. Some scenes, when viewed by humans, may require more than 256 discrete gray levels for each color to give the appearance of a continuum of gray levels from one shade to another.
As an approximation to continuous tone images, conventional digital print engines create pictorial or graphical images via halftone technology. Halftone pictorial or graphical images lower the high contrast between the paper stock and the toned electrographic image and thereby create a more visually pleasing image. Such halftone methods use a basic picture element (also known as a cell) on the recording or display surface. The cell consists of a j×k matrix of sub-elements (pixels or pels) where j and k are positive integers. A halftone image is reproduced by printing the respective sub-elements or leaving them blank. That is, by suitably distributing the printed marks in each cell. Such halftoning technology uses various rendering algorithms, such as those disclosed in U.S. Pat. Nos. 5,198,910, 5,258,849, and 5,260,807, the teachings of which are incorporated herein by reference in their entirety, to form, arrange and/or otherwise orient the marks so as to modulate the contrast between the dots and paper stock background to render the image more visually pleasing.
Halftone image processing algorithms are evaluated, in part, by their capability of delivering a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces the fine detail with lesser or no output contrast.
Another figure of merit of image processing algorithms is the ability to suppress visual details in the output image that are not part of the original image, but are the result of the image processing algorithm. Such details are called artifacts, and include false contours and false textures. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible contour when the input image is truly a smooth, gradual variation from one gray level to another. False textures, and textures that are visual and change with rendered density, are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly but the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level. Commonly used processing algorithms include fixed level thresholding, adaptive thresholding, orthographic tone scale fonts, and electronic screening.
In creating halftone images, two factors are of prime consideration: the line screen frequency and the number of addressable picture elements, i.e., pixels. Once the line screen frequency is determined, the number of addressable pixels determines the number of definable, i.e., theoretical, gray levels. The definable gray levels for a binary system can be calculated by the following formula:
Number of gray levels=(dpi/lpi)
2
+1
The screen frequency (lpi) tends to be set high so that the size of the dots are small and not visually detectable at normal viewing distances. An obvious problem arises when the resolution of the dot matrix on the paper is not very high, for example, 100 dpi or less. In such cases the geometrical patterns for the cell become visible to the eye. In that case the viewer is distracted from the image by artifacts of geometrical patterns themselves and perceives the impression of an image of poor quality. The obvious solution to this problem is to work at very high resolutions, for example, 300 dpi or greater, so that those artifacts are less perceived and their negative effects become less glaring. However, in view of the above formula, having a high screen frequency means there is a tradeoff with resp
Ng Yee S.
Stelter Eric C.
Tai Hwai-Tzuu
Thompson John R.
Zeise Eric
Heidelberger Druckmaschinen AG
Pendegrass Joan
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