Facsimile and static presentation processing – Facsimile – Specific signal processing circuitry
Reexamination Certificate
1995-03-27
2001-03-06
Coles, Sr., Edward L. (Department: 2616)
Facsimile and static presentation processing
Facsimile
Specific signal processing circuitry
C358S451000, C358S296000, C382S254000
Reexamination Certificate
active
06198545
ABSTRACT:
DESCRIPTION
Background of the Invention
The disclosed invention relates to a method and apparatus for synthesizing, displaying and printing colour or black/white halftone images. Such images can be reproduced for example on displays, paper, film or on other matter such as cloth. Apparatuses used for reproducing such images comprise display, printing and latent image transfer devices. Examples of such devices are printers, phototypesetters or computer driven displays.
The case of color image reproduction can be reduced to the case of black/white image reproduction if the color image is considered to be separated into three color planes (red-blue-green or cyan-magenta-yellow) or into four color planes (cyan-magenta-yellow-black), each color plane being treated as though it were a black/white halftone image. Therefore, the present disclosure will mainly relate to black/white halftone images; the color extension is implicit. When the term grayscale pixel or grayscale image is used, a variable intensity pixel or variable intensity image is meant. Therefore, the terms variable intensity and grayscale can be freely interchanged throughout the description and the claims.
In the graphics industry, the most common method for reproducing halftone images using bilevel printing devices is the ordered dither method. This method consists in subdividing the whole output image space into repetitive adjoining areas—screen elements. The inside of each screen element is gradually blackened by screen dots of increasing surface according to the gray level of the original image, thus ensuring the presence of various gray levels in the reproduction.
The terminology used in the description uses (a) the term input or source image for the variable intensity image to be reproduced, (b) the term screen element to describe a single possibly repetitive screen element containing both the white and the black screen element parts, (c) the term discrete screen element to describe a rasterized screen element composed of a set of elementary screen element cells or screen element pixels, (d) the term output halftone image to describe a bitmap or pixmap containing the output halftone image and (e) the terms output pixel space or output image pixel plane to specify the space or plane containing the output halftone image. Both the terms intermediate contour and interpolated contour have the same meaning, i.e. a contour whose generation comprises an interpolation between fixed predefined contours. The scan-conversion and filling of continuous contour shapes in order to obtain discrete shapes in pixel plane memory (bitmaps or pixmaps) is also called “rasterization”.
In the description which follows, large screen elements are considered which may be subdivided into adjoining subscreen elements, the subscreen elements containing subscreen dots. It is also possible to define a plurality of subscreen dots within a large screen element without requiring that the screen element be subdivided into subscreen elements.
In mosts parts of the description, for the sake of simplicity, we assume that intensity level 0 corresponds to white and a that a certain maximal intensity level, for example 1, corresponds to black. People knowledgeable in the art may easily convert this intensity definition to the other definition, where intensity level 0 represents black and a certain maximal intensity level, for example 1, represents white.
Methods for generating screen elements used in industry are generally based on dither threshold matrices. Such dither threshold matrices can be created manually or generated by an algorithmic process. One example of a dither threshold matrix is Bayer's dither threshold matrix, well-known in the graphics art industry [Bayer73]. Dither threshold matrices can be generated automatically from given analytical spot functions [Fink92], by sampling and discretizing the spot function at uniform intervals in both the horizontal and the vertical directions.
When generating screen elements from dither threshold matrices obtained for example by the discretization of an analytical spot function or by some other means, the dither matrix thresholds represent a discrete threshold function which is explicitely defined as a function of the coordinates of each elementary screen element cell. Although relatively flexible, dither matrix based methods only offer limited possibilities when generating sophisticated screen dot shapes, due to the fact that all screen dot shapes generated at different intensity levels need to be imbricated into one another.
We therefore propose a new method based on evolutionary screen dot contours for generating sophisticated screen dot shapes which offers more freedom due to the fact that the generated screen dot shapes at different intensity levels need not be imbricated into one another.
SUMMARY OF THE INVENTION
The disclosed method differs from previously known methods by the fact that, contrarily to methods based on dither threshold matrices, it does not make use of dither thresholds. It is characterized by the fact that intermediate contours bounding the white and black parts of each screen element are obtained by interpolating between fixed predefined contours, by the fact that the intermediate contours are transformed from a screen dot definition space to a screen dot rendition space and by the fact that the desired discrete screen elements are obtained by scan-converting and filling the intermediate transformed contours in the screen dot rendition space. The so obtained discrete screen elements are laid out in adjoining areas so as to pave the output image pixel space.
Alternately, fixed predefined contours can be transformed from the screen dot definition space into the screen dot rendition space and then be interpolated so as to obtain intermediate contours in the screen dot rendition space.
The disclosed method may be used beyond its simple application for generating screen dots with evolutionary screen dot contours. Since this method can produce large screen elements made up of a plurality of adjoining subscreen elements or made of a plurality of distinct subscreen dots, it therefore becomes possible to produce screen elements with a well-defined orientation, i.e. an angle close to a non rational angle. This method can also produce subscreen dots whose shape, period and orientation vary according to their position within the screen element, by choosing an appropriate transformation function between the screen dot definition space and the screen dot rendition space. These variations are useful for preventing unauthorized reproduction of images generated in this manner. The reproduction of small screen dots and large screen dots does not induce the same dot gain. Therefore, when the original image is printed onto the target printing device, this dot gain variation is compensated for by dot gain compensation factors (gamma correction) which take into account the size of the current screen dot. During optical reproduction (photocopying) this variation cannot be compensated for and produces variations in light intensity which depend on the size of the subscreen elements or respectively the size of the subscreen dots. Moreover, the fact that the shape and the period of the subscreen element, respectively subscreen dots vary prevents faithful reproduction by a digital scanning device (scanner). Using a scanner to scan and digitize an image with variable sized subscreen elements, respectively subscreen dots at a scanning frequency close to some of the subscreen element, respectively subscreen dot frequencies produces very visible moiré effects, thus making faithful reproduction almost impossible.
The invented method differs from existing methods for generating evolutionary screen dot shapes using dither matrices in that successive screen dot shapes corresponding to increasing intensity levels do not need to be imbricated or embeddable within one another, as is the case for screen elements produced using dither matrices. This property is essential when generating comple
Hersch Roger D.
Ostromoukhov Victor
Coles Sr. Edward L.
Nguyen Madeleine A-V
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