Half-tone dot generation

Details Half-tone dot generation Half-tone dot generation

1998-06-01

2001-07-24

Nguyen, Thinh (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C347S252000

Reexamination Certificate

active

06266079

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for generating half-tone dots for reproducing images on an ink-jet printer and, more particularly, to the generation of half-tone dots capable of conveying a large number of different tonal values.
In the art of printing it is generally known to reproduce images that originally contain areas of different tonal values by means of a screen-like two-dimensional pattern of inked (e.g. black) dots at regular intervals, the dots and the spaces therebetween being of variable proportions. Such reproduction is also referred to as half-tone reproduction; accordingly, the screen pattern and the dots are also referred to with the modifier “half-tone”. In order to distinguish between half-tone dots and a different type of dot, to be discussed herebelow, the term half-tone spot, or, briefly, spot, will be used in the sequel. For a good quality image, there are, typically, about 150 half-tone spots to the inch. Color printing is typically effected by superposing four images, each with one of the four process-color inks. In order to minimize moire pattern effects, the screen patterns of the four images are mutually inclined at certain angles.
Digital printing devices typically generate an image by marking the printed medium, such as paper, with a regular pattern of contiguous parallel lines, each line varying between the values—“on” (marking) and “off” (blank)—according to the image. In some devices, such as ink-jet printers and printers using pulsed lasers, to be collectively referred to as discrete-dot marking devices, each marking line inherently consists of a series of contiguous elementary dots (to be referred to, briefly, as dots), each dot having a binary value (marking or non-marking—corresponding to having ink applied or not applied at its respective location). In some other devices, such as those using continuously radiating lasers for marking, a marked segment of a line may be continuous, but the algorithm for switching between marking—and non-marking states is based, in effect, on a model of contiguous discrete bi-valued dots—thus, again, forming a two-dimensional array of dots. The necessarily finite spatial frequency of marking lines, and of dots within a marking line, causes certain limitations on achievable image quality—especially with respect to half-tone images, as will be discussed herebelow, and it is the challenge of improving image quality in face of such limitations that is answered by the present invention. It is noted that there are also digital marking devices that, rather than directly print an image on a print medium, serve to create a latent image on a printing plate, which subsequently serves in a printing press. Most of what is discussed and disclosed in the present specification applies to such marking devices as well.
There are many methods known in the art for converting a digitally-represented multi-valued image into a half-tone pattern that is suitable for reproduction by a binary marking device (which conversion process is also referred to as digital screening, or, in short, screening). According to one such method (to be referred to herebelow as method A), in common usage and useful for a marking scheme that is based on the model of discrete dots along marking lines, there is stored a two-dimensional matrix of threshold values, one value per dot location, which represents the screen function at the appropriate angular orientation. For each dot of the half-tone image the corresponding threshold value is compared with the original image value and the result of the comparison determines the (binary) dot value. For certain angles of orientation (namely those whose tangent is a rational number), the screen function can be made to be repetitive in both dimensions, so that a threshold matrix corresponding to only one (repetitive) segment of the screen function need be stored. This method has several drawbacks:
(1) It is practically limited to rational-tangent angles;
(2) for desirable rational-tangent angles, a relatively large amount of values (corresponding to a large segment, as well as to the number of such different screen angles to be implemented) need be stored;, and
(3) there is a variance in size and shape among neighboring spots, resulting in visual artifacts.
Another known rational-tangent angled screening method suitable for discrete dot marking (to be referred to herebelow as method B), has a binary dot pattern over a (repetitive) segment stored for each possible image value. For each image pixel, the dot pattern corresponding to its value and to its relative position in the segment is read out and fed to the marking device. This method shares the first three drawbacks of method A, listed hereabove, and even requires greater storage capacity, but, advantageously, is not limited to monotoniccally increasing spots; rather, there is total independence between the spot shapes at various tone values.
The discrete dot structure entails another drawback for screening, though it is not very pronounced for high resolution devices, such as those using laser beams for marking. In the latter there are, typically, about 2000 lines to the inch, and that same frequency of dots (if indeed so structured). With such devices, each spot of a half-tone screen of, said, 175 spots to the inch (which is typically used for high quality printing) contains, on the average, about (2000/175)×(2000/175)=131 dots. Since the dots are binary valued, each spot can thus have 132 values of proportional area (from totally blank to totally marking). Thus image details as fine as the size of a single spot can be reproduced at any of only 132 shades of gray; and, conversely, an area of an image that has uniformly a shade of any value, out of only 132 values, can be uniformly reproduced down to the screen structure and excluding small variations among neighboring spots due to the screening method (such as the one described hereabove). With devices of continuous marking capability, such as those utilizing continuous-radiation lasers, the number of tonal values and the spot uniformity can be further increased if the dot structure along lines is abandoned and screening methods, such as those taught by the following three patents, are adopted.
U.S. Pat. No. 5,519,792 teaches storing the screen function as a matrix of threshold values that is independent of the marking grid structure and of the orientation angle of the half-tone pattern to be marked, reading out values from the stored matrix along a line that corresponds to a marking line and to the orientation angle, comparing them to a corresponding image value and, where, between any successive points, the sense of the comparison switches, calculating, by interpolation, the exact location, along the marking line, of a boundary between marking- and non-marking segments.
U.S. Pat. No. 5,526,143 teaches a screening technique similar to that of the '792 patent, except that the succession of threshold values is calculated on the fly and that it, as well as the succession of image values, are converted to corresponding successions of analog values; these are interpolated, to create corresponding smooth signals, which are then mutually compared, the point at which the sense of the comparison switches determining the boundary between marking- and non-marking segments.
U.S. Pat. No. 5,691,828 teaches storing the screen function, at any given orientation angle, as relative locations of boundary points of spots along lines parallel to the marking lines (the storage being arranged according to tonal values corresponding to various spot shapes and sizes) and reading out these values to determine boundary points along any marking line.
The methods of the three patents, briefly described hereabove, are suitable for continuous marking devices, such as those using a continuous laser beam, and their main advantage is the possibility of delineating each half-tone spot at a much greater resolution than implied by the spatial line frequency of the

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