Image analysis – Image compression or coding – Quantization
Reexamination Certificate
1997-10-15
2001-08-21
Chang, Jon (Department: 2723)
Image analysis
Image compression or coding
Quantization
C382S299000, C358S001200
Reexamination Certificate
active
06278802
ABSTRACT:
BACKGROUND OF THE INVENTION
Many reproduction methods are only capable of reproducing a small number of stable image tones. For example, offset printing is only capable of printing two stable tone values i.e. deposit ink or not. In order to reproduce images having continuous tones, a halftoning or screening technique is used. In the graphic arts environment, halftoning techniques convert density values of tints and images into a geometric distribution of binary dots that can be printed. The eye is not able to see the individual halftone dots, and only sees the corresponding “spatially integrated” density value. In a more general context, halftoning techniques can be seen as methods to convert “low spatial, high tonal resolution information” into an equivalent of “high spatial, low tonal resolution information”. (The qualifiers “low” and “high” have to be seen on a relative scale in this context).
Two main classes of halftoning techniques have been described for use in the graphic arts field. These two techniques are known as “amplitude modulation” and “frequency modulation” screening. In amplitude modulation screening the halftone dots, that together give the impression of a particular tone, are arranged on a fixed geometric grid. By varying the size of the halftone dots, the different tones of images can be simulated. Consequently, this technique can also be called “dot-size modulation screening”. In frequency modulation screening the distance between the halftone dots is modulated rather then their size, and can also be referred to as “dot-position modulation screening”. This technique, although well known in the field of low resolution plain paper printers, has not obtained much attention for offset printing and other high end printing methods, probably because of the disadvantages to be discussed below.
Both classes of halftoning techniques are used in combination with a digital film recorder. A typical digital film recorder employs a scanning laser beam that exposes a photosensitive material at high resolution. The “grid” that defines the resolution at which the laser beam can be switched on or off, usually has an element size in the range of {fraction (01/1800)} of an inch. The photosensitive material can be a photographic film from which a printing plate is later prepared by means of photomechanical techniques. The smallest addressable unit on a recorder is often called a “micro dot”, “recorder element”, or “rel”. Its size is referred to as the recorder “pitch”. As illustrated in
FIG. 1A
a dot-size modulated halftone dot is made up of a clustered set of recorder elements, while in
FIG. 1B
frequency-modulation halftone dots consist of individual recording elements.
The most important characteristics of a screening or halftoning technique for faithfully reproducing continuous tone information include:
1) The image rendering characteristics, more specifically the capability of the technique to render spatial detail in the original image content without the introduction of artifacts such as moiré, textures and noise, as well as the capability to render a full range of tones;
2) The photomechanical characteristics of the halftone dots produced by the method, which determine how consistently halftone dots can be recorded, copied or duplicated in the different steps of the photomechanical preparation of the printing plates; and,
3) The behavior of the halftones on an offset printing press.
The two classes of halftoning techniques, each with some of their variants, will now be reviewed in the light of the above characteristics, and their advantages and disadvantages will be discussed
Amplitude Modulation Screening
Amplitude modulation screening has as its major advantages that it has excellent photomechanical reproduction characteristics, and that, for screens with rulings up to 200 dots/inch, it prints predictably on offset presses. An important disadvantage of amplitude modulation screening, however, is the fact that unwanted patterns can occur within the halftoned image. Depending on their origin, these patterns are called subject moiré, color moiré or internal moiré. Subject moiré results from the geometric interaction between periodic components in the original subject matter and the halftone screen itself. Methods addressing subject moiré are disclosed in e.g. U.S. Pat. No. 5,130,821, EP 369302 and EP 488324. These methods do not, however, completely solve the problem.
Color moiré results from interferences between the halftones of the different color separations of the image. The use of screen angles for the different color separations shifted by 60 degrees with respect to each other has been suggested to address this problem. Several disclosures relate to the problem of generating screens with these angles or close approximations thereof. See for example U.S. Pat. No. 4,419,690, U.S. Pat. No. 4,350,996, U.S. Pat. No. 4,924,301 and U.S. Pat. No. 5,155,599. Other combinations of angles, frequencies or relative phases of the halftone dot patterns for the different color separations have also been used to overcome the same problem, as described for example in U.S. Pat. No. 4,443,060, U.S. Pat. No. 4,537,470 and EP 501,126.
Internal moiré refers to patterns resulting from the geometric interaction of the halftone screen with the addressable grid on which they are rendered. Methods to reduce internal moiré are usually based on the introduction of a random element that breaks up or “diffuses” the phase error that periodically builds up as a consequence of the frequency and angle relation between the halftone screen and the addressable grid on which it is rendered. Examples of such techniques are disclosed in U.S. Pat. No. 4,456,924, U.S. Pat. No. 4,499,489, U.S. Pat. No. 4,700,235, U.S. Pat. No. 4,918,622, U.S. Pat. No. 5,150,428 and WO 90/04898.
Frequency Modulation Screening
None of the variants of the dot-size modulation screening has proven to be successful in completely eliminating the moiré problems and frequency modulation screening techniques have therefore been suggested to further reduce the problem. Various frequency modulation screening techniques have been disclosed and they can be divided into the following subclasses: (1) Point-to-point thresholding based techniques; (2) Error Diffusion techniques (and their variations); and, (3) Special techniques, such as that disclosed in DE 29,31,092, and further developed in U.S. Pat. No. 4,485,397.
The most representative example of point-to-point thresholding is the halftoning based on the “Bayer” dither matrix. See Bayer, B.E., “An Optimum Method for Two-level Rendition of Continuous-tone Pictures”, Proc. IEEE International Conference on Communications, Conference Record, pp. (26-11), (26-15), 1973. The Bayer dither matrix has a size that is a power of two, and contains threshold values that are arranged in such a fashion that, when thresholded against increasing levels of density, every halftone dot is “as far away as possible” from the halftone dots that are used to render the lower density levels.
Another point-to-point thresholding technique uses a “blue-noise mask” instead of a Bayer dither matrix. It is described in U.S. Pat. No. 5,111,310. The blue-noise mask is the result of an optimization (filtering) performed iteratively (for the subsequent threshold “layers”) between the halftone dot patterns produced by the mask and their Fourier transforms.
The halftone dot patterns produced by the Bayer dither matrix contain strong periodic components, visible as “texture” that can potentially create moiré problems similar to the dot-size modulation algorithms. Because the energy of the periodic dither components is “spread” over the different harmonics, and because most of these harmonics have a relatively high frequency compared to the fundamental frequency of dot-size modulation, the moiré that occurs is less disturbing.
The “Blue-noise mask” threshold matrix produces distributions of halftone dots that are aperiodic. This method is therefore free of the moiré problems that occur with the dot-size modulation method
Delabastita Paul A.
Deschuytere Frank A.
AGFA-GEVAERT
Chang Jon
Prikockis Larry J.
Schmeiser Olsen & Watts
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