Image analysis – Image enhancement or restoration – Intensity – brightness – contrast – or shading correction
Reexamination Certificate
2000-06-01
2004-07-13
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Image enhancement or restoration
Intensity, brightness, contrast, or shading correction
C358S003060, C358S003230, C358S003300, C358S536000, C382S112000, C382S163000, C382S218000
Reexamination Certificate
active
06763144
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for producing a halftone (screened) reproduction of a latent or real image from a digital representation of a black and white or color image by an output device used in the printing and pre-printing industry.
BACKGROUND OF THE INVENTION
Halftone is the reproduction of continuous tone art work, such as a photograph, through a series of dots of various sizes and locations used to simulate grays or color tones.
In general, digital halftoning is accomplished by either binary (1 bit—two levels) or multi-bit (multi-level) halftoning methods. In binary digital halftoning, a continuous tone image is converted into a halftone image consisting of a pattern of dots. Each dot within the halftone image is either “ON” (black) or “OFF” (white). More specifically, binary digital halftoning converts a plurality of digitized intensity values representing a continuous tone image into a plurality of halftone cells, each halftone cell corresponding to an intensity value. The number of dots within each halftone cell is proportional to the magnitude of each corresponding intensity value.
During binary digital halftoning, each intensity value is spatially mapped into a corresponding halftone cell, having a plurality of black and white dots.
In operation, a binary digital halftoning system compares each intensity value to a matrix of threshold levels and generates a halftone cell corresponding to each intensity value. Typically, the threshold matrix has a number of elements equivalent to the number of dots in the halftone cell. To generate the binary halftone cell, a given intensity value is compared to each threshold level in the matrix. Each dot in the halftone cell, that corresponds to a threshold level in the threshold level matrix that is lesser in value than the intensity value, is made black; otherwise, the dot is white. Thus, the intensity value is mapped into an area comprised of an arrangement of black and white dots whose overall intensity is corresponding to the magnitude of the intensity value.
Multi-level halftoning is an extension of binary halftoning. In multi-level halftoning, each dot in the halftone cell has a multi-bit value. Many display devices, some digital printing devices and other marking devices permit multi-level pixel reproduction; multi-level halftoning takes advantage of this capability.
Typically, these devices are limited as to the number of levels that they can produce. In contrast, sampling devices can produce many different output levels. Multi-level halftoning is used to convert a large number of tone levels into a lesser number of levels. For instance, if a display device can accurately display sixteen levels while a scanner can provide a 256 level intensity value, a multi-level halftoning system must distribute each single 256 level value into a halftone cell containing a plurality of sixteen level dots, so that, when viewed, will appear as the 256 level value.
Reference is now made to
FIG. 1
, which is a schematic block diagram of a prior art halftoning technique, as described in U.S. Pat. Nos. 4,350,996 and 4,456,924 assigned to Scitex Corp. Ltd. This halftoning technique is very well known and many vendors such as AGFA, Linotype-Hell, Dainippon Screen etc. have used different versions of it. Screen threshold values are pre-calculated and stored in a screen threshold value matrix
30
—a memory that can be described as a two-dimensional array or matrix. The screen threshold values may be stored permanently, or calculated specifically according to some required parameters defined for a specific job. In either case, the screen threshold values should be stored in the screen threshold value matrix
30
prior to the beginning of the screening (halftoning) process.
During the screening process, screen threshold values are compared, by means of a comparator
60
, with image data, temporarily stored in the image value buffer
40
. During halftoning, the image data is read from the image value buffer
40
and may be modified, e.g. for calibration purposes, by calibration look-up table
50
. In some prior-art embodiments, the calibration LUT
50
may exist in another section of the system, or may not exist at all, and the calibration function is performed by other means.
The output
65
of the comparison performed by the comparator
60
is a binary halftone output, namely, one-bit data that serves as the control for the printing engine. In this case, the printer is capable of either printing a full mark (dot) on the substrate (the halftone output bit is “1” or “ON”) or not printing at this point at all (the halftone output bit is “0” or “OFF”).
The reading of screen threshold values from the screen threshold value matrix
30
may be sequential, value by value and line by line. In such a case, the address calculator
20
performs as a simple count-up counter. There may, however, be more complex address calculations performed by address calculator
20
, including skipping of cells in the screen threshold value matrix, or even more complex address calculations such as angled scanning of the matrix. In case of an angled scanning of the matrix
30
cells, the resulting output halftone image will be a screened (halftone) reproduction that composes an angled screen.
Typically, image values and screen threshold values are each represented by an eight bit digital number. The halftone output, however, is a one-bit number. The number of bits that compose the X address-coordinate
25
and Y address-coordinate
35
may vary, depending on the size of the matrix
30
. A typical small matrix
30
of 16 by 16 entries may be addressed by a four-bit number for the X address-coordinate
25
and another four-bit number for the Y address-coordinate
35
. A large but still typical matrix
30
may be composed of a 1024 by 1024 two-dimensional array, addressed by ten bit numbers for each of the X and Y address-coordinates. Such a large matrix
30
is capable of representing a very accurate and sophisticated halftone cell or a combination of a number of halftone cells in a super cell arrangement. For a super cell arrangement, even a large matrix
30
of 4096 by 4096 entries may be considered. For such a matrix, a 12-bit number is required for each of the X and Y address-coordinates.
FIG. 2
is a simplified example of the arrangement and data composition of the screen threshold value matrix
30
of FIG.
1
. In this simplified example, the matrix
30
is composed of entries
70
in a two-dimensional five by five array. The content of the entries is designed to reproduce a typical square halftone dot. Different contents of the entries
70
will result in different shapes of the output halftone dot. It should be noted that the shapes of the various output halftone dots are a representation of the matrix
30
contents, and a larger image value results in a larger size of the output halftone pixel, since it comprises a larger number of “ON” dots.
FIGS. 3A and 3B
illustrate, by way of example, the output halftone pixels corresponding to intensity values of 10 and 20 respectively, using the threshold matrix of FIG.
2
. In these examples, the matrix contents dictates square dots of a size proportional to the input intensity value.
FIG. 4
schematically illustrates another prior art screening method, as described in U.S. Pat. No. 5,444,551 assigned to Eastman Kodak Company. The method presented is a generalized method for providing a mechanism for performing multi-level half-toning of continuous tone images.
In this prior art system, various dot arrangements are defined and associated with all possible image values. These dot arrangements are stored in a plurality of two-dimensional arrays. A typical variety of 256 different image values requires 256 arrays for storing all possible dot arrangements. The entire halftoning system may be described as a three-dimensional array (memory) in which the X and Y axes correspond to the X and Y axes on the printed media, and the Z (height) axis corresponds to the image value at a locati
Creo IL Ltd.
Desire Gregory
Mehta Bhavesh M.
LandOfFree
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