Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-07-30
2001-03-20
Rogers, Scott (Department: 2724)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S535000
Reexamination Certificate
active
06204934
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processing device and image processing method for converting a color image into a binary black and white image.
2. Description of the Related Art
Recently facsimile machines and copying machines (photocopiers) capable of scanning color images have been developed, and demand for these machines has increased along with the spread of high quality color printers and displays.
These image processing devices capable of processing color images utilize an optical color sensor to scan in an image in the three primary colors (RGB). However, if an associated printer or display is set to a black-and-white output mode or is not provided with a color output or display function, the scanned color image is converted to a black and white image based on lightness of the original color image.
FIG. 4
of the accompanying drawings illustrates processing steps used in such a conversion. Here, image data scanned-in in the three primary colors (RGB) is converted into multi-bit-depth color data according to L*a*b* color, one of the CIE (International Council on Illumination) standard model color systems, and then further converted to binary image data.
Referring to
FIG. 4
, the RGB image data scanned in by a scanner Sc is converted to CIE LAB color-system data using an RGB-Lab converter
100
. Next, lightness L data (multiple bit-depth data) is converted to binary data B&W using a dithering unit
101
, and output from a printer Pr. The dithering unit
101
has only one dither pattern. The values inside the parentheses indicate the bit number for each data.
The dithering process divides an image into a plurality of small areas, and changes the density of the black pixels in each area to provide an intermediary gray-scale image having a plurality of pixel density levels as a whole. Specifically, an area defined by n×n pixels (a dot matrix) is used as a unit of gradation (or an original image is divided into a plurality of matrices), and a dither pattern in which the threshold values between white and black are dispersed is applied to each matrix to obtain a binary image. In the dithering process, the lightness L of each pixel in the matrix is compared with a corresponding threshold value of the dither pattern; the pixel is converted to white if the lightness of the pixel is brighter than the threshold, and the pixel is converted to black if its lightness is darker than the threshold.
FIG. 5
of the accompanying drawings illustrates the color spacing according to the Lab color system. In the Lab color system, lightness is represented as L and chromaticity, showing hue and saturation, is represented as “a” and “b”. Saturation “c” is given by (a
2
+b
2
)
0.5
, and hue “h” is given by tan
−1
(b/a). Further, in the color spacing of the Lab color system, “+a” represents red levels, “−a” represents the green level, “+b” represents the yellow level, and “−b” represents the blue level. As the color values extend to the limits of each range, the colors become more vivid, and as the values approach the center, the colors become softer.
In a conventional image processing system, even when the chromaticity “a”, “b” of dot matrices in a single original image differ, the matrices are converted into the same bilevel black and white data as long as their lightness L is the same. Therefore, the black and white image that is output upon color-to-monochrome conversion can look unnatural in comparison to the original color image.
SUMMARY OF THE INVENTION
An object of the present invention is to propose an image processing device and image processing method which can produce a natural-looking black-and-white image from an original color image.
In order to accomplish this object, the present invention uses different dither patterns according to the hue and/or saturation of an original color image (pixels). Specifically, a plurality of dither patterns are prepared beforehand, and a single dither pattern is then selected based on the hue and/or saturation of the original color image. The lightness is not considered in selecting the dither pattern.
According to one aspect of the invention, there is provided an image processing machine that detects lightness, hue and/or saturation of an original color image, presents a plurality of dither patterns, selects a single dither pattern depending upon the hue and/or saturation of the color image and carries out the dither process (or binary conversion of color image) to the lightness using the selected dither pattern. For example, if an original color image is expressed numerically in the Lab color system, two dither patterns may temporarily be provided, and a single dither pattern is selected depending upon the saturation “c” (c=(a
2
+b
2
)
0.5
). Consequently, the resulting black-and-white image does not appear unnatural when output. The black-and-white image may be output onto recording paper or shown on an LCD or CRT display. The plurality of dither patterns may be stored in a memory of the image processing device beforehand.
The present invention may be modified as follows: only when the lightness of a particular area in the target image is equivalent to that of a neighboring area, hue and/or saturation is used to select the dithering pattern. When the chromaticities of adjoining regions in an original image differ and the lightness remains the same, the eye perceives the difference. Thus when, for example, the total lightness of the pixels in the neighboring dot matrices are equal, the chromaticity (hue and/or saturation) of the image is considered in the selection of the dithering pattern. Particularly, the dither pattern selection depending upon the hue and/or saturation may be performed when the neighboring regions on the same scanning line have the same lightness.
REFERENCES:
patent: 4710822 (1987-12-01), Matsunawa
patent: 5029017 (1991-07-01), Abe et al.
patent: 5153576 (1992-10-01), Harrington
patent: 5822451 (1998-10-01), Spaulding et al.
Hogan & Hartson LLP
Murata Kikai Kabushiki Kaisha
Rogers Scott
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