Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-07-07
2003-09-30
Lee, Thomas D. (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S461000
Reexamination Certificate
active
06628427
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for image processing, and more particularly to a method and apparatus for image processing which improves the performance of gray scale image transformation.
2. Discussion of the Background
A variety of techniques for representing a continuous tone image in a binary format, also known as halftoning, have been developed. Known halftoning techniques include a dithering method and an error diffusion method, each of which represents each pixel of a continuous tone image with one dot. The dithering method uses a threshold matrix and processing occurs by comparing one threshold value to an adjacent threshold value in the matrix so as to create a repetition characteristic which depends on threshold existence in matrix. The repetition characteristic causes the Moire phenomenon to occur when using the dithering process.
Error diffusion is more preferable as compared to the dithering method because the error-diffusion method is not influenced by the repetition characteristic of the threshold matrix because the error diffusion method do not use a threshold matrix in the binarization process. Error diffusion has the advantages of better resolution and non-repeatable data appearance compared to the dithering method.
However, the error diffusion method has problems in that the dots representing an image are not distributed uniformly to produce a clear image. Instead, the dots appear to be arranged in lines rather than uniformly distributed and therefore, the dots are viewed as noise and the dots are clearly visible as a result of the error diffusion process.
FIG. 1
illustrates a block diagram of a general error diffusion circuit for explaining a principle of the error diffusion method. This error diffusion circuit of
FIG. 1
is characterized by the method of error determination. More specifically, an error e(m, n) of an input pixel f(m, n) is determined using a subtracter
113
which is configured to subtract a pixel g(m, n) from a pixel cf(m, n), wherein the pixel cf(m, n) is input to the threshold processing circuit
112
and the pixel g(m, n) is output from the threshold processing circuit
112
.
In the error diffusion circuit of
FIG. 1
, an adder
111
receives an original input pixel f(m, n) from an input source and an error factor from a product/addition circuit
114
. An error weighting mask
115
having a coefficient “a” assigns a weight to an error value e(m−k, n−l) which has been generated during the processing of a pixel (m−k, n−l), wherein the pixel (m−k, n−l) proceeds to the pixel f(m, n) for the number of pixels (k, l). The weighted error value e(m−k, n−l) is sent to the product/addition circuit
114
and then to the adder
111
as an error factor. The adder
111
adds the weighted error value e(m−k, n−l) to the pixel f(m, n) to obtain a corrected value cf(m, n) which is then sent to a threshold processing circuit
112
for outputting a pixel g(m, n). Then, a subtracter
113
generates an error (m, n) using an equation e(m, n)=cf(m, n)−g(m, n).
The above-described error diffusion method has advantages in the aspects of non-periodicity of image and a feature of adaptive modulation.. However, such an error diffusion method also has disadvantages in that a relationship between two adjacent dots are not considered and compensated for, which causes generation of periodic texture such that dots are actually visible to the human eye and causes non-uniform distribution of dots.
The method and apparatus shown in
FIG. 1
does not include any means for correcting the problem of unpredictable, non-uniform distribution of dots also referred to as “periodic texture”.
Japanese Laid-Open Patent Publication No. JPAP62-239667 describes a method for image processing which eliminates the above-mentioned disadvantages of the error diffusion. In principle, this method varies the threshold value in a periodic manner so as to arrange dots such that the dots appear to have been made through a mesh screen. However, this method causes generation of moire fringes and deterioration of image quality by causing a reduction of resolution, similar to dithering.
Another method which is related to the present invention described and claimed hereinbelow and which related method has not been publicly disclosed prior to the filing date of the priority application of the present application,. is referred to as an adaptive cell method. The adaptive cell method is disclosed in Laid-Open Japanese Patent Application 11-27528 published on Jan. 29, 1999.
In the adaptive cell method, the relationship between adjacent dots is considered and compensated. More specifically, in this method, a dot is put into the center of a cell or matrix consisting of a number of pixels. This method determines a cell size in accordance with the sum total of input pixel values and arranges each dot at the center of each cell. This method realizes a gray-scale-like transformation which does not generate factitiously-connected dots that deteriorate image quality, while maintaining a gray image having a relatively high resolution, as with the error diffusion, as a result of its advantageous features such as the non-periodicity of the image and the adaptive modulation.
One superior feature of the adaptive cell method is a better circularity of dots. That is, dots are arranged in an approximately even manner in a highlight portion of a gray image. This feature appears particularly around a highlight portion of a gray image, wherein a highlight portion is defined as an image which has been transformed so that a relatively small number of pixels among all of the pixel's resulting from the binary transformation turn to black or to an ON state.
However, the adaptive cell method has the following drawbacks. One drawback is that a dot of a cell may be inconsistently connected to (D
1
and D
2
in
FIG. 2A
) or spaced apart from (E in
FIG. 2A
) a dot of an adjacent cell in a middle portion (an area where the numbers of black (A in
FIG. 2A
) and white (B in
FIG. 2A
) pixels are nearly equal) of an image. As shown in
FIG. 2A
which illustrates an exemplary bad result of the processing on an image using a three-pixel cell (indicated by a letter C) according to the adaptive cell method, this phenomenon appears particularly when the cell size becomes relatively small (i.e., 3). The reason for the occurrence of this phenomenon is that the adaptive cell method determines the connection of adjacent dots and a location of dot in a cell regardless of dot locations in the surrounding cells.
Another drawback is shown in
FIG. 2B
where an exemplary bad result of the processing on an image using a 20-pixel cell (indicated by a letter C) according to the adaptive cell method. In this case, 9 black pixels (indicated by a letter F) in the center of the cell form a dot. When a minimum cell size is limited, a plurality of adjacent pixels are determined as a dot. Accordingly, lower-resolution output devices (i.e., a laser printer) may produce an image with relatively large-sized dots. Such an image typically produces an inferior circularity of dots. As seen in
FIG. 2
b
, the dot is so big that the image resolution is decreased even though
FIG. 2
b
is improved relative to
FIG. 2
a
since the large white area in the central portion present in
FIG. 2
a
is avoided in
FIG. 2
b.
Also, when a minimum cell size is limited, a problem of lower resolution arises since a cell has a relatively large dot therein and density variations cannot be smaller than the cell.
SUMMARY OF THE INVENTION
The present application provides a method for image processing which can overcome the problems and disadvantages described above while greatly improving the quality and performance of gray scale image transformation.
More specifically, the present application provides a novel method and apparatus for performing error diffusion using a unique adaptive cell error diffusion method which transforms n-step gra
Brinich Stephen
Greenberg & Traurig, LLP
Lee Thomas D.
Manak Joseph M.
Ricoh & Company, Ltd.
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