Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1999-07-12
2003-08-19
Rogers, Scott (Department: 2624)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S536000
Reexamination Certificate
active
06608702
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for generating a threshold pattern and a gradation image binarizing device used for binarizing processing of color images.
2. Description of the Related Art
A photograph image of a newspaper is printed using aggregate of many dots. Such an image is called a “dot image”. To print an image having gradation like a photograph by using a binary output medium such as a dot printer, image data obtained in advance by picture-taking must be converted to a dot image. A color of dot, screen angle and a number of lines are used as parameters indicating the property of the dot image.
FIG. 2
is an explanatory diagram showing a screen angle of a dot image,
FIG.2
(
a
) showing a relation between the screen angle and the number of lines and
FIG. 2
(
b
) showing a typical arrangement of the screen angle.
As shown in the drawing, dots
1
indicated by black round points are so placed as to be aligned at an equal interval in the direction of the arrow. The arrow
2
is referred to as a line. The screen angle is an angle which the line forms with a horizontal line (a line parallel to prime scanning line). In the example shown in the drawing, the dot image is so configured that the screen angle is 45 degrees. The number of lines is one per a inch counted when a lot of lines drawn at an equal interval and in parallel are traced in the vertical direction. In the case of a black-and-white image, if the screen angle is set to 45 degrees, the dot image can be produced which is relatively hard to distinguish when seen by the naked eye.
For color printing, dots each having one of 3 or 4 color components (cyan, magenta, yellow and black) are combined to produce a color. If dots having a different color and/or shape are arranged regularly, streaking called “moire” shows. For the purpose of reducing such moire and making the dot inconspicuous, the use of the screen angle being different for every color component is widely known.
FIG. 2
(
b
) shows a screen angle suitable for every color component which is applicable to printing of a color image. The screen angle for a black color being most perceivable by the naked eye is set to 45 degrees. Further, the screen angle for a yellow color being most inconspicuous and light is set to 0 (zero).
On the other hand, the screen angle for a magenta or cyan color is set to an intermediate angle, for example, 75 and 15 degrees respectively. Each color component is perceived in a different way when seen by the naked eye, and the most suitable screen angle must be selected accordingly.
However, the conventional technologies as described above have the following problems to be solved.
The Dither method and/or Dot pattern method are used for printing a gradation image composed of the dot as described above. The principles of these methods are described below.
Each value of a picture element (i.e., density of a picture element) is represented by, for example, 16 step multivalued data. A gradation of one picture element is represented by a cluster of 4×4 dots. For example, if the value of a picture element is “0” (zero), all 4×4 dots become a white color; and if it is “6”, out of the 4×4 dots, 6 dots become a black color and the remaining dots being a white color. The larger the value of a picture element becomes, the more the black dots increase in number. By selecting the proper position of black dots, the quality of an image obtained when a cluster of dots are seen by the naked eye at a short distance is improved
In order to automatically select either of a black or a white color, the Dither pattern method uses a matrix in which, for example, 4×4 threshold values are assigned therein. For example, if a value of a picture element of an input gradation image is “6”, this value of the picture element is compared with each of 4×4 threshold values. When the value of the picture element exceeds a threshold value, the dot existing at the position is made a black color. If the value of the picture element is less than the threshold value, the dot existing at the position is made a white color. Such binarizing processing allows the selection of either of a black or white color of each dot.
Generally, a dither pattern is represented by a square where a threshold value is assigned at a pitch of dots. In order to align binarized dots in the direction of a screen angle, the rotation of an edge of the dither pattern by an angle of the screen angle is required.
FIG. 3
is an explanatory diagram illustrating a method for rotating a dither pattern.
FIG. 3
(
a
) shows an example of a rotation matrix adapted to be rotated by operational processing of the dither pattern.
FIG. 3
(
b
) shows a result obtained from the rotation of the dither pattern.
As depicted in
FIG. 3
(
a
), for example, a point at a position (X, Y) on the XY coordinates is rotated relative to an origin at an angle of &thgr; in a counterclockwise direction. At this point, the position on the coordinates obtained after being rotated is (X′, Y′). A rotation matrix used to obtain the position (X′, Y′) of the coordinates obtained after being rotated from the position (X, Y) existing before being rotated is shown in the drawing. In the right side of the formula, a coefficient of the position (X, Y) corresponds to the rotation matrix.
By expanding this formula, the following equations are gotten.
X
′=cos &thgr;
X
sin &thgr;
Y
Y
′=sin &thgr;
X
cos &thgr;
Y
Thus, by performing operational processing of a determinant, the conversion from original coordinates (X, Y) to coordinates (X′, Y′) obtained after being rotated is made possible.
An input pattern shown on the left side in
FIG. 3
(
b
) is the dither pattern composed of 4×4 threshold values. The numbers
1
to
16
are assigned sequentially, from the upper left, to each threshold value of this input pattern. The 4×4 dots are binarized then.
For example, the input pattern is rotated relative to a position indicated as “1” at the upper left corner in a counterclockwise direction by an angle of
0
. In this case, the coordinates of the site having the 4×4 threshold values are obtained and converted using the rotation matrix. The results are shown on the right side of the drawing.
Moreover, fractions of the coordinate values obtained by the operational processing are processed by its omission or by counting fractions as one. The site where the dot is arranged is limited to a certain position specified by a structure of a dot image. Each threshold value is always assigned to any of the dots and is used for binarizing processing of the dot. In an area in which a hatching is assigned, a site not corresponding to an input pattern appears. This is because an interval distance between dots remains unchanged even though each side of the dither pattern is made longer due to the rotation of the dither pattern. If nothing is done here, a hole occurs in the dither pattern. To prevent this, the following processing is carried out.
A re-sampling method is introduced by which, in views of convenience of the operational processing, an inverse operation of the coordinates of a site where all threshold values obtained after the rotation are to be assigned is performed and, by using this result, the corresponding threshold value is selected from that of the dither pattern existing before the rotation.
FIG. 4
is an explanatory diagram showing re-sampling processing of the dither pattern,
FIG. 4
(
a
) illustrating the re-sampling and
FIG. 4
(
b
) showing a coordinate system of the dither pattern.
As a first step, coordinates of a site are obtained where all the threshold values of the dither pattern obtained after being rotated are assigned therein. Using the coordinates at 20 positions as shown on the right side in
FIG. 4
(
a
), an inverse operation is performed of the coordinates of a site where a threshold value of the dither pattern existing before being rotated shown on th
Oki Data Corporation
Rogers Scott
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