JPH11331584A - Half-toning device for multilevel image and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To clearly reproduce a thin line low in density by repetitively applying the screen cells having the input gradation value-to-output dot information characteristics which are set to make a dot grow by the adjacent pixels set previously according to the rise of image density against the lowest density range as if those screen cells were spread over. SOLUTION: A screen cell 200 consists of pixels #1 to #5 which are arrayed in a cross shape, and the minimum dot size is decided by dividing a pixel into five parts in the horizontal scanning direction. The first dot is formed at the right edge of the center pixel #1 and the next dot is formed at the left edge of the right pixel #3. Thereafter, the left and right dots are successively formed around the first and second dots in response to the rise of image density. Thus, the dots evenly grow at both sides of a range having the lowest image density with the boundary part set between both pixels #1 and #3 defined as a start point. When the pixels #1 and #3 are completely filled with dots and the image density is heightened by a certain degree, the dots are grown in the order of pixels #2, #4 and #5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a halftoning technique for converting a multi-tone image into a dot pattern.

[0002]

2. Description of the Related Art There are various methods in the halftoning technique. Among them, a screen cell composed of a plurality of pixels of a predetermined geometric shape is repeatedly applied so as to be spread over a multi-tone image, and the screen is screened. In a cell, there is a method of growing dots from small points to large chunks in a certain order as the gradation value increases. This technique is favorably used in electrophotographic printers. As a conventional technique of this kind, as shown in FIG.
In the screen cell 100 composed of # 5 to # 5, dots are grown in the order indicated by the numbers in FIG.
-284343. That is, first,
The dot growth starts from the central pixel # 1, and if the central pixel # 1 is completely filled with dots, then the upper pixel #
2, dot growth is performed in the order of the lower pixel # 4, the right pixel # 3, and finally the left pixel # 5.

[0003]

As described above, the order of dot growth in the prior art is as follows. First, dot growth starts at a central pixel in a screen cell, and after all the central pixels are filled with dots, the surrounding pixels are surrounded by dots. The dot growth shifts to the pixel. The order of dot growth is
This is suitable for the purpose of favorably reproducing halftones.

However, the above-mentioned prior art is not suitable for the purpose of reproducing thin lines included in texts and line drawings, particularly thin lines of low density. For example, as shown in FIG. 9A, a thin line 21 having a width of 1 pixel and a thin line 23 having a width of 2 pixels, which are all low-density on the original image, are converted into a dot pattern by the conventional method shown in FIG. Then, a dot pattern 41 and a dot pattern 43 as shown in FIG. 9C are obtained, and both of the dot patterns 41 and 43 are dots 45 formed only at the central pixel in the screen cell 100.
47. The pitch of the dots 45 and 47 is considerably large, such as 5 pixels for the dot 45 and 2 pixels or 3 pixels for the dot 47.
Thus, according to the prior art, a low density thin line is reproduced as a row of dots interspersed with a large pitch, which appears to the human eye as a "line-like" that is blurred or almost disappearing. Will be.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a halftoning technique which can reproduce a halftone satisfactorily and can reproduce a low-density thin line more clearly than before.

[0006]

According to the halftoning apparatus of the present invention, in a predetermined lowest density range within the entire density range of a multi-valued image, dots are formed at predetermined two or more pixels as the image density increases. By using a screen cell having a growing input tone value-output dot information characteristic and repeatedly applying the screen cell so as to spread it over the multi-valued image, each pixel tone value of the multi-tone image is converted into a dot. Convert to information.

When a screen cell in which dots grow in a plurality of pixels in a low density range is used, a central cell is used.
Compared to the conventional technology using a screen cell as shown in FIG. 1 in which dots grow only in pixels, a thin line with low density is converted into a dot pattern having a narrower dot pitch, so that a clearer line is obtained. Reproduced to be visible.

When dots are grown with two or more pixels in the low density range, dots can be grown substantially uniformly with those two or more pixels.

When pixels at different positions in the main scanning direction of the image are selected as the predetermined two or more pixels, the reproducibility of fine lines in the sub-scanning direction is improved, while the pixels at different positions in the sub-scanning direction are improved. When a certain pixel is selected, the reproducibility of a fine line in the main scanning direction is improved.

Further, if dots are grown as one lump in a screen cell, the same halftone reproducibility as in the prior art can be obtained.

In a preferred embodiment, the data expressing the screen cell includes a plurality of gamma conversion cells respectively corresponding to a plurality of pixels in the screen cell, and each gamma conversion cell has a corresponding pixel in the screen cell. And a large number of indices corresponding to a large number of pixels included in an image area of a predetermined size on the multi-valued image. An index table is prepared in which an index specifies a gamma conversion cell in a gamma table to be applied to a corresponding pixel in the image area. Using these two tables, for each pixel of the multi-valued image, the corresponding index in the index table is referred to, and the gamma conversion cell in the gamma table specified by the index is referred to in a simple manner. The gradation value of each pixel can be converted into dot information.

In addition, instead of the above-mentioned table, a dither threshold matrix corresponding to the above-mentioned characteristics corresponding to a screen cell can be used.

In the preferred embodiment, the halftoning apparatus of the present invention is implemented by a dedicated hardware circuit, but may be implemented by software using a computer instead. In that case, the computer program can be installed or loaded on the computer through various media such as a disk storage, a semiconductor memory, and a communication network.

[0014]

FIG. 2 shows a screen cell used in a halftoning circuit according to a first embodiment of the present invention and the order of dot growth in the cell.

This screen cell 200, like the conventional screen cell 100 shown in FIG. 1, is composed of five pixels # 1 to # 5 arranged in a cross, and the minimum dot size is one pixel for image scanning. 1 divided into 5 in the main scanning direction
One. However, the dot growth order is different from the conventional one. That is, as shown by the numbers in the figure, first, the first dot is formed at the right end of the center pixel # 1, and then the right pixel # 1 is formed.
A second pixel is formed at the left end of No.3, and thereafter, with these two dots as the center, dots are sequentially added to the left and right sides thereof. In short, in the initial stage of dot growth (that is, in the range where the image density is the lowest), starting from the boundary between the central pixel # 1 and the right pixel # 3, it moves to both sides in the main scanning direction (that is, the central pixel # 1). And right pixel # 3
F) The dots grow substantially evenly. Then, when both the center pixel # 1 and the right side pixel # 3 are completely filled with dots, thereafter (that is, after the density of the image has increased to some extent), as in the prior art shown in FIG.
In the upper pixel # 2, a dot grows from the center of the pixel, and when the upper pixel # 2 is completely filled with the dot, the lower pixel # 2
Similarly, when the dot grows from the center at 4 and the lower pixel # 4 is completely filled with the dot, the dot grows at the left pixel # 5 similarly.

FIG. 3 is a graph showing this dot growth order in a graph showing the relationship between the input tone value and the output dot width for each pixel.

As shown in FIG. 3, as the input tone value increases, in the low density range, the center pixel # 1 and the right pixel # 3
As a result, the dots grow substantially uniformly. (Note that FIG. 2
As shown in FIG. 2, the dot actually grows by one dot in the center pixel # 1 ahead of the right pixel # 3 by one dot. However, the growth is uniform from a global perspective. As a modified example, the center pixel # 1 and the right pixel are not added, for example, the first and second dots in FIG. 2 and the third and fourth dots in FIG. The dots may be grown completely evenly in both # 3. Thus, in the lowest density range, the center pixel # 1 and the right pixel #
One lump of dots grows continuously over two adjacent pixels 3. In this low density range, the center pixel #
Since the dot grows around the boundary between 1 and the right pixel # 3, the dot is located at a position shifted to the right by 画素 pixel from the center pixel # 1. When the input tone value further increases and enters the middle density range, the upper pixel # 2, the lower pixel # 4, and the left pixel # 4 until the maximum density is reached.
Dots grow individually in the order of 5.

In the embodiment of the present invention, the order of dot growth shown in FIGS. 2 and 3 is not the only order that can be adopted, but various other orders can be adopted. As an example, as shown in FIG. 4, dots may be grown substantially uniformly in the lower pixel # 4 and the left pixel # 5 in the high density range. For example, in FIG.
The dots are added in the order of No. 22, No.,..., No. 20, No. 25 as the gradation value increases.

Whatever growth order is adopted,
In the low density range, it is important to grow dots across a plurality of pixels in the screen cell 200. For the purpose of favorably reproducing halftones, it is preferable to grow dots as one lump centering on one nucleus. Therefore, it is preferable to grow dots so as to continuously straddle the central pixel # 1 and at least one pixel adjacent thereto.

FIG. 5 shows the entire configuration of a halftoning circuit 1 for an electrophotographic printer according to an embodiment of the present invention, which employs the screen cells 200 and the dot growth order shown in FIGS. 2 and 3. 6 and 7 show the configuration of the index table 7 and the gamma table 9 mounted on the circuit 1, respectively.

The half-toning circuit 1 includes, for example, an SRA
It has an internal memory 33 using M and a processing unit 5 which is a hardware logic circuit. The internal memory 3 stores two tables, an index table 7 and a gamma table 9. The processing unit 5 can express each pixel value of the original multi-tone image from the external image memory 10 using a DRAM (typically, one color component value of one pixel can express 256 tones by an 8-bit word). ) Is read, and by referring to the index table 7 and the gamma table 9 in the internal memory 3, the drawing laser pulse width indicating the dot width corresponding to the read pixel value is determined, and the laser pulse width signal is output. . In accordance with the laser pulse width signal, a laser print engine (not shown) modulates the drawing laser pulse in pulse width, and forms a dot pattern of a colorant on a sheet by electrophotography to reproduce an image.

As shown in FIG. 6, the index table 6 is composed of 5 × 5 indexes 11 corresponding to a 5 × 5 pixel area on an image. Each index 1
1 corresponds to each pixel in the 5 × 5 pixel area, in which the screen cell 20 shown in FIG.
A pixel number (any one of “# 1” to “# 5”) in the screen cell 200 applied to the corresponding pixel when “0” is spread is stored. The index table 6
The corresponding pixel area size is determined by the screen cell used. For example, when a screen cell shown in FIG. 12 described later is used, an index table corresponding to a 13 × 13 pixel area is required.

As shown in FIG. 7, the gamma table 9
Five pixels # 1 in the screen cell 200 shown in FIG.
It has five gamma conversion cells 13 corresponding to # 5. Each gamma conversion cell 13 stores 256 laser pulse width values 15 representing dot widths corresponding to 256 gradations that can be taken by the pixel value (8-bit word for one color component) of the original image. Each laser pulse width value 15 represents each output dot width for each input gradation value according to the characteristics shown in FIG. Each gamma conversion cell 13 also stores dot position information 17 specifying the position (left alignment, right alignment, or center alignment) of the dot within the corresponding pixel.

FIG. 8 shows the operation of the processing section 5 of the halftoning circuit 1.

First, the pixel value of each pixel of the original image is read from the image memory 10 (step S1). Next, the pixel position of the pixel within the 5 × 5 pixel area is determined (step S2). Next, a pixel number is read from the index 11 in the index table 7 corresponding to the pixel position (step S3). Next, a laser pulse width value and dot position information corresponding to the pixel value read in step S1 are read from one gamma conversion cell 13 in the gamma table 9 specified by the pixel number (step S1).
4). Then, a pulse width signal representing a drawing laser pulse having a pulse width corresponding to the read pulse width value and having a pulse position corresponding to the read dot position information is output (step S5). A laser print engine (not shown) performs pulse width modulation on the drawing laser according to the pulse width signal. The above operation is repeated for all the pixels in the original image (step S6).

FIG. 9 shows the halftoning results of this embodiment and the prior art shown in FIG. 1 in comparison.

As described above, according to the related art, the low density (for example, 20%) fine lines 21 and 23 of the original image shown in FIG. 9A are replaced with the dot patterns 41 and 23 shown in FIG.
43. On the other hand, according to the above embodiment, the same fine lines 21 and 23 correspond to the dot pattern 3 shown in FIG.
1, 33. In the dot patterns 31 and 33 shown in FIG. 9B, the dots 35 and 3
The pitch of the dot 7 is considerably narrower than the pitch of the dots 45 and 47 according to the related art shown in FIG. In particular, the dot pattern 31 of the present embodiment, which represents the thin line 21 having a width of one dot, has a dot pitch that is smaller than 1/2 that of the conventional dot pattern 41. As a result, the present embodiment has a higher ability to reproduce a fine line so as to be visually recognized as a clearer line than the related art. On the other hand, the present embodiment and the prior art are substantially equivalent in halftone reproducibility.

FIG. 10 shows another embodiment of the screen cell according to the present invention.

This screen cell 300 is obtained by dividing each pixel of the cross-shaped screen cell 200 composed of five pixels shown in FIG. 2 into four pixels, and has 20 pixels # 1.
~ # 20. This screen cell is shown in FIG.
This is suitable for application to an image having twice the resolution of the image to which the screen cell 200 is applied.

FIG. 11 shows an example of the order of dot growth in the screen cell 300.

In the lowest density range, first, dots grow substantially evenly in the pixel # 1 at the upper right of the center and the pixels # 2 and # 3 on the right and upper sides thereof. In this case, the dots are formed right-aligned in the pixels # 1 and # 3, and the dots are formed left-aligned in the pixel # 2 so that the dot grows as one lump continuously over the three pixels # 1 to # 3. It is desirable to do. When these three pixels # 1 to # 3 are completely filled with dots, next, for example, the lower right pixel # 4 and the upper left pixel #
5, dots grow substantially evenly at the upper right pixel # 6 and the upper right pixel # 7. Also in this case, the pixels # 4 and # 5 are right-aligned so that the dots grow as one lump, and the pixels #
In 6, it is desirable to form dots to the left.

As described above, by reproducing dots substantially evenly in a plurality of pixels in a low density range, reproducibility of a low density thin line is improved. Note that, in the screen cell 200 shown in FIG. 2, as a result of uniformly growing dots at the two pixels # 1 and # 3 at different positions in the main scanning direction, as shown in FIG. ) The reproducibility of fine lines was improved. On the other hand, in the screen cell 300 of FIGS. 10 and 11, a plurality of pixels at different positions not only in the main scanning direction but also in the sub-scanning direction, for example, pixels # 1, # 3,
However, because the dots grow evenly, the sub-scanning direction (vertical)
The reproducibility of not only fine lines but also fine lines in the main scanning direction (horizontal) is improved.

After the density has increased to some extent, dot growth may be performed individually for each pixel, such that when one pixel is completely filled with dots, dot growth for the next pixel is started, or For example, as shown in FIG.
Dot growth may be performed evenly at some pixels as in 9 to # 12. In short, in the middle and high concentration range,
Since the reproducibility of fine lines is originally good, dots may be grown in a conventional manner so that halftones can be reproduced well.

FIG. 12 shows the structure of yet another screen cell 400 according to the present invention. FIG. 13 shows an example of a dot growth order applied to the screen cell 400.

The screen cell 400 is composed of thirteen pixels # 1 to # 13. As shown in FIG.
In the lowest density range, dots are grown substantially uniformly at the four pixels # 1 to # 4 located near the upper left of the center.
This improves the reproducibility of low density vertical and horizontal fine lines.

In each of the above-described embodiments, the dot growth is started at the next pixel after the dot has completely grown at a plurality of pixels where the dot starts to grow at first. That is not to say. If the initially formed lump of dots grows to a stable size (by the way, when the dots are very small, it is unstable that the colorant such as toner is not actually attached), and the surrounding area is unstable. Dot growth can be started at the pixel of. FIG. 14 shows an example of such a growth pattern. This is an example in the screen cell 200 shown in FIG. 2, but first, pixels # 1 and # 3 have 50% (1/1 of the pixel).
At the stage where only 2) dots have grown, dot growth at the next pixel # 2 has started.

The embodiment of the present invention has been described above.
The above embodiment is merely an example for describing the present invention, and is not intended to limit the present invention to only the embodiment. Therefore, the present invention can be implemented in various forms other than the above-described embodiment. For example, the present invention can be applied not only to an electrophotographic printer but also to various types of printers and display devices.

[Brief description of the drawings]

FIG. 1 is a diagram showing a screen cell used in a conventional halftoning technique and a dot growth order in the cell.

FIG. 2 is a diagram showing a screen cell used in the half-toning circuit according to the first embodiment of the present invention, and a dot growth order in the cell.

FIG. 3 is a diagram showing the dot growth order in FIG. 2 in relation to an input dot value and an output dot width.

FIG. 4 is a diagram showing another dot growth order in relation to an input dot value and an output dot width.

FIG. 5 is a block diagram showing an overall configuration of a halftoning circuit 1 for an electrophotographic printer according to an embodiment of the present invention, which employs the screen cell and dot growth order shown in FIGS. 2 and 3;

FIG. 6 is an exemplary view showing the configuration of an index table 7 mounted on the embodiment.

FIG. 7 is an exemplary view showing a configuration of a gamma table 9 mounted on the embodiment.

FIG. 8 is a flowchart showing the operation of the processing unit 5 mounted in the embodiment.

FIG. 9 is a diagram showing the halftoning results of the embodiment and the related art in comparison.

FIG. 10 is a diagram showing another embodiment of the screen cell according to the present invention.

11 is a diagram showing the dot growth order of the screen cell in FIG. 10 in relation to an input dot value and an output dot width.

FIG. 12 is a diagram showing still another embodiment of the screen cell according to the present invention.

FIG. 13 is a diagram showing the dot growth order of the screen cells in FIG. 12 in relation to an input dot value and an output dot width.

FIG. 14 is a diagram showing another example of another dot growth order in the screen cell of FIG. 2;

[Explanation of symbols]

 Reference Signs List 1 halftoning circuit 3 internal memory 5 processing unit 7 index table 9 gamma table 100, 200, 300, 400 screen cell # 1 to # 20 pixel

Claims (10)

[Claims] 1. An input tone value-output dot having a plurality of pixels, wherein a dot grows in two or more predetermined pixels in a predetermined lowest density range as the density of a multi-valued image increases. Screen cells having information characteristics, and screen cell data representing, by using the screen cell data, by repeatedly applying the screen cells to spread over the multi-valued image, each pixel of the multi-tone image And a processing unit for converting the gradation value of the image data into dot information of a corresponding pixel in the screen cell. 2. The halftoning device according to claim 1, wherein the screen cells are configured such that dots grow substantially uniformly in the predetermined two or more pixels. 3. The predetermined two or more pixels are adjacent to each other,
2. The halftoning device according to claim 1, wherein the screen cell is configured such that the dots grow as one lump continuously straddling the predetermined two or more pixels in the low density range. 3. 4. The halftoning device according to claim 1, wherein the screen cell is configured such that the predetermined two or more pixels are located at different positions in one or both of a main scanning direction and a sub-scanning direction. 5. The screen data includes a plurality of gamma conversion cells respectively corresponding to a plurality of pixels in the screen cell, and each gamma conversion cell includes:
A gamma table storing data representing an input tone value-output dot information characteristic of a corresponding pixel in the screen cell; and a plurality of pixels included in an image area of a predetermined size on the multi-valued image. An index table designating a gamma conversion cell in the gamma table to be applied to a corresponding pixel in the image area, the processing unit having a corresponding index. The dot information corresponding to the tone value of each pixel is determined for each pixel of the value image using a gamma conversion cell in the gamma table specified by a corresponding index in the index table. Half toning device. 6. Each of the gamma conversion cells further stores dot position information for specifying a dot formation position in a corresponding pixel in the screen cell.
The halftoning device as described. 7. A plurality of pixels having input tone value-output dot information characteristics such that dots grow at two or more predetermined pixels in a predetermined lowest density range as the density of a multi-valued image increases. A multi-valued image, which is applied to the multi-valued image by repeatedly applying the screen cells, thereby converting the gradation value of each pixel of the multi-valued image into dot information of a corresponding pixel in the screen cell. Half-toning method. 8. An input tone value-output dot having a plurality of pixels, wherein a dot grows in two or more predetermined pixels in a predetermined lowest density range as the density of the multi-valued image increases. Screen cells having information characteristics, and screen cell data representing the, by using the screen cell data, by repeatedly applying the screen cells to spread over the multi-valued image, each pixel of the multi-valued image A halftoning processing unit that converts a gradation value into dot information of a corresponding pixel in the screen cell; and a dot that reproduces the multi-tone image by forming a dot according to the dot information from the halftone processing unit. An image forming apparatus comprising a forming apparatus. 9. An input tone value-output dot having a plurality of pixels, wherein a dot grows in two or more predetermined pixels in a predetermined lowest density range as the density of the multi-valued image increases. Screen cells having information characteristics, and screen cell data representing, by using the screen cell data, by repeatedly applying the screen cells to spread over the multi-valued image, each pixel of the multi-tone image And a processing unit for converting the gradation value of the corresponding pixel in the screen cell into dot information,
A computer-readable recording medium carrying a program for causing a computer to function. 10. A screen cell repeatedly applied to the multi-valued image so as to be spread over the multi-valued image, the screen cell having a plurality of pixels, and having a predetermined value as the density of the multi-valued image increases. Machine-readable data that records screen data having a structure representing an input tone value-output dot information characteristic such that dots grow at two or more predetermined pixels in the lowest density range. recoding media.