JP2002125124A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing unit that reduces moire and prevents a character from being thickened while warranting the density of a consecutive tone image such as a black solid part. SOLUTION: When an edge area detection means 6 detects an edge area, a density conversion processing section 3 selects a density conversion characteristic in which maximum output data take a comparatively lower value and when the edge area detection means 6 detects an edge area, the density conversion processing section 3 selects a density conversion characteristic in which maximum output data take a comparatively higher value.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a digital copying machine and a facsimile machine, and more particularly, to a method for outputting an input image signal to an image output apparatus such as a printer.
The present invention relates to an image processing apparatus that performs conversion so that a predetermined density is obtained by a density conversion unit.

[0002]

2. Description of the Related Art In an image forming apparatus such as a digital copying machine, as shown in FIG. 13, various image processes are performed on image data input from a scanner 1 so as to obtain a desired image quality. Will be

Such image processing includes filter processing 2 for correcting the spatial frequency characteristics of an image, and conversion of density data so as to obtain a desired image density in accordance with the input / output density characteristics of the scanner 1 and the printer 5. There are a density conversion process 3 to be performed, a halftone process 4 to be performed on image data after density conversion, and the like.

The density conversion processing 3 includes a digital value (scanner characteristic) obtained when a document of a certain density is read by a scanner, and an output density obtained when the digital value is subjected to halftone processing and output to a printer. This is performed to match (printer characteristics), that is, to match the document density with the density of a printed image.

Normally, in a monochrome copying machine, the density of a document and an output image is not linear, but a characteristic that gives rise to a gamma, that is, the density of the document is 1.2 to 1.3, and the density of the output image is 1.
In many cases, the output is 4 to 1.5. This is due to the difference between the density range of the original and the density reproduction range of the printer.

[0006] The density conversion is usually performed by a density conversion table having output data corresponding to the quantization number of the input image. For example, if the input image is 8 bits, input 0-25
5 is a density conversion table in which 8-bit output data corresponding to 5 is stored.

Now, for a digital value of 0 to 255,
When the binary error diffusion process was performed, output characteristics as shown in FIG. 14 were obtained. In FIG. 14, the digital value 170
Above, concentration saturation occurs at a concentration of 1.5. When density conversion is performed on such output characteristics, as shown in FIG. 15, the output image data may be a density conversion table in which 170 is the maximum value.

However, in the electrophotographic process, the density characteristics may fluctuate due to environmental fluctuations, component deterioration, and the like. For example, a (dashed line) in FIG. 16 is a case where the density is high, and b (dashed line) in FIG. 16 is a case where the density is low. In the case of a monochrome copying machine, a decrease in the density of a solid black portion results in a remarkable image deterioration. Therefore, when the density decreases as shown in FIG. 16B, the output image density decreases in the density conversion table shown in FIG. Therefore, in order to avoid such a problem, a density conversion table as shown in FIG. 17 is used in consideration of a margin for environmental fluctuation. That is, the maximum value of the output image data is set to a value larger than 170 that can output a saturated density of 1.5 in a normal state, and a margin is taken for the black solid density (FIG. 16).
In the example, the maximum value of the output image data is set to 255 which is the maximum value of the 8-bit data.)

Note that the edge area, that is, a character, a line drawing,
Apply sharp density conversion characteristics to low-contrast line images and parts where the density changes sharply in halftone images,
By applying comparatively gradual density conversion characteristics to non-edge areas, that is, areas with gradual density changes in halftone images, images that sharply reproduce edge areas with sharp density changes and low-contrast line images There is a processing device (see Japanese Patent Application Laid-Open No. 5-153395).

[0010]

By performing the above-described processing, a margin for the density of the solid black portion can be ensured, but problems such as moire and thickening of a character occur in a normal state. Moiré will be described with reference to FIGS.

FIG. 18 shows a change in an image signal when a certain halftone image is subjected to MTF correction by filter processing. For simplicity, it is represented by a one-dimensional image signal. 18 indicate signals read by the scanner, that is, image signals input to the filter processing. Despite being a periodic halftone dot image, a periodic image variation that causes moire is observed at the time of being read by the scanner. On the other hand, when the spatial frequency characteristic is emphasized by the MTF correction filter, the result becomes as shown by a circle in FIG. Since the signal actually output from the filter processing means is 8-bit data, it is cut off at the 0 level and the 255 level as indicated by the triangle in FIG.

FIG. 19 shows an image signal after density conversion.
In FIG. 19, the □ marks indicate that the conversion was performed using the density conversion table (maximum value 170) illustrated in FIG. 15, and the Δ marks indicate that the conversion was performed using the density conversion table (maximum value 255) illustrated in FIG. is there.

As shown in the difference between the two signals in FIG.
The density conversion table with the maximum value of 255 has a larger difference in image signal between the dark portion and the non-dark portion, and conversely, the density conversion table with the maximum value of 170 has a smaller difference in the image signal. Is small. For example, when the sum of the data values of the 3rd and 4th pixels and the 6th and 8th pixels of the data string is calculated, the difference is clearly larger in the density conversion table having the maximum value of 255. Further, when a density conversion table having a maximum value of 170 is combined with a density preserving halftone process such as an error diffusion method, the effect of reducing moire is great.

As described above, with respect to the moire reduction of the halftone dot image, it can be said that the density conversion table as shown in FIG. 15 is preferable. Also, for character images and line drawings, the maximum value is 255.
In the density conversion table, there is a phenomenon that the line becomes thick, and this tendency is particularly strong in an image forming process having a strong edge effect.

The present invention has been made in view of the above circumstances, and an object of the present invention is to guarantee the density of a continuous tone image such as a solid black portion even when the density of a process fluctuates due to environmental fluctuations or the like. It is another object of the present invention to provide an image processing apparatus that performs high-quality image reproduction by preventing the occurrence of moire and the thickening of line drawings (characters) by performing excessive density conversion on a halftone dot image area or a character area.

[0016]

According to the present invention, when an edge area is detected from an input image, a density conversion characteristic in which a maximum output data takes a relatively low value is selected in a density conversion processing section, and a non-edge area is detected. Then, a density conversion characteristic in which the maximum output data takes a relatively high value is selected. As a result, the density of a continuous tone image such as a solid black portion is guaranteed, moiré is reduced, and thickening of characters is prevented.

Further, according to the present invention, when a continuous tone region is detected, the maximum output value of the density conversion characteristic applied to the continuous tone region is made larger than the maximum output value of the density conversion characteristic applied to the non-continuous tone region. Configure to be large.

Further, according to the present invention, when a character area is detected, the maximum output value of the density conversion characteristic applied to the character area is made smaller than the maximum output value of the density conversion characteristic applied to the non-character area. Constitute.

Further, according to the present invention, when a halftone dot area is detected, the maximum output value of the density conversion characteristic applied to the halftone dot area is made smaller than the maximum output value of the density conversion characteristic applied to the non-halftone dot area. It is configured to be small.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be specifically described below with reference to the drawings.

FIG. 1 shows the configuration of an embodiment of the present invention. The configuration is such that an edge area detecting means 6 is added to the configuration of FIG. The edge area detecting means 6 detects an edge area in which the density change is steep (the edge amount is a predetermined value or more) and a non-edge area in which the edge amount is small with respect to the input image signal.

FIG. 2 shows a detailed configuration of the edge area detecting means. In FIG. 2, an input image signal is subjected to edge detection by a primary differential filter 7 of 5 × 5 size. As shown in the figure, edges are detected by the filters shown in FIGS. 3 to 6 in the four directions of the vertical direction, the horizontal direction, and the oblique direction. The maximum value detection unit 8 calculates the obtained edge amounts (a to d).
The maximum value is detected from the data after the absolute value of, and a 5 × 5 size expansion process 9 is performed to further expand the area. Further, the edge amount is corrected by the edge amount correction table 10, and an edge area signal is output.

The edge image area detected by the edge area detecting means 6 has a sharp edge in a continuous tone image such as a character portion (only a character outline portion in the case of a thick character), a halftone dot image, and a photographic paper photograph. Area. For example, an edge image area to be detected for the image shown in FIG. 11 is a black portion shown in FIG. Further, in the present invention, a halftone image having a relatively low number of lines is also processed as an edge region having a large edge amount.

In FIG. 1, the detection result of the edge area detecting means 6 is input to the density conversion processing section 3. FIG. 7 shows a first configuration example of the density conversion processing unit (density conversion means) 3. In FIG. 7, the selector 13 switches between the output of the first density conversion characteristic table 11 and the output of the second density conversion characteristic table 12 based on the edge area signal. Here, the first density conversion characteristic table 11
Is the density conversion characteristic as shown in FIG. 15, and the second density conversion characteristic table 12 is the density conversion characteristic as shown in FIG. 17, and the maximum output data of the second density conversion table 12. Is the first density conversion characteristic table 11
Is larger than the maximum output data of.

The selector 13 operates based on the edge area signal.
In the edge image area, the first density conversion characteristic table 11
Is selected, and in the non-edge image area, the output data of the table 12 of the second density conversion characteristic is selected.

With the above arrangement, the maximum output data has a relatively low value in an edge image area such as a character portion or a halftone dot image portion, so that character fattening is prevented.
In addition, moiré of a halftone dot image can be reduced. In a non-edge image area such as the inside of a bold character or a continuous tone image area, the maximum output data has a relatively high value. Therefore, even if the density is reduced due to environmental fluctuation or the like, the density of the solid black portion is maintained. And suitable image reproduction can be performed.

In the configuration shown in FIG. 7, an example is shown in which two different types of density conversion characteristics are switched based on the edge area signal. However, more density conversion characteristics are prepared, and multiple density conversion characteristics are prepared using multi-stage edge amounts. It is also possible to control the concentration stepwise.

FIG. 8 shows another example of the structure of the density conversion means. The density conversion unit of FIG. 8 includes a correction value generation unit 14 that generates a correction value based on the area signal. The correction value generator 14 is configured to output a large value in the case of an edge region and to output a small value in the case of a non-edge region. The subtraction circuit 16 subtracts the correction value from the maximum output value of the density conversion characteristic 15 (for example, the characteristic table in FIG. 17). In the edge area, a large value is subtracted, and the maximum output value of the density conversion characteristic is smaller than the maximum output value of the density conversion characteristic in the non-edge area.

With this configuration, the same effect as that shown in FIG. 7 can be obtained, and it is not necessary to prepare a plurality of density conversion characteristics.

In the above embodiment, the example in which the density conversion characteristic is controlled by the area determination based on the edge detection result has been described. However, the area determination method is not limited to the above example. Instead of the edge area detecting means in FIG. 1, an image area separating means 17 is provided as shown in FIG. The density conversion processing unit 3 uses the density conversion means of FIG.

When a character area is detected by using the image area separation result, the selector 13 selects the first density conversion characteristic table 11 and controls the maximum output data to have a relatively small value. As a result, character fattening can be suppressed. If no character area is detected (non-character area),
The selector 13 selects the second density conversion characteristic table 12.

As the image area separating method in the image area separating means, for example, the IEICE Transactions' 9
2/1 Vol. J75-D-II No. 1 [Character /
Image area separation method for images with mixed patterns (dots, photos) ”(Ouchi,
(Imao, Yamada).

Similarly, when the halftone dot image area is detected, the selector 13 operates the table 11 of the first density conversion characteristic.
Is selected, and control is performed so that the maximum output data has a relatively small value, whereby moiré can be reduced. When a dot image area is not detected (non-dot image), the selector 13 selects the table 12 of the second density conversion characteristic.

FIG. 10 is a diagram in which the image area separating means 17 is replaced with a continuous tone area detecting means 18.
Has detected the continuous tone image area (solid black area), the selector 13 selects the table 12 of the second density conversion characteristic and controls the maximum output data to be a relatively large value. Even if the density decreases due to fluctuations or the like, the density of the solid black portion can be maintained. When the continuous tone image area (solid black area) is not detected (non-continuous tone image area), the selector 13 selects the table 11 of the first density conversion characteristic. As means for detecting a continuous tone image area (solid black area), for example, Japanese Patent No. 2672583 is used.
The known technique described in the above item is used.

[0035]

As described above, according to the present invention, even when the density of the process fluctuates due to environmental fluctuation or the like, the density of the solid black portion can be sufficiently obtained, and the halftone image area and the character can be obtained. It is possible to perform high-quality image reproduction without performing excessive density conversion on an area and suppressing occurrence of moire and thickening of a line image.

[Brief description of the drawings]

FIG. 1 shows a configuration of an embodiment of the present invention.

FIG. 2 shows a detailed configuration of an edge area detecting means.

FIG. 3 shows a filter for detecting vertical edges.

FIG. 4 shows a filter for detecting horizontal edges.

FIG. 5 shows a filter for detecting an oblique right edge.

FIG. 6 shows a filter for detecting diagonally leftward edges.

FIG. 7 shows a first configuration example of density conversion means.

FIG. 8 shows a second configuration example of the density conversion means.

FIG. 9 shows another configuration example of the embodiment of the present invention.

FIG. 10 shows still another configuration example of the embodiment of the present invention.

FIG. 11 shows an example of an image.

FIG. 12 shows an edge region in the image of FIG.

FIG. 13 illustrates a configuration of an image forming apparatus.

FIG. 14 shows output image density characteristics with respect to image data.

FIG. 15 shows an example of a density conversion table.

FIG. 16 shows an example in which the density characteristics fluctuate.

FIG. 17 shows another example of the density conversion table.

FIG. 18 shows a change in an image signal before and after a filtering process on a halftone image.

FIG. 19 shows an image signal after density conversion.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Scanner 2 Filter processing part 3 Density conversion processing part 4 Halftone processing part 5 Printer 6 Edge area detection means

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2C262 AA24 AA26 AA27 AB13 BB01 BB29 BC01 BC17 DA03 EA04 EA07 EA08 5B057 AA11 CA02 CA07 CA12 CA16 CB02 CB07 CB12 CB16 CC03 CE11 CH07 CH18 DA08 DC16 DC36 5C019 PP27 MP03 LL03 PP43 PP47 PQ08 PQ20 PQ23 TT02 TT06

Claims (6)

[Claims] 1. A feature detecting means for detecting a feature of input image data, a first density conversion characteristic table having a first maximum output value, and a second density conversion characteristic table having a maximum output value larger than the first maximum output value. Density conversion means for performing density conversion on the input image data using the density conversion characteristic table, and density conversion performed by the first or second density conversion characteristic table based on the detected characteristics. An image processing apparatus comprising: a selection unit that selects an output. 2. The method according to claim 1, wherein the feature is an edge area, and when an edge amount of the edge area is larger than a predetermined threshold value, the selection unit selects an output whose density has been converted in the first density conversion characteristic table. 2. The image processing apparatus according to claim 1, wherein when the edge amount of the edge area is small, the selection unit selects an output whose density has been converted in the second density conversion characteristic table. 3. The feature is a continuous tone area, and when the continuous tone area is detected, the selecting means selects an output whose density has been converted in the second density conversion characteristic table, and selects the continuous tone area. 2. The image processing apparatus according to claim 1, wherein when no is detected, the selection unit selects an output whose density has been converted in the first density conversion characteristic table. 4. The feature is a character area, and when the character area is detected, the selecting means selects an output whose density is converted in the first density conversion characteristic table, and the character area is not detected. 2. The image processing apparatus according to claim 1, wherein said selecting means selects an output whose density has been converted in said second density conversion characteristic table. 5. The method according to claim 1, wherein the characteristic is a halftone dot area, and when the halftone dot area is detected, the selecting means selects an output whose density has been converted in the first density conversion characteristic table, and selects the halftone dot area. 2. The image processing apparatus according to claim 1, wherein when no is detected, the selection unit selects an output whose density has been converted in the second density conversion characteristic table. 6. An edge area detecting means for detecting an edge area of input image data, wherein a first correction value is generated when the edge area is detected, and the first correction value is generated when the edge area is not detected. Correction value generating means for generating a small second correction value; density conversion means for performing density conversion on the input image data using a density conversion characteristic table having a predetermined maximum output value; When the edge area is not detected, the first correction value is subtracted from the predetermined maximum output value after density conversion, and when the edge area is not detected, the second correction value is subtracted from the predetermined maximum output value after density conversion. An image processing device comprising: a subtraction unit that performs subtraction.