JP2001292319A - Image processing apparatus and storage medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing apparatus that generates an image with high image quality by compensating weak points of the error diffusion method. SOLUTION: An image feature extract section 130 outputs an edge level for an extended edge and a quantization threshold generating section 140 generates a quantization threshold oscillated with an amplitude width depending on this edge level. A quantization processing section 120 uses the quantization threshold to quantize image data by the error diffusion method. A dither threshold generating section 141 of the quantization threshold generating section 140 uses a proper dither threshold value matrix to generate an image that has excellent stability and smoothness especially at its flat part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus for handling multi-gradation image data, and more particularly, to a digital copying machine,
The present invention relates to an image processing apparatus related to image formation, such as a printer, a facsimile, and a display.

[0002]

2. Description of the Related Art As typical halftone processing methods in an image processing apparatus relating to image formation, there are a dither method and an error diffusion method.

[0003] The dither method has an advantage of being excellent in graininess and being able to smoothly express a halftone image, but also has a disadvantage.
For example, the resolution deteriorates in order to obtain gradation. Also,
In the dither method for generating a periodic image, moire tends to occur in a printed image such as a halftone dot.

On the other hand, the error diffusion method can obtain a resolution faithful to an original image and is suitable for reproducing a character image. However, in a halftone image such as a photograph, isolated dots are dispersed or arranged in an irregularly connected manner, so that granularity is poor and a unique texture may be generated.
Further, in an electrophotographic printer, an image is unstable because an image is formed with isolated dots. In error diffusion, the ratio of the small dots increases, and the stability further decreases. Deterioration and banding are likely to occur.

With respect to the error diffusion method, in order to improve texture due to irregular connection of dots, a dither threshold is used as a quantization threshold, and a method of improving texture by disturbing dot connection is described below. Such improved techniques have been proposed. (1) For the purpose of eliminating the occurrence of pseudo contours and unique stripe patterns, a dither threshold is used, and the larger the edge amount, the greater the amount of error diffusion (JP-A-3-34772). (2) A fixed threshold value is used for an edge portion of an image, a variation threshold value is used for a non-edge portion, and a variation threshold level is used for the purpose of preventing white spots in non-edge low-density portions and preventing the occurrence of character notches. Is reduced as the concentration becomes lower (Japanese Patent No. 2755)
No. 307). (3) In order to prevent the occurrence of moiré and false contours when using a multi-valued printer having three or more values, an edge portion of the image
A dither signal having a size corresponding to the edge amount is added to the image data, and a fixed value is added to the image data in the non-edge portion, and the added image data is subjected to multi-level quantization using a fixed threshold (Japanese Patent No. 2801195). issue).

[0006]

SUMMARY OF THE INVENTION It is a general object of the present invention to provide an image processing apparatus which compensates for the weak point of the error diffusion method and enables formation of a high quality image. An object is to provide an image processing apparatus having the following features. -It is possible to form a high-quality image having periodicity that is inconspicuous to human visual perception and good granularity with little collapse of halftone dots. -It is possible to form a high-quality image with a direction that is hardly noticeable to human vision.・ Smooth, high-quality images with excellent stability can be formed in low density areas. -It is possible to form a smooth and high-quality image with periodicity that is comfortable for humans. A high-quality image with little dot collapse and good graininess can be formed. -It is possible to form a smooth and high-quality image with excellent stability particularly in the flat portion of the image. A high-resolution image is obtained at the changing points of characters and images, and a portion having little change in photographs and images is smooth and stable, and a high-quality image in which both regions are matched without discomfort can be formed.・ Furthermore, the halftone dot image part with relatively low screen ruling has high resolution,
A halftone dot image portion having a high screen ruling can form a smooth and stable high-quality image. -It is possible to form a well-balanced high-quality image for image data having various characteristics.

[0007]

According to another aspect of the present invention, there is provided an image processing apparatus comprising: a quantization threshold generation unit configured to generate a quantization threshold that vibrates periodically; And quantization processing means for quantizing input multi-tone image data by an error diffusion method using a quantization threshold value generated by the quantization threshold value generation means and outputting quantized data, Has a common configuration.

According to the first aspect of the present invention, the quantization is performed so that an image having a periodicity that is inconspicuous to human visual perception and having good granularity with little breakup of halftone dots can be formed. The threshold value generating means is configured to generate a quantization threshold value using a dither threshold value matrix for forming a halftone dot having an image spatial frequency ranging from 100 lines to 250 lines.

According to the second aspect of the present invention, in order to form a high-quality image having a direction that is inconspicuous to human vision, the quantization threshold value generating means preferably includes an image of about 45 °. The configuration is such that a quantization threshold is generated using a dot concentration type dither threshold matrix having a screen angle.

According to the third aspect of the present invention, in order to form a smooth and high-quality image with excellent stability in a low-density part, the quantization threshold generation means includes a basic dither threshold. The quantization threshold is generated by using a dither threshold matrix composed of a plurality of combinations of matrices and obtained by shifting the adjacent basic dither threshold matrices by a half phase relative to the adjacent direction and the orthogonal direction.

According to the fourth aspect of the present invention, in order to form a smooth and high-quality image having a periodicity that is comfortable for human vision, the quantization threshold value generating means includes a main scanning direction. And a combination of a plurality of basic dither threshold matrices each having a size of 4 pixels in the sub-scanning direction, and the adjacent basic dither threshold matrices are relatively half-phase shifted in the direction orthogonal to the adjacent direction. The configuration is such that a quantization threshold is generated using a dither threshold matrix.

According to the fifth aspect of the present invention, in order to form a high-quality image with less graininess and good graininess,
A plurality of basic dither threshold matrices are combined, and the adjacent basic dither threshold matrices are relatively shifted by a half phase in the direction orthogonal to the adjacent direction, and the cycle of the density starting point is 8 pixels in the main scanning direction and 8 pixels in the sub-scanning direction. The configuration is such that a quantization threshold is generated using a dither threshold matrix of four pixels.

According to the sixth aspect of the present invention, in order to form a smooth and high-quality image particularly excellent in the stability of the flat portion of the image, the quantizing threshold value generating means includes: The dither threshold matrix that forms the line is
The configuration is used to generate a quantization threshold.

According to the seventh aspect of the present invention, the changing points of characters and images have a high resolution, the portions where there are few changes in photographs and images are smooth and stable, and both areas have an uncomfortable feeling. Edge detection means for detecting an edge level of the image data inputted to the quantization processing means, so that the image data is inputted to the quantization processing means. The oscillation width of the quantization threshold is changed according to the edge level detected by the detection means.

According to the present invention, the character and the halftone dot image portion having a relatively low ruling have high resolution, and the photograph and the halftone dot image portion having a high ruling have good smoothness and stability. ,And,
In order to be able to form a high-quality image in which both areas are matched without discomfort, an edge detecting means for detecting an edge level of the image data input to the quantization processing means, and an edge level detected by the edge detecting means. A region expansion processing unit that performs a region expansion process on an edge level, wherein the quantization threshold generation unit sets the oscillation width of the quantization threshold according to the edge level after the region expansion process by the region processing unit. It is configured to change.

According to the ninth aspect of the present invention, the image data is input to the quantization processing means so that a high-quality image balanced with respect to image data having various characteristics can be formed. Edge detection means for detecting an edge level of the image data, wherein the quantization threshold value generation means changes the oscillation width of the quantization threshold value according to the edge level detected by the edge detection means; The dither threshold matrix used for generating the threshold is switched according to a mode specified from the outside. According to the tenth aspect of the present invention, for the same image formation, edge detection means for detecting an edge level of image data input to the quantization processing means,
A region extension processing unit for performing a region extension process on the edge level detected by the edge detection unit, wherein the quantization threshold value generation unit responds to the edge level after the region extension process by the region processing unit. In addition to changing the oscillation width of the quantization threshold, a dither threshold matrix used for generating the quantization threshold is switched according to a mode specified from the outside.

[0017]

Embodiments of the present invention will be described below with reference to the accompanying drawings. In order to avoid repetition of the description, the same reference numerals are used for the same or corresponding parts in a plurality of drawings in the accompanying drawings.

Embodiment 1 FIG. 1 shows a block diagram of an image processing apparatus according to Embodiment 1 of the present invention. The image processing apparatus receives multi-gradation image data 100 and outputs quantized data 101.
0, image feature extraction unit 130, quantization threshold generation unit 140,
It comprises a signal delay unit 150 for timing adjustment between the quantization processing unit 120 and the image feature extraction unit 130. The signal delay unit 150 is provided as needed, and is, for example, a line memory having a required number of lines. The input image data 100 is, for example, 60
This is data of 8 bits / pixel read at 0 dpi. Generally, such image data 100 is input after passing through a smoothing filter in order to express halftones smoothly. Normally, since the image cycle is smoothed from an image cycle of about 150 Lpi, 175 L used in gravure printing or the like is used.
The periodicity component of a high-frequency screen image having a pi or more is image data 1
Nothing remains at 00.

The quantization processing unit 120 quantizes the multi-gradation image data by an error diffusion method using the quantization threshold generated by the quantization threshold generation unit 140. , The quantizer (comparator) 12
1, an error calculation unit 122, an error storage unit 123, an error diffusion matrix unit 124, and an error addition unit 125. The timing of the image data 100 is adjusted by the signal delay unit 150 and input to the error addition unit 125. The image data to which the diffusion error has been added by the error adding unit 115 is input to the quantizer 121. The quantizer 121 quantizes the input image data using the quantization threshold value provided from the quantization threshold value generation unit 140, and quantizes the quantization result into the quantized data 1
Output as 01.

For the sake of simplicity, in this embodiment and each of the following embodiments, only one quantization threshold is generated by the quantization threshold generator 140, and the quantizer 121 converts the input image data into quantized data. 1-bit quantized data 101 having a value of “1” when the value is equal to or larger than the threshold value and “0” otherwise.
Is described, but the present invention is not limited to this. For example, the quantization threshold generator 140 generates three quantization thresholds, the quantizer 121 quantizes the image data to four levels using the quantization thresholds, and outputs 2-bit quantized data 101. Such a configuration can be adopted.

The error calculator 122 calculates a quantization error of the quantizer 121. Here, since 8-bit image data is handled, in this error calculation, for example, “1” of the quantized data 101 is changed to 255 (10
Hex) and "0" are treated as 0 (decimal). The calculated quantization error is temporarily stored in the error storage unit 123. The error storage unit 123 stores a quantization error related to a processed pixel around the target pixel. In the present embodiment, for example, a two-line memory is used as the error storage unit 123 to diffuse the quantization error to neighboring pixels two lines ahead as described below.

The error diffusion matrix section 124 calculates a diffusion error to be added to the next target pixel from the quantization error data stored in the error storage section 123. In this embodiment, the error diffusion matrix unit 125 calculates diffusion error data using an error diffusion matrix having a size of three pixels in the sub-scanning direction and five pixels in the main scanning direction as shown in FIG. In FIG. 2, the * mark corresponds to the position of the next target pixel, and a, b,. . . , K, l are coefficients (total 32) corresponding to the positions of the 12 processed pixels in the vicinity. In the error diffusion matrix unit 125, a value obtained by dividing the product sum of the quantization errors and the corresponding coefficients a to l for the twelve processed pixels by 32 is provided to the error addition unit 125 as a diffusion error for the next target pixel. give.

The image feature extraction unit 130 is provided with the edge detection unit 1
31 and an area extension processing unit 132. The edge detection unit 131 detects an edge of the image data 100. In the present embodiment, the edge detection unit 131 outputs 4-bit edge data representing an edge level from 0 level (maximum edge degree) to 8 (non-edge). More specifically, for example, using four types of 5 × 5 differential filters shown in FIG. 3, the edge amount is detected in four directions of the main scanning direction, the sub-scanning direction, and the direction inclined by ± 45 ° from the main scanning direction. Then, the edge amount having the largest absolute value is selected, and the absolute value of the edge amount is set to level 0.
Quantized to 9 edge levels from level to level 8 and output. The area extension processing unit 132 is provided with the edge detection unit 1
The edge detection unit 31 performs an area expansion process of 7 image widths on the edge detected by the edge detection unit 31. The edge data output from the edge detection unit 131 is referred to, and a 7 × 7 pixel area around the pixel of interest (before and after in the main scanning direction) The minimum edge level (maximum edge degree) among the three pixels and the three pixels before and after in the sub-scanning direction is set as the edge level of the pixel of interest, and is output as 4-bit edge data. This edge data is
It is provided to the quantization threshold value generation unit 140.

The quantization threshold generation unit 140 generates a quantization threshold that periodically vibrates in the image space with a vibration width corresponding to the edge level represented by the edge data output from the area expansion processing unit 132. Is given to the quantizer 121 of the quantization processing unit 120, and the dither threshold value generation unit 141
And a coefficient (0 to 8) corresponding to the edge level indicated by the edge data in the output value of the dither threshold generation unit 141.
, And an adder 143 that adds a fixed value to the output value of the multiplier 142.

In this embodiment, the dither threshold generator 141
Uses a dot-concentrated 4 × 4 dither threshold matrix arranged so as to spirally increase a threshold value from −7 to +8 around 0 as shown in FIG. Output a dither threshold that oscillates from -7 to +8. The dither threshold period is 4 pixels, which corresponds to 150 Lpi in the case of 600 dpi image formation. Such a dither threshold generator 141 can be easily realized by a ROM storing the dither threshold matrix, a counter that counts main and sub-scanning timing signals of image data, and generates a read address of the ROM. .

The multiplication unit 142 determines that the edge level indicated by the edge data from the image feature extraction unit 130 is level 8
(Non-edge), coefficient 8; level 7, coefficient 7
, The coefficient 6 at the level 6 and the coefficient 5 at the level 5
, The coefficient 4 at the level 4 and the coefficient 3 at the level 3
, Coefficient 2 at level 2 and coefficient 1 at level 1
Is multiplied by a coefficient 0 at the level 0 (maximum edge degree) with the output value of the dither threshold value generation unit 141. Therefore, the output value of the multiplier 142 oscillates at the maximum oscillation width from +64 to -56 when the edge level is 8 (non-edge). The fixed value added by the adding unit 143 is selected as +128 (decimal) which is the median of the image data width. Therefore, the quantizer 1
The quantization threshold given to 21 oscillates around +128, and its maximum oscillation width is 120 (+192 to −7).
2).

If the quantized data 101 of the image processing apparatus constructed as described above is given to, for example, an electrophotographic printer or the like, a change point of a character or an image, a halftone dot image portion having a relatively low frequency, and the like are high. A photograph, a portion with little change in the image, a halftone image with a high ruling, and the like are smooth and stable at a high resolution, and a high-quality image in which the regions are matched without a sense of incongruity can be formed. This will be described below.

The change in the edge portion of a character or a line drawing in an image is sharp, and the edge level is level 0 (the maximum edge degree).
Since the quantization threshold value generated by the quantization threshold value generation unit 140 is fixed at +128 in the portion of, the quantization processing unit 120 performs the quantization process by the pure error diffusion method using the fixed threshold value. An image can be formed with high resolution.

In a portion having a low edge degree (high edge level) such as a flat portion of a photograph or an image, the amplitude of the quantization threshold generated by the quantization threshold generation section 140 becomes large. The quantization process of 120 is a dot concentration type dither-based process, and the image data is halftoned at a dither threshold period to form a dither-based image with good graininess and stability. In addition, since the halftone dot period is selected to be 150 Lpi, which is less noticeable to human vision, a smooth and high-quality image can be formed. In the image processing apparatus of the present invention, since the dot generation position fluctuates due to the diffusion of the quantization error, the dither threshold matrix only regulates the dot generation position, and the dots are arranged according to the threshold arrangement in the dither threshold matrix. Does not always occur, but in the image area where dither-based processing is performed,
If a dither threshold matrix that forms a halftone dot having an image spatial frequency within the range of 100 lines to 250 lines is used in the dither threshold generator 141, the periodicity is less noticeable to human vision and the Since the dot concentration is performed within the range of the resolving power of a photographic printer or the like, it has been confirmed that a smooth and high-quality image can be formed with little disruption of halftone dots and good granularity.

At the boundary between a region having a large edge degree and a region having a small edge degree, the oscillation width of the quantization threshold is gradually increased or decreased in accordance with the edge degree. In the opposite direction, the characteristics of the quantization processing are smoothly switched, so that an image having no uncomfortable feeling can be formed at the boundary between the two image areas.

Area extension processing unit 1 of image feature extraction unit 130
In the case of 600 dpi, the area expansion width of 7 pixels is about 0.3 mm on the document, which corresponds to a dot period of about 86 Lpi. Therefore, the halftone dot image portion having a screen ruling higher than 86 Lpi is evaluated as an edge portion, and the error diffusion-based processing using the fixed quantization threshold value or the quantization threshold value having a small vibration width is performed in the quantization processing portion 120. Therefore, halftone dots can be faithfully reproduced with high resolution, and moire does not occur.

As described above, since the halftone dot component having a high ruling of 175 Lpi or more is smoothed and does not remain in the image data 100, the halftone dot image portion having such a high ruling has an edge level of level 8 or higher. Level, and is re-dotted at a dither threshold period (150 Lpi) by dither-based processing using a quantization threshold having a large vibration width as in the case of the image flat part.
An image with excellent granularity and stability can be formed,
Further, moire does not occur because the halftone dot component is lost from the image data 100.

In a halftone image having a screen ruling lower than 86 Lpi, the halftone dot boundary evaluated as an edge is subjected to error diffusion-based processing using a fixed or small oscillation width quantization threshold value, so that the halftone dot is faithfully reproduced. The center of the halftone dot that can be reproduced and moiré can be prevented, and is not evaluated as an edge, is subjected to dither-based processing using a quantization threshold with a large vibration width. Can be formed.

<< Embodiment 2 >> According to Embodiment 2 of the present invention,
In the image processing apparatus having the configuration shown in FIG. 1, the dither threshold generation section 141 of the quantization threshold generation section 140 periodically uses an 8 × 8 dither threshold matrix as shown in FIG. Generate a dither threshold that oscillates from -7 to +8. The other configuration is the same as that of the first embodiment.

The dither threshold matrix shown in FIG. 5 is composed of 4 × 4 basic dither thresholds (same as the dither threshold matrix shown in FIG. 4) surrounded by a thick line frame, and is composed of 8 ×
The basic dither threshold matrix adjacent in the main scanning direction is shifted by a half phase in the sub-scanning direction.

When such a dither threshold matrix is used, in an image flat portion or the like where dither-based processing is performed,
A halftone dot equivalent to 150 Lpi is formed at 600 dpi, but since the density start points are arranged in a staggered pattern, an image with excellent stability can be formed especially in a low density portion. Since a screen angle of approximately 63.5 ° is added to the halftone dot arrangement, it is compatible with the error diffusion process, has little dot breakage, and has good graininess. The period of the density start point is as long as 4 pixels in the sub-scanning direction and as long as 8 pixels in the main scanning direction. Enhance.

The dither threshold generator 141 does not necessarily need to have a ROM storing an 8 × 8 dither threshold matrix shown in FIG. 5, but has a ROM storing a 4 × 4 basic dither threshold matrix. The threshold of the 8 × 8 dither threshold matrix may be generated by controlling the read address.

<< Embodiment 3 >> According to Embodiment 3 of the present invention,
In the image processing apparatus having the configuration shown in FIG. 1, the dither threshold generation section 141 of the quantization threshold generation section 140 arranges the thresholds from -7 to +8 in the vertical direction as shown in FIG. Using a × 4 dither threshold matrix, a dither threshold that periodically oscillates from −7 to +8 in image space is generated. The other configuration is the same as that of the first embodiment.

Since such a dither threshold matrix is used, in an image flat portion where dither-based processing is performed, 6
A line-like tone (15 dots) in which dots are continuous in the sub-scanning direction by dot growth at a period of 150 dpi at 00 dpi.
(0 vertical lines), a stable and smooth image is formed.

<< Embodiment 4 >> According to Embodiment 4 of the present invention,
In the image processing apparatus having the configuration shown in FIG. 1, the dither threshold generation section 141 of the quantization threshold generation section 140 arranges the thresholds from -16 to +15 so as to increase in the vertical direction as shown in FIG. Using a × 8 dither threshold matrix,
Generate a dither threshold that periodically oscillates from -16 to +15 in image space. The other configuration is the same as that of the first embodiment.

Since such a dither threshold matrix is used, in the same manner as in the third embodiment, in a flat portion of an image where dither-based processing is performed, smooth lines having excellent stability are obtained by using vertical lines equivalent to 150 lines at 600 dpi. An image is formed.

<< Embodiment 5 >> According to Embodiment 5 of the present invention,
In the image processing apparatus having the configuration shown in FIG. 1, the dither threshold generator 141 of the quantization threshold generator 140 oscillates from −9 to +8 using a 6 × 6 dither threshold matrix as shown in FIG. Generate a threshold. This dither threshold is obtained by combining two basic dither threshold matrices shown by dashed lines in FIG. 8 so as to have a screen angle of 45 °. Four 6 × 6 dither threshold matrices are shown side by side. In the basic dither threshold matrix, -9 to +8
Are arranged in such a manner that the threshold values up to approximately spirally increase.

Since such a dither threshold matrix is used, in an image flat portion or the like where dither-based processing is performed, 6 pixels are used.
Halftone dots corresponding to about 141 lines are formed at 00 dpi, and the halftone dot arrangement has a 45 ° directivity. Such a direction around 45 ° is inconspicuous to human vision. In addition, the image data 100 is stored in the previous stage of
Even if the 0 ° rotation processing is performed (or the 90 ° rotation processing is performed on the quantized data 101 at a later stage of the image processing apparatus), the directionality of the halftone dot arrangement is substantially the same as the case where the rotation processing is not performed. Does not change. That is, the impression of the formed image hardly changes depending on the presence or absence of the rotation processing. Digital copiers and the like may be equipped with a function called rotary sorting that does not use a mechanical sorting mechanism. In this rotation sort, when copying a plurality of copies of a plurality of pages of a document, the first copy is output without performing the rotation process, and the next copy is rotated by 90 ° and the copy is output. A part is rotated alternately such that a copy is output without performing the rotation processing, and the paper feeding direction is switched according to the presence or absence of the rotation processing. When such a rotation sort is performed, it is not preferable that the impression of the copied image differs depending on the presence or absence of the rotation processing. According to the present embodiment, since the directionality of the halftone dot arrangement such as the image flat portion is substantially constant regardless of the presence or absence of rotation, even if the rotation sort is performed, the rotated copy image and the non-rotated copy image are obtained. There is no difference between them that makes them feel uncomfortable.

Embodiment 6 According to Embodiment 6 of the present invention,
In the image processing apparatus having the overall configuration shown in FIG. 1, the quantization threshold generation unit 140 has a configuration as shown in FIG.
Edge detection unit 131 of image feature extraction unit 130 (FIG. 1)
Is changed so that the edge amount is quantized to four edge levels from level 0 (maximum edge degree) to level 3 (non-edge) and output as 2-bit edge data.

As shown in FIG. 9, the quantization threshold generator 140 according to the present embodiment includes a threshold generator 145_0 corresponding to edge level 0, a threshold generator 145_1 corresponding to edge level 1, and an edge level 2 Threshold generation unit 145_2, threshold generation unit 1 corresponding to edge level 3
45_3, a threshold generated by any of the threshold generators 145_0 to 145_3 is selected according to the edge level indicated by the edge data output from the image feature extractor 130, and the selected threshold is used as a quantization threshold.
It comprises a threshold value selector 146 to be given to the quantizer 121 (FIG. 1).

The threshold value generator 145_3 corresponding to the edge level 3 (non-edge) is provided by, for example, the first, second, third, and fourth embodiments.
Using the dither threshold matrix obtained by multiplying each threshold of the dither threshold matrix used in 4 or 5 (FIGS. 4 to 7) by 8 and adding 128, a threshold vibrating with the maximum vibration width is generated. Threshold generator 1 corresponding to edge level 2
45_2 generates a threshold value that vibrates with a smaller vibration width by using a dither threshold value matrix obtained by multiplying each threshold value of the dither threshold value matrix by 5 and adding 128 to the threshold value. The threshold value generation unit 145_1 corresponding to the edge level 1 uses the dither threshold value matrix obtained by multiplying each threshold value of the dither threshold value matrix of the embodiment by 2 and adding 128 to the threshold value, and oscillates with a smaller vibration width. Generate The threshold value generation unit 145_0 corresponding to the edge level 0 (the maximum edge degree) sets the fixed value (+1
28).

Therefore, in this embodiment, it is clear that a high-quality image can be formed by performing the same quantization threshold processing as in the first, second, third, fourth or fourth embodiment.

According to the configuration of the quantization threshold value generating section 140 of this embodiment, means for multiplication which is generally disadvantageous in terms of cost or processing time, regardless of whether it is realized by hardware or software (see FIG. 1 corresponds to the multiplication unit 142)
Can be eliminated. Further, the area extension processing unit 132 needs to temporarily store the edge data for a plurality of lines corresponding to the area extension width, but only temporarily compresses the edge data to 2 bits. The capacity of the line memory and the like can be reduced. Further, since the number of edge levels is as small as 4, the amount of memory required for storing the dither threshold matrix in the threshold generation units 145_0 to 145_2 can be reduced.

Although the memory of the area expansion processing unit 132 is not reduced, a configuration is also possible in which the edge detection unit 131 outputs 9 levels of edge levels, and the area expansion unit 132 converts the edge levels to 4 levels and outputs the converted levels. It is.

Embodiment 7 According to Embodiment 7 of the present invention,
In the image processing apparatus having the overall configuration shown in FIG. 1, a mode signal (not shown) is externally input to a dither threshold generation section 141 in the quantization threshold generation section 140. Then, the dither threshold generator 141 switches the dither threshold matrix used for dither threshold generation according to the mode specified by the mode signal. For example, the dither threshold generator 141
Is a 4 × 4 dither threshold matrix for the “photo mode” as shown in FIG.
2 × 2 dither threshold matrix for “photo mode”, FIG. 1
A 1 × 1 dither threshold matrix such as 0 (c) is held, and a dither threshold matrix corresponding to the mode specified by the mode signal is selected. Note that the dither threshold matrix in FIG. 10A is the same as that shown in FIG.

When the “photo mode” is designated, FIG.
Since the dither threshold matrix of (a) is selected, the non-edge portion forms a smooth image with excellent stability by halftone dots having an image spatial frequency of 150 dpi at 600 dpi. Therefore, this mode is suitable for outputting an image with little change such as a photograph. When the "character mode" is designated, the dither threshold matrix of FIG. 10C is selected, so that the quantization threshold is fixed to +128, and the entire area of the image is quantized by the pure error diffusion method. Thus, an image having excellent resolution is formed. Therefore, this "character mode" is suitable for outputting an image such as a character or a line drawing for which high resolution is desired. "Character / Photo mode"
This mode is suitable for outputting an image having characteristics intermediate between the two modes. When the “text / photo mode” is designated, the dither threshold matrix of FIG. 10B is selected, and an image is formed by a halftone dot having an image spatial frequency of 300 dpi at 600 dpi in the non-edge portion.

According to the configuration in which a plurality of modes can be designated and the dither threshold matrix is switched according to the mode, the image data having various characteristics is balanced by performing appropriate quantization processing. High quality images can be formed.

In order to achieve the same object, the quantization threshold generator 140 shown in FIG.
It is also possible to adopt a configuration in which the dither threshold matrix is switched at 45 according to the mode.

The image processing apparatus of each embodiment described above
It can also be realized by software using a general computer. In this case, a program for realizing the function of each unit of the image processing apparatus on a computer is read from various storage media such as a magnetic disk, an optical disk, a magneto-optical disk, and a semiconductor storage element on which the program is recorded, or The image processing apparatus of the present invention can be realized on a computer by receiving the data from an external computer or the like via a network, loading the data into a main memory of the computer, and causing the CPU to execute the data. For example, a main memory is used as a storage area such as a line memory necessary for storing various data and delaying signals. Various computer-readable storage media on which such a program is recorded are also included in the present invention.

The image processing apparatus of each of the above embodiments can be incorporated in a device such as a printer or a display related to image formation, or a device related to both image reading and image formation, such as a digital copying machine or a facsimile machine. it can. As an example of such an embodiment, an example of a digital copying machine to which the present invention is applied will be described below.

<Eighth Embodiment> FIG. 11 is a schematic sectional view showing a configuration example of an image reading mechanism and an image forming mechanism of a digital copying machine. This digital copier includes a scanner section 400 for optically scanning a document, a laser printer section 411 as an image forming section, and a circuit section 550 (not shown).
(FIG. 12).

The scanner section 400 has a flat document table 403.
The original placed on top is illuminated by the illumination lamp 502,
The reflected light image is passed through mirrors 503, 504, 505 and a lens 506 to an image sensor 507 such as a CCD.
And the illumination lamp 502 and the mirror 50
By sub-scanning the original by moving 3 to 505,
Read the image information of the document. The analog image signal output from the image sensor 507 is supplied to the circuit unit 550 (FIG. 12).
Is input and processed. Image data output from the circuit unit 550 is input to the laser printer unit 411.

In the laser printer unit 411, the writing optical unit 508 converts the image data input from the circuit unit 550 into an optical signal, and exposes an image carrier made of a photoconductor, for example, a photoconductor drum 509. ,
An electrostatic latent image corresponding to the document image is formed. The writing optical unit 508 drives, for example, a semiconductor laser in accordance with the image data by the light emission drive control unit to emit laser light whose intensity is modulated, and deflects and scans this laser light by the rotary polygon mirror 510 to perform f / θ lens. And irradiates the photosensitive drum 509 via the reflection mirror 511. Photoconductor drum 50
Reference numeral 9 is rotated by a driving unit, rotates clockwise as indicated by an arrow, is uniformly charged by a charger 512, and is then exposed by a writing optical unit 508 to form an electrostatic latent image. The electrostatic latent image on the photosensitive drum 509 is developed by the developing device 513 to become a toner image. Further, the sheet is fed to the registration roller 520 from one of the plurality of sheet feeding units 514 to 518 and the manual sheet feeding unit 519. The registration roller 520 is connected to the photosensitive drum 50.
9 is sent out in time with the toner image on the sheet 9. A transfer bias is applied to the transfer belt 521 from a transfer power supply, and the transfer belt 521 transfers the toner image on the photosensitive drum 509 to a sheet and conveys the sheet. The sheet on which the toner image has been transferred is conveyed to the fixing unit 522 by the transfer belt 521, where the toner image is fixed, and then discharged to the discharge tray 523. Further, the photosensitive drum 509 is cleaned by the cleaning device 524 after the transfer of the toner image, and is further discharged by the discharger 525 to prepare for the next image forming operation.

FIG. 10 shows a circuit section 5 of this digital copying machine.
It is a block diagram which shows an example of 50 in simplified form. The input of the circuit unit 550 is an analog image signal read at, for example, 600 dpi by the image sensor 507 of the scanner unit 400. This analog image signal is A
After the level is adjusted by the GC circuit 551, the A / D
The conversion circuit 552 converts the image data into 8-bit digital image data per pixel, and the shading correction circuit 553 corrects variations in sensitivity and illuminance of each pixel of the image sensor 507.

The image data after the shading correction is sent to a filter processing circuit 556, and subjected to MTF correction and filter processing for smoothing. As a result of this filtering, the halftone dot component having a higher screen ruling than about 150 Lpi is smoothed, and the halftone screen component having a higher screen ruling than 175 Lpi is almost completely removed. The image data after the filter processing is sent to a gamma correction circuit 555, where the image data is subjected to gamma correction for conversion into writing density.

Reference numeral 560 denotes a halftone processing section. The halftone processing unit 560 is composed of the image processing device of each of the above embodiments. The image data after the filtering is processed by the image feature extracting unit 13.
0, and the image data after the gamma correction is input to the quantization processing unit 120 via the signal delay unit 150. The quantized data output from the quantization processing unit 120 is sent to the emission drive control unit of the semiconductor laser in the writing optical unit 508.

In the filter processing circuit 556,
By adjusting the timing of the signal output to the image feature extraction unit 130, the signal delay unit 150 can be omitted. In a digital copying machine, scaling processing of image data in the main scanning direction is performed, for example, in a stage preceding the gamma correction circuit 557, and processing such as background removal processing and flare removal processing is performed, for example, with the gamma correction unit 557 and the halftone processing. The processing is performed in the middle of the processing unit 560, and the 90 ° rotation processing is performed, for example, in the preceding stage of the
Although it may be performed after 0, its explanation is omitted.

[0063]

According to the image processing apparatus of the first aspect of the present invention, the image processing apparatus has a periodicity that is inconspicuous to human vision,
In addition, since the dots are concentrated within the range of the resolution capability of an electrophotographic printer or the like, a high-quality image can be formed with little disruption of halftone dots and good graininess.

According to the image processing apparatus of the second aspect of the invention, it is possible to form a high-quality image having a direction that is inconspicuous to human vision. Differences are not noticeable.

According to the image processing apparatus of the third, fourth and fifth aspects of the present invention, the density starting points are arranged in a staggered pattern, and a high-quality image with excellent stability can be formed in a low density portion. It is. According to the image processing apparatus of the present invention, when forming an image at 600 dpi, 1 is used.
The halftone dots are equivalent to 50 lines, and a smooth image having a periodicity that is comfortable for human vision can be formed. Also,
According to the image processing apparatus of the fifth aspect of the present invention, since the intervals between ON and OFF of the dots in the main scanning direction in the low-density image flat portion are long, an image having less halftone dots and good graininess can be obtained. It can be formed.

According to the image processing apparatus of the sixth aspect of the present invention, a smooth and high-quality image excellent in stability particularly in a flat portion of an image is obtained by a linear tone in which dots are continuous in the sub-scanning direction, that is, a vertical line. Formation is possible.

According to the image processing apparatus according to the seventh, eighth, ninth, and tenth aspects of the present invention, the changing points of characters and images have a high resolution, and the portions with little change in photographs and images have smooth and good stability. In addition, it is possible to form a high-quality image in which both areas are matched without any uncomfortable feeling. According to the image processing apparatus of the present invention, the halftone dot image portion having a relatively low ruling has a high resolution, and the halftone dot image portion having a high ruling has a smooth and stable high-quality image. Can be formed. Claims 9 and 1
According to the image processing apparatus of the invention described in Item No. 0, it is possible to form a well-balanced high-quality image suitable for image characteristics by specifying a mode according to the characteristics of the image data to be processed.

According to the storage medium of the eleventh aspect, the above-described image processing apparatus can be easily realized by using a general computer.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an example of a block configuration of an image processing apparatus according to the present invention.

FIG. 2 is a diagram illustrating an example of an error diffusion matrix.

FIG. 3 is a diagram illustrating an example of a differential filter for edge detection.

FIG. 4 is a diagram illustrating an example of a dither threshold matrix for generating a quantization threshold.

FIG. 5 is a diagram illustrating another example of a dither threshold matrix for generating a quantization threshold.

FIG. 6 is a diagram illustrating another example of a dither threshold matrix for generating a quantization threshold.

FIG. 7 is a diagram illustrating another example of a dither threshold matrix for generating a quantization threshold.

FIG. 8 is a diagram illustrating another example of a dither threshold matrix for generating a quantization threshold.

FIG. 9 is a block diagram illustrating another block configuration of the quantization threshold generation unit.

FIG. 10 is a diagram illustrating an example of a dither threshold matrix corresponding to a mode.

FIG. 11 is a schematic sectional view showing a configuration example of a mechanism related to image reading and image formation of the digital copying machine according to the present invention.

FIG. 12 is a block diagram illustrating an example of a circuit unit of the digital copying machine.

[Explanation of symbols]

 REFERENCE SIGNS LIST 100 image data 101 quantized data 120 quantization processing unit 121 quantizer 122 error calculation unit 123 error storage unit 124 error diffusion matrix unit 125 error addition unit 130 image feature extraction unit 131 edge detection unit 132 area extension processing unit 140 quantization Threshold generation unit 141 Dither threshold generation unit 142 Multiplication unit 143 Addition unit 145_0 to 145_3 Threshold generation unit 146 Threshold selection unit 150 Signal delay unit 400 Scanner unit 411 Laser printer unit 551 AGC circuit 552 A / D conversion circuit 553 Shading correction circuit 556 Filter Processing circuit 557 Gamma correction circuit 560 Halftone processing section

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kazunari Tonami 1-3-6 Nakamagome, Ota-ku, Tokyo F-term in Ricoh Co., Ltd. (reference) 2C262 AA24 AA26 AA27 AB13 BB01 BB08 BB22 BB23 BB25 BB27 DA03 5B057 AA11 BA02 BA30 CA02 CA08 CA12 CA16 CB02 CB07 CB12 CB16 CE13 DA08 DC16 5C077 LL19 NN09 NN15 PP01 PP47 PP68 RR08 RR09 RR15 RR16

Claims (11)

[Claims] A quantizing threshold value generating means for generating a quantizing threshold value which periodically oscillates, and input multi-gradation image data is converted by using a quantizing threshold value generated by said quantizing threshold value generating means. Quantization processing means for quantizing by an error diffusion method and outputting quantized data, wherein the quantization threshold value generation means includes a dither threshold value for forming a halftone dot having an image spatial frequency ranging from 100 lines to 250 lines. An image processing apparatus for generating a quantization threshold using a matrix. 2. A quantization threshold generating means for generating a quantizing threshold that oscillates periodically, and input multi-tone image data is converted using a quantization threshold generated by the quantization threshold generating means. Quantization processing means for quantizing by an error diffusion method and outputting quantized data, wherein the quantization threshold value generation means performs quantization using a dot concentration type dither threshold matrix having a screen angle near 45 °. An image processing apparatus for generating a threshold value. 3. A quantizing threshold value generating means for generating a quantizing threshold value which oscillates periodically, and input multi-tone image data is converted by using a quantizing threshold value generated by said quantizing threshold value generating means. Quantization processing means for quantizing by an error diffusion method and outputting quantized data, wherein the quantization threshold value generation means comprises a plurality of combinations of basic dither threshold matrices, An image processing apparatus that generates a quantization threshold value using a dither threshold value matrix that is relatively shifted by a half phase in an adjacent direction and an orthogonal direction. 4. The image processing apparatus according to claim 3, wherein the basic dither threshold matrix has a size of 4 pixels in the main scanning direction and 4 pixels in the sub-scanning direction. 5. The image processing apparatus according to claim 3, wherein in the dither threshold matrix, the cycle of the density start point is 8 pixels in the main scanning direction and 4 pixels in the sub-scanning direction. 6. A quantization threshold value generating means for generating a quantizing threshold value which oscillates periodically, and an error is generated by using the quantization threshold value generated by the quantization threshold value generating means for inputting multi-gradation image data. Quantization processing means for quantizing by a diffusion method and outputting quantized data, wherein the quantization threshold generation means generates a quantization threshold using a dither threshold matrix for forming vertical lines. Image processing device. 7. The image processing apparatus according to claim 1, further comprising an edge detection unit configured to detect an edge level of the image data input to the quantization processing unit, wherein the quantization threshold value generation unit determines an edge level according to the edge level detected by the edge detection unit. The image processing apparatus according to claim 1, wherein a vibration width of the quantization threshold is changed. 8. An edge detecting means for detecting an edge level of image data inputted to said quantization processing means, and an area expanding processing means for performing an area expanding process on the edge level detected by said edge detecting means. 7. The apparatus according to claim 1, wherein the quantization threshold value generating means changes the oscillation width of the quantization threshold value according to the edge level after the area expansion processing by the area processing means. The image processing device according to the item. 9. A quantization threshold value generating means for generating a quantizing threshold value that oscillates periodically, and input multi-gradation image data is converted using a quantization threshold value generated by the quantization threshold value generating means. A quantizing means for quantizing by an error diffusion method and outputting quantized data; and an edge detecting means for detecting an edge level of image data inputted to the quantizing processing means, wherein the quantizing threshold value generating means is provided. Changing the oscillation width of the quantization threshold according to the edge level detected by the edge detection means, and switching a dither threshold matrix used for generation of the quantization threshold according to a mode specified externally. An image processing apparatus characterized by the above-mentioned. 10. A quantization threshold value generating means for generating a quantizing threshold value which periodically oscillates, and input multi-tone image data is converted by using a quantization threshold value generated by said quantization threshold value generating means. Quantizing means for quantizing by an error diffusion method and outputting quantized data; edge detecting means for detecting an edge level of image data input to the quantizing means; and an edge detected by the edge detecting means. Region extension processing means for performing region extension processing on the level, wherein the quantization threshold value generation means changes the oscillation width of the quantization threshold according to the edge level after the region extension processing by the region processing means. An image processing apparatus for switching a dither threshold matrix used for generating a quantization threshold in accordance with a mode specified from the outside. 11. A computer-readable storage medium in which a program for causing a computer to realize the functions of each unit of the image processing apparatus according to claim 1 is recorded.