JPH08114965A - Image forming device - Google Patents

Abstract

PURPOSE: To prevent the occurrence of moire in the output of an image where a dot image and a line image coexist by improving the reproducibility of dot or ground tint at a low density part and the stability of gradation and color reproduction with respect to environment. CONSTITUTION: This device is provided with an image signal low linearization converting means 53 provided with two or more image density signal conversion means 45 and 46 having different characteristic and acting cyclically, and having such characteristic that the output of image density signal to the low density part is set to zero at least in one means, a conversion/non-conversion discriminating device 51 discriminating whether or not conversion is performed for the respective groups of the adjacent image density signals, and a signal selecting device 55 selecting whether the signal is converted and outputted or it is not converted and outputted in accordance with the discriminated result. By detecting the edge of the input image, a dot image part and a natural image part are detected and the input image in the area is smoothed by a smoothing circuit 32 so as to remove the moire. An edge area is emphasized by an edge emphasis circuit 33.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus for scanning a light beam to form a latent image on a photosensitive medium and developing the latent image with toner.

[0002]

2. Description of the Related Art In a printer or a copying machine, a digital electrophotographic system is widely adopted as a system capable of providing high speed and high image quality. In this method, pulse width modulation exposure is often performed using an analog screen generator or the like in order to perform optical scanning of a photosensitive medium using a light beam and reproduce the gradation of an image ( See, for example, Japanese Patent Laid-Open No. 1-280965). In these, from low density part to high density part,
An image is formed with a light beam spot diameter and a constant number of lines.
As a result, the exposure profile in the low-density area has a reduced contrast and becomes analog, and since the exposure amount itself is low, the reproducibility of dots and lines deteriorates, and the stability of the gradation and color reproduction environment is stable. There was a problem that the sex became worse. The above problem can be dealt with by making the light beam spot diameter sufficiently small in order to improve the contrast of the exposure profile in the low density portion. However, an image forming optical system that collects a light beam to form a light beam spot on a photosensitive medium becomes very precise and expensive, and is not suitable for practical use.

In order to solve the above-mentioned problems, a method of stabilizing each element such as stabilization of the light beam light quantity, stabilization of toner concentration in the developing device, temperature / humidity and toner concentration in the developing device are measured to develop bias. And a transfer current value are controlled to increase the stability of gradation and color reproduction with respect to the environment, and a method called process control has been proposed (for example, Japanese Patent Laid-Open No. 4-3).
7882, Japanese Patent Application Laid-Open No. 4-376776).
However, these methods require a highly accurate sensor and control mechanism, and have the drawback of being complicated and expensive.

Further, a method has been proposed in which the diameter of the light beam spot and the light intensity of the light beam are varied to suppress the deterioration of the contrast of the exposure profile and increase the reproducibility of dots and lines (for example, Japanese Patent Laid-Open No. 4-13163). Japanese Patent Laid-Open No. Hei 4
-97374 and Japanese Patent Application Laid-Open No. 4-94261). However, these methods also require a control mechanism for varying the light beam spot diameter and the light emission intensity, which is disadvantageous in that it is complicated and expensive.

In view of the above-mentioned problems, the present inventors convert the image density signal into an analog video signal, and output the pulse width modulating means for modulating the pulse width by comparing with the pattern signal. In an image forming apparatus having an image forming means for forming an image according to a pulse width modulation signal, two or more alternately operating converting means having different characteristics for converting an image density signal are provided, and at least one converting means is an image. An invention has been proposed in which the output of the density signal to the low density portion is set to 0 so that the number of lines in the low density portion of the image formed by the image forming means is substantially reduced (Japanese Patent Application No. 5-58200). -248474), which improves the reproducibility of dots and lines in low-density areas,
In addition, it was possible to improve stability in gradation and color reproduction.

[0006]

However, in the above-mentioned invention, when the halftone image is input, the output image does not exist in the input image because the number of lines in the low density portion is substantially reduced. There is a problem that a periodic striped pattern, that is, a moire is likely to occur. Furthermore, image signals include natural images such as photographs where gradation and color reproducibility are important for reproduction, and line images such as drawings and characters where sharpness is important for reproduction. It is not possible to obtain high-quality image output simply by reducing the number of lines. That is, in the case of outputting an image in which a natural image and a line image are mixed, the line image becomes a clear image when MTF correction that emphasizes the high spatial frequency region of the image is performed in order to increase the sharpness. Moiré is likely to occur in the natural image of the halftone image, and conversely, if MTF correction is performed to remove the halftone frequency, the moire is less likely to occur, but the line image becomes blurred.

The present invention eliminates the problems and drawbacks of the prior art described above, improves the reproducibility of dots and lines in low density areas, and improves the stability of gradation and color reproduction in the environment. An object of the present invention is to provide an image forming apparatus that can obtain a clear line image without generating moire in the halftone image portion when outputting an image in which a halftone image and a line image are mixed.

[0008]

In order to achieve the above object, an image forming apparatus of the present invention (claim 1) comprises a pulse width modulating means for converting an image signal into an analog video signal and performing pulse width modulation, In an image forming apparatus having an image forming means for forming an image in accordance with a pulse width modulation signal output from the pulse width modulating means, an edge detecting means for detecting an edge portion of the input image signal, and the input image signal are smoothed. An image in which the smoothing means (32), the edge enhancing means (33) for enhancing the edge of the input image signal, and the output of the smoothing means or the edge enhancing means are mixed or selected by the detection output of the edge detecting means. A signal selecting means (35) and two or more periodically operating images having different characteristics for converting the image signal output by said image signal selecting means Image signal converting means (53) having a characteristic that the output to the low-density portion of the image signal is set to 0 or a value in the range that is not visualized, and an image adjacent to the image signal converting means. According to the conversion / non-conversion determining means (51) for determining whether or not the image signal line-reduction converting means performs conversion for each set of signals, and the determination result of the conversion / non-conversion determining means. A selection means (55) for selecting whether to output the set of image signals by the image signal line-reduction conversion means and output or without conversion.

Further, the present invention (Claim 2) is characterized in that, in the image forming apparatus, image smoothing means for converting the image signal is provided in the smoothing means. is there.

According to the present invention (claim 3), in the image forming apparatus of each of the above inventions, the image forming means includes a light beam scanning means for relatively scanning a light beam with respect to a photosensitive medium, and An image forming optical system for condensing a light beam to form a light beam spot of a predetermined size on the photosensitive medium, and between adjacent pixels in the main scanning direction when forming the low density portion. The distance is d _p (mm), and the spot diameter of the light beam on the photosensitive medium in the main scanning direction (1 /
When the e ²⁾ was d _B (mm), and is characterized in that the _{d B ≦ (1/3) d p} .

[0011]

The light beam scanning means scans the light beam relative to the photosensitive medium. The imaging optical system forms a light beam spot of a predetermined size on the photosensitive medium. Then, the pulse width modulation means determines the on / off time of the light beam according to the image density signal, and thereby a latent image corresponding to the image density signal is formed on the photosensitive medium. This latent image is later visualized with powder toner or liquid toner to form an image. 2A, 2B, and 2C are exposure energies on the photosensitive medium when the photosensitive medium is exposed using the light beam scanning means, the imaging optical system, and the pulse width modulation means. It shows a profile. When the ratio of the distance d _p (mm) between adjacent pixels and the light beam spot diameter d _B is D, the results are obtained when the values of D are 1/1, 1/2, and 1/3, respectively. . The results are obtained when the light beam spot diameter d _B (mm) is constant. Further, in electrophotography, a bias potential is applied during development in order to prevent toner from adhering to the base. In FIG. 2, the boundary line corresponding to the bias potential is also shown by a broken line as the reversal development for developing the exposed portion.

As is obvious in FIG. 2A, as the pulse width (%) is reduced, the contrast of the exposure energy profile is lowered and becomes analog. The amount exceeding the boundary line corresponding to the bias potential is reduced, and dots and lines are not reproduced. As can be seen from FIG. 2, as the value of D is reduced to 1/1, 1/2, and 1/3, the decrease in contrast is suppressed. As a result, when the light beam spot diameter d _B is constant, the number of lines N is reduced and the value of D is reduced to improve the reproduction of dots and lines in the low density portion, and to improve the gradation for the environment. -It can be seen that the stability of color reproduction increases. Further, as is conventionally known, it is important to reproduce the low density portion in designing the image quality, and it is necessary to reproduce at least a pulse width of 10%. As can be seen from FIG. 2, when the value of D is 1/1, dots and lines are not reproduced at a pulse width of 10%. When the value of D becomes 1/3, dots and lines start to be stably reproduced at a pulse width of 10%. From this, it sets the value of D to 1/3 or less, i.e., by a _{d B ≦ d p (1/3)} , stably image reproduction in the low density portion is performed.

Based on these considerations, the present inventors made the invention of Japanese Patent Application No. 5-248474 as described above, added the conversion to the image density signal, and substantially reduced the number of lines in the low density portion. I tried to be. That is, the two or more image density signal converting means have different characteristics as shown in FIGS. 10 (a) and 10 (b), for example. One conversion means is shown in FIG.
Since it has a characteristic that the output of the image density signal to the low density portion is 0 as in (b), there is no output when the low density signal comes. These conversion means operate alternately in a time-divisional manner, and as shown in FIG. 11A, the pulse width modulated output has a low number of lines for the low-density portion and a medium-high density portion. The number of lines has been increased.

However, since the number of lines in the low-density portion is substantially reduced, when a halftone image is input, the output image has a periodic striped pattern or moire that does not exist in the input image. It turned out that is likely to occur. Furthermore, image signals include natural images such as photographs where gradation and color reproducibility are important for reproduction, and line images such as drawings and characters where sharpness is important for reproduction. It has been found that there is a problem that a high-quality image output cannot be obtained by simply reducing the number of lines.

In the present invention, the edge portion of the input image signal is detected by the edge detecting means. The input image is smoothed by the smoothing means. The edge enhancing means enhances the edges of characters and line drawings in the input image. The image signal selecting means mixes or selects the output of the smoothing means or the edge enhancing means according to the edge detection result of the edge detecting means. Specifically, according to the edge detection result,
When the input image is a halftone image or a natural image such as a photograph which is not a halftone image, the output of the smoothing means is selected. As a result, when the input image is a halftone image,
Since halftone dots are removed by smoothing, it is possible to prevent moire from occurring in the output due to the reduction in the number of lines. When the input image is a natural image such as a photograph that is not a halftone image, smoothing can remove high-frequency noise, so that a smooth image output can be obtained. When the edge detecting unit detects an edge region such as a character or a line drawing, the image signal selecting unit selects the output of the edge emphasizing unit in which the edge of the character or the line drawing is emphasized by the edge emphasizing unit. As a result, a clear output image with sharpened edges can be obtained. Further, the image signal selecting means, when the input image is an image in which a halftone image, characters and line drawings and a natural image are mixed,
According to the edge detection result, the output of the smoothing means is selected when the halftone dot image portion and the natural image portion are detected, and the output of the edge emphasizing means is selected when the character or line drawing portion is detected. , The output of the smoothing means and the output of the edge enhancing means are set to an appropriate ratio (for example,
(See FIG. 9). As a result, the optimum image output can be obtained for each image portion.

As described above, the reproducibility of dots and lines in the low density area is improved, the stability of gradation and color reproduction with respect to the environment is improved, and the nets generated by the reduction in the number of lines are achieved. Prevents moiré from dot images, and when outputting an image in which halftone images and characters and line drawings are mixed,
It became possible to obtain a clear line image without the occurrence of moire.

In the above description, for the sake of simplicity, the characteristics of the image density signal converting means are the characteristics in which the output to the low density portion of one converting means is zero. Generally, in an electrophotographic apparatus that involves scanning with a light beam, the laser diode does not respond to a minute input signal, and the developing bias potential is given to suppress toner adhesion to the base, Even if the output of the means is not 0, there is a value in the range that is not visualized. That is, the characteristic of the image density signal converting means is not an absolute condition that it has a characteristic that the output to the low density portion of one of the converting means is 0. For example, FIG.
As shown in (b), the output to the low-density portion of one of the converting means may be converted into a value in a range not visualized.

[0018]

【Example】

(First Embodiment) FIG. 3 shows a first embodiment of the image forming apparatus of the present invention.
It is a figure which shows the structure of the Example of this. A charger 2, a rotary developing device 3, a transfer drum 4, a cleaner 5 and the like are arranged around the photoconductor 1 rotating in the direction of the arrow. Photoreceptor 1
In the dark area, the charger 2 uniformly charges the toner. The light beam scanning device 20 scans the light beam on the photoconductor 1. Further, the light beam is modulated by the light beam pulse width modulator 3 according to the density signal supplied from the document reading unit 10 or the like.
Turned on and off by 0. As a result, the photoconductor 1 is exposed and an electrostatic latent image is formed. The spot diameter (1 / e ² ) of the light beam in the main scanning direction on the photoconductor 1 is 42 μm.
set to m.

The rotary developing device 3 is composed of four developing devices having yellow, cyan, magenta, and black toners, respectively. Each developing device adopts a reversal developing method using two-component magnetic brush development. Average toner particle size is 7μ
m was used. The rotary developing device 3 is appropriately rotated to develop the electrostatic latent image with toner of a desired color. At this time, a bias voltage is applied to the developing roll to suppress toner adhesion to the white background portion.

The transfer drum 4 is rotated by mounting a sheet on the outer periphery thereof. The developed toner image on the photoconductor is transferred to the transfer device 4b.
Is transferred to the sheet 4P. For each color of yellow, cyan, magenta and black, electrostatic latent image formation, development,
Transfer each. The toner on the paper obtained by this operation is fixed by the fixing device 9 to form a multicolor image.

FIG. 4 is a detailed view of the light beam scanning device 20. The light beam scanning device 20 is composed of a semiconductor laser 21, a collimator lens 22, a polygon mirror 23, an fθ lens 24, and the like, and SO for detecting the light scanning start timing.
A scanning start signal generation sensor 26 that generates an S signal is provided.

The image processing apparatus 30, as shown in FIG.
Smoothing circuit 32, edge enhancement circuit 33, edge detection circuit 34, image signal selection circuit 35, triangular wave oscillator 41, comparison circuit 42, image signal acquisition device 43, conversion / non-conversion determination device 51, non-conversion path 52, conversion It is composed of a device 53, a signal selection device 55, and a D / A converter 48. The conversion device 53 includes a calculation device 44, a first look-up table (LUT) 45, and a second look-up table (LU).
T) 46 and an LUT selection device 47.

First, the document is read by the image input device 31 by 40
It is input with a resolution of 0 dpi, and an 8-bit brightness signal si
g (0) is obtained. The brightness signal sig (0) is captured by the smoothing circuit 32, the edge emphasizing circuit 33, and the edge detecting circuit 34. In the smoothing circuit 32, a digital filter having a filter size of 5 × 7 functions as a low-pass filter that removes halftone dot components existing in the print document,
An image signal sig (1) from which the halftone dot component of the document is removed is generated. By removing the halftone dots, the image is smoothed and moire is prevented from occurring. The coefficients of the 5 × 7 digital filter of the smoothing circuit 32 used in this embodiment are shown in FIG.
Shown in The spatial frequency characteristic of this digital filter is shown in FIG. From FIG. 5B, it is understood that the halftone dot components of 175 lines or more are removed by using this smoothing filter. The smoothing circuit 32
The filter size and the coefficient of the smoothing filter used in (2) may be any ones as long as desired halftone dots can be removed, and are not limited to those shown in this embodiment.

In the edge emphasis circuit 33, a digital filter having a filter size of 5 × 5 functions as a filter for correcting MTF in which characters and thin lines are deteriorated by the image input device, and an image signal sig ( 2) is generated. By carrying out edge enhancement, it is possible to prevent the sharpness of characters and fine lines from decreasing. The coefficients of the 5 × 5 digital filter of the edge enhancement circuit 33 used in this embodiment are shown in FIG. The spatial frequency characteristic of this digital filter is shown in FIG. 6 (b). From FIG. 6B, it can be seen that the edge enhancement filter is used to correct the MTF of high-frequency components such as characters and thin lines, and the edge portion of the document is enhanced. The edge enhancement filter used in the edge enhancement circuit 33 may have any filter size and coefficient as long as it can correct a desired MTF, and is not limited to that shown in this embodiment.

The edge detection circuit 34 functions as a filter for detecting an edge portion of an image such as a character or a thin line by a 5 × 5 second derivative digital filter, and generates a detection signal sig (3) of an edge portion of a document. To do. FIG. 7 shows the coefficients of the 5 × 5 second-order differential digital filter of the edge detection circuit 34 used in this embodiment. The filter size and the coefficient of the digital filter used in the edge detection circuit 34 can be any as long as a desired edge can be detected.
Any Laplacian filter or first-order differential digital filter that is commonly used may be used, and the present invention is not limited to the second-order differential digital filter shown in this embodiment.

In the image signal selection circuit 35, the image signal sig (1) generated by the smoothing circuit 32 and the image signal generated by the edge enhancement circuit 33 are based on the detection result sig (3) of the edge detection circuit 34. Image signal sig (4) is generated by mixing sig (2) with an appropriate ratio. FIG. 8 shows a block diagram of the image signal selection circuit 35.
The image signal selection circuit 35 includes LUT A351 and LUT.
It is composed of two LUTs B352, two multipliers A353 and B354, and an adder 355. LUT A351 and LUT B35 are shown in FIG.
2 shows the conversion characteristic of 2. As a result, since the edge region and the non-edge region are continuously connected, there is no texture change at the boundary. Further, in this embodiment, the edge region and the non-edge region are continuously connected,
The image signal sig (1) generated by the smoothing circuit 32 and the image signal sig (2) generated by the edge enhancement circuit 33 may be switched based on the edge detection result. Further, the conversion characteristics of LUTA and LUT B may be any as long as the edge area and the non-edge area can be continuously mixed, and are not limited to the conversion characteristics shown in this embodiment. With the above configuration, it is possible to obtain the image signal sig (4) without generating moire while maintaining the sharpness of characters and line drawings. The edge detector, the smoothing circuit, and the edge emphasizing circuit can be configured by a conventional technique (for example, Japanese Patent Publication No. 06-005879).

The digital image signal sig (4) arranged in the main scanning direction is transferred to the 2 by the signal capturing device 43.
Conversion / non-conversion determination device 5 that captures and holds data one by one
1, the non-conversion path 52, and the conversion device 53. The conversion / non-conversion determination device 51 compares each value and difference of the captured image signal with a threshold value to perform conversion / non-conversion determination, and transmits the conversion / non-conversion determination signal to the signal selection device 55. The non-conversion path 52 is directly connected to the signal selection device 55,
The captured image signal is transmitted to the signal selection device 55 without being converted.

The signal sent to the converter 53 is first sent to the arithmetic unit 44. In the arithmetic unit 44, the captured 2
An image signal sig (5) representing data is generated. In the present embodiment, the two pieces of captured data are averaged and generated. The generated image signal sig (5) representative of the captured two data is the first look-up table (LU
T) 45, and the second lookup table (LU
T) 46 produces two transformed image signals. The conversion characteristics of the first look-up table (LUT) 45 and the second look-up table (LUT) 46 have the characteristics shown in FIGS.
The output of the lookup table (LUT) 46 for low density image signals is set to zero. The LUT selection device 47 uses the first look-up table (LUT) 4
5, and a second look-up table (LUT) 46
The two image signals converted by are sequentially selected and sent to the signal selection device 55.

The signal selection device 55 selects the unconverted image signal transmitted from the non-conversion path 52 or the image signal converted by the conversion device 53 according to the conversion / non-conversion determination signal. Selected image signal sig
(6) is transmitted to the D / A converter 48. The D / A converter 48 converts the digital image signal sig (6) transmitted from the signal selection device 47 into an analog image signal sig.
Convert to (7). The comparison circuit 42 compares the size of the triangular waveform pattern signal with the size of the analog image signal to generate a pulse width modulation signal. In this embodiment, one cycle of the triangular pattern signal is set to be the same as one data cycle of the image signal.

FIG. 11 is a diagram for explaining the process of pulse width modulation signal generation for uniform density image input. In the example of FIG. 11, the digital image density signal sig (4) input from the image signal selection circuit 35 has low density, medium density, and high density portions, and the low density portion is indicated by a broken line. It is smaller than a predetermined threshold, and the others are over the threshold. The conversion / non-conversion determination device 51 determines that the conversion is performed by the lookup tables 45 and 46 when the image density signal is smaller than the threshold value and does not perform the conversion when the image density signal is larger than the threshold value. In the example of the image density signal of FIG. 11, conversion is performed for the low density portion, but not for the middle density portion and the high density portion. The signal selection device 55 selects the signal from the non-conversion path 52 for the medium concentration and high concentration portions based on the determination signal 54 of the conversion / non-conversion determination device 51, and the LUT selection circuit 47 for the low concentration portion. By selecting the output signal, the image density signal sig (6) in FIG. 11 is output. The LUT selection device 47 alternately selects the outputs of the first and second look-up tables 45 and 46. L
The output of the UT selector 47 has the conversion characteristic shown in FIG. 10A when the first lookup table 45 is selected.
Since the conversion characteristic is such that the signal in the low-density portion is expanded, the second lookup table 46 has a certain size as shown in the corresponding portion of the image density signal sig (6) in FIG. When is selected, the conversion characteristics are shown in FIG.
As shown in (b), it has a characteristic that the output is 0 with respect to the low density signal, so that the output value becomes 0 as shown in the corresponding portion of the image density signal sig (6) in FIG. The image density signal sig (6) output from the signal selection device 55 is converted into an analog signal by the D / A converter 48, and the triangular wave oscillator 4
1 and the pulse width modulation by the comparator 42 to obtain the pulse width modulation signal shown in FIG. In this way, the pulse width modulation signal for uniform image input has a form in which the input image signal is thinned out for the low density portion, the number of lines is reduced for the low density portion, and the line number is reduced for the middle and high density portions. It is possible to form an image with a higher number.

FIG. 12 is a diagram for explaining the process of pulse width modulation signal generation for thin line image input. This Figure 1
In the example of 2, the input digital image density signal sig
(4) has the portions of the thin line images (1) and (2),
The portion of the thin line image (1) is three data, that is, portions a1 and a2 where the image density exceeds the threshold value and a portion a which is smaller than the threshold value a.
The thin line image (1) is composed of one piece of data that exceeds the threshold value. Conversion / non-conversion determination device 51
Determines that there is conversion when two data items each are smaller than the threshold value, but in thin line images (1) and (2), one of the two data items exceeds the threshold value. It is determined that there is no conversion. Therefore, the signal selection device 55 selects the signal from the non-conversion path 52 for the thin line image portion based on the discrimination signal 54 of the conversion / non-conversion discrimination device 51, so that the image density signal si of FIG.
Output g (6). The image density signal sig (6) output from the signal selection device 55 is converted into an analog signal by the D / A converter 48, and the pulse width modulation signal shown in FIG. 12 is obtained by the pulse width modulation by the triangular wave oscillator 41 and the comparator 42. . If all of the data a3 portion of the thin line image (1) is passed through the conversion device, it is converted into 0 by the second look-up table and the pulse width modulation signal c3 is missing, resulting in a thin line in the output image formed. However, in this embodiment, since the thin line image (1) is not converted, the pulse width modulation signal c3 is not lost, and the problem that the thin line becomes thin is solved. That is, by setting the threshold value small, it is possible to prevent the thin line from being converted into a low number of lines and prevent the reproducibility of the thin line or the edge portion from being deteriorated.

FIG. 13 is a diagram for explaining a process of generating a pulse width modulation signal for inputting a low density thin line image. In the example of FIG. 13, the image density signal sig (4) includes the thin line images (1), (2) and (3). These thin line images have a low density, and their value is smaller than a predetermined threshold, and 2
It is isolated across data. Therefore, each 2 data including those thin line images captured by the signal capturing device 43 has 0 density except the thin line images, and both are lower than the threshold value. Therefore, the conversion / non-conversion determination device 31
Determines that conversion is performed on two data including these line images. The arithmetic unit 44 calculates the representative value by calculating the average value of the two data, and the image density signal sig of the representative value is calculated.
Output (5). 2 including line images (1) (2) (3)
The data are both representative values as shown in the figure. This representative value is sent to each of the first and second look-up tables 45 and 46 over two data cycles, but since it is low density, as shown in the image density signal sig (6) of FIG. Only the lookup table 45 of 1 has an output. Since the thin line images (2) and (3) correspond only to the cycle for selecting the second lookup table, the thin line image density signal must be obtained unless the representative value conversion processing by the arithmetic unit 44 is performed as in the present embodiment. Should be missing, but
As shown in FIG. 13, it can be seen that fine lines are generated without being missing. As described above, it is possible to prevent deterioration of reproducibility of low density thin lines.

Further, as can be seen from FIG. 11, it is understood that the line number is reduced to 200 dpi for a uniform low density image. As mentioned above, the laser beam diameter is 42μ.
Yes set to m, satisfies _{d B ≦ (1/3) d p} . The contrast of the latent image is increased, and the stability of the gradation and color reproduction in the low density area is also improved.

In this embodiment, the characteristic of the image density signal converting means is that the output to the low density portion of one converting means is as shown in FIG.
Although it is set to 0 as shown in 0 (b), the characteristic of the image density signal converting means may be a value in a range in which the output to the low density portion of one converting means is not visualized. Figure 16 (a)
The characteristic (b) shows an example of the characteristic of the image density signal converting means.

As described above, in the first embodiment, by reducing the number of lines in the highlight portion, the stability of the gradation / color reproduction in the environment is improved, and at the same time, the reproduction of fine lines of low density is achieved. It is possible to obtain an image forming apparatus that prevents deterioration and that does not cause moire in a state where sharpness of characters and fine lines is maintained.

In the first embodiment described above, the conversion / non-conversion discriminating device 51 compares each value of the captured image signal sig (4) with a preset threshold value, and captures the captured image signal. However, the conversion is performed when all are smaller than the threshold value, and no conversion is performed in the other cases. However, the method for determining the signal and the threshold value for the determination can be changed as follows.

In the conversion / non-conversion discriminating device 51, the difference between the captured image signals sig (4) is calculated and compared with a preset threshold value, and when the difference between the captured image signals is all smaller than the threshold value, the image signal Can be converted, and in other cases no conversion can be performed. FIGS. 14A and 14B show the waveform generation process in the case of such a configuration. By setting the threshold value small, it is possible to prevent the thin lines from being converted into a low number of lines and prevent the reproducibility of the thin lines from deteriorating.

In the conversion / non-conversion discriminating apparatus 51 according to another example, each value of the captured image signal sig (4) and the difference between the image signals are obtained, compared with a preset threshold value, and the captured image signal The image signal may be converted when the differences between the respective values and the image signal are all smaller than the threshold value, and in other cases, no conversion may be performed. FIGS. 15 (a) and 15 (b) show the waveform generation process in such a configuration. By setting the threshold value small, it is possible to prevent the thin lines from being converted into a low number of lines and prevent the reproducibility of the thin lines from deteriorating.

(Second Embodiment) FIG. 17 shows the arrangement of an image processing apparatus 30 according to the second embodiment of the present invention.
As shown in FIG. 17, the image processing device 30 includes a smoothing circuit 62, an edge enhancement circuit 63, an edge detection circuit 64, an image signal selection circuit 65, a triangular wave oscillator 71, a comparison circuit 72,
It is composed of a D / A converter 48. Smoothing circuit 62
Is a smoothing filter 81, a signal acquisition device 83, a conversion / non-conversion determination device 87, a non-conversion path 89, a conversion device 9.
0, a signal selection device 88, a calculation device 83, a first look-up table (LUT) 84, a second look-up table (LUT) 85, and a LUT selection device 86. Compared with the first embodiment, this embodiment is characterized in that the image signal line lowering conversion means is provided in the smoothing means.

First of all, the original is read by the image input device 61.
It is input with a resolution of 0 dpi, and an 8-bit brightness signal si
g (0) is obtained. The brightness signal sig (0) is captured by the smoothing circuit 62, the edge emphasizing circuit 63, and the edge detecting circuit 64. In the smoothing circuit 62, the 5 × 7 smoothing filter 81 functions as a low-pass filter that removes the halftone dot component existing in the printed document, and the image signal sig (1) from which the halftone dots of 175 lines or more of the document have been removed. To generate. By removing the halftone dots, the image is smoothed and moire is prevented from occurring.

The digital image signal sig (1) arranged in the main scanning direction is transferred to the 2 by the signal capturing device 82.
Data conversion and non-conversion determination device 8
7, the non-conversion path 89, and the conversion device 90. The conversion / non-conversion determination device 87 compares each value and difference of the captured image signal with a threshold value to perform conversion / non-conversion determination, and transmits the conversion / non-conversion determination signal to the signal selection device 88. The non-conversion path 89 is directly connected to the signal selection device 88,
The captured image signal is transmitted to the signal selection device 88 without being converted.

The signal sent to the converter 90 is first sent to the arithmetic unit 83. In the arithmetic unit 83, the captured 2
An image signal sig (2) representing data is generated. In the present embodiment, the two pieces of captured data are averaged and generated. The image signal sig (2) representing the generated two captured data is the first look-up table (LU
T) 84 and the second look-up table (LU
T) 85 produces two transformed image signals. The conversion characteristics of the first look-up table (LUT) 84 and the second look-up table (LUT) 85 are similar to those in the first embodiment shown in FIGS.
The second lookup table (L
The output for the low-density image signal of the (UT) 85 is set to 0. The LUT selection device 86 sequentially selects the two image signals converted by the first look-up table (LUT) 84 and the second look-up table (LUT) 85 and sends them to the signal selection device 88.

The signal selection device 88 selects the unconverted image signal transmitted from the non-conversion path 89 or the image signal converted by the conversion device 90 according to the conversion / non-conversion determination signal. Selected image signal sig
(3) is transmitted to the image signal selection circuit 65. Through the above processing, the image signal sig generated by the smoothing circuit 62
(3) is an image signal in which halftone dots are removed and the low-density portion has a reduced number of lines.

In the edge emphasizing circuit 63, a 5 × 5 digital filter functions as a filter for correcting the MTF in which characters and fine lines are deteriorated by the image input device, and an image signal sig (4) emphasizing the edge portion of the document.
Generate By carrying out edge enhancement, it is possible to prevent the sharpness of characters and fine lines from decreasing. The edge detection circuit 64 is
The 5 × 5 second-order differential digital filter functions as a filter for detecting an edge portion of an image such as a character or a thin line, and generates a detection signal sig (5) of an edge portion of a document.

In the image signal selection circuit 65, based on the detection result sig (5) of the edge detection circuit 64, the image signal sig (3) generated by the smoothing circuit 62 and the image signal generated by the edge enhancement circuit 63. The image signal sig (6) is generated by mixing sig (4) with an appropriate ratio. As a result, since the edge region and the non-edge region are continuously connected, there is no texture change at the boundary.

Edge enhancement circuit 63, edge detection circuit 64
For the detailed configuration of the image signal selection circuit 65,
The same one as in the first embodiment was used. The D / A converter 73 converts the digital image signal sig (6) transmitted from the image signal selection circuit 65 into an analog image signal sig.
Convert to (7). The comparison circuit 72 compares the size of the triangular waveform pattern signal with the size of the analog image signal to create a pulse width modulation signal. In this embodiment, one cycle of the triangular pattern signal is set to be the same as one data cycle of the image signal.

With the above structure, the number of lines in the highlight portion is reduced to improve the stability of gradation / color reproduction in the environment and prevent the deterioration of the reproduction of low density fine lines. It was possible to obtain an image forming apparatus in which moire did not occur in the state where the sharpness of characters and fine lines was maintained.

In this embodiment, since the image signal line number reducing means is provided inside the smoothing means, only the natural image portion such as a photograph in which the reproduction of the highlight portion is important has the line number reducing. Therefore, there is an effect of preventing the resolution from being deteriorated by unnecessarily reducing the number of lines and the characters and fine line portions in which the reproduction of the highlight portion is not important. Further, since only the smoothed image signal has a low number of lines, it is possible to reliably prevent the occurrence of moire. Further, since the character and line drawing parts are not subjected to the line number reduction processing, the sharpness is not lowered by the line number reduction.

In the above two examples, the black and white digital copying machine is shown as an example, but the present invention may be applied to a black and white printer and further to a color digital copying machine and printer. It is clear that the same effect can be obtained.

[0050]

According to the present invention, image conversion means and conversion /
By performing appropriate line reduction by the non-conversion determination means,
The stability of gradation and color reproduction in the low-density area is improved without the need for complicated and expensive process controls, variable emission intensity devices, and precise and expensive beam imaging optical systems. It is possible to improve the reproducibility of dots and lines and the stability of gradation and color reproduction in the environment. When the input image is a halftone dot image, the halftone dot image portion is detected by the edge detecting means, the halftone dot is removed by the smoothing means, and the edge area such as a character or line drawing is extracted by the edge detecting means. Since the edges of characters and line drawings are emphasized and output by the edge emphasizing means, moiré that occurs due to the reduction in the number of lines is prevented, and when outputting halftone images and images in which characters and line drawings are mixed. In addition, it became possible to obtain the optimum image output for each.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an image processing apparatus in an image forming apparatus according to a first embodiment.

FIG. 2 is an explanatory view of the operation of the present invention.

FIG. 3 is a diagram showing a schematic configuration of a first embodiment of the image forming apparatus of the invention.

FIG. 4 is a diagram showing a configuration of a light beam scanning device in the first embodiment.

5A is a diagram showing an example of coefficients of a digital filter of a smoothing circuit, and FIG. 5B is a diagram showing a spatial frequency characteristic of the smoothing circuit.

FIG. 6A is a diagram showing an example of coefficients of a digital filter of an edge enhancement circuit, and FIG. 6B is a diagram showing spatial frequency characteristics of the edge enhancement circuit.

FIG. 7 is a diagram showing an example of coefficients of a digital filter of an edge detection circuit.

FIG. 8 is a diagram showing an example of an image signal selection circuit.

FIG. 9 is a diagram showing an example of LUT data conversion characteristics in an image signal selection circuit.

FIG. 10A is a diagram showing an example of the data conversion characteristic of the first LUT, and FIG. 10B is a diagram showing an example of the conversion characteristic of the second LUT.

FIG. 11 is a diagram showing an example of a pulse width modulation signal generation process for a uniform density image input.

FIG. 12 is a diagram showing an example of a pulse width modulation signal generation process for a thin line image input.

FIG. 13 is a diagram showing an example of a pulse width modulation signal generation process for a low density thin line image input.

14A and 14B show another example of the waveform generation process of the image processing apparatus, wherein FIG. 14A is an example of a pulse width modulation signal generation process for a uniform density image input, and FIG. 14B is a pulse width modulation signal generation for a thin line image input. Process example

15A and 15B show another example of the waveform generation process of the image processing apparatus, wherein FIG. 15A is an example of a pulse width modulation signal generation process for a uniform density image input, and FIG. 15B is a pulse width modulation signal generation for a thin line image input. Process example

16 (a) and 16 (b) are diagrams showing examples of data conversion characteristics of the LUT of the present invention.

FIG. 17 is a diagram showing the configuration of an image processing apparatus in the image forming apparatus of the second embodiment.

[Explanation of symbols]

31 ... Image input device, 32 ... Smoothing circuit, 33 ... Edge enhancement circuit, 34 ... Edge detection circuit, 35 ... Image signal selection circuit, 41 ... Triangle wave oscillator, 42 ... Comparator, 43 ... Signal acquisition device, 44 ... Operation Device, 45 ... First LUT,
46 ... Second LUT, 47 ... LUT selection device, 48 ... D
/ A converter, 51 ... Conversion / non-conversion determination device, 55 ... Signal selection device.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location H04N 1/409 (72) Inventor Kazuhiko Arai 2274 Hongo, Ebina-shi, Kanagawa Fuji Xerox Co., Ltd. ( 72) Inventor Yasuhiro Oda 2274 Hongo, Ebina City, Kanagawa Prefecture, Fuji Xerox Co., Ltd. (72) Inventor Masayo Higashi, 2274 Hongo, Ebina City, Kanagawa Prefecture, Fuji Xerox Co., Ltd.

Claims (3)

[Claims] 1. An image forming apparatus having pulse width modulation means for converting an image signal into an analog video signal for pulse width modulation, and image forming means for forming an image in accordance with the pulse width modulation signal output from the pulse width modulation means. In, edge detection means for detecting an edge portion of the input image signal, smoothing means for smoothing the input image signal, edge enhancement means for enhancing the edges of the input image signal, and detection output of the edge detection means An image signal selecting means for mixing or selecting the output of the smoothing means or the edge enhancing means, and two or more periodic operations having different characteristics for converting the image signal output by the image signal selecting means. An image signal converting means is provided, and at least one of the image signal converting means makes the output of the image signal to the low density portion 0 or visualizes it. A conversion / non-conversion for determining whether or not the image signal line-reduction conversion unit performs conversion for each pair of image signal line-reduction units having a characteristic in which the value is in the range Selection for selecting whether to output the set of image signals converted by the image signal lowering conversion unit or output without conversion according to the determination unit and the determination result of the conversion / non-conversion determination unit. And an image forming apparatus. 2. The image forming apparatus according to claim 1, wherein the smoothing unit is provided with an image signal line lowering conversion unit for converting the image signal. 3. The image forming means comprises: a light beam scanning means for scanning the light beam relative to the photosensitive medium; and a light beam spot of a predetermined size on the photosensitive medium for condensing the light beam. The image forming optical system is formed, and the distance between adjacent pixels in the main scanning direction when forming the low-density portion is d _p (mm), and the main scanning of the light beam on the photosensitive medium is performed. when the direction of the spot diameter (1 / e ²⁾ was d _B (mm), the image forming apparatus characterized by a _{d B ≦ (1/3) d p} .