JPH0219669B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a halftone image output device, and particularly to a device for improving resolution and gradation in, for example, a laser beam printer that outputs halftone images.

In general, scanning systems using rotating polygon mirrors or oscillating mirrors have a large scanning angle and low chromatic dispersion, so facsimile machines using lasers,
It is widely used in various display devices, printing devices, etc. In particular, when a rotating polygon mirror is used, it is widely used as a high-speed scanning device.

Conventionally, a systematic dither method is known as a method for recording or displaying halftone images in such a scanning system. This means that the recording display medium is white,
This is effective when only black binary values can be obtained and a halftone image is expressed.

FIG. 1 is a simplified illustration of such a systematic dithering method. The quantized and sampled input image signal x generally has density information of K bits per pixel, and each pixel is compared with a certain threshold value C. If it is above the threshold value, it is displayed as 1, and if it is below the threshold value, it is displayed as a binary value. Output y
and get 0. Here, the threshold value C takes a value consisting of K bits. Therefore, an input image signal of K bits/pixel is converted into a binary display image signal of 1 bit/pixel after passing through the comparator.

The threshold value C is generally displayed in a matrix corresponding to pixels. In the organized dithering method,
Judice's matrix was proposed, and the matrix for one pixel is D ₂ = 0 2/4 3/4 1/4 4 x 4 = 16 dots when the pixel is formed by 2 x 2 = 4 dots. D ⁴ = 0 8/16 2/16 10/16 12/16 4/16 14/16 6/16 3/16 11/16 1/16 9/16 15/16 7/ 16 13/16 5/16 Here, the area of each dot is 1/4 (in the case of formula (1)) or 1/16 (in the case of formula (2)) of the pixel.
It is. Generally, the larger the dimension of these matrices, the richer the gradation, and in general, an n×n matrix can express n ² +1 gradations (including 0).

The advantages of this halftone image method are (1) the system is simple because it is only necessary to record one dot in two values, white or black, and (2) therefore, even if the gamma of the photoreceptor is non-linear, It doesn't really depend on the type of photoreceptor.

etc. On the other hand, according to this method, (1) one pixel is composed of multiple dots, so the resolution deteriorates, and (2) the more you try to increase the gradation, the more
The area of the pixel becomes larger and the resolution becomes worse.

Such disadvantages arise.

The present invention has been made in view of the above points, and it is an object of the present invention to provide a halftone image output device capable of reproducing stable halftone images with high resolution and high gradation.

That is, the present invention allocates threshold values for each of the first and second dither matrices to common input pixel information, performs gradation processing on the input pixel information, and performs gradation processing on the first and second dither matrices. A fine pixel matrix corresponding to the matrix is formed, and each fine pixel in the fine pixel matrix is set to a first density and the first density.
In a halftone image output device that reproduces a halftone image using a second density higher than the density of 3 levels or more and outputs a halftone image, a beam is generated to form the fine pixels on the photoreceptor. beam generating means; input means for inputting a density signal representing the gradation of the fine pixel; modulating the beam generated by the beam generating means based on the density signal input from the input means; modulating means for switching the driving current value of the beam generating means to generate beams of first and second intensities corresponding to the concentrations of 2; means for forming an electrostatic latent image; and a detection means for detecting the potential of the electrostatic latent image of the fine pixel matrix formed on the photoreceptor, and the detection means includes a means for forming an electrostatic latent image on the fine pixel of the second density. a potential of a first electrostatic latent image corresponding to a first gradation micropixel matrix formed of micropixels of the first density and the micropixels of the second density, and The method is configured such that the potential of a second electrostatic latent image corresponding to a fine pixel matrix of a second gradation that is the next gradation after the first gradation can be detected, and the first intensity of the beam is changed. The first,
A drive current that determines the first intensity of the beam based on the potentials of the first and second electrostatic latent images detected by the detection means in order to correct the potential difference between the second electrostatic latent images. The present invention provides a halftone image output device provided with a calculation means for calculating and determining a value.

The present invention will be explained in detail below based on the drawings.

In order to simplify the explanation, an example will be described below in which a halftone image is obtained by a so-called ternary method using three levels of brightness: white, black, and one level of halftone brightness between white and black.

For example, a gray dot or a dot with an area half the area of black is defined between white and black,
When an n×n dither matrix is configured with a ternary configuration, the obtained gradation is 2n ² +1, which is approximately twice as large as that in the above case. Figures 2 and 3 show examples of 2 x 2 dither matrices configured with such a ternary configuration, and Figure 2 shows one with intermediate density,
Figure 3 shows the case where different elements are added to change the black area of the halftone dots. In addition, in FIGS. 2 and 3, the size of a pixel relative to the size of a matrix is shown as one pixel.

In the example shown in FIG. 2, a pixel having nine gradations of brightness is constructed using four fine pixels that can have three brightness levels: white, black, and gray.

In addition, in the example shown in Figure 3, by assigning a white, a large black circle, and a small black circle that is half the area of the large black circle to one fine pixel, a pixel with nine gradations of lightness is created using four fine pixels. It consists of

In this way, using fine pixels of three types of brightness,
When one pixel is composed of an n×n fine pixel matrix, an image having a gradation level of 2×n×n+1 can be obtained. Therefore, it is possible to obtain a gradation level that is approximately twice that of n x n + 1, which is the gradation level obtained when using an n x n micro pixel matrix using micro pixels displayed in binary format as in the past. . In addition, when outputting halftones using this method, the size of one pixel can be made smaller compared to the conventional method for the same gradation level, and therefore high resolution and sharp halftones can be output. You can get the image.

The dither matrix used in the case of Figures 2 and 3 is: D ₁ ² = 0 2/8 3/8 1/8 (3) D ₂ ² = 4/8 6/8 7/8 5/ 8 (4) are considered, and the corresponding pixels are compared. For example, input image information has uniform density X (i) = i/8 i/8 i/8 i/8 (i = 1, 2, ..., 8)
(5), the above dither matrix (3) and (4)
In comparison, 1 is output when the image information is large, and 0 is output in other cases. When i=2, this output image information is Y ₁ (2)=1 0 0 1 (6) Y ₂ (2)=0 0 0 0 (7). Here, the output corresponding to Y ₁ is displayed for the intermediate density or intermediate halftone dot size of the fine pixel, and the output corresponding to Y ₂ is displayed for the high density (or larger halftone dot size) of the fine pixel. Then, the image output shown in 2 of FIG. 2 or 3 is obtained.

When i = 6, Y ₁ (6) = 1 1 1 1 (8) Y ₂ (6) = 1 0 0 1 (9), high density output corresponding to Y ₂ , intermediate density output corresponding to Y ₁ We get the output, 6 in FIG. 2 or 3.

Here, processing is performed to prioritize high-density output,
Then, if it is decided that "where high density is to be output, intermediate density is not output", Y ₁ (6) = 0 1 1 0 (10), and this image output is shown in 6 in Fig. 2.

Figure 4 shows one of the configurations of an output device that obtains halftone images.
Give an example. Here, 12 is a semiconductor laser,
An optical deflector 14 that converts the output beam emitted from the semiconductor laser 12 into parallel light beams using a collimator lens 13 and forms the parallel light beams using a rotating polygon mirror.
incident on the The light beam reflected and deflected by the optical deflector 14 passes through an imaging lens 15 such as an Fθ lens.
reaches the top of the photosensitive drum 16 through the photosensitive drum 1.
The image is formed on 6. Here, one-dimensional scanning of the light beam is performed by rotating the rotary polygon mirror 14, and two-dimensional light scanning is performed over the entire circumferential surface of the photosensitive drum 16 by rotating the photosensitive drum 16. Here, by modulating the semiconductor laser 12 using a drive circuit to be described later, an image pattern including a desired halftone image can be formed on the photosensitive drum 16.

The image forming process in the output device shown in FIG. 4 will be explained based on FIG. Photosensitive drum 16
has a photoconductor coated on its surface, and in the so-called NP process used in this example, an insulating layer is provided on the photoconductor layer. First, as a first step, the primary charger 17 uniformly charges the insulating layer positively. Next, AC corona discharge is performed by the static eliminator 18 while irradiating the photosensitive drum 16 with a laser beam 21 modulated by the semiconductor laser 12 . Further, by uniformly irradiating the photosensitive layer using the entire surface exposure lamp 22, image information can be accumulated on the photosensitive drum 16 as a surface potential distribution, that is, as an electrostatic latent image. Next, the electrostatic latent image described above is developed and transferred to the paper 20 by depositing toner particles of opposite polarity in the developer 19 onto the photosensitive drum 16. Here, surface potential sensor 2
3 to measure the surface potential of the electrostatic latent image,
A desired image is stably obtained as described below.

The image formation process has been described above, but
Next, the operation of the surface potential sensor 23 will be described.

Generally, the relationship between the exposure amount E and the surface potential V is as shown in FIG. 6, where the surface potential decreases where the amount of light is large.
Here, as shown in FIG. 2 or 3, an image pattern as shown in FIG.
That is, when the aforementioned laser beam 21 is modulated so as to obtain an image pattern having 10 gradations of brightness, its surface potential changes stepwise as shown in Figure 8. Here, the horizontal axis r is the number of filled-in fine pixels.

FIG. 9 shows a drive circuit for the semiconductor laser 12 shown in FIG. In this example, the semiconductor laser 12 provides three different optical output values including zero. Here, 12 is the aforementioned semiconductor laser, 31 is an input terminal for the image density signal S1 in the form of a binary signal, and 32 is an input terminal for the image density signal S2 in the form of a binary signal. As shown in FIG. 2, a pair of signals S1 and S are used to specify the gradation for each fine pixel in the form of the ternary dither method according to the present invention.
2 is supplied to these input terminals 31 and 32, and the semiconductor laser 12 is controlled based on these signals S1 and S2 to obtain an optical output L according to the ternary dither method.
Obtain 0, LM, LH. That is, the two transistors 33 and 34 forming a differential switch drive the semiconductor laser 12 in response to the image density signal S1. At this time, the current flowing through the semiconductor laser 12 is determined by the collector current of the current source transistor 35. On the other hand, by the two transistors 36 and 37 forming the differential switch,
The semiconductor laser 12 is activated in response to the image density signal S2.
to drive. At this time, the current flowing through the semiconductor laser 12 is determined by the collector current of the current source transistor 38. Therefore, the image density signal S1,
The semiconductor laser 12 can be driven with different current values depending on which of the terminals 31 and 32 is supplied with a signal from S2.

In this example, from the viewpoint of protecting the semiconductor laser 12, low-level image density signals S1 and S2 are sent to the terminals 3.
1 and 32 at the same time, the semiconductor laser 1
2, the circuit constants are set so that the image obtained by the laser beam emitted corresponding to this current value has the maximum density, and the image density signal S1 is turned off to obtain an intermediate density. It is preferable to form a . That is, as shown in FIG. 10, when the image density signals S1 and S2 are both at high level, the optical output of the semiconductor laser 12 is set to L.
0, the optical output is the intermediate output LM when the signal S1 is low level and the signal S2 is high level,
When both signals S1 and S2 are at low level, the optical output is set to the maximum output LH.

In order to stably obtain such different light outputs L0, LM, and LH in the image forming process, the ninth
It is preferable to control the light output LM for obtaining the intermediate image by controlling the input signal from the terminal 39 shown in the figure, that is, the base potential of the transistor 35. Here, the reason why the optical output LM in the intermediate state is controlled to stabilize the optical output is that it is superior to controlling the maximum output LH in the following points. Here, the relationship between the exposure amount E and the laser light output D shown in FIG. 11 will be described.

(1) In order to obtain stable output, the maximum optical output should be at point A, where the ED curve is saturated.

(2) E-D shown in Figure 11 for environmental changes
Even if the curve changes as shown by the dotted line, the position of the maximum output is not easily affected. On the other hand, the intermediate density position B is greatly affected.

Therefore, it is desirable to fix the maximum light output LH with a constant current and control the intermediate light output LM.

In this way, the three-value optical output L0, LM, LH
Therefore, when an image pattern as shown in FIG. 7 is drawn, if the fine pixels of the intermediate tone can be configured correctly, the amount of change in surface potential for each tone will be approximately constant. However, as shown in FIGS. 2 and 3, gradation levels 4 to 5 are obtained by changing the configuration of fine pixels.
As shown in FIGS. 13A and 13B, the surface potential may change discontinuously.
Therefore, the surface potential on the photosensitive drum 16 is measured using a surface potential sensor 23 shown in FIG. 5 to control the light output. In addition, when measuring the surface potential,
Since it is necessary to extract the potential on the photosensitive drum 16 as a macroscopic surface potential distribution in units of one pixel, the sensor 23 is designed so that it cannot resolve portions of minute pixels. Therefore, it is necessary to draw the image pattern over a relatively large area.

FIG. 12 shows a block diagram for controlling the optical output described above, in which the surface potential output from the surface potential sensor 23 (see FIG. 5) is amplified by an amplifier 50 and then digitized by an A/D converter 51. , the surface potential information is taken into the memory 54 from an input port controlled by the central processing unit CPU53. This series of procedures is performed by a drum clock obtained from a drum clock generator 52 in synchronization with the rotation of the photosensitive drum 16, and surface potential information averaged for each tone of the halftone image is sequentially stored in a memory 54.
be incorporated into In other words, using the test pattern signal for image adjustment (pattern that produces gradation),
The semiconductor laser 12 (see FIG. 9) is driven, and the latent image potential is sequentially taken into the memory 54. Now, assuming that the image spot is not appropriate, Fig. 13A,
As in B, the continuity of each gradation is poor. 13th
Figure A shows 0 to 4 when the intermediate optical output LM is too weak.
The potential change up to 4 is slight, but it changes rapidly from 4 to 5. Figure 13B shows intermediate light output.
LM too strong (but less than maximum light output)
There is little difference between cases 4 and 5.

Therefore, in order to obtain the optimum light output, the 13th
The base potential of the transistor 35 shown in FIG. 9 is controlled to obtain the optimal intermediate optical output LM from the surface potential changes represented by 0 to 4 in FIGS. A and B and the surface potential changes represented by 5 to 8. do it. This can be done by following the steps below.

FIG. 14 is a flowchart showing the procedure for making each gradation uniform. First, find the average value ₀ , ₁ , ..., _N of the potentials measured for each gradation. 2×
For a matrix of 2, _N = ₈ . Therefore, each difference ΔVi (i=1, 2, . . . , 8) is determined. Next, the average of each of the first half and the second half where fine pixels of intermediate brightness appear is calculated. That is, ΔV _A = 1/4 ₄ 〓 ⁱ⁼¹ ΔVi ΔV _B = 1/4 ₈ 〓 ⁱ⁼⁵ ΔVi Here, if the difference between ΔV _A and ΔV _B is minute and smaller than the tolerance range ε, each floor When the difference in key is relatively uniform, the movement can be continued in that state.
This tolerance range ε can be determined depending on the accuracy of the device.

On the other hand, if the difference between ΔV _A and ΔV _B is larger than the allowable range ε, it is necessary to correct it. The amount to be corrected is
When ΔV _A −ΔV _B >0, the intermediate optical output LM is decreased; when ΔV _A −ΔV _B <0, the intermediate optical output LM is increased; |ΔV _A −ΔV _B | It should be controlled so that it is proportional. That is, the current i _A supplied to the semiconductor laser 12 for the intermediate optical output LM is controlled so that it becomes i _A 〓i _A −α·(ΔV _A −ΔV _B ).

Here, α is a positive proportionality constant and may be determined according to the characteristics of the device.

The control of the current value determined by the above procedure is carried out in the 12th
It is converted into an analog by the D/A converter 55 shown in the figure.
It is executed based on the output voltage value output from the terminal 39. This output voltage is input to terminal 39 in FIG. 9 to control the collector current of transistor 35. Further, in the above-described control, the constant α may be determined to be optimal according to the characteristics of the device, the visual characteristics of the human, and the like. In the case of the electronic photograph mentioned above,
Since various characteristics may be nonlinear, and proximity effects may appear depending on the size of the pixel, the size of the micropixel, etc., it is preferable to determine it based on experiments.

Note that the above-described control needs to be performed in advance before outputting the image. This control may be performed, for example, immediately after the power is turned on and when the drum rotation speed reaches a constant number, and does not need to be performed every time. Alternatively, it may be performed manually as needed. At this time, it is not necessarily necessary to transfer the image pattern shown in FIG. 7 onto paper and take it out, but the image pattern may be stored and controlled. Furthermore, in the above example, the surface potential was measured and the collector current of the current source transistor was automatically controlled to obtain an intermediate optical output. The intermediate tone may be adjusted by changing the base current of the transistor. In such a case, a knob such as a variable resistor for adjustment may be arranged in advance on the outer frame of the apparatus so as to be operable from the outside, and a test chart image may be outputted and adjustments made as necessary.

As explained above, according to the present invention, the latent image potential of the photoreceptor is actually detected, the beam intensity is corrected, and the gradation is corrected. It is possible to reproduce halftone images.

Note that the above explanation was given using three values as an example, but
Of course, a desired halftone image can be obtained using a similar method using four or more values of brightness. Moreover, although the present invention has been explained above mainly using a laser beam printer as an example, the present invention is not limited to this, and can be effectively applied to printing using a facsimile machine, a printing machine, etc.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the normal organized dither method, Figure 2
The figure and FIG.
FIG. 4 is a block diagram showing an example of the halftone image output device of the present invention, FIG. 5 is an explanatory diagram of the image forming process, and FIG. 6 is an exposure diagram. A diagram showing the relationship between the quantity E and the surface potential V, FIG. 7 is a diagram showing a standard image pattern showing 10 levels of brightness, FIGS. 8 and 13 A,
B is a diagram showing the relationship between the number of filled-in fine pixels and the surface potential, FIG. 9 is a circuit diagram for driving a semiconductor laser that obtains three-level optical output, FIG. 10 is a timing chart of the waveforms of each part, and FIG. 11 is a diagram showing the relationship between the exposure amount E and the laser light output D; FIG. 12 is a block diagram for controlling the semiconductor laser shown in FIG. 9;
FIG. 14 is a flowchart showing the procedure for making each gradation of brightness uniform. 10... Pixel, 11... Fine pixel, 12... Semiconductor laser light source, 13... Collimator lens, 14... Light deflector, 15... Imaging lens, 16... Photosensitive drum, 17
...Primary charger, 18...Static eliminator, 19...Developer, 2
0... Paper, 21... Laser beam, 22... Exposure lamp, 23... Sensor, 31, 32, 39... Terminal, 3
3 to 38...transistor, 50...amplifier, 51...
A/D converter, 52... drum clock generator, 5
3...CPU, 54...Memory, 55...D/A converter.

Claims (1)

[Scope of Claims] 1. Threshold values of the first and second dither matrices are assigned to common input pixel information, gradation processing is performed on the input pixel information, and the first and second dither A fine pixel matrix corresponding to the matrix is formed, and each fine pixel in the fine pixel matrix is reproduced in three or more gradations using a first density and a second density higher than the first density. A halftone image output device that outputs a halftone image, comprising: a beam generating means for generating a beam to form the fine pixels on a photoreceptor; and an input means for inputting a density signal representing the gradation of the fine pixels. , modulating the beam generated by the beam generating means based on the concentration signal input from the input means, and modulating the beam to generate beams with first and second intensities corresponding to the first and second concentrations. a modulating means for switching the driving current value of the generating means; a means for forming an electrostatic latent image on the photoreceptor based on the beam modulated by the modulating means; and a fine pixel matrix formed on the photoreceptor. and a detection means for detecting the potential of the electrostatic latent image, the detection means detecting the first gradation formed by the first density micropixels without using the second density micropixels. micropixels of a second gradation, which are formed by the potential of the first electrostatic latent image corresponding to the micropixel matrix and the micropixels of the second density, and are the next gradation of the first gradation; The configuration is such that the potential of the second electrostatic latent image corresponding to the matrix can be detected, and the first intensity of the beam is changed to
A drive current that determines the first intensity of the beam based on the potentials of the first and second electrostatic latent images detected by the detection means in order to correct the potential difference between the second electrostatic latent images. What is claimed is: 1. A halftone image output device comprising: calculation means for calculating and determining a value.