JPH0241266A - Image forming device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus such as a laser beam printer, and particularly to an improvement in a technique for compensating for deviations in scanning speed between the main scanning direction and the sub-scanning direction. Regarding.

[Prior Art] In image forming apparatuses, especially laser beam printers,
The scanner motor for driving the polygon mirror that scans the laser beam and the motor that drives the photoreceptor on which the laser scans are driven while maintaining a constant speed relationship. That is, the scanning of the laser beam corresponds to main scanning, and the rotational driving of the photoreceptor corresponds to sub-scanning.

[Problems to be Solved by the Invention] However, relative speed deviations tend to occur in both of the above-mentioned motors, and this causes irregularities in the formation of laser dots, resulting in uneven dot formation. This unevenness is called "pitch unevenness." This pitch unevenness causes problems such as deterioration of the image quality, and for example, in a color image, three or four colors are overlapped, and the pitch unevenness covers multiple colors, resulting in an image that is difficult to see.

The present invention has been made in view of the above-mentioned problems of the prior art.The purpose of the present invention is to provide an image forming apparatus that forms an image using a light beam even when a deviation occurs in the scanning speed in the main scanning direction and the sub-scanning direction. , proposes an image forming apparatus that does not cause deterioration of reproduced images.

[Means for Achieving the Object] The configuration of the present invention for achieving the above object includes a modulating means for modulating an input image signal into a light beam for image formation;
main scanning means for scanning the light beam in the main scanning direction; sub-scanning means for causing the light beam to scan relative to the medium to which the light beam is irradiated in the sub-scanning direction; The present invention is characterized by comprising a calculation means for calculating the correlation between the scanning speed and the scanning speed in the sub-scanning direction, and a correction means for correcting the power of the light beam based on this correlation.

A speed deviation between the scanning speed in the main scanning direction and the scanning speed in the sub-scanning direction is compensated by correcting the power of the light beam.

[Examples] Examples of the present invention will be described in detail below with reference to the accompanying drawings. In this embodiment, the present invention is applied to a color copying machine having a color separation plate output mode. First, the overall picture of this embodiment will be explained, and then the pitch unevenness removal method and configuration used in this embodiment will be explained.

<Outline of Apparatus Configuration> FIG. 2 is an internal sectional view of a color copying apparatus 8o of this embodiment using a contact type color 〇〇D sensor.

The copying device 80 includes a reader section 100 and a printer section 2000.
It is composed of. Reference numeral 83 denotes a document scanning unit, which scans the image of the document 84 on the document table by moving the arrow A.
At the same time, the document scanning unit 83
The exposure lamp 85 inside is turned on. The light reflected from the original is
The light is guided by a focusing rod lens array 86 and focused on a contact type color CD sensor 87 . Close-contact color CCD
The sensor 87 has 5 chips of 1024 pixels arranged in a staggered manner, each pixel being 62.5 μm (1/16 mm), and each pixel has 3 chips of 15.5 μm x 62.5 μm.
Divided into C (cyan), G (green), and Y (yellow) respectively.
Color filters are pasted.

The optical image focused on the contact type color 〇〇D sensor 87 is
Each color is converted into an electrical signal. These electrical signals are subjected to predetermined processing, which will be described later, by the image processing block 88. The color separation image electrical signals formed by image processing block 88 are sent to printer 2000 for printing.

The color image data from the league club 100 is PWM (
After processing (binarization using pulse width modulation), etc., it is finally driven by a laser. The laser light modulated in accordance with the image data is scanned at high speed by a polygon mirror 2289 rotating at high speed, and is reflected by a mirror 2290 to expose dots corresponding to the image on the surface of the photosensitive drum 2900.

One horizontal scan of the laser beam corresponds to one horizontal scan of the image,
In this embodiment, the width is 1/16 mm. On the other hand, since the photosensitive drum 2900 is rotating at a constant speed in the direction of the arrow, a planar image is sequentially exposed by the above-described laser beam scanning in the main scanning direction and by constant rotation of the photosensitive drum 290o in the sub-scanning direction. The photosensitive drum 2900 is charged with a charger 22 prior to exposure.
97, and a latent image is formed by exposing the charged photoreceptor to light. A developing device 22 corresponding to a predetermined color for a latent image based on a predetermined color signal.
92-2295.

For example, considering the first original exposure scan in Color League, a dot image of the yellow component of the original is first exposed onto the photosensitive drum 2900 and developed by the yellow developer 2292 .

Next, this yellow toner image is transferred onto the paper wound on the transfer drum 2296 by a transfer charger 2298 at the contact point between the photosensitive drum 2900 and the transfer drum 2296. By repeating the same process for M (magenta), C (cyan), and Bk (black) and superimposing each image on the paper, a color image using four color toners is formed.

Next, the functional block configuration of the color copying apparatus of the embodiment will be explained with reference to FIG. Figure 3A is League Division 10
FIG. 3B shows the configuration of the printer section.

In FIG. 3A, reference numeral 1 denotes a synchronization signal processing unit which synchronizes with the horizontal synchronization signal 22 output from the gradation control circuit 2160 based on the signal from the BD (beam detector) detector 2220 of the printer unit 2000. Create various timing signals. 2 is a close contact type COD sensor block,
The league horizontal synchronization signal (RH
SYNC) and drive signal 4, the original is read, the image signal is converted into an electrical signal, and the electrical signal is output. 3 is a signal processing section that performs waveform shaping processing to prevent high frequency components of the electrical signal 5 from attenuating.

6 is an image processing block, and the image signal from the signal processing section 3 is first input to the analog processing section 7. In the analog processing unit 7, since the signal from the contact type COD sensor block 2 is configured to sequentially output cyan (C), green (G), and yellow (Y) signals for each pixel, first, C,G,
Separate each color of Y. Next, since each developer in the printer section 200 supports yellow (Y), magenta (M), and cyan (C), the image signals are first processed into red (R), green (G), and
Convert to blue (B) signal. This is CG=B.

This is performed by arithmetic processing according to the formula Y-G=R. Also, these R
, G, and B, the output voltage of which varies linearly with the concentration, so the A/D converter converts the signals into 8
It is converted into a bit density signal. The above processing is executed by the analog processing section 7.

The image signals for each color digitized by the analog processing section 7 are divided into five channels, and the video signals of each channel are not synchronized.

Therefore, the data are combined into one image data by the connection memory 8. It is synthesized by the connection memory 8 and Y
The image data converted into MC signals is sent to an image processing unit (IPU) 9 in synchronization for each color. IPU9
Then, shading processing to correct light distribution, masking processing to correct color tone, etc. are performed. In addition, league part 100
A desired color signal is selected by the control section 10 of the image forming apparatus 1, and is sent to the printer section 2000 from the IPO 2 through the 8-bit image data 11 that has been subjected to a predetermined color conversion process.

Further, apart from the -blade side image signal, the control unit 10 drives a motor driver 13 for operating the original document to control the rotation of the motor 12. In addition, a CV that controls lighting of the exposure lamp 14
It also controls the R15 and "copy" keys and the operating section 16 for performing other operations.

Here, to make the text sharp and the photo to have gradation,
Although not shown, it has a switching mode SW, and the control section 10 takes in the switch output as information from the operation section 16 and transmits it to the printer.

On the other hand, the control section of the printer controls a selector of a binarization circuit, which will be described later, using a signal from the CPU 2110.

The image data 11 output from the league section 100 is input to the gradation control circuit 2160 of the printer section 2000.

The gradation control circuit 2160 has a function of making it correspond to the image clock of the printer section 100 and the color reproduction density of the printer section 2000.

Output data from the gradation control circuit 2160 is input to the laser driver 2200, which drives the laser 2300 to form an image.

The control unit 2500 of the printer unit that communicates with the league unit 100 via the communication control line 24 is connected to the printer unit 2000.
Controls each control element. 2600 is a photoreceptor 290
potential sensor for detecting zero-charged charge, 27
00 is a potential measurement unit that converts the output from the potential sensor 2600 into a digital signal and inputs it to the control section 2500.

The potential data input to the control unit 250o is
It is read by the CPU 2501 of 00 and used for control. On the other hand, a signal from the sensor 2800 for detecting the image leading edge signal (ToP) is input to the control unit 2500 and used for control. Further, the outputs of the humidity sensor 2298 and temperature sensor 2299 for correcting the development characteristics are inputted through the A/D converter 25o3 of the controller 2500.

Overall configuration of image processing> FIG. 4 shows the image processing unit 216 in the printer 2000.
0 is shown. This will be explained below with reference to the drawings.

The image data read from the reader section 100 is sent to the printer 2000 as 8-bit image data 11. In the printer 2000, the 8-bit video data 11 is stored in γ-RAM (LUT) for halftone processing.
2101 , and then subjected to halftone processing in a halftone processing circuit 2102 to be halftone dotted according to a predetermined algorithm, and becomes six inputs of a selector 2103 . Here, γ−R
In the AM 2101, when image data halftone-dotted by the halftone processing circuit 2102 is outputted by an electrophotographic process, table conversion is performed to convert the image data to a desired halftone rate.

Note that the halftone processing will be described later. Video data 11, on the other hand, becomes the B input of selector 2103 for times when halftone processing is not required. The selector 2103 selects either the halftone processing data or the raw video signal from the league based on the select signal 2123 from the CPU 2110. The raw video signal or halftone-processed video signal selected by the selector 2103 is further input to one input A of the selector 2104. The other B input of the selector 2104 is O″Y, 11 fMJl fC, jrK, It from the font ROM 2108 controlled by the font control circuit 21o9.
This is font data such as Ir#J.

An outline of this font data output method will be explained. C
The PU 2110 sets the output position of the font image in the image image, that is, the main scanning direction address, the sub scanning direction address, and the type of character symbol in the font control circuit 21Q9. These six types of position information are known.

After this, when the copying operation starts, the paper leading edge signal I
A counter inside the font control circuit 2109 starts counting in synchronization with the TOP and horizontal periodic signal RH3YNC. When the addresses in the sub-scanning direction and the main scanning direction reach the output address of the font set above, the control circuit 2109 accesses the font ROM 2108, and the R
The font data of the OM 2108 is output to the input B of the selector 2104. As mentioned above, the character to be output is 1
Since the characters are already selected, at this time, font R
The output from the OM 2108 is multivalued (for example, 8 bits) font data for one pixel for each pixel in the character output area.

When font data is being output from the ROM 2108, a control signal SEL for selecting font data from the font ROM 2108 is output from the font control circuit 2109 to the select input of the selector 2104.

Next, the 8-bit output image signal selected and outputted by the selector 2104 is sent to the buffer memory (
FIFO) 2105. That is, the selector 21
04 selects the output of the selector 2103 at times other than the character symbol output timing, and selects the output of the font ROM2.108 at the character symbol output timing.

The image data stored in the buffer memory 2105 is read out from the buffer memory in synchronization with the HSYNC and CLK signals from the synchronization control unit 2113. As a result, the period shift and speed conversion between the reader unit 100 and the printer 2000 are performed, and the results are input to the γ-RAM (LUT) 2106. In this γ-RAM 2106, the buffer 21o5
The 8-bit image signal from is corrected so that the density becomes linear in accordance with the output characteristics of the printer. The converted 8-bit image signal is converted into an analog signal by a D/A converter 2107, and is input to one input terminal of each of comparators 2117 and 2118. A pattern signal for binarizing the video signal (PWM modulation) is input to the other input terminal of each of the comparators 2117 and 2118. Since the pattern signal input to the comparator 2117 is for line drawing and halftone dot generation, resolution is important, so each pixel is It is a signal that changes periodically. Furthermore, since the pattern signal input to the comparator 2118 is a pattern signal for a halftone image, it is necessary to increase the gradation, and therefore the frequency is 1/2 that of the pattern signal for line drawing signals (200 lines in this example). The settings are as follows.

Next, the binarization using the pattern signal described above will be described.

Photoreceptor (not shown) is scanned by laser beam.
As is well known in a digital copying machine that scans the image to obtain a copy by an electrophotographic process, a synchronization signal in the main scanning direction is obtained by the BD detection circuit 2111. This BD double signal, a master clock signal from the crystal oscillator 2112 with a frequency four times or more higher than the image signal, and an ITOP signal, which is a vertical synchronization signal in the sub-scanning direction, are input to a synchronization control circuit 2113, and a horizontal synchronization signal is H3YNC, vertical synchronization signal ITOP, and video clock (VCLK) are synchronized. Furthermore, clocks 2124 and 2125 for pattern generation signals are also simultaneously connected to HSYNC and ITO.
Synchronized with P, a clock with twice the frequency of the video signal is output to 2124, a clock with 2/3 times the frequency of the video signal is output to 2125, and a pattern signal with a duty ratio of 50% is output by the frequency divider circuit of 2114. CLK. This 50
The % duty clock is input to a pattern generation circuit 2115 and a pattern generation circuit 2116. Pattern generation circuits 2115 and 2116 generate analog pattern signals with a predetermined period. This pattern signal is, for example, a triangular wave pattern signal. These two pattern signals are input to the input terminals of comparators 2117 and 2118, and compared with the analog video signal converted by the D/A converter 2107, resulting in two pulse width modulation signals 2126 and 2127, which are input to the selector 2119, respectively. is input to the input terminal of

The selection signal to the selector 2119 is selected so that the input signal of the A terminal is output from the output terminal Y when the original is a line drawing or halftone-processed video data. On the other hand, if the original is in halftone, the input signal of the B input terminal is selected to be output.

As described above, the video signal selected according to the image tone of the original is input to the laser driver 2121 with the output position matched with the output paper by the gate circuit 2120. The semiconductor laser 2122 is driven with a constant current for an ON time corresponding to the pulse width of the video signal, and a photoreceptor (not shown) is scanned to obtain a copy original through an electrophotographic process.

The above is an overview of the entire copying apparatus according to this embodiment.

<Removal of pitch unevenness> FIG. 1 is an overall diagram of a removal system for removing pitch unevenness as described in the prior art section in the copying machine shown in FIG. 2. The feature of pitch unevenness removal in this embodiment is that when it is detected that the rotation speed of the scanner motor 2300 that rotationally drives the polygon mirror 2289 and the rotation speed of the photosensitive drum 2290 are not synchronized in a predetermined relationship,
The purpose is to correct input video data 11.

In FIG. 1, 2160 is an image processing unit (gradation control circuit in FIG. 2) that receives the video signal from the signal line 11 from the reader unit 100, and 2161 is the image processing unit that receives the video signal 11 from the signal line 11 from the reader unit 100. This is a conversion table (lookup table RAM) for correcting input video data 11. Reference numeral 2162 is a circuit unit for performing image modulation for expressing density, and corresponds to the fourth metal body. 2200 is a laser driver for controlling laser lighting in response to the image signal output from the modulation circuit 2162; 2223 is a laser driver 2200;
This is a laser that emits laser light based on the signal. 228
9 is a polygon mirror for scanning laser light;
2300 is a scanner motor for rotating the polygon mirror. 2301 is attached to the shaft of the scanner motor, detects the rotational speed of the motor 2300, and outputs a signal ECD.
RL is the output encoder. 2302 is a scanner driver circuit for rotating the scanner motor 2300.

2900 is a photoreceptor that performs laser scanning using a polygon 2289. 2296 is a transfer drum that superimposes the photoreceptor images; 2285 is a motor that drives the photoreceptor 2900;
2304 is a rotational speed encoder attached to the photoreceptor.

2305 is a motor driver circuit for controlling the drive motor 2285. 2800 is an ITOP sensor for detecting the reference position (home position) of one rotation of the transfer drum.

Now, to explain the detailed blocks of the motor driver circuit 2305, 2308 is a frequency divider that receives the input of the encoder output E CD RL of the scanner motor and performs frequency division. 2309 is a counter that starts counting with the reference signal of the ITOP signal 2800 and counts the output of the frequency divider. 2307 is a counter for locking the PLL with the output of the frequency divider 2308 and the output encoder 2304 of the drive motor 2285. This is a PLL circuit. 2309 is a counter that counts the encoder signal ECDRM of the drive motor encoder 2304; 2310 is a counter that counts the encoder signal ECDRM of the drive motor encoder 2304;
This is a circuit for comparing the counter signals of counter 09 and counter 2311. A latch 2312 latches the difference between the comparator 2312. 2313 is the latch 2312
represents the output data path of

The speed control of the photoconductor drive motor 2285 in the photosensitive mode is performed as follows.

The scanner motor encoder output E CD RL output from the scanner motor driver 2302 is the frequency divider 230
The frequency is divided by 8. Since the encoder signal ECD RL is considerably higher in frequency than the same encoder signal E CD RM, if ECDRL is divided appropriately, if the polygon mirror motor 2300 and photoreceptor motor 2285 are rotating synchronously, E CD Rv period and frequency divider 230
Since the period of the output of 8 should match, the phase difference represents the amount of synchronization deviation.The PLL circuit 2307 detects this phase difference, further amplifies this phase difference, and sends it to the driver 2306. Output. In this way, PLL00
A path 2307 controls the speed of the photoreceptor motor 2285.

Regarding the video data, even if the PLL circuit 2307 can detect a rotational deviation, it cannot correct the signal dropout that occurs due to it. In this embodiment, if a rotational deviation occurs between both motors, the photosensitive Since the laser power per unit dot irradiated onto the body fluctuates, the laser power is corrected according to the rotational deviation. In this case, it is conceivable to vary the output of the laser 2223 itself in accordance with the rotational deviation, but since this poses a problem in responsiveness, in this embodiment, the density of the video data is corrected.

The operation performed by motor driver circuit 2305 for correcting this video data will be explained.

As described above, the counter 2309 receives the ITOP signal and then counts the output of the frequency divider 2308. Similarly, the counter 2311 counts the E CD RM after inputting the ITOP signal. Since the difference between the two counts reflects the rotational deviation, this difference is calculated by the comparator 231.
Calculate with 0. At this time, the comparator 231O outputs a signal if a borrow or carry occurs.

FIG. 5 shows the format of the output of comparator 2310. If the dividing ratio of the 8 divider which means the above-mentioned borrow or carry is 1/n, bit B/C is B/C" 1 (ECDRL/n > EC
DRM)B/C=O(ECDRL/n>
ECDRv). These data are stored in latch 2
312. Latch 2312 outputs these data to image processing circuit 2160. At this time, counters 2309 and 23118. The reference counter is cleared at tITOP2800.

Now, based on the output of the latch 2312, the input video data 11 is corrected by the LUT 2161 of the image processing circuit 2160. The principle of this correction will be explained.

If the linear velocity around the photoreceptor 2290 by the drive motor 2285 is υ1, and the laser scanning speed υ, then t/L CCE CD RL V, CCECDRIJ. On top of that, if , the rotational speed of the photoreceptor 2290 is slower, so the laser energy per hour irradiated onto the photoreceptor becomes larger. On the other hand, if , the energy becomes smaller. Appropriately υL/
After normalizing υ, when α・(υL/υv),=t, the energy of the laser light per time is also normalized to "1", and the horizontal axis shows υL/υ, (= speed difference),
When the vertical axis represents the laser power, the relationship between speed difference and power is as shown in FIG. 6A.

In other words, the characteristics shown in FIG. 6A are such that when the speed difference becomes large,
That is, as υ becomes larger than υ, the laser power also tends to become relatively larger. In other words, the curve slopes upward to the right.

Therefore, in order to compensate for fluctuations in laser power due to rotational deviation, if υL/υ〉1 is a normalized value, then the laser power will be relatively large, so the dot of the laser beam will be The video data 11 is corrected by the LUT 2161 so as to make it smaller. Conversely, if υL/v&l<1, the laser power becomes relatively small, so the video data 11 is corrected by the LUT 2161 so as to make the laser beam dot larger. For this purpose, the serial video data 11 is transferred to the shift register 2.
163 into a parallel value.

FIG. 6B shows an example of an input/output curve of the LUT 2161.

If υL/υ=a (<1), take the top curve above. Then, since the laser output takes a value higher than the normal level vL/υ,=1, the power becomes effectively stronger. Conversely, when υL/υv=b (>1), the output is smaller than the input because it is below the normal curve, so the effective power becomes smaller.

In the actual circuit shown in Figure 1, the normal curve (υL/υM=1
), whether to take the curve above or below the
It may be determined by the B/C bits mentioned above. Also, the 6th B
In addition to the three characteristics shown in the figure, many characteristic curves can be changed depending on the value of the B/C bit (“O” or “1”).
may be provided in each LUT 2161, and the DI
The larger the value of F, the more normal the curve (υL/
What is necessary is to select a curve whose characteristics are far from υ2=1). That is, as shown in FIG. 5, B/C (upper address) and DIF (
(lower address).

In this way, pitch unevenness due to rotational deviation between the polygon mirror motor and the photoreceptor motor can be compensated for.

<Another Example of Pitch Unevenness Removal> FIG. 7 shows another example, and only the differences from FIG. 1 will be described.

The control unit 2500 receives data from the output latch 2312 of the motor driver 2305 and outputs selector data corresponding to the data through a signal line 2314. The output address is given to the address of LUT2161, and the 6th
Data is varied and controlled as shown in Figure B.

Further, 2165 is a selector in the image processing circuit 2160, which switches the address of the CPU when there is no signal in the video data 11 from the control unit 2500 via the control line 2314. 2166 is CPU data and LUT216
This is a selector for switching one output data. The role of this selector is that when there is no signal in the VIDEO signal 11, that is, when there is no PRINT signal, the parameters instructed by the scanning unit 16 are transmitted from the control unit 10 to the control unit 250o by communication, and the LUT of 2161 is This allows the internal data to be changed.

By the way, 2161 in Figure 1 is ROM, and Figure 7 is RAM.
The configuration is shown below.

Although the embodiments described above have been explained using color LBPs, it is obvious that they can be applied to ordinary black and white LBPs. or,
In LED printers and LCD printers, each LED
, and may be controlled by providing an α curve control circuit in the modulation circuit of the liquid crystal. The same applies to thermal transfer printers and inkjet printers.

Further, although the comparator 2310 is used to detect the speed deviation, a divider may also be used.

Furthermore, the video data values (“1” and “0”) and the on/off state of the laser beam can be freely combined. That is, in the above embodiment, when the rotational speed of the motor on the polygon mirror side becomes relatively high, the gradation correction by LUT 2161 increases the gradation density; ” and “0”) and the on/off state of the laser beam, the opposite may occur.

[Effects of the Invention] As described above, the present invention includes a modulation device that modulates an input image signal into a light beam for image formation, a main scanning device that scans the light beam in the main scanning direction, and a modulation device that modulates the input image signal into a light beam for image formation. sub-scanning means for causing the medium to be irradiated with the light beam to scan relative to the light beam in the sub-scanning direction; a calculation means for calculating the correlation between the scanning speed in the main-scanning direction and the scanning speed in the sub-scanning direction;
and a correction means for correcting the power of the light beam based on this correlation.

That is, the speed deviation between the scanning speed in the main scanning direction and the scanning speed in the sub-scanning direction is compensated by correcting the power of the light beam. Therefore, even if a speed deviation occurs, there is no deterioration in the reproduced image.

According to one aspect of the present invention, the power correction of the light beam is performed by correcting the gradation density of the input image signal.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the entire system for pitch unevenness compensation according to the present embodiment, Fig. 2 is an internal perspective view of the color copying machine, Fig. 3A is a circuit block diagram of the reader section of the embodiment, and Fig. 3B is A circuit block diagram of the printer unit of the embodiment. Figure 4 is an overall block diagram of image processing in the printer unit. Figure 5 is a diagram showing the relationship between speed deviation data and LUT. Figure 6A is a diagram showing the relationship between relative speed and laser power. FIG. 6B is a diagram showing an example of the characteristics of the LUT, and FIG. 7 is a diagram showing the configuration of another embodiment. Patent applicant: Canon Co., Ltd. Figure 3B Figure 6A Figure 6B

Claims (3)

[Claims] (1) A modulation device that modulates an input image signal into a light beam for image formation, a main scanning device that scans the light beam in a main scanning direction, a medium to which the light beam is irradiated, and the light beam. sub-scanning means for relatively scanning in the sub-scanning direction; calculation means for calculating the correlation between the scanning speed in the main-scanning direction and the scanning speed in the sub-scanning direction; An image forming apparatus characterized by comprising a correction means for performing correction. (2) The image forming apparatus according to claim 1, wherein the correction means corrects the power of the light beam by correcting the gradation of the image signal input to the modulation means. (3) When the scanning speed in the sub-scanning direction is relatively high with respect to the scanning speed in the main-scanning direction, the correction means adjusts the power of the light beam in the direction in which the power per unit time becomes relatively large.
The image forming apparatus according to claim 2, wherein the image forming apparatus corrects gradation density.