JP3303427B2 - Digital image forming equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to gradation control of a digital image forming apparatus in a digital printer, a digital copying machine and the like.

[0002]

2. Description of the Related Art In an electrophotographic process in a digital printer, a digital copying machine, or the like, an image is reproduced by modulating laser light emission in accordance with an image reading density (multi-value digital value). In image reproduction, it is desirable that the density of an output image is proportional to the image reading density. The gradation characteristic, which is the relationship between the output image density and the image reading density, is a factor that largely affects the impression of a pictorial image. Therefore, the input image density is processed, and the emission characteristics are corrected so that the density of the formed image is proportional to the input image density. This is called gradation correction. In color image reproduction, it is basically required that an output image linearly changes in image density, and therefore, image stabilization is required. The gradation characteristics are the photoreceptor sensitivity of the electrophotographic process,
Subtle changes occur due to changes in surface potential, development bias potential, development characteristics, and the like. Therefore, image reproduction is stabilized by automatic density control, gradation correction, and the like. In a digital image forming apparatus, the read image density is converted into a multi-valued digital value. However, since the non-linear conversion of the multi-valued data is easy by a look-up table process or the like, the digital image forming apparatus requires various stable values. (For example, an apparatus described in Japanese Patent Application Laid-Open No. 3-271664 by the present applicant).

[0003]

However, in reality, since image stabilization is not perfect, the quality of reproduced images by stabilization control may not be satisfactory to some professional users. Therefore, it is considered that if the user can positively change the gradation characteristics, the user can be satisfied, and the user can output an image with a desired gradation. In order to allow the user to change the gradation characteristics, it is necessary to operate the gradation correction and the image stabilization system in conjunction with each other, and a process control system corresponding to each image generation process is required. However, the gradation characteristics also change depending on the gradation expression method of the electrophotographic process itself. For example, in the laser intensity modulation method, the light intensity of one dot of a laser beam is modulated according to an image signal. Here, when the unit of light emission is set to a plurality of dots instead of one dot, or when the duty ratio during the light emission period is set to be smaller than 100% of the normal state, the toner adhesion state depends on each setting condition. Changes, and the image quality also changes. The gradation characteristics are also affected by the color of the toner. In order to allow the user to arbitrarily select the gradation characteristics, it is necessary to cope with the case where there are various factors on the electrophotographic process side that affect such gradation expression.

An object of the present invention is to enable a user to select a gradation characteristic even in a digital image forming apparatus that performs various gradation expressions.

[0005]

Means for Solving the Problems] digital image forming apparatus according to the present invention, the image type by gradation representation having a periodicity
Forming a digital image forming apparatus for performing a first selection means for user tone characteristic of the target from a plurality of gradation characteristics to select, a desired plurality of parameters related to the period p
A second selecting means for the user to select the meter , a setting means for setting the basic tone characteristic according to the parameter relating to the cycle selected by the second selecting means, and a basic tone characteristic set by the setting means. Originally, a calculating means for calculating gradation correction data for obtaining a target gradation characteristic selected by the first selecting means, and converting an image signal based on the gradation correction data obtained by the calculating means. And a tone correcting means for outputting.

[0006]

In the digital image forming apparatus, a target gradation curve can be selected. Further, a plurality of gradation expression methods can be selected corresponding to various gradation expressions by different light emission conditions, different colors, and the like. On the other hand, in the image density control system, a gradation correction curve corresponding to the electrophotographic image reproduction condition is stored. Then, in response to the selection of the target gradation characteristic and the gradation expression method by the user, during image formation (when the electrophotographic image reproduction condition is changed by the image stabilization system), the gradation corresponding to the electrophotographic image reproduction condition is changed. Tone correction characteristics can be switched.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings in the following order. (A) Configuration of digital color copier (b) Printer control unit and image signal processing (c) Image stabilization (d) Gradation control (e) Gradation selection (f) Multiple gradation expression patterns (g) ) Calculation of tone correction data (h) Tone setting in color image (i) Printer control flow

(A) Configuration of Digital Color Copier FIG. 1 is a sectional view showing the overall configuration of a digital color copier according to an embodiment of the present invention. Digital color copying machines are broadly divided into an image reader unit 100 for reading a document image and a printer unit 200 for reproducing an image read by the image reader unit. Image reader unit 100
Scanner 10 for irradiating a document,
A rod lens array 13 for condensing reflected light from the original,
And a contact-type CCD that converts the collected light into electrical signals
The sensor 14 is provided. The scanner 10 is driven by the motor 11 when reading a document, moves in the direction of the arrow (sub-scanning direction), and scans the document placed on the platen 15. As shown in FIG.
Are controlled by the image reader control unit 101. The image reader control unit 101 controls the exposure lamp 12 via the drive I / O 103 in accordance with a position signal from the position detection switch 102 indicating the position of the document on the platen 15, and also controls the drive I / O 103 and the parallel I / O 103.
The scan motor driver 105 is controlled via / O104. The scan motor 11 is driven by a scan motor driver 105.

Returning to FIG. 1, the image on the document surface irradiated by the exposure lamp 12 is photoelectrically converted by the CCD sensor 14. The multi-valued electrical signals of the three colors red (R), green (G), and blue (B) obtained by the CCD sensor 14 are as follows:
Yellow (Y), magenta,
The data is converted into 8-bit gradation data of any one of (M), cyan (C), and black (K), and output to the printer control unit 201. As shown in FIG. 3, the image control unit 106
D color image sensor 14 and image signal processing unit 20
Are connected to each other by a bus. The image signal from the CCD color image sensor 14 is input to the image signal processing unit 20 and processed. As shown in FIG. 2, in the image signal processing unit 20, the image signal photoelectrically converted by the CCD sensor 14 is converted by the A / D converter 21 into R, G, B multi-value digital image data. To
Each of them is subjected to shading correction in the shading correction circuit 22. Since the shading-corrected image data is the reflected light data of the original, the log data is converted by the log conversion circuit 23 into density data of an actual image. Further, the undercolor removal / black printing circuit 24 removes unnecessary black color development and generates true black data K from R, G, B data. Then, the masking processing circuit 25 converts the data of the three colors R, G, and B into data of the three colors Y, M, and C. The density correction circuit 26 multiplies the converted Y, M, and C data by a predetermined coefficient in the density correction circuit 26, and the spatial frequency correction processing in the spatial frequency correction circuit 27. Output as a density signal.

Returning to FIG. 1, in the printer unit 200, the print head unit 31 performs gradation correction on input image data in accordance with the gradation characteristics of the photosensitive member, and then performs correction. The subsequent image data is D / A converted to generate a laser diode drive signal, and the semiconductor laser 264 (FIG. 4) is caused to emit light by this drive signal. The laser beam generated from the print head unit 31 by modulating the light emission intensity corresponding to the image data passes through a polygon mirror,
Photoreceptor drum 41 that is driven to rotate via reflection mirror 37
Is exposed. The photoreceptor drum 41 is irradiated by an eraser lamp 42 before receiving exposure for each copy, and is uniformly charged by a charger 43. When exposure is performed in this state, an electrostatic latent image of the document is formed on the photosensitive drum 41. Only one of the cyan, magenta, yellow, and black toner developing units 45a to 45d is selected to develop the electrostatic latent image on the photosensitive drum 41. On the other hand, the copy paper is fed from a paper cassette 50 and wound on a transfer drum 51. The developed toner image is transferred to copy paper by a transfer charger 46. The above printing process is repeated for four colors of yellow (Y), magenta (M), cyan (C) and black (K). At this time, the photosensitive drum 41 and the transfer drum 51
The scanner 10 repeats the scanning operation in synchronization with the above operation. Thereafter, the copy paper is separated from the transfer drum 51 by operating the separation claw 47, is fixed through the fixing device 48, and is discharged to the discharge tray 49.

(B) Printer control unit and image signal processing FIGS. 3 and 4 are block diagrams showing the entire control system of the digital color copying machine. The printer unit 200 includes a printer control unit 201 that controls general printing operations. A printer control unit 201 having a CPU includes a control ROM 202 storing a control program and a data ROM 20 storing various data (such as gradation correction data).
3 and the RAM 204 are connected. Printer control unit 2
Reference numeral 01 controls the printing operation based on the data in the ROM and RAM. The printer control unit 201, V ₀ sensor 44, AIDC sensors 210, ATDC sensor 21
1. Analog signals from the temperature sensor 212 and the humidity sensor 213 and signals from the fog input switch 214, the color balance switch 216, and the photoconductor lot switch 218 are input. Here, the V ₀ sensor 44 detects the potential of the photoconductor surface. Also, the AIDC sensor 210
For each color, the toner amount of the reference toner image on the photoconductor developed under the standard image forming conditions (photoconductor surface potential V ₀ , developing bias potential V _B , exposure amount) is detected, and V ₀ , V _B , exposure Set volume to optimal conditions. Various data are input to the printer control unit 201 via the parallel I / O 222 by key input on the operation panel 221. Similarly, an input value in the tablet editor 232 (see FIG. 13) described later in detail is input to the printer control unit 201.

The printer control unit 201 controls the copy control unit 231 and the operation panel 221 in accordance with the contents of the control ROM 202 in accordance with various input data, and further controls the charging charger 43 via the parallel I / O 241 and the drive I / O 242. V _G generated for the high-pressure unit for generating the grid potential V _G 243 and the developing unit V _B for generating a high-pressure unit 2 for generating a developing bias potential V _B of 45a~45d
44 is controlled. The printer control unit 201 is also connected to the image signal processing unit 20 of the image reader unit 100 by an image data bus, and will be described later based on an image density signal input via the image data bus. The light emission level is determined by referring to the contents of the data ROM 203 storing the gradation correction table, and the drive I / O 26
1 and the parallel I / O 262 to control the semiconductor laser driver 263. The light emission of the semiconductor laser 264 is driven by the semiconductor laser driver 263. The gradation expression is performed by modulating the emission intensity of the semiconductor laser 264.

FIG. 5 shows the appearance of the operation panel 221.
Here, the LCD display unit 301 displays the mode set by the operation, explains the operation procedure to the user, and displays a status such as a jam display and a copy operation display. A panel reset key 302 is a key for initializing all modes. A key 303 is a numeric key for setting the number of copies and a clear key for clearing. The start key 304 is a key for instructing the start of copying. Image quality menu key 305
By pressing, a menu for image quality adjustment is displayed on the LCD display unit 301. The user can adjust the image quality by operating this. Create menu key 30
When 6 is pressed, a setting menu for various create functions is displayed on the LCD display unit 301. The user can perform various function settings and mode settings by operating this. An enter key 307 is used as an enter key and a next screen key in each operation screen of the image quality menu and the create menu. River Ski 30
Reference numeral 8 is used as a cancel key and a previous screen key.
A cursor key 309 is a key for selecting a menu and setting a level on each operation screen. The multi function key 310 is
Is a key whose meaning can be changed depending on each selection menu displayed in the menu. This panel has IC card insertion ports 311 and 312, and up to two IC cards can be inserted at the same time. A program call / registration key 313 and an IC card ejection key 314 are provided corresponding to each insertion slot. The barcode reader pen 315
It is possible to perform various mode settings by reading a barcode using.

FIG. 6 is a block diagram of image data processing in the printer control unit 201. Here, the image data (8 bits) from the image signal processing unit 20 is input to a first-in first-out memory (hereinafter, referred to as a FIFO memory) 252 via the interface unit 251. The FIFO memory 252 is a line buffer memory capable of storing gradation data of a predetermined number of lines of images in the main scanning direction, and absorbs a difference in operation clock frequency between the image reader unit 100 and the printer unit 200. It is provided in order to. Next, the data in the FIFO memory 252 is input to the gamma correction unit 253. Data ROM2
03 various γ correction data are sent to the γ correction unit 253 by the printer control CPU 250, and the γ correction unit 253 corrects the input data and sends the light emission level to the D / A conversion unit 254. The data ROM 203 stores various tone correction data. The analog voltage converted from the light emission level (digital value) by the D / A converter 254 is
In the gain switching section 255, the switches SW1, SW2,... (Corresponding to different powers P1, P2,...) Are switched and set by the gain switching signal generation circuit section 256 in accordance with the gain set value from the printer control section 201. After being amplified by the gain, the signal is sent to the semiconductor laser driver 263 via the drive I / O 261 and the semiconductor laser 26
4 emits light at the light intensity of that value. On the other hand, the printer control CPU 250 sends a signal to the light emission signal generation circuit 257 to generate a clock signal indicating a dot cycle and a duty ratio of light emission, which will be described later, and sends it to the semiconductor laser driver 263 via the parallel I / O 262. The image data is modulated by the clock. Thereby, a gradation expression pattern is selected.

(C) Image stabilization The gradation characteristics basically include the sensitivity characteristics, development characteristics,
Further, it is determined by setting the charging potential V ₀ , the developing bias potential V _B , and the attenuation potential V _S of the electrostatic latent image. In color image reproduction, it is basically required that the output image linearly changes to the document density, and therefore, image stabilization is required. Although the present invention allows a user to select a gradation characteristic, a gradation control system operates in conjunction with an image stabilization system and can always supply the selected gradation characteristic stably. Must be something.

Before describing image stabilization, an outline of the electrophotographic process will be described. FIG. 7 schematically shows the arrangement of the charging charger 43 and the developing unit 45r around the photosensitive drum 41. Here, the photoreceptor 41, a charger 43 for discharge potential V _G is installed to face. The grid of the charging charger 43 includes a grid potential generating unit 24.
Negative grid potential V _G is applied by 3. Relationship between the grid potential V _G and the surface potential V ₀ photoreceptor drum,
Because regarded as substantially V ₀ = V _G, potential V ₀ at the photosensitive drum 41 surface can be controlled by the grid potential V _G. The surface potential V ₀ is detected by a V ₀ sensor 44 which is a surface voltmeter. First, before the laser exposure, a negative surface potential V ₀ is applied to the photosensitive drum 41 by the charging charger 43, and a low negative developing bias potential V ₀ is applied to the roller of the developing unit 45 r by the developing bias generating unit 244. _B (| V _B | <| V ₀ |). That is, the developing sleeve potential of the developing device 45r is V _B. The potential of the irradiation position on the photosensitive drum 41 is reduced by the laser exposure, and the potential changes from the surface potential V ₀ to the attenuation potential V _S of the electrostatic latent image. When decay potential V _S is lower potential than the developing bias voltage V _B, toner charged negatively came carried on the sleeve surface of the developing device 45r is deposited on the photosensitive drum 41.

Here, if the difference between V ₀ and V _B is too large, the carrier adheres to the non-exposed area, and if it is too small, toner fogging occurs. The larger the developing potential difference ΔV = | V _B −V _S |, the larger the toner adhesion amount. On the other hand, the attenuation potential V _S changes as the surface potential V ₀ changes even with the same exposure amount. Therefore, if the surface potential V ₀ and the developing bias potential V _B are changed while maintaining the difference between V ₀ and V _B within a certain range, for example, while keeping the difference constant, the difference between V _B and V _S changes. Therefore, the amount of toner adhesion can be changed, and the density can be controlled (for example, see Japanese Patent Application Laid-Open No. 3-271664). The gain of the lasing, V ₀ sensor 44
Is switched according to the sensitivity information of the photoreceptor obtained by the above.

In addition, since the electrophotographic process handles static electricity, it is affected by the environment. For this reason, mainly the development characteristics and the photoconductor characteristics change, so this compensation is necessary. Therefore, the AIDC sensor 210 detects the amount of toner developed for each of the four colors under standard image forming conditions. That is, a reference toner image, which is the basis of density control, is formed outside the image area on the photosensitive drum 41, and the toner amount is detected by the AIDC sensor 210 provided near the photosensitive drum 41. In response to this detection value, by changing the developing bias potential V _B and grid potential V _G, by selecting the developing potential difference ([Delta] V), by performing automatic density control to keep the amount of toner adhesion at a maximum concentration level constant Can be. Also, background fog must be removed.

(D) Tone Control Next, a description will be given of standard tone correction in which the value of an input image signal and the density of an actually printed image are linear.
Particularly, a color image is required to have basically linear characteristics. FIG. 8 is a diagram of sensitometry in reversal development. Image input level OD from image reader
Is output linearly with respect to the document density. When the laser emission amount P (Lx) is linearly changed with respect to the image input level value Lx, the gradation characteristics (the relationship between the output image density ID and the image input level OD) become non-linear. The surface potential V _S of the photoconductor is attenuated in response to the laser emission. In other words, the surface potential gradually and non-linearly attenuates as the amount of laser emission increases. Further, the developing bias potential V
_B is determined with respect to the photoconductor charging potential V ₀ so as to remove background fog, and the development potential difference (V _B −V _S (L
Although in response to x)) output image density ID (V _S) is that obtained, the developing property, also shows the nonlinearity. Therefore, instead of linearly changing the light emission amount P of the laser, the respective non-linearities of the photoconductor characteristics and the development characteristics are corrected so that the output image density becomes linear with respect to the input level, which will be described later. The light emission characteristics are corrected in a non-linear manner.
This allows the output image density to be linear with respect to the image input level.

FIG. 9 is a diagram showing how to obtain gradation correction data for realizing standard gradation characteristics for making the output image density ID linear with respect to the image input level OD. When the image input data is directly converted to laser exposure as shown on the lower side without any conversion and exposed, the gradation curve shown on the upper side (relationship between the image input level and the output image density) is shown as a broken line. Becomes nonlinear. The emission characteristic for converting this into a target gradation curve shown by a solid line is a curve shown by a solid line in the figure. That is, to convert the point A on the dotted line (image input level L ₁ ) into the point A ′ on the solid line, the laser exposure amount P at the point A ″ on the broken line having the same output image density as the point A ′
(L ₁ ) may be output corresponding to the input image data L ₁ . Similarly, in order to convert the point B on the dotted line into the point B 'on the target solid line, the laser exposure amount P (L ₂ ) at the point B "on the broken line having the same output image density as the point B' is output. Thus, the laser exposure amount corresponding to the image input level, that is, the gradation correction data is obtained.

(E) Selection of Gradation In the above description, a standard gradation curve for outputting an image density linearly with respect to input image data in order to obtain an output faithful to a document image has been described. In this embodiment, the user can select a different gradation curve in addition to the standard linear gradation curve. In selecting the target gradation curve,
In this embodiment, in order to facilitate selection by the user,
The gradation curve is specified by inputting at two levels, that is, the shape of the gradation curve and the level of the shape.

FIG. 10 schematically shows the concept of the shape of the target gradation curve and the stage of the change in the shape. The following four types (a) to (d) can be considered as combinations of selection of the gradation curve based on the relative relationship with the standard curve. Also,
By changing the step (level) of each shape change, an infinite gradation curve can be realized. In the low concentration emphasis type (a),
Make the gradation curve convex upward. Using this gradation curve gives a solid feeling. In the high density emphasis type (b),
Make the gradation curve convex downward. By using this gradation curve, a pastel feeling can be obtained. Also, it is possible to correct a dark image as a whole. In the halftone density part emphasizing type (c), the gradation curve has a large upward convex on the high level side, but has a small convex downward on the low level side. When this gradation curve is used, a feeling such as “colorful” or “sharp” is obtained. In the halftone density part non-emphasized type (d), the gradation curve is
It is small and convex upward on the low level side, but largely convex downward on the high level side. By using this gradation curve, feelings such as "moist" and "smooth" can be obtained. The point where the gradation curves of the halftone density portion emphasized type (c) and the halftone density portion non-emphasized (d) intersect a linear straight line may be the same as, for example, the case of the standard tone curve (FIG. 8). .

Next, a specific method of selecting these gradation curves will be described. First, the operation panel 2 shown in FIG.
The selection of the gradation curve according to 21 will be described. In the setting of the operation panel 221, the selection is made by two-stage input of selection of the gradation curve and the degree of change of the shape. By operating the key 306, a screen for selecting a gradation curve is called on the display unit 301. FIG. 11 is a diagram of a selection screen displayed on the display unit 301. On the selection screen, the standard tone curve and the tone curves of the respective types shown in FIG. 10 are words representing the characteristics of the tone curve (“standard”, “smooth”, “colorful”, “bright” , "Heavy"). A desired gradation curve is selected from among the displayed five gradation curves by a key 310 provided below the display unit 301. When one of the gradation curves is selected by the key 310, words ("weak", "standard", "strong") indicating the degree of change of the gradation curve are displayed on the display unit as shown in FIG. Displayed at 301. The user sets the degree of change by selecting one of them with the key 310 as before. That is,
The change degree can be selected from three levels, "strong"
Is selected, a gradation curve of the type selected in FIG. 11 is selected, and a gradation curve farther from the standard gradation curve shown in FIG. 10 is selected. When the closest gradation curve is selected, and when "standard" is selected, an intermediate gradation curve is selected.

In the above selection, after selecting the type of the desired gradation curve, a display prompting the user to input the level of the gradation curve is output. When no level is input, the level is set to the standard target level "0". The curve is selected. Also, the gradation curve can be selected by the tablet editor 232 shown in FIG. FIG. 13 shows the appearance of the tablet editor 232. In the coordinate input unit 320, a position on a document can be designated by pointing using the coordinate input pen 321. As a result, partial editing designation of various editing functions can be performed. Also, this coordinate input unit 3
20 is provided with a group of keys 322 and 323 for mode setting. That is, the mode setting keys 322 and 323, the gradation curve setting unit 324, and the color pallet 325 are printed on the coordinate input unit 320.
It can be used as a mode setting section and level setting section. The mode setting keys 322 and 323 are keys for setting various modes, respectively, and can be set by pressing them with the coordinate input pen 321. Therefore, it is also possible to select the type and degree of the gradation curve using the key groups 322 and 323.

(F) Plurality of gradation expression patterns In this embodiment, in the laser intensity modulation method, the dot period and the duty ratio of laser emission can be selected to enable a plurality of gradation expression patterns. I have. The reason is as follows. In gradation expression by the laser intensity modulation method, the light intensity of one dot of a laser beam is modulated according to an image signal. This method basically has high resolution and enables smooth gradation expression. However, the laser intensity modulation method has the following problems. (1) Pitch noise in the sub-scanning direction unique to laser exposure is easily reproduced. (2) Even the noise of the image data of the image reader is faithfully reproduced. (3) The non-linearity of the gradation characteristic is likely to increase.
Further, in color copying, a pictorial image is often used as a document, so that image quality and noise are particularly important.

For this reason, in the present embodiment, in the intensity modulation method, a light emission period of N dots in the main scanning direction (light emission is possible) and a non-light emission period in which light emission is not performed are provided so that N can be changed. As a result, the image density is reduced during the non-emission period, the pitch noise peculiar to laser exposure is not noticeable, and the smoothness of the image is improved. Even at the same duty ratio X of light emission per time, when the light emission cycle N is increased, the pattern of the vertical line is more strongly felt.

Further, the entire light emitting period (= light emitting period +
Light emission duty ratio X, which is a time ratio to the non-light emission period)
(%) Can also be selected. By setting N and X, the image quality of re-development can be set according to the user's preference. That is, the duty ratio X is the time ratio of the light emitting period in which the semiconductor laser 34 can emit light to one period in one cycle composed of consecutive N dots. Corresponding to the duty ratio X, in the main scanning direction, the amount of toner is large in the center of the dot (light emission period), and is small in the gap between dots (non-light emission period). When this is continuously exposed and developed in the sub-scanning direction, a vertical line pattern (gap) is formed. As the duty ratio X of light emission is reduced, the gap becomes clearer, resulting in a remarkable vertical line pattern. As described above, the laser intensity modulation method has pitch noise in the sub-scanning direction, which is peculiar to the sub-scanning direction.However, since there is a gap in the vertical line direction, the noise in the sub-scanning direction is less noticeable, and a smooth image as a whole is obtained. can get. When the duty ratio is reduced while the light emission intensity remains the same, the amount of toner adhesion decreases. Therefore, the emission intensity is increased as the duty ratio decreases. This compensates for the decrease in image density. The printer control CPU 250
The number of dots N and the emission duty ratio X are determined by the emission signal generation circuit 2
57, and generates a clock corresponding to the gradation expression pattern selected by N and X.

As an example, FIG. 14 shows that in the gradation expression pattern (N, X), X = 80% and the dot period N is 1
4 shows a light emission state at a certain halftone density when changing from to 4. Here, (a) shows the light emission signal when N = 1, and (b) shows the light emission intensity of the laser. here,
The laser emission intensity is controlled to be slightly higher than the laser intensity at the time of 100% emission (X = 100%) indicated by the broken line according to the 80% duty ratio. As a result, (a) of FIG.
As shown in (1), a gap is formed at the center of the dot with a relatively large amount of toner and a small amount of toner between the dots. When this exposure is continuously performed in the sub-scanning direction and developed, a vertical line pattern is formed.

FIGS. 14C, 14D and 14E show N =
The emission signals in the cases of 2, 3, and 4 are shown. Thus, the period N
Are increased, (b), (c), and (d) of FIG.
As shown in the figure, the pattern of the vertical line is strongly felt by human eyes, noise in the sub-scanning direction is not noticeable, and smooth image quality can be obtained as a whole. Similarly, FIG. 16 shows a light emission state at a certain halftone density when X = 60% and the dot period N changes from 1 to 4 in the gradation expression pattern (N, X). FIG. 17 shows a toner adhesion state corresponding to this.

Normally, one dot period (N = 1), 100
By modulating the light emission intensity with the light emission duty ratio of% (X = 100%), 256 gradations are expressed. 1
In the example, the N dot cycle can be selected in four stages of 1 to 4 dots, and the duty ratio (X%) of light emission can be selected in three stages of 100%, 80%, and 60%. In order to facilitate selection for the user, it is set so that continuous smoothness can be obtained subjectively by combining (N, X). This pattern condition is shown in Table 1. Thus, the standard conditions (S
It is possible to select nine gradation expression patterns including (N, X) of (TD) = (1, 100).

[0031]

[Table 1]

The laser emission intensity P is set by the following equation. P = (P ₀ / X) × 100 × α where P ₀ is the light emission intensity of normal (STD), α
Is set to make a gap well. When the period N is small, if the intensity is too high, no gap is formed between dots, and a vertical line pattern cannot be formed.
, And approaches 1 as N increases. It is also possible to provide the upper limit of the P ₀ increases.

FIG. 18 shows the operation panel 221 shown in FIG.
5 shows an example of a selection key on the sub operation panel 340 provided on the side of the sub-operation panel. Initially, the standard (STD) (N,
X) = (1,100) is selected, and the LED display 334 below the STD mark is turned on. The user can use the increase key 32
By pressing the 7 or decrease key 326, the number of dots (pitch) can be selected in four steps, and the increase ratio 329 or the decrease key 328 can be selected to select the duty ratio between 60% and 80%. Further, for resolution selection, normal (no processing), smooth 1 (average processing), and smooth 2 (anamo processing) keys 330, 331, and 332, which will be described later, are provided, and LEDs corresponding to the selected keys are provided. The display lights up. The STD key 330 operates in the standard mode (N = 1,
X = 100%). By default,
Normal is set.

As described above, the degree of smoothness can be changed by switching the pattern by selecting the gradation expression pattern (N, X). However, in digital copying, the MTF is further corrected in the processing of image data. Processing (spatial filtering) is possible. In the present invention, in order to form a vertical line pattern in the main scanning direction at an N dot cycle,
The resolving power and smoothness of the reproduced image differ depending on whether or not the image data of N dots is subjected to the smoothing process and output. In the present invention, smoothing is basically performed only by pattern control without performing smoothing processing. However, the smoothing method can be selected and the resolution can be selected. A smoother image can be obtained by the smoothing process in the pattern smoothing processing unit 27 described below.

FIG. 19 shows an example where N = 4 dots and X = 80%. (A) shows image data D1, D2, D3,... Sequentially obtained in the main scanning direction. Here, the light emission signal is generated at a duty ratio of 80% in a 4-dot cycle. Therefore, assuming that the basic period of one dot is T,
The light emission signal is output during the period of 3.2T and is not output during the next 0.8T. Here, (b) shows the laser exposure level and the pulse when the image data is output without being smoothed ("normal" in FIG. 18). Here, in a 4-dot cycle, the image data D1,
D2 and D3 are output as they are, but in the fourth dot, a pulse is output only for a period of 20%.

(C) shows the laser exposure level and the pulse when the averaging process is performed on the image data of four dots ("smooth 1" in FIG. 18). As a result, a laser exposure level obtained by averaging the four dot image data is output. For example, for the first four dots, an average value (D1 + D2 + D3 + D4) / 4 is obtained, and this value is output for a period of 3.2T. As a result, a smoother image is obtained, but of course, the resolution is reduced. On the other hand, in (b), basically 4 × 0.8 =
The image for 3.2 dots is the read image data (a)
Is faithfully reproduced, so that there is little decrease in resolution. Further, in the anamo smoothing process (“smooth 2” in FIG. 18) illustrated in (d), the value of each dot is the same,
The period of each dot is shortened according to the duty ratio.
That is, the data of each dot is output in a period corresponding to the duty of 80%. For example, each of the image data D1, D2, D3, and D4 is continuously output for a period of 0.8T, and no signal is output for the remaining period.

(H) Tone setting in color image In the case of a color image, one copy is sequentially processed in the order of cyan, magenta, yellow, and black. Therefore, the selection of the gradation curve and the calculation of the gradation correction are repeated for each color. Thus, a gradation curve is set for each color. As a result, different gradation conversions can be performed for each color, so that the color tone can be corrected according to the density level. Further, the gradation correction color balance for each color can be stored, and delicate color adjustment that cannot be set by the entire color balance processing can be performed. In addition, this setting
204 can be stored so that it can be used repeatedly. Also,
The user may be able to select all color batch processing for converting all colors with the same target gradation curve and the above-described division processing for each color. In the all-color batch processing, it is not necessary to input the same target tone curve many times.

(G) Calculation of Gradation Correction Data As described above, the gradation characteristics slightly differ depending on the setting of the gradation expression pattern (N, X) and the color. Then, N, X
The gradation characteristic (γ characteristic) is changed in accordance with the selected value or color. However, in order to store all of a large number of tone correction data for such various tone expression patterns,
Requires large storage capacity. Therefore, only a small number of basic data (original tone curve) is stored, and tone correction data is calculated according to the selection of tone. FIG. 20 is a diagram illustrating the concept of gradation control according to the present embodiment. In this embodiment, divided into levels of toner detected amount 28 by AIDC sensor 210 for each color, the original data before correction respectively 28 pairs of selected corresponding to each level (V _G, V _B) (The upper broken line in FIG. 9) is stored. Also, since the relationship between the adhesion amount and the density differs depending on each color and each gradation expression pattern 0, 1, 2, ..., 8, each gradation expression pattern 0, 1, 2, ..., 8 is associated with cyan, magenta, yellow, black. The gradation characteristic data before correction is stored for each of the colors. Therefore, 9 × 28 × 4 original tone curves are stored in the data ROM 203 in advance. On the other hand, FIG.
In addition, a total of 13 types of gradation curves, ie, three stages of each of the four types of gradation curves shown in FIG.
03 is stored. The original data before correction is stored as data normalized to 0 to 1 (× the number of operation bits) for the input level I of 255 levels. Similarly, the target curve is set to 0 to 1 with respect to the input level L of each of the 255 levels.
It is stored as data normalized to (× the number of operation bits). In this embodiment, the original gradation curve is
At each level of the AIDC sensor 210, a tone curve when printing is performed by linearly converting the value of an input image signal into an exposure amount is stored.

First, (A) the user operates the operation panel 221.
To set the gradation curve by using
A tone expression pattern is set from 0. The setting may be performed by the tablet editor 232. On the other hand, (B) detection value levels 1-2 of the AIDC sensor 210
The original tone curve corresponding to the set tone expression pattern 8 is called. Next, (C) tone correction data is calculated from the called original tone curve and the set tone curve in the same manner as in the case of the standard tone curve in FIG.

FIG. 21 is a graph showing a procedure for calculating the gradation correction data. In FIG. 21, a certain AIDC
The original tone curve at the level is indicated by. Is a non-linear gradation curve when the laser exposure amount is linear with respect to the image input level OD (the straight line indicated by the lower dotted line).
Called by OM203. Further, denotes a gradation curve set by the operator from the operation panel 221. Next, a procedure of calculating gradation correction data for realizing the set gradation curve using these two gradation curves will be described. The case of the image input level L11 will be described. When the image input level L11 is given, it is necessary to print at the image density indicated by the point C 'in the set gradation curve. The point on the original tone curve that reproduces the same image density as point C ′ is point C ″, and it can be seen that this point C ″ is reproduced when the laser exposure amount is P (L11). Accordingly, when the image input level L11 is given and printing is performed with the laser exposure amount of P (L11), printing is performed with the gradation characteristics indicated by the set gradation curve. In this manner, conversion is performed for other image input levels, and tone correction data indicating the relationship between the image input level and the laser exposure amount for realizing the set tone curve is calculated. (D) The correction curve thus obtained is stored in the RAM 20
4 and are reused when the same settings are made.

(I) Printer Control Flow FIGS. 22 to 24 show the main flow of the printer control unit 201. First, after performing initial setting (S1), key input processing of the operation panel 221 is performed, and a gradation expression pattern (S2), a gradation curve shape (S3) and a degree (S1).
4) Enter. At that time, each code is set according to the input contents. Then, the operation panel 221
Wait for the start key 304 to be pressed (S
5). When the start key 304 is pressed, a sensor input process is performed (S6). Next, input signals from various switches of the operation panel 221 are taken into the RAM in the printer control unit 201 (S7). Next, step S4
The gain of the gain switching circuit 255 in FIG. 5 is switched according to the setting values obtained in S5 and S5, and the light amount level of the semiconductor laser 264 is set (S8). Next, AIDC measurement processing is executed, and the toner adhesion amount is obtained by the AIDC sensor 210 (S9). In the AIDC process, a reference toner image is formed on a photoreceptor, and the image reproduction density is determined based on the toner adhesion amount of the reference toner image.
10 and the RAM in the printer control unit 201
Take in 204. Thereafter, copying of cyan, magenta, yellow, and black is performed.

Next, it is determined whether or not the copy is cyan (S11). If it is determined that the copy is cyan,
Next, based on a density detection level corresponding to the measured cyan toner adhesion amount, a grid potential correction value, a development bias potential correction value, and a code of an original gradation curve which are set in advance corresponding to this level are selected. (S12, see FIG. 25). Next, the original gradation curve is called from the code selected in steps S2 and S12, and the gradation correction data is obtained by the above-described method so that the target gradation curve selected in steps S3 and S4 is obtained. (S13, see FIG. 19). Next, the developing bias potential V and the grid voltage V _G which is selected in step S12
_{Based on B} and the gradation correction data calculated in step S13, a copying operation by a known electrophotographic method is executed (S14). Next, it is determined whether the copying has been completed (S15). If the copying is not completed, the process proceeds to S21, and if completed, the process returns to S3 to perform the next copying.

Next, it is determined whether or not the copy is magenta (S21). If it is determined that the copy is a magenta copy, the grid potential correction value and the developing bias potential set in advance corresponding to this level are then determined based on the measured density detection level corresponding to the measured magenta toner adhesion amount. Select the correction value and the code of the original gradation curve (S2
2, see FIG. 25). Next, the original gradation curve is called from the code selected in steps S2 and S22, and the gradation correction data is obtained by the above-described method so that the target gradation curve selected in steps S3 and S4 is obtained. Is calculated. Next, to perform the copy operation by known electrophotographic method based on the gradation correction data obtained by calculating the grid potential V _G and the developing bias potential V _B and step S23 which has been selected in step S22 ( S24). Next, it is determined whether the copying has been completed (S25). If the copying has not been completed, the process proceeds to S31, and if completed, the process returns to S3 to perform the next copying.

Next, it is determined whether or not the copy is a yellow copy (S31). If it is determined that the copy is a yellow copy, a grid potential correction value and a developing bias potential set in advance in accordance with the density detection level corresponding to the measured yellow toner adhesion amount are determined based on the measured density detection level. Select the correction value and the code of the original gradation curve (S3
2, see FIG. 25). Next, the original gradation curve is called from the code selected in steps S2 and S32, and the gradation correction data is obtained by the above-described method so that the target gradation curve selected in steps S3 and S4 is obtained. Is calculated (S33, see FIG. 19). Next, the above step S
Executing a copying operation by known electrophotographic method based on the gradation correction data obtained by calculating 32 the grid potential V _G selected by a developing bias voltage V _B and the step S33 (S34). Next, it is determined whether the copying has been completed (S35). If the copying is not completed, the process proceeds to S41, and if completed, the process returns to S3 to perform the next copying.

If it is determined in S31 that the image is not a yellow copy, the image is a black copy. Then, based on the density detection level corresponding to the measured black toner adhesion amount, a preset value corresponding to this level is set in advance. The selected grid potential correction value, developing bias potential correction value, and original tone curve code are selected (S41, see FIG. 25). Next, the original gradation curve is called from the code selected in steps S2 and S42, and the gradation correction data is obtained by the above-described method so that the target gradation curve selected in steps S3 and S4 is obtained. (S42, FIG. 1)
9). Then, developing the grid potential V _G selected in the step S41 the bias potential V _B and Step S42
A copying operation by a known electrophotographic method is executed based on the gradation correction data obtained by the calculation in (S43). Since the copying has been completed, the process returns to S3 to perform the next copying. Note that an upper limit of P0 increase may be provided.

[0046] Figure 25 is, V _G, V _B, the gradation data selection (FIG. 2
3S12, S22, FIG. 24, S32, S41). Grid voltage V _G on the basis of the detected density level of AIDC sensor 210, selects a developing bias voltage V _B (S81). Then, select the Motokaicho curve corresponding to _{(V G, V B) (} S82), the process returns. In the present embodiment, the gradation curve is stored for each type and color of the selected pattern, and the calculation is performed in accordance with each. However, only the pattern type may be used, or the pattern may be located between the colors. If there is a relative relationship, tone data may be stored for each pattern, and data may be corrected by processing in a fixed relationship according to the color difference.

[0047]

According to the digital image forming apparatus for performing various gradation expressions, the gradation characteristics can be selected by the user. Here, since different gradation conversion can be performed for each different gradation expression (for example, color), it is possible to correct the color tone and the like according to the density level.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an entire configuration of a digital color copying machine.

FIG. 2 is a block diagram of an image signal processing unit.

FIG. 3 is a block diagram of a part of a printer control unit.

FIG. 4 is a block diagram of a part of a printer control unit.

FIG. 5 is a perspective view of an operation panel.

FIG. 6 is a block diagram of image data processing in a printer control unit.

FIG. 7 is a diagram schematically illustrating an arrangement of a charging charger and a developing device around a photosensitive drum.

FIG. 8 is a diagram of sensitometry in reversal development.

FIG. 9 is a diagram showing how to obtain standard tone correction data.

FIG. 10 is a diagram schematically showing the concept of the shape of a target gradation curve and the stage of the shape change.

FIG. 11 is a diagram of a screen for selecting a type of a gradation curve.

FIG. 12 is a diagram of a screen for selecting a level of a gradation curve.

FIG. 13 is a diagram of a panel of a tablet editor.

FIG. 14 is a diagram illustrating a light emission state at a certain halftone density when X = 80% and the dot cycle N changes from 1 to 4.

FIG. 15 is a diagram illustrating a toner adhesion state corresponding to FIG. 14;

FIG. 16 is a diagram showing a light emission state at a certain halftone density when X = 60% and the dot cycle N changes from 1 to 4.

FIG. 17 is a diagram illustrating a toner adhesion state corresponding to FIG. 16;

FIG. 18 is a diagram illustrating an example of a selection key for gradation expression.

FIG. 19 is a diagram showing various smoothing methods.

FIG. 20 is a diagram showing data stored for gradation correction.

FIG. 21 is a diagram illustrating calculation of light emission data.

FIG. 22 is a flowchart of calculation of light emission data.

FIG. 23 is a diagram of a part of a main flow of the printer control unit.

FIG. 24 is a diagram illustrating a part of a main flow of the printer control unit.

FIG. 25 is a diagram of a part of a main flow of the printer control unit.

[Explanation of symbols]

201: Printer control unit, 203: Data ROM, 2
10: AIDC sensor, 232: Tablet editor, 253: γ correction unit, 322, 323: Mode setting key.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yoshinobu Hata Osaka International Building Minolta Camera Co., Ltd. 2-3-13 Azuchicho, Chuo-ku, Osaka-shi, Osaka In-house (72) Inventor Hiroyuki Deyama Azuchi, Chuo-ku, Osaka-shi, Osaka 2-3-1, Machimachi, Osaka International Building Minolta Camera Co., Ltd. In-house (56) References JP-A-4-96571 (JP, A) JP-A-4-40752 (JP, A) JP-A-62-97474 (JP) JP-A-2-268074 (JP, A) JP-A-3-163958 (JP, A) JP-A-1-151370 (JP, A) JP-A-60-125064 (JP, A) 61-53874 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60

Claims (1)

(57) [Claims] 1. An image forming method using a gradation expression having periodicity.
Forming a digital image forming apparatus for performing a first selection means for the user to select the tone characteristics of the target from a plurality of gradation characteristics, a desired plurality of parameters related to the periodic parameter
A second selection means for the user to select the data, and setting means for setting the reference grayscale characteristics according to the parameter <br/> for the selected period in the second selection means, basal set by the setting means Calculating means for calculating tone correction data for obtaining a target tone characteristic selected by the first selecting means based on the tone characteristics; and an image signal based on the tone correction data obtained by the calculating means. And a gradation correcting unit for converting and outputting the converted image.