JP2958290B2 - Image processing method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gradation correction table for an image forming apparatus which forms given image data on a medium.
The present invention relates to an image processing method for adjusting an image. 2. Description of the Related Art In this type of apparatus, density characteristics and gradation characteristics of an output image are important evaluations. As a method of forming a halftone image based on halftone image data, a dither method, a density pattern method, or PWM based on multi-valued image data is used.
A method of performing multi-level output with modulation or the like is adopted.
In addition, as the image quality at that time, it is required that the image quality be faithful to the original image quality, or that the original image quality be enhanced or weakened so as to be processed to a more easily viewable image quality. A typical device of this type includes, for example, a reader unit for reading and inputting a document image and a printer unit for outputting the read and input image signal on a recording material.
Alternatively, some printers are configured as only a printer unit that outputs image information artificially formed by computer graphics or the like onto a recording material. In such an apparatus, each of the reader unit and the printer unit has its own density and gradation characteristics, whereby the density characteristics and gradation characteristics of a reproduced image or an output image are greatly changed. Especially,
In the printer section, there are an electrophotographic system, an ink jet system, a thermal transfer system, and many other systems, and the output characteristics differ greatly depending on each system.
Further, in the electrophotographic system, its output characteristics are largely influenced by minute variations in characteristics of a photosensitive member and toner used for development. Under such circumstances, a technique of forming a reference image for output adjustment by a printer unit has been conventionally known. [0005] However, conventionally,
Recognize the status of the image forming apparatus based on the output adjustment result.
And couldn't. The present invention relates to gradation correction of an image forming apparatus.
Make table adjustments automatic
And the image format based on the obtained gradation correction table.
An object of the present invention is to make it possible to recognize the state of a forming device . The present invention has, for example, the following constitution as one means for achieving the above object. That is, an image forming apparatus that forms given image data on a medium
When adjusting the tone correction table, the gradation correction table
A reference image for adjusting the Le is formed on the image forming apparatus, resulting read a reference image which is the formed image
Signal, and a gradation correction table based on the image signal.
And means for displaying the contents of the gradation correction table . [0007] For example, in the gradation correction table,
The display of the contents is performed at the time of maintenance .
Alternatively, based on the gradation correction table, the image
A means for performing a self-diagnosis process for determining the state of the forming apparatus
It is characterized by doing. In the above configuration, the gradation correction of the image forming apparatus is performed.
Make table adjustments automatic
And the image format based on the obtained gradation correction table.
It can recognize the state of the forming device. Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. <Embodiment 1> FIG. 1 is a block diagram of an image forming apparatus according to Embodiment 1 of the present invention, in which the present invention is applied to an electrophotographic digital copying machine. Here, A is a reader unit, and B is a printer unit. A normal copying operation according to the configuration shown in the figure is as follows. First, the original 1 is read by the reading element of the reader unit A.
(CCD) 2 to convert the original image into an analog electric signal. The electric signal is transmitted to an amplifier (AMP) 3
, And 8 bits by the A / D converter 4 (= 256 gradations)
, And input to the RAM 10 for use gradation correction via the latch 5. Further, the RAM 10
D / A converter 1 of the printer unit B
3 and converted into an analog image signal, and the signal is input to the pulse width modulation circuit 14. FIG. 2 is a diagram for explaining an example of the pulse width modulation operation. In the figure, “image signal” is an analog image signal output from the D / A converter 13, and “pattern signal” is a triangular wave signal generated in the pulse width modulation circuit 14. The "image signal" and the "pattern signal" are synchronized as shown in the figure, and a pulse signal obtained as a result of level comparison of these by a level comparator in the circuit 14 is a PWM signal output from the pulse width modulation circuit 14. is there. The above is an example of the pulse width modulation operation, and is suitable for relatively high-speed image signal processing. On the other hand, when the image signal is relatively slow, D / A
Even without using the converter 13, for example, a pulse width modulation signal can be directly generated from the digital image signal by generating a digital pattern signal sufficiently faster than the digital image signal and comparing these digitally. Further, the PWM of the output of the pulse width modulation circuit 14
The signal is amplified by the laser driver 15 and the laser generation circuit 1
6 to control ON / OFF of the laser beam. The laser light emitted from the laser generation circuit 16 is irradiated on the photoreceptor 19 via an optical system including a polygon mirror 17 and an f-θ lens 18 and the like. The photoreceptor 19 is uniformly charged by the corona charger 20, and then receives the above-mentioned laser exposure to form an electrostatic latent image on the surface. After the electrostatic latent image is visualized by the developing device 21, it is transferred onto a transfer material 27 by a transfer charger 24, and the transfer material 27 is separated from the photoreceptor 19 by a separation charger 25. Thereafter, the image is fixed by the fixing device 26. on the other hand,
The toner remaining on the photoreceptor 19 without being transferred is the cleaner 2
2, and the electrical history of the photoconductor 19 is erased by the pre-exposure lamp 23, and the next print cycle is started again. Next, a method of testing the characteristics of the printer unit will be described. In order to test the characteristics of the printer section, first, a test image signal is generated by the pattern generator 12 and is input to the D / A converter 13 instead of a normal image signal.
In the present embodiment, the pattern generators 12
Test image signals of 256 levels up to FH (H is hexadecimal display) can be generated. However, at the time of an actual test, a test image signal for every 16 levels is output. That is, when the density level is changed from the white level of 00H to the black level of FFH, the test image signals are 00H, 10H, 2
A total of 17 levels of 16 levels of 0H, 30H,..., E0H, F0H and the last FFH level are generated. Next, the density pattern based on the test image signal is transferred onto the transfer paper 27 in exactly the same manner as in a normal printing operation. FIG. 3 is a diagram showing an example of a test pattern formed on transfer paper. In the figure, a density pattern from a white level to a black level in the paper passing direction is represented in 17 levels. The positions where these density patterns are generated are positions that can be sequentially read by a densitometer 28 described later. Next, the thus transferred density patterns are sequentially read by a reflection type densitometer 28. Then, the address line of the buffer 8 and the data write line (downward direction) of the bidirectional buffer 9 are selected and energized by the select signal from the CPU 6, and are sequentially read through the I / O circuit 11. CPU 6 stores the density pattern detection data.
Is written to the table 2 of the RAM 10 by the write signal from the RAM 10. FIG. 4 is a diagram showing output characteristics (table 2) obtained by testing the printer section. In the figure, the horizontal axis is the D / A input level given by the pattern generator 12, and the vertical axis is the output density detected by the densitometer 28. still,
The output density is standardized so that the white level is 0.0 and the black level is 1.0. Also, 01H, 02H,... Not included in the test image signal detected by the densitometer 28.
Levels such as are interpolated by appropriate calculations. Furthermore, in this embodiment, since the output density is processed by an 8-bit signal, the output density level can be assigned to 0.000 to 1.275 in 0.005 steps. That is, it is possible to cope with the case where the black density is higher or lower than the standard. Next, based on the test results, the RAM
A method for correcting the used gradation correction table (table 1) in 10 will be described. For this purpose, it is necessary to first describe the contents of various tables accommodated in the RAM 10 and their functions. That is, in the RAM 10, a used gradation correction table (Table 1 in FIG. 9) for performing gradation correction of a digital image signal at the time of actual printing, and a test printer output table (FIG. 4) in which the above-described tested printer output characteristics are written. Table 2) and a test gradation correction table (Table 3 in FIG. 8) used to correct the used gradation correction table based on the tested printer output characteristics. The RAM 10 has a separate backup power supply so that the contents of the RAM 10 are retained even when the main body power supply is turned off. A standard tone correction table (Table 4) as shown in FIG. 5 and a standard printer output table (Table 5) as shown in FIG. 6 are stored in advance in the ROM 7 of FIG. I have. The standard gradation correction table is RAM1
At 0, the input image signal level is a standard characteristic that should normally be subjected to gradation correction (for example, γ correction) to a D / A input level having characteristics as shown in the figure. In the standard printer output table, when a linear D / A input level (00H to FFH) is given to the horizontal axis, the printer output density on the vertical axis should be as shown in the figure. This is a reference for comparing the output characteristics of the test printer shown in FIG. Next, the CPU 6 creates a test gradation correction table shown in FIG. 8 from the standard printer output characteristics shown in FIG. 6 and the test printer output characteristics shown in FIG. That is, in FIG. 7 showing the standard printer output characteristic and the test printer output characteristic simultaneously, the standard printer output characteristic D corresponding to the same output density is shown.
/ A Input level ns and D / A of test printer output characteristics
The correspondence between the input level nm and the input level nm is obtained as shown by an arrow, and in FIG. 8, a test gradation correction table as shown in FIG. 8 is created by setting ns on the horizontal axis and the corresponding nm on the vertical axis. In FIG. 7, it can be seen that when the output density exceeds dr, the value of ns is inherently present even if it exceeds nsr, but the value of nm is already saturated as FFH. . This relationship is clearly shown in FIG. Next, the CPU 6 reads the D / A input level ns corresponding to each image signal level from 00H to FFH from the standard gradation correction table of the ROM 7 as shown by the arrow in FIG. This operation is a standard γ (gamma) correction conversion operation. However, since the above-described test gradation correction table was created in a non-linear form, a standard gamma correction conversion operation was performed in the ROM 10 even in actual printing.
It can be seen that a standard printer output as shown in FIG. 6 cannot be obtained. Therefore, the CPU 6 creates or modifies the used gradation correction table shown in FIG. 9 by the following method. That is, the fifth
With the value ns of the vertical axis (D / A input level) read by the standard gradation correction table shown in FIG. 8 being applied to the horizontal axis of the test gradation correction table shown in FIG. Then, each ns is converted to a corresponding nm, and the value nm is made to correspond to the D / A input level on the vertical axis in FIG. That is, according to the gradation correction table used in FIG. 9, the image signal level actually used is set on the horizontal axis, and the vertical axis is subjected to characteristic conversion so as to always obtain an ideal output characteristic for the printer. D / A input level is obtained. In general, a sufficiently high image quality can be obtained by the use gradation correction table described above. However, if the output characteristics of the printer greatly deviate from the standard characteristics, for example, the black level (FF)
If the density in H) becomes much darker than the standard black density, or if it becomes much lighter, the gradation of the dark part of the original image may be impaired. In order to prevent this to some extent, it is more effective to make the following additional modifications. That is, for example, in the case of the test printer output characteristic as shown in FIG. 4, the black level is slightly thinner than the standard, so that the test gradation correction table in FIG. For example, in the range where ns.gtoreq.nsr, nm is always at the FFH level, and the gradation of the input image signal from the nsr to the FFH level is lost. In order to prevent this, as shown in FIG. 10, for example, as shown in FIG. 10, when the value of mn or ns exceeds a certain threshold value nmt or nst, ns or ns is maintained until FFH.
What is necessary is to interpolate by replacing the relationship between と and nm with a specific functional relationship. FIG. 10 shows an example in which this section is linearly complemented. In addition, if interpolation is performed using a quadratic curve or the like, the gradation characteristics can be further smoothed. The above method is also effective when the test output characteristic of the printer is darker than the standard at the black level. In this case, the dynamic range of the output can be expanded. In the above example, the black (FFH) level of the test printer characteristic fluctuates. On the other hand, when the white (00H) level fluctuates, a threshold value nmt ', for example, NM≤nmt' is provided. , 00H level and nmt 'level. However, fluctuations in the vicinity of the white level are liable to cause fogging and skipping even if they are delicate, and the visual characteristics are not effective. Therefore, a method other than linear approximation, for example, a quadratic curve is used as an interpolation method. It needs some ingenuity.
Also, the test gradation correction table as shown in FIG. 8 can be used as a guide for checking the current state of the printer as it is. For example, at the time of maintenance, this correction table is output to an appropriate display means, and the current state of the printer can be known by checking the deviation from the ideal linear state of ns = nm. In particular, if the correlation between the deterioration of the developer in the developing unit 21 and the deterioration of the photosensitive member 19 and the change in the correction table is known in advance, it is possible to detect an image trouble in advance. The cause can be easily determined. Alternatively, it is more effective to use the contents of the correction table in combination with the automatic self-diagnosis function of the printer. As described above, according to the present invention, since the state of the machine can be known based on the correction table, the serviceability is significantly improved. <Embodiment 2> In Embodiment 1 described above, a densitometer 28 as shown in FIG. 1 was required. This configuration will be extremely effective if the present invention is configured as a printer only. However, there are drawbacks that the densitometer itself becomes a cost-up factor and that the density test cannot be performed over a wide range. Therefore, a second embodiment in which this point is further improved will be described below. The device of the second embodiment is the reader A shown in FIG.
Is used in place of the densitometer 28. FIG. 11 is a block diagram showing the arrangement of an image forming apparatus according to the second embodiment of the present invention. The same components as those shown in FIG. First, a method for testing printer characteristics will be described. Also when detecting the printer characteristics, a test image signal is formed by the pattern generator 12 and input to the D / A converter 13 instead of a normal image signal. In the pattern generator 12, as in the case of the first embodiment, the 00H level (white) is changed to 10H, 20H, 30H.
, E0H, FOH, and a total of 17 levels of the FFH level (black) are generated. Next, the test image based on the test image signal is transferred onto the transfer paper 27 by a normal printing operation in the same manner as in the first embodiment. FIGS. 12A to 12D are diagrams showing some examples of density patterns on the transfer paper 27. FIG. For example, FIG. 12 (a) provides margins a and b at the leading edge and the trailing edge of the paper, and in other portions, changes the density stepwise from 00H (white) to FFH (black) every 16 levels. The difference from FIG. 3 of the first embodiment is that the density pattern area is made wider by outputting the respective densities widely over the entire area in the main operation direction. Then, returning to FIG. 11, the original 31 of the test density pattern prepared as shown in FIG. 12 is read again from the reader unit A by the CCD 2 as shown in FIG. At this time, there is no need to operate the printer unit B. In addition,
At this time, the original 31 needs to be placed on the original platen in parallel with the paper passing direction at the time of print output as shown in FIG. However, which of the FFH level and the 00H level should be set at the beginning may be determined in advance. The arrangement of the test density pattern itself does not necessarily have to be such that the density changes in one direction. For example, dark and light ones may be arranged alternately, or may be random. In short, if the order of generation of the test patterns by the pattern generator 12 is known, it is possible to always cope with the sub-scanning position at the time of reading. In this way, by always setting the original 31 at a fixed position on the original platen in a fixed direction, the reader unit A associates what level of density is always read during scanning with the sub-scanning position. You can know that. FIGS. 12 (b) to 12 (d) show another example in which a test density pattern is formed. Among them, for example, FIG. 12 (c) shows the channel information (density information) corresponding to each test density pattern written in a bar code beside the pattern. It is possible to always know what level of density is being read without detecting the scanning position. Next, after the test image signal read from the CCD 2 is digitally converted by the A / D converter 4, in this case, the density data of the same group is cumulatively added through the adder 33 and the register 34. Calculates the average value for each density level. In this way, if concentration data is collected over a certain wide area and the average value is obtained,
The influence of partial density unevenness and noise can be significantly reduced. Then, the data of the average value of the image signal levels obtained by reading the respective density levels is plotted on the vertical axis to obtain the first data.
A test printer output table (Table 6) as shown in FIG. 4 is created. Again, untested horizontal axis 0
The levels on the vertical axis corresponding to 1H, 02H,... Are interpolated by appropriate calculations. As can be seen from FIG. 14, the CCD
Since the test image data read from step 2 includes all the characteristics of the CCD 2, the amplifier 3, the A / D converter 4, etc., the vertical axis indicates the reflection density as shown in FIG. It is different from the one that is standardized based on physical quantities taking into account various visual characteristics. Of course, the test printer output table of FIG. 14 obtained as described above may be used as it is, but here, in order to make the physical meaning more clear, this information is temporarily stored in FIG. It is converted into a form such as the test printer output characteristic (Table 2). To this end, a table (Table 7) indicating the input characteristics of the reader unit A as shown in FIG. 15 is prepared in the ROM 32 in advance. Table 7 shows what the input density is relative to the value of the read image signal level in a standardized manner. Standard black level to 1.00
After rating, 00H is set to 0.0 and FFH is set to 1.70. Thus, by converting the table 6 in FIG. 14 with the table 7 in FIG. 15, a test printer output characteristic (table 8) as shown in FIG. 16 is obtained. Therefore, this actually shows exactly the same relationship as the printer output characteristics (Table 2) in FIG. Therefore, the subsequent processing may be the same as that described in the first embodiment. <Embodiment 3> In Embodiments 1 and 2 described above,
Using a test density pattern as shown in FIG. 3 or FIG. 12 (a) to (d), a test image signal value generated by the pattern generator 12 is known from the sub-scanning position or code display of the pattern. Using. Therefore, it is necessary to arrange the test density patterns with some level of margin (step) so as not to cause misreading between adjacent patterns. On the other hand, although the entire configuration of the third embodiment is not shown, for example, the test image signal from 00H to FFH is continuously and finely divided to some extent by the pattern generator 12 in FIG. 11 of the second embodiment. In such a manner, a gradation test density pattern as shown in FIG.
And the cumulative frequency histogram at this time (18th
Figure) is created and the output characteristics of the printer are created using this. Also in FIG. 18, the ordinates of “frequency distribution” and “cumulative frequency distribution” are normalized from 0.0 to 1.0. An example of a method for creating the output characteristics of the test printer is as follows. First, when creating the gradation pattern shown in FIG. 17, the time for outputting each test image signal from 00H to FFH is made equal. , And the test image signal is changed linearly by one gradation. Then, the ordinate of the cumulative frequency in FIG.
The same characteristics as those in the case of Table 6 in FIG. 14 in the second embodiment can be obtained only by normalizing according to 55. As described above, the third embodiment is similar to the second embodiment until a test density pattern is output from the printer unit B.
However, when the output test density pattern is read again from the reader unit A, it is only necessary to count the number of data read for each input level. Therefore, it is not necessary to make correspondence between the generated test image signal and the read density data. Furthermore, even if shading due to mechanical or electrical noise occurs when creating a test density pattern,
Due to the nature of the cumulative frequency histogram, since it always becomes an increasing function, it is possible to prevent the occurrence of a decrease such as reversal of the correction curve, and also smooth the unevenness of the curve due to noise,
There is an advantage that it becomes resistant to noise. In the case where the gradation density pattern as shown in FIG. 17 is read from the reader unit A, the test image signal may be read from the 00H side or from the FFH side. Since it is not necessary to pay attention to the right and left directions of the density original when setting the original, it is possible to prevent a mistake during service. This is also due to the nature of the cumulative frequency histogram, and for the same reason, when a gradation pattern is created, it is not always necessary to set 00 as shown in FIG.
Irregular arrangement may be made without linear change from H to FFH. However, if an extremely different level is arranged adjacently, an erroneous density may occur due to an edge effect of development. As described above, according to the present invention, the floor
The reference image for adjusting the tone correction table
And read the formed reference image.
An image signal is input, and a gradation correction table is generated based on the image signal.
The tone correction table of the image forming apparatus.
Adjustment can be performed automatically. In addition, the requested floor
Since the contents of the key correction table are displayed, the image forming apparatus
Can be recognized.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an image forming apparatus according to a first embodiment of the present invention. FIG. 2 is a diagram illustrating an example of an operation principle of pulse width modulation. FIG. 3 is a diagram illustrating an example of a density pattern formed on a transfer sheet according to the first embodiment; FIG. 4 is a diagram illustrating a tested printer output characteristic according to the first embodiment. FIG. 5 is a diagram illustrating standard tone correction characteristics of the printer according to the first embodiment. FIG. 6 is a diagram illustrating standard output characteristics of the printer according to the first embodiment. FIG. 7 is a diagram illustrating a comparison between a standard printer output characteristic and a test printer output characteristic of the printer according to the first embodiment. FIG. 8 is a diagram illustrating test gradation correction characteristics of the printer according to the first embodiment. FIG. 9 is a diagram illustrating a used gradation correction characteristic of the printer according to the first embodiment. FIG. 10 is a diagram illustrating another test gradation correction characteristic of the printer according to the first embodiment. FIG. 11 is a block diagram of an image forming apparatus according to a second embodiment of the present invention. FIG. 12 is a diagram illustrating some examples of density patterns on transfer paper 27 according to the second embodiment. FIG. 13 is a diagram illustrating a relationship between a sub-scanning direction of a reader unit and a test density document. FIG. 14 is a diagram illustrating output characteristics of the tested printer of the embodiment. FIG. 15 is a diagram illustrating density conversion characteristics according to the second embodiment. FIG. 16 is a diagram illustrating a tested printer output characteristic according to the second embodiment. FIG. 17 is a diagram illustrating an example of a density pattern on transfer paper according to the third embodiment. FIG. 18 is a diagram illustrating histogram characteristics of a tested printer output according to the third embodiment. [Description of Signs] 1 Original 2 CCD 3 Amplifier 4 A / D converter 5 Latch 6 Microprocessor 7 ROM 8 Buffer 9 Bidirectional buffer 10 RAM 11 I / O port 12 Pattern generator 13 D / A converter 14 Pulse Width modulation circuit 15 Laser driver 16 Semiconductor laser 17 Polygon mirror 18 f-θ lens 19 Photoconductor 20 Corona charger 21 Developer 22 Cleaner 23 Pre-exposure lamp 24 Transfer charger 25 Separate charger 26 Fixing roller 27 Transfer paper 28 Densitometer 29 A / D converter 30 Bus 31 Reference density output pattern 32 ROM 33 Adder 34 Register

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-56-72774 (JP, A) JP-A-60-242343 (JP, A) JP-A-61-142427 (JP, A) 28652 (JP, U) Actually open 63-63,848 (JP, U) (58) Fields investigated (Int. Cl. ⁶ , DB name) B41J 29/46 B41J 2/52 G06F 3/12 G06T 5/00 H04N 1/00 106

Claims (1)

(57) [Claims] An image processing method for adjusting a tone correction table of an image forming apparatus that forms given image data on a medium, wherein a reference image for adjusting the tone correction table is formed in the image forming apparatus. And read the formed reference image.
An image processing method comprising: inputting an obtained image signal; obtaining a gradation correction table based on the image signal; and displaying the contents of the gradation correction table . 2. Of the image forming apparatus based on the gradation correction table.
2. The image processing method according to claim 1, wherein a self-diagnosis process for determining a state is performed . 3. The display of the contents of the gradation correction table is performed by the maintenance
2. The image processing method according to claim 1, wherein the method is performed at the time of scanning .