KR0132879B1 - Thermal transfering printer - Google Patents

Abstract

The present invention discloses a thermal transfer printer apparatus for printing with a constant energy for each gray level.

The present invention provides a constant heating energy for each gray level according to the distribution of the number of heat generating heads for each gray level calculated from the sum of the thermal low head and the line data of the gray level and the line data and the predetermined number of pixel data. Compensation means for controlling the thermal transfer head to generate heat can control the heating time, adjust the applied voltage of the heating element, or change the gradation data to perform common resistance correction to print high quality images.

Description

Thermal Transfer Printer Device

1 is a block diagram of a conventional thermal transfer printer.

FIG. 2 is a detailed circuit diagram of some blocks of the thermal transfer printer apparatus shown in FIG.

3 is a view for explaining the sensitivity curve of the transfer film.

4 is a view for explaining the relationship between the current time flowing through the thermal transfer head (TPH) and the print concentration shown in FIG.

5 is a diagram for explaining a common common resistance correction.

6 is a block diagram according to an embodiment of the thermal transfer printer apparatus according to the present invention.

7A to 7C are operation timing diagrams of the thermal transfer printer apparatus shown in FIG.

FIG. 8 is a waveform diagram of the strobe signal generated by the halftone control unit shown in FIG.

9 is a block diagram according to another embodiment of the thermal transfer printer apparatus according to the present invention.

10 is a block diagram according to another embodiment of the thermal transfer printer apparatus according to the present invention.

* Explanation of symbols for main parts of the drawings

701, 710: first and second integrated circuits 702, 703: first and second counters

704: RAM 705: address generator

706, 707: first and second reset circuits 708: comparator

709: level generator 711: common resistance correction ROM

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal transfer printer apparatus, and more particularly, to a thermal transfer printer apparatus for printing in continuous tones for each gradation by overcoming deterioration in image quality of a printed image due to the number of heating elements of a thermal transfer head, that is, a load variation.

In general, an apparatus for expressing the energization factor using a thermal print head (hereinafter referred to as TPH) may include a thermal transfer printer, a color copying machine, a faxmile, and the like. A printer is a device which applies energy to TPH and sublimates the dye of the film to which the dye is applied by the energy which TPH generates, and prints a desired image or picture by the amount of dye transferred onto the recording paper.

In the conventional thermal transfer printer apparatus, as shown in FIG. 1, an analog image signal transmitted from a signal input source (not shown), such as a video camera or a television, by the A / D conversion unit 10 is red (R). ), Green (G) and blue (B) signals are input and converted into digital signal types.

In the first selector 20, a signal output from the A / D converter 10 or a digital graphic image transmitted from a digital signal input source such as a personal computer or a graphic computer through a protocol such as GP-IB, SCSI, CENRONICS, or the like. Select the data.

Under the control of the memory controller 40 which controls the timing of writing and reading data, the selection signal of the first selection unit 20 is stored in the image memory 30 for image data for a frame or a field.

The second selector 50 composed of a multiplexer sequentially switches one signal for the R, G, and B data stored in the image memory 30 so that the color converter 60 has a complementary Y for the B signal. Yellow) signal is converted into a M (Magenta) signal having a complementary color relation to the G signal, and a C (Cyan) signal having a complementary color relation to the R signal.

In addition, the correction unit 70 performs various corrections such as GAMMA correction, color correction for Y, M, and C, resistance of the heating element, and resistance of the heating element. 80).

The data read in line units from the line memory 80 is compared with the gray level preset in the halftone control unit 90, and then generates a pulse width modulation signal PWM corresponding to the heating time for each gray level. Color printing is performed by driving the TPH 100 for a time period. That is, when performing color printing, color printing can be performed by making Y, M, and C colors into surface order.

In detail, when the data is read from the image memory 30, the data of one vertical line is first read out by the second selecting unit 50 with respect to the B signal, and the line memory is read as the Y signal through the color converter 60. Data written in (80) and written in this line memory (80) is subjected to halftone conversion via the halftone control unit (90), and then transferred to the TPH (100) to perform one-line printing. In the NTSC thermal transfer printer, printing about 500-600 lines per screen completes one-color printing of Y in complementary relationship of B.

Next, the data of one screen for the G signal is read out from the image memory 50 by the second selection unit 50 by one vertical line through the above-described process and applied to the line memory 80, and then the complementary color of G is obtained. After printing one color of M in relation, the R data for one screen is read out one by one vertical line by the second selecting unit 50, and the one color of C in complementary color relation of R is processed through the above-described process. Complete printing.

Here, the A / D converter to the memory controller 10-40 correspond to the image signal processing circuit 1, and the second selector to the TPH 50-100 correspond to the print control circuit 2. An image display circuit for processing the output of the signal processing circuit 1 to be displayed on a display device such as a monitor may be added.

FIG. 2 is a detailed circuit diagram of some blocks of the thermal transfer printer apparatus shown in FIG.

According to FIG. 2, when data of one vertical line is first written to the line memory 80, the address generator 91 enables the gradation level generator 92 and the line memory 80 can perform the read operation. Output the read address.

The gradation level generator 92 outputs data 0000 0001 for one gradation to an ErROMable Programmed ROM (EPROM) 93 and a gradation comparator 96.

Here, as shown in FIG. 4, the energy density of one gradation level, that is, the optical density of 0000 0001 is 0.2, as shown in FIG. 3 to apply the energy E1 to the transfer film through the TPH 100, as shown in FIG. As described above, the strobe signal corresponding to the energization time T1 corresponding to the energy E1 is generated in the time width generator 95 and applied to the latch register 102, and the heating element 103 generates heat by t1 with respect to the energy E1. Express

Next, the gradation level generator 92 outputs 0000 0010 in the heat generation of the two gradations, and the gradation comparator 96 performs gradation comparison with the data of the line memory 80.

If the data in the line memory 80 is higher than the gradation data of the gradation level generator 92, the operation of outputting a high signal and a low signal is repeated 512 times in the case of A6 size for one line. The data are shifted and stored in the shift register 101 in order.

Here, the heat generation time is programmed in advance corresponding to the gradation data generated by the gradation level generator 92 of the EPROM 93, and the electronic switch 94 is driven to generate a different heat generation time depending on the gradation in the time width generator 95. A strobe signal STROBE is generated and output to the heating element 103.

The output of the shift register 101 is transmitted to the latch register 102 to generate heat during the time width of T2 generated by the time width generator 95.

In this way, when 256 gradations finish the heat generation in the same way as described above, one line of printing is completed, and for one screen, for example, A6 size, the heat of 500-600 lines is completed, and Y, M, C 3 Color printing can also be performed in the same manner as the above process.

At this time, the exothermic energy that generates heat with respect to the factor concentration for each gradation is proportional to the S curve shape as shown in FIG. 3, and as the heat generation time is longer as shown in FIG. 4, the factor concentration increases.

Next, common resistance correction among various corrections performed by the correction unit 70 shown in FIG. 1 will be described with reference to FIGS. 2 and 5.

The number of heating elements of the TPH 100 is about 500-600 resistors for the NTSC thermal transfer printer device, and about 2000-4000 heating elements for the thermal transfer printer device for graphics.

When the voltage applied to the heating element is V and the application time is T, the energy E applied can be expressed by the following Equation (1).

E = (V2 / R) T ------- (1)

Since the voltage applied to the TPH 100 changes when the heat generating element generates 1 heat and 3,000 heats, the energy generated by the TPH 100 is changed. Compensating this is called common resistance correction.

As shown in FIG. 5, a conceptual diagram in which a resistance from the power supply unit 110 to the TPH 100 is formed is illustrated.

Here, assume that the resistors R1-Rn are 3 Kohm, the output resistance Rour of the power supply unit is 1 ohm, and the wiring resistance Rw is 0.1 ohm. When only R1 generates heat, the voltage V2 applied directly to the heating element is V2 = (R1 * V1) / (Rout + Rw + R1) = 24.99 VOLT. When heating to R2, the voltage V2 applied directly to the heating element is V2 = (R1 // R2) * V1 / (Rout + Rw) (R1 / R2)) = 24.98 (V).

Here, R1 // R2 represents the parallel resistance of R1 and R2. Here, assuming that 500 heating elements generate heat, the parallel resistance value is 3K / 500.

Therefore, V2 = (3K / 500) * 25 / (R1 + Rw + 3K / 500)) = 21.126 (V).

Since V2 varies according to the number of heat generating elements, E is changed by substituting V2 instead of V in (1). To compensate for this, when there are more heating elements that heat than the preset center value (threshold value), when the image data is printed, the heating time, i.e., the width of the strobe signal, is increased. Shortening the width of the signal shortens the heating time.

In order to control the heating time by adjusting the strobe signal width for each gray level according to the number of heating elements to generate heat, the common resistance correction unit 73 shown in FIG. 2 is required.

The image data output from the color converter 60 shown in FIG. 2 is input to the line memory 80 and input to the gray-scale heating element calculator 71 to generate heat for each gray level from 1 to the maximum gray. Calculate the sum.

When expressing one gradation from the gradation number of heating elements calculator 71, the sum of the number of heating elements for one gradation is input to the correction rom 72 as an address signal, and the correction rom 72 inputs n-bit data for n-bit data. Change to. That is, since the number of heat generating elements of TPH is about 12 bits in case of 3000-4000, it is changed to 6 bits in case of having 64 quantization steps for economical correction.

This n-bit data is input to the upper address terminal of the EPROM 93 of the intermediate tone control unit 90, and is output from the tone data generated by the tone generator 92 and the correction ROM 72 in this EPROM 93. The strobe signal, that is, the heating time, corrected by the time width generator 95 by inputting the quantized data as an address signal and outputting correction data according to the number of heating elements that generate heat for each gradation stored at this address. Is applied to the TPH 100 to print one gradation.

When the next two-gradation printing is performed, the sum of the number of the heating elements for the two grayscales is output from the gray-scale heating element number calculator 71, and the following operation is the same as in the first grayscale.

The number of heating element calculators for each gray level uses a RAM, and the common resistance correction unit 73 shown in FIG. 2 when parallel processing is performed in order to prevent the calculation time from becoming longer to calculate the total amount for each gray level. There was a disadvantage in that hardware was complicated because two or more bays should be used.

Since the control is performed for each gradation, if the correction data of the correction ROM 72 is fed back to the power supply unit, the V1 voltage is increased when there are more heat generating elements than the center value, and the V1 voltage is decreased when the resistance is low. Even if the voltage of the power supply unit was changed for each gray level, since the actual printing speed was fast, there was a problem that the voltage of the power supply unit did not vary according to the printing speed.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermal transfer printer apparatus which prints with a constant energy for each gray level by performing common resistance correction.

Another object of the present invention is to provide a thermal transfer printer apparatus for performing common resistance correction by controlling the heating time by the total information of one vertical line and the distribution of the heating elements to be generated for each gray level.

Still another object of the present invention is to provide a thermal transfer printer apparatus for controlling common voltage by controlling a voltage of a power supply unit based on total data of one vertical line and controlling a heat generation time by a distribution of heat generating elements for each gray level. have.

It is still another object of the present invention to provide a thermal transfer printer apparatus for performing common resistance correction by changing the data value of the vertical line by the total information of one vertical line data and the distribution of the heating elements to be generated for each gray level.

In order to achieve the above-mentioned objects, the thermal transfer printer apparatus according to the present invention includes a thermal transfer printer apparatus for performing an expression of the current factor after the gray level comparison and the gray level comparison of image data in line units; A thermal transfer head for generating and expressing grayscale-compared data; And correction means for controlling the thermal transfer head to generate heat with a constant heating energy for each gray level according to the distribution of the number of heating elements for each gray level calculated from the sum of the image data in line units and the sum of the predetermined number of pixel data. Doing.

Hereinafter, with reference to the accompanying drawings will be described a preferred embodiment of the thermal transfer printer apparatus according to the present invention.

6 is a block diagram according to an embodiment of the thermal transfer printer apparatus according to the present invention.

According to FIG. 6, although not shown in the drawing, the image processing circuit is configured in the same manner as the image processing circuit 1 shown in FIG. 1, and an image display circuit can also be added.

Here, the print control circuit is denoted by the same reference numerals for the same parts as the print control circuit 2 shown in FIG. 1, and the description of the configuration will be omitted.

The input terminal of the first sum circuit 701 of the common resistance correction unit 700, which is part of the correction unit, is connected to the output terminal of the color converter 60, and the output terminal of the first and second counters 702 and 703 is provided. It is connected to each clock terminal.

The input terminal of the RAM 704 is connected to the output terminal of the second counter 703, and the output terminal thereof is connected to the first input terminal of the comparator 708.

The clock terminal of the address generator 705 is connected to the second control signal output terminal of the gradation level generator 92 of the halftone control unit 90, and the output terminal thereof is connected to the input terminal of the RAM 704, respectively.

The enable terminal of the first reset circuit 706 is connected to the second control signal output terminal of the gradation level generator 92, and the output terminal thereof is connected to the reset terminal of the first counter 702.

The enable terminal of the second reset circuit 707 is connected to the output terminal of the address generator 91, and the output terminal thereof is connected to the reset terminal of the second counter 703.

The input terminal of the level generator 709 is connected to the second control signal output terminal of the gradation level generator 92, the output terminal thereof is connected to the second input terminal of the comparator 708, and the output terminal of the comparator 708 is the second conference. It is connected to the input terminal of the furnace 710.

The output terminal of the first counter 702 is connected to the uppermost address terminal of the EPROM 93 of the halftone control unit 90, and the output terminal of the second sum circuit 710 is connected to the next higher address terminal of the EPROM 93. .

Next, the operation of the apparatus shown in FIG. 7 will be described.

First, image data of one vertical line is written to the line memory 80 via the color converter 60 and input to the first sum circuit 701.

The first sum circuit 701 calculates the sum in the order of the input image data, and generates a carry when a certain value (for example, the data value reaches 50) is generated and the carry signal is transmitted to the first and second counters 702,. The counting operation is controlled by outputting the clock signal 703.

The first counter 702 counts the carry counts for one vertical line, outputs the total value of the data of one vertical line to the highest address terminal of the EPROM 93, and outputs a predetermined number of times at the second counter 703. For example, by counting 10 pixels, a predetermined number of pixel unit data is written to the RAM 704.

The reason for using the first counter 702 is that the hardware becomes huge in order to calculate and store the total of image data of one vertical line. Thus, it is the effect of adding up all the data transmitted through the color converter 60 and dividing it by a predetermined value.

The first sum circuit 701 continuously sums the incoming input (SUM) and generates a carry when it reaches a predetermined value. When this carry occurs, the first sum circuit 701 is initialized and the first counter ( 702 counts another, and the value of counting this carry number before being reset by the first reset circuit 706 is the sum of data for one line.

On the other hand, the output of the second counter 703 that outputs the data sum in ten pixel units is written to the RAM 704 in accordance with an address incremented by ten pixels in the address generator 705.

Whenever an address occurs in the address generator 91, the reset circuit 707 inputs it to initialize the second counter 703 when an address for 10 pixels is generated.

Again, when 10 pixels of image data is input, its sum value is written to the next address of the RAM 704. When the sum of the image data of one vertical line is written to the first counter 702 and the RAM 704, it means that the data transfer of one vertical line is finished in the line memory 80.

At this time, the output of the first counter 702 is input to the highest address of the EPROM 93.

Here, assuming that printing in one gradation, the RAM 704 stores the sum data of 10 pixel units and outputs the data to the comparator 708 while reading out each data, and the level generator 709, for example, weights 10 for one gradation. Data is generated and output to the comparator 708.

In this case, the level generator 709 generates a preset weight for each gray level whenever gray data is generated by the gray level generator 92.

In the second sum circuit 710, data that is compared with the gray level weights generated by the level generator 709 and the output of the RAM 704 in the comparator 708 are summed for each gray level to an upper next address of the EPROM 93. Is entered. The next upper address is to know how much distribution the data to be printed currently has in each gradation.

In this way, common resistance correction is performed while printing 256 gradations. When 256 gradations are generated by the gradation generator 92 when printing 256 gradations of one vertical line, the signal is input to the first reset circuit 706 as an enable signal to delay time for a predetermined time, that is, After the printing is completed, the first counter 702 is initialized through the first reset circuit 706.

In summary, in the conventional art, the common resistance correction was performed one by one gradation. However, in the present invention, the overall one line correction is performed by the total value of one vertical line in the first counter 702, and the gray scale is generated by the output of the second sum circuit 710. Detailed corrections are made to the distribution of the heating elements that generate heat.

7A to 7C are operation timing diagrams of the thermal transfer printer apparatus shown in FIG.

As shown in FIG. 7A, one vertical line of data is written to the line memory 80, and as shown in FIG. 7B, the first sum circuit 701 sums up one vertical line data. Operation is performed, and the counting operation is performed at the first and second counters 702 and 703 as shown in FIG. 7C.

After printing one vertical line of data in the line memory 80 (Fig. 7A), the counter 702 and the RAM 704 output correction data for the common resistance (Fig. 7C).

FIG. 8 is a diagram for explaining the strobe signal generated by the time width generator 95 shown in FIG.

A1 is the time duration of the reference strobe signal for controlling one gradation, A2 is the time duration of the reference strobe signal for controlling two gradations, and A258 is the time duration of the reference strobe signal for controlling 256 gradations.

B is a time width to be corrected by the output of the first counter 702, and a constant pulse width is allocated in the entire grayscale of one line by the value of the total for one vertical line. This value has the effect of smoothing the tone between lines.

C1 is the correction time width by the second sum circuit 710 when printing one gradation, C2 is the correction time width by the second sum circuit 710 when printing two gradations, and C258 prints 256 gradations. Is a correction time width by the second sum circuit 710.

9 is a block diagram according to another embodiment of the thermal transfer printer apparatus according to the present invention.

According to FIG. 9, the configuration of the thermal transfer printer apparatus shown in FIG. 6 is the same, except that the output terminal of the first counter 702 is connected to the control signal input terminal of the voltage regulating unit 111 of the power supply unit 110. The only difference is that the output terminal of the second sum circuit 710 is connected to the highest address terminal of the EPROM (not shown) of the halftone control unit 90.

The operation of FIG. 9 is the same as that of the apparatus shown in FIG. 6, and the output of the first counter 702 of the correction unit 700 is input to the voltage adjusting unit 111 of the power supply unit 110 so that one vertical line is provided. The overall tone of the image printed by this voltage is adjusted by adjusting the voltage V1 input to the TPH 100 according to the total value of the data of the EPH OM 93 according to the output of the second sum circuit 710. Detailed resistance correction is performed by The strobe signal width is shortened when the number of heating elements is high, that is, when the number of heating elements is large, and the strobe signal width is increased when the number of heating elements is small.

10 is a block diagram according to another embodiment of the thermal transfer printer apparatus according to the present invention.

In the thermal transfer printer apparatus of FIG. 10, all configurations except for the common resistance correcting unit 700 are the same as those of FIG. 6.

A common resistance correction ROM 711 is further configured such that an input terminal of the ROM 711 is connected to an output terminal of the line memory 80, and the output terminal of the gray level comparator (not shown) of the intermediate tone control unit 90 is shown. Connected to an input terminal, an output terminal of the first counter 702 is connected to an uppermost address terminal of the common resistance correction ROM 711, and an output terminal of the second sum circuit 710 is next to the common resistance correction ROM 711; Since the configuration of the common resistance corrector 700 except for the point connected to the upper address terminal is the same as that of FIG. 6, it will be omitted.

Referring to FIG. 10, the output of the line memory 80 is input to the lower address of the common resistance compensation ROM 711, and the output of the first counter 702 is input to the upper address of the common resistance compensation ROM 711. The output of the second sum circuit 710 is input to the next address above the common resistance correction ROM 711, and the common resistance correction ROM 711 outputs the corrected data.

The common resistance correction ROM 711 stores data that can increase the value of the image data when the number of the heating elements is large, and decrease the value of the image data when the number of the heating elements is small.

As described above, the thermal transfer printer apparatus according to the present invention is capable of printing with ambient lines and soft tones at high speed by performing overall correction with the total data one by one vertical line, thereby improving image quality.

In addition, the present invention is effective to perform the common resistance correction by matching the actual printing speed by adjusting the voltage with the total data by one vertical line.

The present invention has the effect of reducing the hardware by controlling the heat generation time by the predetermined number of pixels in the heating element number distribution for each gray level.

Claims (8)

10. A thermal transfer printer apparatus for performing an expression of an electricity supply factor after comparing gray levels with image data preset in line units; A thermal transfer head for generating and expressing grayscale-compared data; And correction means for controlling the thermal transfer head to generate heat with a constant heating energy for each gray level according to the distribution of the number of heating elements for each gray level calculated from the sum of the image data in line units and the sum of the predetermined number of pixel data. Thermal transfer printer device. Image signal processing circuit for converting image signal from signal input source into red, green, and blue signal, image display circuit for displaying signal processed signal, and comparing the gray level value with preset signal level in line unit A thermal transfer printer apparatus having a print control circuit for performing energization factors by a thermal transfer head, the print control circuit comprising: a line memory for storing image signals processed from the image signal processing circuit as vertical lines; An intermediate tone control unit for supplying the thermal transfer head with a strobe signal that controls the data stored in the line memory and the tone comparison data after the tone comparison and the heat generation time for each tone; And a heat generation time control unit for varying the time width of the strobe signal by calculating the distribution of the number of heating elements for each gray level by the sum of the data of one line of the image signal processed by the image signal processing circuit and the sum of a predetermined number of pixel data. Thermal transfer printer device characterized in that it comprises a. The heating unit of claim 2, wherein the intermediate tone control unit generates heat differently for each gray level according to a predetermined heating time width for each gray level, a predetermined heating time width for a line corresponding to a total of image data in a line unit, and distribution of the number of heating elements for each gray level. A thermal transfer printer apparatus for generating the strobe signal at a sum of time widths and outputting the generated stub signal to the thermal transfer head. 3. The apparatus of claim 2, wherein the heat generation time control unit comprises: a first summation circuit which sums up the image data output from the line memory until it reaches a predetermined value; A first counter which inputs an output which is the first sum circuit and outputs a sum of data of one line to the intermediate control unit; A second counter for outputting a sum of data in a predetermined number of pixel units from the output of the first sum circuit; A memory for storing the output of the second counter; A level generator for generating a level according to the gray level weights; A comparator comparing the output stored in the memory with a weight of the level generator; And a second sum circuit configured to sum up the result compared by the comparator for each gray level and output the sum result to the halftone control unit. Image signal processing circuit for inputting image signal from signal input source and converting it into red and green signal, image display circuit for displaying signal processed signal, and comparing the signal processed signal with preset gray level value by line unit A thermal transfer printer apparatus having a print control circuit for performing energization factor expression by a thermal transfer head, the print control circuit comprising: a line memory for storing image signals processed from the image signal processing circuit as vertical lines; An intermediate tone control unit for supplying the thermal transfer head with a strobe signal that controls the data stored in the line memory and the tone comparison data after the tone comparison and the heat generation time for each tone; A power supply unit supplying a supply voltage of the thermal transfer head; And controlling the voltage of the power supply unit according to the sum of the data for one line of the image signal processed from the image signal processing circuit, calculating the distribution of the number of heating elements for each gray level by the sum of a predetermined number of pixel data, And a correction unit for varying the time width. 6. The apparatus of claim 5, wherein the correction unit comprises: a first combining circuit for generating a carry when the image data output from the line memory reaches a predetermined value; A first counter counting the carry signal when the carry signal is generated and counting the total data value of one line to output the voltage control signal of the power supply unit; A second counter for outputting a data sum in a predetermined number of pixel units from the output of the first sum circuit; A memory for storing the output of the second counter; A level generator for generating a level according to the gray level weights; A comparator comparing the output stored in the memory with a weight of the level generator; And a second sum circuit configured to sum up the result compared with the comparator for each gray level and output the sum result to the halftone control unit. Image signal processing circuit for converting image signal from signal input source into red, green, and blue signal, image display circuit for displaying signal processed signal, and comparing the gray level value with preset signal level in line unit A thermal transfer printer apparatus comprising a print control circuit for performing energization factor expression by a thermal transfer head thereafter; The print control circuit includes a line memory for storing the image signal processed from the image signal processing circuit as vertical lines; An intermediate tone control unit for supplying the thermal transfer head with a strobe signal that controls the data stored in the line memory and the tone comparison data after the tone comparison and the heat generation time for each tone; And calculating the distribution of the number of heating elements for each gray level by the sum of the data for one line of the image signal processed from the image signal processing circuit and the sum of the predetermined number of pixel data to vary the data of the line memory to change the data of the line memory. Thermal transfer printer device characterized in that it comprises a data conversion unit for outputting. 8. The apparatus of claim 7, wherein the data converter comprises: a first summation circuit which generates a carry when the image data output from the line memory reaches a predetermined value; A first counter that counts the carry signal when the carry signal occurs and counts a data value of one line; A second counter for outputting a data sum in a predetermined number of pixels from the output of the first sum circuit; A memory for storing the output of the second counter; A level generator for generating a level according to the gray level weights; A comparator comparing the output stored in the memory with a weight of the level generator; A second sum circuit for adding up the result compared by the comparator for each gray level; And a correction memory configured to input data of the first counter and output of the second sum circuit as an address, and to store data for varying the data of the line memory at this address.