JPH10217527A - Thermal printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a density of print images from being decreased by a banding phenomenon without complicating a circuit constitution in a thermal head, by correcting a density of original image data corresponding to a heat- generating element in the vicinity of an end part through control by software. SOLUTION: Multigradational original image data are input to a data- processing part 41 via an original image data input means 9 and stored in an original image data buffer memory 100. In order to prevent an image density from being decreased because of a decrease of a dot density per unit area caused by a banding phenomenon, a density-correcting means 13 preliminary corrects a density of the multigradational original image data before the data are changed to two gradations, ion the basis of a density value of each pixel included in the original image data stored in the original image data buffer memory 10 and positional information at a thermal head of a heat-generating element corresponding to each pixel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of scanning a print head having a plurality of heating elements in a paper transport direction in a direction perpendicular to the paper transport direction, such as a thermal printer for performing printing using an ink ribbon. And an image recording apparatus for performing printing.

[0002]

2. Description of the Related Art In an image recording apparatus which performs printing by scanning a print head having a plurality of heating elements in a direction perpendicular to a sheet conveying direction, a general thermal head employs a print element for forming print characters and figures. A plurality of heating elements are arranged in the paper transport direction, and the print head is moved for a predetermined time in a direction perpendicular to the paper transport direction while selectively heating any of the plurality of heating elements. In this way, various print characters and figures are formed.

However, as the heat radiation characteristics of the heat generating element on the thermal head and the heat radiating plate joined to the thermal head,
Among the plurality of heating elements, the heat radiation characteristics of the element near the end and the heat radiating plate facing the element near the end are smaller than those of the central part and the heat radiating plate facing the central part. The efficiency tends to be high and the temperature tends to be low. For this reason,
A so-called banding phenomenon occurs in which the density of a portion printed by the heat generating element near the end is reduced.

That is, in the thermal printer, it is determined whether or not the ink on the ink ribbon facing the heating element is heated and melted depending on whether or not the heating element is energized, and the melted ink selectively adheres to the paper. Thereby, a dot image is formed. In such a thermal printer, the image density is changed by changing the dot density per unit area by increasing or decreasing the number of heating elements to be energized. That is, the number of heating elements to be energized in the thermal head increases or decreases according to the image density. However, when a banding phenomenon occurs near the upper and lower ends of the thermal head, the ink ribbon is not sufficiently heated by the energized heating element, and the heating element cannot melt the ink in the opposing portion, thereby forming on the paper. The dot density per unit area decreases, and the image density decreases.

Japanese Unexamined Patent Publication (Kokai) No. 62-233267 discloses a method in which a heating resistor at an end in a column direction among a plurality of heating resistors is detected by an end detecting means, and based on a detection result of the end detecting means. There is disclosed a configuration in which a heating value larger than the heating value of the other heating resistors is generated by the heating control means when the heating resistors at the ends in the row direction operate.

[0006] Japanese Patent Application Laid-Open No. 2-81647 discloses that
A ladder circuit over a print control unit of a thermal head for forming a printed figure, an addition circuit for superimposing a temperature distribution correction level generated according to a voltage division ratio of the ladder circuit on a reference voltage level, and a circuit generated by the addition circuit. A constant current control circuit for each print control unit that keeps a current proportional to the control level constant, and a current value that flows through the resistor of the thermal head is varied by an output current of the constant current circuit. Is disclosed.

That is, in the configurations disclosed in JP-A-62-233267 and JP-A-2-81647,
By changing the circuit configuration, the amount of heat generated by the heating elements in the vicinity of the end of the plurality of heating elements constituting the thermal head is increased to prevent the banding phenomenon.

[0008]

However, in the above-mentioned conventional image recording apparatus, since the additional circuit is provided to change the circuit configuration to increase the amount of heat generated by the heating element near the end, There is a problem that the circuit configuration becomes complicated, the size of the thermal head increases, and the cost increases. Particularly, in the configuration disclosed in Japanese Patent Application Laid-Open No. 62-233267, a circuit for detecting the end of the heating element to be energized is also required, and the circuit configuration of the thermal head is further complicated.

An object of the present invention is to correct the density of original image data corresponding to a heating element near an end by software control, and to prevent a circuit configuration in a thermal head from becoming complicated without causing a printed image due to a banding phenomenon. An object of the present invention is to provide a thermal printer capable of preventing a decrease in density beforehand.

[0010]

According to the first aspect of the present invention, original image data having a multi-gradation density is converted into print image data having two gradations, and the print image data is converted in the paper transport direction based on the print image data. In a thermal printer that performs a printing process by selectively driving a plurality of heating elements provided in the paper transport direction on a thermal head that moves in the orthogonal direction, the density of the original image data for each pixel is changed to print image data. Before you do
It is characterized in that density correction means for correcting in accordance with a correction amount determined based on the position of the heating element corresponding to each pixel in the paper transport direction is provided.

In the first aspect, the density of the original image data for each pixel is corrected based on the position of the heating element corresponding to each pixel in the paper transport direction. Therefore, the density of the print image facing the vicinity of both ends in the paper transport direction may be reduced due to the banding phenomenon caused by the different heat radiation state of the thermal head in the paper transport direction. Is prevented beforehand.

According to a second aspect of the present invention, the density correction means adjusts the position of the heating element corresponding to each pixel in the paper transport direction before changing the density of the original image data for each pixel into the print image data. , And correction is performed according to a correction amount determined based on the density of the original image data of each pixel.

In the present invention, the density of the original image data for each pixel is corrected based on the position of the heating element corresponding to each pixel in the paper transport direction and the density of the image data of each pixel. Is done. Therefore, excessive correction is not performed on image data having a density close to white or black.

According to a third aspect of the present invention, in the image processing apparatus, the density correction means may include an original image data for a pixel corresponding to a heating element located near both ends in the sheet conveying direction among a plurality of heating elements provided in the thermal head. Characterized by increasing the concentration of

According to the third aspect of the invention, the density of the original image data is increased for the pixels corresponding to the heating elements located near both ends in the sheet transport direction. Therefore, by decreasing the density of the original image data in advance in the pixels near both ends in the paper transport direction, the decrease in the density of the printed image due to the banding phenomenon is offset.

According to a fourth aspect of the present invention, the density correction means includes a pixel corresponding to a heating element excluding heating elements located near both ends in the sheet conveying direction among a plurality of heating elements provided in the thermal head. Is characterized in that the density of the original image data is reduced.

According to the fourth aspect of the present invention, the density of the original image data is reduced for the pixels corresponding to the heating elements except for the heating elements located near both ends in the sheet conveying direction. Therefore, by lowering the density of the original image data in advance for the pixels other than those near the both ends in the paper conveyance direction, the decrease in the density near the both ends in the paper conveyance direction due to the banding phenomenon is relatively corrected.

According to a fifth aspect of the present invention, the density correction means determines the position of the heating element in the sheet conveying direction based on one or a plurality of printing conditions which affect the heat radiation state of the thermal head in the sheet conveying direction. And the relationship between the density of the original image data of each pixel and the correction amount of the image data is changed.

According to the fifth aspect of the present invention, the position of the heating element in the sheet conveying direction and the original image data of each pixel are determined based on one or a plurality of printing conditions which affect the heat radiation state in the sheet conveying direction. The relationship between the density and the correction amount of the image data is changed. Therefore, the density of the original image data is corrected in consideration of these printing conditions. The invention described in claim 6 is that the printing condition is a single or a plurality of printing conditions of the total number of the driven heating elements, the printing speed, the presence or absence of the ink ribbon, or the type of the ink ribbon. Features.

According to the present invention, the heat generation is performed based on the total number of the driven heating elements, the printing speed, the presence or absence of the ink ribbon, or one or a plurality of printing conditions of the type of the ink ribbon. The relationship between the position of the element in the paper transport direction, the density of the original image data of each pixel, and the correction amount of the image data is changed. Therefore, the density of the original image data is corrected in consideration of the total number of driven heating elements, the printing speed, the presence or absence of the ink ribbon, or one or a plurality of printing conditions among the types of the ink ribbon.

[0021]

FIG. 1 is a plan view of a serial thermal printer according to an embodiment of the present invention. In the serial thermal printer 31, the thermal head 5 is mounted on the carriage 1 together with the ribbon cassette 2. A part of a belt 29 stretched over pulleys 32 and 33 is fixed to the carriage 1. To one pulley 32 on which the belt 29 is stretched, rotational force is supplied from a pulse motor 28 via a gear 34 and a gear 35. As the pulse motor 28 rotates in both the forward and reverse directions, the carriage 1 reciprocates in the directions of arrows A and B.

At a position facing the thermal head 5 in the printer 31, a platen 36 is fixed in parallel with the moving direction of the carriage 1. Ink ribbon 40 stored in ribbon cassette 2 mounted on carriage 1
Passes between the thermal head 5 and the platen 36. In the printer 31, a rotation shaft 37 is arranged in parallel with the moving direction of the carriage 1. This rotating shaft 37
Are provided with a plurality of paper feed rollers 24. The rotation of the motor 38 is supplied to the rotation shaft 37 via a transmission mechanism 39. By the rotation of the motor 38, the paper feed roller 24 rotates together with the rotation shaft 37, and feeds a paper (not shown) between the ink ribbon 40 and the platen 36 in a direction orthogonal to the moving direction of the carriage 1.

The thermal head 5 has, for example, 64 heating elements arranged in a line in the paper transport direction. The heating element selectively energized among the plurality of heating elements heats the back surface of the ink ribbon 40 and melts the ink applied to the surface of the ink ribbon 40. The thermal head 5 moves in a direction approaching the platen 36 while the carriage 1 moves in the direction of arrow A during the printing operation, and presses the ink ribbon 40 against the platen 36 across the paper. By selectively energizing the plurality of heating elements during the forward movement of the carriage 1, the ink melted in the ink ribbon 40 may be arranged such that the arrangement range of the plurality of heating elements in the thermal head 5 is set to a width in the vertical direction. Each time it is transferred to the surface of the paper.

By selectively energizing a plurality of heating elements of the thermal head 5, a binary image such as a character image is formed, and the number of dots per unit area is increased / decreased to change a figure having a shading. Form an image.

When thermal recording paper is fed instead of paper, the ribbon cassette 2 is not mounted on the carriage 1 and the surface of the thermal recording paper is heated by a heating element selectively energized in the thermal head 5. Is heated directly to develop color. Further, only a part of the plurality of heating elements provided in the thermal head 5, for example, only 60 out of 64 heating elements may be used for a normal printing operation.

FIG. 2 is a block diagram showing the configuration of the data processing section of the serial thermal printer. The serial thermal printer 31 includes, as a data processing unit 41, an original image data input unit 9, an original image data buffer memory 10, a density correction unit 13, a corrected image data buffer memory 11, a binarization unit 14, and a binary image data. A buffer memory 12, a thermal head driving means 4, and a control means 7 for controlling them collectively. Among them, the original image data input means 9, the original image data buffer memory 10, the binarization means 14, the binarized image data buffer memory 12, and the thermal head driving means 4 have the same configuration as the conventional serial thermal printer. It is.

The data processing section 41 receives, via the original image data input means 9, multi-gradation original image data for one line of X × Y, where X is the number of columns and Y is the number of heating elements. Are stored in the original image data buffer memory 10.
The density correction means 13 is a buffer memory 1 for original image data.
Based on the density value of each pixel included in the original image data stored in 0 and the position information of the heating element corresponding to each pixel on the thermal head 5, the density value of each pixel is determined by a process described later. Buffer memory 1 for corrected image data
Numeral 1 stores corrected image data for one row of multi-gradation for each pixel.

The binarizing means 14 converts the density value of each pixel contained in the corrected image data stored in the corrected image data buffer memory 11 into a two-tone print image by an image processing method such as a dither method. Binary conversion to data. 2
The binarized image data buffer memory 12 stores one line of binarized image data binarized for each pixel.
The thermal head driving means 4 drives the heating elements of the thermal head 5 based on the binarized image data stored in the binarized image data buffer memory 12.

That is, the serial thermal printer 1 converts multi-gradation image data for each pixel of the original image into two-gradation image data, and assigns each pixel based on the converted two-gradation image data. By energizing the corresponding heating elements, a multi-gradation original image is represented by a 2-gradation dot image.
The density correction means 13 corrects in advance the density of original image data of multiple gradations before conversion into two gradations, in order to prevent a decrease in image density due to a decrease in dot density per unit area due to the banding phenomenon. .

FIG. 3 is a flowchart showing a part of the processing procedure in the data processing unit of the serial thermal printer. Control means 7 constituting data processing unit 41
Determines whether there is image data waiting to be printed in the original image data buffer memory 10 (s1), and if there is no image data, ends the printing process for one page. If the original image data buffer memory 10 has image data waiting for print processing, the control means 7 reads out the image data and performs density correction processing in the density correction means 13 (s2). The data is stored in the corrected image data buffer memory 11.

Next, the control means 7 reads the corrected image data from the corrected image data buffer memory 11,
The binarization processing is performed by the binarization means (s3), and the binarized image data after the binarization processing is stored in the binarized image data buffer memory 12. Further, the control means 7 reads out the binary image data from the binary image data buffer memory 12 and supplies the read binary image data to the thermal head driving means 4 to drive the thermal head 5. (S4). The control means 7 repeatedly executes the processing of s2 to s4.

FIG. 4 is a flowchart showing the details of the density correction processing in the data processing section of the serial thermal printer. In the density correction processing of s2, the control unit 7 of the data processing unit 41 first initializes a counter Cx for specifying a column to be subjected to density correction calculation (s11), and then calculates the count value of the counter Cx as an original. A comparison is made with the number of columns X of the image data (s12). The control means 7
In this comparison, if the count value of the counter Cx is equal to or more than the number of columns X of the original image data, the density correction processing ends.

If the count value of the counter Cx is less than the number X of columns of the original image data, the control means 7 initializes a counter Cy for specifying the number of the heating element in the thermal head 5 (s13), and then the counter Cy Is compared with the total number Y of the heating elements in the thermal head 5 (s1).
4). When the count value of the counter Cy is equal to or more than the total number Y of the heating elements in this comparison, the control means 7 increments the count value of the counter Cx and returns to s12 (s
17).

The control means 7 determines that the count value of the counter Cy is Y
If it is less than the above, the density B of the pixel at the column x and the heating element position y specified by the count values of the counters Cx and Cy is read from the original image data buffer memory 10, and the read density B and the heating element position y , The value of the corrected density Br is calculated by the following first equation, and the calculation result is stored in the corrected image data buffer memory 11 (s1).
5). Thereafter, the control means 7 increments the count value of the counter Cy and returns to s14 (s16).

Br = B + B × f1 (y) × f2 (B) Expression 1 In Expression 1, f1 (y) is a correction coefficient corresponding to the position y of the heating element with the position y of the heating element as an input. , And f2 (B) is a correction amount function that receives the density B of the target pixel in the original image as an input and outputs a correction amount corresponding to the density B of the target pixel. That is, the right side of the first expression means that the correction density determined by multiplying the correction coefficient determined by the position y of the heating element and the correction amount determined by the density B of the target pixel is added to the density B of the target pixel in the original image. doing.

The correction coefficient function f1 (y) and the correction amount function f
2 (B) is determined according to the state of occurrence of the banding phenomenon due to uneven heat radiation characteristics in the vertical direction of the thermal head 5. FIG. 5 shows an example of the correction coefficient function f1 (y), and FIG. 6 shows an example of the correction amount function f2 (B).

FIG. 5 shows an example of a correction coefficient function f1 (y) when printing is performed using 60 heating elements in the thermal head 5. The horizontal axis represents the correction coefficient, and the vertical axis represents the element position. Is shown. FIG. 5A shows a correction coefficient function f1 (y) when the banding phenomenon occurrence range is different between the upper end and the lower end of the thermal head 5.
Is a diagram showing a case where 100% correction is performed for 8 elements near the upper end and 5 elements near the lower end.

FIG. 5B is a diagram showing a correction coefficient function f (y) when the degree of occurrence of the banding phenomenon is different between the upper end and the lower end of the thermal head 5. Indicates a case where 80% correction is performed, and 100% correction is performed for the five elements near the lower end.

FIG. 5C shows a correction coefficient function f (y) in the case where the degree of banding phenomenon increases as the temperature of the heating element in a predetermined range near the upper end and lower end of the thermal head 5 approaches the end. ) Is a diagram illustrating a case where (100-20i)% correction is performed for five elements near the upper end and the lower end, where i is the number of elements from the end.

FIG. 5D shows an example of a plurality of heat generating elements of the thermal head 5 which are based on the print density of the element which generates the maximum banding phenomenon and which elements have a low banding phenomenon. Correction coefficient function f1 when performing a negative correction for an element that does not generate
It is a figure showing (y). FIG. 6 shows an example of the correction amount function f2 (B) when printing is performed using 60 heating elements in the thermal head 5, and the horizontal axis represents the original image density, and the vertical axis represents the correction amount. , Is shown. FIG.
(A) shows that the correction amount is 0 when the original image density is in the range of 0 to 20%, and that the correction amount is f2 when the original image density is in the range of 20 to 40%.
(B) = The correction amount is set to 0 to 20% by B, the correction amount is set to 20% in the range of the original image density of 40 to 80%, and f2 (B) = − in the range of the original image density of 80 to 100%. B shows a case where the correction amount is set to 20 to 0%.

Thus, the correction amount changes based on the original image density. This is because a fixed amount of correction is performed irrespective of the original image density. For example, when a correction value of 10% is added to a pixel of the original image whose original image density is 90%, the corrected density is corrected. Is 100%, which results in complete black, and when the original image density is near 0 or 100%, excessive correction is prevented from being performed.

FIG. 6B shows an element having a low banding phenomenon and a banding phenomenon based on the print density of the element which generates the banding phenomenon which is the largest in the plurality of heating elements of the thermal head 5. FIG. 9 is a diagram illustrating a correction amount function f2 (B) when performing a negative correction on an element that does not generate a phenomenon.

FIG. 7 is a flowchart showing a part of a processing procedure of a serial thermal printer according to another embodiment of the present invention. In the density correction process, the control unit 7 firstly sets the counter Cd and the counter Cx
Is cleared (s21, s22). Counter C
d counts the total number of heating elements that are energized while printing an image for one line. Next, the count value of the counter Cx is compared with the column number X of the original image data (s23). In this comparison, if the count value of the counter Cx is equal to or greater than the number of columns X of the original image data in this comparison, the control unit 7 ends the density correction processing.

If the count value of the counter Cx is less than the number X of columns of the original image data, the control means 7 initializes the counter Cy for specifying the number of the heating element in the thermal head 5 (s24), and then the counter Cy Is compared with the total number Y of the heating elements in the thermal head 5 (s2).
5). When the count value of the counter Cy is equal to or more than the total number Y of the heating elements in this comparison, the control means 7 binarizes the image data (s31), and
The number of heating elements that have been energized this time is added to the count value of d (s3
2), the count value of the counter Cx is incremented and s2
It returns to 3 (s33).

When the count value of the counter Cy is less than the total number Y of the heating elements, the control means 7 compares the count value of the counter Cd with a predetermined reference value D1 (s2).
6). When the count value of the counter Cd is smaller than the reference value D1 in this comparison, the control unit 7 sets the counter Cx
And the density B of the pixel at the column x and the heating element position y specified by the count values of Cy and Cy from the original image data buffer memory 10, and read the density B and the heating element position y.
, The value of the corrected density Br is calculated by the above-described first equation, and the calculated result is stored in the buffer memory 1 for corrected image data.
1 (s28).

When the count value of the counter Cd is equal to or more than the reference value D1, the control means 7 determines the density B of the pixel at the column x and the heating element position y specified by the count values of the counters Cx and Cy. The value is read from the data buffer memory 10 and the value of the corrected density Br is calculated based on the read density B and the heating element position y by the following equation (2), and the calculation result is stored in the corrected image data buffer memory 11 ( s29).

Br = B + B × g1 (y) × g2 (B) (2) The control means 7 calculates the correction density B in s28 or s29.
After the calculation of r, the coefficient value of the counter Cy is incremented and the process returns to s25 (s30).

In the second equation, g1 (y) is the first equation.
Similar to f1 (y) in the equation, this is a correction coefficient function in which the position y of the heating element is input and the correction coefficient corresponding to the position y of the heating element is output. g2 (B) is a correction amount function that receives the density B of the target pixel in the original image as an input and outputs a correction amount corresponding to the density B of the target pixel, similarly to f2 (B) in the first equation. The right side of the second expression is
Similarly to the right side of the first equation, adding a correction density obtained by multiplying a correction coefficient determined by the position y of the heating element and a correction amount determined by the density B of the target pixel to the density B of the target pixel in the original image. Means.

It should be noted that the correction coefficient function g1 (y) and the correction amount function g2 (B) correspond to the vertical direction of the thermal head 5 when a heat storage phenomenon occurs when a large number of heating elements exceeding the reference value D1 are energized. Is determined according to the state of occurrence of the banding phenomenon due to the unevenness of the heat radiation characteristics.

That is, when a large number of heating elements are driven, the heat generated by the energized heating elements causes the thermal head 5 to generate heat.
A heat storage phenomenon occurs in which the temperature rises, and the occurrence state of the banding phenomenon also changes accordingly. Therefore, the count value of the counter Cd that cumulatively counts the number of heating elements driven since the start of the printing process is compared with a reference value D1 that is a reference for discriminating whether or not a heat storage phenomenon occurs.
Is determined to be a heat storage phenomenon when the count value of the reference value is equal to or more than the reference value D1, and the correction amount of the image density is determined using the correction coefficient function g1 (y) and the correction amount function g2 (B). .

Thus, the image density can be finely corrected in consideration of the change in the occurrence state of the banding phenomenon according to the presence or absence of the heat accumulation phenomenon in the thermal head 5.

By comparing the coefficient value of the counter Cd with a plurality of reference values and selectively using any combination of three or more types of correction coefficient functions and correction amount functions based on the comparison result, The image density can be corrected more finely.

Further, at s28, only one of the correction coefficient function and the correction amount function may be changed.

Further, in the example shown in FIG. 7, the density correction is performed in consideration of the change in the state of occurrence of the banding phenomenon due to the number of driven heating elements. It also changes depending on printing conditions such as the difference between ink ribbons.

Therefore, instead of, or in addition to, the number of heating elements to be driven, some of a plurality of other printing conditions that change the state of occurrence of banding phenomena such as printing speed, printing method, and differences in ink ribbon. Alternatively, the correction coefficient function and the correction amount function used in the density correction processing may be changed according to all of them. In this case, there is provided a means for detecting the printing speed, a means for detecting the presence or absence of the ink ribbon in the carriage, or a means for detecting the type of the ink ribbon mounted on the carriage, and the density based on the detection output of these detecting means. The correction coefficient function and the correction amount function used for the correction processing are changed.

[0056]

According to the first aspect of the invention, the density of the original image data for each pixel is corrected based on the position of the heating element corresponding to each pixel in the paper transport direction.
Preventing a decrease in the density of the print image facing the vicinity of both ends in the paper transport direction due to the difference in the heat radiation state of the thermal head in the paper transport direction is prevented beforehand by correcting the density of the original image data by software in advance. Therefore, the configuration of the driving circuit of the thermal head is not complicated.

According to the second aspect of the present invention, the density of the original image data for each pixel is determined based on the position of the heating element corresponding to each pixel in the paper transport direction and the density of the image data of each pixel. By performing the correction, it is possible to prevent the image data having the density close to white or black from being excessively corrected.

According to the third aspect of the present invention, the density of the original image data is increased for the pixels corresponding to the heating elements located near both ends in the sheet conveying direction, so that the pixels near both ends in the sheet conveying direction are increased. For the pixels, the density of the original image data is increased in advance, so that the decrease in the density of the printed image due to the banding phenomenon can be offset.

According to the fourth aspect of the present invention, the density of the original image data is reduced for the pixels corresponding to the heating elements except for the heating elements located near both ends in the sheet conveyance direction, thereby reducing the density in the sheet conveyance direction. The density of the original image data is reduced beforehand for the pixels other than those near the both ends, and the reduction in the density near the both ends in the paper transport direction due to the banding phenomenon can be relatively corrected.

According to the fifth aspect of the present invention, the position of the heating element in the paper transport direction and the original image data of each pixel are determined based on one or a plurality of printing conditions which affect the heat radiation state in the paper transport direction. By changing the relationship between the density of the image data and the correction amount of the image data, the density of the original image data can be corrected in consideration of these printing conditions.

According to the present invention, the total number of driven heating elements, the printing speed, the presence or absence of the ink ribbon,
Alternatively, the relationship between the position of the heating element in the paper transport direction, the density of the original image data of each pixel, and the correction amount of the image data is changed based on one or a plurality of printing conditions of the type of the ink ribbon. By doing so, the density of the original image data can be finely corrected in consideration of these printing conditions.

[Brief description of the drawings]

FIG. 1 is a plan view of a thermal printer as an example of an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration of the thermal printer.

FIG. 3 is a flowchart showing a processing procedure in a printing process of the thermal printer.

FIG. 4 is a flowchart illustrating details of the print processing.

FIG. 5 is a diagram illustrating an example of a correction coefficient function in the thermal printer.

FIG. 6 is a diagram illustrating an example of a correction amount function in the thermal printer.

FIG. 7 is a flowchart illustrating details of print processing of a thermal printer according to another embodiment of the present invention.

[Explanation of symbols]

 1-carriage 2-ribbon cassette 4-thermal head driving means 5-thermal head 7-control means 11-corrected image data buffer memory 13-density correcting means

Claims (6)

[Claims] An original image data having a multi-gradation density is converted into print image data having two gradations, and based on the print image data,
In a thermal printer that performs printing processing by selectively driving a plurality of heating elements provided in the paper transport direction on a thermal head that moves in a direction perpendicular to the paper transport direction, the density of the original image data for each pixel is printed. A thermal printer comprising density correction means for correcting a heating element corresponding to each pixel in accordance with a correction amount determined based on a position of the heating element corresponding to each pixel in a sheet conveying direction before changing to image data. 2. The image forming apparatus according to claim 1, wherein the density correction unit changes the density of the original image data of each pixel into a print image data before changing the position of the heating element corresponding to each pixel in the sheet conveying direction and the original density of each pixel. 2. The thermal printer according to claim 1, wherein the correction is performed according to a correction amount determined based on the density of the image data. 3. The density correction means corrects the density of original image data for a pixel corresponding to a heating element located near both ends in the paper transport direction among a plurality of heating elements provided in a thermal head. 3. The thermal printer according to 1 or 2. 4. The density correction means according to claim 1, wherein said density correction means determines the density of the original image data for pixels corresponding to the heating elements excluding the heating elements located near both ends in the sheet transport direction among the plurality of heating elements provided in the thermal head. Claim 1 or 2 to be corrected
The thermal printer according to 1. 5. A method according to claim 1, wherein said density correction means determines a position of the heating element in the paper transport direction and original image data of each pixel based on one or a plurality of printing conditions which affect a heat radiation state of the thermal head in the paper transport direction. And changing the relationship between the density of the image and the correction amount of the image data.
The thermal printer according to 1. 6. The printing condition according to claim 5, wherein the printing condition is a single or a plurality of printing conditions of the total number of driven heating elements, a printing speed, the presence or absence of an ink ribbon, and the type of the ink ribbon. Thermal printer.