JPH05261963A - Thermal transfer type image forming device - Google Patents

Abstract

PURPOSE:To enable both recording of a sublimating ink sheet and a melting ink sheet by thinning data for printing so that dots are arrayed at a specific angle when the ink sheet is discriminated as the melting ink sheet by a discriminating means. CONSTITUTION:Gamma conversion is conducted comparatively from low density to high density by a gamma conversion means 34 having a gamma-characteristic conversion function, and data for printing are output. Whether or not a sublimating ink sheet or a melting ink sheet is used as the ink sheet set is discriminated by a discriminating means 38. When the ink sheet is discriminated as the melting ink sheet, the ink sheet is changed over by a changeover means 39, and printing data are thinned so that dots are arrayed at a specific angle by a thinning means 40. Accordingly, gamma-converted data are thinned and output in the melting ink sheet, which may have low energy, though gamma-converted data are output as they are in the sublimating ink sheet requiring high energy, thus allowing recording by low energy on an average.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a sublimable or fusible ink sheet in which a plurality of heating resistance elements are arranged in a line on the surface of a substrate and the heating resistance elements are selectively heated according to pixel data. And an image forming apparatus for forming dots on a recording sheet by sublimating or melting the ink.

[0002]

2. Description of the Related Art As a conventional example of an image forming apparatus for forming dots on a recording paper by sublimating or melting ink of a sublimable or fusible ink sheet, for example, a thermal transfer type disclosed in Japanese Patent Laid-Open No. 204059/1990. There is a color image recording apparatus, which will be described with reference to the block diagram of FIG. Reference numeral 101 is an image data reading section for reading image data from a memory (not shown), and 102 is based on a mark (not shown) in which the set ink sheet is a fusible ink sheet or a sublimable ink sheet. An ink sheet determination sensor unit 103 determines a sheet feeding control unit for a recording sheet that performs variable speed control based on information detected by the sensor unit 102, and a thermal conversion unit 104 converts image data into gradation data to perform thermal conversion. A gradation conversion unit for outputting to the head 106, and 105 for converting the image data into 2 by area pseudo gradation.
Convert to value data and output to thermal head 106 2
The value conversion unit is composed of, for example, a dither circuit.

Further, 107 is a changeover switch (changeover means).
The changeover switch 107 outputs the image data to the gradation conversion unit 104 when the sensor unit 102 determines that the ink is a sublimable ink sheet, and outputs the image data when the sensor unit 102 determines that the ink is a fusible ink sheet. It is output to the binary conversion unit 105.

[0004]

Since the conventional thermal transfer type color image recording apparatus is constructed as described above, the recording speed is different when the sublimable ink sheet and the fusible ink sheet are used, and the paper feed control for that purpose. There is a problem that a mechanism and the like are required, and when the fusible ink sheet is used, the image quality is largely different from the image when the sublimable ink sheet is used due to the dither processing of binary conversion.

The present invention has been made in view of the above problems, and an object thereof is to provide a special control mechanism or the like regardless of whether a sublimable ink sheet or a meltable ink sheet is set. It is an object of the present invention to provide a novel thermal transfer type image forming apparatus which can perform recording with no need and with little difference in image quality between the two.

[0006]

In order to solve such a problem, according to the present invention, a print head having a plurality of heating resistance elements arranged in a line, and a recording sheet and an ink sheet are conveyed at a predetermined pitch. Means, image data reading means for continuously reading pixel data forming the input image data for M lines, and pixel data for a plurality of lines of M × N pixels (M, N ≧ 2 integers). ) Sampling means for sampling in matrix form, and in order to express the pixel density for each pixel of M × N in dot size, gamma conversion is proportionally performed from low density to high density to output printing data. Gamma conversion means including a gamma characteristic conversion function, a determination means for determining whether the set ink sheet is a sublimable ink sheet or a fusible ink sheet, and a determination means And a thinning-out means for thinning the printing data so that the dots are aligned at a specific angle after switching by the switching means when the fusible ink sheet is discriminated by the discriminating means. A means for supplying electric energy corresponding to the printing data to the heating resistance element of the print head is provided. Further, the γ conversion means is provided with a plurality of types of gamma characteristic conversion functions, and further, the thinning means is set so that the aligned angles of the thinned dots are different for each primary color.

[0007]

In the thermal transfer type image forming apparatus according to the present invention,
The image processing method is automatically switched depending on the type of the set ink sheet. A sublimable ink sheet that requires high energy outputs γ-converted data as it is, whereas a low-energy fusible ink sheet that outputs only γ-converted data and outputs the average data. It will be recorded at low energy, and it will be performed at the same recording speed. Further, by setting the thinning-out means so as to change the angle at which the dots after thinning are aligned for each primary color, the generation of mesh noise and the prevention of color turbidity are prevented.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will now be described based on the illustrated embodiments.

FIG. 2 shows an embodiment of the present invention, which comprises a thermal transfer printing mechanism and a control device. The thermal transfer printing mechanism includes a paper feed mechanism 4 that pulls out one recording sheet from a stacker 2 stocking the recording sheets 1 by a pickup roller 3 and conveys the recording sheet to a printing area, and a constant speed of the recording sheets 1 and the ink sheet 9. , A thermal transfer type print head 6 which is brought into pressure contact with the platen 5 at the time of printing, and the printed recording paper is again fed to the stacker 2
It is composed of a conveying roller 8 that returns to the side, a stock roller 10 that supplies the ink ribbon 9, and a take-up roller 11. A circuit board 12 incorporating a control circuit, which will be described later, is housed in the lower part of the housing.

As shown in FIG. 3, the thermal transfer print head 6 includes a heating resistor element 14 and a heating resistor element 14 on the surface of the substrate 13 at a constant pitch.
, 14 are formed in a line, and lead wires 15, 15, 15, ..., 16, 16, 16 ...
It is constructed by pulling out.

FIG. 4 shows an embodiment of the above-mentioned control circuit. In the figure, reference numeral 17 is a microcomputer constituting the central portion of the control device, which is a CPU 18, a control program and data processing described later. Program, and further, a ROM 19 storing a gamma conversion characteristic and a RAM 20 constituting a buffer and a frame memory for data processing, and an external device such as a personal computer via the interfaces 21 and 22, and a print head drive circuit 23. , Connected to the motor drive circuit 24.

The print head drive circuit 23 outputs electric energy corresponding to the print data output from the interface 22, for example, print data D _{0 as} shown in FIG.
Times T ₀ , T ₁ , corresponding to D ₁ , D ₂ , D ₃ ,.
The heat generating resistance elements 14, 14, 14 of the thermal transfer print head 6 are supplied with pulsed electric power in which T ₂ , T ₃ , ...
Is configured to feed.

The motor drive circuit 24 drives the motor 25 connected to the pickup roller 3, the platen 5, and the conveyance roller 8, and has a rotation direction and rotation corresponding to a command output from the interface 22. It is configured to output the drive pulse so as to have the speed.

FIG. 1 shows an embodiment represented by a function fulfilled by the microcomputer 17 necessary for performing color printing by making use of the above-mentioned basic printing function, and reference numeral 30 in the drawing. Is an image memory for storing image data output from an external device, and is configured to store a certain amount of image data for each color output from the external device, for example, one page. 31 is an image data reading means,
Image data stored in the image memory 30 is extracted for M lines (an integer of M ≧ 2), and in this embodiment, pixel data for 4 lines is extracted while being moved by M lines and output to the line buffer 32. Is. A sampling unit 33 samples N digits (an integer of N ≧ 2), that is, M × N dots, in this embodiment, 4 × 4 pixels in the main scanning direction from the pixel data stored in the line buffer 32. Output to the gamma conversion means 34 described later.

Reference numeral 34 denotes the above-mentioned gamma conversion means, which is used for actual recording of the pixel data as shown in FIG. 6 so that the pixel data from the sampling means 33 can function as a filter and a print density setting means. Data for defining the relationship between pixel data and printing data is stored so as to specify the size of dots (hereinafter referred to as printing data).

FIG. 6 shows an embodiment of the gamma characteristic conversion function set in the aforementioned gamma conversion means 34, in which the gamma characteristic conversion functions γ0, γ1 and γ2 are set. FIG. 14 is a matrix diagram for explaining the allocation of the gamma characteristic conversion function of the gamma conversion means 34, which is configured by storing the gamma characteristic conversion function γ0 in all 4 × 4 pixels. Further, FIG. 18 is configured by storing the gamma characteristic conversion functions γ1 and γ2 as shown in the figure. Further, the gamma characteristic conversion function γ1 or γ2 is stored (not shown) in all 4 × 4 pixels.

Reference numeral 35 is a conversion characteristic selection means for storing data for appropriately selecting from among the various gamma characteristic conversion functions of the above-mentioned gamma conversion means 34 corresponding to the primary colors output to the image memory 30. It is configured.

Further, in order to uniformly correct the output density of each heating resistance element 14 of the thermal transfer print head 6, the gamma conversion characteristic is selected by the head density correction data 36, and the thermal influence is corrected by the history of each print. In order to do so, the gamma conversion characteristic is selected by the thermal history correction data 37.

Reference numeral 38 is a discriminating means for discriminating whether the set ink sheet is a sublimable ink sheet or a fusible ink sheet, and the discriminating means 38 switches the subsequent processing by a switching means 39. .. Reference numeral 40 denotes a thinning-out means for thinning out the printing data so that the dots are aligned at a specific angle after being switched by the switching means 39 when the determination means 38 determines that the ink is a fusible ink sheet.
At that time, the thinned portion becomes non-energized. Reference numeral 41 denotes a print buffer. When it is determined that the sheet is a sublimation ink sheet, it is switched by the switching unit 39, and then the printing data is temporarily stored as it is. When it is determined that the sheet is a fusible ink sheet, the switching unit 39 is used. After the switching, the print data processed by the thinning means 40 is temporarily stored in the print buffer 41.

Next, in order to facilitate the understanding of the present invention, the relationship between the driving mode of the heating resistance element and the dots formed by the heating resistance element will be described.

FIG. 7 is a transfer characteristic diagram for each ink sheet and each recording method. When recording with a sublimable ink sheet, the transfer rate is gentle with respect to the input energy as indicated by the characteristic 26.

On the other hand, in the case of recording with the fusible ink sheet, when the heating resistance element 14 of the thermal transfer print head 6 is recorded simultaneously and continuously also in the sub-scanning direction, the input energy is changed as shown in the characteristic 27. On the other hand, the transfer rate is extremely steep. In addition, the heat generating resistance element 1 of the thermal transfer print head 6
When recording is performed in a checkered pattern with every other 4 and the other heating resistance element 14 is energized in the sub-scanning direction, the transfer rate is recorded with the sublimable ink sheet with respect to the input energy as indicated by the characteristic 28. It exhibits similar characteristics to those of Further, when recording is performed by energizing every other heat generating resistance element 14 of the thermal transfer print head 6 and every other one in the sub-scanning direction, the transfer rate becomes gentle with respect to the input energy as indicated by the characteristic 29. A large input energy is required to obtain a solid image (transfer rate 100%). Recording with this fusible ink sheet will be described in more detail with reference to FIGS. 8 to 13.

As shown in the explanatory view of FIG. 8, when only one heating resistance element 14 is energized, while the adjacent heating resistance elements 14 'and 14 "are suspended and printing is performed, the driven heating resistance element is driven. Since the element 14 does not receive thermal interference from the heat generating resistance elements 14 'and 14 "adjacent to it,
It is possible to use up to the area of the other adjacent heating resistance elements 14 'and 14 "as a dot forming area. As a result, the relative input energy E supplied to the heating resistance element is" as shown in FIG. Even if it is divided into multiple stages of "32", the size of the dots formed is proportional to the relative input energy, so that extremely high gradation can be expressed.
Therefore, dots having a density proportional to the density of pixel data are formed. As a result, as shown in FIG. 10, each dot is independently formed in the vicinity of medium density and high density as the energy increases, and a solid image is formed at the maximum density.

Further, as shown in FIG. 11, while the two heating resistance elements 14 and 14 'are driven at the same time, while the heating resistance elements adjacent to these elements are printed in a stopped state, the heating resistance elements are not separated from each other. Since it is possible to sufficiently prevent the heat interference in practical use, it is possible to form dots with a gradation that is substantially the same as in the case where the heating resistance element described above is driven alone.

On the other hand, when the adjacent heating resistor elements are also driven, the thermal interference between the heating resistor elements becomes large as shown in FIG. 12, so that as shown in FIG. In the density range, dots that should originally be formed are already connected in places, which causes variations in gradation expression (Fig. (I)), and in the vicinity of high density, the problem of complete connection occurs. (Figure (II)).

In consideration of the above phenomenon, the thinning means 40 is adapted to obtain smooth gradation. This will be described with reference to FIGS. 15, 16 and 17.

FIG. 15 shows data obtained by thinning out half of the dots in the checkered pattern in the matrix formed by storing the gamma characteristic conversion function γ0 in all 4 × 4 pixels in FIG. Show. The part thinned out here becomes non-energized. As a result, the total energy consumed for recording is half that in the case of the sublimable ink sheet, and the recording is performed with almost the proper density and the quality close to that of the sublimable ink sheet without being affected by the peripheral dots. .. As a result of this,
Dots are formed at a screen angle of 45 degrees.

FIG. 16 shows a matrix formed by storing the gamma characteristic conversion function γ0 in all 4 × 4 pixels in FIG. 14 and combining two dots in the sub-scanning direction, and half the dots in a checkered pattern. The data after thinning by the thinning means 40 is shown. As a result, the total energy consumed for recording is half that in the case of the sublimable ink sheet, and the recording is performed with almost the proper density and the quality close to that of the sublimable ink sheet without being affected by the peripheral dots. .. As a result of this,
Dots are formed at a screen angle of 63.4 degrees.

FIG. 17 shows a matrix formed by storing the gamma characteristic conversion function γ0 in all of the 4 × 4 pixels in FIG. 14, which is a combination of 2 dots in the main scanning direction and a half dot in a checkered pattern. The data after thinning by the thinning means 40 is shown. As a result, the total energy consumed for recording is half that in the case of the sublimable ink sheet, and the recording is performed with almost the proper density and the quality close to that of the sublimable ink sheet without being affected by the peripheral dots. .. As a result of this,
Dots are formed at a screen angle of 26.6 degrees.

Next, the case of FIG. 18 stored in the gamma conversion means 34 will be described with reference to FIGS. 19, 20 and 21.

In FIG. 18, 8 gamma characteristic conversion functions γ1 and γ2 are stored in 4 × 4 pixels.

FIG. 19 shows a matrix composed of eight 4 × 4 pixel gamma characteristic conversion functions γ1 and γ2 in FIG.
The data after thinning out with 0 are shown. As a result, the gamma characteristic conversion function γ1 is 7, and the gamma characteristic conversion function γ2 is 1, so that the total energy consumed for recording is less than half that in the case of the sublimable ink sheet. As a result, the characteristic 26 closer to that of the sublimable ink sheet shown in FIG. 7 is obtained without being affected by the peripheral dots, and high-quality recording rich in gradation can be obtained. In addition, dots are formed with a screen angle of 45 degrees.

FIG. 20 shows a matrix composed of eight 8 × 4 × 4 pixel gamma characteristic conversion functions γ1 and γ2 in FIG. The data after thinning the dots by the thinning means 40 is shown. This gives the gamma characteristic conversion function γ
There are four 1s and four gamma characteristic conversion functions γ2, and the total energy consumed for recording is 1/2 that in the case of a sublimable ink sheet. As a result, the characteristic 26 similar to that of the sublimable ink sheet shown in FIG. 7 is obtained without being affected by the peripheral dots, and high-quality recording rich in gradation can be obtained. Also,
Dots are formed at a screen angle of 63.4 degrees.

FIG. 21 shows a matrix consisting of eight 4 × 4 pixel gamma characteristic conversion functions γ1 and γ2 in FIG. The data after thinning the dots by the thinning means 40 is shown. This gives the gamma characteristic conversion function γ
The number of 1's is 5, the number of gamma characteristic conversion functions γ 2 is 3, and the total energy consumed for recording is 1/2 or less that in the case of the sublimable ink sheet. As a result, the characteristic 26 closer to that of the sublimable ink sheet shown in FIG. 7 is obtained without being affected by the peripheral dots, and high-quality recording rich in gradation can be obtained. Further, dots are formed with a screen angle of 26.6 degrees.

The combination of these gamma characteristic conversion functions γ1 and γ2 is made in consideration of the transfer efficiency of each pattern arrangement, and the image quality becomes closer to that of a sublimable ink sheet. Even in the case of sublimable ink sheets, by combining different gamma characteristic conversion functions,
In particular, it enhances gradation at low density.

Similarly, only the gamma characteristic conversion function γ1 or only γ2 can be arranged, and the gamma characteristic conversion functions γ0, γ1, and γ2 can be freely selected and arranged.

As described above, when recording with the three colors of the meltable ink sheet, yellow, magenta, and cyan, when applying FIGS. 15, 16 and 17 or FIGS. 19, 20 and 21, respectively, a screen angle of 45 is obtained. Degree, 63.4
With a combination of 2 and 66.6 degrees, clear color printing can be performed by utilizing the advantage of the screen angle setting that is performed in gravure printing or the like.

FIG. 22 shows another embodiment represented by the function fulfilled by the microcomputer 17 required for performing color printing by making use of the basic printing function described above. Reference numeral 30 is an image memory for storing image data output from the external device, and is configured to store a certain amount of image data for each color output from the external device, for example, for one page. An image data reading unit 31 extracts the image data stored in the image memory 30 by M lines (an integer of M ≧ 2), and in this embodiment, moves the pixel data of four lines by M lines. Output to the line buffer 32. Reference numeral 33 is a sampling means, which has N digits (an integer of N ≧ 2) in the main scanning direction from the pixel data stored in the line buffer 32.
Minutes, that is, M × N dots, that is, 4 × 4 pixels in this embodiment, are sampled and output to the switching means 39.

Reference numeral 38 is a discriminating means for discriminating whether the set ink sheet is a sublimable ink sheet or a fusible ink sheet, and the discriminating means 38 switches the subsequent processing, and the discriminating means 39. When the means 38 determines that the sheet is a fusible ink sheet, the switching means 39
After the switching, the printing data is processed by the thinning means 40 for thinning out so that the dots are aligned at a specific angle, the pixel data is processed by the gamma conversion means 34 described above, and is temporarily stored in the print buffer 41. The pixel data of the thinned portion corresponds to d0 in FIG. On the other hand, when the determination unit 38 determines that the ink is a sublimable ink sheet, the switching unit 39 switches the printing data and the printing data is directly changed to the gamma conversion unit 34.
And the image is temporarily stored in the print buffer 41.

Numeral 35 is a conversion characteristic selection means for storing data for appropriately selecting from among the various gamma characteristic conversion functions of the above-mentioned gamma conversion means 34 corresponding to the primary colors output to the image memory 30. It is configured.

Further, in order to uniformly correct the output density of each heating resistance element 14 of the thermal transfer print head 6, the gamma conversion characteristic is selected by the head density correction data 36, and the thermal effect is corrected by the history of each print. In order to do so, the gamma conversion characteristic is selected by the thermal history correction data 37.

[0042]

As described above, according to the present invention, a print head having a plurality of heating resistance elements arranged in a line, a means for conveying a recording paper and an ink sheet at a predetermined pitch, and an input image Image data reading means for continuously reading pixel data forming data for M lines, and pixel data for the plurality of lines are M × N pixels (M,
Sampling means for sampling in a matrix form of (N ≧ 2), and in order to express the pixel density for each M × N pixel by the dot size, γ conversion is performed proportionally from low density to high density. A gamma conversion unit that includes a gamma characteristic conversion function that outputs printing data, a determination unit that determines whether the set ink sheet is a sublimable ink sheet or a fusible ink sheet, and a determination unit that performs subsequent processing. Switching means for switching, and when the determination means determines that the sheet is a fusible ink sheet, after switching by the switching means, thinning-out means for thinning out the printing data so that dots are aligned at a specific angle, and the printing data Means for supplying corresponding electric energy to the heat generating resistance element of the print head, and the gamma conversion means comprises a plurality of types of guns. By providing a characteristic conversion function, and further, the thinning means is set so that the angle at which the dots after thinning are aligned is different for each primary color.
Sublimable ink sheet, without significant changes to the device
Both recordings of the fusible ink sheet can be enabled.

Further, the sublimable ink sheet which requires high energy outputs the γ-converted data as it is, whereas the fusible ink sheet which requires low energy has the γ value.
By thinning and outputting the converted data, the data is recorded with low energy on average, and the recording can be performed at the same recording speed.

Further, by setting the thinning-out by the thinning-out means so that the angles of the dots after the thinning-out are different for each primary color, it is possible to prevent the generation of mesh noise and the color turbidity.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention with the functions to be performed by a microcomputer.

FIG. 2 is a sectional view showing an embodiment of a thermal transfer printer to which the present invention is applied.

FIG. 3 is an enlarged front view showing an example of a print head used in the present invention.

FIG. 4 is a block diagram showing an example of a control device of the thermal transfer printer shown in FIG.

FIG. 5 is a diagram showing an example of signals for driving the print head in the present invention.

FIG. 6 is a diagram showing an embodiment of a γ function used in the present invention.

FIG. 7 is a transfer characteristic diagram of each ink sheet.

FIG. 8 is a diagram showing a relationship between energy supplied to the print head and a heat generation region.

9 is a diagram showing the relationship between the energy supplied to the heating resistance element and the dot size by the driving method shown in FIG.

10 is a diagram showing the relationship between the driving method shown in FIG. 8 and a print pattern.

FIG. 11 is a diagram showing a relationship between supplied energy and a heat generation region in a print head driving method applied to the present invention.

FIG. 12 is a diagram showing an example of a conventional driving method of a print head.

FIG. 13 is a diagram showing a relationship between the conventional driving method shown in FIG. 12 and a print pattern.

14 is a diagram showing an application example of the gamma characteristic conversion function shown in FIG. 6 used in the present invention.

FIG. 15 is a diagram showing an application example of a gamma characteristic conversion function after performing a thinning process in FIG.

FIG. 16 is a diagram showing another application example of the gamma characteristic conversion function after performing the thinning process in FIG.

FIG. 17 is a diagram showing another application example of the gamma characteristic conversion function after performing the thinning process in FIG.

FIG. 18 is a diagram showing another application example of the gamma characteristic conversion function shown in FIG. 6 used in the present invention.

FIG. 19 is a diagram showing an application example of the gamma characteristic conversion function after performing the thinning process in FIG. 18;

FIG. 20 is a diagram showing another application example of the gamma characteristic conversion function after performing the thinning process in FIG. 18;

FIG. 21 is a diagram showing another application example of the gamma characteristic conversion function after performing the thinning process in FIG. 18.

FIG. 22 is a block diagram showing another embodiment of the present invention with the function to be performed by the microcomputer.

FIG. 23 is a block diagram showing functions of a conventional thermal transfer type color image recording apparatus.

[Explanation of symbols]

 1 Recording Paper 2 Stacker 3 Pickup Roller 4 Paper Feeding Mechanism 5 Platen 6 Thermal Transfer Printing Head 9 Ink Sheet 12 Control Circuit Board 14 Heating Resistance Element 31 Image Data Reading Means 33 Sampling Means 34 Gamma Converting Means 35 Conversion Characteristic Selecting Means 38 Discriminating Means 39 Switching means 40 Thinning means

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical indication location B41J 17/36 Z 9211-2C H04N 1/032 D 9070-5C 1/23 102 B 9186-5C 8907 -2C B41J 3/20 109 Z

Claims (3)

[Claims] 1. A print head having a plurality of heating resistance elements arranged in a line, a means for conveying a recording paper and an ink sheet at a predetermined pitch, and pixel data constituting input image data is M. Image data reading means for continuously reading lines, and pixel data for a plurality of lines M ×
Sampling means for sampling in a matrix of N pixels (M, an integer of N ≧ 2) and pixel density for each M × N pixel are expressed in dot size. Gamma conversion means that includes a gamma characteristic conversion function that performs gamma conversion and outputs printing data; determination means that determines whether the set ink sheet is a sublimable ink sheet or a meltable ink sheet; Switching means for switching the subsequent processing by means, and thinning means for thinning the printing data so that the dots are aligned at a specific angle after switching by the switching means when the determination means determines that the sheet is a fusible ink sheet. And a means for supplying electric energy corresponding to the printing data to the heating resistance element of the print head. Image forming apparatus. 2. The thermal transfer image forming apparatus according to claim 1, wherein the gamma conversion unit includes a plurality of types of gamma characteristic conversion functions. 3. The thermal transfer image forming apparatus according to claim 1, wherein the thinning-out means is set so that the aligned angles of the thinned dots are different for each primary color.