JPH0646274A - Image forming device - Google Patents

Abstract

PURPOSE:To provide the thermal transfer type printer in which the gradation and resolution are improved with screen angle setting data as less as possible. CONSTITUTION:The device is provided with a picture data read means 31 reading picture element data consecutively by N lines, a sampling means 33 sampling picture element data by plural lines into a matrix of NXN picture elements, a gamma conversion means 34 selecting one among plural kinds of gamma characteristic conversion functions with respect to each of the NXN picture elements and expressing the result of conversion in a dot size, and a conversion characteristic selection means 35 applying the selected gamma characteristic conversion function to each of the NXN picture elements sampled so that dots resulting from the gamma characteristic conversion function are arranged to a specific angle. Gamma conversion is implemented almost monotonously and increasingly from a low density to a medium density and implemented almost monotonously and decreasingly from a medium density to a high density to form major dots.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for forming an image by forming dots on a recording sheet. More specifically, it relates to the gradation control technology and the color reproduction technology.

[0002]

2. Description of the Related Art When full-color printing is performed by a printer, a process of using a total of four primary colors of yellow, magenta, cyan, and black, and returning the recording medium to the initial position for printing for each primary color is performed. Must be repeated multiple times. Therefore, there is a problem in that the pixel data to be printed and the printing point on the recording medium are misaligned, and the quality of the printed image deteriorates. Therefore, a method has been proposed in which a screen angle is set with respect to a recording medium to prevent deviation of the dot formation position for each primary color and to prevent color turbidity as much as possible. For example, in the inkjet system or the wire dot system in which the size of dots that can be printed is constant, the density is expressed by the number of dots formed in a fixed area of 4 × 4 dots, and the position where one dot is formed in at least the area is determined. By defining each primary color, the dot forming positions are aligned in a specific direction of the recording medium.

However, in this method, since the density is expressed by the number of dots, if the density level that can be expressed is increased, the number of dots to be assigned per pixel increases and the image becomes rough.

Therefore, it is possible to express the density of the dots themselves formed on the recording medium, and set the threshold value corresponding to the pixel which is the reading unit of the original image, which becomes the core of forming dots at all times. Japanese Patent Laid-Open No. 61-189774 proposes a method of setting density gradation and screen angle by setting a plurality of matrices.

[0005]

However, when the image forming method as described above is adopted, the gradation and the resolution can be improved at the same time, but when the gradation is further increased, a huge amount of matrix data is required. However, there is a problem that the memory for storing the matrix data becomes large.

The present invention has been made in view of such a problem, and an object thereof is to set a screen angle while maintaining high gradation and resolution while using a small amount of screen angle setting data. It is to provide a novel image forming method and apparatus capable of achieving the above.

[0007]

In order to solve such a problem, in the image forming apparatus of the present invention, in the first aspect, the pixel data forming the input image data is for N lines (N ≧). (Integer of 2) image data reading means for continuously reading, and N × N pixel data for the plurality of lines.
Sampling means for sampling in a matrix of pixels, gamma conversion characteristic selecting means for selecting one from a plurality of gamma characteristic functions, and gamma characteristic function selected by the gamma conversion characteristic selecting means are obtained by the sampling means. The sampled N × N is provided with a gamma conversion means for converting the sampled data into print data.
The screen angle is set by using a predetermined gamma characteristic function for each of the pixels.

Further, according to a second aspect of the present invention, in the gamma conversion means, the gamma conversion is performed in a monotonically increasing manner at least from a low density to a medium density, and the gamma conversion is performed in a monotonically decreasing manner from a middle density to a high density. One type of gamma characteristic conversion function that outputs the dots as the print data is included.

Further, according to a third aspect of the present invention, all the gamma characteristic conversion functions output printing data that is substantially equal in the highest density portion.

Further, in claim 4, an image is printed by the thermal transfer type printer.

[0011]

At least 2 × obtained by sampling
Pixel data having two pixels is limited to two main dots and sub dots in the low to medium density range by the gamma conversion means. As a result, thermal interference between the heat generating resistance elements forming the dot forming means is reduced as much as possible, and gradation is expressed by the area of the dot itself to improve gradation and resolution.

Further, by changing the application form of the gamma conversion characteristic, the position where the main dot corresponding to at least 2 × 2 pixels is formed is changed for each primary color, and the generation of mesh noise is prevented. To prevent turbidity.

[0013]

EXAMPLE A thermal transfer type printer, which is one of thermal transfer type image forming apparatuses, comprises an ink sheet or ribbon in which a wax containing a pigment is applied to a polymer film, and minute heating elements are integrated on a substrate. By supplying a signal based on pixel data to the print head, ink in a minute area is melted to form dots on the recording paper. In such a thermal transfer type image forming apparatus, it is easy to configure the heating resistance element, which is a dot forming element, to a minute size, and moreover, the size of the print dot can be changed relatively easily. It is widely used in the field of full color printing that requires.

Therefore, in the embodiment, the details of the present invention will be described by taking a thermal transfer printer as an example.

FIG. 2 shows an embodiment of the present invention, which comprises a thermal transfer printing mechanism and a controller. 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 print head 6 that is pressed against the platen 5 when printing, and a stacker 2 that prints recording paper again.
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 described later is accommodated in the lower part of the housing. The thermal transfer print head 6 is
As shown in FIG. 3, the heating resistor elements 14 are formed in a line on the surface of the substrate 13 at a constant pitch, and lead wires 15 and 16 are drawn out in the paper feeding direction.

FIG. 4 shows an embodiment of the above-mentioned control circuit. In the figure, reference numeral 17 is a microcomputer constituting the central part of the control device, and a CPU 18, a control program, and a data processing described later. An external device such as a personal computer and a print head drive circuit 23, which are composed of a ROM 19 storing a program, further a gamma conversion characteristic, and a RAM 20 constituting a buffer for data processing and a frame memory, and the interface 21 and 22. It is connected to the motor drive circuit 24. The print head drive circuit 23 supplies electric energy that matches the data representing the density output from the interface 22. For example, as shown in FIG.
1, B2, B3, B4, ... Bn corresponding to times T1, T2,
The supply time of the pulsed electric power is changed so that T3, T4, ... Tn are increased, and the electric power is supplied to each heating resistance element 14 of the thermal transfer print head 6. Also,
The motor drive circuit 24 drives a motor 25 connected to the pickup roller 3, the platen 5, and the conveyance roller 8. The motor drive circuit 24 applies a drive pulse so that the rotation direction and the rotation speed correspond to the command output from the interface 22. It is configured to output.

FIG. 1 shows an embodiment for performing color printing by making use of the basic printing function described above. Reference numeral 3 in the figure
An image memory 0 stores image data output from the external device, and stores image data for each color output from the external device in a fixed amount, for example, for one page. 31
Is an image data reading means for extracting the image data stored in the image memory 30 by N lines (an integer of N ≧ 2), and in this embodiment, by moving the pixel data of 2 lines by N lines. The data is output to the line buffer 32. A sampling means 33 samples N digits (an integer of N ≧ 2), that is, N × N dots (2 × 2 pixels in this embodiment) from the pixel data stored in the line buffer 32 in the main scanning direction. It is then output to the gamma conversion means 34 described later.

Reference numeral 34 denotes the above-mentioned gamma conversion means, which stores the data 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. FIG. 6 is data that defines the density of dots (hereinafter referred to as converted print data) with respect to the density of pixel data, and in this embodiment, it is composed of the following four types of functions.

The gamma characteristic conversion function γ1 does not form a transfer dot when the density of pixel data is zero, but supplies a minimum amount of energy that can maintain the temperature of the heating resistance element constant regardless of the surrounding environment. The converted print data Da is set to increase the density to a maximum density Db from here to the maximum density Db, and further decreases to a maximum density Db from the intermediate density area to the high density area. It has a characteristic of decreasing to a density Dc close to the density of data. The density Dc is set to a value that gives a dot density of 11.8 dots / mm when binary printing is performed such as character printing. The density Db is set to a value capable of supplying about twice the energy as the density Dc.

The gamma characteristic conversion function γ2 has a density Dc close to the density of pixel data only near the maximum density value of image data.
It has a characteristic to generate.

The gamma characteristic conversion function γ3 has a characteristic that it increases substantially monotonically in the high density region and becomes a density Dc close to the density of image data at the maximum density.

The gamma characteristic conversion function γ4 has a characteristic that it increases almost monotonically from Da to the high density area and has a density Dc close to the density of pixel data in the maximum high density area.

As a result of the gamma characteristic conversion function having such a characteristic, when the gamma characteristic is assigned to the four sampled matrix-shaped pixels, the dot which becomes the center of the image representation by γ1 is arranged to be adjacent to it. As a result, it becomes possible to re-express the sampling area by adding sub dots by γ4, γ3 in the high density area, and γ2 to the pixel data showing the highest density.

Reference numeral 35 in FIG. 1 is a conversion characteristic selecting means,
Sampled matrix pixel data D11, D
12, ... D14, D21, D22, ..., D24, D31, D32,
Data for allocating four gamma characteristic conversion functions to D34, D41, D42, ... D44 are stored. The allocation at that time corresponds to the primary colors output to the image memory 30.

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

As shown in FIG. 7, only one heating resistor element 14 is energized, while the adjacent heating resistor element 1 is energized.
When printing is performed with 4 ′ and 14 ″ paused, the driven heating resistor element 14 is driven by the heating resistor element 1 adjacent thereto.
Since there is no thermal interference from 4'and 14 ", it is possible to use up to the area of the other adjacent heating resistance elements 14 'and 14" as the dot formation area, and express extremely high gradation. It becomes possible to do. For example, in FIG.
Even when the relative input energy E supplied to the heat generating resistance element is divided into 32 stages as shown in (4), it is possible to form a relative dot size that faithfully represents the density of pixel data. As a result, as shown in FIG. 9, from low density to high density, each dot changes from an independent shape to a solid image as the energy increases.

Further, as shown in FIG. 10, even when two adjacent heating resistance elements 14 and 14 'are driven at the same time and the heating resistance elements adjacent to them are stopped at the same time, thermal interference between the heating resistance elements is caused. Since it can be sufficiently prevented in practical use, it is possible to form dots with a gradation that is approximately the same as when the above-mentioned heating resistance element is driven alone. On the other hand, when the adjacent heat generating resistance elements are also driven, thermal interference between the heat generating resistance elements becomes large as shown in FIG. 11, so that as shown in FIG. In this case, dots that should originally be formed independently are connected in places, which causes variations in gradation expression (FIG. 9B). Further, they are completely connected in the vicinity of high concentration (Fig. C).

Next, the case where the above-mentioned gamma conversion characteristic is applied will be described.

Sampling means 33 as shown in FIG.
The conversion characteristic selecting means 3 is applied to the matrix-like 2 × 2 pixel data D12, D12, D21, D22 extracted by
The case where the conversion characteristics γ1, γ2, γ3, and γ4 are applied as shown in FIG.

The sampled pixel data D11 by the gamma characteristic conversion function γ1, the pixel data D12 by the gamma characteristic conversion function γ2, the pixel data D21 by the gamma characteristic conversion function γ3, and the pixel data D22 by the gamma characteristic conversion function γ4. To be converted. As a result, when the pixel data D11 and D12 of the first line are printed, the pixel data D11 is applied to the pixel data of the intermediate density which frequently appears in a normal image.
Dots are formed due to the characteristic of γ1 only for.
When the printing of the first line is completed, the pixel data D21 and D22 in the same sampling area are printed, but the function γ3 is assigned to the pixel data D21 and the function γ4 is assigned to the pixel data D22. Therefore, only the pixel data D22 is printed in the intermediate density area. In the printing of the second line as well, similar to the printing of the first line described above, the other adjacent heating resistance element is in the idle state, so that the pixel data to be printed is faithful to the density converted by γ4. The dots are formed with various densities. Hereinafter, the form as shown in FIG. 14, that is, the choreographic form of the gamma characteristic conversion function shown in FIG. 13 is alternately repeated to continue printing. As a result, the four pixels specified by the sampling area are printed with the pixel data D11 as the main dots and the pixel data D22 as the sub dots, and the main dots are supplemented by the sub dots. 4 pixel data D11, D12, D21, D22 are expressed by.

That is, when the density of the area composed of the four sampled pixels is low, only the dot corresponding to the pixel D11 to which the gamma characteristic conversion function γ1 is assigned is printed (Fig. 15A). Further, when the density of the sampled area becomes high, the dot 40 corresponding to the pixel D22 to which the gamma characteristic conversion function γ4 is assigned is also printed (FIG. 15B). Further, as the density of the pixel data increases, the size of the dots corresponding to the pixels D11 and D22 increases, and the image is printed so as to narrow the island-shaped blank portion 41 (FIGS. 15C and 15D,
E). As a result of the blank portion 41 being formed like an island,
Even when the high density data is printed, the ink of the ink sheet is not peeled off unnecessarily, and only the ink having the area proportional to the density of the image data is reliably transferred to the recording paper.

FIG. 16 shows another form of designation of the conversion characteristics by the conversion characteristic selecting means 35. For the four pixels D11 to D22 extracted by the sampling means 33, the gamma characteristic conversion is performed on the pixel data D11. The function γ1 is assigned to the pixel data D12, the gamma characteristic conversion function γ3 is assigned to the pixel data D21, and the gamma characteristic conversion function γ2 is assigned to the pixel data D22. According to this, the dots based on the pixel data D11 are preferentially formed, and the dots based on the pixel data D21 are secondary formed. When such a form is applied by shifting one dot for every two lines in the main scanning direction as shown in FIG. 17, when the density of the image data in the sampling area is low as shown in FIG. 2 pixel data D11
Only dots corresponding to are formed. Further, at the stage of moving two lines at this density, the sampling area is shifted by one pixel as described above, so that a sub dot 42 is formed in the middle of the dots formed as the first line. When printing is continued by repeating such an operation, the angle connecting the dots located at the shortest distance, that is, the screen angle becomes 63.4 degrees.

FIG. 19 shows another form of specifying conversion characteristics by the conversion characteristic selecting means 35. As a first form, the pixel data D11 has a gamma characteristic conversion function γ4, and the pixel data D12 has a gamma characteristic conversion function. γ1 is the pixel data D21
Is the gamma characteristic conversion function γ3, and the pixel data D22
Is assigned to the gamma characteristic conversion function γ2 ((a) in the figure),
As a second embodiment, the pixel data D13 has a gamma characteristic conversion function γ3, D14 has a gamma characteristic conversion function γ2, pixel data D23 has a gamma characteristic conversion function γ4, and pixel data D24 has a gamma characteristic conversion function. γ1 is assigned ((B) in the figure), and the first (A) and the second (B) modes are alternately used as shown in FIG.

That is, when the first mode is selected for the four pixel data D11 to D22 in the first extraction area, dots based on the pixel data D12 are printed with priority, and the dots are printed in this sampling area. When the next adjacent pixel data is selected, the pixel data D24 is preferentially printed. As a result, in the low density area, dots are formed at a screen angle of plus or minus 26.6 degrees as shown in FIG. Further, when the image density of the extraction region becomes high, dots of the gamma characteristic conversion function γ4 are also printed in addition to the gamma characteristic conversion function γ1, so that in the odd line, the pixel data D12 is the main dot and the pixel data D11 is the sub dot. In the even line, the pixel data 24 is used as a main dot and D23 is used as a sub dot.

FIG. 22 shows another form of specifying the conversion characteristic by the conversion characteristic selecting means 35. As the first mode, the pixel data D11 is the gamma characteristic conversion function γ1 and D12 is the gamma characteristic conversion function γ2. , D21 is assigned the gamma characteristic conversion function γ3, and D22 is assigned the gamma characteristic conversion function γ4 ((a) in the figure).
The gamma characteristic conversion function γ4 is assigned to 13, the gamma characteristic conversion function γ3 is assigned to D14, the gamma characteristic conversion function γ2 is assigned to D23, and the gamma characteristic conversion function γ1 is assigned to D24 (Fig. (B)). After two lines, as shown in FIG. 23, the first form and the second form are applied in the reverse order.

That is, when the first form is selected for the four pixel data D11 to D22 of the first extraction region,
Dots based on the pixel data D11 are preferentially printed, and when the next pixel data adjacent to this pixel data is selected, the pixel data D24 is preferentially printed. As a result,
In the low density area, dots with a screen angle of 45 degrees are formed as shown in FIG. When the image density of the extraction area becomes high, the gamma characteristic conversion function γ1
In addition to this, dots of the gamma characteristic conversion function γ4 are also printed. Therefore, in the odd line, the pixel data D11 is formed as a main dot, and the next adjacent pixel data D13 is formed as a sub dot with a one-dot interval. To be done. In the even line, the pixel data D24 is the main dot,
Further, the pixel data D22 is formed as a sub-dot one dot before this.

FIG. 25 shows another form of specifying the conversion characteristic by the conversion characteristic selecting means 35. As a first form, the pixel data D11 has a gamma characteristic conversion function γ2, and the pixel data D12 has a gamma characteristic conversion function. γ4 is the pixel data D21
Is the gamma characteristic conversion function γ1 and pixel data D22
Is assigned to the gamma characteristic conversion function γ3 ((a) in the figure),
As a second form, the pixel data D13 has a gamma characteristic conversion function γ3, the pixel data D14 has a gamma characteristic conversion function γ1, and the pixel data D23 has a gamma characteristic conversion function γ4.
Further, the gamma characteristic conversion function γ2 is assigned to the pixel data D24 ((B) in the same figure), and after two lines, the application order of the first form and the second form is reversed as shown in FIG. Is.

That is, when the first form is selected for the four pixel data D11 to D22 of the first extraction region,
Dots based on the pixel data D21 are preferentially printed, and when the next area is selected, the pixel data D14 is preferentially printed. As a result, dots with a screen angle of 135 degrees are formed in the low density area as shown in FIG. Further, when the image density of the extraction region becomes high, dots of the gamma characteristic conversion function γ4 as well as the gamma characteristic conversion function γ1 are printed, so that in the odd line, the pixel data D14 is used as the main dot and one adjacent pixel is used. Pixel data D12 in the previous area is formed as a sub-dot at an interval of one dot, pixel data D21 is printed as a main dot in an even line, and pixel D23 is printed as a sub-dot one dot after this.

Among these embodiments, particularly, FIGS.
6, according to the application form of the gamma conversion characteristic shown in FIG. 19, the blank portion is formed in a shape close to a circle and the blank portions are independent from each other, and the ink in the non-dot forming portion is unnecessarily peeled off. Can be prevented. This makes it possible to represent a gradation that is faithful to the density of pixel data. FIG. 28 shows a second embodiment of the gamma characteristic conversion function. The gamma characteristic conversion function γ1 increases from the low density to the entire intermediate density region substantially monotonically as described above, and the maximum density D
It is set so as to be converted to the converted print data which rises to b, further monotonically decreases from the intermediate density area to the high density area, and decreases to the density Dc close to the density of the image data at the maximum density. Also, the gamma characteristic conversion function γ2,
Both γ3 maintain the density Dd at which energy is applied in the low density region but cannot substantially form dots, and increase substantially monotonically with respect to the density of the pixel data at the middle density or higher, and at the maximum density the image It is converted into converted print data having a density Dc close to the density of the data. Further, the gamma characteristic conversion function γ4 is set to have the density Dc only at the maximum density.

These gamma characteristic conversion functions γ1 to γ4 are represented by pixel data D11, D12,
When applied to D21 and D22 and continuously printed, dots based on the pixel data D11 are formed at a stage where the density of the pixel data in the extraction area is low. Therefore, as shown in FIG. Dots are formed on the screen, and screen angles of 0 and 90 are set. When the density of the pixel data becomes high, dots due to the gamma characteristic conversion functions γ2 and γ3 are formed as shown in FIG. 7C, but the dots due to these are small, and the pixel data D21 reaches the maximum density. Along with the fact that no dots are formed, dots are formed while forming a circular blank portion 45 for approximately one dot. As a result, it is possible to prevent the ink in the blank portion from being unnecessarily peeled off, and to form a dot having a gradation faithful to the density of the pixel data.

The second gamma characteristic conversion function is shown in FIG.
Although the case of applying in such a form has been described, it is also clear that it can be applied in the forms shown in FIGS. 17, 20, 23 and 26 described above.

As described above, by changing the application form of the gamma characteristic conversion function, the position where the main dot is formed also changes, so that for each of the three primary colors for expressing the color corresponding to the pixel data. By setting different application modes in advance, clear color printing can be performed by utilizing the advantage of the screen angle setting performed in gravure printing or the like.

Although the present image processing method is applied to the thermal transfer image forming apparatus in this embodiment, it may be applied to an electrophotographic image forming apparatus, an inkjet recording apparatus, or an impact dot matrix apparatus.

[0044]

As described above, in the present invention, the image data reading means for continuously reading the pixel data forming the input image data for N lines (an integer of N ≧ 2), N × N pixel data for multiple lines
Sampling means for sampling in a matrix of pixels, and one of a plurality of types of gamma characteristic conversion functions is selected and applied to each N × N pixel, and the conversion result is expressed in dot size, thereby making the image light and shade And a conversion characteristic selecting means for selectively applying the gamma characteristic conversion function to each of the sampled N × N pixels so that the dots by the gamma characteristic conversion function are aligned at a specific angle. At least one of the gamma characteristic conversion functions performs gamma conversion in a substantially monotonic increase from low density to medium density, and performs gamma conversion in a substantially monotonic decrease from medium density to high density to form a main dot. Since a common gamma characteristic conversion function combination is set in the conversion characteristic selection means, it is possible to specify the dot formation position and reduce the data size. Screen angle can be set with the key. Furthermore, by changing the size of the dots, while maintaining fine gradation and high resolution, the dots formed for each primary color are aligned at a certain angle and the area of the untransferred ink area is subdivided It is possible to improve separation and realize rich color expression.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention with a function 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.

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

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

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

FIG. 7 is an explanatory diagram showing a relationship between energy supplied to the print head and a heat generation area.

8 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.

FIG. 9 is an explanatory diagram showing the relationship between the driving method and the print pattern shown in FIG. 7.

FIG. 10 is an explanatory diagram showing a relationship between supplied energy and a heat generation region in a print head driving method applied to the present invention.

FIG. 11 is an explanatory diagram showing an example of a conventional driving method of a print head.

FIG. 12 is an explanatory diagram showing a relationship between the conventional driving method shown in FIG. 11 and a print pattern.

FIG. 13 is an explanatory diagram showing an application example of the gamma characteristic conversion function shown in FIG.

14 is an explanatory diagram showing a form in which the gamma characteristic conversion function shown in FIG. 13 is continuously applied.

FIG. 15 is an explanatory diagram showing a print form by applying the gamma characteristic conversion function shown in FIG.

16 is an explanatory diagram showing another embodiment of an application example of the gamma characteristic conversion function shown in FIG.

17 is an explanatory diagram showing a form in which the gamma characteristic conversion function shown in FIG. 16 is continuously applied.

18 is an explanatory diagram showing a printing form by applying the gamma characteristic conversion function shown in FIG.

19 is an explanatory diagram showing another embodiment of an application example of the gamma characteristic conversion function shown in FIG.

20 is an explanatory diagram showing a form in which the gamma characteristic conversion function shown in FIG. 19 is continuously applied.

21 is an explanatory diagram showing a printing form by applying the gamma characteristic conversion function shown in FIG.

22 is an explanatory diagram showing another embodiment of an application example of the gamma characteristic conversion function shown in FIG.

FIG. 23 is an explanatory diagram showing a form in which the gamma characteristic conversion function shown in FIG. 22 is continuously applied.

24 is an explanatory diagram showing a printing form by applying the gamma characteristic conversion function shown in FIG.

FIG. 25 is an explanatory diagram showing another embodiment of the application example of the gamma characteristic conversion function shown in FIG. 6.

26 is an explanatory diagram showing a form in which the gamma characteristic conversion function shown in FIG. 25 is continuously applied.

FIG. 27 is an explanatory diagram showing a printing form by applying the gamma characteristic conversion function shown in FIG. 25.

FIG. 28 is a diagram showing another embodiment of the gamma characteristic conversion function used in the present invention.

FIG. 29 is a diagram showing a print form by applying the gamma characteristic conversion function of FIG. 28.

[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

Claims (4)

[Claims] 1. An image data reading means for continuously reading out pixel data forming input image data for N lines (an integer of N ≧ 2), and pixel data for the plurality of lines is N × N pixels. Sampling means for sampling in a matrix form, gamma conversion characteristic selecting means for selecting one from a plurality of gamma characteristic functions, and gamma characteristic function selected by the gamma conversion characteristic selecting means obtained by the sampling means. And a screen angle is set by using a predetermined gamma characteristic function for each pixel of the N × N sampled pixels. Image forming apparatus. 2. A main dot is printed by performing gamma conversion in the gamma conversion means at least from a low density to a medium density in a substantially monotonous increasing manner and in a gamma conversion from a middle density to a high density in a substantially monotonically decreasing manner. 2. The image processing apparatus according to claim 1, wherein at least one type of gamma characteristic conversion function to be output as use data is included. 3. The image forming apparatus according to claim 1, wherein all the gamma characteristic conversion functions output printing data that is substantially equal in the highest density portion. 4. The image forming apparatus according to claim 1, wherein the image is printed by a thermal transfer printer.