JPS61176273A - Thermal transfer recording system - Google Patents

Abstract

PURPOSE:To improve image resolution and picture quality by securing the multi-value CONSTITUTION:A thermal memory circuit 4 controls the pulse width for each dot of a thernal memory head in response to the actuation.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention is based on pseudo-halftone expression, in which one pixel is constructed in a matrix, and halftones are expressed by changing the area ratio of dots arranged in the matrix. Related to thermal transfer recording method.

[Technical background of the invention and its problems]

In the thermal transfer recording method, a thermal head is pressed against the recording paper via a heat-melting ink film, and the ink on the ink film is transferred to the recording paper using the heat generated when electricity is applied to the heating resistor that makes up the thermal head. It is a method. This recording method is originally suitable for binary density recording, and requires a special density modulation technique when recording images with gradation.

Conventionally, this density modulation technology involves modulating the amount of energy supplied to the heating resistor of the thermal head and controlling the heat diffusion area within the ink film to change the size of the recorded dots. The range of variable density that can be expressed is narrow, and in the case of thermal transfer, the spread of the transferred ink is random, resulting in variations in dot shape and size and unstable gradation. Furthermore, there is also the problem that the recording density itself shifts over time due to the heat accumulation phenomenon unique to thermal recording. Therefore, it has been considered difficult to express halftones using this type of method.

Therefore, attempts have been made to perform pseudo-halftone recording in which a halftone image is expressed by dot density or ink occupancy within a certain area. This method is widely used to express gradation in printers that have only binary density expression capability, and the dither method and density pattern method are typical examples. The dither method is a method of binarizing a grayscale image using a threshold pattern as shown in FIG. 6, and the density pattern method is a method that uses a threshold pattern as shown in FIG. This method records the corresponding pattern.

However, in the case of pseudo-halftone recording, there is a drawback that the image quality is inferior. In other words, it is said that in order to obtain an image comparable to the human visual characteristics, a resolution of 6 lines/# or more and a number of gradations of 32 or more are required. If you try to achieve this by law,
A thermal head with a resolution of at least 32 lines/# is required.

However, the thermal head that can be realized with current technology is
Its resolution is at most 16 lines/mm culm;
It is impossible to satisfy both the number of gradations, and deterioration of image quality is undeniable. Also, in this case, due to the effect of heat storage phenomenon,
Particularly in medium and high density areas, areas that should be whitened out are crushed, causing so-called gradation jumps, and unstable ink adhesion in low and medium density areas causes noticeable roughness in the image. .

[Purpose of the invention]

The present invention has been made based on these problems, and its purpose is to provide a thermal transfer recording method that can express halftone images that are significantly improved in terms of resolution, resolution, and image quality. .

[Summary of the invention]

The present invention is a thermal transfer recording method that expresses the gradation of one pixel using a pseudo-halftone pattern, in which the following specific pseudo-halftone pattern is used at least in a predetermined density area. That is, this pattern is a striped pattern extending in the direction of movement of the thermal head relative to the recording paper. The present invention is characterized in that the gradation of the pixel in the density region is expressed by multileveling the energy injected into the thermal head when forming the striped pattern.

〔Effect of the invention〕

51 As a result of several experiments, the present inventors have found that when a striped pattern extending in the direction of relative movement between the thermal head and the recording paper is used as a pseudo halftone pattern, images with extremely good image quality can be obtained. was confirmed to be obtained.

Firstly, when a striped pattern is formed, only a specific heating resistor is always energized, so the local heat storage effect of the head causes the ink to flow even with a relatively short pulse. One point is that it can be melted at high temperatures. In other words, when the ink is applied to the recording paper at a high temperature in this way, the ink stably adheres to the recording paper and the outer edges of the dots become smooth. Also, the second
In this case, since only a specific heating resistor is energized, the effect of heat accumulation in that part is large, resulting in the so-called trailing phenomenon. However, in the striped pattern shown above, this trailing actually smooths out the side edges of the line. This is because the striped pattern can be prevented from becoming uneven at the side edges. When the side edges of the stripe are smoothed in this way,
Images without so-called roughness can be obtained. in short,
The present invention makes it possible to provide images with stable gradation and good image quality without graininess by making use of the heat storage effect, which has been considered a drawback in the past.

Of course, the heat accumulation phenomenon of the thermal head is concentrated only in a specific heating resistor, and is suppressed to a low level in other heating resistors, so the area that should be blanked in the high concentration area does not collapse, and the heat accumulation phenomenon causes the concentration to decrease. The impact of the shift is also small.

In addition, according to the present invention, since the pseudo halftone recording method and the thermal energy control method are used together at least in a predetermined density area, more tones can be obtained with the same matrix size as before. Not only can you
Since a large number of gradations can be obtained with a relatively small matrix size, a decrease in resolution can also be prevented.

Therefore, according to the present invention, it is possible to obtain a halftone image that is significantly improved in terms of resolution, resolution, and image quality.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the pseudo halftone pattern used in this embodiment and the relationship between injection energy and recording density. Note that in the figure, the horizontal axis indicates the total amount of energy injected into one pixel.

That is, in this embodiment, one pixel is formed by a 3×3 matrix, and three pseudo halftone patterns are prepared corresponding to a low density area, a medium density area, and a high density area.

The low density pattern A is an isolated dot pattern in which one dot is isolated in one pixel. The gradation in the low concentration region is expressed by varying the amount of energy implanted by the thermal head to change the size of the dot.

The medium density pattern B is a striped pattern extending in the direction of movement of the thermal head relative to the recording paper. The gradation in the medium concentration region is expressed by changing the thickness of the stripes by varying the amount of energy implanted by the thermal head.

The high-density pattern C is an L-shaped pattern in which a white portion corresponding to 2×2 dots is formed. High-density gradation is expressed by varying the area of the white portion by varying the energy injected into the thermal head.

The density area that can be expressed by pattern A for low density and the density area that can be expressed by pattern B for medium density partially overlap, and the density area that can be expressed by pattern B for medium density,
It also partially overlaps with the density region that can be expressed by pattern C for high density. In this example, as shown in FIG. 2, the implantation energy amount is varied in 7 steps from 0 to 6 in pattern A for low concentration, and from 7 to 18 in pattern B for medium concentration. A variation range of 12 steps is provided, and furthermore, in pattern C for high concentration, the amount of implantation energy is provided with a variation range of 12 steps from 19 to 30, resulting in a total of 31 steps of gradation in a 3×3 matrix. ing.

Next, a schematic configuration of a thermal printer for obtaining such 31 levels of gradation will be explained based on FIG. 3.

That is, the output of the multi-gradation signal processing circuit 1 and the output of the matrix position specifying circuit 2 are given to the multi-value pattern table 3. The output of the multivalued pattern table 3 is given to a thermal head drive circuit 5 via a thermal recording circuit 4.

The multi-gradation signal processing circuit 1 processes multi-gradation/half-tone image signals input in the form of digital signals from the scanner, A/D converter, image memory, or demodulation or decoding circuit of the transmission system, and converts them into printer specifications and characteristics. This is a circuit that performs signal processing and outputs the signal.

The matrix position specifying circuit 2 is a circuit necessary for digital recording or pseudo halftone recording, and is a circuit for specifying the position of a pixel in a matrix array consisting of a plurality of pixel points. In the case of a normal line printer, it is linked with a dot counter in the main scanning direction and a line counter in the sub-scanning direction.

In some cases, it may also be linked to the image signal input clock used in the multi-gradation signal processing circuit 1.

The multi-value pattern table 3 is the main part of this embodiment, and this part is conventionally constructed using a ROM (R
(ead ONLY MEMORY) This includes calculations for compensating for the heat accumulation phenomenon of the thermal head as necessary. However, in this embodiment, this multi-value pattern table is based on each density pattern A shown in FIG.
, B, C and the amount of energy to be injected into each dot.

The thermal recording circuit 4 is a circuit for controlling the pulse width of each dot of the thermal head in conjunction with the thermal head drive circuit 5, and is controlled by output data from the multi-value pattern table 3.

The thermal head drive circuit 5 is an IC circuit consisting of a shift register, a latch, a gate, and a driver (not shown), and is mounted on a thermal head substrate. This interlocking operation of the thermal head drive circuit 5 and the thermal recording circuit 4 is used to compensate for the heat accumulation phenomenon in conventional fused heat transfer recording, but in this embodiment, the pulses for expressing gradation are used. Used for width control.

゛ In a thermal printer configured in this way, the multilevel pattern table 3 has a correspondence between a large number of gradation levels (31 in this example) and a small number (3 in this example) of recording patterns, that is, a many-to-one correspondence. At the same time, the amount of energy for each recording dot is determined.

In this case, since the above-mentioned three patterns A, B, and C are used as pseudo halftone patterns, the following effects can be obtained.

That is, the present inventors have discovered the following four facts through careful experiments regarding the fusion thermal transfer recording method. That is, (1) the density of the striped pattern perpendicular to the dot row of the thermal head changes depending on the control of the injection energy, and the image quality is clear and free of roughness. This tendency appears most clearly in the medium concentration region.

■ If the white block is a 2×2 dot, the white block will not be crushed at any density. This can be confirmed at a resolution of at least 16 lines/#.

(2) When the amount of implantation energy is changed within the same pattern, the characteristic of recording density with respect to the average energy per dot has a relationship of simple increase.

(2) Regarding (2) above, the higher the resolution of the thermal head, the weaker the pattern dependence tends to be. This can be said about the low and medium concentration regions.

From these four results, it was confirmed that as a result of forming halftone images for the three patterns described above, an image with extremely good image quality could be obtained. This is because the medium-density pattern uses a striped pattern perpendicular to the dot row of the thermal head, and the high-density pattern also has a white portion of 2×2 dots. In addition, in low-density areas, thermal energy is intensively injected into isolated dots, so the ink can be melted at a high temperature, resulting in consistent adhesion to the recording paper and stabilization of the dot shape. can be secured.

In this way, by dividing the density area into a plurality of parts and using the most suitable pattern for the density area, an image with good image quality can be obtained.

According to this embodiment, it is possible to express 31 gradations with a 3 x 3 matrix, which was conventionally only able to express 10 gradations, and moreover, it is possible to express 31 gradations without causing image quality deterioration that is specific to the thermal melt transfer recording method. It is possible to improve poor adhesion of ink, roughness, and collapse of blank areas due to heat accumulation, and it is possible to obtain images with good image quality, excellent gradation, and resolution.

In addition, in the above example, a 3×3 matrix is used, and 3
Although the method of allocating one fixed pattern to each density area has been described, the present invention is not limited to the matrix size, the method of dividing density areas, the number of patterns, etc.

For example, as shown in Figure 4, a 4x4 pattern is
It is also possible to express an image with even more gradations by using one.

In this case, the density region is divided into four density regions: low density, medium low density, medium high density, and high density. Examples of patterns used for each density include the four patterns D, E, F, and G shown in FIG.

■ is possible. The pattern D for low density is a dot pattern that forms four isolated dots in one pixel, and the pattern E for medium and low density is a striped pattern with a width of one dot extending in the direction of relative movement between the thermal head and the recording paper. The pattern F for medium and high density is a striped pattern with a width of 2 dots extending in the same direction, and the pattern G for high density is a 1-character shape with a white area of 3 x 3 dots in the -pixel. It's a pattern. Furthermore, the low-density pattern H is an isolated dot pattern in which only one dot is formed in the -pixel, and the high-density pattern ■ is an isolated dot pattern in which only one dot is formed in the -pixel.
This is a one-character pattern that forms a white portion for two dots.

If such a pattern is used, almost stepless continuous gradation can be expressed.

Furthermore, the present invention overlaps the boundaries of adjacent density areas, and sets the density threshold value for pattern switching to different values depending on whether the recording density moves from a high density side to a low temperature side or vice versa. By doing so, it is possible to make discontinuities caused by pattern switching less noticeable.

Further, each of the above-described patterns may have a form different from the form described above depending on how the matrix is focused, for example, as shown in FIG. 5(a).

The forms shown in (b) and (C) are not essentially different from patterns A, B, and C shown in FIG. 1, respectively.

The present invention can also be applied to a case where the pseudo halftone patterns described above are arranged via a space portion.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a recording pattern used in a thermal transfer recording method according to an embodiment of the present invention and the relationship between the energy field injected into the pattern and the recording density, and FIG. Schematic diagram showing the amount of energy to be injected, 3rd
The figure is a block diagram showing a partial configuration of a thermal printer using the method of this embodiment, FIGS. 4 and 5 are diagrams showing a thermal transfer recording method according to another embodiment of the present invention, and FIG.
7 and 7 are diagrams for explaining the conventional thermal transfer recording method. A, D, l-1... pattern for low density, B... pattern for medium density, C, G, I... pattern for high density, F
...Low-medium density pattern, F...Medium-high density pattern. To the applicant's agent and patent attorney Takehiko Suzue 1) D V'

Claims (3)

[Claims] (1) In a thermal transfer recording method in which the gradation of one pixel is expressed using a pseudo-halftone pattern, at least in a predetermined density area, a striped pattern extending in the direction of relative movement of the thermal head with respect to the recording paper is used as the pseudo-halftone pattern. and the amount of energy injected into the thermal head when forming the striped pattern is multi-valued to express the gradation of the pixel in the density region. method. (2) The pseudo halftone pattern is an isolated dot pattern with no adjacent dots in a low density area, the stripe pattern in a medium density area, and a pattern with adjacent dots in a matrix constituting one pixel in a high density area. Claim 1, characterized in that the pattern is an L-shaped pattern with dots formed on two sides, and the amount of energy injected into the thermal head is multivalued for each pattern. thermal transfer recording method. (3) The thermal transfer recording method according to claim 1 or 2, wherein the pseudo halftone pattern is a pattern including a blank for at least 2×2 dots.