JPH0746828B2 - Thermal transfer recorder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to thermal transfer by pseudo halftone expression in which one pixel is formed by a matrix and halftone is expressed by changing the area ratio of recording dots in the matrix. Recording device

[Technical background of the invention and its problems]

In the thermal transfer recording method, a thermal head is pressed against a recording paper via a heat-melting ink film, and the ink generated on the ink film is transferred to the recording paper by heat generated when a heating resistor forming the thermal head is energized. It is a method. This recording method is originally suitable for binary density recording, and requires a special density modulation technique when recording an image having gradation.

Conventionally, as this density modulation technique, a method has been known in which the amount of energy supplied to the heating resistor of the thermal head is modulated to control the thermal diffusion area in the ink film to change the size of the recording dot. Has a problem in that the density variable range that can be expressed is narrow, and in the case of thermal transfer, the spread of the transferred ink is random, so that the shapes and sizes of the dots vary, and the gradation is unstable. There is also a problem that the recording density itself shifts with time due to a heat storage phenomenon peculiar to thermal recording. Therefore, it has been difficult to express halftones by this type of method.

Therefore, pseudo halftone recording has been attempted in which a halftone image is expressed by the dot density or ink occupancy within a certain area. This method is a method that is widely used for expressing gradation in a dot printer that has only a binary density expression capability, and the dither method is a density pattern method as a typical one. 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 has a one-to-one correspondence with gradation as shown in FIG. 7, for example. This is a method of recording the corresponding pattern.

However, in the case of pseudo halftone recording, there is a drawback that the image quality is inferior. That is, it is said that a resolution of 6 lines / mm or more and a gradation number of 32 gradations or more are required in order to obtain an image comparable to human visual characteristics. In order to realize it by the method, at least a thermal head with a resolution of 32 lines / mm or more is required. However, the resolution of the thermal head that can be realized by the current technology is at most about 16 lines / mm, and it is impossible to satisfy both the resolution and the number of gradations, and the deterioration of the image quality cannot be denied. Also in this case, due to the effect of the heat storage phenomenon, a so-called gradation jump occurs, that is, a gradation jump occurs, in which a white area is crushed, especially in the middle and high density areas, and
There are problems such as the image roughness being conspicuous due to unstable ink adhesion in the medium density region.

[Object of the Invention]

The present invention has been made based on such a problem, and an object thereof is to provide a thermal transfer recording apparatus capable of expressing a halftone image which is remarkably improved in resolution, resolution and image quality. .

[Outline of Invention]

According to the present invention, a matrix composed of a set of dots is assigned to each pixel, and a thermal head having a plurality of heating elements arranged linearly is relatively moved in a direction substantially orthogonal to the direction of arrangement of the heating elements. At the same time, the heating element is selectively heated to thermally transfer the ink to a desired dot position, and the gradation of each pixel is expressed by the area ratio of the thermally transferred ink in the matrix. A plurality of types of dot patterns to be thermally transferred in the matrix, a storage unit that multi-values the amount of energy injected into each dot and stores in advance, and a density corresponding to a given multi-tone image signal for each pixel Based on the above, referring to the storage means, a pattern of dots to be thermally transferred in the matrix assigned to the pixel and a pattern corresponding to each dot to be thermally transferred. The storage means includes: a determining unit that determines the amount of energy injected into the device; and a unit that heats and relatively moves the thermal head according to the determined pattern of each pixel and the amount of energy injected. As a pattern in which a plurality of dots to be thermally transferred exist in a matrix, a pattern in which dots to be thermally transferred are arranged in the relative movement direction is stored.

Further, preferably, the storage means stores the pattern of dots to be thermally transferred in the matrix in correspondence with a low density region, a medium density region and a high density region, respectively, and the pattern in the low density region is stored. Is a pattern in which only one dot to be thermally transferred exists in the matrix, and the pattern in the medium density region is a pattern composed only of patterns in which dots to be thermally transferred are arranged in the relative movement direction, The pattern in the high-density area is characterized by being a pattern in which at least one dot to be thermally transferred is further added to a pattern in which dots to be thermally transferred are arranged by the method of the relative movement.

Further, preferably, the positions of the dots to be thermally transferred of the pattern in the low density region, the positions of the dots to be thermally transferred of the pattern in the medium density region, and the patterns in the high density region are arranged in the relative movement direction. The positions of the dots to be thermally transferred are the same in the arrangement direction in the matrix.

Further, preferably, the pattern in the high-density region has at least four dots that are not thermally transferred in the matrix, and four of the dots that are not thermally transferred are in a 2 × 2 matrix shape adjacent to each other. Whether it is arranged or located at four corners of the matrix, and two are arranged adjacent to one side of the outer circumference of the matrix, and these two are the centerlines of the matrix. In contrast, the other two are arranged at the position of the line object.

Further, preferably, the amount of energy injected into the thermal head is multivalued so as to monotonically increase with respect to the concentration.

〔The invention's effect〕

According to the present invention, the pseudo-halftone recording method and the thermal energy control method are used together, and the dot to be thermally transferred is a thermal head as a pattern in which a plurality of dots to be thermally transferred exist in a matrix for expressing gradation. By using the pattern arranged in the direction of relative movement between the recording paper and the recording paper, it is possible to obtain a halftone image which is remarkably improved in resolution, resolution and image quality. Hereinafter, this point will be described.

As a result of conducting some experiments, the present inventors have found that when a linear pattern (that is, a stripe pattern) extending in the relative movement direction of the thermal head and the recording paper is used as a pseudo halftone pattern, the image quality is extremely high. It was confirmed that a good image was obtained. The reason is, firstly,
When the stripe-shaped pattern is formed, the specific heating resistor is preferentially urged, so that the ink can be melted at a high temperature even with a pulse having a relatively short energization time due to the local heat storage effect of the head. Points are given. That is, when the ink is attached to the recording paper at a high temperature in this way, the ink is stably attached to the recording paper and the outer edge portion of the dot is also smoothed. Secondly, since a specific heat generating resistor is preferentially biased, heat storage in that portion has a large effect, and a so-called tailing phenomenon occurs. However, in the stripe pattern as described above, this tailing is rather a line. This is to smooth the side edges of the stripe pattern and prevent unevenness of the side edges of the stripe pattern. When the side edges of the stripe are smoothed in this way, the so-called roughness is obtained. An image without In short, the present invention can provide an image having a stable gradation and good image quality by utilizing the heat storage effect, which has been conventionally regarded as a drawback.

Of course, the heat storage phenomenon of the thermal head concentrates on a specific heating resistor, and on the other hand, it can be suppressed low,
In the high-concentration region, the portion to be blank is not crushed, and the influence of the concentration shift due to the heat storage phenomenon is small.

Moreover, in the present invention, while eliminating the defects of the thermal transfer recording method as described above, the pseudo halftone recording method and the thermal energy control method are used in combination, so that the image quality is improved,
Not only can more gradations be obtained with the same matrix size as the conventional one, but also a larger number of gradations can be obtained with a smaller matrix size, so a reduction in resolution can be prevented.

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

Example of Invention

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

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

That is, in this embodiment, one pixel is composed of a 3 × 3 matrix, and three pseudo halftone patterns are prepared corresponding to the low density region, the medium density region and the high density region.

The low-density pattern A is an isolated dot pattern formed by isolating one dot in one pixel. The gradation in the low-density region is expressed by changing the amount of energy injected into the thermal head to change the dot size.

The medium density pattern B is a linear pattern (hereinafter, referred to as a stripe pattern) extending in the relative movement direction of the thermal head with respect to the recording paper. The gradation of the middle density area is
It is expressed by changing the implantation energy amount of the thermal head and changing the thickness of the stripe.

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

The density area that can be represented by the low density pattern A and the density area that can be represented by the medium density pattern B partially overlap, and the density area that can be represented by the medium density pattern B and the high density area It partially overlaps with the density area that can be represented by the use pattern C. In this embodiment, as shown in FIG.
In the low-concentration pattern A, the implantation energy amount is 0 to 6 7
The medium-concentration pattern B has a 12-step variation range of the implantation energy amount, and the high-concentration pattern C has a 12-stage variation range of the implantation energy amount.
There is a step change width, and a 3 × 3 matrix is used as a whole to obtain 31 gradations.

Next, a schematic configuration of a thermal printer for obtaining such 31 gradations will be described with reference to FIG.

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-valued pattern table 3. The output of the multi-valued pattern table 3 is given to the thermal head drive circuit 5 via the thermal recording circuit 4.

The multi-gradation signal processing circuit 1 is a printer with specifications and characteristics of multi-gradation / halftone image signals input in the form of digital signals from a scanner and an A / D converter, an image memory or a demodulation or decoding circuit of a transmission system. Is a circuit for performing signal processing and outputting according to.

The matrix position specifying circuit 2 is a circuit required for digital recording or pseudo-halftone recording, and is a circuit for designating the positions of pixels in a matrix array composed of a plurality of image points. In the case of a normal line printer, the dot counter in the main scanning direction and the line counter in the sub scanning direction are interlocked. In some cases, the multi-gradation signal processing circuit 1
It may also be linked to the image signal input clock used in.

The multi-valued pattern table 3 is a main part of the present embodiment, and this part is a ROM (Read On) for generating a binary dither pattern of a table lookup type in the conventional case.
ly Memory), which included calculations to compensate for the thermal storage phenomenon of the thermal head, if necessary. However, in this embodiment, this multi-valued pattern table has the above-mentioned second value.
Each density pattern A, B, C shown in the figure and the amount of energy injected into each dot are stored.

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

The thermal head drive circuit 5 is an IC circuit including a shift register, a latch, a gate, and a driver (not shown), and is mounted on the thermal head substrate. The interlocking operation of the thermal head drive circuit 5 and the thermal recording circuit 4 is used for compensating the heat accumulation phenomenon in the conventional fusion thermal transfer recording, but in the present embodiment, pulse width control for gradation expression. Used for.

In the thermal printer configured in this way, the multi-valued pattern table 3 is used to make 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 three patterns A, B, and C described above are used as the pseudo-halftone pattern, the following effects can be obtained.

That is, the present inventors found out the following four facts from a careful experiment on the melt thermal transfer recording system. That is, the stripe-shaped pattern orthogonal to the dot row of the thermal head has a density that changes in accordance with the control of the implantation energy, and has a clean image quality without roughness. This tendency is most apparent in the medium density region.

If the white block is 2 × 2 dots, the white block is not crushed at all densities. This can be confirmed at a resolution of at least 16 lines / mm.

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

As described above, the higher the resolution of the thermal head, the weaker the pattern dependence tends to be. This is true for low and medium density regions.

From these four results, it was confirmed that as a result of forming a halftone image for the above-mentioned three patterns, an image with extremely good image quality was obtained. This is because a stripe pattern that is orthogonal to the dot row of the thermal head is used as the medium density pattern, and the high density pattern has a white portion of 2 × 2 dots. Further, in the low density area, the thermal energy is intensively injected into the isolated dots, so that the ink can be melted at a high temperature, and stable attachment to the recording paper and stabilization of the dot shape are ensured. it can. In this way, by dividing the density region into a plurality of parts and using the most suitable pattern in the density region, an image with good image quality can be obtained.

According to this embodiment, 31 gradations can be expressed by a 3 × 3 matrix, which can express only 10 gradations in the past, and it is a factor of image quality deterioration peculiar to the thermal fusion transfer recording method. In addition, it is possible to improve ink adhesion failure, roughness, crushing of the blank portion due to heat accumulation, etc., and it is possible to obtain an image with good image quality and excellent gradation and resolution.

In the above embodiment, a 3 × 3 matrix is used.
Although the method of allocating one fixed pattern to each density region has been described, the present invention is not limited to the matrix size, the density region dividing method, the number of patterns, and the like. For example, as shown in FIG.
It is also possible to express four multi-tone images by using four 4 × 4 patterns.

In this case, the concentration region is divided into four concentration regions of low concentration, medium low concentration, medium high concentration, and high concentration. As the pattern used for each density, for example, four patterns D, E, F, G shown in FIG. 9A or four patterns H, E, F, I shown in FIG. The low-density pattern D is a dot pattern that forms four isolated dots in one pixel, and the medium-low-density pattern E is a 1-dot-width stripe pattern extending in the relative movement direction between the thermal head and the recording paper. The medium-high density pattern F is a stripe-shaped pattern having a width of 2 dots extending in the same direction, and the high-density pattern G is an L-shaped pattern having a white portion of 3 × 3 dots in one pixel. It is a pattern. Further, the low-density pattern H is an isolated dot pattern in which only one dot is formed in one pixel, and the high-density pattern I is formed so that 2 × 2 dots of white are formed in one pixel. Done L
It is a letter-shaped pattern.

By using such a pattern, it is possible to express a continuous gradation having almost no steps.

Further, according to the present invention, the boundary between adjacent density areas is overlapped, and the density threshold for pattern switching is set to different values depending on whether the recording density moves from the high density side to the low density side or vice versa. By doing so, it is possible to make the discontinuity due to pattern switching inconspicuous.

Further, each of the above-mentioned patterns may have a form different from the above-mentioned form depending on how the matrix is focused, for example, FIGS. 5 (a), (b) and (c).
The respective forms shown in FIG. 3 are not essentially different from the patterns A, B and C shown in FIG.

In addition, the present invention can be applied to the case where the above-mentioned pseudo halftone patterns are arranged via the space portion.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between a recording pattern used in a thermal transfer recording system according to an embodiment of the present invention and the amount of energy injected into the pattern and the recording density, and FIG. 2 is shown injecting dots in the pattern. FIG. 3 is a schematic diagram showing the amount of energy, FIG. 3 is a block diagram showing a partial configuration of a thermal printer using the method of this embodiment, and FIGS. 4 and 5 are thermal transfer recording methods according to other embodiments of the present invention. FIG. 6, FIG. 6 and FIG. 7 are views for explaining a conventional thermal transfer recording system. A, D, H ... Low density pattern, B ... Medium density pattern, C, G, I ... High density pattern, E ... Low / medium density pattern, F ... Medium / high density pattern.

Claims (5)

[Claims] 1. A thermal head having a plurality of heating elements arranged linearly, wherein a matrix of a set of dots is assigned to each pixel, and the thermal head is arranged in a direction substantially orthogonal to the arrangement direction of the heating elements. In the thermal transfer recording apparatus that moves and moves the heating element selectively to thermally transfer the ink to a desired dot position, and express the gradation of each pixel by the area ratio of the thermally transferred ink in the matrix, A plurality of types of dot patterns to be thermally transferred in the matrix, a storage unit that multi-values the amount of energy injected into each dot and stores in advance, and each pixel corresponds to a given multi-gradation image signal Based on the density, referring to the storage means, it corresponds to the pattern of dots to be thermally transferred and each dot to be thermally transferred in the matrix assigned to the pixel. The storage means comprises a determining means for determining the amount of energy injected into the heating element, and means for performing heat generation and relative movement of the thermal head according to the determined pattern of each pixel and the amount of energy injected. A thermal transfer recording apparatus, wherein a pattern in which dots to be thermally transferred are arranged in the relative movement direction is stored as a pattern in which a plurality of dots to be thermally transferred are present in the matrix. 2. The storage means stores a pattern of dots to be thermally transferred in the matrix in correspondence with a low density region, a medium density region and a high density region, respectively, and the pattern in the low density region is stored. Is a pattern in which only one dot to be thermally transferred exists in the matrix, and the pattern in the medium density region is a pattern composed only of patterns in which dots to be thermally transferred are arranged in the relative movement direction, The pattern in the high-density region is a pattern in which dots to be thermally transferred are arranged in the relative movement direction and at least one dot to be thermally transferred is added to the pattern. The thermal transfer recording apparatus described. 3. The positions of dots to be thermally transferred to the pattern in the low density region, the positions of dots to be thermally transferred to the pattern in the medium density region, and the patterns in the high density region are arranged in the relative movement direction. The thermal transfer recording apparatus according to claim 2, wherein the positions of the dots to be thermally transferred are the same in the array direction in the matrix. 4. The pattern in the high-density region has at least four dots which are not thermally transferred in the matrix, and four of the dots which are not thermally transferred are arranged in a 2 × 2 matrix shape adjacent to each other. Whether it is arranged or located at four corners of the matrix, and two are arranged adjacent to one side of the outer circumference of the matrix, and these two are the centerlines of the matrix. 4. The thermal transfer recording apparatus according to claim 3, wherein the other two are arranged in line-symmetrical positions. 5. The thermal transfer recording apparatus according to claim 1, wherein the amount of energy injected into the thermal head is multivalued so as to monotonically increase with respect to the density. .