JPS6269772A - Thermal transfer recording system - Google Patents

Abstract

PURPOSE:To decrease the picture noise at changing over a pattern, to make the make the half tone recording smooth and to improve the picture quality of printing by providing plural multi-value density patterns where the density area is overlapped and processing them at a multi-value pattern processing circuit. CONSTITUTION:One picture element is composed of a 3X3 matrix in the density area, and corresponding to the low density area, the middle low density area, the middle high density area and the high density area, four multi-value density patterns are provided. The density area which can be expressed by a pattern A for the low density and the density area which can be expressed by a pattern B for the middle low density are partially overlapped, and the pattern B and a middle high density pattern C, and the pattern C and a high density pattern D are also partially overlapped. In a multi-value pattern processing circuit 2, patterns A, B, C and D are changed over by the input picture density from a mulit-gradation signal processing circuit 1 and its changed part, and the energy quantity injected to respective dots is determined. Thus, the pattern changing- over fraquency is eliminated, the noise is decreased, the smooth half tone recording can be executed, and the printing picture quality can be improved.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention is based on a pseudo-halftone expression in which one pixel is composed of a matrix and halftones are expressed by changing the area ratio of recording dots within 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 recording paper via a heat-melting ink film, and the ink on the ink film is transferred to the 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 recording binary density, and requires a special density modulation technique when recording images with gradation.

Conventionally, as a density modulation technique, a method has been known in which the amount of energy supplied to the heat generating resistor of the thermal head is modulated to control the heat diffusion area within the ink film to change the size of the recorded dot. This method has the problem that the variable range of 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. ing. 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 was considered difficult to express midtones 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 the floor 11 with dot 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. 10, and the density pattern method is a method of binarizing a grayscale image using a threshold pattern as shown in FIG. 11. However, the image quality of pseudo-halftone recording is said to be inferior. In other words, in order to obtain an image that is comparable to the human visual characteristics, 6-
It is said that it is necessary to have a resolution of 32 levels υ and a gradation number of 8 or more, but if you try to achieve this using the dither method, for example, a thermal head with a resolution of at least 32 lines/■ or more is required. is necessary. However, currently available thermal heads have a resolution of about 16 lines per barrel, making it impossible to satisfy both the resolution and the number of gradations, and when reproducing images with such a printer, it is inevitable that the image quality will deteriorate. do not have. In addition, due to the heat storage effect in the thermal head, gradation discontinuities occurred, especially in medium and high density areas, where areas that should have been whitened out), and unstable ink adhesion in low and medium density areas caused image distortion. It also includes problems such as noticeable roughness.

In order to solve the above problems, the present inventors have proposed a thermal transfer recording method using a multivalued density pattern (Japanese Patent Application No. 1983).
-16768). This recording method is a method that records images with stable gradation and no graininess by making use of the heat storage effect, which has been considered a drawback in the past.

FIG. 12 is a diagram showing a multilevel density pattern and the relationship between injection energy and recording density. In this example, one pixel is constituted by a 3.times.3 matrix, and three multi-value density patterns are prepared corresponding to a low density region, a medium density region, and a high 11K region (FIG. 13).

Low dark pattern Ad - This is an isolated dot pattern formed by separating one dot in a 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 density region is expressed by varying the thickness of the stripe by varying the amount of energy implanted in the thermal head.

The high-summer pattern C is an L-shaped pattern in which a white portion of 2×2 dots is formed. High-density gradation is expressed by varying the area of the white portion by varying the amount of energy injected into the thermal head. By performing halftone recording using this method, it becomes possible to solve the problems caused by the dither method (b) and the density pattern method. However, according to the studies of the present inventors, the halftone recording method using such a multi-value density pattern also has the following problems. In other words, this recording method assigns several patterns to each density area and switches the patterns depending on the image density, so the pattern changes from pattern to pattern B, from pattern B to pattern C, or vice versa, depending on the image density. However, the printed image appears discontinuous and the quality of the printed image deteriorates. Also,
In order to reduce image noise caused by pattern switching, it is necessary to consider the pattern shape, pattern switching density, etc.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to improve the above-mentioned drawbacks and to provide a thermal transfer recording system that causes less pattern switching noise when halftone recording is performed using a multi-density pattern system.

[Summary of the invention]

The partial density area of the pattern used in the halftone recording method using the multilevel density pattern method is widened, the density area overlaps with other patterns, and the pattern switching density is changed when the image RA increases and when it decreases, and pattern switching is performed. By reducing the frequency and reducing image noise when switching patterns, smooth halftone recording is possible.

〔Effect of the invention〕

According to the present invention, it is possible to reduce the number of pattern changes that tend to impair density continuity, thereby improving the quality of printed images.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 2 is a diagram showing the multilevel density pattern used in this embodiment and the relationship between implantation 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 configured as a 3×3 matrix, and four multi-value density patterns are prepared corresponding to a low density area, a medium-low density area, a medium-high density area, and a high density area.

The pattern for low density is an isolated dot pattern in which one dot is isolated in one pixel.

The gradation of the low concentration region using this pattern is expressed by varying the amount of energy implanted in the thermal head to change the size of the dots.

Pattern B for medium and low concentrations is an extension of the pattern for low concentrations, and three dots are considered as one dot, and by varying the amount of energy injected into the thermal head and changing the size of the dots, pattern B for medium and low concentrations can be achieved. expresses the gradation of

The pattern C for medium and high density is an L-shaped pattern in which a white portion of 2×2 dots is formed. Medium and high density gradations are
It is expressed by changing the area of the white part by varying the energy injected into the thermal head.

The high density pattern D has one printing dot added to the white part of the medium and high density pattern C to increase the printing density. The gradation is controlled by varying the area of the white part by varying the energy injected into the thermal head, similar to the medium-high density pattern C.

The density region that can be expressed by the low-density pattern human and the density region that can be expressed by the medium-low density pattern B partially overlaps, and the medium-low density pattern B and the medium-high Kl l'l! : Partially overlaps with the density region that can be expressed by pattern C. The medium-high density pattern C and the high-density pattern D also partially overlap.

In this implementation row, as shown in FIG.
In this case, the implantation energy amount is varied in 7 steps from 0 to 6, and in the medium and low concentration pattern B, the implantation energy amount is changed from 3 to 18.
26 steps from 10 to 30 t for medium-high density pattern C, and 1 for high density pattern D.
It has a variation range of 31 steps from 8 to 36, and the total is 3.
A ×3 matrix is used to obtain 37 levels of gradation.

In the present invention, these patterns are switched and used as shown in FIG. If the recording density is increasing, pattern switching is performed up to the highest density of the currently selected pattern 1. At the density of El, pattern switching is performed. If the recording density is decreasing, pattern switching is performed at the lowest density of pattern 2. It is used without cutting until a certain point E2, and then it is switched to pattern 1.

Switching between El and E2 is performed so as to maintain the same recording density.

By switching patterns in a hysteretic manner in this manner, even when a plurality of patterns are used, frequent pattern switching is suppressed and noise caused by pattern switching is also reduced.

Butter/1 and pattern 2 overlap between El.829, and which pattern is used to represent the density is determined depending on the direction of density change in the input image.

Further, in this embodiment, a case is explained in which there are four patterns.

Therefore, if the recording density is increasing, as shown in Fig. 2(b)
A pattern is selected as shown in FIG. 2(C), and if the recording density is decreasing, a pattern is selected as shown in FIG. 2(C).

Next, a schematic configuration of a thermal printer for obtaining such 37 levels of gradation will be explained based on FIG. 4.

The output of the multi-gradation signal processing circuit 1 is sent to the multi-value pattern processing circuit 2.
The output of the multi-value pattern processing circuit 2 is supplied to a thermal head drive circuit 4 via a thermal recording circuit 3.

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 according to the specifications and characteristics of the printer. , is a circuit that performs signal processing and outputs the signal.

The multi-value pattern processing circuit 2 is the main part of this embodiment,
This is the part that controls switching of multi-value patterns. Each density pattern shown in FIG. 2 described above is created based on the input image density and the input image density change.

Switch B, C, D and inject energy M into each dot?
This is the circuit that determines.

The thermal recording circuit 3 is a circuit for controlling the pulse width of each dot of the thermal head in conjunction with the thermal head drive circuit 4, and is controlled by output data from the multi-value pattern processing circuit 2.

The thermal head drive circuit 4 is an IC circuit including 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 4 and the thermal recording circuit 3 is used to compensate for the heat accumulation phenomenon in conventional fused thermal transfer recording, but in this embodiment, it is used to control the pulse width for gradation expression. used for.

FIG. 5 shows the internal configuration of the multi-value pattern processing circuit 2. The image signal a (signal representing the density of a 3×3 pixel in FIG. 3) output from the multi-tone signal processing circuit 1 is -
It is input to a latch 10 and a subtracter 11 to calculate the density change between pixels. The latch 10 delays the input image signal by one pixel and inputs it to the subtracter 11 . 1g, the output of the calculator 11 becomes a differentiated signal of the input image signal and is output as an image density change signal. Image 1 density change ε~ is a plurality of shift registers (12-1 to 12-n) (in this embodiment,
For example, let n=5. ) are stored sequentially. Using the stored image density change signal (bx-bn) and the image density change signal, an adder 13 calculates an a degree change signal C between a plurality of pixels (here, a signal representing a density change over seven pixels). . Examples of these signals a, b, and c are shown in Figure 6), (b), and (C). Also, the same figure (
d) is a waveform obtained by superimposing the input signal a and the density change signal C between a plurality of pixels, and is a diagram showing a temperature change from the current image density to after a certain pixel.

The pattern switching table 15 includes the density change signal C and the input image signal d (the delay time from the latch 10 to the output of the adder 13 for the input image signal a (where T is one clock time for transferring the input image signal). , 7T) corrected by the delay circuit 14) and the currently used pattern code f (i- table for manually determining the pattern code to be printed next υ,
A determination table calculated by the determination sequence shown in FIG. 7 is stored. This determination table records input image signal density, density changes between multiple pixels, and current usage.

The pattern to be used for the next printing is determined from the used patterns based on the density range, overlap amount, etc. of each pattern, and a multi-value pattern code e corresponding to the density change is output.

The multi-value pattern switching table output e is stored in a wrap 16 for storing the pattern code currently in use, and the output signal f is fed back to the multi-value pattern switching table 15 and used as data for pattern switching.

The multilevel pattern table 17 stores the density patterns A, B, C, and D shown in FIG. 2 described above and the amount of energy to be injected into each dot.

The matrix counter 18 is a circuit necessary for digital recording or pseudo halftone recording, and is a circuit for indicating the pixel position of 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 addition, in some cases, the present invention may be used to operate the image signal input clock of FIG. FIG. 6 is a diagram showing pattern switching when processing an image signal. The hatched pattern in the figure indicates that it is used. Moreover, FIG. 9 is a diagram showing pattern switching when the overlap between each pattern is eliminated (prior art). Comparing the two, pattern switching when using the present invention is significantly less, and there are fewer occurrences due to pattern switching. This makes it possible to reduce image noise.

In addition, the present invention enables not only determination in one-dimensional direction but also determination in two-dimensional direction, and it is also possible to minimize pattern switching over the entire screen.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining butter/butter switching, which is the basis of the present invention, and Fig. 2 shows a recording pattern used in a thermal transfer recording method according to an embodiment of the present invention and the amount of energy injected into the pattern. Figure 3 is a schematic diagram showing the amount of energy injected into each dot of the same pattern. Figures 4 and 5 are a partial configuration of a thermal printer using the method of this embodiment. FIG. 6 is a block diagram showing the system of the present invention. Pattern person...Pattern for low density Pattern B...Pattern for medium and low fi degree Pattern C...
・Mid-high density pattern Pattern D...High density pattern...Multi-value signal processing circuit 2...Multi-value pattern processing circuit 3...Thermal recording circuit

Claims (2)

[Claims] (1) In a thermal transfer recording method that divides a density area into several partial density areas and obtains pseudo gradation by modulating the size of pixels constituting a certain fixed pattern within each partial density area, A thermal transfer recording method characterized in that the density ranges of fixed patterns used for each density region overlap with each other. (2) The thermal transfer recording method according to claim 1, wherein the transition point of fixed pattern switching due to a change in image density differs depending on the direction of image density transition.