JPH0552271B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention prints one line of pixels on thermal recording paper by energizing the heating element dots of a thermal head to generate heat, and then moves the recording paper, etc. by one line. , relates to an improvement in a thermal image recording device that obtains a two-dimensional image by sequentially printing pixels in each row. [Background of the Invention] A method of reproducing images in the form of hard copies such as photographs from electrical image signals obtained from televisions, video cameras, video disks, electronic cameras, etc. has been actively researched. The most promising method among these is the thermal image recording method, in which thermal recording paper (thermal coloring type (In addition to the above, it also includes, for example, a layered image-receiving sheet and a thermal transfer ink sheet)
While moving the dot, only a predetermined heating element dot is energized to generate heat based on the input image signal, and this heat is used to print one line of pixels on the recording paper. By sequentially printing each line of pixels, one This completes a two-dimensional image (see Figure 1). In this case, the number of heating element dots corresponding to one row of pixels may reach, for example, 1280, depending on the size of the image. However, when there are such a large number of dots, the maximum current that flows when all the dots generate heat becomes extremely large, and therefore a large power source is required. Therefore, a block printing method was proposed in which the group of heating element dots is divided from the center into two blocks Ba and Bb, and each block is printed (heated by energization) at different times in sequence. Although this method doubles the time required to print one row of pixels, the maximum current required is only halved. By the way, the temperature distribution when one heating element dot is made to generate heat shows a mountain-shaped distribution as shown in Fig. 2. If each block is made to generate heat, if all the dots in the block are made to generate heat, As shown in FIG. 3A, the heat generation temperature is low at the boundary between the heat generating block Ba (dot area with hatching) and the non-heat generating block Bb (dot area without hatching). This is a non-heating block outside the boundary.
This is because the temperature is low and therefore heat escapes, and this phenomenon cannot be avoided. This phenomenon also occurs when the energization and heat generation operation of block Ba is completed and the next energization and heat generation operation of block Bb is performed, resulting in a temperature distribution as shown in FIG. 3B. Therefore,
At the boundary when the block is divided, the heat generation temperature is not high enough in either the first operation or the next operation (see Figure 3C), so the printed pixels are discontinuous at the boundary, resulting in white dots. These white spots naturally appear as white streaks (referred to as white lines by the inventors) in the printed image in the direction of movement of the recording paper or the like. This white line becomes a fatal drawback when creating images such as photographs. Therefore, instead of dividing the blocks at the center of the dot group, by numbering the dot groups sequentially from the end, we divide the dots into blocks to which odd-numbered dots belong and blocks to which even-numbered dots belong. An alternating block printing method was proposed in which each block was printed. By the way, not only this alternating method but also other methods, it is not possible to obtain a high-quality image if the image is simply composed of the presence or absence of pixels. It is necessary to be different. It is important to divide the pixel density into different types, generally 16 gradations, 32 gradations, 64 gradations,
It is said to have 128 gradations or 256 gradations. The formation of pixels with such gradation density is
In general, printing is performed by shortening the time for one printing (current application) (this is why thin pixels are printed) and printing over and over again. In other words, to form an image with the highest density of 64 gradations, for example, the same pixel is overprinted 64 times. However, for pixels that are thinner than that, printing is stopped in the middle of 64 times, and further overlapping printing is stopped. Therefore, if pixels of 64 gradations and the highest density are to be formed in that block, the block will undergo energization printing 64 times. Of course, during this time, the power supply to the dots forming pixels with low density is stopped midway. The alternating method proposed earlier was to perform overprinting up to n gradations (for example, n = 64) in the first block, and then perform overlapping printing up to n gradations in the same way in the next block. . Therefore, in the case of batch printing without block printing, if the sum of the time required for one printing (equivalent to one gradation) and the pause time until the next printing is t milliseconds, then 1
It takes n×t milliseconds to print pixels for a row, but in the case of block printing, it takes twice as long as 2×
It will take n×t milliseconds. However, according to the inventors' follow-up experiments, the previously proposed alternating block printing method:
The pixels formed from the dots belonging to the block to be printed later are darker than the previous ones, so the resulting two-dimensional image has dark streaks every other pixel in the direction of movement of the recording paper. I knew it would appear. [Object of the Invention] Therefore, the object of the present invention is to shorten the printing time for one line, which takes 2×n×t milliseconds, and to eliminate dark streaks in the above-mentioned alternating block printing method. An object of the present invention is to provide a thermal image recording device that can perform the following steps. [Summary of the Invention] Coincidentally, the inventors of the present invention discovered that by alternating blocks for each gradation and printing, the printing time required to print one line of pixels was reduced to n x t milliseconds. However, the inventors have discovered that the resulting image is free of streaks and is close to an n-gradation image obtained by batch printing, leading to the completion of the present invention. In the apparatus of the present invention, printing is performed only n/2 times when operating on the same pixel over all gradations. Conventionally, only half the density of n gradations can be obtained by printing n/2 times, but with the device of the present invention, due to the heat accumulation phenomenon in the head, the higher the gradations, the higher the actual heat generation temperature of the dots. As a result, it is thought that an image with n gradations close to the conventional n-time printing can be obtained. In particular, when n is set to 64, 128, or 256 gradations, there is no inferiority at all, and it is almost impossible to tell them apart. EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto. In the following examples, for the sake of simplicity,
It is assumed that the electrical image signal has already been decomposed into monochrome image signals. (Example) ... [Thermal sublimation transfer image recording method] Electrical image signals are input sequentially starting from the first row and ending with the last row. As an example, the image signal of the first row is is shown in Figure 4. This image signal is divided into 1 to 10 sections along the time axis, and each section corresponds to each pixel, that is, an individual heating element dot. Here, we will explain a device that expresses gradations based on the time that electricity is applied to one heating element dot, but to simplify the explanation, the gradations are n = 6 gradations of 0, 1, 2, 3, 4, and 5. The number of pixels per row is 10.
Therefore, first, the input image signal for one line is sequentially compared with five comparison levels L ₁ , L ₂ , . . . L ₅ that are proportional to the density. Is the result equal to the comparison level?
Or if it is higher (darker) than that, set it as “1”,
If it is determined to be "0" when it is lower (thinner) than the comparison level, level data expressed as 0 and 1 is obtained for each comparison level. The level data of the image signal shown in FIG. 4 is as shown in Table 1 below.

〔Effect of the invention〕

As described above, according to the present invention, in the alternating block printing method, a high-quality image close to a high-gradation image can be obtained, there are no streaks, and the printing time for one line, that is, the time required to create the entire image. is only half that of high gradation.

[Brief explanation of the drawing]

FIG. 1 is a conceptual diagram illustrating a general thermal image recording method. FIG. 2 is a conceptual diagram showing the temperature distribution when one heating element dot generates heat. FIG. 3A is a conceptual diagram showing the temperature distribution when the thermal head generates all the heat in block Ba according to the conventional method. FIG. 3B is also a conceptual diagram of block Bb. FIG. 3C is a composite diagram of FIGS. 3A and 3B. Figure 4 shows
7 is a graph showing the waveform or time chart of the first row of image signals used in the embodiment of the present invention. Fifth
The figure is a waveform diagram of a level data signal. FIG. 6 is a driving circuit diagram of the thermal head also used in Example 1. FIG. 7 is a waveform diagram of the strobe signal. FIGS. 8 and 9 are waveform diagrams of a level data signal, a strobe signal, and a block level data signal generated by multiplying the two. [Explanation of symbols of main parts], TH: thermal head, P: platen roll, R: image receiving sheet,
I: thermal transfer ink sheet, E: electrical image signal input terminal, D: heating element dot, 61: level data signal generation circuit, 62: clock signal generation circuit, 63: load signal generation circuit, 64: thermal head drive Signal generation circuit, 65: Shift register, 66: Latch circuit, 67: NAND gate, 6
8: strobe signal switching circuit, 69: strobe signal generation circuit, 70, 71, 72: AND gate, 73: OR gate.

Claims (1)

[Scope of Claims] 1 Using a line-type thermal head consisting of a plurality of heating element dots, the energization time to the head necessary to achieve a predetermined gradation is divided into a plurality of pulses, and the pulses are applied to the head. In a thermal image recording device that reproduces a two-dimensional image with gradation by controlling the number of dots, one of the odd-numbered dot groups and the even-numbered dot group when printing an odd-numbered gradation. A thermal image recording device comprising means for outputting a signal for preventing printing to a group of dots, and outputting a signal for preventing printing to the other group of dots when printing an even-numbered gradation.