JPS6369380A - Density gradation type color printer - Google Patents

Abstract

PURPOSE:To obtain excellent picture quality by deciding a density gradation level of a 3-primary color ink of the titled printer so as to divide the achromatic lightness axis uniformly caused in overlapping each color of the same density gradation level of the ink and quantizing the density of each color to one level among the density gradation levels. CONSTITUTION:The density of each color is quantized by using a density level conversion section 2 in a way that the achromatic color obtained by overlapping three colors of the same density level is arranged on the density level of the ink decided to be arranged at a uniform interval on the axis of lightness, that is, to one of the density gradation levels. The difference in the visual characteristic between the density and lightness is corrected by the processing and the quantized noise in terms visuality is minimized. Each quantized density gradation level is converted into a signal representing a pulse width to drive a head by a picture recording section 3 in response to the drive condition of a thermal head and further converted into an output signal having the pulse width.

Description

[Detailed Description of the Invention] [Summary] The present invention is a density gradation type color printer, in which the density gradation level of the three primary color inks of the printer is changed to the density gradation level that occurs when each color of the above ink having the same density gradation level is superimposed. Good image quality is obtained by defining the brightness axis of achromatic colors to be equally divided and quantizing the density value of each color to one of the density gradation levels.

[Industrial application field]

The present invention relates to a method for setting density gradation levels of three primary color inks in a density gradation type color printer.

[Conventional technology]

In density gradation type color printers, the quantization level, which determines the ink density recorded for each density gradation level, greatly influences the image quality of the output image.

The higher the degree of agreement between this quantization level and the degree to which humans perceive color differences, the smoother the gradation changes and the better the image quality. Therefore, it is necessary to determine the quantization level so that the density value can be easily quantized and is close to the visual characteristics described above.

Gradation expression methods for color printers include a density gradation method and a pseudo-area gradation method.

The former is used when the recording dot itself can express different densities or areas, and each pixel of the image signal is made to correspond to the recording dot.

The latter pseudo-area gradation method is used when neither the density nor the area of the recording dots themselves can be changed, and the number of recording dots in a 0-pixel matrix is used to make each pixel of an image signal correspond to a group of multiple recording dots (pixel matrix). By changing , the area ratio is changed and the apparent density change is obtained.

This is a recording method for performing gradation recording in a binary printer, and since one pixel is composed of multiple recording dots, it is not possible to increase the pixel density (for example, 8 dots) / mm
When recording 16 gradations with a dot density of The former is used for high-resolution, high-quality recording.

Since the present invention relates to a color printer using a density gradation method, the conventional density gradation method will be explained below by taking a thermal transfer printer, which is a typical recording method thereof, as an example.

Heat generating elements (thermal heads) of thermal transfer printers can be broadly classified into those for line recording method and those for serial recording method. A line recording method is used for high-speed recording, and a serial recording method is used for low-speed recording. Since the recording principle is the same, only the line recording method will be explained below.

FIG. 3 shows the main parts of the above-mentioned line recording type thermal printer, in which the thermal head 7 is connected to the ink sheet 8. It faces a platen 10 with a recording paper 9 in between.

By heating this ink sheet 8, the ink on the ink sheet 8 is melted and transferred to the recording paper 9, and recording is performed.

The thermal head 7 has heating elements for one line arranged in a direction perpendicular to the paper surface, and records for one line are performed almost simultaneously. When the recording for one line is completed, the recording paper 9 and the ink sheet 8 are transported in the direction of the arrow.

When a density gradation type recording method is used, recording is performed in a multivalued manner. That is, as shown in the explanatory diagram of the relationship between applied heating energy and recording density in FIG. 4, when the applied thermal energy is changed from O to EN, the recording density becomes D @ t.
It changes from n to □.

Recording density is generally divided into about 16 to 64 levels (S degree gradation levels), and for each level, a representative density value (quantization level) is determined. Look for energy. During actual printing, the recording density is controlled by selecting the thermal energy to be applied. Therefore, the quality of the recorded image changes greatly depending on the quantization level.

For monochrome images, when determining the quantization level density, it has been confirmed through subjective experiments that selecting the quantization level uniformly on the brightness scale is better suited to human visual characteristics than selecting uniformly on the density scale.

Therefore, when outputting after performing complicated processing on a computer, or on expensive special-purpose printers, a method is used that uses quantization levels equally divided on the brightness scale.

However, in color, a uniform color space corresponds to human visual characteristics, and it is necessary to treat each YMC color in three dimensions in terms of density and brightness. Therefore, handling of color images is quite complicated, and therefore the configuration of the device is also complicated and expensive.

For this reason, conventional low-cost printers often use a method of using quantization levels that are divided evenly on the density scale because it is easy to convert monochrome images to ink amounts, while color printers The quantization level was determined by dividing the density equally for each color independently, ignoring the mutual relationship between the colors.

This is because there is a substantially proportional relationship between the amount of ink and the density of each printed color, so the amount of ink can be determined simply by measuring the density.

However, human visual characteristics are not linear with respect to density, and the nonlinearity is particularly significant in low density areas.

For this reason, in low-density areas, moire and false contours are likely to occur in the recorded image, resulting in the problem that an image of sufficient quality is not obtained.

(Problems to be Solved by the Invention) As mentioned above, in conventional density gradation type color printers, in order to obtain good color reproducibility and image quality, the device becomes complicated and expensive. However, it was not possible to obtain sufficiently satisfactory color reproducibility and image quality.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a density gradation type color printer that can obtain good color reproducibility and image quality, has a simple configuration, and is inexpensive.

[Means for solving problems]

In the present invention, the color image input section 1 outputs the density values of the three primary colors of ink used for printing, and the achromatic color that is generated when the same density gradation level of the three primary colors is superimposed is arranged at equal distance intervals on the lightness axis. By providing a density level conversion unit 2 that selects and outputs one of the density values output from the image input unit 1 based on the ink density gradation level table determined as being arranged, the original image and the corresponding The configuration is such that the recorded image obtained by converting the image signal read from the original image substantially matches the image signal.

[For production]

In the recorded image obtained using the present invention, the quantization level is set uniformly with respect to visual characteristics, so visually, quantization noise is minimized, and good image quality can be obtained. Further, since the above-mentioned configuration is extremely simple, an inexpensive density gradation type color printer can be manufactured.

〔Example〕

The present invention will be explained in detail below with reference to Examples.

FIG. 1 is a block diagram illustrating the invention in detail.

In the figure, a color image input circuit 1 is a circuit that converts an input signal into three primary color density values of ink used for printing. The density value obtained here is the density value of the original image, and in order to perform recording, it must be quantized and converted into a density gradation level.

Therefore, the density value of each color is determined by the density level converter 2 so that the achromatic colors obtained by overlapping three colors of the same density level are arranged at equal distance intervals on the brightness axis, that is, the density level of the ink is determined by the density level converter 2. It is quantized by assigning it to one of the density gradation levels. Through this processing, the difference in visual characteristics between density and brightness is corrected, and visual quantization noise is minimized.

Each quantized density gradation level is converted into a signal indicating a pulse width value for driving the head in the image recording section 3 according to the driving conditions of the thermal head, and is further converted into an output signal having this pulse width. converted.

The above pulse width value can be determined from the characteristic curve of applied heating energy and recording density shown in FIG. In other words, the relationship between the pulse width value and the applied heating energy is constant for each type of device, so if this relationship is determined in advance, the applied heating energy corresponding to each concentration level can be determined from the above characteristic curve by determining the required pulse width. It can be converted to a width value.

If the ambient temperature of the thermal head is constant and the recording speed is sufficiently slow, the pulse width value of the drive pulse corresponding to each density level can be uniquely determined. However, the substrate temperature of the thermal head generally changes depending on the ambient temperature of the thermal head and the history of recording density, and the characteristic curve shown in FIG. 4 also changes. Therefore, the image recording section 3 inputs the output density value of the color image input section 1 as well as the output of a temperature detector (not shown) attached to the thermal head, and receives a drive pulse width value that provides a predetermined recording density. Correct.

In this way, in the present invention, pulses optimized for each color to be generated are generated in the image recording section 3, thereby controlling the thermal head and recording good density gradation matching the visual characteristics. will be held.

In the present invention, the density level converter 2 shown in FIG. 1 determines the color reproduction characteristics.

In the present invention, by using the brightness of an achromatic color when three colors are superimposed as a standard for the quantization level of each color, the three-dimensional handling of the three primary colors is reduced to one-dimensional, thereby simplifying the conversion process. Moreover, by providing the density level converter 2 with quantization levels corresponding to the evenly divided brightness scales in the form of a table, an inexpensive and simple configuration is possible.

The method for determining this is described below.

The relationship between the density values of the three colors, such that an achromatic color is obtained when the three colors are superimposed, is determined in advance by theoretical formulas and experiments.

Next, the achromatic color level is divided equally into a brightness scale, and the quantization level of each color is determined. That is, as shown in FIG. 5, the quantization density level of each color is selected so as to equally divide the brightness axis (vertical axis) in the uniform color space.

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

FIG. 2 is a block diagram of an embodiment of the present invention, showing an example of dividing the density into 16 gradations.
OM-3 is the main part of the density level converter 2 shown in FIG. 1, and constitutes the quantization circuit 4.

The density values (8 bits each) of each color (YMC) converted by the color image input unit 1 are stored in memories ROM-1, ROM-2, and ROM-3 that store quantization tables corresponding to each color.
Each is converted into a 4-bit gradation level value. ROM-1, ROM-2, and ROM-3 output quantized density levels that equally divide the brightness axis of the achromatic color obtained when three colors are superimposed, that is, the density value of each color ink. . Both use IK bit ROM with a capacity of 256 x 4 bits.

A method for determining quantization tables for ROM-1, ROM-2, and ROM-3 will be shown.

The characteristic curve of the applied heating energy and recording density when each color ink is superimposed using a thermal transfer printer is as shown in FIG. 4, as described above.

Next, based on theoretical formulas and experiments, the pulse width is adjusted so that an achromatic color is obtained when the three colors are superimposed. The obtained achromatic color level is 1 on the brightness scale as shown in Figure 5.
Divide it equally into 6 gradations and find the pulse width of the density level value of each color. When printing using the determined table, errors from the theoretical values may occur depending on the transfer conditions.

Therefore, characteristic curves are determined again from printing experiments, and each color level value is corrected. The correction is repeated to obtain an optimal pulse width table, which is stored as a quantization table in ROM-1, ROM-2, and ROM-3.

The converted gradation level value is converted into a pulse width (8 bits) in ROM-4, ROM-5, and ROM-6, taking into account the temperature information (8 points) of the thermal head. At this time, the temperature data is used to correct heat accumulation in the head. ROM-4, ROM-5, and ROM-6 are the first
Included in the image recording section 3 in the figure, each with a capacity of 4096X
A 32-bit ROM with 8 pins is used.

The converted pulse width data for each color is converted into a binary signal sequence for driving the thermal head by a binarization circuit 6 in the image recording section 3, and drives the thermal head.

The recorded image of this example obtained as described above has the minimum quantization error perceived by humans in terms of visual characteristics.

As described in the above embodiment, the means for converting the density level can be realized with an extremely simple structure, and therefore, an inexpensive density gradation type color printer can be provided.

〔Effect of the invention〕

As explained above, the present invention selects the quantization level of the density gradation so that the achromatic colors produced when the three primary colors are superimposed are arranged at equal distance intervals on the lightness axis.
You can obtain high-quality images that match your visual characteristics. In addition, since the density level converter 2 can be configured with a small capacity ROM,
The circuit configuration is simple and therefore can be realized at low cost.

[Brief explanation of the drawing]

Figure 1 is a block diagram of the principle of the present invention, Figure 2 is a block diagram of an embodiment of the present invention (an example of 16 gradations), Figure 3 is an explanatory diagram of the main part configuration of a thermal printer, and Figure 4 is an impression heating An explanatory diagram of the relationship between energy and recording density. FIG. 5 is an explanatory diagram of the lightness axis in a uniform color space. In the figure, 1 is a color image input section, 2 is a density level conversion section, 3 is an image recording section, 4 is a quantization circuit, a sensor? Lua・
Wasigushi f etc. line 9Hη Figure 3 OEM and r+le and eye n JAl-γrugiγ;
B Moon Map Figure 4

Claims (1)

[Claims] A color image input unit (1) that outputs the density values of the three primary colors of ink used for printing, and a color image input unit (1) that outputs the density values of the three primary colors of ink used for printing, and a color image input unit (1) that outputs the density values of the three primary colors, such that the achromatic colors that are generated when the three primary colors are superimposed are arranged at equal distance intervals on the brightness axis. a density level conversion unit (2) that selects and outputs one of the density gradation level values according to the determined ink density gradation level table and the density value output from the color image input unit; 1. A density gradation type color printer, comprising: a density gradation type color printer configured to record each of the three primary colors according to the output density gradation level.