JPH09254424A - Method and apparatus for processing image - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve gradation reproducibility and to reproduce characters and edges correctly in multi-gradation printing. SOLUTION: Multi-garadation initial image data inputted externally by an image data input circuit 11 are taken in sequence in an initial image data division circuit 12 while being divided into block units, and the average of the gradation value of each picture element in a block is calculated in an average value operation circuit 13. Next, in a resolution selection circuit 15, by judging whether the average is in a range which is determined for each of resolutions set in advance or not, for the higher average, the higher resolution is selected, and the initial image data in the block is converted into printing data of the selected resolution in a data conversion circuit 16 to be outputted outside from an image data output circuit 19.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing method and an image processing apparatus for producing print data corresponding to original image data of multiple gradations, and more particularly to improving gradation reproducibility and eliminating characters and edges. The present invention relates to an image processing method and an image processing device that can be accurately reproduced.

[0002]

2. Description of the Related Art As a method of printing a multi-gradation image by a printer or a copying machine, for example, in the case of electrophotography, Japanese Patent Laid-Open No. HEI-1
However, the pulse width modulation exposure method described in Japanese Patent Publication No. 280965, etc.
The divided printing method in the sub-scanning direction, which is described in Japanese Patent Laid-Open No. 11274, can be cited.

FIG. 12 is a diagram showing an example in which multiple gradations are expressed by the sub-scanning division method in printing by the thermal transfer recording method. In the figure, a rectangle indicated by a broken line indicates a 1-pixel region, and a blackened portion indicates an actually printed dot. In this printing method, based on the original image data input from an image sensor or the like, print data of a numerical value that increases or decreases corresponding to the density of each pixel is created, and line-shaped in the main scanning direction. Printing is performed by applying a pulse corresponding to the numerical value indicated by the print data to each of the plurality of heat generating elements 1 arranged in. The heating element 1 relatively moves from the top to the bottom in the pixel area in the drawing while contacting the ink sheet and the recording paper or the thermal recording paper, and at this time, heat is generated by applying the pulse, and the ink sheet The ink is melted and transferred to recording paper, or heat is applied to the thermal recording paper to print the corresponding pixels.

At this time, a small number of pulses are applied as shown on the right side of the pixel area in FIG. 12 (a) when printing a low gradation portion, while FIG.
By applying a large number of pulses as shown in (b), the length of the dots to be printed in the sub-scanning direction changes.
Therefore, the number of pulses applied to the corresponding heating element is changed according to the gradation value of each pixel of the input original image data, and the printing area of each pixel is changed by a line to obtain multiple gradations. Can be expressed.

FIG. 13B shows the result of printing the multi-gradation original image shown in FIG. 13A by the above printing method. In the original image, one section formed by orthogonal vertical and horizontal straight lines is defined as one pixel, and the gradation value increases in the sub-scanning direction to 0, 32/256, 64/256, 128/256 in four stages. In the gradation image of, the line portion of the gradation value 1 (256/256) for representing a character or the like is diagonally overlapped. In the print result shown in FIG. 13B, the high gradation value line portion is printed as in the original image, but dots are generated in the gradation portions with gradation values 0, 32/256, 64/256. It does not appear, and the binary print result has low reproducibility in the low gradation part. This is because if the heating element 1 is not applied with a certain number of pulses or more (in this example, a maximum of 256 pulses are applied per pixel printing, and dots are printed only when 65 pulses or more are applied. This is because the adhesive force required for transferring the ink cannot be obtained, and in printing in the low gradation portion, dots do not appear even if a corresponding small number of pulses are applied to the heating element 1.

With respect to the above problems, there is known a method of reducing the resolution in the sub-scanning direction in order to improve the reproducibility of dots and lines in the low gradation area and smoothly reproduce the intermediate gradation area. . In order to express a smooth gradation from a lower gradation portion, it is necessary to reduce the minimum dot area ratio and increase the number of reproducible gradations. Here, the minimum dot area ratio is represented by the ratio of the minimum dot area to the print unit area. The minimum dot area is the value when the pulse is sequentially applied to the heating element and the ink starts stable transfer. The area is the area that is uniquely determined (constant) in the system in a manner that depends on the ink characteristics and the structure of the heating element, etc., so by increasing the printing unit area, that is, by decreasing the resolution, the minimum dot area The rate can be reduced. In the sub-scanning division method, the resolution in the main scanning direction cannot be controlled, so that the resolution in the sub-scanning direction is lowered.

FIG. 14 shows the original image of FIG.
The results of printing by the method of reducing the resolution in the sub-scanning direction are shown below. Four pixels continuous in the sub-scanning direction of the original image are set as one printing unit, and the boundaries of the printing units are arranged so as to coincide with the boundaries of the gradation density. At this time, the resolution in the sub-scanning direction during printing is 1/4 times the resolution of the original image. A value obtained by adding the gradation value indicated by the original image data of 4 pixels in the print unit is sequentially assigned as the print data from the first pixel,
Printing is performed by applying a pulse corresponding to the numerical value indicated by the print data to the heating element corresponding to each pixel.

[0008]

According to the above-mentioned printing method, the resolution in the sub-scanning direction is lowered when printing data is created, and the application of pulses is concentrated from the first pixel of the printing unit with a plurality of dots as the printing unit. By doing so, it is possible to reproduce from a lower gradation region. Therefore, in the printing result shown in FIG. 14, the gradation part reproduces the gradation as the original image. However, the line portion loses its characteristic continuity and is not reproduced exactly as the original image. This is because printing is performed with the resolution in the sub-scanning direction being reduced for the entire original image, so that in contrast to the improvement in reproducibility of the low gradation part due to the increase in the number of reproducible gradations, in the high gradation part. This is because the deterioration of the image quality is remarkable. Therefore, reproduction may be inaccurate when a region having a rapid density change such as the above line portion is printed. Further, when a high density area is printed without abrupt density change, each dot size is large and the pixel structure is conspicuous, resulting in an image with roughness.

The present invention has been made in view of the above circumstances, and image processing capable of improving gradation reproducibility, accurately reproducing characters and edges, and obtaining a printed image with an inconspicuous pixel structure. A method and an image processing apparatus are provided.

[0010]

In order to solve the above-mentioned problems, an image processing method according to a first aspect of the present invention is configured such that a plurality of heating elements arranged in a line in the main scanning direction are caused to selectively generate heat by printing data, When performing multi-gradation printing by adjusting the printing area in the scanning direction, the print data corresponding to the multi-gradation original image data is created by the following method. First, the two-dimensional original image data is divided into a plurality of blocks including at least a plurality of pixels continuous in the sub-scanning direction. Next, the resolution of the print data corresponding to each block is selected from a plurality of preset resolutions based on the gradation value indicated by each original image data in each block. Subsequently, the original image data in each block is converted into print data of the selected resolution.

In the image processing apparatus according to the second aspect, a plurality of heating elements arranged in a line in the main scanning direction are selectively caused to generate heat by print data, and the printing area in the sub-scanning direction is adjusted to adjust the multi-story. An image processing apparatus that creates the print data according to the original image data of multiple gradations when performing the tonal printing is characterized by including the following means. Original image data dividing means. The original image data dividing unit divides the two-dimensional original image data into a plurality of blocks including at least a plurality of pixels continuous in the sub-scanning direction. Resolution selection means. The resolution selecting means selects the resolution of the print data corresponding to each block from a plurality of preset resolutions based on the gradation value indicated by each original image data in each block. is there. Data conversion means. This data conversion means
The original image data in each block is converted into print data of the selected resolution.

According to the image processing method and the image processing apparatus described above, the resolution is selected for each block according to the characteristic indicated by the gradation value of the original image in each block, so that the block requiring gradation reproducibility By setting the resolution to be selected for the lower than the resolution to be selected for the block requiring high resolution, it is possible to improve the gradation reproducibility and accurately reproduce the characters and edges.

An image processing apparatus according to a third aspect is the image processing apparatus according to the second aspect.
In the image processing device according to the item 1, the resolution selecting unit determines whether an average value of gradation values indicated by the original image data of each pixel in the block is within a range determined for each of the plurality of types of resolutions. Thus, the resolution of the print data corresponding to each block is selected so that the resolution increases as the average value increases.

According to the above image processing apparatus, when printing a low gradation area in the original image, the resolution of the corresponding block is selected to be low to reduce the dot area ratio and increase the number of reproducible gradations. , It is possible to express smooth gradation from low gradation. On the other hand, when printing a high gradation area, by selecting a high resolution for the corresponding block, it is possible to prevent the dot area from increasing and to make the pixel structure inconspicuous, and to accurately reproduce characters and edges. .

An image processing apparatus according to a fourth aspect is the second aspect.
In the image processing device described in the paragraph 1, the resolution selecting unit determines whether the variation value of the gradation value indicated by the original image data of each pixel in the block is within a range determined for each of the plurality of types of resolutions. Thus, the resolution of the print data corresponding to each block is selected so that the resolution increases as the variation value increases.

According to the above image processing apparatus, when printing the uniform density portion (area where the density change is small) in the original image, it is possible to reproduce by reducing the resolution of the corresponding block to reduce the dot area ratio. Increase the number of gradations,
It is possible to express a smoother gradation. On the other hand, at the time of printing an area accompanied by a sudden change in density, it is possible to accurately reproduce characters and edges by selecting a high resolution for the block.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION An example of an embodiment of an image processing apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an image processing apparatus of the present invention, and FIG. 2 shows a thermal transfer recording type printing apparatus for printing using print data created by this image processing apparatus. As shown in FIG. 2, the printing apparatus prepares a thermal head 8, a pulse applying means 9 for driving the thermal head 8 and print data according to the original image data, and sends it to the pulse applying means 9. The image processing apparatus 10 for outputting. A plurality of heating elements 1 that generate heat when a pulse is applied are arranged on the thermal head 8, and each heating element 1 is discretely positioned so as to form a line along the front-back direction of the drawing. . A recording paper 3 and an ink sheet 4 are provided between the thermal head 8 and the platen roller 2.
And the hot-melt ink 5 is applied onto the ink sheet 4. The recording paper 3 and the ink sheet 4 are
The thermal head 8 and the platen roller 2 are conveyed by the conveying means (not shown) in the directions of the respective arrows.
After passing through the space, they are separated by the peeling roller 6.

Each heating element 1 of the thermal head 8 generates heat when selectively energized by the pulse applying means 9. That is, in the image processing apparatus 10, print data corresponding to multi-tone original image data input from an image scanner or the like.
Data is created, and in the pulse applying means 9, the printing data is printed.
Pulse is generated and applied to each of the plurality of heating elements 1 to selectively energize and generate heat, and the ink 5a of the image portion having a size corresponding to each pulse is melted and the recording paper 3 Image formation process of adhering to the substrate (Fig.
And the ink 5a in the image area adhered to the recording paper 3 is peeled off.
The image 7 is formed by the peeling process (FIG. 2B) where the image 7 is separated from the ink sheet 4 by the liner 6.

Next, the structure of the image processing apparatus 10 will be described. The image processing apparatus 10, as shown in FIG. 1, includes an image data input circuit 11, an original image data division circuit 12, and
An average value calculation circuit 13, a resolution selection circuit 15, a data conversion circuit 16, and an image data output circuit 19 are provided. The image data input circuit 11 inputs the original image data from an external device such as an image scanner and outputs the original image data as two-dimensional data composed of a plurality of pixels in the main scanning direction and the sub scanning direction. Is temporarily held. The original image data division circuit 12 divides the original image data from the image data input circuit 11 into block units each having a plurality of pixels continuous in the sub-scanning direction as one block, and sequentially captures them as data. It is a thing. Average value calculation circuit 13
Is to calculate the average value of the gradation values indicated by the original image data of each pixel in the block. The resolution selection circuit 15
The resolution of the print data corresponding to each block is selected according to the average value. The data conversion circuit 16 is
The original image data in the block is converted into the print data of the selected resolution, and the image data output circuit 1
Reference numeral 9 is for outputting the created print data to the outside.

Further, the data conversion circuit 16 uses the original image data.
Resolution conversion data generation circuits 17a, 17b, 17c for converting data into the data of the selected resolution, and print data generation for converting the resolution conversion data into a value per applied pulse number at the time of printing. It is composed of circuits 18a, 18b and 18c. The present invention relates to creation of print data used when performing multi-gradation printing by the sub-scanning division method,
Since the resolution in the main scanning direction is not controlled, the portion simply described as the resolution indicates the resolution in the sub scanning direction.

In the image processing apparatus 10, the original image data corresponding to one block (a plurality of pixels in the sub-scanning direction) is taken in, and the average value of the gradation values indicated by each pixel in the block is calculated. The print data is created by a procedure in which the resolution is selected according to the average value and the original image data is converted into the selected resolution. Therefore, it is necessary that the block size in the original image data division circuit 12 and the correspondence between the plurality of resolutions and the range of the average value and the resolution in the resolution selection circuit 15 are set in advance. The setting procedure of will be described.

The setting of the resolution depends on the desired reproducible gradation number. Since the reproducible gradation number is determined by the minimum dot area ratio, the minimum dot area is first determined. In the thermal transfer recording system shown in FIG. 2, when the recording paper 3 and the ink sheet 4 are conveyed at a constant speed and the number of pulses applied to the heating element 1 is changed, the same number of pulses are printed. It is a figure which shows the relationship between the average dot area in the said several dot, and the value which divided the dot area variation in the said several dot by the average dot area. As shown in the figure, when a large number of pulses are applied to print a dot having a large area, the hot-melt ink 5 and the recording paper 3
The adhesive force with the ink is increased, the ink is transferred well, and stable dots with a small area variation are formed. vice versa,
As the number of applied pulses decreases and the dot area decreases, ink transfer becomes unstable due to the effects of irregularities on the surface of the recording paper, and the dot area variation becomes extremely large. Here, the upper limit of the dot area variation at which the ink is recognized to start transferring stably by visual evaluation of the printed sample is defined as σ _max, and the average dot area at that time is defined as the minimum dot area A as shown in FIG. Determined as _min .

Once the minimum dot area A _min is determined, the number of reproducible gradations can be increased as the resolution is lowered and the printing unit area is increased. It is necessary to set the minimum resolution so that the minimum dot area ratio at that time can reproduce a gradation number equal to or larger than a desired gradation number. The relationship between the resolution and the number of reproducible gradations at that time is derived below.

The relationship between the dot area A and the print density D is expressed as follows by using the Yule-Nielsen equation considering the influence of the dot gain. A = Adot (10 ^{-D / n} -1) / (10 ^{-Di / n} -1) ... (1) Adot: Print pixel area Di: Maximum density n: Constant At this time, D and Di are inks on the recording paper. It is not the density but the density of the ink only. In the equation (1), when the dot area A is the minimum dot area A _min , the first gradation and the print density at this time is D1, A _min = Adot (10 −D1 ^{/ n} −1) / (10 ^{−Di / n} −1) ... (1 ′)

On the other hand, the number of reproducible gradations N is expressed as follows. N = D _PP / D1 (2) D _PP : Ink density dynamic range From the equations (1 ') and (2), the number of gradations N is expressed as follows. N = −D _PP / nlog (1+ (A _min / Adot) (10 ^{−Di / n} −1)) (3) Here, the print pixel area Adot [μm ² ] has a resolution in the main scanning direction of Rm [dpi ] (Constant), the resolution in the sub-scanning direction is Rs [d
pi], Adot = (25400) ² / (RsRm) (4) (3), (4) the number of gradations N from _{equation, N = - D PP / nlog} (1 + A min · Rs · Rm (10 -Di / n -1) / (25400) 2) ... (5) Table To be done.

FIG. 4 is a diagram showing the relationship between the sub-scanning direction resolution Rs in equation (5) and the number of gradations N at that time. The number of gradations increases as the resolution decreases, and at this time, reproducibility of low gradation parts naturally improves. Therefore, it is necessary to reduce the sub-scanning direction resolution Rs as the desired number of reproduced gradations increases. For example, as shown in FIG.
In the case of a, the resolution in the sub-scanning direction needs to be Rs _min or less. In practice, the resolution of the original image data is divided by a multiple of 2 in consideration of the efficiency of data conversion.
Here, assuming that the resolution of the original image data is 300 dpi, 300 dpi (hereinafter referred to as Rs 3), 150
dpi (hereinafter referred to as Rs 2), 75 dpi (hereinafter referred to as Rs 1)
And three resolutions are set. Compared with the resolution of the original image data, each resolution is 1 time, 1 /
2 times and 1/4 times. At this time, the print pixel size (print unit area) is 1 pixel, 2 pixels, and 4 pixels, and 4 pixels, which is the print pixel size at the minimum resolution Rs1, is set as the block size of one block.

The three resolutions Rs1, Rs2, and Rs3 determined by the above procedure are set in the resolution selection circuit 15, and the block size (4 pixels) is set in the original image data division circuit 12.

Next, a method of setting the range of the average value for selecting each of the plurality of resolutions will be described. FIG. 5 shows the print pixel size at each resolution for one pixel of the original image data. Here, 4 dots adjacent to the original image data in the sub-scanning direction are regarded as one block. In the figure, assuming that the minimum dot area is A _min and the one-pixel area of the original image data is A, the minimum dot area ratio at each resolution is A _min / 4A, A at resolutions Rs1, Rs2, and Rs3, respectively.
_min / 2A and A _min / A.

Since the higher the gradation the average value calculated by the average value calculation circuit 13 is, the higher the resolution is selected, the resolution selected when the average value is the highest gradation is Rs3. Considering the case where the average value gradually decreases from the highest gradation, in the printing performed by selecting the resolution Rs3, the number of pulses applied to the heat generating element 1 is applied to the gradation having the gradation value A _min / A or less. Is insufficient and cannot be reproduced as dots. Therefore, it is necessary to select and print one resolution Rs2 lower by one. If the average value further decreases and becomes A _min / 2A or less, dots cannot be reproduced similarly in printing by selecting the resolution Rs2. Therefore, it is necessary to select and print one resolution Rs1 lower. However, the minimum resolution Rs1
In the printing for selecting, it is not possible to print with dots below the variation upper limit value.

As described above, in each resolution (Rs2, Rs3) other than the lowest resolution (Rs1), when the gradation value of the average value is A _min / (print pixel size) or less, the resolution one lower is selected. If the gradation value indicated by the average value is a, the resolution Rs1 is set when the gradation range is a = 0 to A _min / 2A, and the resolution Rs2 is set when a = A _min / 2A to A _min / A. , A = A _min / A to 1, if the resolution Rs3 is set to be selected, continuous gradation can be reproduced. At this time, A _min becomes the threshold value.

However, the minimum dot area A _min is a value determined on the basis of the upper limit of the area variation by visual evaluation, and if the number of pulses applied to the heating element 1 is reduced until it approaches this area, the dot area variation will occur. When the gradation value further decreases and becomes less than A _min / (print pixel size), the dot area variation temporarily decreases in printing performed by selecting a resolution one lower. Therefore, when the minimum dot area A _min is used as a threshold value, smooth gradation reproduction is hindered. Therefore, in practice, it is preferable to set αA _min (α> 1) as the threshold in consideration of some margin. For example, in FIG. 3, 2A _min
= 3000 μm ² is used as the threshold value, compared to the case where A _min is used as the threshold value, it is possible to sufficiently suppress the variation in dot area before and after the change of the resolution and realize smooth gradation reproduction.

The streets threshold above the 2A _min, A _min = A
In the case of / 4, the gradation range of the average value a for selecting each resolution is
As shown by the arrows in FIG. 6, a = 0~A _min / resolution in the case of _{A Rs1, a = A min /} A~2A min / in the case of A resolution Rs2, a = 2A _min / For A~1 is The resolution is set to Rs3. However, in the printing performed by selecting the lowest resolution Rs1, it is not possible to print with dots below the variation upper limit value.

FIG. 7 is a diagram showing original image data and print data corresponding thereto when the range of the average value a corresponding to each resolution is set as described above. At this time, the number of gradations of the original image data is 256, and the ratio of the minimum dot area A _min to the area A of one pixel is A _min / A = 65/256.
The number of pulses applied to the heating element 1 at the time of printing is 128 pulses per pixel of the original image data. The threshold value of the range of the average value a is 2A _min = 130, and the gradation range of the average value a for selecting each resolution is resolution Rs1 when a = 0 to 65/256, and a = 65/256 to 130/256. In this case, the resolution is Rs2, and in the case of a = 130/256 to 1, the resolution is Rs3. Correspondence between the gradation range and the resolution is preset in the resolution selection circuit 15.

In the case of FIG. 7A, the gradation values indicated by the original image data of each pixel in the block are 24/256 and 32/2.
56, 48/256, 40/256, and the average value calculating circuit 13 calculates the average value a. a = 3
The resolution becomes 6/256, and the resolution Rs1 is selected by referring to the correspondence with the resolution in the resolution selection circuit 15. Since the print pixel size at the resolution Rs1 is 4 pixels, the resolution conversion data generation circuit 17a uses the average value a as it is to obtain the resolution conversion data over 4 pixels of the gradation value 36/256. Then, in the print data generation circuit 18a, a value 14
4/256 is sequentially assigned from the first pixel of the print pixel unit, the gradation value of the first pixel is 144/256, the gradation value of the second to fourth pixels is 0/256, and
Since 128 pulses are input to each pixel of the original image data, the gradation values are converted to 128 pulses per input, and the gradation value of the first pixel is 72/128, and the gradation value of the second to fourth pixels is The print data with the gradation value of 0/128 is obtained. The pulse applying means 9 generates a pulse corresponding to the print data, applies the pulse 72 times to the heating element 1 corresponding to the first pixel, and corresponds to the second to fourth pixels. No pulse is applied to the heating element 1.

In the case of FIG. 7B, the average value a is calculated in the average value calculation circuit 13 as in the case of FIG. a = 89 /
Then, the resolution Rs2 is similarly selected by the resolution selection circuit 15. Since the print pixel size at the resolution Rs2 is 2 pixels, the resolution conversion data generation circuit 17b
In, the average value 96/256 of the gradation values of the first and second pixels is calculated, and the resolution conversion data of the gradation value 96/256 over the first and second two pixels is obtained.
In addition, the average value 82 of the gradation values of the third and fourth pixels
/ 256 is calculated, and the resolution conversion data of the gradation value 82/256 over the third and fourth two pixels is obtained. Then, in the print data generation circuit 18b, a value 1 obtained by doubling the average gradation value 96/256 of the first and second pixels is obtained.
92/256 is the value 164 / that is obtained by doubling the average gradation value 82/256 of the first and third and fourth pixels.
256 is set as the grayscale value of the third pixel, and further converted into the grayscale value per 128 pulse input, and the grayscale values of the first to fourth pixels are set to 96/128, 0/12.
Print data of 8, 82/128, 0/128 is obtained.

In the case of FIG. 7C, the average value is calculated in the average value calculation circuit 13 in the same manner as in FIGS. 7A and 7B, and the average value a = 205/256, and the resolution is similarly determined in the resolution selection circuit 15. Rs3 is selected. Since the print pixel size at the resolution Rs3 is 1 pixel, the gradation value of the original image data is used as the resolution conversion data in the resolution conversion data generation circuit 17c. Further, in the print data generation circuit 18c, the resolution conversion data is converted into gradation values per 128 pulse inputs, and the gradation values of the first to fourth pixels are 113/128, 120. / 128, 93 /
Printing data of 128, 84/128 is obtained.

According to the above image processing apparatus, the resolution selection circuit 15 selects a higher resolution as the average gradation value of each block of the original image data is higher, and a lower resolution as the average gradation value is lower. While improving the reproducibility of the gradation part, the pixel structure of the high gradation part can be made inconspicuous,
In addition, it becomes possible to accurately reproduce characters and edges.

FIG. 8 is a block diagram showing another example of the embodiment of the image processing apparatus according to the present invention. 8, the same components as those in FIG. 1 are designated by the same reference numerals and the description thereof will be omitted.
This image processing apparatus 10 'is provided with a standard deviation calculation circuit 14 which replaces the average value calculation circuit 13 and calculates the standard deviation of the gradation values indicated by the original image data of each pixel in the block, and the resolution. 1 in that the selection circuit 15 'selects the resolution of the print data corresponding to the block by referring to the correspondence between the preset standard deviation range and the resolution.
The image processing device 10 shown in FIG. Further, while the resolution conversion data generation circuit 17a uses the average value calculated by the average value calculation circuit 13 to create the resolution conversion data, the resolution conversion data generation circuit 17a '
The resolution conversion data is created using the gradation value of each pixel transferred from the original image data division circuit 12.

In the image processing apparatus 10 ', the block size and the plurality of resolutions Rs1, Rs2, Rs3 are set.
Both are set to the same value as that of the image processing apparatus 10, and the correspondence between the range of the variation value of the gradation value indicated by the original image data of each pixel in the block and the resolution is set as follows. As the variation value, the sum of squared deviations, variances, standard deviations, ranges, and the like can be used. In this example, the standard deviations shown below are used. s = [{Σ (xi−X) ² } / n] ^1/2 (6) xi: gradation value of each pixel X: average gradation value in block n: number of pixels in block Characters with large variation value and High edge resolution,
The range of the standard deviation s for selecting each resolution is s = 0 so that the resolution of the uniform gradation part having a small variation value is selected to be low.
When s is 0.1, the resolution is Rs1, when s is 0.1 to 0.2, the resolution is Rs2, and when s is 0.2, the resolution is Rs3.
The correspondence between the range of the standard deviation and the resolution is preset in the resolution selection circuit 15 '.

FIG. 9 is a diagram showing original image data and print data corresponding thereto when the range of the standard deviation s corresponding to each resolution is set as described above. At this time, the number of gradations of the original image data is 256, and the minimum dot area A _min and 1
The ratio of the pixel area A is A _min / A = 65/256.
The number of pulses applied to the heating element 1 at the time of printing is 128 pulses per pixel of the original image data.

In the case of FIG. 9A, the gradation values indicated by the original image data of each pixel in the block are 87/256 and 89/2.
56, 95/256, 97/256, and the standard deviation calculation circuit 14 calculates these standard deviations s. s
Therefore, the resolution Rs1 is selected by referring to the correspondence with the resolution in the resolution selection circuit 15 '. Since the print pixel size at the resolution Rs1 is 4 pixels, the average value of each pixel is calculated in the resolution conversion data generation circuit 17a 'and the gradation value 9 over 4 pixels is calculated.
Obtain resolution conversion data of 2/256. Subsequently, in the print data generation circuit 18a, a value 368/256 obtained by multiplying the average value by 4 is sequentially assigned from the first pixel of the print pixel unit, and the gradation values of the first to fourth pixels are set to 256/25.
6, 112/256, 0/256, 0/256, and further converted into gradation values per 128 pulse input, and the gradation values of the first to fourth pixels are respectively 12
Print data of 8/128, 56/128, 0/128, 0/128 is obtained. The pulse applying means 9 generates a pulse corresponding to the print data, and the heat generating element 1 corresponding to the first and second pixels is 128 times, respectively.
The pulse is applied 56 times, and no pulse is applied to the heating elements 1 corresponding to the third and fourth pixels.

In the case of FIG. 9B, the standard deviation calculating circuit 14 calculates the standard deviation s as in the case of FIG. 9A. s =
The value becomes 0.14, and the resolution Rs2 is similarly selected by the resolution selection circuit 15 '. Since the print pixel size at the resolution Rs2 is 2 pixels, the resolution conversion data generation circuit 1
In 7b, the average value 42/256 of the gradation values of the first and second pixels is calculated and used as the resolution conversion data of the gradation value 42/256 over the first and second two pixels. Further, the average value 94/256 of the gradation values of the third and fourth pixels is calculated and used as the resolution conversion data of the gradation value 94/256 over the second pixels of the third and fourth pixels.
Subsequently, in the print data generation circuit 18b, a value 84/25 obtained by doubling the average gradation value of the first and second pixels
6 is the value 188/256 obtained by doubling the average gradation value of the third and fourth pixels of the first pixel, and the gradation value of the third pixel is 188/256. Converted to gradation values, the gradation values of the first to fourth pixels are changed to 42/128, 0/128, 94/128, 0/1.
The printing data of 28 is obtained.

In the case of FIG. 9 (c), the standard deviation s is calculated in the standard deviation calculation circuit 14 as in (a) and (b),
Since s = 0.41, the resolution Rs3 is similarly selected by the resolution selection circuit 15 '. Since the print pixel size at the resolution Rs3 is 1 pixel, the gradation value of the original image data is used as the resolution conversion data in the resolution conversion data generation circuit 17c. Further, in the print data generating circuit 18c, the resolution conversion data is converted into the gradation value per 128 pulse input, and the gradation values of the first to fourth pixels are changed to 23/128, 128. / 128, 128/12
Print data of 8, 128/128 is obtained.

According to the above image processing apparatus, the resolution selecting circuit 15 'selects the higher resolution as the variation value of the gradation value of each block of the original image data is higher, and selects the lower resolution as the variation value is lower. Characters and edges can be accurately reproduced, and at the same time, smooth gradation can be reproduced in the equal density portion (area where density change is small).

In the image processing apparatus of each example described above,
Four pixels continuous in the sub-scanning direction are set as the block size of one block. However, for example, eight pixels each including two pixels in the main scanning direction and four continuous pixels in the sub-scanning direction are set as the block size of each block. The image processing may be performed in which the resolution of the print data corresponding to is selected and the original image data is converted into print data of the selected resolution, and the pixels forming one block form a plurality of images that are continuous in at least the sub-scanning direction. If included, it is possible to improve the reproducibility of the low gradation portion in the sub-scanning direction.

[0046]

Next, an embodiment of the image processing apparatus of the present invention will be described with reference to FIGS. FIG.
FIG. 3 is a block diagram of an image processing apparatus according to an embodiment. 10, the same components as those in FIGS. 1 and 8 are designated by the same reference numerals and the description thereof will be omitted. This image processing device 10 ″ is
The original image data division circuit 12 simultaneously transfers the gradation value indicated by the original image data of each pixel in the block to the average value operation circuit 13 and the standard deviation operation circuit 14, and the operation by both operation circuits is performed. The results are simultaneously transferred to the resolution selection circuit 15 ″. The resolution selection circuit 15 ″ is set to select the corresponding resolution by the combination of the range of the average value a and the standard deviation s. Here, when the standard deviation s is large, the standard deviation s is selected, and when the standard deviation s is small, the average value a is prioritized to select the resolution.

FIG. 11 shows the image processing apparatus 1 shown in FIG.
It is a figure which shows the printing result performed using the printing data produced by 0 ″, and used the original image of FIG. 13 (a).
Here, the number of gradations of the original image data is 256, and the ratio between the minimum dot area A _min and the one pixel area A of the original image is A _min / A = 6.
It is 5/256. The block size was set to 4 pixels, and in each block, 4 pixels adjacent to each other in the sub-scanning direction of the original image were arranged so that the boundary of the gradation part and the boundary of the block coincided with each other. The resolution preset in the resolution selection circuit 15 ″ is Rs1 to Rs3 similar to the above description, and
Regarding the correspondence between the average value a and the standard deviation s and the resolution, the resolution Rs3 is selected regardless of the value of a when s ≧ 0.2, and a = 0 to 0 when 0 ≦ s <0.2. 65/25
If 6, the resolution Rs1, a = 65/256 to 130 /
If 256, resolution Rs2, a = 130 / 256-1
Then, the resolution Rs3 is set to be selected.

Since the resolution Rs3 is selected for s ≧ 0.2 for all blocks including the line portion and s = 0 for all other blocks, the average value of gradation values is low. A lower resolution is selected to print.
Therefore, in the printing result shown in FIG. 11, the line portion is reproduced exactly as the original image, while the gradation portion is printed with dots having the area ratio from 32/256 gradation to the gradation value. , It reproduces smooth gradation from the low gradation part.

According to the above image processing apparatus, the selection of the resolution based on the average gradation value and the dispersion value of the original image in the block is combined, and if the dispersion value is equal to or more than a certain value, a high resolution is obtained regardless of the average gradation value. If the variation value is less than a certain value and the average gradation value is lower, the lower resolution is selected, and the print data of the selected resolution can be created. By performing printing using this print data, it is possible to accurately reproduce the characters and edges that are accompanied by a sudden change in density as the original image. Further, in the part where the density change is gentle, a smooth gradation can be reproduced in the low gradation part, and the pixel structure can be made inconspicuous in the high gradation part.

[0050]

According to the printing using the printing data created according to the present invention, the reproducibility can be improved by selecting the resolution for each block according to the characteristics indicated by the gradation value in each block of the original image. An excellent printed image can be obtained.
For example, the resolution selecting unit selects a higher resolution as the average gradation value of each block of the original image is higher, and a lower resolution as the average gradation value is lower, thereby improving the reproducibility of the low gradation portion and at the same time. The pixel structure of the high gradation part can be made inconspicuous, and characters and edges can be accurately reproduced. Further, the resolution selecting unit selects a higher resolution as the variation value of the gradation value of each block of the original image is higher, and a lower resolution as the variation value is lower, whereby the character or the edge can be accurately reproduced. At the same time, a smooth gradation can be reproduced in the equal density portion (area where the density change is small). Furthermore, by combining the selection of the resolution based on the average gradation value and the variation value of each block of the original image, it is possible to obtain an excellent printing result that is compatible with the gradation reproducibility and the resolution.

[Brief description of drawings]

FIG. 1 is a block diagram of an image processing apparatus according to the present invention.

2A and 2B are configuration explanatory views showing a thermal transfer recording type printer.

FIG. 3 is a diagram showing a relationship between an average dot area and a dot area variation rate.

FIG. 4 is a diagram showing the relationship between sub-scanning direction resolution and the number of reproduced gradations.

FIG. 5 is a diagram showing a relationship between a resolution and a print pixel size set in the image processing method of the present invention.

FIG. 6 is a diagram showing a gradation range of an average value corresponding to each resolution.

FIGS. 7A, 7B, and 7C are diagrams showing an example of creating print data when the resolution is selected based on an average value.

FIG. 8 is a block diagram of an image processing apparatus according to the present invention.

9A, 9B, and 9C are diagrams showing an example of creating print data when a resolution is selected according to a variation value.

FIG. 10 is a block diagram of an image processing apparatus according to the present invention.

FIG. 11 is a diagram showing a print result using print data created by the image processing apparatus of the present invention.

12A and 12B are diagrams showing an example of representation of multi-gradation by the sub-scan division method.

FIG. 13 is a diagram showing a print result using print data created by a conventional image processing method, in which (a) is an original image and (b) is a print result.

FIG. 14 is a diagram showing a print result using print data created by a conventional image processing method.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Heating element, 2 ... Platen roller, 3 ... Recording paper, 4 ... Ink sheet, 5 ... Thermal melting ink, 6
... peeling roller, 7 ... image, 8 ... thermal head,
9 ... Pulse applying means, 10, 10 ', 10 "... Image processing device, 11 ... Image data input circuit, 12 ... Original image data division circuit, 13 ... Average value calculation circuit, 14 ...
Standard deviation calculation circuit, 15, 15 ', 15 "... Resolution selection circuit, 16, 16' ... Data conversion circuit, 17a, 1
7a ', 17b, 17c ... Resolution conversion data generation circuit,
18a, 18b, 18c ... Print data generation circuit, 1
9 ... Image data output circuit

Claims (4)

[Claims] 1. A plurality of heating elements arranged in a line in the main scanning direction are selectively caused to generate heat by print data, and a printing area in the sub scanning direction is adjusted to perform multi-gradation printing. In the image processing method for creating the print data according to the original image data of gradation, the two-dimensional original image data is divided into a plurality of blocks including at least a plurality of pixels continuous in the sub-scanning direction, Of the original image data, the resolution of the print data corresponding to each of the blocks is selected from a plurality of preset resolutions, and the original image data in each of the blocks is selected. And an image processing method, wherein the image data is converted into print data having the above resolution. 2. A plurality of heating elements arranged in a line in the main scanning direction are selectively caused to generate heat by print data, and a printing area in the sub scanning direction is adjusted to perform multi-gradation printing. In the image processing apparatus for creating the print data according to the original image data of gradation, the original image data dividing means for dividing the two-dimensional original image data into a plurality of blocks including at least a plurality of pixels continuous in the sub-scanning direction. A resolution selecting unit that selects the resolution of the print data corresponding to each block from a plurality of preset resolutions based on the gradation value indicated by each original image data in each block, An image processing device, comprising: data conversion means for converting original image data in each block into print data of the selected resolution. 3. The resolution selecting means determines whether the average value of the gradation values indicated by the original image data of each pixel in the block is within a range determined for each of the plurality of types of resolutions. The image processing apparatus according to claim 2, wherein the resolution of the print data corresponding to each of the blocks is selected so that the resolution increases as the average value increases. 4. The resolution selecting means determines whether the variation value of the gradation value indicated by the original image data of each pixel in the block is within a range determined for each of the plurality of types of resolutions. The image processing apparatus according to claim 2, wherein the resolution of the print data corresponding to each block is selected so that the resolution increases as the variation value increases.