JP3489288B2 - Image processing method and image processing apparatus - Google Patents

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 used for multi-tone printing in which divided printing is performed to express gradation, and in particular, the density is reduced in order to improve gradation. The present invention relates to an image processing method and an image processing apparatus that perform gradation processing when printing is performed.

[0002]

2. Description of the Related Art In writing processing in a printer or a copying machine, its resolution and gradation are important factors.
In order to express the gradation, in electrophotography, for example, there is a pulse width modulation exposure method described in JP-A-1-280965. Further, in the fusion thermal transfer method, gradation expression can be performed by applying a divided printing method in the sub-scanning direction as described in, for example, Japanese Patent Publication No. 3-11274.

FIG. 11 is an illustration of an example of a gradation printing method by a thermal transfer method. In the figure, 91 is a heating element.
In FIG. 11, the rectangle indicated by the broken line indicates one dot, and the heating element 91 corresponding to this dot generates heat when moving from top to bottom as indicated by the arrow in the figure, for example,
The ink of the ink ribbon is melted and transferred to the recording paper, or heat is applied to the thermal paper to perform recording. At this time, the length of the drive pulse applied to the heating element 91 can change the printing length in the arrow direction in the drawing, that is, the sub-scanning direction. Corresponding this to the print density, when printing a thin portion, the shaded portion is printed by reducing the pulse width as shown in FIG. 11 (A).
Further, when printing a dark portion, the pulse width is increased as shown in FIG. As a result, the shaded area in FIG. 11B is printed, and the printing area is larger than in FIG. 11A. Therefore, the dark portion can be expressed. As described above, in this printing method, the area gradation can be expressed for each dot by the lines.

Such a printing method has a problem in reproducibility in a low gradation area. On the other hand, as described in, for example, Japanese Patent Application Laid-Open No. 6-62248 and Japanese Patent Application No. 5-248474, improvement in reproducibility of dots and lines in a low gradation region and smoothing of halftones. It is known that it is effective to reduce the line density in the sub-scanning direction in order to reproduce.

FIG. 12 is an explanatory diagram of an example of a printing method when the resolution in the sub-scanning direction is lowered in gradation printing by the conventional thermal transfer method. The method shown in FIG. 12A shows an example in which the second pixels are thinned out in order to reduce the resolution in the sub-scanning direction. Further, the method shown in FIG. 12B shows an example in which the interpolation values of the first pixel and the second pixel are used. 12A and 12B, the left side shows the printing result when printing is performed at the normal resolution. Here, the first pixel, the second pixel, and the first 'pixel are the low-density portion. The second 'pixel is a high density portion, and an edge is present in this portion.

In the method of thinning out the second pixel as shown in FIG. 12A, the area of two pixels of the first and second pixels is printed by the area corresponding to the density of the first pixel. . Therefore, as shown on the right side of FIG.
Areas corresponding to the densities of the pixels of No. 1 and the pixels of No. 1'are printed. In this case, the high density pixel of the 2'th pixel is not reflected in the printing, and the edge of this portion cannot be reproduced.

As shown in FIG. 12B, in the method using the interpolated value of the density of the first pixel and the density of the second pixel, the first
And the area of two pixels of the second pixel, the first and second
Only the area corresponding to the interpolated value of the density of the pixel, for example, the average value is printed. Therefore, the printing is performed as shown on the right side of FIG. However, in this case, since the densities are averaged, the edge existing in the second 'pixel portion cannot be reproduced and is expressed as a gradual density change.

FIG. 13 is an explanatory diagram of an example of a print result when the resolution in the sub-scanning direction is lowered when a high density edge exists in a low density portion in gradation printing by the conventional thermal transfer method. Now, there is an image printed as shown in FIG. 13A when printed at a normal resolution. In this image, high-density edges such as characters and line segments are present in the low-density background. This image is shown in Figure 12.
When the resolution in the sub-scanning direction is reduced by the method of thinning out the second pixels as shown in FIG. 13A, printing is performed as shown in FIG. Here, the first pixel and the second pixel are alternately arranged in the main scanning direction. As can be seen from FIG. 13B, the edge existing in FIG. 13A recedes to the lower left, and the position of the character or line segment is displaced, resulting in poor reproducibility.

Further, the image shown in FIG. 13A is shown in FIG.
As shown in FIG. 13B, when the resolution in the sub-scanning direction is reduced by using the method of using the interpolated value of the density of the first pixel and the density of the second pixel, printing is performed as shown in FIG. To be done.
In this case, the density of the character or line portion is reduced, or white lines are generated, and the image quality is deteriorated.

As described above, when the resolution in the sub-scanning direction is lowered in order to improve the reproducibility of dots and lines in the low gradation region and smoothly reproduce halftones, characters and lines are broken. The image quality deteriorates. As a countermeasure for preventing the deterioration of image quality, Japanese Patent Application No. 5-248474 reduces the resolution only in the low density portion. In addition, JP-A-6
In the -62248 publication, a means for enhancing an image edge in the image space is provided.

However, when such a means is used, there is a problem that the image processing becomes complicated and the cost becomes high. Therefore, the means for preventing the deterioration of the image quality cannot be used in a recording device such as a low-priced copying machine or a printer.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and it is possible to print the outline of a character or an image whose gradation greatly changes, with the resolution of the original image data, and to obtain a halftone. An object of the present invention is to provide an image processing method and an image processing apparatus that realizes data conversion that can be printed without impairing the gradation of an area by a simple method at low cost.

[0013]

According to a first aspect of the present invention, in an image processing method for multi-gradation printing, original image data is divided into a set of a plurality of continuous pixels, Different gradation ranges are allocated according to the position of each pixel in the set, and the gradation number of the pixel is included in the gradation range assigned to the pixel for each pixel in the set. It is determined whether the gradation range is smaller or larger than the gradation range. If the gradation range is smaller than the gradation range assigned to the pixel, the converted gradation number is converted to a first predetermined value and allocated to the pixel. When the gradation range is larger than the specified gradation range, the converted gradation number is converted into a second predetermined value,
When included in the gradation range assigned to the pixel, within the set according to the gradation number and within the range of the maximum gradation number smaller than the maximum gradation number that the original image data can take. The gradation number of each pixel is converted to determine the gradation number of the pixel of the print data.

According to a second aspect of the present invention, in an image processing apparatus for generating an image used for multi-gradation printing, a dividing means for dividing the original image data into a set composed of a plurality of continuous pixels, and dividing means. Different gradation ranges are allocated according to the position of each pixel in the set, and for each pixel in the set, the number of gradations of the pixel is included in the gradation range assigned to the pixel, or A determination unit that determines whether the gradation range is smaller or larger than the allocated gradation range, and if the gradation range is smaller than the allocated gradation range for the pixel, converts the converted gradation number into a first predetermined value. If the gradation range assigned to the pixel is larger, the converted number of gradations is converted into a second predetermined value, and if it is included in the gradation range assigned to the pixel, Depending on the number of gradations, It is characterized in that it has a print data generation means for converting the number of gradations of each pixel in the set in the maximum gradation range maximum gradation number of less than the number that can be taken of the data.

[0015]

[0016]

[0017]

[0018]

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, the gradation change of adjacent dots is small in a halftone or a highlight part, and the gradation change is large in an adjacent part. The number is assigned to a predetermined number of gradations. At this time, a set is generated by a plurality of pixels,
The total number of gray levels is assigned to each pixel in the set. In each pixel, it is determined whether the assigned gradation range includes the number of gradations of the pixel, or is smaller or larger than the allocated gradation range. If the gradation range is smaller than the assigned gradation range, the converted gradation number is the first predetermined value, and if it is larger than the allocated gradation range, the converted gradation number is the second prescribed value. To decide. When the gradation number of the pixel exists in the allocated gradation range, the gradation number is converted into the gradation number corresponding to the gradation number.

FIG. 1 is a flow chart for explaining an embodiment of the image processing method of the present invention. Now, a set of k pixels is set as a first pixel, a second pixel, ...
., The kth pixel. Further, the total number of gradations is divided into k gradation ranges. The thresholds for division are a1, a2, ...
Let a (k-1). Here, a1 <a2 <... <a
(K-1). The first pixel is assigned a gradation range of a1 or less, the second pixel is allocated a gradation range of a1 to a2, and similarly, the i-th pixel is assigned a (i-1)-
The gradation range of ai is assigned, and a (k-
1) Assign a larger gradation range. Then, when the gradation number x of each pixel is included in the allocated gradation range, the conversion functions of the gradation number are f1 (x) and f2, respectively.
(X), ..., Fk (x). Further, when the gradation number x of the i-th pixel is smaller than the allocated gradation range, it is converted into the first predetermined gradation number ni, and when it is larger than the allocated gradation range, the second gradation value ni is converted. Is converted into a predetermined gradation number mi. Note that ni <mi.

As for the first pixel, as shown in FIG. 1A, when the gradation number x of the first pixel is a1 or less,
The converted gradation number x1 = f1 (x), and the gradation number x is a1.
If it is larger than the above, the number of gradations after conversion x1 = m1. Regarding the second pixel, as shown in FIG.
When the gradation number x of the second pixel is larger than a2, the converted gradation number x2 = m2, and when the gradation number x is larger than a1 and a2 or less, the converted gradation number x2 = f2
(X) and when the gradation number x is a1 or less, the converted gradation number x2 = n2. The third pixel to the (k−1) th pixel are the same as the second pixel. For the kth pixel, as shown in FIG. 1C, the number of gradations x of the kth pixel x
Is less than or equal to a (k-1), the gradation number after conversion xk
= Nk, and when the gradation number x is larger than a (k-1), the converted gradation number xk = fk (x).

As a concrete example, consider the case of k = 2. In this case, the first pixel follows the flowchart shown in FIG. 1A, and the second pixel follows the flowchart shown in FIG. Each pixel has a gradation of 0 to m. Here, m1 is the maximum gradation number m,
n2 is 0. Further, f1 (x) is to convert the gradation from 0 to a1 into the gradation from 0 to the maximum gradation number m, and f2 (x) is the gradation from a1 to the maximum gradation number m. 0 to
It is assumed that the gradation is converted up to the maximum gradation number m.

FIG. 2 shows k = by the image processing method of the present invention.
6 is an explanatory diagram of an example of print pixels in the case of 2. FIG. First, let us consider a portion where the gradation change in the halftone region is small. For example, when the gray scale number x of the first and second pixels is a1 or less, the gray scale number x1 after conversion is f1 (x because the first pixel is within the assigned gray scale range. ).
Since the second pixel is smaller than the assigned gradation range, the converted gradation number x2 is 0. for that reason,
When printing is performed with the converted number of gradations, as shown in FIG. 2A, the number of gradations f1 is displayed in the portion corresponding to the first pixel.
The printing of (x) is performed, and the portion corresponding to the second pixel is converted into the gradation number 0, so that the printing is not performed.

For example, the gradation number x of the first and second pixels
Is larger than a1, the assigned gradation range for the first pixel is exceeded, so the converted gradation number x1 is converted to the maximum gradation number m. Since the second pixel is within the assigned gradation range, the converted gradation number x2 is f2 (x). Therefore, when printing is performed with the converted gradation number, as shown in FIG.
The portion corresponding to the first pixel is printed in all black, and the portion corresponding to the second pixel is printed with the gradation number f2 (x).

On the other hand, consider a case where the gradation change is large and, for example, an edge exists between the first pixel and the second pixel. Now, the gradation number of the first pixel is a1 or less, and the second pixel
If the number of gradations of the pixel is larger than a1, for example, the maximum number of gradations is m, the number of gradations after conversion of the first pixel x1
Becomes f1 (x), and the number of gradations after conversion of the second pixel is x2
Is f2 (x) = m. Therefore, as shown in FIG. 2C, the number of gradations f1 (x) is printed for the first pixel, all black is printed for the second pixel, and the edge is preserved. It is possible to obtain a printed image.

On the contrary, when there is an edge such that the gradation number of the first pixel is larger than a1 and the gradation number of the second pixel is a1 or less, after the conversion of the first pixel Number of gradations x
1 is the maximum gradation number m, and the number of gradations after the conversion of the second pixel is x2
Is 0, so printing is performed as shown in FIG. Also in this case, the edge is preserved.

Specifically, the number of gradations that each pixel can take is 0.
.About.255, and a1 = 127. At this time, if the converted gradation number is 0 to 127, f1
In (x), the gradation number of 0 to 127 is directly used as the gradation number of 0 to 127. Further, f2 (x) converts the number of gradations of 128 to 255 into the number of gradations of 0 to 127. That is, f
Let 2 (x) = x−128. As a result, in a portion where the number of gradations does not change so much, each pixel expresses 128 gradations, so that the number of gradations 0 to 255 can be expressed by two pixels. Further, in the portion where the change in gradation is large, the number of gradations is converted according to the number of gradations of each pixel.

In FIG. 2, one pixel is represented by a rectangle, but the pixel on the left side original image is one rectangle with 25 pixels.
6 gradations are shown, and the pixels of the print image after conversion on the right side are 128 gradations in one rectangle. Therefore, even if the number of gradations is the same before and after conversion, the area of the black part that is expressed is
After conversion is twice as much as before conversion.

Of course, each pixel is not limited to 128 gradations, and for example, it is possible to control so that each pixel performs recording with an apparatus capable of recording 256 gradations. The recording device that actually performs recording is only required to be capable of expressing gradations of 128 gradations or more, and the function fi (x) for conversion may be set according to the number of gradations that can be expressed.
Conversely, for example, even if the original image data has 256 gradations, the number of gradations after conversion can be printed by a recording device that can express only gradations smaller than 256 gradations.

FIG. 3 is an explanatory diagram showing an example of the arrangement of the first and second pixels when k = 2 in the embodiment of the image processing method of the present invention. In the figure, ∘ indicates the first pixel, and · indicates the second pixel. Then, the number of gradations after gradation conversion is expressed by the first pixel and the second pixel which are adjacent to each other in the sub-scanning direction. Actually the first as above
When processing the second pixel and the second pixel, in order to prevent the occurrence of moire, as shown in FIG. 3, the first pixel and the second pixel are arranged so as not to be adjacent to each other.
And a second pixel is arranged. With such an arrangement, for example, in the low gradation area, the pixels to be printed are arranged in a staggered pattern, so that the pixels are printed like a halftone dot image and the halftone can be expressed well.

FIGS. 4 and 5 are flowcharts showing an example of processing when k = 2 in the embodiment of the image processing method of the present invention. In the conversion process for each pixel as shown in FIG. 1, if it is known which pixel among the first to kth pixels each pixel corresponds to, the conversion process for that pixel is performed regardless of the values of other pixels. It is possible. For example, in the example shown in FIG. 3, if the number of the pixel in each line is known, it is determined whether the pixel is the first pixel or the second pixel,
Processing suitable for each pixel can be performed. In particular, in the case of the pattern shown in FIG. 3, it is possible to determine whether it is the first pixel or the second pixel depending on whether each line is an odd number or an even number and whether the pixel position is an odd number or an even number. Can be converted. In the example shown in FIGS. 4 and 5, pixels are sequentially accessed in the main scanning direction, and when one line is completed, the next line is sequentially accessed in the main scanning direction to perform conversion processing.

First, in step S11, an image file to be printed by performing conversion processing is opened. In S12, it is determined whether the line is an odd line or an even line, and further, S1
3. In S20, it is determined whether the pixel position is an odd number or an even number. In the case of an odd number of odd lines, the pixel is determined to be the first pixel, and the processing of FIG. 1A is performed in S14 to S16. Here, in S14, it is determined whether the gradation number x is 127 or less or 128 or more. If 128 or more, the converted gradation number x'is 127, and if 127 or less, the gradation number x'is 127. The number x is directly used as the converted gray scale number x ′.

In the case of an even number of odd lines, the pixel is determined to be the second pixel, and the processing of FIG. 1C is performed in S17 to S19. Here, it is determined in S17 whether the gradation number x is 127 or less or 128 or more, and if it is 127 or less, S18.
The gradation number x ′ after conversion is set to 0, and when it is 128 or more, 128 is subtracted from the gradation number in S19 to obtain the gradation number x ′ after conversion.
And

In the even-numbered line, the odd-numbered pixel becomes the second pixel, and therefore, in S21 to S23, the same processing as in S17-S19 is performed. In addition, since the even-numbered pixel is the first pixel, S2
The same processing as in S14 to S16 is performed in 4 to S26.

In S27, it is determined whether or not the processing of one line of pixels in the main scanning direction is completed. If there is an unprocessed pixel, that pixel is selected in S28 and the process returns to S12. In S29, it is determined whether or not the processing of each line in the sub-scanning direction has been completed. If there is an unprocessed line, that line is selected in S30 and the process returns to S12. Further, in S31, it is determined whether or not images of all colors have been processed. If there is an unprocessed color, the image of that color is selected in S32 and the process returns to S12. When the processing of all the pixels is completed, the image file is closed in S33, and the image processing is completed.

As described above, according to the image processing method of the present invention,
A multi-valued image for printing can be obtained only by a simple conversion process for each pixel. Such processing can be realized not only by software but also by hardware.
It can be realized by a simple circuit.

The image to be processed is, for example, 5
91 × 591 pixels × 3 colors (Y, M, C) with 256 pixels for each pixel
It can be gradation (8 bits). Of course, the size and the number of colors of the image are arbitrary. It should be noted that the number of gradations after conversion of each pixel is 128 gradations, and 256 gradations are obtained by 2 pixels.
It expresses gradation.

FIG. 6 shows k = by the image processing method of the present invention.
It is explanatory drawing of an example of the image before and behind conversion in the case of 2. Figure 6
13A shows an image in which the original image before conversion is printed as it is, and is the same image as FIG. 13A. That is, in the image shown in FIG. 6A, high density edges such as characters and line segments exist in the low density background. Figure 6
(B) shows an image printed using the number of gradations after conversion by the image processing method of the present invention as described above.

As can be seen from FIG. 6B, the number of gradations of the low-density background portion is expressed by two pixels adjacent in the sub-scanning direction, and the high-density edge portion is also expressed. Is saved as the original image.

Such a printed image is printed by, for example, a printing device for dividing printing in the sub-scanning direction with a parallel line using a thermal head in which the sub-scanning width of the strip-shaped resistor is narrower than the main scanning width. Obtainable. For example, when printing is performed using a 300 dpi thermal head having a band-shaped resistor with a sub-scanning width of 50 μm, as shown in FIG. 6B, the halftone in which the number of tones of 2 dots in the sub-scanning direction hardly changes. In the highlight area, printing can be performed with 256 gradations in the sub-scanning direction of 150 dpi, and if there is an edge such as a character in the background of the highlight, the character part is always black, and the edge with a large change in character or gradation Copies can be printed at the original data resolution of 300 dpi. It should be noted that with this method, even with white letters,
It can be printed at 00 dpi.

Next, consider the case of k = 3. In this case, the first pixel follows the flowchart shown in FIG. 1A, the second pixel follows the flowchart shown in FIG. 1B, and the third pixel shows the flowchart shown in FIG.
Follow the flowchart shown in (C). When k = 3, the gradation width is divided by two threshold values a1 and a2, and 0
To gradation a1 to the first pixel, gradations a1 to a2 to the second pixel, gradations a2 to maximum gradation m to the third.
To the pixel of. In the first to third pixels, if the gradation number is within the allocated gradation width, f
Gradation conversion by 1 (x), f2 (x), f3 (x),
If it is smaller than the gradation width, it is converted to 0, and if it is larger than the gradation width, it is converted to the maximum gradation m.

FIG. 7 shows k = by the image processing method of the present invention.
6 is an explanatory diagram of an example of print pixels in the case of 3. FIG. First, let us consider a portion where the gradation change in the halftone region is small. For example, when the gradation number x of the first to third pixels is a1 or less, the converted gradation number x1 is f1 (x) because the first pixel is within the allocated gradation range. ).
Since the second and third pixels are smaller than the assigned gradation range, the converted gradation numbers x2 and x3 are zero. Therefore, when printing is performed with the converted gradation number, as shown in FIG. 7A, the gradation number f1 (x) is printed in the portion corresponding to the first pixel, and the second and The portion corresponding to the third pixel is converted to the gradation number 0, so that printing is not performed.

When the gradation number x is in the range of a1 to a2,
Since the assigned gradation range is exceeded for the first pixel, the converted gradation number x1 is converted to the maximum gradation number m. Since the second pixel is within the assigned gradation range, the converted gradation number x2 is f2 (x). Further, for the third pixel, since the assigned gradation is smaller than that, the converted gradation number x3 is zero. Therefore, when printing is performed with the converted number of gradations, as shown in FIG. 7B, the portion corresponding to the first pixel is printed in all black, and the portion corresponding to the second pixel is printed. The number of gradations f2 (x) is printed. The portion corresponding to the third pixel is not printed.

When the gradation number x is larger than a2, the first
Further, since the assigned gradation range is exceeded for the second pixel, the converted gradation numbers x1 and x2 are converted to the maximum gradation number m. Since the third pixel is within the assigned gradation range, the converted gradation number x3 is f3.
(X). Therefore, when printing is performed with the converted number of gradations, as shown in FIG. 7C, the portions corresponding to the first and second pixels are printed in all black, and corresponding to the third pixel. In the portion to be printed, the gradation number f3 (x) is printed.

On the other hand, there is a large change in gradation, and for example, a line segment such as a character having a gradation number of a2 or more is applied to the background having a gradation number of a1 or less, and a line segment between the second pixel and the third pixel. Consider the case where an edge exists. Now, the number of gradations of the first and second pixels is a1 or less, and the number of gradations of the third pixel is a1.
If it is larger than 2, for example, the maximum gradation number m, the converted gradation number x1 of the first pixel is f1 (x), and the converted gradation number x2 of the second pixel is It becomes 0. Further, since the third pixel falls within the assigned gradation range, the converted gradation number x3 is f3 (x) = m.
Therefore, as shown in FIG. 7D, the number of gradations f1 (x) is printed for the first pixel, nothing is printed for the second pixel, and all of the third pixel is printed. It is possible to obtain a printed image in which edges are preserved by performing black printing. Also, when a line segment such as a character overlaps the first pixel or the second pixel, an image can be obtained by printing with the edge preserved by the same process.

Specifically, the number of gradations that each pixel can take is 0.
˜255, and a1 = 85 and a2 = 170. At this time, if the number of gradations after conversion is set to 0 to 84, f1 (x) is 0 as it is,
In the case of ~ 85, x-1 is set. Also, f2 (x) is
F2 (x) = x−86 is set so that the gradation number of 86 to 170 is converted to the gradation number of 0 to 84. Furthermore, f3
(X) may be set to f3 (x) = x-171 so that the gradation number of 171 to 255 is converted into the gradation number of 0 to 84. As a result, in the part where the number of gradations does not change much,
Since each pixel expresses 85 gradations, the number of gradations from 1 to 255 can be expressed by 3 pixels.
Further, in the portion where the change in gradation is large, the number of gradations is converted according to the number of gradations of each pixel.

When k = 3, 256 gradations cannot be equally divided into 3 pixels. Therefore, each pixel is set to 85 gradations, and the gradation numbers 0 and 1 in the first pixel are converted into the gradation number 0 after conversion. Have been assigned. However, the present invention is not limited to this, and various conversions are possible, for example, converting the number of gradations 254 and 255 in the third pixel to 84 after conversion. Further, the number of gradations after conversion for each pixel may be set to 86 gradations, or may be converted to 128 gradations or 256 gradations, and the conversion may be performed according to the number of gradations that can be expressed by the recording device. Of course, it is not necessary to assign the same gradation range to each pixel, and the thresholds a1 and a2 may be appropriately set to determine the function fi (x) for conversion.

FIG. 8 is an explanatory diagram showing an example of the arrangement of the first to third pixels when k = 3 in the embodiment of the image processing method of the present invention. In this example, each row of pixels arranged in the main scanning direction is a row of first, second, and third pixels, and three pixels adjacent in the sub scanning direction are one group. In this case, for example, each pixel on the first row is processed as the first pixel, each pixel on the next second row is processed as the second pixel, and each pixel on the third row is processed. May be processed as the third pixel. Therefore, it is not necessary to switch the conversion process depending on the position of the pixel in each row, and the process can be simplified. Of course, as in the case of k = 2 shown in FIG. 3, the first pixels, the second pixels, and the third pixels may not be adjacent to each other.

FIG. 9 shows k = by the image processing method of the present invention.
6 is an explanatory diagram of an example of images before and after conversion in the case of 3. FIG. Figure 9
Now, the original image shown in FIG. 6A is grouped as shown in FIG. 8, and an image printed using the converted gradation number when the conversion process is performed is shown. There is. here,
The low density background of the original image has a gradation number of a1 or less, and the high density portion such as characters and line segments has the maximum gradation number. As can be seen from FIG. 9, the number of gradations of the low-density background portion is expressed by three pixels adjacent in the sub-scanning direction, and the high-density edge portion is stored as the original image. Has been done.

The case where k = 2 and k = 3 has been described above. The present invention is not limited to these, and the conversion processing can be performed in the same manner even when k = 4 or more. At this time, in addition to setting the first to kth pixels by forming a set of k pixels continuous in the sub-scanning direction, for example, k = p × q
In this case, k pixels having various arrangement configurations can be set as a set, such as making a set of pixels arranged in a p × q matrix.

Further, with respect to the k pixels belonging to each set, the gradation range assigned to each pixel is not limited to equally dividing the total number of gradations, and a pixel with a wide gradation range or a pixel with a narrow gradation range may be used. May exist. That is, the threshold ai that divides the gradation range is not limited to the case where all gradations are set to be equally divided. In the converted image, the total number of gradations is represented by k pixels.

Also, fi (x) for converting the number of gradations in the gradation range assigned to each pixel does not need to be a linear function as in the above-mentioned example, and for example, nonlinear conversion may be performed. Can be any function. For example, if the density actually printed does not match the visual density, it is possible to set the function to perform conversion so as to match the density.

FIG. 10 is a block diagram showing an example of a thermal transfer recording apparatus including the image processing apparatus of the present invention. In the figure, 41 is a photo sensor, 42 is an A / D conversion unit, 43 is a color conversion unit, 44 is a UCR / black plate generation unit, 45 is an image data storage unit, 46 is a recording control unit, 47 is an image processing unit, 48
Is a thermal head drive IC, and 49 is a thermal head. The thermal transfer recording apparatus shown in FIG. 10 is an example of a thermal transfer recording apparatus capable of color recording with four colors of cyan, magenta, yellow, and black.

The photo sensor 41 converts an image into an electric signal for each color of red, green and blue and reads it. The A / D converter 42 converts an analog signal of an image read by the photo sensor 41 into a multi-valued digital signal. The color conversion unit 43 converts signals of respective colors of red, green and blue into signals of respective colors of cyan, magenta and yellow. The UCR / black plate generation unit 44 removes the ground color, extracts the black portion from the signals of each color of cyan, magenta, and yellow, and creates image data of four colors of cyan, magenta, yellow, and black. The image data storage unit 45 stores image data of four colors of cyan, magenta, yellow, and black, respectively.

The recording control section 46 includes an image data storage section 45.
The image data is read out from the device, and the image processing unit 47 and the thermal head drive IC 48 are controlled. The image processing unit 47 is a device that realizes the above-described image processing method of the present invention, and performs the above-described gradation conversion processing for each image data of each color. For example, the position of the image data read from the recording control unit 46 is received, and the gradation conversion is performed according to the pixel position as in the flowcharts shown in FIGS. In the conversion processing, as described with reference to FIG. 1, it is determined whether or not it is within the gradation width of the pixel assigned according to the position in the set, and the gradation number is converted according to each case.

The thermal head drive IC 48 sends a drive signal to the thermal head 49 according to the image data converted by the image processing unit 47. Thermal head 4
In No. 9, the heating elements corresponding to the dots driven by the thermal head driving IC 48 generate heat, and as shown in FIG. 11, the recording is actually performed on the transfer paper. The thermal head 49 may be capable of recording only gradations smaller than the gradations that the original image data can take.

The data R1, G1, B1 read from the photo sensor 41 and the like are converted into digital signal data R2, G2, B2 by the A / D converter 42, and the color converter 43.
Data C1 and M corresponding to the ink colors used for recording
1, color converted to Y1. Further, the image data is passed through the UCR / black plate generation unit 44, converted into image data Y2, M2, C2, K2 corresponding to recording colors including black, and stored in the image data storage unit 45. Then, the image data D2 of each color
In the image processing section 47, gradation conversion processing is performed by the above-described image processing method, and converted into converted image data D3. When the recording start enable signal S3 is input from the recording controller 46, the thermal head drive IC 48 receives the recording energy D based on the number of gradations of the converted image data D3.
4 is sent to the thermal head 49, and multi-tone recording is performed by the area gradation method using lines.

In each of the above-mentioned specific examples of the image processing method and the thermal transfer recording apparatus, the case where the gradation printing is performed by performing the division printing in the sub-scanning direction has been shown. This system is effective, for example, in a thermal transfer recording apparatus as shown in FIG. 10, but the present invention is not limited to this. For example, the present invention can be applied to the case where printing is performed at a reduced resolution in all recording devices capable of multi-gradation printing, such as a recording device that expresses gradation by changing the dot diameter.

[0059]

As is apparent from the above description, according to the present invention, the original image data is divided into a group consisting of a plurality of consecutive pixels, and each group is divided according to the position of each pixel in the group. A different gradation range is allocated, and for each pixel in the set, whether the gradation number of the pixel is included in the gradation range assigned to the pixel, or is smaller or larger than the allocated gradation range is determined. If it is smaller than the gradation range assigned to the pixel, the converted number of gradations is converted to the first predetermined value, and if it is larger than the gradation range assigned to the pixel, , The converted number of gradations is converted into a second predetermined value, and when included in the gradation range assigned to the pixel, the maximum number that can be taken by the original image data depends on the number of gradations. Within the range of maximum gradation number less than the gradation number, It converts the gradation number of each pixel in the,
The number of gradations of the pixels of the print data is determined and the total number of gradations is expressed. This reduces the resolution in the highlight part and the halftone area to obtain smooth gradation, and the outline of the character or image can be expressed with the original resolution. It is possible to improve gradation stability such as reproducibility in a low gradation region, save the edge portion, and obtain a high-quality recorded image.

Further, simple gradation processing which does not require a complicated image processing function such as detecting the edges of characters or images and changing the resolution in the sub-scanning direction depending on the number of gradations as in the prior art and which does not cost much is performed. Therefore, it is possible to reduce the cost required for gradation processing and to apply it to low-priced copying machines and printers. Furthermore, it is possible to perform recording with a reduced resolution even for a recording device that can print only a smaller number of gradations than the number of gradations that the original image data can take, and a high-quality image with a high number of gradations can be obtained. There are various effects such that printing can be performed with a low-priced recording device having a low gradation number.

[Brief description of drawings]

FIG. 1 is a flowchart illustrating an embodiment of an image processing method of the present invention.

FIG. 2 is an explanatory diagram showing an example of print pixels when k = 2 according to the image processing method of the present invention.

FIG. 3 is an explanatory diagram showing an example of an arrangement of first and second pixels when k = 2 in the embodiment of the image processing method of the present invention.

FIG. 4 is a flowchart showing an example of processing when k = 2 in the embodiment of the image processing method of the present invention.

FIG. 5 is a flowchart (continuation) showing an example of processing when k = 2 in the embodiment of the image processing method of the present invention.

FIG. 6 is an explanatory diagram of an example of images before and after conversion when k = 2 according to the image processing method of the present invention.

FIG. 7 is an explanatory diagram showing an example of print pixels when k = 3 according to the image processing method of the present invention.

FIG. 8 is an explanatory diagram showing an example of an arrangement of first to third pixels when k = 3 in the embodiment of the image processing method of the present invention.

FIG. 9 is an explanatory diagram of an example of images before and after conversion when k = 3 according to the image processing method of the present invention.

FIG. 10 is a block diagram showing an example of a thermal transfer recording apparatus including the image processing apparatus of the present invention.

FIG. 11 is an explanatory diagram of an example of a gradation printing method by a thermal transfer method.

FIG. 12 is an explanatory diagram of an example of a printing method when the resolution in the sub-scanning direction is lowered in gradation printing by the conventional thermal transfer method.

FIG. 13 is an explanatory diagram of an example of a print result when the resolution in the sub-scanning direction is reduced when a high-density edge exists in a low-density portion in gradation printing by the conventional thermal transfer method.

[Explanation of symbols]

41 ... Photo sensor, 42 ... A / D conversion section, 43 ... Color conversion section, 44 ... UCR / black plate generation section, 45 ... Image data storage section, 46 ... Recording control section, 47 ... Image processing section, 48 ... Thermal Head drive IC, 49 ... Thermal head.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-6-62248 (JP, A) JP-A-5-284341 (JP, A) JP-A-6-178088 (JP, A) JP-A-7- 87317 (JP, A) JP 9-121283 (JP, A) JP 8-235353 (JP, A) JP 7-159904 (JP, A) JP 55-95146 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) B41J 2/36 B41J 2/52 H04N 1/405

Claims (2)

(57) [Claims] 1. An image processing method for multi-gradation printing, wherein original image data is divided into a group consisting of a plurality of continuous pixels, and different floors are set according to the position of each pixel in the group. Allocating a tonal range, for each pixel in the set,
It is smaller than the gradation range assigned to the pixel by determining whether the number of gradations of the pixel is included in the gradation range assigned to the pixel, or is smaller or larger than the allocated gradation range. If the number of gradations after conversion is the first
Is converted to a predetermined value and is larger than the gradation range assigned to the pixel, the converted number of gradations is converted to a second predetermined value and included in the gradation range assigned to the pixel. In the case where the number of gradations is changed, the number of gradations of each pixel in the set is converted in accordance with the number of gradations and within the range of the maximum number of gradations smaller than the maximum number of gradations that the original image data can take. An image processing method characterized by determining the number of gradations of pixels of print data. 2. In an image processing apparatus for generating an image used for multi-gradation printing, dividing means for dividing the original image data into a group composed of a plurality of consecutive pixels, and each pixel in the divided group. A different gradation range is assigned according to the position of, and for each pixel in the set, the number of gradations of that pixel is included in the gradation range assigned to that pixel, or Is smaller or larger, and if the gradation range is smaller than the gradation range assigned to the pixel, the converted number of gradations is converted to a first predetermined value and assigned to the pixel. If it is larger than the gradation range, the converted gradation number is converted into a second predetermined value, and if it is included in the gradation range assigned to the pixel, according to the gradation number, , The maximum number of gradations that the original image data can take An image processing apparatus comprising print data generating means for converting the gradation number of each pixel in the set within a range of a maximum gradation number smaller than that.