JP2002252764A - Image processor and image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processor that can reproduce a color image with high image quality through the combination of the multi-valued error diffusion system and the area gradation conversion system so as to enhance the reproducibility of color thin lines. SOLUTION: The image processor combines the gradation conversion by the multi-valued error diffusion system with the gradation conversion by the area gradation conversion system and excludes intense black and white pixels or a minimum value and a maximum value in consecutive gradation from an object of averaging in the case of averaging processing for the resolution conversion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image processing apparatus and an image processing method for reproducing a color image by using a multi-level error diffusion method.

[0002]

2. Description of the Related Art Conventionally, a multilevel error diffusion method has been known as a processing method suitable for achieving both gradation and resolution. FIG. 10 shows an image processing circuit for performing a multi-level error diffusion process. The image processing circuit shown in the figure inputs an image signal from an input terminal 91 to an adder 92 and adds an integration error. The image signal to which the integration error has been added is input to the comparator 93 and compared with the slice level, whereby the image signal is encoded into a multilevel signal and output from the output terminal 95. On the other hand, the output of the comparator 93 is input to a subtractor 96 to obtain a quantization error.
It is stored in the error memory 97. The error filter 98 replaces the error (E (p)) of the pixel of interest output from the subtractor 96 with the error (E (a) 〜) of the already multivalued neighboring pixels a〜d stored in the error memory 97. E (d)) are weighted and added. The integrated error calculated in this way is output to the adder 92. As described above, the image quality can be improved by diffusing the quantization error when the image signal is multi-valued to the surrounding pixels.

[0003]

However, when a color image is reproduced using the above-described multi-valued error diffusion method, C
When four colors of MYK or three colors of RGB are superimposed, there is a possibility that the colors may be overlapped or separated too much, and in such a case, the quality of the reproduced color image may be degraded. there were.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and can reproduce a high-quality color image by combining a multi-value error diffusion method and an area gradation conversion method. An object of the present invention is to provide an image processing apparatus and an image processing method having excellent reproducibility.

[0005]

SUMMARY OF THE INVENTION The present invention combines tone conversion by multi-valued error diffusion and tone conversion by area tone conversion so that even if the image is a color image, the colors are too overlapped or too far apart. Image quality degradation due to the black and white pixels that are forcibly processed during the averaging process, or the minimum value and maximum value pixels in the continuous tone,
By excluding the image from averaging, the original image density is secured, and a color image with little deterioration in image quality due to color omission of a thin color line image is reproduced.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first aspect of the present invention is to divide a continuous tone input pixel into N.times.N pixel blocks and determine whether a white pixel or a black pixel exists in the pixel block. Determining means, averaging means for calculating an average value of pixels other than white pixels or black pixels from the pixel block, and resolution conversion of the input pixel to 1 / N in pixel block units, and the average value of the pixel block As a pixel value of a pixel of interest, and a multi-level converter that converts the pixel value output from the resolution converter to a multi-level level and diffuses a quantization error during the gradation conversion to peripheral pixels. Error diffusion means, area gradation conversion means for replacing the gradation-converted target pixel with an N × N pixel area gradation pattern corresponding to the multi-value level, and resolution among pixels replaced with the area gradation pattern Before conversion And a black and white compulsory processing unit for forcibly converting a pixel position where the input pixel is a white pixel or a black pixel into a white or black pixel.

According to this image processing apparatus, by combining the gradation conversion based on the multi-valued error diffusion and the gradation conversion based on the area gradation conversion, the colors of the color image may be overlapped or separated too much. As a result, it is possible to suppress deterioration of the image quality.

Further, by excluding black or white pixels which are forcibly processed at the time of averaging of pixel blocks from the object of averaging, the original image density can be secured at the time of black and white forced processing, and the color of the color thin line image can be secured. A color image with little deterioration in image quality due to omission can be reproduced.

Further, since the resolution conversion (1 / N) and the area gradation conversion are performed, the resolution which increases with the area gradation conversion can be canceled by the prior resolution conversion, and the output having the same resolution as the input image can be canceled. Images can be obtained.

According to a second aspect of the present invention, a continuous tone input pixel is divided into N × N pixel blocks, and a pixel having a minimum value or a maximum value in the continuous tone exists in the pixel block. Determining means for determining whether or not the pixel block has an average value of pixels other than a pixel having a maximum value or a minimum value from the pixel block; and Resolution conversion means for outputting the average value of the block as the pixel value of the pixel of interest; gradation conversion of the pixel value output from the resolution conversion means to a multivalued level; Error diffusion means for diffusing the pixel of interest into gradation, area gradation conversion means for replacing the gradation-converted target pixel with an area gradation pattern of N × N pixels corresponding to the multi-value level, and area gradation pattern And a forcible black-and-white processing unit for forcibly converting a pixel position where the input pixel before resolution conversion was the maximum value or minimum value pixel into white or a processing color.

According to this image processing apparatus, by combining the gradation conversion by multi-valued error diffusion and the gradation conversion by area gradation conversion, the colors of the color image may overlap or separate too much. As a result, it is possible to suppress deterioration of the image quality.

In addition, by excluding the minimum value (process color) or the maximum value (white pixel) which is forcibly processed at the time of pixel block averaging from the averaging target, the original image density is secured at the time of black and white forced processing. This makes it possible to reproduce a color image in which the image quality is hardly deteriorated due to color omission of a color thin line image.

Further, since the resolution conversion (1 / N) and the area gradation conversion are performed, the resolution which increases with the area gradation conversion can be canceled by the resolution conversion in advance, and the output having the same resolution as the input image can be canceled. Images can be obtained.

According to a third aspect of the present invention, in the image processing apparatus according to the first or second aspect, the area gradation conversion means comprises four types of area gradation of N × N pixels for each multi-valued level. A pixel having a pattern, the gradation-converted pixel is divided into a pixel block of N × N pixels, and is replaced with one of four types of area gradation patterns according to the pixel position in the pixel block of the target pixel to be subjected to the area gradation conversion.

According to this image processing apparatus, since the area gradation pattern selected from the four types is selected according to the pixel position of the pixel of interest in the pixel block, the gradation of a photographic image or the like (halftone) is obtained. Reproduces color images with excellent properties.

According to a fourth aspect of the present invention, in the image processing apparatus of the third aspect, the four types of area gradation patterns prepared for each multi-valued level are the area gradation patterns at a certain position on the pixel block. And a pattern rotated by 90 degrees with respect to.

According to this image processing apparatus, the area gradation pattern is set to 90% with respect to the area gradation pattern at a certain position.
Since it is constituted by a pattern rotated every time, it is possible to arrange the print dots in a concentrated manner, and it is possible to prevent the gradation property from being deteriorated by the print dot shape.

According to a fifth aspect of the present invention, in the image processing apparatus according to any one of the first to fourth aspects, the area gradation conversion means comprises:
Four types of area gradation patterns defined for each multi-level level are provided for each processing color, and the area gradation pattern for each multi-level level is based on the area gradation pattern for a processing color at each multi-level level. It is composed of a pattern shifted in the main scanning direction, the sub-scanning direction or the main-sub-scanning direction.

According to this image processing apparatus, the area gradation pattern is a pattern shifted in the main scanning direction, the sub-scanning direction, or the main / sub-scanning direction based on the area gradation pattern of a processing color at each multi-valued level. , It is possible to prevent the image quality from being degraded due to the processing colors being too far apart or overlapping.

According to a sixth aspect of the present invention, in the image processing apparatus of the first to fifth aspects, additional pattern generation means for outputting additional data at a pixel position corresponding to a target pixel according to a predetermined additional pattern. And level correction means for level-correcting the additional data in accordance with a function that maximizes the additional data at a gray level at which a pseudo contour occurs.

According to this image processing apparatus, the level of the additional data is corrected according to the function that maximizes the additional data at the gray level at which the pseudo contour occurs. Easy pseudo contours can be suppressed.

According to a seventh aspect of the present invention, a continuous tone input pixel is divided into N × N pixel blocks, and it is determined whether or not a white pixel or a black pixel exists in the pixel block. The average value of pixels other than white pixels or black pixels is obtained from the pixel block, the resolution of the input pixel is converted to 1 / N for each pixel block, and the average value of the pixel block is output as the pixel value of the pixel of interest, The output pixel value is subjected to gradation conversion to a multi-level level, and at the same time, a quantization error at the time of gradation conversion is diffused to peripheral pixels. And the pixel position where the input pixel before resolution conversion was a white pixel or a black pixel among the pixels replaced with the area grayscale pattern is forcibly converted to a white or black pixel. Image processing method .

According to an eighth aspect of the present invention, a continuous tone input pixel is divided into N × N pixel blocks, and a pixel having a minimum value or a maximum value in the continuous tone exists in the pixel block. It is determined whether or not the average value of the pixels other than the pixel of the maximum value or the minimum value is determined from the pixel block, the resolution of the input pixel is converted to 1 / N in pixel block units, and the average value of the pixel block is noted. The output pixel value is output as a pixel value, the output pixel value is subjected to gradation conversion into a multi-valued level, and a quantization error at the time of gradation conversion is diffused to peripheral pixels. N × N according to level
The pixel position is replaced with the area gradation pattern of the pixel, and the pixel position where the input pixel before the resolution conversion is the pixel of the maximum value or the minimum value among the pixels replaced with the area gradation pattern is forcibly converted to white or the processing color. An image processing method characterized in that:

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram illustrating an overall configuration of an image processing apparatus according to the present embodiment. FIG. 3 shows a block configuration in the case of performing four-color-plane sequential processing.
In this image processing apparatus, even lines of a multi-valued input image (for example, 256 values) are directly input to the black and white pixel determination unit 10,
The odd-numbered lines are input to the monochrome pixel determination unit 10 via the line memory 2 having a length of one line. The black-and-white pixel determining unit 10 determines whether the pixel is a white pixel and a black pixel or a pixel other than the black and white pixels in the main / sub 2 × 2 pixel range, and outputs pixel data determined to be other than a black-and-white pixel to the pixel averaging processing unit 1. The pixel averaging processing unit 1 averages multi-valued image data other than black and white pixels input in the main / sub 2 × 2 pixel range from the black and white pixel determination unit 10 by the number of input pixel data, and outputs the averaged result as the main value. It applies to all the sub 2 × 2 pixels and outputs to the range resolution converter 3. However, when there is no input data in the pixel averaging processing unit 1,
That is, when the black and white pixel determination unit 10 determines that all of the main and sub 2 × 2 are white or black, the pixel averaging processing unit 1 sets the output data to the range resolution conversion unit 3 to 0. The range resolution conversion unit 3 outputs the average value of the 2 × 2 pixel range averaged by the pixel averaging processing unit 1 as a pixel value for one pixel, so that the resolution of the input image data in the main and sub directions is 1 / 2.

The resolution-converted image data output from the range resolution converter 3 is added to the integration error by the first adder 4, and the additional data is added by the second adder 5, and then input to the comparator 6. You. The comparator 6 calculates the four slice levels provided from the threshold value generating unit 7 and the multi-valued image data (256
), And multi-valued image data output from the second adder 5 is quinary-valued.

The area gradation converter 8 receives the quinary data of the current line every other line from the comparator 6 and the quinary data of the previous line from the line memory 9. The area gradation conversion unit 8 converts the input image into the same resolution as that of the input image by converting the area gradation pattern into 2 × 2 pixels based on the level and position of the pixel of interest (quinary data). The area gradation pattern storage unit 11 is a part for storing the area gradation pattern selected by the area gradation conversion unit 8. The area gradation pattern will be described later.

The black-and-white compulsory processing section 12 is a section for applying a black-and-white compulsory process for preventing the resolution of a mixed character image or a thin line image from lowering in resolution-converted image information. When the input pixel (multi-value) at the pixel position of the resolution-converted image information is the minimum, it is corrected to 0 (white), and when it is the maximum, it is corrected to 1 (processing color). In the case of a black-and-white image, correction is made to 0 (white) when the input pixel (multi-valued) is pure white, and to 1 (black) when the input pixel (multi-value) is pure black. Therefore, the data densities of the pixels having the minimum and maximum input pixels (multi-valued) are stored, and the pixel averaging unit 1 averages only the data other than the minimum and maximum pixels to perform the error diffusion area gradation processing. Output data, the density of the input data D0 can be reflected in the output data D10,
In addition, the resolution of mixed character images and fine line images decreases,
Output without color characters and fine lines missing.

On the other hand, the multivalued image information (256 values) output from the first adder 4 and the quinary data output from the comparator 6 are input to the subtractor 13. The subtracter 13 calculates a quantization error (E2) when the target pixel is quinary-coded and outputs the calculated error to the error distribution integrated operation unit 14. The error distribution integration operation unit 14 multiplies the quantization error (E2) of the target pixel by a coefficient and distributes the result to peripheral pixels, and accumulates the current distribution error in the integration error of the peripheral pixels stored in the error buffer 15. Return to the error buffer 15. The coefficients used when distributing the quantization error to the peripheral pixels are randomized by random numbers generated by the random number generator 16. By randomizing the distribution coefficient, it is possible to suppress the occurrence of a texture pattern and prevent a decrease in gradation characteristics and resolution. The integration error (E1) of the target pixel is read from the error buffer 15 and output to the first adder 4.

The image processing apparatus further includes an additional pattern generator 17 and a level corrector 18 for removing a false contour.
And The additional data generated according to the additional pattern is corrected by the level corrector 18 in accordance with the gradation level of the pixel of interest, and the corrected data is output to the second adder 5 and added to the multi-valued image information.

Next, the processing contents of the image processing apparatus configured as described above will be described in detail. FIG. 2 is a flowchart showing processing from image input to color image reproduction by the image processing apparatus. FIG. 3A shows input data (D0) for one color among image data input to the image processing apparatus in the order of four color planes. Image data (D
0) is a 256-value image signal represented by 8 bits.

Input data is written to the line memory 2 while the input image is in the odd-numbered line period (S1, S2). When the input image is switched to the even-numbered line, the current input data (even-numbered line) is input to the black-and-white pixel determination unit 10, and the previous line (odd-numbered line) image data is input from the line memory 2 (S3).

The black-and-white pixel determination unit 10 extracts pixel values in units of two pixels from the beginning of each of the odd and even lines and determines whether the pixel is a white pixel or a black pixel in the main / sub 2 × 2 pixel range, or any other pixel. Then, the pixel data determined to be other than the monochrome pixel is output to the pixel averaging processing unit 1 (S4). The pixel averaging processing unit 1 averages multi-valued image data other than black and white pixels input in the main / sub 2 × 2 pixel range from the black and white pixel determination unit 10 by the number of input pixel data (S5), and averages The result is applied to all the main and sub 2 × 2 pixels, and is output to the range resolution converter 3. However, when there is no input pixel data in the pixel averaging processing unit 1, that is, when all of the main and sub 2 × 2 pixels are determined to be white or black by the monochrome pixel determination unit 10,
The pixel averaging unit 1 sets the output data to the resolution conversion unit 3 to 0. FIG. 4A shows a state in which two lines including an odd line and an even line are formed into pixel blocks, and FIG. 4B conceptually shows a result of averaging the individual pixel blocks.

The range resolution converter 3 converts the multivalued data (D1) having been subjected to resolution conversion into half using the average result of each pixel block (2 × 2 pixels) output from the pixel averaging processor 1 as a pixel value. ) Is output (S6). As a result, the resolution of the input image in the main scanning direction and the resolution
Has been converted to

Next, multi-value data (D 1) whose resolution in the main scanning direction and the sub-scanning direction has been converted to に is input to the first adder 4. The integrated error (E1) from the determined pixel is read from the error buffer 15, and the input adder error (E1) and the multi-level data (D1) are added by the first adder 4 (S7). On the other hand, the level of the additional data output according to the additional pattern is corrected and added to the added data (D2) by the second adder 5 (S8). The data obtained by adding the additional data (P2) is output to the comparator 6 by the comparator 6.
Binarization by one slice level (S9, 1
0). FIG. 5A is a diagram showing quinary data. In the figure, the 0 level (Lv
0), 1 level (Lv1), 2 levels (Lv2), 3 levels (Lv3), and 4 levels (Lv4). For the last two pixels, odd lines have 2 levels, 3 levels, and even lines have 1 level. There are two levels.

The output of the comparator 6 is input to the area gradation conversion unit 8 via the line memory 9 if it is an odd line, and is directly input to the area gradation conversion unit 8 if it is an even line (S11, 12). .

In the area gradation conversion section 8, the quinary data (D
Each of the odd-numbered lines and the even-numbered lines constituted in 7) is divided into two pixels from the head, and 2 × 2 pixels are treated as one block. One block is composed of four pixels from a first pixel to a fourth pixel. The first on the upper line (odd number) is called the first pixel position, the second on the lower line (even number) is called the third pixel position, and the second on the lower line (even number) is called the fourth pixel position. And

In this embodiment, the quinary data (D7)
The area gradation pattern used to convert the area gradation
It is determined based on the color, level, and pixel position of the quantified data (D7). Since the color of the quinary data (D7) is sequentially processed in four color planes, it can be determined by knowing the current processing number. The level of the quinary data (D7) is
This is the level of the quinary data (D7) that is the pixel of interest. The pixel position of the quinary data (D7) can be determined from information such as whether the line to which the pixel of interest belongs is odd or even and information such as whether the pixel number in the main scanning direction is odd or even.

Here, a specific example of the area gradation pattern stored in the area gradation pattern storage section 11 will be described. FIG. 6 is a diagram showing a specific example of the area gradation pattern.
Four patterns of A, B, C, and D are prepared corresponding to the four colors of CMYK. Pattern sets corresponding to levels 0 to 4 are prepared for patterns A to D, respectively. If the processing color is RGB + K, the colors A, B, C,
A pattern in which four patterns of D correspond to four colors of RGB + K is prepared.

In this embodiment, before the multi-level error diffusion processing, the resolution in the main scanning direction and the sub-scanning direction is reduced to half, but one area gradation pattern is composed of 2 × 2 pixels. Because there is. That is, since the resolution is doubled by the area gradation conversion, the resolution is reduced to 1/2 in advance.
Thereby, the resolutions of the input image and the output image can be made the same.

One pattern set is formed by four area gradation patterns corresponding to the first pixel position, the second pixel position, the third pixel position, and the fourth pixel position, respectively. further,
In the area gradation pattern of 2 × 2 pixels, pixels which are painted black are recording pixels for recording the processing color, and the other pixels are non-recording pixels. One set is composed of four patterns each rotated by 90 degrees with respect to the area gradation pattern at the first pixel position. By using such a pattern, concentrated arrangement of print dots is enabled.

In the level 0 pattern, the first pixel position, the second pixel position, the third pixel position, and the fourth pixel position are all recording pixels. In the level 1 pattern, only one pixel is a recording pixel in the area gradation pattern of 2 × 2 pixels. In the level 2 pattern, two pixels are recording pixels in the area gradation pattern of 2 × 2 pixels. In the level 3 pattern, only one pixel is a non-recording pixel in the area gradation pattern of 2 × 2 pixels. Level 4 pattern is 1st
All of the pixel position, the second pixel position, the third pixel position, and the fourth pixel position are non-printed pixels. Thus, the number of recording pixels decreases as the level increases.

In the patterns A to D of different types, recording pixels are evenly distributed so that recording pixels do not overlap between patterns (between processing colors). In the example shown in FIG. 6, pattern A is Y color, pattern B is C color, and pattern C is M
Color and pattern D are assigned to K color. The dot arrangement is such that the dot pattern is shifted in the main scanning direction, the sub scanning direction, or both the main and sub directions with reference to the Y color pattern A. This can prevent the CMYK four colors from overlapping too much.

The area gradation conversion unit 8 fetches an odd line as a previous line from the line memory 9 (S1).
3) The even line that is the current line is fetched from the comparator 6. The 2 × 2 pixels on the odd and even lines are converted into 4 × 4 pixels by area gradation conversion processing.

For this purpose, an area gradation pattern is selected based on the quinary level of the pixel of interest. Note that which of the patterns A to D is selected is determined in advance by the current processing color. For example, if the current processing color is the Y color, it is selected from the pattern A in FIG. 6 according to the level (S14).

Now, it is assumed that the target pixel is a pixel P1 indicated by oblique lines in FIG. In this case, a pattern set of level 1 in the pattern A shown in FIG. 6 is selected. Furthermore, since the pixel position of the pixel of interest P1 is the first pixel position in 2 × 2 pixels, the upper left dot pattern (L1) in the pattern set of level 1 is selected (S15).
The selected dot pattern (L1) is output as the area gradation conversion result of the target pixel P1 in the Y color. If the target pixel is P2, the dot pattern (L2) is selected and output as the area gradation conversion result.

The multi-valued data (5-valued) D7 shown in FIG.
FIG. 5 (b) shows the result of area gradation conversion of the area gradation with the area gradation pattern shown in FIG. The resolution returns to the same resolution as the input image by the area gradation conversion processing.

Next, the black and white compulsory processing section 12 performs black and white compulsory processing on each pixel after the area gradation conversion processing (S16). The black-and-white forced processing unit 12 captures the pixel value at the time of input of the target pixel, and determines whether the input pixel value (256 values) is the minimum value (0) or the maximum value (255). Then, the pixel value at the time of input of the target pixel (256
Value) is the minimum value (0), the target pixel is set to 0 (processing color).
In the case of the maximum value (255), the target pixel is set to 1 (white).
To be corrected. Accordingly, when the target pixel before the resolution conversion is the minimum value or the maximum value of the processing color, it is forcibly corrected to white or the processing color, so that it is possible to prevent the resolution of the mixed character image or the thin line image from lowering. FIG. 5 shows a result (D10) obtained by forcibly processing the data (D9) shown in FIG.
FIG. 3B shows the output data shown in FIG. 5C and from which the blocking in FIG. 5C is eliminated.

Also, in the present embodiment, the pixel value when the black-and-white pixel determination unit 10 inputs the minimum value (0) and the maximum value (25).
Since the pixel 5) is excluded from the averaging target in the pixel averaging processing unit 1, even if the black and white forced processing is performed by the black and white forced processing unit 12, the original image density can be secured. Therefore, color omission of a color thin line image can be effectively prevented. Particularly, as in the present embodiment, the area gradation conversion of the multi-valued image by the area gradation conversion unit 8 and the black and white forced processing unit 12 are performed.
This is effective in preventing color omission of a color thin line image in image processing combined with black-and-white forced processing.

Here, the principle of preventing color omission of a fine line image by excluding white pixels and black pixels from the averaging target in the pixel averaging processing section 1 will be described.

As shown in FIG. 7, consider the case where 2 × 2 pixels in a region corresponding to a thin line image are taken in by the black and white pixel judging section 10. Now, the 2 × 2 pixels captured by the monochrome pixel determination unit 10 are (first pixel position, second pixel position, third pixel position, fourth pixel position) = (0, 0, 128, 128)
Is assumed to be held.

In this case, if the white and black pixels (or the maximum and minimum values) are excluded from the averaging target as in the present embodiment, the average value in the 2 × 2 pixel range is 128. However, when all pixel values of four pixels are simply averaged, 2
The average value in the × 2 pixel range is 64, and the average value is greatly reduced.

The image whose resolution has been reduced by half in the main and sub directions, respectively, is converted to a multi-valued level (level 0 to level 4) while maintaining the density of the above average value while being subjected to error diffusion processing. . In particular, the pixel converted to level 1 or level 3 in FIG. 6 is converted into one of the area grayscale patterns (four patterns) corresponding to level 1 in FIG. It is converted into one of the corresponding area gradation patterns (four patterns). For example, in the area gradation pattern corresponding to level 3, the pixel density of the pixel subjected to the multi-value conversion is stored in only one of the 2 × 2 pixels. The pixel position where the pixel density of the multi-value converted pixel is stored is determined independently of the pixel position of the white pixel in the original input image (2 × 2 pixels). Therefore, when the pixel in which only the pixel density of the multi-value converted pixel is stored corresponds to the pixel position of the white pixel in the original input image (2 × 2 pixels), the white pixel is forcibly subjected to the black and white forcible treatment. And the density information of the original pixel is not stored at all. This is considered to be one of the causes of color loss of the fine line image.

Therefore, in this embodiment, as shown in FIG. 7, the remaining pixels (the third pixel position and the fourth pixel position) except for the white pixels (the first pixel position and the second pixel position) are forcibly set. (Pixel position). As a result, the average value was from 64 to 12
The multi-value level will also change from level 3 to level 4 after a significant rise to 8. In the area gradation pattern corresponding to level 4, as shown in FIG. 6, since the pixel density is stored in at least two pixels, the two pixels in which the pixel density is stored are simultaneously white pixel positions and the pixel density is completely stored. The probability of not being done is greatly reduced. Therefore, it is possible to greatly reduce the probability of occurrence of color omission of a fine line image due to the fact that the density information of the original pixel is not stored at all.

Next, the level correction of the additional pattern will be described. In the multi-level error diffusion processing, the quantization error (E2)
Pseudo contours are likely to occur at gradation levels where = 0. In the present embodiment, the second adder 5 adds the additional data corrected so that the amplitude becomes maximum at the gradation level at which the pseudo contour occurs.

The additional pattern generator 17 outputs additional data based on the additional pattern. The additional pattern is 8
It is composed of a matrix of × 8 pixels, and additional data is set at predetermined pixel positions. The added value of the additional data is set to plus or minus zero in the entire matrix. The additional data (P1) at the pixel position corresponding to the pixel position of the target pixel is output to the level correction unit 18.

FIG. 8 shows that the level correction section 18 outputs additional data (P
FIG. 9 is a waveform diagram of an additional pattern amplitude function used to correct 1). The additional pattern amplitude function takes a pixel value before multi-level error diffusion, which is an error diffusion input level in the horizontal axis direction, and has a sawtooth waveform having the maximum amplitude at a gradation level at which a pseudo contour occurs.

The level correction unit 18 obtains a correction value by substituting the pixel value of the pixel of interest into the additional pattern amplitude function of FIG. 8 for the additional data (P1) given from the additional pattern generation unit 17. The corrected additional data (P2) is input to the second adder 5. As a result, the largest additional data at the gradation level at which the pseudo contour occurs is added,
It is possible to prevent the generation of a pseudo contour at the gradation level where the quantization error (E2) = 0.

Although the area gradation pattern in FIG. 6 differs depending on the position, a plurality of types of area gradation patterns are prepared for each level like the area gradation pattern shown in FIG. 9 (level 0). And level 4), and the area gradation pattern may be selected according to the level and the processing color.

In the above description, a case where a color image is processed in a frame-sequential manner has been described. However, a monochrome image can be similarly processed. In this case, only one of the area gradation patterns A to D needs to be prepared. Alternatively, any type of pattern may be used when the monochrome mode is selected.

If it is not necessary to make the resolution of the input image coincide with the resolution of the output image, the resolution conversion process performed before the multi-level error diffusion may be omitted. In this case, the resolution of the output image is twice that of the input image. Further, the black-and-white compulsory processing is performed on 2 × 2 pixels after area gradation conversion for one input pixel.

Further, the multi-level error diffusion processing is performed with nine values,
By setting the number of levels of the area gradation pattern to nine, 3
A format for outputting value data may be used.

[0062]

As described above in detail, according to the present invention, it is possible to provide an image processing apparatus and an image processing method capable of reproducing a high-quality color image by combining a multi-level error diffusion method and an area gradation conversion method. .

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram of an image processing apparatus according to an embodiment of the present invention;

FIG. 2 is a flowchart showing processing contents of the image processing apparatus according to the embodiment;

3A is a diagram illustrating a specific example of an input image according to the embodiment; FIG. 3B is a diagram illustrating a specific example of an output image with respect to the input image according to the embodiment;

4A is a conceptual diagram of a pixel block to be averaged, and FIG. 4B is a conceptual diagram of averaging.

5A is a conceptual diagram of image data that has been averaged and converted in resolution, FIG. 5B is a conceptual diagram of image data subjected to area gradation conversion, and FIG. 5C is a conceptual diagram of an area gradation converted image subjected to black and white forced processing.

FIG. 6 is a conceptual diagram of an area gradation pattern used in the area gradation conversion in the embodiment.

FIG. 7 is a diagram showing a pixel block of a thin line image portion;

FIG. 8 is a waveform chart of a function for level correction of additional data in the embodiment.

FIG. 9 is a conceptual diagram of a modification of the area gradation pattern.

FIG. 10 is a configuration diagram of a multi-level error diffusion circuit in a conventional image processing apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Pixel averaging processing part 2, 9 Line memory 3 Resolution conversion part 4 First adder 5 Second adder 6 Comparator 7 Threshold generation part 8 Area gradation conversion part 10 Black-and-white pixel judgment part 11 Area gradation pattern storage part 12 Black and white forced processing unit 13 Subtractor 14 Error distribution integrated operation unit 15 Error buffer 17 Additional pattern generation unit 18 Level correction unit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) G09G 3/36 B41J 3/00 A 5C082 5/00 B 5/02 G09G 5/00 520J (72) Inventor Shinichi Sato 2-3-8 Shimomeguro, Meguro-ku, Tokyo Matsushita Electric Transmission System Co., Ltd. (72) Inventor Fumiko Koshimi 2-3-8 Shimomeguro, Meguro-ku, Tokyo Matsushita Electric Transmission System Co., Ltd. F-term (reference) 2C262 AA24 AA26 AB13 BB03 BB08 BB14 CA09 DA16 GA23 5B057 CA01 CA08 CA12 CA16 CB01 CB07 CB12 CB16 CD05 CE13 5C006 AA13 AA22 AF44 BB11 FA56 5C077 LL19 MP02 MP08 NN11 PP20 PP32 PP33 PP43 PG08 A03 RR08 PP08 BA13 BA34 BA35 BB02 BB15 BD10 CA12 DA53 DA87 DA89 MM10

Claims (8)

[Claims] And a determination unit for determining whether or not a white pixel or a black pixel exists in the pixel block by dividing an input pixel of continuous tone into N × N pixel blocks. Averaging means for calculating an average value of pixels other than pixels or black pixels;
/ N, and converts the average value of the pixel block into a pixel value of a pixel of interest as a pixel value. Multi-level error diffusion means for diffusing the quantization error at the time to peripheral pixels; and area gradation conversion means for replacing the gradation-converted target pixel with an area gradation pattern of N × N pixels corresponding to the multi-value level. Black and white forced processing means for forcibly converting a pixel position where the input pixel before resolution conversion was a white pixel or a black pixel among the pixels replaced by the area gradation pattern into a white or black pixel. Processing equipment. 2. A determination means for dividing a continuous tone input pixel into N × N pixel blocks and determining whether or not a pixel having a minimum value or a maximum value in the continuous tone exists in the pixel block. And averaging means for calculating an average value of pixels other than a pixel having a maximum value or a minimum value from the pixel block, and converting the resolution of the input pixel to 1 / N for each pixel block and paying attention to the average value of the pixel block. Resolution conversion means for outputting a pixel value of a pixel, and multi-level error diffusion for converting a pixel value output from the resolution conversion level to a multi-valued level and diffusing a quantization error at the time of the gradation conversion to peripheral pixels Means, an area gradation conversion means for replacing the gradation-converted pixel of interest with an area gradation pattern of N × N pixels corresponding to the multi-value level, and a resolution changing method among the pixels replaced with the area gradation pattern. Image processing apparatus comprising a monochrome forced processing means before the input pixel is converted to force a white or process color pixel position was pixel of the maximum value or the minimum value. 3. The area gradation conversion means has four area gradation patterns of N × N pixels for each multi-valued level, and converts the gradation-converted pixels into a pixel block of N × N pixels. The image processing apparatus according to claim 1, wherein the pixel is replaced with one of four types of area gradation patterns according to a pixel position in a pixel block of a target pixel to be divided and subjected to area gradation conversion. 4. The four area gradation patterns prepared for each multi-valued level are constituted by patterns rotated by 90 degrees with respect to the area gradation pattern at a certain position on the pixel block. The image processing apparatus according to claim 3, wherein: 5. The area gradation conversion means has four area gradation patterns determined for each multi-valued level for each processing color, and the area gradation pattern for each multi-valued level corresponds to each multi-valued level. 5. The image processing apparatus according to claim 1, wherein the pattern comprises a pattern shifted in the main scanning direction, the sub-scanning direction, or the main-sub-scanning direction based on the area gradation pattern of a processing color. 6. Image processing device. 6. An additional pattern generating means for outputting additional data at a pixel position corresponding to a pixel of interest in accordance with a predetermined additional pattern, and according to a function which maximizes the additional data at a gradation level at which a pseudo contour occurs. The image processing apparatus according to claim 1, further comprising a level correction unit that performs level correction on the additional data. 7. A continuous tone input pixel is divided into N × N pixel blocks, and it is determined whether a white pixel or a black pixel exists in the pixel block. The average value of the pixels other than the pixel is obtained, the resolution of the input pixel is converted to 1 / N in units of a pixel block, the average value of the pixel block is output as the pixel value of the pixel of interest, and the output pixel value is The gradation conversion is performed to the multi-value level, and the quantization error at the time of the gradation conversion is diffused to the peripheral pixels.
A pixel area gradation pattern is replaced, and a pixel position where an input pixel before resolution conversion is a white pixel or a black pixel among the pixels replaced with the area gradation pattern is forcibly converted to a white or black pixel. Characteristic image processing method. 8. A continuous tone input pixel is divided into N × N pixel blocks, and it is determined whether or not a pixel having a minimum value or a maximum value in the continuous tone exists in the pixel block. The average value of pixels other than the maximum value or minimum value pixel is obtained from the pixel block, the resolution of the input pixel is converted to 1 / N in pixel block units, and the average value of the pixel block is output as the pixel value of the pixel of interest. The output pixel value is subjected to gradation conversion to a multi-level level, and at the same time, the quantization error at the time of gradation conversion is diffused to peripheral pixels, and the target pixel subjected to the gradation conversion is converted to an N × N The pixel position is replaced with the area gradation pattern of the pixel, and the pixel position where the input pixel before the resolution conversion is the pixel of the maximum value or the minimum value among the pixels replaced with the area gradation pattern is forcibly converted to white or the processing color. Picture characterized by the fact that Processing method.