JP2001326818A - Image processing unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image processing unit that improves missing of an image by quickening a response to an error and realizes a small scale circuit by furthermore simplifying correction processing required for the improvement of missing image. SOLUTION: The image processing unit is provided with a low error discriminator 16 that discriminates whether or not an error caused by applying quantization processing to corrected density resulting from adding an accumulated error of density data of a target pixel (error before correction) is low and with a correction data generating section 15 that generates correction data depending on the density of the target pixel. Only in the case that no dot forming is decided and it is discriminated that the error before correction is low through the quantization processing by a threshold processing circuit 12, a selector 17 replaces the error before correction with correction data and provides an output of the replaced data to a weighting circuit 14. In other conditions than above, the error before correction is outputted to the weighting circuit 14 without any modification.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image processing apparatus provided for an image forming apparatus such as a digital copying machine and a printer.

[0002]

2. Description of the Related Art An image processing apparatus provided in an image forming apparatus such as a digital copying machine or a printer converts an image signal of a read original image into two or more values in order to reproduce an original image having multiple gradations. Needs to be quantized to multivalues. Conventionally used methods for quantizing an image signal into multi-valued binary signals include a dither method and an error diffusion method. Among them, the error diffusion method is also called an error dispersion method, and is a method of dispersing an error between the gradation of the quantized image and the gradation of the original image to surrounding pixels. FIG.
, The error diffusion method will be specifically described.

In the above-mentioned error diffusion method, density data of a target pixel in an original image having multiple gradations (indicated as multi-valued pixel density data in the figure) is added from a processed pixel to the target pixel. The added error (integrated error) is called the corrected density of the pixel of interest. As shown in FIG. 15, the corrected density is quantized by a threshold processing circuit 100, and a difference between the quantized data and the corrected density data is calculated as an error by an error generation circuit 101. The error calculated in this way is distributed to unprocessed peripheral pixels by the weighting circuit 102, and the result is stored in the error memory 103 as an integrated error. The processing of one pixel is sequentially performed for each pixel of the original image.

[0004]

However, when the above-mentioned conventional error diffusion method is used, in an input multi-tone image, in a low density portion or a rapid density change portion, an image in which no dot is formed is missing. Had occurred. This is because the diffusion error is small in the low-density area, and no dot is formed while the correction density, which is the sum of the diffusion error and the pixel density of the input image, increases until the threshold value for forming the dot is reached. is there.

On the other hand, Japanese Patent Application Laid-Open No. Hei 10-294866 discloses an image output apparatus with improved follow-up to a change in density. The image output device includes a comparator that detects a change in density, and a correction data generation unit that performs correction based on the density data after the change in density. Instead, it is possible to select the correction data of the correction data generation unit, and to make the same error as when the error is sufficiently diffused from the beginning of the density change.

However, the technique described in the above-mentioned conventional publication requires a comparator for detecting a change in density, so that a line memory for storing data to be compared must be separately provided. Therefore, since the number of configurations increases accordingly, there is a problem that the circuit configuration also becomes complicated.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has been made to solve the problem of missing images by accelerating an error response.
It is another object of the present invention to further simplify the correction processing for improving the lack of an image to realize a smaller circuit.

[0008]

In order to solve the above-mentioned problems, an image processing apparatus according to the present invention provides an image processing apparatus for quantizing an original multi-tone image into a multi-value image by using an error diffusion method for each pixel. The processing apparatus further includes a low error determination unit that determines that the target pixel error is a low error when the target pixel error generated when the target pixel is quantized is smaller than a predetermined error value. If a dot is not formed as a result of the conversion and the low error determination means determines that the target pixel error is a low error, the target pixel error is corrected.

According to the above arrangement, when no dot is formed for the pixel of interest and the low error determination unit determines that the pixel error of interest is a low error, the correction of the pixel error of interest is performed. Will be Accordingly, in the error diffusion method, the error response can be accelerated in a low-density portion region or a region where the density changes rapidly, which is generally regarded as a slow error response. That is, it is possible to improve the delay which occurs in the low density portion area or the rapid density change portion, until the addition of the diffusion error reaches the dot formation threshold.

As a result, it is possible to obtain a good image with almost no image defects in the low density portion area or the rapid density change portion.

Further, in order to solve the above-mentioned problem, the image processing apparatus of the present invention further comprises a correction data generating means for generating correction data for correcting the target pixel error in accordance with the density of the target pixel. When a dot is not formed by the quantization result of the target pixel and the low error determination unit determines that the target pixel error is a low error,
It is preferable that the apparatus further includes a correction unit that corrects the target pixel error using the correction data.

According to the above arrangement, when no dot is formed for the pixel of interest and the error of the pixel of interest is determined to be low by the low error determination means, the correction data is generated by the correction data generation means. The target pixel error is corrected using the corrected data. Therefore, it is possible to improve the delay, which occurs in the low-density portion area or the rapid density change portion, until the addition of the diffusion error reaches the dot formation threshold value, simply by replacing the target pixel error with the correction data.

[0013] With this, it is possible to prevent the occurrence of image chipping in the low-density portion region or the rapid density change portion using a simple circuit configuration.

Further, in order to solve the above-mentioned problem, the image processing apparatus of the present invention quantizes a multi-tone original image into a multi-valued image by using an error diffusion method, and does not correct a target pixel error. When the minimum value of the error in the error stabilizing unit of the formed image is set as the minimum error, it is preferable that the low error determination unit uses a value lower than the minimum error value as the predetermined error value.

According to the above arrangement, the low error determining means uses a value lower than the minimum error of the error stabilizing unit as the predetermined error value, and sets the target value only when the target pixel error is lower than the predetermined error value. The pixel error is determined to be low. Therefore, the pixel portion having a stable error (error stabilizing portion) is corrected, and there is no possibility that the density of this portion is unnecessarily increased.

Further, it is general that the minimum error of the low density part is larger than the minimum error of the high density part. Since the present invention is particularly for preventing the image of the low density portion area from being chipped, the correction condition of the low density portion area is widened by using the minimum error. Therefore, it is possible to more effectively reduce the lack of an image in the low density area than in the high density area.

This makes it possible to more reliably prevent the occurrence of image chipping in the low density area.

Further, in order to solve the above-mentioned problem, the image processing apparatus of the present invention quantizes a multi-tone original image into a multi-valued image by using an error diffusion method, and does not correct a pixel error of interest. In the case where a value obtained by averaging the errors in the error stabilizing unit of the formed image is defined as an average error, the correction data generation unit includes:
It is preferable to create correction data based on the average error.

The above-mentioned error stabilizing section is an error area where errors of processed pixels are sufficiently integrated. Therefore, the average error is the center value of the error of the pixel located in the error stabilizing section, and is most appropriately used for creating correction data. As the correction data based on the average error,
For example, a value of the average error as it is, a value near the average error, or a value depending on the average error is used.

This makes it possible to perform appropriate error correction in accordance with the density of each pixel of interest, and to more reliably prevent the occurrence of image chipping in the low-density area or in the rapidly changing density area.

Further, in the image processing apparatus according to the present invention, in order to solve the above-mentioned problem, the low-error determining means determines that the difference between the target pixel error and the correction data is equal to or larger than a predetermined value. Preferably, the pixel error is determined to be low.

According to the above arrangement, it is determined whether or not the target pixel error is a low error by using the difference between the correction data created based on the average error of the error stabilizing unit and the target pixel error. You. Therefore, by appropriately setting the predetermined value,
It is possible to effectively correct an error in a low-density area in which an image is likely to be chipped.

This makes it possible to more reliably prevent the occurrence of image chipping in the low density area.

Further, in order to solve the above problem, the image processing apparatus of the present invention further comprises an error diffusion means for determining a ratio of distributing a target pixel error to unprocessed peripheral pixels,
The error diffusion means, when a dot is not formed by the quantization result of the pixel of interest and the low error determination means determines that the error of the pixel of interest is a low error, the error spread to the unprocessed pixel. It is also possible to increase the sum.

According to the above configuration, the error of the pixel of interest is corrected by increasing the sum of the errors distributed to the unprocessed pixels, so that it is not necessary to separately provide a means for generating correction data. Further, the error diffusion means of this configuration can be realized by simply changing the error diffusion means used in the conventional error diffusion method. For example, when a dot is not formed due to the quantization result of the target pixel and the low error determination unit determines that the target pixel error is a low error, the weighting used in the conventional error diffusion unit is used. By setting to use a coefficient larger than the coefficient of the matrix, or by setting the number of shifts of the weighting matrix to be small,
It can be easily realized.

Thus, with a simpler circuit configuration, it is possible to improve the delay until the addition of the diffusion error reaches the threshold value for dot formation, which occurs in the low-density portion area or the rapid density change portion. A good image with almost no chipping can be obtained.

Further, in order to solve the above-mentioned problem, the image processing apparatus of the present invention further comprises a density determining means for determining whether or not the density of the target pixel is within a desired density range.
It is preferable to correct the target pixel error only when the density of the target pixel is within the desired density range.

According to the above configuration, when determining whether or not to correct the target pixel error, the result of the density determination means is further used, so that the image is largely lost and the correction is required in the density area. Error correction can be performed only for this.

As a result, it is possible to prevent unnecessary correction for pixels in the density region that does not require correction.

Further, in order to solve the above-mentioned problem, the image processing apparatus according to the present invention provides a method in which a target pixel error generated when quantizing a target pixel is larger than a predetermined error value.
The apparatus further includes a high error determination unit that determines the target pixel as a high error, a dot is formed based on a quantization result of the target pixel, and the high error determination unit determines that the target pixel error is a high error. Also in this case, it is preferable to correct the target pixel error.

According to the above configuration, a high error determination is also used for determining whether or not to correct a target pixel error,
It is also possible to obtain an effect that it is possible to prevent a delay in starting the appearance of white spots in a dark image and to prevent the image from being chipped when the image is inverted and used. That is, according to the present configuration, it is possible to cope with the case where the image is used in either normal rotation or reverse rotation.

Further, according to the image processing apparatus of the present invention, in an image processing apparatus for quantizing a multi-tone original image into a multi-valued image by an error diffusion method for each pixel, a pixel of interest generated when the pixel of interest is quantized When the error is larger than a predetermined error value, the image processing apparatus includes high error determination means for determining the pixel of interest as a high error, a dot is formed based on a quantization result of the pixel of interest, and the high error determination means When it is determined that the target pixel error is a high error, the target pixel error is corrected.

According to the above arrangement, the target pixel error is corrected only when a dot is formed for the target pixel and the high error determination means determines that the target pixel error is a high error. Therefore, even when an image is used after being inverted, the response of an error in the error diffusion method can be quickened.

As a result, even when the image is used upside down, it is possible to obtain a good image with almost no chipped image. Further, it is possible to improve the delay of the appearance of white spots in a dark image.

Furthermore, in order to solve the above-mentioned problems, the image processing apparatus of the present invention preferably corrects a target pixel error for each color when the multi-tone original image is a color image.

According to the above arrangement, for example, even when an image having the same density for each color is processed, the position at which the dot where the transition from the lack of the image to the error stabilizing portion begins to be formed is shifted for each color. become. Therefore, it is possible to express a color image smoothly.

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS [First Embodiment] The first embodiment of the present invention will be described below with reference to FIGS.

First, a configuration of the image processing apparatus according to the present embodiment and an error diffusion processing operation will be described with reference to FIGS. Here, FIG. 1 is a block diagram showing a configuration of the image processing apparatus according to the present embodiment.
9 is a flowchart illustrating an error diffusion process when the image processing device quantizes a multi-tone image into a multi-value image (including a binary value).

The image processing apparatus 1 according to the present embodiment includes an error memory 11, a threshold processing circuit 12, an error generation circuit 13,
It includes a weighting circuit (error diffusion means) 14, a correction data generation unit (correction data generation means) 15, a low error determiner (low error determination means) 16, and a selector (correction means) 17. For each of these components, the image processing device 1
A description will be given below with reference to the flowchart of FIG.

First, to the density data (multi-valued pixel density data) of the multi-tone target pixel, an integration error stored in the error memory 11 and distributed from the processed pixel to the target pixel is added ( S1). The sum of the density of the target pixel and the integration error is referred to as the corrected density of the target pixel.

Next, with respect to the corrected density of the target pixel,
A quantization process is performed (S2). Here, the quantization processing means that a difference between the quantized data output by performing threshold processing and the like on the corrected density in the threshold processing circuit 12 and the corrected density is calculated as an error in the error generation circuit 13. Is output as an error of the target pixel (hereinafter, referred to as an error before correction).

It is determined whether a dot is formed based on the quantized data (S3). Here, in order to determine whether or not to correct the pre-correction error, the low error determiner 16 determines the pixel density of interest and the error generation circuit 13.
It is determined whether or not the pre-correction error is a low error using the pre-correction error output from (S4). Here, the case where the error before correction is determined to be a low error refers to a state in which correction is considered necessary because the processed error is not sufficiently integrated as the integrated error of the target pixel. is there. A specific low error determination method of the error before correction by the low error determiner 16 will be described later.

Next, only when no dot is formed and the pre-correction error is determined to be low, the pre-correction error is corrected (error correction) (S5). That is, the selector 17 replaces the correction data created by the correction data generation unit 15 with the above-described error before correction as a correction error, and outputs the error to the weighting circuit 14 as an error after correction. Under the other conditions (when there is dot formation or when the pre-correction error is not a low error), no error correction is performed, and the pre-correction error is directly input to the weighting circuit 14. The specific description of the correction data creation in the correction data generation unit 15 will be described later.

The weighted circuit 14 distributes the corrected error to unprocessed peripheral pixels, and stores the distributed error in the error memory 11 as an integrated error for each pixel (S6).

The above processing from S1 to S6 is performed on all pixels (S7, S8). The scanning order of the pixels to be processed is shown in FIG.
As shown in, processing is performed in the main scanning direction, and when the last pixel in the main scanning direction is completed, the pixel is moved by one pixel in the sub-scanning direction.
The process is performed again from the first pixel in the main scanning direction to the main scanning direction. In the drawing, processed pixels are indicated by solid lines, and unprocessed pixels are indicated by broken lines.

Next, detection of a low error by the low error determiner 16 and creation of correction data by the correction data generator 15 will be described in detail.

First, the image processing apparatus 1 according to the present embodiment
An image when no error correction is performed in will be described. According to FIG. 4, which shows how the integration error changes in the sub-scanning direction, a constant multi-gradation density (here, 25
The image of 6 gradations and a density of 7 is shown).
In the case of binarization at 28, the integration error of the pixel located at the beginning of the processing is small. That is, the pixel region at the beginning of the process is a region where dots are not formed, and such a region is referred to as a missing portion of an image. Thereafter, the process proceeds, and the errors are sufficiently integrated. When the integrated error + the target pixel density 7 exceeds the threshold value 128, dot formation starts. In this way, a region of a pixel in which a sufficient error is distributed from the processed pixel is referred to as an error stabilizing unit. FIG.
(A) shows a missing part of an image and an error stabilizing part.

FIG. 6 is a diagram showing the relationship between the pixel density of interest and the error before correction, and shows the error range of the error stabilizing section. When the error before correction of the target pixel calculated by the error generation circuit 13 is within the error range of the error stabilizing unit, it is considered that the errors of the processed pixels are sufficiently integrated and added. When the minimum value of the error range in the error stabilizing unit is the minimum error, if the error before correction of the pixel of interest is smaller than the minimum error, the error of the processed pixel is not sufficiently integrated, so that correction is necessary. (Corresponding to the error area 1 requiring correction in the figure). The average value of the errors within the error range of the error stabilizing unit is defined as the average error.

In the image processing apparatus 1 according to the present embodiment, the low error determination for the error before correction is performed by the low error determination unit 1.
6 is performed by determining whether the error before correction of the target pixel is smaller than a predetermined value (predetermined error value). As described above, whether or not the pre-correction error needs to be corrected can be considered based on the minimum error. Therefore, it is preferable that the predetermined value serving as a criterion be lower than the minimum error. Therefore, here, it is preferable to use, for example, either the value of the minimum error as it is, a value near the minimum error smaller than the minimum error, or a value depending on the minimum error.

In the present embodiment, an LUT (Look Up Tab-le) method or an arithmetic method is used for the low error determination as described above.

As shown in FIG. 7, the LUT method is a method in which an LUT provided with output values corresponding to respective input values is held, and stored output values are extracted from the LUT input values. It is. The low error determiner 16 in the present embodiment compares the output value with the target pixel density as an input value and the pre-correction error to determine whether the pre-correction error of the target pixel is a low error. That is, an output value having the target pixel density as an input value is the predetermined value. It should be noted that the input values and output values specifically shown in FIG. 7 are merely examples, and the present invention is not limited thereto.

On the other hand, a case where an arithmetic method is used for low error determination will be described below. Assuming that the target pixel density is X and the output value is Y, for example, the following arithmetic expression can be used.

Y = −128 / 121 × X + 53 (1) By comparing the output value Y of the above equation (1) with the pre-correction error, the pre-correction error of the target pixel can be reduced to a low level requiring correction. It is determined whether or not there is an error. Also, instead of the above equation (1), the following equation (2) can be used.

Y = -X + 53 (2) The above equation (2) expresses the minimum error as it is. If this equation (2) is used, a multiplication circuit is not required, and as a result, the circuit of the image processing apparatus 1 can be reduced, and the processing speed can be increased. The above equations (1) and (2) are both arithmetic equations created using the minimum error.

Next, a method of creating correction data in the correction data generator 15 will be described. In the present embodiment, the correction data generation unit 15 generates correction data using, for example, an LUT method or an arithmetic method. Here, the correction data is created based on the average error of the error stabilizing unit shown in FIG. 6, and for example, depends on the value of the average error itself, a value near the average error, or the average error. Values can be used. This is because the average error is the error center value of the pixel located in the error stabilizing section, and thus it is determined that the average error is most appropriate to use as correction data.

When the LUT method is used, the correction data generation unit 15
Are provided with LUTs having output values only for the types of input values. Using this LUT, a target pixel density is set as an input value.
The correction data is output.

On the other hand, when an arithmetic method is used to generate correction data, for example, the following arithmetic expression is used. Here, the target pixel density is the input value X, and the correction data value is the output value Z. Further, in the present embodiment, since the gradation of the image is 256 gradations, 128 is the central value.

Z = −X + 128 (3) The above equation (3) uses the average error of the error stabilizing section shown in FIG. 6 as it is for the correction data. When equation (3) is used, it is not necessary to use a multiplication circuit, so that the circuit can be reduced in size, and the processing speed can be increased. For example, when it is determined that correction is necessary according to FIG. 6 due to the pre-correction error -10 at the target pixel density 20, the correction data value Z is 108.

When it is desired to make the missing portion of the image smaller, an equation having a larger intercept and inclination than equation (3) may be used. For example, the following equation (4) may be used. . Also in equation (4), the pixel density of interest is the input value X
Where the correction data value is the output value Z.

Z = −128 / 121 × X + 128 (4) In the above equation (4), there is a case where the correction data value Z becomes larger than 128, but this is distributed to unprocessed peripheral pixels. For example, each peripheral pixel has an error of 128 or less. Therefore, there is no problem even if the error memory 11 can store, for example, only 128 or less.

The arithmetic expressions used in generating the correction data are not limited to the above-mentioned expressions (3) and (4), but may be expressions for calculating values near the average error or values depending on the average error. If so, other arithmetic expressions can be used. However, it is desirable that the correction data value Z is 0 when the target pixel density X is 128.

As described above, in the image processing apparatus 1 according to the present embodiment, no dot is formed for the target pixel, and the error before correction of the target pixel is a low error by the low error determiner 16. Is determined, the correction data generation unit 1
By correcting the pre-correction error of the target pixel using the correction data value created in step 5, the response of the error can be accelerated. In other words, it is possible to improve a delay that is likely to occur in a low density portion area or a rapid density change portion, and that occurs until the addition of a diffusion error reaches a dot formation threshold. As a result, as shown in FIG. 5B, a good image having almost no missing portions can be obtained in the low-density portion region or the rapid density change portion. Furthermore, if the processing in the correction data generation unit 15 is simplified, the circuit can be reduced in size.

Further, since the image processing apparatus 1 according to the present embodiment is configured to perform error correction of the target pixel after quantization, the error correction value is set to a value larger than the threshold value used in the quantization processing. Can be set to a value. As a result, a larger error can be distributed to the peripheral pixels, and the pixels to be processed thereafter are easily formed with dots. As a result, the missing portion of the image can be reduced.

The image processing apparatus 1 according to the present embodiment
Is also used for a color image.
Each color of color image, C (cyan), M (magenta), Y
In the (yellow) and K (black) error diffusion processes, the low error determination performed by the low error determiner 16 and the generation of correction data performed by the correction data generator 15 are performed by C,
Perform separately for each of M, Y, and K. Therefore, the weighting circuit 14 as the error diffusion means is also provided for each of C, M, Y, and K.

Thus, for example, when processing images having the same density of C, M, Y, and K, the image shifts from a missing portion of the image to an error stabilizing portion due to the difference in the correction data of C, M, Y, and K. The positions where the dots begin to be formed are C, M, Y,
It will be shifted by K. Therefore, it is possible to express an image smoothly.

The image processing apparatus 1 according to the present embodiment
Then, the error correction of the target pixel is performed after quantization,
A configuration before quantization is also possible. By performing error correction before quantization as described above, it is possible to intentionally set correction data equal to or larger than a threshold value and perform dot formation on a target pixel. Therefore, the missing portion of the image can be reduced. However, in this case, it is necessary to control the dot formation using a random number or the like so that dots are not formed more than necessary.

[Second Embodiment] The second embodiment of the present invention is described below with reference to FIG. Note that, for convenience of description, the same reference numerals are given to components having the same functions as those described in the first embodiment, and description thereof is omitted.

The image processing apparatus 2 according to the present embodiment is different from the image processing apparatus 1 according to the first embodiment in the method of determining whether or not the pre-correction error of the target pixel is a low error. FIG.
As shown in (2), the image processing apparatus 2 uses the low error determiner (low error determination means, correction means) 21 to separate the error before correction from the correction data created by the correction data generation unit 15 by a certain value or more. In this case, the error before correction is determined to be a low error. Further, the low error determiner 21
When the pre-correction error is determined to be a low error, the pre-correction error is replaced with a correction data value and output to the weighting circuit 14, and when the pre-correction error is not determined to be the low error, the pre-correction error is output to the weighting circuit 14 as it is. .

For example, considering the case where correction data is created using an arithmetic method, the equation (3) described in the first embodiment is used.
Is used and the correction is performed when the correction data value and the error before correction are separated by 75 or more.
The following equation is used at 1. If the error before correction is smaller than the output value Y in this equation,
1 determines that the error before correction is a low error.

Y = −X + 53 (5) If the low error determination is made using the above equation (5), it is the same as the case of using the equation (2) in the low error determination of the first embodiment. The result is obtained. Therefore, the image processing apparatus 2 of the present embodiment can also achieve the same operation and effect as the image processing apparatus 1 of the first embodiment.

[Third Embodiment] The following will describe a third embodiment of the present invention with reference to FIGS. 9 and 10. Note that, for convenience of explanation, components having the same functions as those described in the first or second embodiment are denoted by the same reference numerals, and description thereof is omitted.

The image processing apparatus 3 according to the present embodiment differs from the first and second embodiments in the method of error correction. FIG.
As shown in (2), the image processing apparatus 3 does not use the correction data created by the correction data generation unit 15 at the time of error correction. Instead, the image processing apparatus 3 uses a weighting circuit 31 to calculate the error to be distributed to unprocessed pixels. Correction to increase the sum is performed.
The determination as to whether or not to correct the pre-correction error of the target pixel is performed in the same manner as in the image processing apparatus 1 according to the first embodiment. That is, the low error determiner 16 compares the output value with the target pixel density as an input value and the pre-correction error, and determines whether the pre-correction error is a low error requiring correction, The result of the determination is input to the weighting circuit 31, and error correction is performed.

Normally (when the error before correction of the target pixel is not corrected), the weighting circuit 31 multiplies the input error by the coefficient of the weighting matrix of the unprocessed peripheral pixel and calculates the sum of the coefficients. Division is performed by bit shift by the amount. However, when the low error determiner 16 determines that the pre-correction error is a low error, the weighting circuit 31 replaces the normal weighted matrix risk coefficient with
A coefficient of a correction weighting matrix stored in advance is used. The correction weighting matrix is set such that the sum of its coefficients is larger than that of a normal weighting matrix.

FIG. 10A shows an example of a coefficient of a normal weighting matrix, and FIG. 10B shows an example of a coefficient of a weighting matrix for correction. FIG.
By simply changing the matrix coefficients shown in FIG. 10A to the matrix coefficients shown in FIG.
Increase to 1. Therefore, the magnitude of the error allocated to the unprocessed pixels can be increased to 21/16 times.

Further, it is also possible to correct by reducing the number of bit shifts by dividing by the bit shift after multiplying the coefficient of the weighting matrix by the error before correction while keeping the coefficient of the weighting matrix as it is. . If the number of bit shifts is reduced, the sum of the errors scattered to the unprocessed pixels can be increased twice, four times,....

A method for reducing the number of bit shifts will be described. For example, since the sum of the coefficients of the weighting matrix shown in FIG. 10A is 16, a normal 4-bit shift equal to $ 16 is performed. However, when the low error determiner 16 determines that the pre-correction error is a low error, the weighting circuit 31 shifts by 3 bits (÷) to increase the sum of the errors distributed to the unprocessed pixels.
Perform 8). This makes it possible to double the sum of the errors spread to the unprocessed pixels.

As described above, by increasing the total sum of the coefficients of the weighting matrix or reducing the number of bit shifts to increase the total sum of the errors scattered to the unprocessed pixels, correction data generation for generating correction data is performed. Part 1
5 need not be provided, and the processing only requires simple modifications to the conventional weighting circuit. Therefore, with a simpler circuit configuration, it is possible to improve the delay until the addition of the diffusion error reaches the threshold value for dot formation, which occurs in the low-density part region or the abrupt density change part region. And a good image having almost no image quality can be obtained.

[Fourth Embodiment] The following will describe a fourth embodiment of the present invention with reference to FIG. 11 and FIG. For the sake of convenience, the same reference numerals are given to the components having the same functions as those described in any of the first to third embodiments, and description thereof will be omitted.

As shown in FIG. 11, the image processing apparatus 4 according to the present embodiment is different from the image processing apparatus 1 according to the first embodiment in that
In this configuration, a density determiner (density determination means) 41 is further provided, and when determining whether to correct the pre-correction error of the target pixel, the density determination of the target pixel is also added.

The density determiner 41 determines whether the density of the target pixel is within a desired range by threshold processing of a fixed value or threshold processing by random number generation, and outputs the result to the selector 17. The selector 17 determines the pre-correction error only when the dot is not formed and the target pixel is in a desired density range and has a lower error.
Error correction is performed by substituting the correction data created in step (1). Here, as the desired density range, a density at which the image is likely to be chipped is set.

Referring to the flowchart of FIG. 12, it is determined whether or not a dot is formed for the pixel of interest (S13).
The density is detected as being within the desired density range by threshold processing (S14), and is detected as having a low error (S1).
Only in the case of 5), error correction is performed on the error before correction of the target pixel (S16). The corrected error is distributed to peripheral pixels (S17). Under the other conditions, the error correction is not performed, and the error before correction is directly allocated to the peripheral pixels (S17). The other processing operations are the same as the processing operations of the image processing apparatus 1 shown in FIG.

As described above, when deciding whether or not to perform error correction on the target pixel, the density of the target pixel is also added to the determination criterion, so that the density of the image is large and correction is needed (low density portion). Error correction can be performed only on the pixels of the (region). As a result, error correction is performed on pixels having a density that does not need to be corrected, and as a result, there is no possibility that the error is unnecessarily increased.

When the density determination unit 41 performs threshold processing using a fixed threshold value, the density of the target pixel can be determined with a simple circuit configuration, and the density determination range can be easily changed by changing the threshold value. You can choose.

In the case where the density judgment unit 41 performs a threshold process by generating a random number, the threshold value is not constant.
The change between the corrected density image and the uncorrected density image becomes smooth. That is, it is possible to prevent a boundary of an image from being formed in an image having a density near the density determination value due to a change in the error due to the error correction.

A generally known example of the random number generation circuit is an LFSR (Linear Feedback Shift Reg).
ister). The LFSR holds a fixed bit length,
The number of bits of a desired random number is assigned from a fixed bit length. When the random number is changed, the bit data is shifted and, at the same time, the EXOR (exclusive OR) of a specific two bits from a fixed bit length is inserted into one bit at the free end.

As described above, the image processing apparatus 4 according to the present embodiment further includes the density judgment unit 41,
In addition to the effects of the image processing apparatus 1 of the first embodiment, the effect that error correction can be performed only on pixels having a density that needs to be corrected can be realized at the same time.

In this embodiment, the image processing apparatus 1 according to the first embodiment is provided with the density determiner 41, but is provided in the image processing apparatuses 2 and 3 according to the second and third embodiments. It is also possible to achieve the same effect.

[Fifth Embodiment] The following will describe a fifth embodiment of the present invention with reference to FIGS. 6, 13 and 14. For the sake of convenience, the same reference numerals are given to the components having the same functions as those described in any of Embodiments 1 to 4, and description thereof is omitted.

As shown in FIG. 13, an image processing apparatus 5 according to the present embodiment has a configuration in which a high error decision unit 51 is further provided in the image processing apparatus 1 according to the first embodiment. And the low error determination unit 16
In the same manner as in the determination method described above, the error correction is also performed when it is determined that the pre-correction error is a high error larger than a predetermined error value.

As described above, when there is an error within the error range of the error stabilizing unit shown in FIG. 6, the errors of the processed pixels are sufficiently integrated. Assuming that the maximum value of the error range of the error stabilizing unit is the maximum error, if the pre-correction error of the target pixel is larger than the maximum error, a negative error or a small positive error is not fully integrated. Is determined. Therefore, the target pixel having an error larger than the maximum error is also considered to be in a state where error correction is required (in the figure, it is indicated as “error area 2 requiring correction”). Therefore, the predetermined error value used in the high error determination is obtained based on the maximum error.

The processing operation of the image processing apparatus 5 according to the present embodiment will be described with reference to the flowchart of FIG.
It is determined whether or not a dot is formed with respect to the target pixel (S23). If no dot is formed and it is determined that the error is low (S24), and if a dot is formed. And a high error is determined (S
25) In the case, error correction is performed using the correction data created by the correction data generation unit 15 (S26). The post-correction error that has been replaced by the correction data is then allocated to peripheral pixels (S27). Under other conditions, the error correction is not performed, and the error before correction is directly scattered to the surrounding pixels (S27). Other processing operations are the same as those of the image processing apparatus 1 shown in FIG.

The correction data generated by the correction data generating unit 15 can be different data in two cases of low error determination, no dot formation, high error determination, and dot formation. It is preferable to use the same correction data based on the average error of the indicated error stabilizing section. This allows
Since the processing in the correction data generation unit 15 is simplified, the circuit can be downsized.

As described above, by using the high error determination to determine whether or not to perform the error correction, it is possible to delay the appearance of white spots in a dark image or to reduce the image when the image is used after being inverted. Chipping can be prevented. That is, the image processing apparatus 5 according to the present embodiment can cope with the case where the image is used in normal rotation or inversion.

The image processing apparatus 5 according to the present embodiment
The high error determiner 51 determines whether the pre-correction error of the target pixel is a high error by comparing the output value obtained from the target pixel density with the pre-correction error, similarly to the low error determination. . However, the present invention is not limited to such a determination method, and may be configured to determine a high error if the pre-correction error is at least a certain distance from the value of the correction data.

In the present embodiment, the image processing apparatus 1 according to the first embodiment is provided with the high error determiner 51. However, the image processing apparatus 2 according to the second, third, and fourth embodiments is used. , 3, and 4 can be provided, and the same function and effect can be obtained.

[0096]

As described above, according to the image processing apparatus of the present invention, the target pixel error generated when the target pixel is quantized is
A low error determining unit that determines the target pixel error to be a low error when the target pixel error is smaller than a predetermined error value; a dot is not formed based on a quantization result of the target pixel; When the pixel error is determined to be low, the target pixel error is corrected.

Therefore, it is possible to improve the delay which occurs in the low density portion area or the rapid density change portion, until the addition of the diffusion error reaches the threshold value for dot formation. As a result, there is an effect that in a low density portion area or a sharp density change portion, it is possible to obtain a good image with almost no image defects.

Further, the image processing apparatus according to the present invention comprises a correction data generating means for generating correction data for correcting the target pixel error in accordance with the density of the target pixel, and a correction data generating means for obtaining the target pixel error. When no dot is formed, and the low error determination unit determines that the target pixel error is a low error, the correction unit corrects the target pixel error using the correction data. Preferably, there is.

Therefore, it is possible to improve the delay, which occurs in the low-density portion area or the rapid density change portion, until the addition of the diffusion error reaches the threshold value for dot formation, by simply replacing the target pixel error with the correction data. Can be. As a result, there is an effect that it is possible to prevent the occurrence of image chipping in a low-density portion area or a sharp density change portion using a simple circuit configuration.

Further, the image processing apparatus of the present invention quantizes a multi-tone original image into a multi-valued image by using an error diffusion method, and converts an image formed without correcting a pixel-of-interest error into an error stabilizing unit. When the minimum value of the error is set as the minimum error, it is preferable that the low error determination means uses a value lower than the minimum error as the predetermined error value.

Therefore, there is no possibility that the pixel portion having a stable error (error stabilizing portion) is corrected and the density of this portion is unnecessarily increased. Further, since the correction conditions for the low-density portion area where the image lack occurs are broadened, the image lack of the low-density portion area can be improved as compared with the high-density portion area. As a result, there is an effect that it is possible to more reliably prevent the occurrence of image chipping in the low density portion region.

Further, the image processing apparatus of the present invention quantizes a multi-tone original image into a multi-valued image by using an error diffusion method, and converts an image formed without correcting a pixel-of-interest error into an error stabilizing unit. In a case where a value obtained by averaging the errors is used as an average error, it is preferable that the correction data generating means is configured to generate correction data based on the average error value.

Therefore, it is possible to perform an appropriate error correction in accordance with the density of each target pixel, and it is possible to more reliably prevent the occurrence of image chipping in a low-density area or a rapidly changing density area.

Further, in the image processing apparatus according to the present invention, when the difference between the target pixel error and the correction data is equal to or greater than a predetermined value, the low error determining means determines that the target pixel error is a low error. It is preferable that the configuration is such that:

Therefore, by appropriately setting the predetermined value, the low-density portion region in which the image is likely to be chipped can be reduced.
Error correction can be performed effectively. As a result, there is an effect that occurrence of chipping of the image in the low density portion region can be more reliably prevented.

Further, the image processing apparatus of the present invention further comprises an error diffusion means for determining a ratio of distributing a pixel error of interest to unprocessed peripheral pixels, and the error diffusion means comprises a dot based on a quantization result of the pixel of interest. Is not formed, and when the low error determination unit determines that the target pixel error is a low error, the total sum of the errors distributed to the unprocessed pixels may be increased.

Therefore, it is not necessary to separately provide a means for generating correction data. Further, the error diffusion means of this configuration is
It can be realized only by adding a simple change to the error diffusion means used in the conventional error diffusion method.
As a result, with a simpler circuit configuration, it is possible to improve the delay until the addition of the diffusion error reaches the threshold value for dot formation, which occurs in the low-density part area or the rapid density change part, and the image lacks. There is an effect that it is possible to obtain a good image having almost no image.

Further, the image processing apparatus of the present invention further comprises a density determining means for determining whether or not the density of the pixel of interest is within a desired density range, wherein the density of the pixel of interest is within the desired density range. It is preferable that the target pixel error is corrected only at a certain time.

Therefore, it is possible to perform error correction only on a density region in which the image is large and correction is required. As a result, there is an effect that unnecessary correction can be prevented from being performed on pixels in a density region that does not require correction.

Further, the image processing apparatus according to the present invention further comprises a high error judging means for judging the target pixel as a high error when the target pixel error generated when quantizing the target pixel is larger than a predetermined error value. A configuration in which a dot is formed based on the quantization result of the target pixel, and the target pixel error is corrected even when the target pixel error is determined to be a high error by the high error determination unit. It is preferable that

Therefore, it is possible to obtain an effect that a delay in starting the appearance of a white point in a dark image and a loss of an image when the image is inverted and used can be prevented. That is, according to this configuration, an effect is provided that the image can be used regardless of whether the image is rotated forward or inverted.

The image processing apparatus according to the present invention further comprises a high error determining means for determining that the target pixel is a high error when the target pixel error generated when quantizing the target pixel is larger than a predetermined error value. Wherein a dot is formed based on the quantization result of the target pixel, and when the target error is determined to be a high error by the high error determination means, the target pixel error is corrected. Can also.

Therefore, even when an image is used after being inverted, the error response can be made faster in the error diffusion method. As a result, even when the image is used inverted, an advantageous effect that a good image with almost no chipping of the image can be obtained can be obtained. Furthermore, the effect that the delay of the appearance of white spots in a dark image can be reduced is also achieved.

Further, the image processing apparatus of the present invention is preferably configured such that, when the original multi-tone image is a color image, the target pixel error is corrected for each color.

Therefore, it is possible to smoothly represent a color image.

[Brief description of the drawings]

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

FIG. 2 is a flowchart illustrating an image processing operation performed by the image processing apparatus.

FIG. 3 is an explanatory diagram showing a processing direction of an image in the image processing apparatus.

FIG. 4 is an explanatory diagram showing how an integrated error changes in the sub-scanning direction when a conventional error diffusion method is used.

FIG. 5A is an explanatory diagram showing an image formed when an error diffusion method without performing error correction is used,
(B) is an explanatory view showing an image formed by the image processing device.

FIG. 6 is a relationship diagram showing a relationship between a target pixel density and an error distribution of an error stabilizing unit when a conventional error diffusion method is used.

FIG. 7 is an explanatory diagram for explaining an LUT method.

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

FIG. 9 is a block diagram illustrating a configuration of an image processing apparatus according to a third embodiment of the present invention.

FIGS. 10A and 10B are explanatory diagrams showing coefficients of a weighting matrix by a weighting circuit of the image processing apparatus. FIGS.

FIG. 11 is a block diagram illustrating a configuration of an image processing apparatus according to a fourth embodiment of the present invention.

FIG. 12 is a flowchart illustrating an image processing operation by the image processing apparatus.

FIG. 13 is a block diagram illustrating a configuration of an image processing apparatus according to a fifth embodiment of the present invention.

FIG. 14 is a flowchart showing an image processing operation by the image processing apparatus.

FIG. 15 is a block diagram showing a configuration of a conventional image processing apparatus using an error diffusion method.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 image processing device 2 image processing device 3 image processing device 4 image processing device 5 image processing device 14 weighting circuit (error diffusion means) 15 correction data generation unit (correction data generation means) 16 low error determiner (low error determination means) Reference Signs List 17 selector (correction means) 21 low error determiner (low error determination means, correction means) 31 weighting circuit (error diffusion means) 41 density determiner (density determination means) 51 high error determiner (high error determination means)

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takahiro Kawakami 22-22, Nagaikecho, Abeno-ku, Osaka, Osaka Inside Sharp Corporation (72) Inventor Naoyuki Kamei 22-22, Nagaikecho, Abeno-ku, Osaka, Osaka Sharp Co., Ltd. (72) Inventor Hiroki Eto 22-22, Nagaikecho, Abeno-ku, Osaka-shi, Osaka Sharp Co., Ltd. (72) Inventor ▲ Yoshi ▼ mura Souichi 22-22, Nagaikecho, Abeno-ku, Osaka-shi, Osaka In-house F term (reference) 5B057 CA01 CA02 CA08 CA12 CA16 CB01 CB02 CB06 CB12 CB16 CC01 CE13 CH01 CH07 5C077 LL17 LL19 MP01 MP08 NN12 NN13 PP33 PP43 PQ08 PQ12 PQ18 PQ20 PQ23 RR02 RR08 5C009 LA02 PA02 PA03

Claims (10)

[Claims] An image processing apparatus for quantizing a multi-tone original image into a multi-valued image using an error diffusion method for each pixel, wherein a pixel error of interest generated when quantizing a pixel of interest is a predetermined error. When the value is smaller than the value, a low error determination unit that determines the target pixel error as a low error is provided, and a dot is not formed based on a quantization result of the target pixel,
An image processing apparatus for correcting the target pixel error when the low error determination unit determines that the target pixel error is a low error. 2. A correction data generating means for generating correction data for correcting the target pixel error in accordance with a density of the target pixel, wherein a dot is not formed by a quantization result of the target pixel.
And a correction means for correcting the pixel error of interest using the correction data when the pixel error of interest is determined to be a low error by the low error determination means. An image processing apparatus according to claim 1. 3. A multi-gradation original image is quantized into a multi-valued image using an error diffusion method, and a minimum value of an error in an error stabilizing section of an image formed without correcting a target pixel error is defined as a minimum error. In this case, the low error determination means sets a value lower than the minimum error,
The image processing device according to claim 1, wherein the image processing device uses the error value as the predetermined error value. 4. An image obtained by quantizing a multi-tone original image into a multi-valued image by using an error diffusion method and averaging an error in an error stabilizing part of an image formed without correcting a target pixel error. 3. The image processing apparatus according to claim 2, wherein the correction data generation unit generates correction data based on the average error. 5. The apparatus according to claim 1, wherein said low error determination means determines that said pixel error of interest is a low error when a difference between said pixel error of interest and said correction data is equal to or greater than a predetermined value. 5. The image processing device according to 4. 6. An error diffusion means for determining a ratio of distributing a target pixel error to unprocessed peripheral pixels, wherein said error diffusion means does not form a dot due to a quantization result of said target pixel, and 2. The image processing apparatus according to claim 1, wherein when the error determination unit determines that the target pixel error is a low error, the total sum of the errors allocated to the unprocessed pixels is increased. 7. A density determining means for determining whether or not the density of the target pixel is within a desired density range, wherein the target pixel error is determined only when the density of the target pixel is within the desired density range. The image processing apparatus according to claim 1, wherein the correction is performed. 8. A high-error judging means for judging the pixel error of interest as a high error when the pixel error of interest generated when quantizing the pixel of interest is larger than a predetermined error value, 2. The method according to claim 1, further comprising the step of correcting the target pixel error even when a dot is formed on the basis of the result of the quantization and the high error determining means determines that the target pixel error is a high error. 8. The image processing device according to any one of items 7 to 7. 9. An image processing apparatus for quantizing a multi-tone original image into a multi-valued image for each pixel by an error diffusion method, wherein a target pixel error generated when the target pixel is quantized is smaller than a predetermined error value. If the target pixel error is large, a high error determination unit that determines the target pixel error as a high error is provided. A dot is formed based on the quantization result of the target pixel, and the target pixel error is a high error by the high error determination unit. If it is determined that there is, the image processing apparatus corrects the target pixel error. 10. When the multi-tone original image is a color image,
The image processing apparatus according to claim 1, wherein the target pixel error is corrected for each color.