JP2003274176A - Image processor and image processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processor and an image processing method, capable of obtaining a higher quality image by eliminating the difference between the image density set in a patch shape and the output density at an isolated point. <P>SOLUTION: This image processor for outputting digital image data to an image output device is provided with a determining means for calculating a value corresponding to the output density in outputting a pixel under consideration and its ambient pixels to determine whether the pixel under consideration is located at an isolated point where the pixel under consideration is higher than the ambient pixels in output density, and a correcting means for applying correction for increasing the output density to the ambient pixels except the pixel under consideration when the pixel under consideration is located at the isolated point. Otherwise, the image processor is provided with a determining means for calculating the value corresponding to the output density in outputting the pixel under consideration and the ambient pixels to determine whether the pixel under consideration is located at an isolated white point where the pixel under consideration is lower than the ambient pixels in output density, and a correcting means for applying correction for reducing output density to the pixel under consideration when the pixel under consideration is located at the isolated white point. <P>COPYRIGHT: (C)2003,JPO

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 and an image processing method, and more particularly to an image processing apparatus and an image processing method capable of improving the output stability of an image including isolated points.

[0002]

2. Description of the Related Art Conventionally, as a method of correcting an output density with respect to an image density, a method of adjusting a gradation of an output image is known. Taking the electrophotographic printer unit as an example, the gradation adjustment of the output image in the electrophotographic printer unit is normally performed by exposing a patch-like pattern having a certain size as shown in FIG. 6A. As shown in FIG. 6B, from the density measurement data obtained by outputting a plurality of gradations on the body D (P1, P2, P3) and measuring the reflected light amount by the measurement sensor S, the input gradation level Accordingly, a correction curve for obtaining a constant output density is calculated by inverse conversion and corrected. In the conventional method, this correction curve is constant regardless of the shape of the output image, in the expectation that the relationship between the input gradation level and the output density is constant regardless of the shape of the output image.

However, in the case of an electrophotographic printer, for example, when there are other output pixels around the pixel of interest, such as the patch pattern or linear pattern, and when there is an isolated point, There is a problem that the output density is different even when the same input gradation level is output, and the density becomes lighter at the isolated point than when there is no other output pixel around the pixel.

In the case of the electrophotographic system, as shown in FIG. 7, one pixel in image processing is a square, but one pixel is optically one by a laser or an LED.
Since the pixels (optical dots) are usually circular, the diameter of the circle must be equal to or greater than the length of the diagonal of the square to completely fill the square, which is one pixel in image processing. This solves the problem that when the optical dot becomes smaller than one pixel in image processing, a gap is generated even if an image is written in all pixels, and the maximum density (so-called solid image density) becomes low. This is because. Therefore, inevitably, a portion (a hatched portion in FIG. 7) that overlaps the peripheral pixels occurs.

Due to the presence of this overlap, when an image exists around the pixel of interest, such as a patch-like image, as shown in FIG. 8, overlap with adjacent pixels (hatched portion). Will increase the overall concentration. As a result, even if the correction curve matched with the output density of the patch image is used, the density at the isolated point portion becomes lighter than the expected density.

Such a phenomenon is not limited to the electrophotographic printer, but also occurs in, for example, an ink jet printer that employs an image recording method in which pixel overlap occurs.

Such a phenomenon becomes a problem especially when the output density is light, and a situation occurs in which an image is output / not output due to the above phenomenon. That is, as shown in FIG. 9, when the output density is highlight, that is, near the development potential level, the output density exceeds the development potential level due to the overlap with the adjacent pixels in the patch image. The image is output. However, in the case of an isolated point, the output density does not exceed the development potential level and an image may not be output because there is no overlap with adjacent pixels even if the input gray level is the same.

Such a situation not only causes a serious problem that the information output device is missing, which should not be present in the information output device, but also causes an analog error (for example, a development error in the electrophotographic process). The image is output / not output in places, the gradation stability is lost in the visually sensitive highlight portion, and the image quality is significantly deteriorated.

Such an isolated point of the highlight portion is a technique derived from the gradation expression by the error diffusion method, particularly the flow of high image quality of the printer, "multi-valued error diffusion (more than two output levels are used. When the method of “used error diffusion” is used, an isolated point with a small output level appears with high probability as an expression of the highlight part. In addition, the size of the isolated point is drastically reduced with the increase in the resolution of the printer, and this phenomenon becomes more prominent. Therefore, when aiming at higher image quality, output stability of isolated points in the highlight part is a very important issue.

On the other hand, when the peripheral pixels have a high density and the target pixel is light (white isolated point), contrary to the above phenomenon, there is a problem that the output density becomes higher than the desired output density due to the overlap with the adjacent pixels. (See FIG. 10). In this case, in an image with white spots in the high-density area, the white spots are crushed, and so-called image dropout (white image reproducibility in the black peripheral area)
There is a problem that it becomes worse.

[0011]

Therefore, an object of the present invention is to eliminate the difference between the image density set as a patch and the output density at an isolated point, and to obtain an image of higher quality. And to provide an image processing method.

Another object of the present invention is to solve the problem of instability of the output image particularly when the highlight part is output in multiple gradations, and an image processing apparatus capable of obtaining a higher quality image. An object is to provide an image processing method.

Still another object of the present invention is to improve the reproducibility of white spots existing in a high density portion, improve so-called image omission (self-image reproducibility of black peripheral portion), An object of the present invention is to provide an image processing device and an image processing method capable of obtaining a high quality image.

[0014]

Each of the above problems can be solved by the following inventions.

(Claim 1) In an image processing device for outputting digital image data to an image output device, a value corresponding to the output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel outputs more than the peripheral pixels. And a correction unit that determines whether or not the pixel is an isolated point having a high density and a correction unit that, when the pixel of interest is an isolated point, corrects the peripheral pixels other than the pixel of interest to increase the output density. A characteristic image processing device.

According to a second aspect of the present invention, in an image processing apparatus for outputting digital image data to an image output apparatus, a value corresponding to an output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel outputs more than the peripheral pixels. The present invention is characterized by further comprising: a determination unit that determines whether or not a white isolated point has a low density, and a correction unit that, when the target pixel is a white isolated point, corrects the output density of the target pixel. Image processing device.

(Claim 3) The image processing apparatus according to claim 1 or 2, wherein the correction means performs the correction only when the pixel of interest or the peripheral pixel is in a predetermined density range.

(Claim 4) The correction means controls the pulse duration of the laser on a pixel-by-pixel basis.
The image processing apparatus according to claim 1, 2 or 3, wherein the output density is corrected by M).

According to a fifth aspect of the present invention, in an image processing method of an image processing apparatus for outputting digital image data to an image output apparatus, a value corresponding to an output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel is a peripheral pixel. An image characterized by determining whether it is in an isolated point state in which the output density is higher than that of the pixel, and when the target pixel is the isolated point, a correction for increasing the output density is added to peripheral pixels other than the target pixel. Processing method.

According to a sixth aspect of the present invention, in an image processing method of an image processing apparatus for outputting digital image data to an image output apparatus, a value corresponding to an output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel is a peripheral pixel. An image processing method characterized by determining whether or not there is a white isolated point whose output density is lower than that of the pixel, and when the pixel of interest is a white isolated point, correction is made to reduce the output density of the pixel of interest. .

(Claim 7) The image processing method according to claim 5 or 6, wherein the correction means performs correction only when the pixel of interest or a peripheral pixel is within a predetermined density range.

(Claim 8) The correction means is a pulse width modulation (PW) for controlling the lighting time of the laser for each pixel.
8. The image processing method according to claim 5, wherein the output density is corrected by M).

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below.

FIG. 1 is a block diagram showing an example of the main configuration of the image processing apparatus according to the present invention. Here, a case of a monochrome printer will be described as an example.

In FIG. 1, 1 is a buffer, 2 is a multi-valued error diffusion circuit, 3 is an isolated point discrimination circuit, 4 is an isolated point correction circuit, 5 is a gamma correction circuit, 6 is a timing adjustment circuit, and 7 is a circuit.
Is a PWM conversion circuit.

Image input data is sequentially stored in the buffer 1 and sent to the multi-valued error diffusion circuit 2. The multi-valued error diffusion circuit 2 multi-values a multi-valued input image and distributes and adds the quantization error generated at that time to peripheral pixels to generate an average density.

The isolated point discrimination circuit 3 checks the value corresponding to the output density at the time of outputting the pixel of interest and its peripheral pixels,
A determination means is configured to determine whether or not the pixel of interest is in an isolated point state in which the output density is higher than that of the peripheral pixels. That is, the isolated point determination circuit 3 checks the input image data for a value corresponding to the output density of the pixel of interest and its peripheral pixels at the time of output, and determines the presence or absence of an isolated point. The method of discriminating an isolated point in the isolated point discriminating circuit 3 is as follows.

As shown in FIG. 2, the determination of an isolated point is set in advance for a pixel of interest (indicated by a black square) with respect to the input image data and a peripheral pixel (indicated by an open square) consisting of eight pixels around the pixel of interest. It is performed in comparison with the threshold value (Th).

For example, if the gradation of the input image is 8 bits (0 to 0
255), four thresholds of Th1 = 255, Th2 = 10, Th3 = 10, and Th4 = 0 are set, and these are compared with the target pixel and peripheral pixels. Then, when the conditions of Th1 ≧ pixel of interest ≧ Th2 and Th3 ≧ sum of peripheral pixels ≧ Th4 are satisfied, it is determined that the pixel of interest is in an isolated point state in which the output density is higher than that of the peripheral pixel. That is, when the target pixel = 10 to 255 (gray to black pixel) and the total sum of the peripheral pixels is around 0 (most white pixels from 0 to 10), the target pixel is determined to be an isolated point.

When calculating the total sum of the peripheral pixels, not only the total sum of all eight pixels around the target pixel is calculated, but it is also preferable to be able to select the peripheral pixel to be calculated. For example, when calculating the sum of two pixels or six pixels located vertically in the sub-scanning direction of the pixel of interest, or when calculating the sum of two pixels vertically located in the sub-scanning direction of the pixel of interest and left and right along the main scanning direction. In the case of calculating the total of 4 pixels of 2 pixels in total, the total can be set from minimum = 1 total of pixels to maximum 8 total pixels.

The threshold value which is a criterion for determining such an isolated point can be set, for example, by providing threshold value setting means in the isolated point discriminating circuit 3 and based on the threshold value set in the threshold value setting means. Note that the set value of the threshold value is not limited to the above example, and can be changed appropriately according to the input image data.

When the pixel of interest is determined to be an isolated point in this way, the isolated point correction circuit 4 makes a correction to increase the output density of the peripheral pixels other than the pixel of interest. In the isolated point correction circuit 4, for example, in the above example,
For the peripheral pixel (0) of the pixel of interest (255) determined to be an isolated point, the input gradation level is added by 10 levels, for example, as shown below, and a correction for increasing the output density is added.

[0033]       0 0 0 10 10 10       0 255 0 → 10 25 5 10       0 0 0 10 10 10

As a result, as shown in FIG. 3A, the target pixel in the state of an isolated point is output as much as the overlap with the adjacent pixel (peripheral pixel) as shown in FIG. 3B. As a result, the output density at the isolated point increases. As a result, the development potential level is exceeded, and the reproducibility (output stability) of isolated points is improved. Therefore, the difference between the image density set as a patch and the output density at the isolated point can be eliminated, and the problem that the output density at the isolated point becomes light can be solved, and a higher quality image can be obtained. Like

The correction value added to the peripheral pixels is set to such a value that the corrected peripheral pixels do not reach the development potential level. As a result, the peripheral pixels to which the correction has been applied are not developed, and the dots at isolated points do not become huge.

Further, as shown in FIG. 4A, since the isolated point has a small potential area, the toner is not stably loaded in the case of electrophotography, resulting in poor reproducibility. b)
As shown in (4), the potential of the peripheral pixels to which the correction has been applied rises slightly, so that the area of the portion where the potential has risen expands at the isolated point, and the toner with a certain volume is in a stable and easy-to-carry state. Can be produced and is particularly effective in the case of electrophotography using toner.

When the target pixel is an isolated point, the correction method for increasing the density of the target pixel itself does not change the fact that the target pixel is an isolated point even if the density of the target pixel itself is increased. However, in the target pixel, the area of the portion where the potential has risen due to the correction remains unchanged and remains small. Therefore, in the case of electrophotography, the toner is not stably applied, and the above-described reproducibility improving effect cannot be expected.

The correction of such isolated points works particularly effectively for the gradation expression of the highlight part in the image, especially for the image stability of the highlight when multi-valued error diffusion is used.

In this case, by setting Th1 = 80 Th2 = 20 Th3 = 10 Th4 = 0 as the threshold value, the target pixel = 20 to 80 (gray pixel) and the sum total of eight peripheral pixels is around zero. In the case of (almost white pixels), the pixel of interest is determined to be an isolated point by the isolated point determination circuit 3. Therefore, in the isolated point correction circuit 4, as shown below, for example, 20 levels of the input gradation level are added to the peripheral pixels around the pixel of interest (80) to correct the output density.

[0040]       0 0 0 20 20 20       0 80 0 → 20 80 20       0 0 0 20 20 20

As a result, the target pixel overlaps with the adjacent pixel (peripheral pixel) in the same manner as described above, so that the reproducibility (output stability) of the isolated point is improved. Therefore, the isolated point of the highlight having a low potential and a small area can be effectively corrected, and the potential rising area is widened, so that the toner can be loaded well in the case of electrophotography, and the stability of the output image is remarkably improved. Improve to.

In this case, even if the peripheral pixels are developed, the isolated points of highlight often appear in an image for which resolution is not required, so that there is no particular problem.

In the isolated point correction circuit 4, it is also preferable to perform correction only when the pixel of interest or the peripheral pixel is in a predetermined density range. For example, in the above setting, the threshold value of the pixel of interest is defined as the halftone (20 to 80). Therefore, in the case of the printer characteristic that the correction of the isolated point is not required in the high density portion, the isolated point correction is performed only for the halftone. It becomes possible to act. Further, the effect of isolated point correction can be expected even if the peripheral pixels to be added are selectively (for example, only in the vertical and horizontal directions).

Further, in order to prevent the so-called "dust" existing on the background of the original (white) from being emphasized, it is necessary to set Th1 = 80 Th2 = 10 Th3 = 10 Th4 = 1 as a threshold value. When the number of pixels is 10 to 80 (gray pixel), and all the eight surrounding pixels are 0, the total sum is smaller than Th4, so the isolated point determination circuit 3
Since the pixel of interest is not judged to be an isolated point in, the isolated point correction circuit 4 does not correct the isolated point. Therefore, it is possible to prevent dust on the background of the document (white) from standing out.

As a method of adding the correction, regardless of the isolated point, the input gradation level (0 level-offset) equal to or lower than the development potential level is applied to the entire surface of the original background level, so that the isolated point Although the same effect can be intended, in this method, in addition to increasing the density other than the isolated point, in the case of the electrophotographic method or the inkjet method, the writing operation for outputting an image, that is, the laser or the piezoelectric method is performed. Although there is a drawback that the EMI level rises remarkably due to electrical noise generated when driving the element, according to the present invention, the isolated point correction circuit 4 is used.
Then, as described above, since the correction is applied only to the peripheral pixels around the target pixel which is determined to be the isolated point by the isolated point determination circuit 3, the input gradation level added by the correction is partial, For example, if the entire surface including the pixel of interest = 0, it is not judged as an isolated point and no correction is applied, so that the above-mentioned drawback does not occur.

The above effect is approximately established even in the algorithm of adding / subtracting only the pixel of interest.

For example, by setting the thresholds as Th1 = 5 Th2 = 0 Th3 = 255 Th4 = 10, the pixel of interest = 0 to 5 (almost white pixel) and the total sum of 8 peripheral pixels = 10 to 255 ( When there are a relatively small number of black pixels), the isolated point determination circuit 3 can determine that there is a high possibility that there are isolated points around the pixel of interest. In this case, as shown below, by adding, for example, 10 levels of the input gradation level to the pixel of interest, it is possible to improve the reproducibility (output stability) of the isolated point due to the overlap with the adjacent pixel (peripheral pixel). it can.

[0048]       0 0 0 0 0 0   255 0 0 → 255 10 0       0 0 0 0 0 0

This has the same effect when gray pixels are dispersed as shown below.

[0050]       0 0 80 0 0 80     80 0 → 80 10 0       0 0 0 0 0 0

Such patterns often occur when multi-valued error diffusion is applied.

After the correction by the isolated point correction circuit 4 as described above, the gamma correction circuit 5 performs gamma correction according to a predetermined gamma characteristic. At this stage, since the isolated point is corrected by the isolated point correction circuit 4 as described above, the problem that the output density of the isolated point becomes lighter no longer occurs.

After that, the gamma-corrected image data is adjusted to a predetermined timing suitable for image recording in the timing adjustment circuit 6, and then is set to 1 in the PWM conversion circuit 7.
It is changed to a control signal for controlling pulse width modulation (PWM) that controls the lighting time of the laser for each pixel. The PWM conversion circuit 7 corrects the output density by controlling the lighting time of the laser for each pixel based on the correction made by the isolated point correction circuit 4, and writes a desired image on the recording medium. I do. Therefore, the PWM conversion circuit 7 constitutes a correction means together with the isolated point correction circuit 4 in the present invention.

The isolated point discrimination circuit 3 determines whether or not the target pixel is in an isolated point state in which the output density is higher than that of the peripheral pixels, and the target pixel is a white isolated point whose output density is lower than that of the peripheral pixels. It is also possible to judge whether or not the state.

As a method of determining the white isolated point, for example, thresholds are set as Th1 = 200 Th2 = 20 Th3 = 2040 (255 × 8) Th4 = 1785 (255 × 7), and the target pixel = 20 to 200 (gray pixel) and all eight peripheral pixels are close to black pixels,
The isolated point determination circuit 3 determines that this pixel of interest is an isolated point.

Next, in the isolated point correction circuit 4, as shown below, by subtracting, for example, 20 levels of the input gradation level from the pixel of interest, white isolated points are generated due to overlap with adjacent pixels (peripheral pixels). It is possible to prevent the density of the image from increasing and improve the omission of the image.

[0057]     255 255 255 255 255 255 255     255 80 255 → 255 60 255     255 255 255 255 255 255 255

Even in this case, it is also preferable that the isolated point correction circuit 4 performs the correction only when the target pixel or the peripheral pixels are in the predetermined density range. This makes it possible to prevent the isolated point correction from being applied when the printer characteristic does not require the correction of the isolated point.

In the above description, the isolated point determination circuit 3 is used.
When it is determined that the pixel of interest is an isolated point in
This is a method of adding or subtracting the correction value of the input gradation level to the peripheral pixels in the isolated point correction circuit 4, but instead of adding or subtracting the correction value of the input gradation level by the isolated point correction circuit 4 as described above, A plurality of gamma tables are prepared in the gamma correction circuit 5, and when the isolated pixel determination circuit 3 determines that the pixel of interest is / is not an isolated point, the gamma correction circuit 5 uses, for example, peripheral pixels. Alternatively, the gamma table to be used may be switched (see FIG. 5).

Thus, according to the level of the input image,
It is possible to easily change the addition / subtraction level.

Further, a changeover switch for switching whether or not the isolated point discriminating circuit 3 and the isolated point correcting circuit 4 are made to function may be provided so that they can be switched as needed.

Further, although the case of a monochrome printer is assumed in the above description, the image processing apparatus and the image processing method according to the present invention can be similarly applied to a color printer. Further, it is needless to say that the present invention is also applicable to a so-called copier in which a scanner for reading a document is combined.

[0063]

According to the first and fifth aspects of the present invention, an image processing apparatus capable of obtaining a higher quality image by eliminating the difference between the image density set in the form of a patch and the output density at an isolated point. And an image processing method can be provided.

Further, it is possible to provide an image processing apparatus and an image processing method which can solve the problem of instability of an output image particularly when a highlight part is output in multiple gradations and can obtain a higher quality image. it can.

Further, according to the inventions of claims 2 and 6, the reproducibility of the white spots existing in the high density portion is improved,
It is possible to provide an image processing apparatus and an image processing method capable of improving so-called image dropout (self-image reproducibility in a black peripheral portion) and obtaining a higher quality image.

Further, according to the inventions of claims 3 and 7, it is possible to prevent the isolated point correction from being applied in the case of a printer characteristic in which the correction of the isolated point is not required.

According to the invention described in claims 4 and 8, it is possible to correct the output density for each pixel.

[Brief description of drawings]

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

FIG. 2 is a diagram illustrating a target pixel and peripheral pixels.

FIG. 3A is a diagram illustrating a target pixel in an isolated point state, and FIG. 3B is a diagram illustrating a target pixel whose output is increased by an amount of overlap with an adjacent pixel (peripheral pixel).

FIG. 4A and FIG. 4B are views for explaining the electric potential and the toner riding state.

FIG. 5 is a diagram showing an example of a gamma table.

FIG. 6A is a diagram showing a state in which a patch-shaped pattern output on a photoconductor is measured, and FIG. 6B is a diagram showing a correction curve according to an input gradation level.

FIG. 7 is a diagram illustrating a relationship between one pixel in image processing and one optical pixel.

FIG. 8 is a diagram illustrating a state in which the overall density is increased due to the overlap with adjacent pixels.

FIG. 9 is a diagram illustrating a state in which the output density exceeds the development potential level due to the overlap with an adjacent pixel when the output density is near the development potential level.

FIG. 10 is a diagram illustrating a state in which when a peripheral pixel has a high density and a target pixel is light (white isolated point), the output density is higher than a desired output density due to an overlap with an adjacent pixel.

[Explanation of symbols]

1: buffer 2: Multilevel error diffusion circuit 3: Isolated point discrimination circuit 4: Isolated point correction circuit 5: Gamma correction circuit 6: Timing adjustment circuit 7: PWM conversion circuit

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Claims (8)

[Claims] 1. In an image processing apparatus for outputting digital image data to an image output apparatus, a value corresponding to the output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel has a higher output density than its peripheral pixels. It is characterized by further comprising: a determination unit that determines whether or not the state is an isolated point, and a correction unit that, when the pixel of interest is an isolated point, performs correction to increase output density for peripheral pixels other than the pixel of interest. Image processing device. 2. In an image processing apparatus for outputting digital image data to an image output apparatus, a value corresponding to the output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel has a lower output density than its peripheral pixels. Image processing characterized by comprising a judging means for judging whether or not the state is a white isolated point, and a correcting means for adding a correction for lowering the output density to the target pixel when the target pixel is a white isolated point. apparatus. 3. The image processing apparatus according to claim 1, wherein the correction means performs the correction only when the pixel of interest or the peripheral pixel is in a predetermined density range. 4. The correction means corrects the output density by pulse width modulation (PWM) for controlling the lighting time of the laser for each pixel.
The image processing device described. 5. In an image processing method of an image processing apparatus for outputting digital image data to an image output apparatus, a value corresponding to an output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel is higher than the peripheral pixels. An image processing method, comprising: determining whether or not an output point is an isolated point with a high output density, and adding correction to increase output density to peripheral pixels other than the target pixel when the target pixel is an isolated point. 6. An image processing method of an image processing apparatus for outputting digital image data to an image output apparatus, wherein a value corresponding to an output density of a target pixel and its peripheral pixels at the time of output is checked, and the target pixel is higher than the peripheral pixels. An image processing method, comprising: determining whether or not a white isolated point has a low output density, and correcting the output density of the target pixel when the target pixel is a white isolated point. 7. The image processing method according to claim 5, wherein the correction means performs the correction only when the pixel of interest or the peripheral pixel is in a predetermined density range. 8. The correction means corrects the output density by pulse width modulation (PWM) for controlling the lighting time of the laser on a pixel-by-pixel basis.
The described image processing method.