JP4177486B2 - Image processing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention, for example, reads an image of a document by an image input means such as a scanner, inputs an image signal, performs signal processing on the input image signal in accordance with the output characteristics of the image output means, and then In an image forming apparatus such as a digital copying machine that outputs an image signal onto a sheet by image output means such as an electrophotographic printer, the signal processing according to the output characteristics of the image output means is performed on the input image signal. The present invention relates to an image processing apparatus.
[0002]
[Prior art]
For example, a process of converting an image signal input by a scanner or the like into an image signal that matches the output characteristics of an image output device (image output means) is called gradation processing and is widely used. For example, when an 8-bit input image signal read by a scanner is output by an image output device having only a multi-value expression capability of about 4 values per pixel, a multi-value is used to express the gradation of the input image signal. A gradation process such as a pseudo gradation process such as a dither method or a multi-value error diffusion method is often used.
[0003]
In recent years, there is a gradation process that considers not only the multi-value expression capability of an image output device but also the gradation expression capability considering the stability of the image output device (see, for example, Japanese Patent Application No. 7-204714). When quantizing the value of each pixel during multi-level error diffusion, look at the surrounding pixel pattern to determine whether the pattern is a pattern that can be output stably by the image output device. Is a method of converting to a stable pattern. However, these techniques are limited to the case where the input image signal is higher than the image multilevel representation capability, and are not compatible with the case where the number of gradations of the image signal is lower than the image multilevel representation capability. .
[0004]
Recently, with the development of multimedia technology, the opportunity to pass image signals between image input / output devices is increasing. In that case, another image output device outputs an image signal that has undergone gradation processing for that device by another image output device. In this case, if the gradation processing has already been performed and these are output as they are, the image output device Does not match and is recorded unstable.
[0005]
For example, when an image that has been subjected to binary error diffusion of 600 dpi with an image input / output device is recorded with an image output device that cannot stably record a binary isolated point of 600 dpi, 600 dpi isolated points frequently appear in the image. Rough noise is generated in the highlight image, gradation is skipped, pseudo contour and density are changed, and the image quality is remarkably deteriorated. However, the conventional technique cannot cope with an input image signal whose number of gradations has already decreased.
[0006]
[Problems to be solved by the invention]
As described above, in the conventional technique, when gradation processing having different characteristics is performed and the number of gradations is reduced, there is a problem in that noise is generated when an image signal is recorded. Therefore, according to the present invention, gradation processing having different characteristics has already been performed, the number of gradations is reduced, and an image signal of an isolated black pixel that is unstable when output as it is can be output stably. An object is to provide an image processing apparatus.
[0007]
[Means for Solving the Problems]
The present invention includes a peripheral pixel state observation unit and a determination unit, and determines whether or not the density of each pixel should be converted with respect to the input image signal, and the input image signal based on the peripheral pixel state signal representing the pixel state of the periphery of the target pixel specified as the pixel density pattern in the window by the pixel state observing unit has a higher concentration than the target pixel is the threshold value Th1, a periphery of a target pixel pixel and pixel density conversion condition determining means for determining whether or not an isolated black pixels as is at a lower concentration than all the threshold Th2, and a density conversion unit and the selector unit, the input image signal the concentration of pixels determined to be the isolated black pixels to be converted to concentration by the pixel density conversion condition determining means, and density conversion by the density conversion unit is a look-up table or calculator A pixel density conversion means for selecting either one of the conversion signal or the input image signal density conversion by the density conversion unit, is the density conversion by the input image signal and the pixel density conversion means The density error accumulating unit and the density error which obtain the accumulated density error signal by adding the density conversion error signal from the subtracting unit and the subtracting unit which takes the difference from the converted image signal and outputs the difference as the density conversion error signal It is determined whether or not to compensate the accumulated density error signal from the density error accumulation unit and output the density compensation signal from the accumulated density error signal obtained from the accumulation unit and the peripheral pixel state signal from the peripheral pixel state observation unit. storage density and a prediction error decision unit, and a concentration change detecting means for detecting the concentration variation of the density conversion transform signal in the density conversion unit of the pixel density conversion unit, the density conversion unit and determine that And a section, when the concentration variation detected by said concentration change detecting means is within a predetermined condition, to compensate for the density variation of the density conversion transform signal by the density conversion unit of the pixel density conversion means It converts the image density of the pixel of interest and outputs one of the density conversion transform signal by the density conversion unit compensation pixel signal or the pixel density conversion unit, the accumulation density error from the density error accumulation unit It is an object of the present invention to provide an image processing apparatus comprising: the density compensation signal that compensates the signal; and density compensation means that outputs either the density-converted image density of the target pixel.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First, before describing a configuration example of an image processing apparatus according to an embodiment of the present invention, a processing method of the image processing apparatus will be briefly described.
[0009]
An outline of a processing example by the image processing apparatus according to the embodiment of the present invention will be described with reference to FIG. FIG. 1A shows an image before processing, which is composed of isolated black pixels (for example, pixel A). In an image forming apparatus in which isolated black pixels are not stably recorded (formed), noise such as unevenness occurs when this image is recorded.
[0010]
Therefore, as shown in FIG. 1B, an isolated black pixel such as the pixel A is detected, and the next processing pixel (usually, the pixel is processed serially from the left to the right. Pixel) white pixel A ′ is density-converted to generate a half dot. As a result, the isolated black pixel is expanded. Since this conversion processing times × A ′ is darker than the input image, this darkened amount E is counted, and the counted amount E is 100% in density, that is, more than the amount corresponding to one isolated black pixel. In this case, the isolated black pixel is changed to a white pixel as shown in FIG. At the same time, the density of whitened from E is reduced by 100%.
[0011]
As a result, the isolated black pixels are expanded, a stable image is recorded, and the density is compensated. The reason why the isolated black pixel becomes stable when expanded will be described with reference to FIG. Here, as an example of the image forming apparatus, an image forming apparatus such as a digital copying machine having an electrophotographic system using a laser beam as an exposure system will be described. The image forming apparatus scans a charged photoconductive drum with a laser beam controlled to be turned on / off, discharges only a portion where the laser beam is lit, forms an electrostatic latent image on the photoconductive drum, In this system, toner particles charged in the opposite polarity to the photosensitive member of the photosensitive drum are attached to the latent image by electrostatic attraction, thereby developing the latent image.
[0012]
FIG. 2A shows the amount of light energy on the photosensitive drum for a short pulse a and a pulse a + a ′ obtained by adding a pulse a ′ thereto. Due to the electrostatic attraction mechanism, a normal electrophotographic image forming apparatus has two energies E1 and E2 in which the toner is extremely unstable.
[0013]
FIGS. 2B and 2C schematically show the state of toner attached to the exposure energies E1 and E2. Since the spot of the laser beam has a spread, the toner adheres in a circular shape within a short range of the lighting pulse. Here, the gray region corresponds to a region where toner adhesion between the exposure energies E1 and E2 in FIG. 2A is unstable. As shown in FIG. 2B, in the case of only the short pulse a, all the toner adheres in an unstable manner, whereas the pulse a ′ is applied as shown in FIG. In addition, in the expanded pulse a + a ′, a region where the toner is stably attached is formed at the center, and the region where the toner is unstablely attached is reduced. For this reason, it is advantageous in terms of stability to expand the pulse.
[0014]
Next, an image processing apparatus according to an embodiment of the present invention will be described.
FIG. 3 schematically shows the configuration of the image processing apparatus 1 that performs processing based on the processing method described above. The image processing apparatus 1 includes a pixel density conversion condition determination unit 2 as a pixel density conversion condition determination unit, a pixel density conversion unit 3 as a pixel density conversion unit, a density error calculation unit 4 as a density change detection unit, and a density It comprises a density compensation unit 5 as compensation means.
[0015]
The pixel density conversion condition determination unit 2 serially receives each pixel signal of the image signal 6 input by an image input means such as a scanner, determines whether or not the density of each pixel should be converted, and the determination result 7 Is output. Also, a peripheral pixel state signal 8 representing a pixel state around the target pixel is output.
[0016]
The pixel density conversion unit 3 receives each pixel signal of the input image signal 6 serially, converts the pixel density of the input image signal 6 based on the determination result 7 of the pixel density conversion condition determination unit 2, and converts the converted image Signal 9 is output.
[0017]
The density error calculation unit 4 receives the input image signal 6, the converted image signal 9 from the pixel density conversion unit 3, and the peripheral pixel state signal 8 from the pixel density conversion condition determination unit 2, and receives the input image signal 6 and the image signal 9 are calculated, and the density compensation signal 10 is output based on the difference (error).
[0018]
The density compensation unit 5 performs density compensation by converting the pixel density of the converted image signal 9 from the pixel density conversion unit 3 based on the density compensation signal 10 from the density error calculation unit 4, and outputs an output image signal. 11 is output.
[0019]
FIG. 4 shows a configuration example of the pixel density conversion condition determination unit 2. The pixel density conversion condition determination unit 2 includes a peripheral pixel state observation unit 12 and a determination unit 13. The peripheral pixel state observation unit 12 receives the input image signal 6 and outputs a peripheral pixel state signal 8. The determination unit 13 receives the peripheral pixel state signal 8 from the peripheral pixel state observation unit 12 and outputs a determination result 7.
[0020]
The peripheral pixel state observation unit 12 will be described with reference to FIG. The peripheral pixel state observation unit 12 has N line buffers that form a window of M × N pixels centering on the target pixel 15 in the input image signal 6. The peripheral pixel state signal 8 at that time is a pixel density pattern in an M × N pixel window.
[0021]
As an example of processing in the determination unit 13, it is output whether or not the pixel (preprocessed pixel) on the left side of the target pixel 15 is an isolated black pixel that is unstable in recording. The condition for the isolated black pixel is that the pixel has a density higher than a certain threshold value Th1, and pixels whose peripheral pixels are all lower than a certain threshold value Th2 are defined as isolated black pixels. When the input image signal 6 is binary, the isolated black pixel is a black pixel surrounded by white pixels.
[0022]
FIG. 6 shows a configuration example of the pixel density conversion unit 3. The pixel density conversion unit 3 includes a density conversion unit 16 and a selector unit 17. The density conversion unit 16 converts the density of the input image signal 6 of the target pixel using a known technique such as a look-up table (LUT) or a four arithmetic unit, and outputs the converted signal 18. The selector unit 17 switches between the input image signal 6 and the conversion signal 18 based on the determination result 7 of the pixel density conversion condition determination unit 2 and outputs the image signal 9 after conversion.
[0023]
FIG. 7 shows a configuration example of the density error calculation unit 4. The density error calculation unit 4 includes a subtraction unit 19, a density error storage unit 20, and a storage density error determination unit 21. The subtractor 19 takes the difference between the input image signal 6 and the converted image signal 9 and outputs the difference as a density conversion error signal 22. The density error accumulating unit 20 adds the density conversion error signal 22 from the subtracting unit 19 by a known addition technique and outputs it as the accumulated density error signal 23.
[0024]
The density error accumulation unit 20 also receives the density compensation signal 10 from the accumulation density error determination unit 21 and resets the accumulation density error signal 23 when the density compensation signal 10 is on. As an example of resetting, in the example of FIG. 1, 100% density is subtracted from the accumulated density error signal 23.
[0025]
The accumulated density error determination unit 21 receives the accumulated density error signal 23 and the peripheral pixel state signal 8, and when the accumulated density error signal 23 has a certain condition, in the example of FIG. 1, the density exceeds 100%, and The density compensation signal 10 is output in a specific state in which the peripheral pixel state signal 8 is in a certain state, for example, in a state where the target pixel 15 is a white pixel and there are no black pixels around it.
[0026]
FIG. 8 shows a configuration example of the density compensation unit 5. The density compensation unit 5 includes a density conversion unit 24 and a determination unit 25. The density conversion unit 24 performs density conversion on the image signal 9 after conversion from the pixel density conversion unit 3 using a lookup table (LUT) or a four arithmetic unit, which is a known technique, and outputs it as a compensated image signal 26. As an example of conversion, a process of converting a black pixel into a white pixel as shown in FIG.
[0027]
The determination unit 25 selects the compensation image signal 26 from the density conversion unit 24 or the converted image signal 9 from the pixel density conversion unit 3 based on the density compensation signal 10 from the density error calculation unit 4, and outputs an output image signal. 11 is output. In this case, the compensated image signal 26 is selected only when the density compensation signal 10 is on, and the converted image signal 9 is selected otherwise.
[0028]
FIG. 9 is a flowchart showing the above-described processing flow. This flowchart includes a pixel density conversion condition step, a pixel density conversion step, a density error calculation step, and a density compensation step.
[0029]
That is, first, in step S1, the density error E and the flag flag are initialized, and the process proceeds to step S2. In step S2, it is determined whether or not the preprocessed pixel of the target pixel (left adjacent pixel) is an isolated black pixel. If the preprocessed pixel of the target pixel is determined to be an isolated black pixel, the process proceeds to step S3. In step S3, the white pixel of the target pixel is converted into a half dot of density H, and the process proceeds to step S4.
[0030]
In step S4, the density error E resulting from the density conversion in step S3 is accumulated, and in step S5, it is determined whether or not the density error E has reached 100% worth a black pixel. As a result of this determination, if the density error E has reached 100% worth a black pixel, the flag flag is set to “1” in step S6, and if the density error E has not reached 100% worth a black pixel, In step S7, the flag flag is set to “0”, and the process ends.
[0031]
If it is determined in step S2 that the preprocessed pixel of the target pixel is not an isolated black pixel, the process proceeds to step S8. In step S8, it is determined whether the target pixel is an isolated black pixel. If it is determined that the target pixel is an isolated black pixel, the process proceeds to step S9. In step S9, it is determined whether or not the flag flag is “1” (that is, whether or not the density error E has reached 100% worth a black pixel), and the flag flag is “1”. If yes, go to Step S10.
[0032]
In step S10, the black pixel of the target pixel is converted into a white pixel, and the process proceeds to step S11. In step S11, a process of reducing the density 100% corresponding to the black pixel from the density error E is performed, and the process proceeds to step S5.
[0033]
If it is determined in step S8 that the target pixel is not an isolated black pixel, or if the flag flag is not “1” in step S9, the process proceeds to step S5.
[0034]
Through the above processing, image signals containing isolated black pixels that become unstable when recorded as they are can be stably recorded by expanding the isolated black pixels, and the macroscopic density is compensated by the density compensation function. can do.
[0035]
Therefore, for example, it is possible to stably form and output an image that has already been subjected to gradation processing corresponding to an image forming apparatus having different characteristics, particularly an image signal in which the gradation level of the image has been lowered by gradation processing.
[0036]
【The invention's effect】
As described above in detail, according to the present invention, gradation processing having different characteristics has already been performed, the number of gradations has been reduced, and an image signal that is unstable when output as it is can be stably output. It is possible to provide an image processing apparatus capable of performing the above.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining an outline of a processing example performed by an image processing apparatus according to an embodiment of the present invention;
FIG. 2 is a diagram for explaining a reason why an isolated black pixel becomes stable when expanded.
FIG. 3 is a block diagram schematically showing the configuration of the image processing apparatus according to the embodiment of the present invention.
FIG. 4 is a block diagram illustrating a configuration example of a pixel density conversion condition determination unit.
FIG. 5 is a diagram for explaining a peripheral pixel state observation unit.
FIG. 6 is a block diagram illustrating a configuration example of a pixel density conversion unit.
FIG. 7 is a block diagram illustrating a configuration example of a density error calculation unit.
FIG. 8 is a block diagram illustrating a configuration example of a density compensation unit.
FIG. 9 is a flowchart illustrating a processing flow of the image processing apparatus.
[Explanation of symbols]
1 …… Image processing device,
2... Pixel density conversion condition determination unit (pixel density conversion condition determination means)
3. Pixel density conversion unit (pixel density conversion means)
4 …… Density error calculation unit (density change detection means),
5. Concentration compensation unit (concentration compensation means)
12 ...... Peripheral pixel state observation unit,
13 …… Determination part,
16: Density converter
17 …… Selector section,
19 …… Subtraction part,
20: Concentration error accumulation unit,
21 …… Accumulated density error determination unit,
24 …… Density converter
25 …… Determining unit.

Claims (3)

A peripheral pixel state observation unit and a determination unit, the result of determining whether the density of each pixel should be converted with respect to the input image signal, and the peripheral pixel state observation unit for the input image signal based on the peripheral pixel state signal representing the pixel state of the periphery of the target pixel specified as the pixel density pattern in the window by the pixel of interest has a higher density than the threshold value Th1, the pixel around the target pixel are all and a pixel density conversion condition determining means for determining whether or not an isolated black pixel such that at lower concentrations than the threshold value Th2,
A density conversion unit and a selector unit, and a look-up table or an arithmetic unit for calculating a density of a pixel which is determined to be an isolated black pixel whose density is to be converted by the pixel density conversion condition determination unit with respect to an input image signal a pixel density conversion means for density conversion, selects and outputs either one of the conversion signal or the input image signal density conversion by the density conversion unit by the density conversion unit is,
The difference between the input image signal and the converted image signal converted by the pixel density conversion means, and outputs the difference as a density conversion error signal. The density conversion error signal from the subtractor 19 From the density error accumulator obtained from the density error accumulator and the density error signal obtained from the density error accumulator and the peripheral pixel status signal from the peripheral pixel status observation unit. An accumulated density error determination unit that determines whether or not to output a density compensation signal by compensating the error signal, and detects a density change amount of the converted signal converted by the density conversion unit of the pixel density conversion unit Concentration change detecting means for
And a a determination unit density converter, the concentration of the when the concentration variation detected by the concentration change detecting means is in a predetermined condition, the converted signal density conversion by the density conversion unit of the pixel density conversion means converts the image density of the pixel of interest and outputs one of the density conversion transform signal by the density conversion unit compensation pixel signal or the pixel density conversion means to compensate for variation, the density error accumulation unit Density compensation means for outputting either one of the density compensation signal that compensates the accumulated density error signal from the image density of the target pixel that has undergone density conversion ;
An image processing apparatus comprising:   2. The accumulated density error determining unit of the density change detecting unit determines whether or not a result obtained by adding the output of the subtracting unit by the density error accumulating unit reaches a predetermined condition. The image processing apparatus described. The determination unit of the density compensation unit is configured to compare the conversion signal converted by the density conversion unit of the pixel density conversion unit based on the density compensation signal from the density difference determination unit of the density change detection unit. The image processing apparatus according to claim 1 , wherein density conversion is performed by (LUT) or by calculation .

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