JP2009189003A - Image processing apparatus, image processing method, and image forming apparatus - Google Patents

Abstract

An image processing apparatus, an image processing method, and an image forming apparatus capable of performing smoothing processing and enhancement processing according to an attribute of an image signal with a simple configuration.
An image area identification signal generation unit generates an image area identification signal indicating an attribute of an image. The control unit 10 inputs the image signal to the smoothing processing unit 201 and the input signal conversion unit 202, respectively. The smoothing processing unit 201 extracts a low frequency component from the image signal and performs smoothing processing. The input signal conversion unit 202 performs gradation conversion (compression or expansion) suitable for the attribute of the image signal. Next, the enhancement amount extraction unit 203 extracts a high frequency component generated at the edge of the character or the patch from the image signal subjected to the gradation conversion, and removes a low frequency component generated at a flat portion of the patch. By doing so, the emphasis component that emphasizes the edge portion is extracted. Next, the enhancement amount conversion unit 204 converts the obtained enhancement amount of the edge according to the size. The signal synthesis unit 205 synthesizes the smoothed image signal and the converted edge enhancement amount.
[Selection] Figure 2

Description

  The present invention relates to an image processing apparatus, an image processing method, and an image forming apparatus that correct sharpness of an image signal.

  In an image processing apparatus incorporated in a digital color multifunction peripheral, a color printer, or the like, filter processing is one of the processes for correcting a read image. Moire by controlling the gain of periodic structures such as halftone dots according to image spatial frequency by filtering (smoothing of halftone dots by weakening the enhancement of the image spatial frequency corresponding to the number of halftone lines of the printed material as a document) It is possible to obtain a suppression effect and an emphasis (sharpness) effect on character edges in continuous tone.

  As an image signal enhancement (sharpness adjustment) method in filter processing, N × N filter coefficients corresponding to a filter matrix of N × N pixels are recalculated for the sharpness adjustment value designated by the user input, and the corresponding ASIC A technique for setting a register is known (for example, see Patent Document 1). In Patent Document 1, filter processing is performed with an appropriate filter coefficient on image signals having different sampling rates, and moire can be suppressed without requiring excessive unsharpness processing. It is disclosed.

Japanese Patent Laid-Open No. 2003-248821

  However, in the case of the conventional filter processing, there is a restriction on the amount of enhancement because smoothing and sharpness adjustment are performed with only a single filter, and there is an effect of uniformly enhancing portions where the change in the image signal in continuous tone is large. there were. Therefore, the character portion is preferably emphasized, but there are cases where over-emphasis is applied even to a portion that is not so emphasized, such as a patch edge (an edge portion of a uniform density patch region). On the contrary, if adjustment is made to suppress over-emphasis, this time, the image signal becomes lower to the base level of the image signal, and the image becomes rough. Further, the technique of Patent Document 1 is complicated because the filter coefficient must be changed depending on the resolution, requires more parameters, and requires a huge memory.

  The present invention has been made in consideration of such points, and an image processing apparatus, an image processing method, and an image forming apparatus capable of performing smoothing processing and enhancement processing according to attributes of an image signal with a simple configuration. The purpose is to provide.

  An image processing apparatus according to the present invention includes an identification signal generation unit that generates an image area identification signal that identifies an attribute of the image signal in the image processing apparatus that performs smoothing and enhancement processing of an image signal, and the identification signal generation unit A smoothing processing unit for smoothing the image signal based on the image area identification signal generated by the step, and further compressing the gradation of the image signal having a gradation lower than a predetermined gradation, An input signal conversion unit that converts the gradation of the image signal so as to further expand the gradation of the image signal having a gradation higher than the predetermined gradation, and a gradation by the input signal conversion unit An enhancement amount extraction unit that extracts an enhancement amount of an edge included in the image signal from the converted image signal, and the enhancement amount is further increased by the magnitude of the enhancement amount extracted by the enhancement amount extraction unit. To be smaller An enhancement amount conversion unit for conversion, and a signal synthesis unit for synthesizing the enhancement amount converted by the enhancement amount conversion unit and the image signal smoothed by the smoothing processing unit are provided. And

  An image forming apparatus according to the present invention includes the image processing apparatus according to claim 1 and an image recording unit that forms an image based on the image signal combined by the signal combining unit.

  An image processing method according to the present invention is an image processing method that performs smoothing and enhancement processing of an image signal, generates an image area identification signal that identifies an attribute of the image signal, and based on the generated image area identification signal , Smoothing the image signal, further compressing the gradation of the image signal having a gradation lower than a predetermined gradation, and applying the compression to the image signal having a gradation higher than the predetermined gradation. Converts the gradation of the image signal so as to further expand the gradation, extracts the enhancement amount of the edge included in the image signal from the image signal subjected to gradation conversion, and extracts the enhancement amount extracted The amount of enhancement is converted so that the enhancement amount is further increased or decreased, and the converted enhancement amount and the smoothed image signal are synthesized.

  According to the image processing apparatus, the image processing method, and the image forming apparatus according to the present invention, the smoothing process and the enhancement process according to the attribute of the image signal can be performed with a simple configuration.

1 is a block diagram showing a schematic configuration of an image forming apparatus according to an embodiment of the present invention. The block diagram which shows schematic structure of the sharpness process part in embodiment same as the above. (A) is a figure which shows the frequency characteristic showing the filter coefficient set to the smoothing process part in embodiment same as the above, (B) is the frequency characteristic showing the filter coefficient set to the enhancement amount extraction part in embodiment same as the above. FIG. The flowchart which shows the operation | movement of the emphasis process in embodiment same as the above. The figure which shows the setting of the gradation conversion of the input conversion part in embodiment same as the above. The figure which shows the enhancement amount of the image signal in embodiment same as the above. The figure which shows the setting of the enhancement amount conversion of the enhancement amount conversion part in embodiment same as the above. The figure which shows the setting of the enhancement amount conversion part by a user input in embodiment same as the above.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a diagram illustrating a system configuration example of an image forming apparatus 1 according to an embodiment of the present invention, for example, a digital MFP (Digital Multi-Function Peripheral). The image forming apparatus 1 functions as a digital color multifunction peripheral. For example, a copy function, a printer function, a scanner function, a FAX function, a storage function, and the like are realized.

  The image forming apparatus 1 includes a reading unit 11, an image processing unit 12, a page memory unit 13, and an image recording unit 14, and a copy function is realized by these configurations. The reading unit 11 optically reads a document with, for example, a three-line CCD sensor, and converts it into red (R), green (G), and blue (B) color digital image signals.

  The image processing unit 12 includes a scanner system image processing unit 21, a printer system image processing unit 22, and a sharpness processing unit 20.

  The scanner system image processing unit 21 performs various image processing such as shading correction for correcting non-uniformity of the signal level in the main scanning direction. Further, when operating as a copy function, the page memory unit 13 converts the primary colors of R, G, and B into, for example, cyan (C), magenta (M), yellow (Y), and black (K) color signals. On the other hand, when it operates as a scanner function, it outputs to the page memory unit 13 as three primary colors of R, G, and B.

  The printer system image processing unit 22 performs image processing for printing, for example, γ correction processing and gradation processing on the image signal output from the page memory unit 13, and outputs the image signal to the image recording unit 14.

  The sharpness processing unit 20 performs smoothing and enhancement (sharpness) processing on the image signal input from the reading unit 11 or the page memory unit 13. Details will be described later.

  The page memory unit 13 temporarily stores the image signal output by the scanner system image processing unit 21 in units of pages, for example, and outputs the image signal to the image processing unit 12 again.

  The image recording unit 14 is a component that prints an image on a recording sheet by, for example, an electrophotographic method, and includes an exposure device, a photosensitive drum, a developing device, and the like (all not shown).

  The image forming apparatus 1 includes a storage unit 15, a printer controller unit 16, a FAX controller unit 17, an electronic data creation unit 18, and an external I / F unit 19. These components are used when the image forming apparatus 1 is operated as a storage function, a printer function, a FAX function, a scanner function, and the like.

  The storage unit 15 is configured by, for example, an HDD (Hard Disk Drive), and stores an image signal read by the reading unit 11 and an image signal input from an external personal computer or the like. The image signal stored in the storage unit 15 can be read out as necessary, printed by the image recording unit 14, or output to the outside as scan data.

  When the image forming apparatus 1 operates as a scanner function, the electronic data creation unit 18 converts the image signal into a predetermined data format and generates scan data. The generated scan data is output to the outside via the external I / F unit 19.

  The FAX controller unit 17 operates the image forming apparatus 1 as a FAX function. The image signal read by the reading unit 11 is converted into a data format for FAX and output to an external telephone line via the external I / F unit 19. In addition, FAX data input from a telephone line can be converted into an image signal, and printing can be performed by the image recording unit 14.

  The printer controller unit 16 performs various controls and various processes necessary for the print function when the image forming apparatus 1 operates as a printer function. The control unit 10 performs control related to the entire image forming apparatus 1.

  FIG. 2 is a diagram illustrating details of the sharpness processing unit 20 in the image processing unit 12. The sharpness processing unit 20 includes an image area identification signal generation unit 200, a smoothing processing unit 201, an input signal conversion unit 202, an enhancement amount extraction unit 203, an enhancement amount conversion unit 204, and a signal synthesis unit 205.

  The image area identification signal generation unit 200 performs at least one of “edge detection processing”, “achromatic color determination processing”, and “halftone determination processing” on the image signal of the pixels constituting the image to be processed. By doing so, an image area identification signal for identifying the type of image formed by each pixel is generated. Here, the image area identification signal generated by the image area identification signal generation unit 200 identifies pixels constituting at least one of “black characters”, “color characters”, “patches”, and “photos”. It is information (attribute) to do. The “patch” refers to an image such as a graphic image in which a certain density is distributed almost uniformly within a predetermined range. For example, for an image signal identified as a character, the values of RGB or CMYK color image signals are compared, and if the signal value difference is within a preset threshold value, it is determined that the character is a black character, and the image area identification signal is A distinction is made between “black letters” and “color letters”. The image area identification signal generated by the image area identification signal generation unit 200 is transmitted to a smoothing processing unit 201, an input signal conversion unit 202, an enhancement amount extraction unit 203, and an enhancement amount conversion unit 204, which will be described later.

  The smoothing processing unit 201 has a low-pass filter, performs a smoothing process on the input image signal, extracts only low frequency components from the image signal, and outputs them.

  The input signal conversion unit 202 is configured to include, for example, a lookup table, and performs conversion that compresses or expands the gradation of the input image signal.

  The enhancement amount extraction unit 203 has a band-pass filter, performs enhancement processing on the image signal whose tone has been converted by the input signal conversion unit 202, and performs processing for extracting the enhancement amount at the edge. Specifically, the sum of the filter coefficients is set to be 0, and only the emphasized component is extracted.

  The enhancement amount conversion unit 204 is configured to include, for example, a lookup table, and converts the edge enhancement amount extracted by the enhancement amount extraction unit 203 nonlinearly.

  Note that the smoothing processing unit 201, the input signal conversion unit 202, the enhancement amount extraction unit 203, and the enhancement amount conversion unit 204 switch parameters according to the image area identification signal generated by the image area identification signal generation unit 200, respectively. Is possible. For example, the smoothing processing unit 201 and the enhancement amount extraction unit 203 change the filter coefficient parameters according to the image area identification signal. FIG. 3A shows an example of frequency characteristics set in the smoothing processing unit 201, and FIG. 3B shows an example of frequency characteristics set in the enhancement amount extraction unit 203. FIG. 3B shows that the filter coefficient of the enhancement amount extraction unit 203 is switched between “for text” and “for photograph”, and different frequency characteristics are set. Since the frequency characteristic of “for text” has a high gain peak, a strong enhancement amount can be obtained. The frequency characteristics for “photographic” have a low gain peak and a low cut-off frequency, so that a high moire suppression effect can be obtained.

  In the sharpness processing unit 20, a smoothing processing unit 201 and an enhancement amount extraction unit 203 are arranged in parallel, and an input signal conversion unit 202 is provided upstream of the enhancement amount extraction unit 203, and an enhancement amount conversion unit 204 is provided downstream. The signal synthesis unit 205 adds and synthesizes the output of the smoothing processing unit 201 and the output of the enhancement amount conversion unit 204 and outputs the result.

  Next, the operation of the sharpness processing unit 20 in the above-described enhancement processing will be described with reference to FIG.

  The control unit 10 inputs an image signal output from the reading unit 11 or the page memory unit 13 to the image area identification signal generation unit 200. The image area identification signal generation unit 200 generates an image area identification signal indicating the attribute of the image (step S101). The generated image area identification signal is sent to the smoothing processing unit 201, the input signal conversion unit 202, the enhancement amount extraction unit 203, and the enhancement amount conversion unit 204 in the sharpness processing unit 20. The attribute of the image is at least one of black characters, color characters, patches, and photographs.

  Further, the control unit 10 inputs the image signal sent from the reading unit 11 or the page memory unit 13 to the smoothing processing unit 201 and the input signal conversion unit 202, respectively (step S103).

  The smoothing processing unit 201 performs smoothing processing by extracting low frequency components from the image signal (step S104). At this time, as shown in the example of FIG. 3A described above, the smoothing processing unit 201 preliminarily sets the filter coefficient of the low-pass filter based on the image area identification signal generated in step S101.

  The input signal conversion unit 202 performs gradation conversion of the image signal using, for example, the gradation conversion characteristics shown in FIG. 5 (step S105). The characteristics shown in FIG. 5 are obtained by further compressing the gradation for an image signal whose gradation is lower than a predetermined gradation, and for the image signal having a gradation higher than the predetermined gradation. The image signal has a characteristic of converting the gradation of the image signal so as to further extend the gradation. Due to the tone conversion characteristics, when the attribute of the image signal is low to medium gradation such as a photograph, the signal range (gradation) of the image signal is compressed, and is less susceptible to the enhancement processing in the subsequent stage. On the other hand, when the attribute of the input image signal is high gradation such as characters, the signal range (gradation) of the image signal is expanded so that sufficient edge enhancement is obtained. The signal range is also extended for patches with high gradation. In this way, conversion (compression or expansion) suitable for the attribute of the image signal is performed according to the setting of FIG. 5 according to the gradation of the image signal.

  Next, the enhancement amount extraction unit 203 extracts a high frequency component generated at the edge of the character or the patch from the image signal subjected to the gradation conversion in step S105, while lowering it generated at a flat portion of the patch. By removing the frequency component, an enhancement component that emphasizes the edge portion is extracted (step S107). At this time, as shown in the example of FIG. 3B described above, the enhancement amount extraction unit 203 is preset with filter coefficients of a bandpass filter based on the image area identification signal generated in step S101.

  The output after passing through the enhancement amount extraction unit 203 becomes a spike-like signal that appears at the edge of a character or patch, as shown by a one-dot chain line in FIG. The magnitude of this spike signal is the amount of enhancement at the edge. From FIG. 6, it can be seen that the emphasis amount (output level) of characters appears large (sharp), but the emphasis amount of patch edges does not appear as large as characters, and the emphasis amount differs depending on the attribute of the image signal. The reason for this is that characters are usually two-dimensionally isolated and the edge of the character has a high frequency component in both the main scanning direction and the sub-scanning direction, whereas the edge of the patch is in the main scanning direction. One of the direction and the sub-scanning direction is discontinuous and has a high frequency component, but the other direction is continuous and thus has a low frequency component. For this reason, the output level of the two-dimensional bandpass filter is higher at the edge of the character than at the edge of the patch.

  Next, the enhancement amount conversion unit 204 converts the enhancement amount of the edge obtained in step S107 according to the size (step S109). FIG. 7 is a diagram illustrating an example of the enhancement amount conversion, in which the horizontal axis is the level of the edge enhancement amount input to the enhancement amount conversion unit 204, and the vertical axis is the post-conversion output output from the enhancement amount conversion unit 204. Is the edge enhancement amount. The enhancement amount conversion characteristic shown in FIG. 7 is a non-linear conversion characteristic. In the region where the input enhancement amount is large, the enhancement amount is not changed (linear conversion characteristic with a slope of 1), while the input enhancement is performed. In a region where the amount is small, the gradient of the conversion characteristic is set small so that the output of the enhancement amount is smaller than the input.

  Generally, even if the edge of a character is emphasized too much, the image does not become so unnatural. Rather, if the edge is sufficiently emphasized, the character image becomes clearer and clearer. On the contrary, if the edge of the patch is emphasized too much, only the edge portion becomes a dark image, so that a better image can be obtained by suppressing the enhancement of the patch edge.

  As described above, the edge enhancement amount output from the enhancement amount extraction unit 203 is larger for normal characters than for patches. The enhancement amount conversion unit 204 outputs the enhancement amount of the character for which an edge enhancement effect is desired as the enhancement amount as it is, and further reduces the enhancement amount of the patch edge for which enhancement is to be suppressed.

  In FIG. 6, the fact that the enhancement amount of the patch edge is further reduced by the enhancement amount conversion unit 204 is indicated by a dotted line in the drawing.

  In FIG. 7, the slope of the conversion characteristic is set to be small only for the region where the input enhancement amount is small, and the patch edge enhancement amount is reduced. However, the conversion is performed only for the region where the input enhancement amount is large. The characteristic slope may be set to be large so that the amount of emphasis on the edge of the character is increased.

  The characteristic of enhancement amount conversion is not limited to that shown in FIG. 7, and various parameter settings may be made according to the image area identification signal. For example, in the case of an image area identification signal for black characters, the reproducibility is regarded as important, so the setting emphasizes characters, and in the case of an image area identification signal for color characters, settings that take into account the balance between reproduction of characters and other parts. If the set value is changed to, the degree of enhancement can be distinguished between black characters and color characters.

  Then, the signal synthesis unit 205 synthesizes the image signal smoothed in step S104 and the edge enhancement amount converted in step S109 (step S111).

  In FIG. 4, the enhancement amount conversion processing in steps S105 to S109 is performed after the smoothing processing in step S104. However, even if the processing in step S104 is performed after the processing in steps S105 to S109, Processing may be performed in parallel with steps S105 to S109.

  Further, the setting of the enhancement amount in the enhancement amount conversion unit 204 can be adjusted by an input from the user. FIG. 8 shows an example of adjustment settings. For example, when the user uses the external I / F unit 19 and designates the sharpness adjustment value as “+2”, the control unit 10 sets the enhancement amount conversion setting in the enhancement amount conversion unit 204 in the sharpness processing unit 20 in FIG. Change to the slope of “+2”. Therefore, for example, when the user wants to output characters more clearly, the setting of enhancement amount conversion can be output according to the user's preference.

  As described above, according to the present embodiment, since the smoothing processing of the image signal and the enhancement processing of the image signal are performed independently, the portions to be emphasized such as characters are emphasized, and the patch edges and the like are not so emphasized. Processing according to the image signal such as suppressing the moire without enhancing the portion can be performed.

  As mentioned above, although embodiment of this invention was explained in full detail with reference to drawings, the specific structure is not restricted to this embodiment, The design change etc. of the range which does not deviate from the summary of this invention are included.

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus 10 ... Control part 11 ... Reading part 12 ... Image processing part 13 ... Page memory part 14 ... Image recording part 15 ... Storage part 16 ... Printer controller part 17 ... FAX controller part , 18 ... Electronic data creation unit, 19 ... External I / F unit, 20 ... Sharpness processing unit, 21 ... Scanner system image processing unit, 22 ... Printer system image processing unit, 200 ... Image area identification signal generation unit, 201 ... Smoothing , Processing unit 202, input signal conversion unit 203, enhancement amount extraction unit 204, enhancement amount conversion unit 205, signal synthesis unit.

Claims (10)

In an image processing apparatus that performs smoothing and enhancement processing of an image signal,
An identification signal generator for generating an image area identification signal for identifying an attribute of the image signal;
A smoothing processing unit that smoothes the image signal based on the image area identification signal generated by the identification signal generation unit;
The gradation is further compressed for the image signal whose gradation is lower than a predetermined gradation, and the gradation is further expanded for the image signal whose gradation is higher than the predetermined gradation. An input signal conversion unit for converting the gradation of the image signal,
An enhancement amount extraction unit that extracts an enhancement amount of an edge included in the image signal from the image signal subjected to gradation conversion by the input signal conversion unit;
An enhancement amount conversion unit for converting the enhancement amount so as to be larger or smaller depending on the magnitude of the enhancement amount extracted by the enhancement amount extraction unit;
An image processing apparatus comprising: a signal synthesis unit that synthesizes the enhancement amount converted by the enhancement amount conversion unit and the image signal smoothed by the smoothing processing unit. The enhancement amount conversion unit
When the enhancement amount extracted by the enhancement amount extraction unit is smaller than a predetermined value, the enhancement amount is converted so that the enhancement amount is further reduced,
The image processing apparatus according to claim 1, wherein when the enhancement amount extracted by the enhancement amount extraction unit is larger than a predetermined value, the enhancement amount is converted so that the enhancement amount is further increased. The image processing apparatus according to claim 1, wherein the image area identification signal is a black character, a color character, a patch, or a photograph. The image processing apparatus according to claim 1, wherein the smoothing processing unit uses a low-pass filter having a filter coefficient set based on the image area identification signal. The image processing apparatus according to claim 1, wherein the enhancement amount extraction unit uses a band-pass filter having a filter coefficient set based on the image area identification signal. The input signal conversion unit compresses the image signal when the image area identification signal is a photograph, and expands the image signal when the image area identification signal is a character.
The image processing apparatus according to claim 1. The enhancement amount conversion unit reduces an enhancement amount other than characters in the image signal emphasized by the enhancement amount extraction unit.
The image processing apparatus according to claim 1. The image processing apparatus according to claim 1, wherein the enhancement amount conversion unit can arbitrarily change the enhancement amount. An image processing apparatus according to claim 1;
An image recording unit for forming an image based on the image signal synthesized by the signal synthesis unit;
An image forming apparatus comprising: In an image processing method for performing smoothing and enhancement processing of an image signal,
Generating an image area identification signal for identifying an attribute of the image signal;
Smoothing the image signal based on the generated image area identification signal;
The gradation is further compressed for the image signal whose gradation is lower than a predetermined gradation, and the gradation is further expanded for the image signal whose gradation is higher than the predetermined gradation. So as to convert the gradation of the image signal,
Extracting an edge enhancement amount included in the image signal from the gradation-converted image signal,
Depending on the size of the extracted enhancement amount, the enhancement amount is converted to be larger or smaller,
An image processing method comprising combining the converted enhancement amount and the smoothed image signal.

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