JP2011024050A - Image processing apparatus, and image processing method - Google Patents

Abstract

When image data after MTF correction has a value exceeding the output image data range, processing such as clipping is required, and sufficient correction cannot be performed.
A correction amount calculation unit calculates a correction amount for each frequency component based on MTF data of an image output apparatus. When the corrected image data obtained by performing the MTF correction using each correction amount exceeds the image output data range, the correction amount limiting unit 108 limits the correction amount of the frequency component and calculates the correction limit amount. Then, another frequency component is selected by the compensation frequency component selection unit 109 so that the correction limit amount is compensated by another frequency component different from the frequency component, and the correction limit amount compensation unit 110 compensates for each. The amount of correction for use is calculated. Then, the frequency conversion unit 112 converts each frequency component based on each correction amount calculated as described above.
[Selection] Figure 1

Description

  The present invention relates to an image processing apparatus and an image processing method for performing MTF correction corresponding to an image output apparatus on image data output to the image output apparatus.

  In recent years, with the spread of personal computers and the like, digital image data has become widespread, and the opportunity to output digital image data with an image output device such as a printer has increased. In general, in an image output device, the frequency characteristics of an output image are modulated by MTF (Modulation Transfer Function) which is the frequency characteristics. This modulation causes image quality deterioration related to sharpness such as thinning, fading, and cutting of characters and image edge portions. Therefore, in order to cope with image quality degradation due to MTF of the image output apparatus, MTF correction (edge enhancement or the like) by image processing is performed. In general, as MTF correction, an image signal is corrected by filtering in real space by convolution integration. For example, the level of MTF correction is controlled based on the level difference between the target pixel and the pixels before and after the target pixel, and the MTF correction calculation is performed on the target pixel, thereby reducing image quality degradation of a reproduced image in a region having a high spatial frequency. A method is known (see, for example, Patent Document 1).

  However, in the MTF correction method based on the above filtering, for example, when an 8-bit image is processed and the corrected image data is less than 0 or exceeds 255, this is output as 0 to 255 output data by clipping or the like. Must be within range. Accordingly, it is not possible to sufficiently compensate for the maximum density and minimum density degradation caused by the MTF of the image output device.

  The following methods are known as processing when the corrected image data exceeds the output data range. For example, since the density change of an image occurs due to clipping accompanying edge enhancement processing, a method of storing density by reflecting a difference from an upper limit value or a lower limit value after clipping in surrounding pixels is disclosed (for example, , See Patent Document 2). In addition, in order to reduce the thinning, blurring, cutting, etc. of characters and image edges, which are image quality degradation due to MTF, the image data and its delay data, or the maximum density data of the target pixel and adjacent pixels are used as the target pixel data. (For example, refer to Patent Document 3). According to this method, since edge enhancement is not performed, a density change does not occur. Therefore, an image of an edge portion can be expanded without performing clipping.

JP 05-075859 A JP 05-344339 A JP-A-06-152932

  However, since the degree of image quality degradation due to the MTF of the image output device differs depending on the frequency component of the image, the MTF correction amount also changes according to the amount of degradation, and the degree of clipping that occurs during the MTF correction also depends on the frequency component and the image. It changes depending on the data. As described above, when the degree of clipping changes depending on the frequency component and the image data, there is a problem that the above-described conventional correction method for clipping cannot compensate for image quality degradation for each frequency component that occurs during image output. It was.

  Also, in the method of reflecting the difference between the upper limit or the lower limit value described in Patent Document 2 on the surrounding pixels, the density in the local region of the image can be stored, but the maximum density after image degradation or Compensating for the minimum concentration was not considered.

  In addition, the method of expanding the image of the edge portion described in Patent Document 3 can reduce image quality deterioration in high-frequency components such as characters and image edges, but can be used in all frequency bands generated by the MTF of the image output device. The image quality degradation of was not considered. Further, compensation for the maximum density and the minimum density after image degradation has not been considered.

  SUMMARY An advantage of some aspects of the invention is to provide an image processing apparatus and an image processing method having the following functions. That is, appropriate MTF correction can be performed by compensating the correction amount of the frequency component that needs to be limited at the time of MTF correction with other frequency components.

  As a means for achieving the above object, an image processing apparatus of the present invention comprises the following arrangement.

  That is, an image processing device that performs MTF correction according to an image output device for image data to be output to the image output device, the MTF acquisition means for acquiring the MTF characteristics of the image output device, and the image data A frequency component acquisition unit for acquiring a frequency component, and a correction amount for performing the MTF correction for each frequency component acquired by the frequency component acquisition unit based on the MTF characteristics acquired by the MTF acquisition unit A correction amount calculation means for performing the correction, the post-limit correction amount limiting the correction amount for the first frequency component that needs to be limited among the frequency components, and the change in the correction amount due to the limitation Correction amount limiting means for calculating a correction limit amount that is a minute, and the second frequency so that the correction limit amount is compensated with a second frequency component different from the first frequency component. A correction limit amount compensation means for calculating a compensation correction amount for several components, and combining the compensation correction amount with the correction amount for the second frequency component to calculate a post-compensation correction amount; and the correction amount, MTF correction means for performing the MTF correction based on the post-limit correction amount and the post-compensation correction amount.

  According to the present invention having the above-described configuration, it is possible to perform appropriate MTF correction by compensating the correction amount of the frequency component that needs to be limited at the time of MTF correction with the other frequency components.

The block diagram which shows the structure of the image processing apparatus in one Embodiment which concerns on this invention, The flowchart which shows the MTF correction process in this embodiment, A flowchart showing compensation processing for a correction amount in the present embodiment, The block diagram which shows the structure of the image processing apparatus in 2nd Embodiment, The flowchart which shows the MTF correction process in 2nd Embodiment, A graph showing an example of frequency components in the correction amount compensation processing of the present embodiment, A graph showing a luminance profile example of each frequency component in the correction amount compensation processing of the present embodiment, A graph showing an example of a luminance profile before and after compensation for correction amount in the present embodiment, The figure which shows the example of the sine wave image before and after correction amount compensation in this embodiment, It is a flowchart which shows the acquisition process of the MTF characteristic in 2nd Embodiment.

  Hereinafter, the present invention will be described in detail based on preferred embodiments with reference to the accompanying drawings. The configurations shown in the following embodiments are merely examples, and the present invention is not limited to the illustrated configurations.

<First Embodiment>
System Configuration The configuration of the image processing apparatus according to this embodiment will be described with reference to the block diagram of FIG. First, 101 is an image processing apparatus of the present embodiment, 102 is an image holding unit that holds image data to be subjected to MTF correction processing, and 103 is an image such as an inkjet printer or an electrophotographic printer for printing image data. Output device.

  In the image processing apparatus 101, reference numeral 104 denotes an MTF holding unit that holds the MTF characteristics of the image output apparatus 103. Reference numeral 105 denotes a frequency component acquisition unit that converts image data held in the image holding unit 102 into frequency space data. Reference numeral 106 denotes a correction amount calculation unit that calculates the MTF correction amount of each frequency component based on the MTF characteristics held in the MTF holding unit 104. Reference numeral 107 denotes a restriction determination unit that determines whether or not it is necessary to limit the correction amount calculated by the correction amount calculation unit 106 based on a range of image data that can be output by the image output apparatus 103.

  108, when it is determined that the correction amount needs to be limited by the limit determination unit 107, for a corresponding frequency component, a post-limit correction amount that limits the correction amount and a change in the correction amount due to the limitation. This is a correction amount limiting unit that calculates the correction limit amount. Reference numeral 109 denotes a compensation frequency component selection unit that selects another frequency component used to compensate for the correction restriction amount when the correction amount is restricted. Reference numeral 110 denotes a correction limit amount compensation unit that calculates a compensation correction amount for the frequency component selected by the compensation frequency component selection unit 109 based on the correction limit amount calculated by the correction amount restriction unit 108. The compensation correction amount calculated here is combined with the correction amount of the same frequency component calculated by the correction amount calculation unit 106, thereby calculating a post-compensation correction amount.

  111 denotes MTF correction using the correction amount calculated by the correction amount calculation unit 106, the post-limit correction amount calculated by the correction amount limit unit 108, and the post-compensation correction amount calculated by the correction limit amount compensation unit 110. This is an MTF correction unit to be performed. Reference numeral 112 denotes an output image data calculation unit that converts frequency space data that has been subjected to MTF correction by the MTF correction unit 111 into real space image data that is output by the image output device 103.

MTF correction overall processing Hereinafter, the MTF correction processing in the image processing apparatus 101 of the present embodiment having the above-described configuration will be described with reference to the flowchart of FIG.

  First, in step S <b> 201, processing target image data held in the image holding unit 102 is acquired. In step S202, the frequency component acquisition unit 105 converts the image data in the real space acquired in step S201 into frequency space data by performing processing such as Fourier transform. In step S <b> 203, MTF data of all frequency components to be corrected is acquired from the MTF characteristics of the image output apparatus 103 held in the MTF holding unit 104. In step S204, the correction amount calculation unit 106 calculates the correction amount used for MTF correction of the image for all frequency components to be corrected based on the MTF data acquired in step S203. The correction amount here is calculated by MTF correction so that the frequency characteristic of the image becomes equal to that before the deterioration of the image output apparatus 103 due to MTF. Details of this correction amount calculation processing will be described later.

  Next, in S205, when the limit determination unit 107 performs MTF correction using the correction amount for each correction amount calculated in S204, it is determined whether or not the correction amount needs to be limited. To do. This determination is made based on whether or not the corrected image data that would be obtained by frequency conversion based on the respective correction amounts, that is, MTF correction, is within the image data range that can be output by the image output device 103. That is, if the image data after MTF correction is out of the image data range that can be output, there is a possibility that sufficient correction cannot be performed by clipping. Therefore, it is necessary to limit the correction amount for such frequency components. To be judged. If it is determined that the restriction is necessary, the process proceeds to S206. If it is determined that the restriction is not necessary, the process proceeds to S208.

  In S206, the correction amount restriction unit 108 restricts the correction amount calculated in S204 so that the image data after MTF correction falls within the range that can be output by the image output device 103, and the correction by the restriction. A correction limit amount that is a change in the amount is calculated. In step S207, the compensation frequency component selection unit 109 and the correction limit amount compensation unit 110 compensate the correction limit amount calculated in step S206 using other frequency components. Details of this compensation processing will be described later.

  In S208, the MTF correction unit 111 performs MTF correction for each frequency component using the correction amount calculated in S204, the correction amount limited by the correction limit calculated in S206, and the correction amount compensated in S207. Do. Details of the MTF correction process will be described later.

  In step S209, the output image data calculation unit 112 converts the frequency space data subjected to MTF correction in step S208 into real space data by performing processing such as inverse Fourier transform, and the image output device 103 receives the real space data. Data is transmitted as output image data. In S210, the image output apparatus 103 outputs (prints out) the output image data transmitted in S209, and the process ends.

Correction Amount Calculation Processing Hereinafter, the processing for calculating the correction amount based on the MTF data in S204 will be described in detail.

  The image quality degradation due to the MTF of the image output apparatus 103 is expressed by the following equation (1) as a multiplication value of the output image data and the MTF representing the degradation in the frequency space.

G (u, v) = F (u, v) × H (u, v) (1)
In Expression (1), F (u, v) represents image data to be output input to the image output apparatus 103, and H (u, v) represents the MTF characteristic of the image output apparatus 103, that is, a deterioration function. G (u, v) represents an image output from the image output apparatus 103 with deterioration due to MTF.

  An image restoration method using an image restoration filter is known as a method for performing MTF correction for image degradation represented by the equation (1). As an image restoration filter used for MTF correction, an inverse filter or a Wiener filter is generally used. For example, the inverse filter is represented by I (u, v) using an inverse function of H (u, v), which is a degradation function, as shown in the following equation (2).

I (u, v) = 1 / H (u, v) (2)
However, since the inverse filter is not designed in consideration of the noise component, when the degraded image has noise, the noise component may be emphasized. Therefore, a Wiener filter represented by the following expression (3) is known as an image restoration filter in which a noise component is controlled.

W (u, v) = Hc (u, v) / (| H (u, v) | ² + Γ) (3)
In Equation (3), W (u, v) is a Wiener filter, H (u, v) is a degradation function, Hc (u, v) is a complex conjugate of H (u, v), and Γ is a noise component. Represents the parameter to be controlled.

  The correction amount in the present embodiment corresponds to the filter coefficient of the MTF correction filter, and is calculated by using the MTF characteristic of the image output apparatus 103 as the deterioration function H (u, v) in any one of the image restoration filters. . However, the format of the image restoration filter to be used is not limited to the above two types of filters as long as the degradation of the image can be restored.

  It is also possible to create a two-dimensional MTF correction filter using a one-dimensional MTF correction filter coefficient. As a creation method, a method of rotating a one-dimensional MTF correction filter coefficient around the origin, a method of interpolating between two directions of MTF correction filter coefficients, or a method of using MTF correction filter coefficients in a plurality of directions. There is a method of interpolating between them. However, the method of creating a two-dimensional filter is not limited to these methods.

Correction amount compensation processing Hereinafter, the processing for compensating the correction limit amount calculated in S206 using other frequency components in S207 will be described in detail with reference to the flowchart of FIG. Hereinafter, the frequency component for which the correction limit amount is calculated is referred to as a target frequency component (first frequency component), and other frequency components used for compensation of the correction limit amount of the target frequency component are used as compensation frequency components (second frequency component). Frequency component).

  First, in S301, the compensation frequency component selection unit 109 selects another frequency component used for compensation of the correction limit amount in the frequency component of interest as the compensation frequency component.

  In step S302, the correction limit amount compensation unit 110 calculates a compensation correction amount to be assigned to each of the compensation frequency components selected in step S301 based on the correction limit amount calculated in step S206.

  The compensation frequency components determined here and the respective compensation correction amounts are adaptive based on the difference between the frequency component of interest and the image data range that can be output in the image data after MTF correction by the compensation amount before the compensation. Adjusted. For example, an optimal frequency component and its correction amount may be set based on experiments or the like.

  In S303, the correction limit compensation unit 110 synthesizes the compensation correction amount calculated in S302 with the compensation amount corresponding to the compensation frequency component among the compensation amounts of the frequency components acquired in S204. Thereby, the post-compensation correction amount used for the MTF correction of the present embodiment is calculated for the compensation frequency component.

  As described above, in this embodiment, the correction limit amount with respect to the frequency component of interest is compensated by the compensation frequency component. At this time, the correction amount in a certain frequency component is modeled as shown in the following equation (4).

C (f) = {k (f, f) + k (f, f2) + k (f, f3) + ...} × D (f) (4)
In equation (4), D (f) represents the frequency component f before MTF correction, and C (f) represents the frequency component f after MTF correction. K (f, f) represents the post-limit correction amount of the frequency component f calculated in S206, and k (f, f2) and k (f, f3) are correction limits for the frequency components f2 and f3, respectively. The compensation amount for compensation of the frequency component f when the frequency component f is used to compensate the amount is shown.

  Here, FIG. 6 shows a graph in which specific numerical values are applied to the above equation (4) and the correction limit amount of the frequency component of interest is compensated by using other frequency components (compensation frequency components). That is, FIG. 6 shows an example of a frequency spectrum in an image before and after MTF correction according to the present embodiment, and shows an example in which a high-frequency component of 1/3 period of the frequency of interest is used as a compensation frequency component. In FIG. 6, reference numeral 601 denotes a target frequency component, reference numeral 602 denotes a compensation frequency component used for the compensation process, and when the target frequency component 601 is subjected to MTF correction, a correction amount is also generated for the compensation frequency component. Is shown. That is, in this case, the attention frequency component 601 is corrected up to the correction amount limited by the correction limit amount (post-limit correction amount), and this correction limit amount is the correction amount of the compensation frequency component 602 (after compensation). It can be seen that the compensation is made by the correction amount.

  FIG. 7 shows a luminance profile of each frequency component when the MTF correction shown in FIG. 6 is applied to sine wave data. In FIG. 7, reference numeral 701 denotes sine wave data of the frequency component of interest before MTF correction. Reference numeral 702 denotes a post-limit correction amount for the frequency component of interest, which corresponds to 1 / x of the amplitude of the sine wave data 701 in this example. Reference numeral 703 denotes a compensation amount of compensation frequency component (compensation compensation amount). In this example, a high frequency component of 1/3 period of the frequency component of interest is used as a compensation frequency component. The correction amount of the compensation frequency component is determined in accordance with the correction limit amount of the frequency component of interest and the like. Here, an example in which the compensation correction amount, that is, the amplitude, is equal to the post-limit correction amount 702 of the frequency component of interest. Show.

  As described above, according to the compensation processing of the present embodiment, the correction amount of the compensation frequency component shown in 703 is combined with the correction amount of the same frequency component acquired in S204 and used for MTF correction. Then, the final correction amount for the target frequency f by this MTF processing is equivalent to the sum of the sine waves of 702 and 703 as indicated by 704, and the MTF correction is performed on the original sine wave data 701. Later sine wave data 705 is obtained. According to 705, it can be seen that the concentration at the peak peripheral portion is converted to a value close to the peak concentration without changing the value of the sine wave peak portion of the frequency component of interest and the center of the sine wave. According to the MTF correction of the present embodiment, the change amount of the sine wave peak portion is reduced according to the correction limit amount, and is not necessarily zero.

  For example, in the case of an ink jet printer, the density in the highlight portion increases due to the influence of the dot gain of surrounding pixels, and as a result, the MTF deteriorates. Therefore, as shown in FIG. 7, by making the density of the peak peripheral part close to the peak density, it is possible to reduce the influence of the dot gain of the peripheral pixels and to compensate the correction limit amount as a result.

  FIG. 8 is a graph showing an example of a luminance profile before and after MTF correction in the present embodiment. According to FIG. 8, as described above, it can be seen that the MTF correction is performed on the upper limit value and the lower limit value of the luminance so that the respective peripheral portions approach the upper limit value and the lower limit value. FIG. 9 shows an example of a sine wave image before and after MTF correction. According to FIG. 9, it is understood that the contrast is enhanced and the deterioration due to MTF is reduced by correcting the peripheral portions to approach the upper limit value and the lower limit value of the luminance as shown in FIG. 8. .

  However, the method for compensating the correction limit amount of the frequency component of interest in the present invention is not limited to the above compensation method.

MTF correction processing The MTF correction processing in S208 will be described in detail below. As MTF correction processing of this embodiment, it is assumed that image restoration using a general image restoration filter is performed.

  The filtering in the frequency space is expressed as the following equation (5) as a multiplication value of the target image of the MTF correction process and the MTF correction filter in the real space.

Q (u, v) = P (u, v) × I (u, v) (5)
In Equation (5), P (u, v) represents the target image data for the MTF correction processing, I (u, v) represents the inverse filter as the MTF correction filter, and Q (u, v) is the MTF corrected Represents the image data. However, the frequency conversion process in the present embodiment is not limited to this method.

  As described above, according to the present embodiment, in the image data to be corrected, other frequency components than the frequency components that are expected to be unable to be corrected sufficiently because the normal MTF correction exceeds the output possible range. Is adjusted to compensate for the deterioration amount of the frequency component. This makes it possible to perform appropriate MTF correction even for frequency components close to the upper limit or lower limit of the image data range that can be output by the image output apparatus.

Second Embodiment
Hereinafter, a second embodiment according to the present invention will be described. In the first embodiment described above, when the image data after MTF correction with respect to the frequency component of interest exceeds the image output data range, an example in which compensation is performed using another frequency component has been described. That is, when determining whether or not the compensation is necessary, it is necessary to convert to real space data based on the values after MTF correction for all frequency components to obtain the values of the image output data. It was. In the second embodiment, an upper limit value after frequency conversion by MTF correction is set in advance for all frequency components based on the MTF of the image output apparatus, and compensation by other frequency components is required for each frequency component. It is characterized by determining whether or not. The second embodiment also shows an example in which the MTF characteristics of the image output apparatus are acquired by measurement.

System Configuration The configuration of the image processing apparatus according to the second embodiment will be described with reference to the block diagram of FIG. First, 401 is an image processing apparatus according to the present embodiment, 402 is an image holding unit that holds image data to be subjected to MTF correction processing, and 403 is an image such as an ink jet printer or an electrophotographic printer for printing image data. Output device.

  In the image processing apparatus 401, reference numeral 404 denotes an MTF holding unit that holds the MTF characteristics of the image output apparatus 403. Reference numeral 405 denotes a frequency component acquisition unit that converts the image data held in the image holding unit 402 into frequency space data. A correction amount calculation unit 406 calculates an MTF correction amount of each frequency component based on the MTF characteristics held in the MTF holding unit 404.

  Reference numeral 407 calculates an upper limit value after frequency conversion for each frequency component based on the MTF characteristic held in the MTF holding unit 404, performs a limit on the correction amount using the upper limit value, and performs a post-limit correction amount. And a correction amount limiting unit that calculates a correction limit amount corresponding to the change. Reference numeral 408 denotes a compensation frequency component selection unit that selects a compensation frequency component used to compensate the correction limit amount for the frequency component of interest. A correction limit amount compensation 409 calculates a correction amount (compensation correction amount) of the compensation frequency component selected by the compensation frequency component selection unit 408 based on the correction limit amount calculated by the correction amount restriction unit 407. Part. The compensation correction amount calculated here is combined with the correction amount of the same frequency component calculated by the correction amount calculation unit 406, thereby calculating a post-compensation correction amount. 410, MTF correction is performed using the correction amount calculated by the correction amount calculation unit 406, the post-limit correction amount calculated by the correction amount limit unit 407, and the post-compensation correction amount calculated by the correction limit amount compensation unit 409. This is an MTF correction unit to be performed. Reference numeral 411 denotes an output image data calculation unit that converts frequency space data that has been subjected to MTF correction by the MTF correction unit 410 into real space image data that is output by the image output device 403.

  Reference numeral 412 denotes a chart measuring device such as a scanner or a microscope for measuring the MTF characteristics of the image output device 103 from the frequency chart output using the image output device 403. The MTF characteristic measured by the chart measuring device 412 is stored in the MTF holding unit 404.

MTF Correction Overall Processing Hereinafter, the MTF correction processing in the image processing apparatus 401 according to the second embodiment having the above configuration will be described with reference to the flowchart of FIG.

  First, in S501, the chart measurement device 413 is used to acquire the MTF characteristic of the image output device 403 and store it in the MTF holding unit. Details of this MTF characteristic acquisition processing will be described later.

  In step S502, the image data held in the image holding unit 402 is acquired. In step S503, the image data in the real space acquired in step S502 is converted into frequency space data by performing processing such as Fourier transform. In step S <b> 504, MTF data of all frequency components to be corrected is acquired from the MTF characteristics of the image output apparatus 403 held in the MTF holding unit 404. In step S505, the correction amount calculation unit 406 calculates a correction amount used for MTF correction of the image for all frequency components to be corrected based on the MTF data acquired in step S504. This correction amount is calculated so that the frequency characteristic of the image becomes equal to that before the deterioration by the MTF of the image output device 403 by the MTF correction, as in S204 in the first embodiment described above.

  In step S <b> 506, the correction amount restriction unit 407 calculates an upper limit value (correction upper limit value) after frequency conversion for each frequency component based on the MTF characteristic of the image output device 403 held in the MTF holding unit 404.

  In S507, the correction amount limiting unit 407 limits the correction amount calculated in S505 based on the correction upper limit value calculated in S506, and calculates the post-limit correction amount and the correction limit amount. That is, for the frequency component of interest whose frequency conversion result by the correction amount exceeds the correction upper limit value, the correction limit amount is calculated so that the frequency conversion result does not exceed the correction upper limit value.

  In step S508, the compensation frequency component selection unit 408 and the correction limit amount compensation unit 409 compensate the correction limit amount calculated in step S507 with another frequency component (compensation frequency component). The compensation processing for the correction amount here is the same as S207 in the first embodiment described above.

  In S509, the MTF correction unit 410 performs MTF correction for each frequency component using each correction amount calculated up to S508. In step S <b> 510, the output image data calculation unit 411 converts the frequency space data converted in step S <b> 509 into real space data by performing processing such as inverse Fourier transform, and outputs the real space data to the image output device 403. Send as image data. In step S511, the image output apparatus 403 outputs (prints out) the output image data transmitted in step S510, and the process ends.

MTF Characteristics Acquisition Processing Hereinafter, the MTF characteristics acquisition processing of the image output apparatus 403 in S501 will be described in detail with reference to the flowchart of FIG.

  First, in step S101, the image output device 403 is used to output a frequency chart for MTF measurement. Next, in S102, the frequency chart output in S101 is measured by the chart measuring device 413. In S 103, the MTF characteristic of the image output device 403 is calculated based on the chart measurement value in S 102, and this is stored in the MTF holding unit 404.

  In the second embodiment, the following method can be considered as a method of measuring the MTF of the image output apparatus 403. First, there is a method of calculating an MTF characteristic from a maximum density value and a minimum density value of the sine wave chart using a sine wave chart as a frequency chart for MTF measurement. Further, a rectangular wave chart is used as a frequency chart for MTF measurement, and a CTF (Contrast Transfer Function) is calculated from the maximum density value and the minimum density value of the rectangular wave chart. There is a method of converting the CTF into MTF characteristics using a Coltman correction equation. However, the method for measuring the MTF characteristic in the second embodiment is not limited to the above method, and any method that can measure the MTF characteristic is applicable.

  A method of calculating MTF characteristics by converting the frequency chart for MTF measurement output using the image output device 403 into image data is also conceivable. For example, the output chart is first converted into data using an image input device such as a scanner or a microscope, and the density distribution of the chart is measured. Then, the MTF characteristic of the image output device 403 alone can be calculated by calculating the MTF characteristic from the maximum density value and the minimum density value of the chart and canceling the MTF characteristic of the image input device from the calculated MTF characteristic. However, the chart image data conversion method and the MTF characteristic calculation method from the image data are not limited to the above method.

  In the second embodiment, the MTF measurement of the image output device 403 is performed by executing the above MTF measurement method.

  As described above, according to the second embodiment, in the image data to be corrected, in the normal MTF correction, for the frequency component that exceeds a predetermined upper limit value after frequency conversion, the first embodiment described above is used. In the same manner as described above, the amount of deterioration is compensated by adjusting other frequency components. In the second embodiment, since it is determined based on the upper limit value after frequency conversion whether or not the correction amount needs to be limited for a certain frequency component, the real space as in the first embodiment described above. It is not necessary to reproduce the upper image data, and a faster determination is possible.

  In the first and second embodiments described above, an example in which the correction amount of each frequency component is calculated based on the MTF characteristic of the image output apparatus has been described. However, since the MTF characteristic varies depending on the output medium, It is desirable to consider the MTF characteristics for each medium.

<Other embodiments>
The present invention can also be realized by executing the following processing. That is, software (program) that realizes the functions of the above-described embodiments is supplied to a system or apparatus via a network or various storage media, and a computer (or CPU, MPU, or the like) of the system or apparatus reads the program. It is a process to be executed.

Claims (9)

An image processing device that performs MTF correction according to an image output device for image data output to the image output device,
MTF acquisition means for acquiring MTF characteristics of the image output device;
Frequency component acquisition means for acquiring a frequency component of the image data;
A correction amount calculating unit that calculates a correction amount when performing the MTF correction on the basis of the MTF characteristics acquired by the MTF acquiring unit for each frequency component acquired by the frequency component acquiring unit;
Among the frequency components, for the first frequency component that requires the correction amount to be limited, a post-limit correction amount that limits the correction amount, and a correction limit amount that is a change in the correction amount due to the limitation. Correction amount limiting means for calculating,
A compensation correction amount for the second frequency component is calculated so that the correction limit amount is compensated with a second frequency component different from the first frequency component, and the correction frequency for the second frequency component is calculated. Correction limit amount compensation means for calculating the compensation amount after compensation by combining the compensation amount with the compensation amount;
MTF correction means for performing the MTF correction based on the correction amount, the post-limit correction amount, and the post-compensation correction amount;
An image processing apparatus comprising:   The correction amount limiting means includes, for the frequency components in which the image data after MTF correction by the correction amount exceeds the image data range that can be output by the image output device among the frequency components, the image data after MTF correction is The image processing apparatus according to claim 1, wherein the post-limit correction amount and the correction limit amount are calculated so as to fall within the outputable image data range.   The correction amount limiting means calculates an upper limit value after frequency conversion by the MTF correction for each frequency component, and a frequency component whose frequency conversion result by the correction amount calculated by the correction amount calculation means exceeds the upper limit value. The image processing apparatus according to claim 1, wherein the post-limit correction amount and the correction limit amount are calculated so that the frequency conversion result does not exceed the upper limit value.   The correction correction amount compensation unit calculates the compensation correction amount so that a change amount of a peak portion of the sine wave corresponding to the first frequency component is reduced. The image processing apparatus according to any one of the above.   The image processing apparatus according to claim 4, wherein the correction limit amount compensation unit calculates the compensation correction amount so that a center of a sine wave corresponding to the first frequency component does not change. Furthermore, it has MTF holding means for holding a plurality of MTF characteristics,
The image processing apparatus according to claim 1, wherein the MTF acquisition unit acquires an MTF characteristic corresponding to the image output apparatus from the MTF holding unit. Furthermore, it has a measuring means for measuring the MTF characteristic of the image output device,
The image processing apparatus according to claim 6, wherein the MTF characteristic measured by the measuring unit is held by the MTF holding unit. An image processing method for performing MTF correction according to an image output device for image data output to the image output device,
An MTF acquisition step of acquiring an MTF characteristic of the image output device;
A frequency component acquisition step of acquiring a frequency component of the image data;
A correction amount calculating step for calculating a correction amount for performing the MTF correction on the basis of the MTF characteristics acquired in the MTF acquisition step for each frequency component acquired in the frequency component acquisition step;
Among the frequency components, for the first frequency component that requires the correction amount to be limited, a post-limit correction amount that limits the correction amount, and a correction limit amount that is a change in the correction amount due to the limitation. A correction amount limiting step to be calculated;
A compensation correction amount for the second frequency component is calculated so that the correction limit amount is compensated with a second frequency component different from the first frequency component, and the correction frequency for the second frequency component is calculated. A correction limiting amount compensation step of calculating a post-compensation correction amount by combining the compensation correction amount with the correction amount;
An MTF correction step for performing the MTF correction based on the correction amount, the post-limit correction amount, and the post-compensation correction amount;
An image processing method comprising:   A computer program for causing a computer to function as each unit of the image processing apparatus according to any one of claims 1 to 7 by being executed by the computer.