JP2008067071A - Signal processing apparatus and signal processing method - Google Patents

Abstract

An optimum gradation conversion is performed on a color image signal in accordance with a focal length and photographing sensitivity at the time of photographing.
An image feature amount extraction circuit for extracting an image feature amount of an image signal output from an image sensor according to a feature amount extraction characteristic set according to focal length information indicating a focal length at the time of shooting; A gradation conversion characteristic selection circuit 14 that selects a gradation conversion characteristic from predetermined gradation conversion characteristics according to the extracted image feature amount and focal length information, and a gradation conversion characteristic selection circuit 14 that selects the gradation conversion characteristic. And a luminance gradation conversion circuit 5 that performs gradation conversion on the image signal output from the image sensor 1 based on the gradation conversion characteristics that have been obtained, and the feature amount extraction characteristics and the The gradation conversion characteristic is changed so that gradation conversion corresponding to the focal length can be performed.
[Selection] Figure 1

Description

  The present invention relates to a signal processing apparatus and a signal processing method, and more particularly, to a signal processing technique for performing gradation conversion on an image signal obtained by shooting according to shooting conditions.

An example of creating a luminance signal in a conventional imaging device will be described.
FIG. 9 is a block diagram showing a configuration of a signal processing unit in a conventional imaging apparatus. A white gain circuit 2 adjusts the white gain of a signal (color image signal) from the image sensor 1 such as a CCD image sensor or a CMOS image sensor, and the color signal generation circuit 3 performs processing such as color conversion matrix and gradation conversion. The color signal is created and output.

  As luminance signal processing, a signal from the white balance circuit 2 is subjected to processing such as interpolation and edge enhancement by the luminance signal generation circuit 4 to generate a luminance signal. After that, gradation conversion is performed by the luminance gradation conversion circuit 5, and luminance signal correction corresponding to the hue of the luminance signal is performed by the luminance correction circuit 6, which is output as a luminance signal.

  The created color signal and luminance signal are compressed by the image compression circuit 7 and stored in the recording medium 9 by the image recording circuit 8.

Here, luminance gradation conversion in recent digital cameras is a reverse characteristic of the monitor characteristic.
Y ′ = Y ^γ : γ = 0.45
In this way, the method is defined by numerical values, and gradation conversion is performed according to a predetermined gradation conversion characteristic using a method of performing gradation conversion along the arithmetic expression, a lookup table (hereinafter referred to as LUT), or the like. These gradation characteristics are also converted by setting a predetermined gradation conversion characteristic in the digital camera in advance according to the shooting mode of the digital camera, for example, the portrait mode or the landscape mode. ing. In these methods, gradation conversion is performed using only gradation conversion characteristics set in advance in the digital camera.

  On the other hand, when shooting with a camera, flare may occur depending on the shooting conditions, resulting in a whitish foggy image or a clearer image required for scenes such as landscapes There is.

  In other words, the optimum gradation characteristics vary depending on the shooting scene and subject, and it is very important to select the optimum gradation conversion characteristics corresponding to the shooting scene in order to obtain a more preferable image. On the other hand, some digital cameras have a function that allows the user to select contrast.

  Patent Document 1 discloses that “an information combining circuit that combines information on a dynamic range acquired by an information acquisition unit, a histogram calculation circuit that calculates a histogram of combined information combined by the information combining circuit, and the histogram An imaging apparatus comprising: a conversion characteristic calculation circuit that calculates a gradation conversion characteristic based on a histogram having a frequency equal to or higher than a predetermined value among histograms calculated by the calculation circuit. ”(Refer to claim 4) Is disclosed. Japanese Patent Application Laid-Open No. 2004-228561 proposes a method for changing gradation conversion characteristics by calculation with respect to basic gradation conversion characteristics as a technique for automatically correcting gradation.

  Japanese Patent Application Laid-Open No. 2004-228561 proposes a technique of adaptively varying the gradation conversion characteristics according to the histogram in the image and the shooting mode of the imaging device.

JP 2002-135648 A JP 2004-120589 A

  As described above, since the optimum gradation conversion characteristic for an image varies depending on the subject and the shooting scene, it is necessary to reset the gradation conversion characteristic every time shooting is performed. However, since current digital camera liquid crystal displays are different from those seen on a personal computer (PC) monitor, it is difficult to predict a finished image, and it is difficult to select the optimum gradation conversion characteristics. difficult.

  Some application software for viewing images on a PC has an automatic contrast correction and flare removal function. However, these corrections are performed on an image compressed by a digital camera or the like. JFIF (JPEG File Interchange Format), which is a file format generally used in digital cameras, is composed of 8 bits for each color. If 8-bit to 8-bit gradation conversion is performed on the image, gradation skipping or dithering to suppress gradation skipping is performed, resulting in image degradation. End up.

  In addition, the user can reduce troublesomeness by automatically selecting the optimum gradation conversion characteristics as a function in the digital camera without selecting the gradation conversion characteristics or retouching correction in the PC application. Needless to say, it is more preferable.

In the case of the method as described in Patent Document 1, since it is necessary to obtain the gradation conversion characteristic by calculation, it can be changed only within a predetermined mathematical calculation range. In addition, in order to change to a flexible gradation conversion characteristic, that much computation is required, and the computation scale becomes very large.
In addition, when gradation conversion is performed with reference to the LUT, generally, the correction amount from the LUT is defined by calculation or the like to calculate the gradation conversion characteristics. For the above reasons, it has been difficult to change the flexible gradation conversion characteristics.

In addition, in the case of an imaging apparatus such as a digital camera, there are many devices that have an optical zoom function in which the focal length can be varied in photographing. The occurrence frequency of flare varies greatly depending on the focal length. In addition, the contrast of the lens varies depending on the focal length.
In the case of the method described in Patent Document 2, it is possible to select an optimum gradation conversion characteristic to some extent, but detection of flare and contrast and gradation for each focal length according to the lens performance. There is a problem that it is difficult to optimally control the above.

  The present invention has been made in view of such circumstances, and an object of the present invention is to make it possible to perform optimum gradation conversion in accordance with shooting conditions for an image signal obtained by shooting.

The signal processing apparatus according to the present invention is acquired by the information acquisition unit from an information acquisition unit that acquires focal length information indicating a focal length at the time of shooting and an image signal output from an imaging unit that converts an optical image into an electrical signal. Acquired by the feature amount extraction means for extracting the image feature amount based on the feature amount extraction characteristic set according to the focal length information, the image feature amount extracted by the feature amount extraction means, and the information acquisition means In accordance with the focal length information, a gradation conversion characteristic selection unit that selects a gradation conversion characteristic from predetermined gradation conversion characteristics, and a gradation conversion characteristic selected by the gradation conversion characteristic selection unit And a gradation converting means for performing gradation conversion on the image signal output from the imaging means.
The signal processing apparatus according to the present invention includes a focal length acquisition unit that acquires a focal length at the time of shooting, and a feature amount extraction characteristic setting unit that sets a feature amount extraction characteristic according to the focal length acquired by the focal length acquisition unit. And feature quantity extraction means for extracting an image feature quantity from an image signal output from an imaging means for converting an optical image into an electrical signal based on the feature quantity extraction characteristic set by the feature quantity extraction characteristic setting means, A gradation conversion characteristic selection means for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics according to the image feature amount extracted by the feature amount extraction means; and the gradation conversion characteristic selection means. Based on the selected gradation conversion characteristics, gradation conversion means for performing gradation conversion of the image signal output from the imaging means, and the image signal subjected to gradation conversion by the gradation conversion means are compressed and recorded. Characterized in that it comprises a image compression recording means.
The signal processing method of the present invention includes an information acquisition step of acquiring focal length information indicating a focal length at the time of shooting, and an image signal output from an imaging unit that converts an optical image into an electrical signal. A feature amount extraction step for extracting an image feature amount based on a feature amount extraction characteristic set according to the acquired focal length information, an image feature amount extracted in the feature amount extraction step, and the information acquisition step. A gradation conversion characteristic selecting step for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics according to the focal length information acquired in step S3, and a level selected in the gradation conversion characteristic selection step. And a gradation conversion step of performing gradation conversion on the image signal output from the image pickup means based on the tone conversion characteristics.
The program of the present invention is acquired in the information acquisition step from an information acquisition step of acquiring focal length information indicating a focal length at the time of shooting and an image signal output from an imaging unit that converts an optical image into an electrical signal. A feature amount extraction step for extracting an image feature amount based on a feature amount extraction characteristic set according to the focal length information, an image feature amount extracted in the feature amount extraction step, and an information acquisition step. A gradation conversion characteristic selecting step for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics according to the focal length information, and the gradation conversion selected in the gradation conversion characteristic selection step Based on the characteristics, the computer is caused to execute a gradation conversion step of performing gradation conversion on the image signal output from the imaging means.
The computer-readable recording medium of the present invention is characterized in that the program is recorded.

  According to the present invention, the feature amount extraction characteristic and the gradation conversion characteristic are changed in accordance with the focal length at the time of shooting, and an optimum level corresponding to the focal length at the time of shooting is obtained for the image signal output from the imaging unit. Key conversion can be performed. In particular, when an optimal gradation conversion characteristic is selected before irreversible compression in an imaging device such as a digital camera, an optimal image can be obtained without image degradation. It becomes.

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

(First embodiment)
FIG. 1 is a block diagram illustrating a configuration example of an imaging apparatus to which the signal processing apparatus according to the first embodiment of the present invention is applied. FIG. 1 shows a configuration of a signal processing unit in the imaging apparatus of the first embodiment. In FIG. 1, blocks having the same functions as those shown in FIG. 9 are denoted by the same reference numerals.

  A white gain circuit 2 adjusts a white gain of a signal (color image signal) output from the image sensor 1 such as a CCD image sensor or a CMOS image sensor, and the color signal generation circuit 3 performs color conversion matrix, gradation conversion, and the like. The color signal is created and output. As luminance signal processing, a signal from the white balance circuit 2 is subjected to processing such as interpolation and edge enhancement by the luminance signal generation circuit 4 to generate a luminance signal. Thereafter, the luminance signal is subjected to gradation conversion by the luminance gradation conversion circuit 5 in accordance with the gradation conversion characteristic selected by the gradation conversion characteristic selection circuit 14, and the luminance correction circuit 6 is adapted to the hue of the luminance signal. The luminance signal is corrected and output as a luminance signal. The created color signal and luminance signal are compressed by the image compression circuit 7 and stored in the recording medium 9 by the image recording circuit 8.

  A signal (color image signal) from the image sensor 1 is a feature of the image according to a feature amount extraction characteristic (feature amount extraction setting value) set by the feature amount extraction characteristic setting unit 12 in the image feature amount extraction circuit 11. The quantity is extracted. At this time, the feature amount extraction characteristic setting unit 12 determines an image feature amount extraction characteristic (extraction parameter related to image feature amount extraction) by a setting method described later based on the focal length information from the focal length control unit 13. To do.

  The feature amount of the image extracted by the image feature amount extraction circuit 11 is supplied to the gradation conversion characteristic selection circuit 14. Further, the gradation conversion characteristic selection circuit 14 includes focal length information from the focal length control unit 13, exposure control information (signal) when shooting is performed from the exposure control unit 15, and shooting mode from the shooting mode control unit 16. Information (signal) is supplied. The gradation conversion characteristic selection circuit 14 obtains an optimum gradation conversion characteristic from the gradation conversion characteristic candidate storage unit 17 based on the supplied feature amount, focal length information, exposure control information, and shooting mode information. The gradation conversion characteristic is selected and the luminance gradation conversion circuit 5 is set. Here, the focal length information is information indicating the focal length of a lens (not shown) at the time of shooting, the exposure control information is information related to the exposure at the time of shooting, and the shooting mode information is a set shooting mode. It is information to show.

  FIG. 2 is a diagram illustrating an example of gradation conversion characteristic candidates stored in the gradation conversion characteristic candidate storage unit 17 in the present embodiment. In the example shown in FIG. 2, Gam0 is the highest contrast gradation conversion characteristic, and Gam3 is the low contrast gradation conversion characteristic.

Hereinafter, a method for selecting gradation conversion characteristics will be specifically described.
Hereinafter, the operation will be specifically described with an example in which the flare phenomenon occurs. FIG. 3B is an example showing a histogram of the luminance of the image as shown in FIG. When flare occurs in the scene of FIG. 3A, the entire image is whitish and has no contrast. In such a case, the histogram changes as shown in FIG. 3C with respect to the histogram shown in FIG. 3B and 3C, the horizontal axis represents the luminance value, and the vertical axis represents the frequency. Comparing FIGS. 3B and 3C, when flare occurs, it is characterized in that it floats black due to flare and the frequency of the low-luminance part decreases.

  As a scene where flare occurs, it often occurs when a bright scene contains strong light in the image or when there is diffusely reflected light coming from the water surface. In such a case, a more optimal image can be obtained by selecting a gradation conversion characteristic that reproduces a low-brightness portion darkly with high contrast.

  Further, in the imaging apparatus, the possibility that flare occurs increases as the zoom position of the lens is on the telephoto side, that is, as the focal length increases. Therefore, in the present embodiment, as the focal length becomes longer, the flare detection level in the image feature amount extraction circuit 11 is increased in accordance with the setting by the feature amount extraction characteristic setting unit 12, thereby more actively performing flare detection. It is possible.

  FIG. 4 is a graph showing an example of a change in the flare detection threshold according to the focal length. In FIG. 4, the horizontal axis represents the focal length, and the vertical axis represents the pixel value. In this embodiment, the image feature extraction circuit 11 can detect the occurrence of flare by calculating the total number of frequencies (the number of dark pixels) whose pixel value is equal to or smaller than the detection threshold from the histogram. In the present embodiment, the image feature quantity extraction circuit 11 calculates the frequency of the low luminance part using the histogram, but the image feature quantity extraction circuit is a low luminance part frequency calculation circuit that calculates the frequency below a certain threshold. It goes without saying that it may be used as 11. Further, the ratio of the number of dark pixels to the total number of pixels of the image may be calculated instead of the frequency total (number of dark pixels) whose pixel value is equal to or less than the detection threshold.

  FIG. 5 is a diagram showing an example in which selection of the gradation conversion characteristic (γ curve) is varied based on the ratio of the number of dark pixels detected by the flare detection described above. FIG. 5 shows, as an example, a case where the gradation conversion characteristic is selected from the gradation conversion characteristics shown in FIG. 2 according to the ratio of the number of detected dark pixels. In FIG. 5, the horizontal axis represents the focal length, and the vertical axis represents the number of dark pixels (ratio).

  The gradation conversion characteristic selection circuit 14 selects the gradation conversion characteristic Gam0 having the highest contrast when the ratio of the number of dark pixels is the lowest (when it is lower than the contrast high level C1). In addition, when the ratio of the number of dark pixels is the highest (when the contrast is lower than the weak contrast level C3), the gradation conversion characteristic Gam3 having the lowest contrast is selected. Similarly, the gradation conversion characteristic Gam1 is selected when it is higher than the high contrast level C1 and lower than the medium contrast level C2, and when it is higher than the medium contrast level C2 and lower than the low contrast level C3, the gradation conversion characteristic is selected. Select Gam2.

  As shown in FIG. 5, in the present embodiment, the boundary value of the ratio of the number of dark pixels that determines the gradation conversion characteristics to be selected changes according to the focal length. By setting the selection characteristics of the gradation conversion characteristics by the gradation conversion characteristic selection circuit 14 in this way, it becomes possible to select the optimum gradation conversion characteristics according to the occurrence frequency of flare, that is, the focal length, and a good image can be obtained. Obtainable.

In the present embodiment, adaptive gradation conversion may be switched on / off (execution / non-execution) depending on the shooting mode. For example, it can be realized by performing gradation conversion processing when the shooting mode is auto, and not performing gradation conversion processing when the shooting mode is set to manual or gradation conversion characteristics. is there.
In addition, flare often occurs in a scene with relatively bright brightness such as a backlight scene. Therefore, on / off of adaptive gradation conversion processing may be switched according to subject brightness.

(Second Embodiment)
Next, a second embodiment of the present invention will be described.
FIG. 6 is a block diagram illustrating a configuration example of an imaging apparatus to which the signal processing apparatus according to the second embodiment is applied. FIG. 6 shows the configuration of the signal processing unit in the imaging apparatus of the second embodiment. In FIG. 6, blocks having the same functions as those shown in FIGS. 1 and 9 are denoted by the same reference numerals, and redundant description is omitted.

  In the second embodiment, the exposure control unit 15 transmits sensitivity information at the time of shooting to the feature amount extraction characteristic setting unit 12. The feature quantity extraction characteristic setting unit 12 uses the focal length information from the focal length control unit 13 and the sensitivity information from the exposure control unit 15 to determine a low-luminance portion detection threshold for flare detection. Here, the sensitivity information is information indicating photographing sensitivity at the time of photographing.

  FIG. 7 is a diagram illustrating an example of a threshold value for detection according to the focal length for each photographing sensitivity set by the feature amount extraction characteristic setting unit 12. In FIG. 7, the horizontal axis represents the focal length, and the vertical axis represents the pixel value. FIG. 7 shows an example of setting the detection threshold for high sensitivity and the detection threshold for low sensitivity as an example. The operation in the second embodiment is the same as the operation in the first embodiment, except that the flare detection threshold is set according to the focal length and the imaging sensitivity.

  According to the second embodiment, it is possible to select an optimum gradation conversion characteristic according to the focal length. In addition, in this embodiment, in order to compensate for the disadvantage that noise is conspicuous when the gradation conversion characteristic that sinks low luminance is selected when the photographing sensitivity is high, the dark part is not sinked so much at high sensitivity. By doing so, it is possible to select an optimum gradation conversion characteristic even at high sensitivity.

  In this embodiment, the case where two types of threshold values for detection, one for high sensitivity and one for low sensitivity, are described as an example. However, the present invention is not limited to this, and three or more different shooting sensitivities can be used as detection threshold values. Thus, a threshold value for detection may be provided. For example, the low-sensitivity detection threshold is for ISO 50, the high-sensitivity detection threshold is for ISO 400, and the detection threshold for the imaging sensitivity between them is linearly interpolated using the detection thresholds for ISO 50 and ISO 400. You may do it.

  In the present embodiment, the detection threshold is changed according to sensitivity information in addition to focal length information. However, the sensitivity information is similarly applied when the gradation conversion characteristic is selected by the gradation conversion characteristic selection circuit 14. It goes without saying that the gradation conversion characteristics to be selected may be changed accordingly.

(Other embodiments of the present invention)
In order to operate various devices to realize the functions of the above-described embodiments, to realize the functions of the above-described embodiments for a device (CPU or MPU) in an apparatus or system connected to the various devices. Supply software programs. And what was implemented by operating the said various devices according to the program stored in the computer of the system or the apparatus is also contained under the category of this invention.
In this case, the software program itself realizes the functions of the above-described embodiments, and the program itself constitutes the present invention. Further, means for supplying the program to the computer, for example, a recording medium storing the program constitutes the present invention. As a recording medium for storing such a program, for example, a flexible disk, a hard disk, an optical disk, a magneto-optical disk, a CD-ROM, a magnetic tape, a nonvolatile memory card, a ROM, or the like can be used.
In addition, such a program is included in the embodiment of the present invention when the function of the above-described embodiment is realized in cooperation with an operating system or other application software running on a computer. Needless to say.
Further, after the supplied program is stored in a memory provided in a function expansion board or a function expansion unit related to the computer, a CPU or the like provided in the function expansion board or the like based on an instruction of the program may be part of the actual processing Do everything. Needless to say, the present invention includes the case where the functions of the above-described embodiments are realized by the processing.

For example, the signal processing apparatus shown in the first and second embodiments has a computer function 800 as shown in FIG. 8, and the CPU 801 performs operations in the first and second embodiments.
As shown in FIG. 8, the computer function 800 includes a CPU 801, a ROM 802, and a RAM 803. Further, a controller (CONSC) 805 of the operation unit (CONS) 809 and a display controller (DISPC) 806 of a display (DISP) 810 as a display unit such as a CRT or LCD are provided. Furthermore, a hard disk (HD) 811 and a controller (DCONT) 807 of a storage device (STD) 812 such as a flexible disk, and a network interface card (NIC) 808 are provided. These functional units 801, 802, 803, 805, 806, 807, and 808 are configured to be communicably connected to each other via a system bus 804.
The CPU 801 generally controls each component connected to the system bus 804 by executing software stored in the ROM 802 or the HD 811 or software supplied from the STD 812. That is, the CPU 901 reads out a processing program for performing the operation as described above from the ROM 802, the HD 811, or the STD 812, and executes it, thereby performing control for realizing the operation in the first and second embodiments. Do. The RAM 803 functions as a main memory or work area for the CPU 801.
The CONSC 805 controls an instruction input from the CONS 809. The DISPC 806 controls the display of the DISP 810. The DCONT 807 controls access to the HD 811 and the STD 812 that store a boot program, various applications, user files, a network management program, the processing program in the first and second embodiments, and the like. The NIC 808 exchanges data with other devices on the network 813 in both directions.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed as being limited thereto. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

It is a figure which shows the structural example of the imaging device to which the signal processing apparatus by 1st Embodiment is applied. It is a figure which shows an example of the gradation conversion characteristic candidate in this embodiment. (A) is a diagram showing a sample image, (B) is a diagram showing a histogram of luminance relating to the sample image, and (C) is a diagram showing a histogram of luminance relating to the sample image in which the flare phenomenon has occurred. It is a figure which shows the example of a setting of the threshold value for flare detection according to a focal distance. It is a figure for demonstrating the selection method of a gradation conversion characteristic. It is a figure which shows the structural example of the imaging device to which the signal processing apparatus by 2nd Embodiment is applied. It is a figure which shows the example of a setting of the threshold value for flare detection according to imaging | photography sensitivity and a focal distance. It is a block diagram which shows the computer function which can implement | achieve the signal processing apparatus in this embodiment. It is a figure which shows the structure of the signal processing unit in the conventional imaging device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image pick-up element 2 White balance circuit 3 Color signal creation circuit 4 Luminance signal creation circuit 5 Luminance gradation conversion circuit 6 Luminance correction circuit 7 Image compression circuit 8 Image recording circuit 9 Recording medium 11 Image feature amount extraction circuit 12 Feature amount extraction characteristic setting Unit 13 focal length control unit 14 gradation conversion characteristic selection circuit 15 exposure control unit 16 photographing mode control unit 17 gradation conversion characteristic candidate storage unit

Claims (10)

Information acquisition means for acquiring focal length information indicating a focal length at the time of shooting;
A feature quantity for extracting an image feature quantity from an image signal output from an imaging means for converting an optical image into an electrical signal based on a feature quantity extraction characteristic set according to focal length information acquired by the information acquisition means Extraction means;
A gradation conversion characteristic for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics in accordance with the image feature amount extracted by the feature amount extraction unit and the focal length information acquired by the information acquisition unit A selection means;
Signal processing comprising: gradation conversion means for performing gradation conversion on an image signal output from the imaging means based on the gradation conversion characteristics selected by the gradation conversion characteristic selection means apparatus. The information acquisition means further acquires sensitivity information indicating photographing sensitivity,
The feature amount extraction unit extracts an image feature amount from an image signal output from the imaging unit based on a feature amount extraction characteristic set according to the focal length information and sensitivity information acquired by the information acquisition unit. The signal processing apparatus according to claim 1, wherein:   The gradation conversion characteristic selection unit selects the gradation conversion characteristic according to the image feature amount extracted by the feature amount extraction unit and the focal length information and sensitivity information acquired by the information acquisition unit. The signal processing apparatus according to claim 2. A focal length acquisition means for acquiring a focal length at the time of shooting;
Feature quantity extraction characteristic setting means for setting a feature quantity extraction characteristic in accordance with the focal length acquired by the focal length acquisition means;
Feature quantity extraction means for extracting an image feature quantity from an image signal output from an imaging means for converting an optical image into an electrical signal based on the feature quantity extraction characteristic set by the feature quantity extraction characteristic setting means;
A gradation conversion characteristic selecting means for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics according to the image feature amount extracted by the feature amount extraction means;
Gradation conversion means for performing gradation conversion of an image signal output from the imaging means based on the gradation conversion characteristics selected by the gradation conversion characteristic selection means;
An image compression recording means for compressing and recording the image signal subjected to gradation conversion by the gradation conversion means.   5. The signal processing apparatus according to claim 4, wherein the feature quantity extraction characteristic setting unit sets the feature quantity extraction characteristic according to the focal length and photographing sensitivity. The feature amount extraction means detects the number of pixels whose pixel value is equal to or less than a detection threshold in the image or the ratio to all pixels,
The signal processing apparatus according to claim 4, wherein the feature quantity extraction characteristic setting unit sets the detection threshold.   The gradation conversion characteristic selection unit selects a gradation conversion characteristic from predetermined gradation conversion characteristics in accordance with the output of the feature amount extraction unit and the focal length acquired by the focal length acquisition unit. The signal processing device according to claim 4, wherein the signal processing device is a signal processing device. An information acquisition step of acquiring focal length information indicating a focal length at the time of shooting;
A feature that extracts an image feature amount from an image signal output from an imaging unit that converts an optical image into an electrical signal based on a feature amount extraction characteristic that is set according to the focal length information acquired in the information acquisition step A quantity extraction process;
A gradation for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics according to the image feature amount extracted in the feature amount extraction step and the focal length information acquired in the information acquisition step A conversion characteristic selection process;
A gradation conversion step of performing gradation conversion on the image signal output from the imaging means based on the gradation conversion characteristic selected in the gradation conversion characteristic selection step. Processing method. An information acquisition step of acquiring focal length information indicating a focal length at the time of shooting;
A feature for extracting an image feature amount from an image signal output from an image pickup unit that converts an optical image into an electrical signal based on a feature amount extraction characteristic set according to the focal length information acquired in the information acquisition step A quantity extraction step;
A gradation for selecting a gradation conversion characteristic from predetermined gradation conversion characteristics according to the image feature amount extracted in the feature amount extraction step and the focal length information acquired in the information acquisition step A conversion characteristic selection step;
A program for causing a computer to execute a gradation conversion step for performing gradation conversion on an image signal output from the imaging unit based on the gradation conversion characteristic selected in the gradation conversion characteristic selection step. .   A computer-readable recording medium having the program according to claim 9 recorded thereon.

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