JPH1175212A - Electronic still camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress deterioration in image quality due to data compression, by controlling frequency characteristics and gradation characteristics as specified by a band correction part and a γ correction part respectively according to image data compression which is set before an image is picked up. SOLUTION: The band correction part 14 adjusts the levels of respective signals of R, G, and B according to gains set in amplification parts for high- range correction and intermediate-range correction and the respective signal frequency characteristics are controlled. Therefore, when the gains are set large, the high and intermediate frequency components of the respective signals are emphasized. The γ correction part 16 converts the gradations of the respective signals of R, G, and B which are controlled and normalized by the band correction part 14 by using a 1024- and 256-gradation look-up table. Gradation characteristics based upon a γ curve are set in the γ correction part 16 and image data has some of gradation characteristics of the γ curve converted according to the state. Therefore, the γ correction part 16 selects a specific γcurve according to various conditions and converts the gradations of the image data with the gradation characteristics, thereby making the output image easier to see.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electronic still camera for recording a photographed image as digital data.

[0002]

2. Description of the Related Art In recent years, electronic still cameras (so-called digital cameras) for recording images in a memory as digital data have begun to spread widely. This electronic still camera generally deals with image information having a larger data amount than character information, and thus often compresses data to reduce the data amount. As such an electronic still camera capable of data compression, there is known an electronic still camera capable of setting a compression ratio of image data before photographing. According to this type of camera, the user himself / herself sets a desired data compression ratio depending on the situation,
It is possible to control the data amount of each image, especially
If the user wants to store a large number of shots in a fixed-capacity storage medium or wants to speed up data transfer in data processing, the user sets a high compression ratio,
Compression can be performed so that the amount of image data is reduced.

[0003]

In such an electronic still camera, the higher the compression ratio is set, the smaller the image data is compressed, but on the other hand, the lower the image quality becomes. In general, a conventional electronic still camera generally includes, for example, a band correction unit that controls a frequency characteristic of a signal to suppress noise, and a gamma correction unit that controls a gradation characteristic to adjust contrast of an image. Are provided, and each control section controls signal characteristics that affect image quality. However, conventionally, such a characteristic control is not performed in accordance with the setting of the compression ratio, and particularly when photographing under the setting of a high compression ratio, a high resolution for obtaining a fine image can be obtained. Even in photographing, it is inevitable that the image quality is degraded due to the change in the image quality due to the data compression.

Therefore, according to the present invention, in accordance with the compression ratio of image data set by the user, the band control unit and the gamma correction unit respectively control good frequency characteristics and gradation characteristics, and perform image compression by data compression. An object of the present invention is to provide an electronic still camera capable of suppressing a decrease in quality.

[0005]

An invention according to claim 1 of the present application (hereinafter, referred to as a first invention) includes a band correction unit that controls the frequency characteristic of each color data that constitutes image data; A gamma correction unit for controlling tone characteristics, and in the electronic still camera capable of setting a compression ratio of image data before photographing, the band correction unit selects a specific frequency characteristic from a plurality of frequency characteristics. The gamma correction unit is provided with control means for selecting a specific gradation characteristic from a plurality of gradation characteristics, and according to the compression ratio of image data set before photographing, In the band correction section and the gamma correction section, predetermined frequency characteristics and gradation characteristics are respectively selected.

[0006] The invention according to claim 2 of the present application (hereinafter referred to as the invention)
The second aspect of the present invention is characterized in that, when the compression ratio of the image data is set high in the band correction section, a frequency characteristic for suppressing a high frequency component is selected.

Further, the invention according to claim 3 of the present application (hereinafter referred to as “the invention”)
A third aspect of the present invention is characterized in that, in the gamma correction section, when the compression ratio of the image data is set high, a gradation characteristic for enhancing the contrast of the image is selected.

[0008]

Embodiments of the present invention will be described below in detail with reference to the accompanying drawings. As shown in FIG.
The electronic still camera 1 according to the present embodiment has a shutter button 2, an imaging lens 6, and a built-in CCD 8. The electronic still camera 1 has a CCD
Memory card for storing image data obtained by subjecting the electric signal converted in 8 to a predetermined process (not shown: see FIG. 2 described later)
Card insertion slot 5 for inserting and connecting
The memory card connected to the card insertion slot 5 is removed from the card insertion slot 5 by pressing the card ejection button 4. In the electronic still camera 1, when the shutter button 2 is pressed, an image is formed on the CCD 8 by the imaging lens 6, and the CCD 8 converts a light signal into an electric signal. This electric signal is converted into digital data as image data and processed.

FIG. 2 is a block diagram of the electronic still camera 1. As shown in FIG. 2, the electronic still camera 1 includes the imaging lens 6, an optical diaphragm 7 for controlling the amount of incident light from the imaging lens 6, and a CCD 8 for photoelectric conversion.
A camera control CPU 24 for controlling the photographing operation of these optical systems; a CDS 9 for sampling the electric signal obtained by the CCD 8 to remove noise; and an AGC (auto gain control) for automatically controlling the gain and correcting the sensitivity. )
10 and analog / digital conversion of an electric signal (hereinafter A
/ D conversion), and an image processing CPU 12 for performing predetermined image processing on the digital signal obtained through each of the above components.

The image processing CPU 12 includes a pixel interpolating unit 13 for performing an interpolation process on image data of each color obtained by color-separating the image data, and a band control for each frequency for the image data of each color. A band correction unit 14 that performs
A color balance control unit 15 that independently corrects each color,
A gamma correction unit 16 that performs gradation conversion of an input signal; an image compression unit 17 that compresses image data; a video encoder 18 that encodes image data and outputs it to a built-in monitor 20 or an external monitor 21; And a memory card driver 19 for supplying image data to the memory card driver 19.

On the input side of the camera control CPU 24 for controlling the photographing operation of the optical system, the shutter button 2 (see FIG. 1) and image condition setting for setting the photographed image size and compression ratio before photographing are provided. A camera operation switch 27 including a switch, a photometric / colorimetric sensor 28 for measuring light quantity and color at the time of photographing, and a flash 29 are connected, while an aperture for driving the optical aperture 7 is provided on the output side. A driver 25, a timing generator CCD drive 26 for controlling the exposure time of the CCD 8, and the AGC 10 are connected. This camera control CP
In U24, exposure control data is calculated based on the light quantity and color measured by the photometric / colorimetric sensor 28, and the aperture value of the optical aperture 7, the CCD
8 (ie, electronic shutter speed) and AG
The gain of C10 is controlled. The camera control CPU
Reference numeral 24 denotes the image processing CPU 12 via a data bus.
It is connected to the.

In the electronic still camera 1 having the above configuration, the user can set the following image conditions before photographing. That is, in the present electronic still camera 1, as the image conditions, it is possible to set a captured image size for determining the resolution of an image and a data compression ratio in the image compression processing.

The size of the photographed image is “512 ×
384 "," 640x480 "and" 1024x768 "
Are provided, and the user can select a desired image size according to the situation. On the other hand, as the image compression mode for determining the compression ratio of the image data,
There are three types, "no compression mode", "1/8 JPEG compression mode", and "1/20 JPEG compression mode". Like the above image size, the user can select a desired one before shooting. Can be.

In the electronic still camera 1 according to the present embodiment, the image recording mode representing the type of the subject is set as
"Natural image mode" for natural objects such as natural people and landscapes, "Gray text mode" for capturing natural objects or their combination of letters and numbers in black and white, "Binary" for letters and numbers Three types of “text mode” are provided, and the user can select one desired mode from among them according to the type of the subject before shooting. Further, there is provided a CRT on / off mode for setting whether to output the image signal to the built-in monitor side or to the external monitor side.

Hereinafter, a basic photographing operation of the electronic still camera 1 will be described with reference to a flowchart of FIG. In the electronic still camera 1 according to the present embodiment,
Various conditions such as a photographed image size and a compression ratio in a compression process are set by a user before photographing.
First, these set various conditions are fetched (step S10).

Basically, image processing in the electronic still camera 1 at the time of photographing is performed when the shutter button 2 is half-pressed (steps S11 to S21) and when the shutter button 2 is fully pressed (steps S11 to S21). Steps S22 to S33) are included. In the present embodiment, the electronic still camera 1 has a preview function of displaying an image incident from the imaging lens on the built-in monitor 20 when the shutter button 2 is half-pressed,
The user looks at the image to be shot with the displayed image
(Such as the balance and composition of the picture). If the shutter button 2 is further depressed to the fully depressed state, the image incident at that time from the imaging lens 6 is processed and then recorded on the memory card 22.

First, the operation (steps S11 to S21) when the shutter button 2 is half-pressed will be described. In this operation, an image to be photographed is treated as a preview image, and predetermined image processing is performed. Step S11
When the shutter button 2 is half-pressed, the camera 1 first measures the amount of light and color in the photometric / colorimetric sensor 28 (step S12). Subsequently, based on the measurement data, exposure setting for the aperture value of the optical diaphragm 7, the accumulation time of the CCD 8 (that is, the shutter speed), and the like is performed (step S13). After the exposure is set, light enters from the imaging lens 6 and is converted into an electric signal by the CCD 8. The camera 1 converts the electric signal from analog to digital and sends it to the image processing CPU 12 as digital data (step S14).

Thereafter, in the image processing CPU 12, first, the pixel interpolating unit 13 interpolates missing pixels for each color data (step S15). This pixel interpolation unit 1
3 is provided with a plurality of interpolation means.
According to the set values of the captured image size and the image compression ratio captured in 10, the optimal interpolation means is selected from these.

The image data after pixel interpolation is supplied to a band correction unit 1.
The contour is corrected in step 4 (step S16), and step S17
After color correction is performed for each color data by the color balance control unit 15, gradation conversion is performed by the gamma correction block 16 (step S18). Subsequently, the image data is temporarily written into the image memory 23 by the image processing CPU 12 (step S19).

Thereafter, the image data is read from the image memory 23, and the video encoder 18 performs the NTSC / P
AL (step S20) and output to the built-in monitor 20 as a preview image (step S2).
1). When the shutter button 2 is maintained in a half-pressed state, the image incident from the imaging lens 6 is updated at a predetermined frame cycle and displayed on the built-in monitor 20 as a moving image. After confirming the preview image, the user can determine whether or not to shoot, and YES
In the case of, the shutter button 2 is completely depressed.

When the shutter button 2 is fully pressed, image processing is performed when the shutter button 2 is fully pressed in steps S22 to S31. The flow of this processing is half that of the shutter button 2 described above. Since this is the same as in the pressed state, the description is omitted here. However, in this process, image data to be recorded on the memory card 22 is processed.

The image data formed by the image processing in steps S22 to S31 is compressed in step S32. Here, the image data is compressed based on the compression ratio of the image data captured in step S10. This image compression requires more time as the compression ratio is higher. The compressed image data is recorded on the memory card 22 as a recorded image in step S33. In step S34, the user determines whether or not to end shooting,
In the case of YES, the photographing operation ends, and in the case of NO,
Steps after step S11 are repeated. The image to be photographed is processed according to the above flow, and is finally recorded as digital data.

Exposure Control Processing Next, the exposure control processing in the electronic still camera 1 will be described. In the present embodiment, the camera control CPU 2 uses the light amount and color measured by the photometric / colorimetric sensor.
4, the aperture driver 25, the timing generator CCD drive 2
6, the AGC 10 is controlled, and the aperture value of the optical aperture 8, the accumulation time of the CCD 8 (so-called electronic shutter speed), and the gain of the AGC 10 are set in each section.

FIG. 4 shows the automatic exposure control characteristics programmed in the camera control CPU 24. A solid line R and a broken line S in the figure are characteristic lines representing exposure control characteristics used at the time of capturing each of a recorded image and a preview image. This exposure control characteristic is determined by the aperture value of the optical aperture 7, the accumulation time of the CCD 8, and the gain of the AGC 10. The oblique lines rising to the right in the figure represent the relationship between the shutter speed and the aperture value in an environment of a predetermined brightness, respectively, in order to obtain an image of the same brightness. It represents.

According to the exposure control characteristic lines R and S, when viewing the environment of a predetermined brightness, the interlocking range of the aperture
In the range of (F number 2.8 to 11), the aperture value F number is set smaller at the time of preview image shooting than at the time of shooting a recorded image. For example, when shooting a recorded image, the aperture value F
Number 4, shutter speed 1/125 sec (point r1 in the figure)
Image shooting performed under the exposure setting described above, when the preview image is shot, the aperture value F number is 3
Then, the shutter speed is set to about 1/180 second (s1
point). According to this exposure characteristic, the exposure amount does not change between when the preview image is shot and when the recorded image is shot,
While the aperture value F-number is reduced to increase the aperture of the optical aperture 8, the shutter speed is set to a higher speed. As described above, the F-number is reduced, and
If the aperture of the lens is increased, the depth of field in that case becomes shallower. As a result, the focusing range is narrowed, and it is easy to confirm the focus by the preview image output to the built-in monitor 20.

When the optical stop 8 is fully opened
In (F number 2.8), the shutter speed is adjusted according to the brightness of the environment so that the brightness of the captured image is kept constant. The minimum shutter speed at the time of capturing a recorded image is set to 1/30 second within a range where blurring is not noticeable. Shutter speed when taking recorded images is 1/3 minimum speed
The image data is adjusted by the gain of the AGC 10 in a range where it takes 0 second and the brightness of the captured image cannot be adjusted optically. Also in this range, as in the case of the aperture interlocking range, the aperture value F-number is set smaller than that at the time of recording image recording, and the aperture of the optical aperture 8 is increased. For example, if the shutter speed is 1/30 second,
With respect to recording image photographing based on the set value (r2 point) of the AGC 10 having a gain of 6 dB, the shutter speed is set to be faster than 1/30 second when photographing a preview image.
(s2 points). According to such a setting, blurring that is more likely to occur as the shutter speed becomes lower at the time of shooting the preview image can be reduced, and focus confirmation using the preview image output to the built-in monitor 20 becomes easier.

As described above, in the present embodiment, the depth of field is reduced by making the aperture of the optical diaphragm 8 larger at the time of preview image shooting than at the time of recording image shooting,
The built-in monitor 20 can easily confirm the focus. In general, the lower the shutter speed, the greater the effect of camera shake on the image. In this way, on the lower shutter speed side, the shutter speed at the time of preview image shooting is set to be higher than at the time of recording image shooting. By doing so, the effect of blurring is reduced and the built-in monitor 20
The focus can be easily confirmed using the preview image output to the user.

The electronic still camera 1 also has an exposure characteristic for reproducing a preview image more naturally. FIG. 5 shows a camera control CPU 2 of the electronic still camera 1.
4 shows a characteristic line representing the programmed exposure characteristic. A solid line P and a broken line Q in the figure represent exposure characteristics applied at the time of capturing a recorded image and at the time of capturing a preview image, respectively. As can be seen from FIG. 5, according to this characteristic, in a bright environment, the shutter speed is set higher at the time of shooting a preview image than at the time of shooting a recorded image, and the image is reproduced brighter. On the other hand, in a dark environment, the shutter speed is set lower at the time of preview image shooting than at the time of recording image shooting, and the image is reproduced darker.

For example, when "white cloud" is the subject,
Shutter speed for recorded images during preview image shooting
The preview image is reproduced brighter in a bright environment by overexposure using a shutter speed (point q1) slower than (point p1). On the other hand, when the subject is “Night Festival”, the preview image is darker in a dark environment by using a shutter speed (point q2) faster than the shutter speed (point p2) at the time of recording the preview image. Reproduce. In this way, at the time of preview image shooting, the exposure amount of the monitor, which is influenced by the shooting environment, is corrected,
By playing back brighter in a bright environment and darker in a dark environment, a more natural preview image can be obtained.

In the exposure control processing according to the present embodiment, first, as described above, based on the exposure characteristics shown in FIG.
After setting the aperture of the optical diaphragm 8 at the time of photographing the preview image larger than that at the time of photographing the recorded image so that the exposure amount does not change, the state shown in FIG. By setting the shutter speed at the time of preview image shooting to be lower than at the time of recording image shooting so as to overexpose based on the exposure characteristics, a more natural image can be obtained when the preview image is output to the built-in monitor. In addition to the above, it is possible to facilitate focusing with the obtained preview image. In this embodiment, the case where the preview image is output to the built-in monitor 20 will be described. However, the output destination of the image data can be set to either the built-in monitor 20 or the external monitor 21. In the mode, the above-described exposure control characteristics may be used also when shooting with the output destination set to the external monitor 21.

Pixel Interpolation Processing Next, the pixel interpolation processing (steps S15 and S25 in FIG. 3) in the pixel interpolation section 13 will be described in detail with reference to a subroutine in FIG. In the electronic still camera 1 according to the present embodiment, the arrangement of pixels is R (red), G (green), B
A Bayer-array CCD 8 of (blue) is mounted, and Bayer-array image data is obtained. The pixel interpolating unit 13 interpolates missing pixels for each color in the image data of the Bayer array.

As shown in FIG. 6, first, in step S51,
In, image data in a Bayer array is input. This image data is color-separated into R, G, and B color data in step S52. The pixel interpolation unit 13 is provided with a plurality of interpolation means. In step S53, one of the plurality of interpolation means is selected in accordance with the image conditions captured in step S10 in FIG. Then, an interpolation process is performed based on the selected interpolation means (step S54). The image data after the interpolation processing is stored in step S
At 55, each color data is output to the band correction unit 14 (see FIG. 2).

Electronic still camera 1 according to the present embodiment
Are three types of interpolation means as interpolation means in the pixel interpolation unit 13, that is, interpolation means a using an average filter.
(A so-called averaging method), an interpolation means b using a median filter (a so-called median method), and an interpolation means c for performing simple interpolation using adjacent pixels. Hereinafter, these interpolation means a, b, and c will be described.

FIG. 7 is an explanatory diagram of the pixel interpolation means a.
CCD output pixel pattern input to pixel interpolation unit 13
The (Bayer array) image data 31 is separated into color data 32, 34, and 36 by performing a masking process with a different filter pattern for each of R, G, and B pixels. These color data 32, 34, and 36 are masked for pixels having colors other than the target color, that is, for missing pixels (black portions in the figure). In the pixel interpolation means a, each of the color data 32, 3 having these missing pixels
Interpolation filters 33, 35 and 37 having a 3 × 3 filter matrix are applied to 4 and 36, and for each color,
Missing pixels are interpolated by replacing each pixel value with an appropriate average of neighboring pixel values.

For example, the missing pixel 32 for the G data
Considering the case where A is obtained using the interpolation filter 33,
The value G of the pixel located at the top, bottom, left and right_Four, G₆, G₇, G₉From
Can be obtained by the following equation. For missing pixels,
The value is calculated as 0. G_Four× 1/4 + G₆× 1/4 + G₇× 1/4 + G₉× １ ／ = (G_Four+ G₆+ G₇+ G ₉ ) / 4 (1) Such interpolation processing is performed for all the missing images in the G data.
If the process is executed for the element, all the pixel data of G can be obtained.
Similarly, for the R and B color data, the filters 35,
By performing the interpolation process by R, B,
Pixel data is obtained. This pixel interpolation means a
The processing speed is relatively high.

FIG. 8 is an explanatory diagram of the pixel interpolation means b. The image data 41 of the CCD output pixel pattern (Bayer array) input to the pixel interpolation unit 13 is subjected to a masking process with a different filter pattern for each of R, G, and B pixels, as in the case of the interpolation means a. Is separated into the respective color data 42, 44 and 46. This interpolation means b
Then, for each of these color data 42, 44 and 46,
For G color data having pixels up to the high band, a median (intermediate value) filter 43 is applied, and the value of the missing pixel is replaced by the average value of the intermediate values of the four surrounding pixels, while R,
As for the color data B, the average filters 45 and 47 are applied as in the case of the interpolation means a, and the value of each pixel is replaced with an appropriate average of the values of neighboring pixels.

For example, the missing pixel 42 for the G data
When A is obtained using the median filter 43, first, the magnitudes of the values G ₄ , G ₆ , G ₇ , and G ₉ of the adjacent pixels located above, below, left, and right of the missing pixel 42A are compared. At this time, G ₆ <
If G ₄ <G ₉ <G ₇ , the intermediate two values are G ₄ and G ₉ , and the value of the pixel 42A is represented by (G ₄ + G ₉ ) / 2.
If this interpolation process is performed for all missing pixels,
The G color data is reproduced. This pixel interpolation means b
This is called a median method, and the processing speed is slower than that of the interpolation means a.

In this embodiment, pixel interpolating means c as shown in FIG. 9 may be used in place of pixel interpolating means a and b for obtaining the value of a missing pixel using an interpolation filter. In this interpolation means c, after the image data 51 is color-separated by filter patterns 52, 54, 56, 53, 55 and 57
As shown by the bold line in FIG. 3, after dividing into 2 × 2 pixels, one pixel is selected for each section, and the pixel value is given to the other three pixels. Are made simple interpolation. The interpolation means c is inferior to the interpolation means a and b in image quality, but has the highest processing speed.

The pixel interpolation process is performed for each of the preview image and the recorded image.
At the time of the preview image interpolation process (step S18 in FIG. 3), the pixel interpolation unit 13 uses an interpolation means having a high processing speed. Specifically, the image data is stored in the built-in monitor 2 according to the type of the output destination of the preview image.
When the image data is output to the external monitor 21, the interpolation means c is used.
Is used. According to this, the preview image data can be processed faster through all the processes, and the user can immediately confirm the state of the image to be shot through the built-in monitor 20 or the external monitor 21.

On the other hand, during the interpolation processing of the recorded image (step S25 in FIG. 3), various conditions set by the user before photographing are set.
According to the (captured image size and image compression ratio), the pixel interpolation unit 13 selects a predetermined interpolation unit from the above-described interpolation units having different processing speeds. Note that the interpolation means c having the highest processing speed has a greater effect on image quality than others, and is not used for interpolation processing of a recorded image. "Table 1"
The relationship between the photographed image size and the compression ratio of the image data set by the user at the time of the recording image data processing, and the interpolating means selected in accordance therewith is shown.

[0041]

[Table 1]

According to Table 1, for example, when the user sets the photographed image size to “512 × 384” and the image compression mode to “１ ／ JPEG compression”, the pixel interpolation unit 13 performs interpolation. Means a is used. In the present embodiment, especially when the compression ratio in the image compression unit 17 is set to be high, the interpolation unit a having a high processing speed is selected in the pixel interpolation unit 13. This is because the higher the compression ratio set by the user, the longer the compression process in the image compression unit 17 takes.
3 to compensate.

In the present embodiment, when the image quality is not regarded as important for the purpose of saving memory, in particular, the photographed image size is set to “512 × 384” and the image compression mode is set to “1”.
When “/ 8 JPEG compression” or “1/20 JPEG compression” is set, the interpolation unit a is selected in the pixel interpolation unit 13. In this case, since each process is performed at a relatively high speed in order to handle an image having a small amount of data, and the time loss in the image compression unit 17 is compensated in the pixel interpolation unit 13, so that the image data is Processed quickly. In this way, by increasing the image processing speed, particularly for shooting in which image quality is not regarded as important, it is possible to shoot more frames per time.

In this embodiment, the case where any one of the interpolation means a, b, and c is selected in the pixel interpolation unit 13 is described. However, more interpolation means having different processing speeds are provided. Then, a desired one may be selected from them. In this case as well, the higher the compression ratio set by the user, the faster the processing speed is selected by the interpolation means, so that a processing speed suitable for the image quality can be achieved.

[0045] As described above bandwidth control process, when comparing the frequency characteristic of each color data immediately after the pixel interpolation processing, the frequency characteristic of G, extends to the high-frequency than other colors R, and B I know that. This is due to the arrangement of the RGB filters based on G in the CCD 8 (the Bayer arrangement in the present embodiment). In order to correct such a decrease in resolution due to the missing pixels of R, G, and B, the band of the image data is controlled by the band correcting unit 14.

Hereinafter, the band control processing (steps S16 and S26 in FIG. 3) in the band correction section 14 will be described. FIG. 10 is a diagram showing an example of the flow of the band control process for each color data. First, the same band limitation as RB is applied to the G signal after the pixel interpolation processing using the GL-low-pass filter 61 provided with a 5 × 5 filter matrix, and a signal (low-band signal) composed of low-frequency components is obtained. Extract. The low-frequency signal is subtracted from the original signal of G by the subtraction circuit 62, so that a signal (middle-frequency signal) including a middle-frequency component is extracted. This mid-range signal is input to amplification sections 63 and 68. The signal amplified with a predetermined gain in the amplifying section 63 is added to the original signal for G in an adding circuit 64. On the other hand, the signals amplified by the amplifying section 68 are added to the R and B signals composed of low frequency components in the adders 69 and 71.

In this band correction processing, a signal (high-frequency signal) composed of high-frequency components is obtained by using a Laplacian filter 65 having a 3 × 3 filter matrix for the G signal after pixel interpolation. Is extracted. The extracted high-frequency signal is input to the amplifying unit 66, amplified by a predetermined gain, and then the base side of the waveform amplitude is removed by the clipping circuit 67. As a result, a desired high-frequency signal is obtained. The high-frequency signal is added to the original signal for G in the adding circuit 64, or added to the R and B signals in the adding circuits 70 and 72, respectively. As described above, the R, G, and B signals are band-controlled and output to the color balance control unit 15.

In the band correction section 14, the RGB signals are level-adjusted in accordance with the gain setting values (α, β) in the high-frequency correction amplification section 66 and the mid-range correction amplification sections 63, 68. The frequency characteristics of the signal are controlled. For example, when the gain α of the amplification unit 66 is set to a large value, the high frequency component of each signal is emphasized, and when the gain β of the amplification units 63 and 68 is set to a large value, each signal is Will be emphasized.

FIG. 11 shows four sets of gain setting values (α,
Regarding β), the frequency characteristic curves of the output signals obtained in each case are shown. Here, the horizontal axis represents frequency, and the vertical axis represents response. The characteristic curve a (solid line) indicates that the gain α in the high-frequency correction amplification unit 66 is 0,
3, 68 when the gain β is 0. The characteristic curve b (broken line) is obtained when the gain α in the high-frequency correction amplification unit 66 is 0.3 and the gain β in the middle-frequency correction amplification units 63 and 68 is 0. In this case, the high frequency components of each signal are emphasized. The characteristic curve c (dashed-dotted line) indicates the gain α in the amplifier 66 for high-frequency correction.
Is 0, and the gain β in the amplifiers 63 and 68 for mid-range correction.
Is 0.3. In this case, the middle frequency component of each signal is emphasized. The characteristic curve d (two-dot chain line) is obtained when the gain α in the high-frequency correction amplification unit 66 is 0.3 and the gain β in the middle-frequency correction amplification units 63 and 68 is 0.3. In this case, both the middle band component and the high band component of each signal will be emphasized.

As described above, the electronic still camera 1 according to the present embodiment has an image recording mode in which the type of the subject can be set. (Natural image mode) used when it is (natural image), "Gray text mode" used when it is a black and white graphic or its combination with characters and numbers, and "Binary text" used when it is only characters and numbers Mode ". In the present embodiment, the band correction unit 14 first sets a predetermined gain setting value according to the image recording mode set by the user.
(α, β) is selected.

The "natural image mode" is used as the image recording mode.
Is set, the gain setting value (α, β) in the band correction unit 14 is selected according to the photographed image size and the image data compression ratio set by the user. When “Natural image mode” is set in “Table 2”, the photographed image size and the image compression ratio set by the user, and the high-frequency amplification gain α in the high-frequency correction amplification unit 66 selected in accordance with the setting. And the relationship with the mid-range amplification gain β in the mid-range correction amplification units 63 and 68.

[Table 2] In the electronic still camera 1 according to the present embodiment, predetermined gain setting values (α, β) are set based on “Table 2” according to the image shooting size and the compression ratio of the image data set by the user.
Is selected. For example, if the captured image size is “512 × 3
84 pixels "and the image compression mode is" 1/8 JP "
When “EG compression” is set, the high-frequency amplification gain α takes a value of 1.5 and the mid-frequency amplification gain β takes a value of 0.1, and the mid-frequency component of each signal is emphasized.

As can be seen from Table 2, the lower the photographed image size, the larger the mid-range amplification gain α is set. As a result, the mid-range component of each signal is emphasized, and the correction with emphasis on contrast that suppresses the noise of the high-band component that originally has no information can be performed. On the other hand, when the photographed image size is high, the high-frequency amplification gain β is set large,
High-frequency emphasis is performed so that high-frequency signals can be reproduced.
Also, the higher the compression ratio of the image data, the smaller the high-frequency amplification gain β is set, so that the high-frequency characteristics are reduced and the occurrence of high-frequency noise is suppressed.

As described above, when the "natural image mode" is set, the band control is performed based on the set value of the predetermined amplification gain according to the size of the photographed image and the compression ratio of the image data. This makes it possible to compensate for the drop in the high frequency range of R and B due to the Bayer array based on G, and to obtain the frequency characteristics suitable for the characteristics of the subject while suppressing the coloring of the edges and the rotation of the colors. Can be.

Next, when the image recording mode is set to "gray text mode" or "binary text mode",
The gain setting value (α,
β) will be described. In the electronic still camera 1 according to the present embodiment, when “gray text mode” or “binary text mode” is set as the image recording mode, the captured image size is automatically set to “1024 × 768 pixels” Is done. Also, in this case, regardless of the compression ratio of the image data,
(α, β) is constant. Table 3 shows amplification gain setting values (α, β) used when setting each mode.

[Table 3] In the present embodiment, when the “gray text mode” is set as the image recording mode, the mid-range component is emphasized while the high-range component is suppressed. As a result, a sharply contoured image with high contrast and low noise is provided while maintaining color reproduction. Also, when the "binary text mode" is set, the mid-range component is maximized while the high-range component is suppressed.
By emphasizing the contour under these settings and then binarizing the image data, it is possible to reproduce an edge with little dot noise by the Bayer arrangement.

Next, the gradation conversion processing in the gamma correction section 16 will be described. In the gamma correction unit 16, the R, G, and B signals normalized by the color balance control block 14 are
Tone conversion is performed by a look-up table of 1024 tones and 256 tones. The gamma correction unit 16 includes:
The gradation characteristics according to the gamma curves A to D shown in FIG. 15 are set, and the image data is subjected to gradation conversion based on one of the gradation characteristics according to the gamma curves A to D according to the situation.

FIG. 12 shows a gamma curve A representing a normal characteristic used when the image output monitor is, for example, a personal computer CRT. The brightness of the surface of the output monitor is corrected in accordance with such a curve without being proportional to the input voltage, thereby obtaining a substantially linear light emission gradation. FIG.
3 shows a gamma curve B representing a gradation characteristic used when the output monitor is a personal computer CRT, similarly to the case of the gamma curve A. The gamma curve B is a curve that appears above the gamma curve A on the high input voltage side and below the gamma curve A on the low input voltage side. According to the gradation characteristic represented by the gamma curve B, the image is brighter on the bright side than when the normal characteristic of the gamma curve A is used.
On the dark side, a darker, higher-contrast, sharpened image is obtained.

The gamma curve C shown in FIG. 14 is used when the output monitor is the built-in monitor 20. The gradation characteristic of the gamma curve C is for setting an image to have a higher contrast than the normal characteristic for a personal computer CRT. With this setting, image data particularly suitable for output to the liquid crystal built-in monitor 20 is set. Is obtained. Further, the gamma curve D shown in FIG. 15 is used when the output monitor is an external monitor 21 such as a television. The curve D generally describes a curve appearing above the gamma curve A. According to the gradation characteristics of the gamma curve D, an image that is overall brighter than the normal characteristic of the gamma curve A is obtained. Obtainable.

The gamma curve D is selected and the curve D
, The image data is subjected to gradation conversion based on the gradation characteristics. As described above, since the preview image data is converted into an image suitable for the type of the output destination,
A preview image that is easy to see on the monitor can be obtained. This also allows the state of the image to be recorded to be confirmed more accurately.

At the time of the gradation conversion processing of the preview image,
While the gradation characteristic by the gamma curve C or D is used,
The gamma curve A
Or B is used. In the gradation conversion processing of the recorded image data, first, the gamma curve A or B is selected according to the type of the image recording mode set by the user. In the present embodiment, when the image recording mode is set to “gray text mode” or “binary text mode”, gradation conversion processing is performed based on the gradation characteristics by the gamma curve B. This makes it possible to reproduce a high-contrast image when capturing a text image such as character data.

On the other hand, when the image recording mode is set to the "natural image mode", one of the gamma curves A and B is further selected according to the size of the photographed image and the type of compression ratio of the image data. Table 4 shows the relationship between the photographed image size and the image data compression ratio set by the user, and the gamma curve used for the gradation conversion process in accordance with those. For example, when the image size is set to “640 × 480” and the image compression mode is set to “1/20 JPEG compression”, the gamma curve B is selected, and the gradation conversion is performed based on the gradation characteristics represented by the curve B. Processing is performed.

[Table 4] As can be seen from Table 4, when the compression ratio of the image data is set to be high, processing is performed based on the gradation characteristics by the gamma curve B, and the image is laid out to increase the contrast. In addition, it is possible to make the deterioration of the image quality at the time of compression / decompression inconspicuous. Similarly, even when the photographed image size is set to be small, the processing is performed based on the gradation characteristics based on the gamma curve B, so that the deterioration of the image quality on the monitor can be minimized.

As described above, the gamma correction section 16 selects a predetermined gamma curve in accordance with various conditions, and performs gradation conversion on the image data based on the gradation characteristic, thereby outputting the output image. Can be improved.

In this embodiment, the gamma correction unit 1
6 describes a case where any one of the four types of gamma curves A to D is selected. However, more gamma curves having different gradation characteristics are set, and a desired one is selected from among them. One may be selected. Also in this case, it is possible to select a desired one from the plurality according to the situation, and to make the image output on the monitor easier to see.

The present invention is not limited to the above-described embodiment, and it goes without saying that various improvements or design changes can be made without departing from the scope of the invention.

[0064]

As is apparent from the above description, according to the first aspect of the present invention, the band correcting section and the gamma correcting section respectively set the image data in accordance with the compression ratio of the image data set before the photographing by the user. Since the predetermined characteristics are selected, the frequency characteristics and the gradation characteristics of the image data can be favorably controlled. As a result, for each setting of the compression ratio, frequency characteristics and gradation characteristics that provide good image quality are obtained,
It is possible to suppress a decrease in image quality due to data compression.

Further, according to the second aspect of the present invention, the high frequency component noise of the image signal, which becomes larger as the data compression ratio is set higher, is suppressed, and the image quality by the data compression under the higher compression ratio setting is reduced. Roughness can be made inconspicuous.

Further, according to the third aspect of the present invention, when the user sets a high compression ratio before photographing, the gamma correction unit makes the bright part of the image brighter and the dark part darker. In addition, it is possible to make the image smoother and make the deterioration of the image quality due to the set compression ratio inconspicuous.

[Brief description of the drawings]

FIG. 1 is a perspective view of an electronic still camera according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of the electronic still camera.

FIG. 3 is a flowchart of a shooting operation of the electronic still camera.

FIG. 4 is a diagram showing exposure characteristics of the electronic still camera.

FIG. 5 is a diagram showing exposure characteristics of the electronic still camera.

FIG. 6 is a flowchart showing a subroutine executed by a pixel interpolation unit of the electronic still camera.

FIG. 7 is a diagram showing a flow of interpolation means a in the pixel interpolation section.

FIG. 8 is a diagram showing a flow of an interpolation means b in the pixel interpolation section.

FIG. 9 is a diagram showing a flow of interpolation means c in the pixel interpolation section.

FIG. 10 is a diagram showing a flow of each color data in a band correction unit of the electronic still camera.

FIG. 11 is a diagram illustrating frequency characteristics after processing in the band correction unit.

FIG. 12 is a gamma curve A representing a gradation characteristic used in a gamma correction unit of the electronic still camera.

FIG. 13 is a gamma curve B representing gradation characteristics used in the gamma correction unit.

FIG. 14 is a gamma curve C representing gradation characteristics used in the gamma correction unit.

FIG. 15 is a gamma curve D representing gradation characteristics used in the gamma correction unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Electronic still camera 6 ... Imaging lens 7 ... Optical aperture 8 ... CCD 10 ... AGC 12 ... Image processing CPU 13 ... Pixel interpolation part 14 ... Band correction part 16 ... Gamma correction part 17 ... Image compression part 20 ... Built-in monitor 21 ... External monitor 22 Memory card 24 Camera control CPU 27 Camera operation switch

Claims (3)

[Claims] An image processing apparatus includes a band correcting section for controlling frequency characteristics of each color data constituting image data, and a gamma correcting section for controlling gradation characteristics of image data. In the electronic still camera, the band correction unit is provided with control means capable of selecting a specific frequency characteristic from a plurality of frequency characteristics, and the gamma correction unit is provided with a specific gradation from a plurality of gradation characteristics. A control means for selecting characteristics is provided. In the band correction unit and the gamma correction unit, predetermined frequency characteristics and gradation characteristics are selected according to the compression ratio of image data set before photographing. An electronic still camera, characterized in that: 2. A frequency characteristic for suppressing a high frequency component when a high compression ratio of image data is set in the band correction unit.
An electronic still camera as described. 3. The electronic device according to claim 1, wherein the gamma correction unit selects a gradation characteristic for enhancing image contrast when a high compression ratio of the image data is set. Still camera.