JP3433653B2 - Electronic still camera - Google Patents

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 that records captured images 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. Since this electronic still camera generally handles image information having a larger amount of data than character information, it is often performed to compress the data to reduce the amount of data. As such a data-compressible electronic still camera, 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 sets the desired data compression rate depending on the situation,
It is possible to control the amount of data in each image, especially
If you want to save a large number of frames in a fixed capacity storage medium, or if you want to speed up the data transfer in data processing, the user can set a high compression rate,
It can be compressed so that the amount of image data becomes small.

[0003]

In such an electronic still camera, the higher the compression rate is set, the smaller the image data is compressed, but on the other hand, the deterioration of the image quality becomes large. By the way, a conventional electronic still camera generally has, for example, a band correction unit that controls a frequency characteristic of a signal to suppress noise, a gamma correction unit that controls a gradation characteristic to adjust the contrast of an image, and the like. Is provided for controlling the signal characteristics that affect the image quality. However, conventionally, such characteristic control is not performed according to the setting of the compression rate, and particularly when shooting is performed under the setting of the high compression rate, a high resolution for obtaining a fine image is obtained. Even in shooting, since the deterioration of the image quality due to the change in the content of the image quality due to the data compression is unavoidable, the original picture making is often not useful.

Therefore, according to the present invention, in the band control section and the gamma correction section, good frequency characteristics and gradation characteristics are respectively controlled according to the compression rate of the image data set by the user, and the image is compressed by data compression. An object of the present invention is to provide an electronic still camera capable of suppressing deterioration in quality.

[0005]

The invention according to claim 1 of the present application (hereinafter referred to as the first invention) includes a band correction unit for controlling the frequency characteristic of each color data forming image data, and a floor of the image data. In an electronic still camera that has a gamma correction unit that controls the tonal characteristics and is capable of setting the compression ratio of image data before shooting, select a specific frequency characteristic from a plurality of frequency characteristics in the band correction unit. In addition to the control means for obtaining the same, the gamma correction section is provided with control means for selecting a specific gradation characteristic from a plurality of gradation characteristics, and according to the compression ratio of the image data set before photographing, In the band correction unit and the gamma correction unit, predetermined frequency characteristics and gradation characteristics are selected, respectively.

The invention according to claim 2 of the present application (hereinafter,
The second invention) is characterized in that, in the band correction unit, when the compression rate of the image data is set high, the frequency characteristic for suppressing the high frequency component of the frequency is selected.

The invention according to claim 3 of the present application (hereinafter,
The third invention) is characterized in that the gamma correction section selects a gradation characteristic that emphasizes the contrast of the image when the compression rate of the image data is set high.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. As shown in Figure 1,
The electronic still camera 1 according to the present embodiment has a shutter button 2, an image pickup lens 6, and a built-in CCD 8. In addition, the electronic still camera 1 has a CCD
A memory card that stores image data obtained by performing a predetermined process on the electric signal converted in 8 (not shown: see FIG. 2 described later)
There is a card insertion slot 5 for inserting and connecting
The memory card connected to the card insertion slot 5 is ejected 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 an image pickup lens 6, an optical diaphragm 7 for controlling the amount of light incident from the image pickup 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 electrical signals (hereinafter, A
It has an A / D conversion unit 11 that performs a / D conversion), and an image processing CPU 12 that performs a predetermined image processing on the digital signal obtained through each of the above components.

The image processing CPU 12 has a pixel interpolating unit 13 for interpolating image data of each color obtained by color-separating the image data, and band control of each frequency for the image data of each color. A band correction unit 14 for
A color balance control unit 15 for independently correcting 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 the built-in monitor 20 or the external monitor 21, and the memory card 22. A memory card driver 19 for supplying image data to

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 the image condition setting capable of setting the photographed image size and the compression rate before photographing are set. A camera operation switch 27 including a switch, a photometric / colorimetric sensor 28 for measuring the amount of light and color at the time of shooting, 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 amount and color measured by the photometric / colorimetric sensor 28, and the exposure control data is used to calculate the aperture value of the optical aperture 7 and the CCD.
8 accumulation time (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 the data bus.
It is connected to the.

With 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, the captured image size that determines the image resolution and the data compression rate in the image compression process can be set as the image conditions.

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 rate of the image data,
There are three types of "no compression mode", "1/8 JPEG compression mode", and "1/20 JPEG compression mode", and the user can select the desired one before shooting, as with the above image size. You can

In the electronic still camera 1 according to the present embodiment, the image recording mode showing the type of subject is
"Natural image mode" for natural objects such as people and landscapes in natural colors, "Gray text mode" for capturing natural objects or a combination of letters and numbers in black and white, "Binary value" for characters and numbers Three types of “text mode” are provided, and the user can select one desired mode from them according to the type of 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 the external monitor side.

The basic photographing operation of the electronic still camera 1 will be described below with reference to the flowchart of FIG. In the electronic still camera 1 according to the present embodiment,
The user sets various conditions such as the captured image size and the compression rate in the compression process before the image capturing.
First, these set various conditions are fetched (step S10).

The image processing in the electronic still camera 1 at the time of shooting is basically the image processing when the shutter button 2 is half-pressed (steps S11 to S21) and the image processing when the shutter button 2 is fully pressed (steps S11 to S21). Steps S22 to S33). In the present embodiment, the electronic still camera 1 has a preview function of displaying the image incident from the imaging lens on the built-in monitor 20 when the shutter button 2 is half-pressed.
The state of the image that the user wants to take with the displayed image
You can check (balance and composition of the picture). When the shutter button 2 is further pressed to bring it into a completely pressed state, the image incident from the imaging lens 6 at that time is processed and then recorded in 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, the image to be captured is treated as a preview image and subjected to predetermined image processing. Step S11
When the shutter button 2 is half-depressed, the camera 1 first measures the light amount and color in the photometric / colorimetric sensor 28 (step S12). Then, based on the measurement data, exposure setting is performed for the aperture value of the optical aperture 7, the storage time of the CCD 8 (that is, shutter speed), etc. (step S13). After the exposure is set, light is incident from the image pickup lens 6 and converted into an electric signal by the CCD 8. The camera 1 A / D converts this electric signal 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 the missing pixel for each color data (step S15). This pixel interpolator 1
3 is provided with a plurality of interpolation means, and step S
The optimum interpolating means is selected from these in accordance with the set values of the captured image size and the image compression rate captured in 10.

The image data after pixel interpolation is processed by the band correction unit 1.
The contour is corrected in step 4 (step S16), and step S17
The color balance controller 15 performs color correction for each color data, and the gamma correction block 16 performs gradation conversion (step S18). Then, the image data is temporarily written in the image memory 23 by the image processing CPU 12 (step S19).

After that, the image data is read from the image memory 23, and the video encoder 18 sends the NTSC / P data.
It is encoded into AL (step S20) and output as a preview image to the built-in monitor 20 (step S2).
1). When the shutter button 2 is maintained in the 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. The user can determine whether or not to shoot after confirming the preview image, and YES.
In the case of, the shutter button 2 is pushed in completely.

When the shutter button 2 is completely pressed, the image processing when the shutter button 2 is completely pressed is performed in steps S22 to S31. The flow of this processing is half that of the shutter button 2 described above. Since it is the same as in the pressed state, it is omitted here. However, in this processing, image data to be recorded in 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 rate of the image data captured in step S10. This image compression requires more time as the compression rate is higher. The compressed image data is recorded in the memory card 22 as a recording image in step S33. In step S34, the user determines whether to end the shooting,
If YES, the shooting operation ends, and if NO,
The steps after step S11 are repeated. The image to be captured is processed according to the above flow, and 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 based on the exposure control data calculated by
6 and the AGC 10 are controlled, and the aperture value of the optical diaphragm 8, the storage time of the CCD 8 (so-called electronic shutter speed), and the gain of the AGC 10 are set in each unit.

FIG. 4 shows the automatic exposure control characteristic programmed in the camera control CPU 24. A solid line R and a broken line S in the figure are characteristic lines representing the exposure control characteristics used at the time of capturing each image of the recorded image and the preview image, respectively. This exposure control characteristic is determined by the aperture value of the optical aperture 7, the storage time of the CCD 8, and the gain of the AGC 10. The diagonal 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. To represent.

According to these exposure control characteristic lines R and S, when viewed in an environment of a predetermined brightness, the interlocking range of the diaphragm
Within (F number 2.8 to 11), the aperture value F number is set smaller when the preview image is captured than when the recorded image is captured. For example, when shooting a recorded image, the aperture value F
Number 4, shutter speed 1/125 second (r1 point in the figure)
With regard to the image shooting performed under the exposure setting, the aperture value F number is 3 when the preview image is shot.
, 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 taken and when the recorded image is taken,
While the aperture value F number is reduced and the aperture of the optical aperture 8 is increased, the shutter speed is set higher. In this way, the F-number is reduced and the optical aperture 8
The larger the aperture of, the smaller the depth of field in that case. As a result, the focus range is narrowed, and it becomes easier to confirm the focus with the preview image output to the built-in monitor 20.

When the optical diaphragm 8 is fully opened
For (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 seconds within a range where blurring is not noticeable. Shutter speed when recording images is 1/3 minimum
The image data is adjusted by the gain in the AGC 10 within a range where the brightness of the photographed image cannot be optically adjusted for 0 seconds. 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, shutter speed is 1/30 second,
Regarding the recording image shooting based on the setting value (r2 points) where the gain of the AGC 10 is 6 dB, the shutter speed is set to a speed higher than 1/30 seconds at the time of shooting the preview image.
(s 2 points). With such a setting, when a preview image is captured, it is possible to reduce blurring that tends to occur as the shutter speed becomes slower, and it becomes easier to confirm the focus with the preview image output to the built-in monitor 20.

As described above, in the present embodiment, the depth of field is made shallower by making the aperture of the optical diaphragm 8 larger when photographing a preview image than when photographing a recorded image.
The focus of the built-in monitor 20 can be easily confirmed. Also, generally, the lower the shutter speed, the greater the influence of camera shake on the image. Thus, on the low shutter speed side, the shutter speed at the time of preview image shooting is set higher than that at the time of recording image shooting. By doing so, the influence of blurring is reduced, and the built-in monitor 20
This makes it easy to check the focus with the preview image output to.

The electronic still camera 1 also has an exposure characteristic for reproducing the 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 drawing represent the exposure characteristics applied at the time of shooting the recorded image and at the time of shooting the preview image, respectively. As can be seen from FIG. 5, according to this characteristic, in a bright environment, the shutter speed is set to a higher speed during preview image shooting than during recording image shooting, and the image is reproduced brighter. On the other hand, in a dark environment, the shutter speed is set to be slower when the preview image is captured than when the recorded image is captured, and the image is reproduced darker.

For example, when the "white cloud" is the subject,
Shutter speed for recorded images when shooting preview images
The shutter speed (point q1) slower than (point p1) is used to overexpose to reproduce the preview image brighter in a bright environment. On the other hand, when the "night festival" is the subject, the preview image is made darker in a dark environment by using the shutter speed (point q2) faster than the shutter speed (point p2) at the time of recording the image when capturing the preview image. Reproduce. In this way, when shooting a preview image, the monitor's visible exposure amount, which is affected by the shooting environment, is corrected,
By reproducing the image 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.
In order to prevent the exposure amount from changing, the aperture of the optical diaphragm 8 at the time of shooting a preview image is set to be larger than that at the time of capturing a recorded image, and then the aperture of the optical diaphragm 8 is kept constant, as shown in FIG. By setting the shutter speed at the time of shooting the preview image to be slower than that at the time of recording the image to prevent overexposure based on the exposure characteristics, a more natural image can be displayed when the preview image is output to the built-in monitor. In addition to being able to obtain the image, it is possible to facilitate focusing with the obtained preview image. In the present embodiment, the case where the preview image is output to the built-in monitor 20 is described, but the CRT on / off in which the output destination of the image data can be set to either the built-in monitor 20 or the external monitor 21 is described. In the mode, the exposure control characteristics described above may be used also when shooting is performed 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 unit 13 will be described in detail with reference to the subroutine in FIG. In the electronic still camera 1 according to the present embodiment, the pixel arrangement is R (red), G (green), B.
The Bayer array CCD 8 (blue) is mounted, and Bayer array image data can be obtained. The pixel interpolating unit 13 interpolates the missing pixels for each color in the image data of the Bayer array.

As shown in FIG. 6, first, step S51.
At, the Bayer array image data is input. This image data is color-separated into R, G, and B color data in step S52. The pixel interpolating unit 13 is provided with a plurality of interpolating means, and in step S53, one of the plurality of interpolating means is selected in accordance with the image condition fetched in step S10 of FIG. Then, the interpolation processing based on the selected interpolation means is executed (step S54). The image data after the interpolation processing is performed 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 interpolating means as the interpolating means in the pixel interpolating unit 13, that is, an interpolating means a using an average filter.
It has a so-called averaging method, an interpolating means b using a median filter (so-called median method), and an interpolating means c for performing simple interpolation using adjacent pixels. Below, these interpolation means a, b, and c will be described.

FIG. 7 is an explanatory diagram of the pixel interpolating means a.
CCD output pixel pattern input to the pixel interpolation unit 13
The (Bayer array) image data 31 is separated into color data 32, 34, and 36 by being masked with different filter patterns for each of the R, G, and B pixels. The respective color data 32, 34 and 36 are masked for pixels having colors other than the target color, that is, missing pixels (black portions in the figure). In the pixel interpolating means a, each 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 the value of each pixel with an appropriate average of neighboring pixel values.

For example, the missing pixel 32 for G data
Considering the case of obtaining A using the interpolation filter 33,
The value G of the pixels located vertically and horizontally_Four, G₆, G₇, G₉From below
It can be obtained by the formula. Regarding missing pixels
The value is calculated as 0.   G_Four× 1/4 + G₆× 1/4 + G₇× 1/4 + G₉× 1/4 = (G_Four+ G₆+ G₇+ G ₉ ) / 4 ... (1) Such an interpolation process can be applied to all missing images in G data.
If it is executed for the prime, all pixel data of G is obtained.
Similarly, for the R and B color data, the filters 35,
By executing the interpolation processing by 37, all R and B
Pixel data is obtained. This pixel interpolating means a is
It is called and has a relatively high processing speed.

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

For example, the missing pixel 42 for G data
When A is obtained by 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 on the upper, lower, left, and right sides 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 executed for all missing pixels,
All G color data are reproduced. This pixel interpolation means b is
This is called the median method, and the processing speed is slower than that of the interpolation means a.

In the present embodiment, pixel interpolating means c as shown in FIG. 9 may be used in place of the pixel interpolating means a and b for obtaining the value of the missing pixel using the interpolation filter. In the interpolation means c, the image data 51 is color-separated by the filter patterns 52, 54 and 56, and then 53, 55 and 57.
As shown by the thick line in FIG. 1, after partitioning into 2 × 2 pixels, one existing pixel is selected for each partition, and the pixel value is given to the other three pixels. A simple interpolation is performed to make This interpolating means c is inferior to the above-mentioned interpolating means a and b in image quality, but has the highest processing speed.

Pixel interpolation processing is carried out for each of the preview image and the recorded image, but in the present embodiment,
At the time of the interpolation processing of the preview image (step S18 in FIG. 3), the pixel interpolating unit 13 always uses the interpolating means having a high processing speed. Specifically, the image data is stored in the built-in monitor 2 according to the type of the preview image output destination.
When the image data is output to 0, the interpolating means c is used, and when the image data is output to the external monitor 21, the interpolating means a 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 captured through the built-in monitor 20 or the external monitor 21.

On the other hand, at the time of interpolation processing of the recorded image (step S25 in FIG. 3), various conditions set by the user before photographing are set.
According to (captured image size and image compression rate), the pixel interpolating unit 13 selects a predetermined interpolating unit from the above-described interpolating units having different processing speeds. The interpolation means c, which has the highest processing speed, has a greater effect on the image quality than the others, and is not used for the interpolation processing of the recorded image. In "Table 1",
The relationship between the photographed image size and the image data compression rate set by the user at the time of processing the recorded image data 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 "1/8 JPEG compression", the pixel interpolation unit 13 performs interpolation. Means a are used. In the present embodiment, particularly when the compression rate in the image compression unit 17 is set to be high, the pixel interpolation unit 13 selects the interpolation means a having a high processing speed. This is because the higher the compression rate set by the user, the longer the compression process in the image compression unit 17, and the pixel interpolation unit 1
This is to compensate in 3.

Further, in the present embodiment, when the image quality is not emphasized for the purpose of saving the memory, 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 pixel interpolating unit 13 selects the interpolation means a. In this case, since each image is processed with a small amount of data, each process is performed at a relatively high speed and the image compression is performed.
Since the time loss in the R> section 17 is compensated in the pixel interpolating section 13, the image data is processed faster throughout. In this way, by improving the image processing speed, especially for shooting where image quality is not important,
More frames can be taken per hour.

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

Band control processing As described above, when the frequency characteristics of the respective color data immediately after the pixel interpolation processing are compared, the frequency characteristics of G extend to a higher range than the other colors R and B. I know that. This is due to the array of RGB filters (Bayer array in this embodiment) based on G in the CCD 8. In order to correct the decrease in resolution due to such missing of R, G, and B pixels, the band correction unit 14 controls the band of the image data.

The band control process (steps S16 and 26 in FIG. 3) in the band correction unit 14 will be described below. FIG. 10 is a diagram showing an example of the flow of band control processing for each color data. First, a G signal after pixel interpolation processing is subjected to the same band limitation as RB by using a GL-low pass filter 61 having a 5 × 5 filter matrix, and a signal composed of low frequency components (low frequency signal) is obtained. Extract. This low-frequency signal is subtracted from the original signal for G by the subtraction circuit 62, so that a signal including a middle-frequency component (middle-frequency signal) is extracted. This mid-range signal is input to the amplification units 63 and 68. The signal amplified by the amplifier 63 with a predetermined gain is added to the original signal for G by the adder circuit 64. On the other hand, the signals amplified by the amplifier 68 are added to the R and B signals composed of low frequency components in the adder circuits 69 and 71.

Further, in this band correction processing, by using the Laplacian filter 65 provided with a 3 × 3 filter matrix for the G signal after pixel interpolation, a signal composed of high frequency components (high frequency signal) is generated. To be extracted. The extracted high frequency signal is input to the amplification unit 66, amplified by a predetermined gain, and then the clip circuit 67 removes the base side of the waveform amplitude, so that a desired high frequency signal is obtained. This high frequency signal is added to the original signal for G in the adder circuit 64, or added to the R and B signals in adder 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 unit 14, the RGB signals are level-adjusted according to the gain setting values (α, β) in the high-frequency correction amplification unit 66 and the middle-frequency correction amplification units 63, 68, and The frequency characteristics of the signal are controlled. For example, when the gain α of the amplifier 66 is set to a large value, the high frequency component of each signal is emphasized, and when the gain β of the amplifiers 63 and 68 is set to a large value, each signal is increased. The midrange component of is emphasized.

FIG. 11 shows four sets of gain setting values (α,
For β), the frequency characteristic curve of the output signal obtained in each case is shown. Here, the horizontal axis represents frequency and the vertical axis represents response. The characteristic curve a (solid line) shows that the gain α in the high-frequency correction amplification section 66 is 0, and the mid-frequency correction amplification section 6 is
This is the case where the gain β at 3,68 is 0. The characteristic curve b (broken line) is for a case where the gain α in the high-frequency correction amplification section 66 is 0.3 and the gain β in the mid-frequency correction amplification sections 63 and 68 is 0. In this case, the high frequency component of each signal is emphasized. The characteristic curve c (dashed-dotted line) is the gain α in the amplification unit 66 for high frequency correction.
Is 0, the gain β in the amplifying units 63 and 68 for midrange correction
Is for 0.3. In this case, the middle frequency component of each signal is emphasized. The characteristic curve d (two-dot chain line) is for a case where the gain α in the amplification unit 66 for high frequency correction is 0.3 and the gain β in the amplification units 63, 68 for medium frequency correction is 0.3. In this case, the mid-range component and high-range component of each signal are emphasized together.

By the way, as described above, the electronic still camera 1 according to the present embodiment has the image recording mode in which the type of the subject can be set. As the image recording mode, the subject has a natural color graphic. "Natural image mode" used when it is (natural image), "Gray text mode" used when it is a monochrome graphic or a combination of it and letters or numbers, and "Binary text" used when it is only letters or 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.

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

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

As can be seen from "Table 2", the lower the captured 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 it is possible to perform the contrast-oriented correction for suppressing the noise of the high-range component that originally has no information. On the other hand, when the captured image size is high, the high-frequency amplification gain β is set large,
High-frequency emphasis is performed so that even high-frequency signals can be reproduced.
Further, the higher the image data compression rate, the smaller the high-frequency amplification gain β is set, so the high-frequency characteristic is lowered, 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 captured image size and the compression ratio of the image data. As a result, it is possible to compensate for the lowering of high frequencies of R and B due to the Bayer arrangement based on G, and to obtain the frequency characteristic suitable for the characteristic of the subject while suppressing the coloring of the edges and the color rotation. You can

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 the “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”. To be done. In this case, the gain setting value for each mode is set regardless of the image data compression ratio.
(α, β) is constant. “Table 3” shows the 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 middle frequency component is emphasized while the high frequency component is suppressed. As a result, a high-contrast, low-noise sharp image with a contour is provided while maintaining color reproduction. Also, when the "binary text mode" is set, the midrange component is maximized while the highrange component is suppressed.
By emphasizing the contour under such a setting and then binarizing the image data, it is possible to reproduce an edge with less dot noise by the Bayer array.

Gradation Conversion Processing Next, the gradation conversion processing in the gamma correction unit 16 will be described. In the gamma correction unit 16, the R, G, B signals normalized by the color balance control block 14 are
The gradation conversion is performed by the look-up table of 1024 gradations and 256 gradations. The gamma correction unit 16 has a configuration shown in FIG.
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 any of the gradation characteristics according to the gamma curves A to D depending on the situation.

FIG. 12 shows a gamma curve A representing the 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 not proportional to the input voltage and is corrected by such a curve to obtain a substantially linear light emission gradation. Also, FIG.
Similarly to the case of the gamma curve A, FIG. 3 shows a gamma curve B representing the gradation characteristic used when the output monitor is a personal computer CRT. The gamma curve B depicts 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 brighter side is brighter than the case where the normal characteristic of the gamma curve A is used.
On the dark side, a darker high-contrast image with a sharper contrast can be obtained.

Further, 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 sets the image to have a higher contrast than the normal characteristic intended for a personal computer CRT, and by this setting, image data particularly suitable for output to the liquid crystal built-in monitor 20. Is obtained. Furthermore, the gamma curve D shown in FIG. 15 is used when the output monitor is the external monitor 21 such as a television. The curve D generally represents a curve that appears above the gamma curve A. According to the gradation characteristics of the gamma curve D, a brighter image can be obtained as a whole as compared with the normal characteristics of the gamma curve A. Obtainable.

For the gamma curve D, the curve D is selected.
The image data is subjected to gradation conversion based on the gradation characteristics of In this way, the preview image data is converted into an image suitable for it according to the type of the output destination,
A preview image that is easy to see on the monitor can be obtained. Further, this makes it possible to more accurately check the state of the image to be recorded.

During 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 is used for gradation conversion processing of recorded image data.
Or B is used. In the gradation conversion process 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 the “gray text mode” or the “binary text mode”, the gradation conversion process is performed based on the gradation characteristic of the gamma curve B. This makes it possible to reproduce a high-contrast image when shooting a text image such as character data.

On the other hand, when the image recording mode is set to the "natural image mode", either the gamma curve A or B is selected according to the size of the captured image and the type of compression rate of the image data. "Table 4" shows the relationship between the photographed image size and the image data compression rate set by the user, and the gamma curve used for the gradation conversion processing in accordance with them. 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 characteristic represented by the curve B. Processing is performed.

[Table 4] As can be seen from "Table 4", when the compression rate of the image data is set to be high, the processing is performed based on the gradation characteristic of the gamma curve B, the image is sharpened, and the contrast is increased. 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 can be performed based on the gradation characteristic by the gamma curve B, and the deterioration of the image quality on the monitor can be made inconspicuous to the maximum extent.

As described above, the gamma correction unit 16 selects a predetermined gamma curve according to various conditions and performs gradation conversion of the image data based on the gradation characteristic of the output image. The visibility of can be improved.

In the present embodiment, the gamma correction unit 1
6 describes the 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 gamma curve is selected from them. One may be selected. Also in this case, it is possible to select a desired one from a plurality of ones according to the situation and make the image output on the monitor easier to see.

Further, it is needless to say that the present invention is not limited to the above embodiments, and various improvements and design changes can be made without departing from the gist thereof.

[0064]

As is apparent from the above description, according to the first invention of the present application, the band correction unit and the gamma correction unit respectively adjust the image data compression ratio set by the user before photographing. Since the predetermined characteristics are selected, the frequency characteristics and gradation characteristics of the image data can be controlled well. As a result, for each compression rate setting, frequency characteristics and gradation characteristics that provide good image quality are obtained,
It is possible to suppress deterioration of image quality due to data compression.

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

Further, according to the third invention of the present application, when the user sets a high compression rate before photographing, in the gamma correction section, a bright part of an image becomes brighter and a dark part becomes darker. , It is possible to add sharpness to the image and make the deterioration of the image quality due to the set compression rate inconspicuous.

[Brief description of 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 subroutine by a process of a pixel interpolation unit of the electronic still camera.

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

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

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

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 showing 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 a gradation characteristic used in the gamma correction unit.

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

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

[Explanation of symbols]

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

─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-5-75978 (JP, A) JP-A-5-153439 (JP, A) JP-A-7-336718 (JP, A) JP-A-8- 30781 (JP, A) JP-A-8-163503 (JP, A) Actually developed 60-177565 (JP, U) (58) Fields investigated (Int.Cl. ⁷ , DB name) H04N 9/68 H04N 5 / 202 H04N 5/208 H04N 5/232

Claims (3)

(57) [Claims] 1. A compression ratio of image data before image capturing, comprising a band correction unit for controlling frequency characteristics of each color data forming image data and a gamma correction unit for controlling gradation characteristics of image data. In an electronic still camera capable of setting, the band correction unit is provided with a control unit 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 capable of selecting a characteristic is provided, and a predetermined frequency characteristic and a gradation characteristic are respectively selected by the band correction unit and the gamma correction unit according to the compression ratio of the image data set before photographing. An electronic still camera characterized in that 2. The band correction unit selects a frequency characteristic that suppresses a high frequency component of a frequency when the compression rate of image data is set high.
Electronic still camera described. 3. The electronic device according to claim 1, wherein in the gamma correction unit, a gradation characteristic that enhances image contrast is selected when the compression rate of image data is set high. Still camera.