JP4385749B2 - Electronic camera having white balance adjustment function and program - Google Patents

Description

The present invention relates to an electronic camera having a white balance adjustment function.
The present invention also relates to a program for causing a computer to perform white balance adjustment.

  In general flash photography, a subject is illuminated with light in a state in which flash light emitted from an electronic flash device is mixed with external light such as sunlight or streetlight. The irradiation ratio of the flash light and the external light slightly changes depending on the distance to the subject and the angle of the subject. Therefore, the color temperature of the mixed light varies greatly depending on the position in the screen.

For example, when flash photography is performed on a human subject that is backlit by sunlight in the evening, the human subject is mainly illuminated by flashlight. As a result, the color temperature of the human subject shows a high color temperature of flash (such as 5000K). On the other hand, the surrounding background portion is illuminated by evening sunlight. Therefore, the color temperature of the background portion is a low color temperature in the evening (eg, 3000K).
If the color temperature is uneven in the same screen as described above, it is very difficult to perform an appropriate white balance adjustment.

Japanese Patent Application Laid-Open Publication No. 2005-228561 discloses a conventional apparatus that performs such white balance adjustment during flash photography in response to such a problem.
In this conventional apparatus, preliminary shooting without flash and main shooting with flash are performed. Next, the conventional apparatus obtains a luminance difference in pixel units between the two captured images. The region where the luminance difference is larger is the region that is brightly illuminated by the flash, and the region where the color temperature of the flash is stained. On the other hand, the smaller the luminance difference, the smaller the influence of the flash light, and the region dyed by the color temperature of external light.
In view of this, this conventional apparatus performs white balance adjustment based on the flash for an area having a large luminance difference. On the other hand, white balance adjustment based on outside light is performed for an area where the luminance difference is small. Further, for an area with an intermediate luminance difference, white balance adjustment is performed based on the intermediate light between the flashlight and the outside light.
JP-A-8-051632 (Claim 3)

  As described above, the conventional apparatus obtains the luminance change in pixel units caused by the presence or absence of flash, and estimates the light source color temperature by determining the degree of flash hit from the magnitude of the luminance change.

  However, in this conventional apparatus, it is impossible to capture a captured image without flashing at the same time. Therefore, a time difference always occurs between these two captured images. If subject movement, camera shake, or camera shake occurs during this time difference, the images of the two captured images do not exactly match. Since a luminance difference also occurs due to such a picture shift, it is impossible to accurately detect only a luminance change caused by flash emission. Therefore, it has been impossible for the conventional apparatus to perform appropriate white balance adjustment on a moving subject or the like.

  In order to avoid such a problem of the conventional apparatus, the subject must be limited to a stationary object, and the electronic camera must be fixed to a tripod. There was a point.

  In view of such problems, an object of the present invention is to provide a technique for more reliably determining the effect of flash emission even under severe conditions such as a moving subject and appropriately performing white balance adjustment during flash photography. Is to provide.

<Claim 1>
An electronic camera according to a first aspect includes an imaging unit, an imaging control unit, a histogram calculation unit, a histogram correlation detection unit, a white balance calculation unit, and a white balance adjustment unit as described below.
The imaging unit captures a subject image.
The imaging control unit controls the imaging unit to generate a test image that is imaged without flashing and a main image that is captured with flashing light emitted.
The histogram calculation unit obtains a histogram distribution for each of the test image and the main image.
The histogram correlation detection unit detects a correlation for these two histogram distributions.
When the degree of this correlation is low, the white balance calculation unit determines that the flash illumination is not uniform, and changes the white balance adjustment value to emphasize the color temperature of the flash. On the other hand, when the degree of correlation is high, it is determined that the flash illumination is uniform (including the case where the illumination is not uniformly reached), and is changed to a white balance adjustment value that emphasizes the color temperature of the main image.
The white balance adjustment unit performs white balance adjustment on the main image according to the white balance adjustment value obtained by the white balance calculation unit.

<Claim 2>
According to a second aspect of the present invention, in the electronic camera of the first aspect, the white balance calculation unit obtains the first white balance adjustment value determined based on the color temperature of the flash and the color temperature of the main image. This is a means for obtaining a weighted composite value with the second white balance adjustment value to obtain a white balance adjustment value. In this case, the white balance calculation unit increases the weighted distribution of the first white balance adjustment value when the degree of correlation is low. Conversely, when the degree of correlation is high, the weighted distribution of the second white balance adjustment value is increased.

<Claim 3>
According to a third aspect of the present invention, in the electronic camera according to the first or second aspect, the white balance calculating unit obtains the frequency of overexposure and / or blackout for the main image. When this frequency exceeds a predetermined threshold value, the white balance calculation unit stops the determination of the uniformity of the flash illumination.

<Claim 4>
According to a fourth aspect of the present invention, in the electronic camera according to any one of the first to third aspects, the imaging unit performs a photometric image sensor for performing an image-capturing operation for split photometry and an image-capturing operation for recording and storage. And an image sensor for recording to be performed. On the other hand, the imaging control unit controls the photometric imaging element in the absence of flash to capture a test image. In addition, the imaging control unit controls the recording imaging element in the presence of the flash to capture the main image.

<Claim 5>
According to a fifth aspect of the present invention, in the electronic camera according to the fourth aspect, the histogram calculation unit suppresses a difference between two histogram distributions caused by an element difference between the photometric image sensor and the recording image sensor.

<Claim 6>
An electronic camera according to a sixth aspect is the electronic camera according to any one of the first to fifth aspects, wherein the histogram correlation detecting unit determines a predetermined frequency value from two histogram distributions calculated by the histogram calculating unit. After removing the following, the correlation between two histogram distributions is detected.

<Claim 7>
The program according to claim 7 uses a test image captured without flashing and a main image captured with flashing. This program causes a computer to execute the following processes. In the histogram calculation process, a histogram distribution is obtained for each of the test image and the main image. In the histogram correlation detection process, the correlation between the two histogram distributions calculated by the histogram calculation process is detected. In the white balance calculation process, when the degree of correlation is low, it is determined that the flash illumination is non-uniform, and the white balance adjustment value is changed with emphasis on the color temperature of the flash. Also, in the white balance calculation process, when the degree of correlation is high, it is determined that the flash illumination is uniform (including the case where the illumination has not been uniformly reached), and the white balance adjustment value that emphasizes the color temperature of the main image Change to In the white balance adjustment processing, white balance adjustment is performed on the main image according to the white balance adjustment value obtained in the white balance calculation processing.

<Claim 1>
The electronic camera according to claim 1 generates a test image without flash and a main image with flash. Next, the electronic camera obtains histogram distributions of these two images and determines how similar the histogram distributions of both images are.

  At this time, it can be determined that as the histogram distributions of the two images are different, the change in the luminance distribution from the test image to the main image is uneven, and the flash illumination is uneven. In this case, in the screen of the main image, there are a mixture of an image area where the flash illumination is well applied and an image area where the flash illumination does not reach.

  If the light source color temperature is estimated from the main image in this state, the light source color temperature that does not exist in the screen is erroneously output by, for example, forcibly averaging the uneven light source color temperature in the screen. If this incorrect light source color temperature is used as a reference, improper white balance adjustment will be performed in both areas where the flash illumination is well applied and areas where the flash illumination is not applied well.

  Therefore, the present invention switches to white balance adjustment with an emphasis on the color temperature of the flash when it detects the non-uniform state of the flash illumination. In this case, a good white balance adjustment is performed for an image area that is well lit.

  Usually, the user of an electronic camera takes care to sufficiently flash the main subject. Therefore, the above-mentioned “image area that is well lit” is naturally more likely to be the main subject in the screen. Therefore, by placing importance on the color temperature of the flash, the probability that white balance adjustment can be appropriately performed on the main subject increases, and the color reproducibility of the main subject can be increased with a high probability.

On the other hand, as the histogram distributions of both images are similar, the change in the luminance distribution from the test image to the main image is more uniform across the screen, and the degree of flash illumination is more uniform throughout the screen. Can be judged close.
In such a situation, the present invention switches to white balance adjustment that emphasizes the color temperature of the main image. In this case, since the degree of flash hit is uniform within the screen, the light source color temperature estimated from the main image well reflects the actual light source color temperature. Accordingly, it is possible to perform an appropriate white balance adjustment on the entire screen by the white balance adjustment that emphasizes the color temperature of the main image.

  In this way, in the present invention, it is possible to accurately estimate how uniform the flash illumination is in the screen by comparing the histogram of the test image and the main image, and perform appropriate white balance adjustment according to the estimation. become.

Furthermore, the present invention has the following excellent advantages as compared with the prior art of Patent Document 1. That is, in the present invention, the histogram distribution is compared between the test image and the main image. Normally, when the main subject moves on the screen or camera shake or camera shake occurs, the positions of the patterns of the test image and the main image slightly change. However, the histogram distribution hardly changes to such an extent that the pattern is slightly moved.
Therefore, in the histogram comparison, unlike the above-described conventional apparatus, the pattern change that occurs between the test image and the main image can be skillfully ignored.
Therefore, according to the present invention, even if subject movement, camera shake, or camera shake occurs to some extent, it is possible to accurately determine the uniformity of flash illumination and perform appropriate white balance adjustment from the determination result.

<Claim 2>
The electronic camera according to claim 2 obtains a weighted composite value of the first white balance adjustment value based on the color temperature of the flash and the second white balance adjustment value based on the color temperature of the main image. In this weighted composition, the weighted distribution of the first white balance adjustment value is increased as the degree of correlation is lower. Conversely, as the degree of correlation is higher, the weighted distribution of the second white balance adjustment value is increased.
Such an operation makes it possible to obtain an appropriate white balance adjustment value in consideration of the intermediate degree of uniformity / non-uniformity even if the illumination distribution of the flash is intermediate / uniform in the screen. Become.

<Claim 3>
The electronic camera according to claim 3 stops the determination of the uniformity of the flash illumination when the frequency of whiteout and / or blackout of the main image increases to some extent.
This overexposure or blackout occurs due to a non-linear change in the signal level of the main image, for example, when the flash illumination strikes extremely strongly. Even if such a main image is compared with a test image, it is difficult to determine the uniformity of flash illumination.
Therefore, the invention of claim 3 stops the determination of the uniformity of the flash illumination when the frequency of whiteout and / or blackout of the main image increases to some extent. Therefore, the probability of misjudging the uniformity of flash illumination can be greatly reduced.

<Claim 4>
The electronic camera according to the fourth aspect is particularly suitable for a single-lens electronic camera. That is, the single-lens electronic camera includes a photometric image sensor for division photometry and a recording image sensor for generating a main image for recording and storage.
Among these, a test image is taken with a photometric image sensor. In general, this photometric image sensor has a smaller number of pixels than the recording image sensor, and takes less time to transfer and read a captured image and to process it.
Therefore, by capturing a test image with this photometric image sensor, it is possible to simplify and speed up the flash photographing operation sequence, rather than separately capturing a test image with a recording image sensor.

<Claim 5>
The electronic camera according to claim 5 suppresses a difference between two histogram distributions caused by the element difference between the above-described photometric image sensor and recording image sensor.
For example, there are element differences such as a difference in the number of pixels in the output image, a gradation difference, and a quantization bit number between the photometric image sensor and the recording image sensor. By suppressing such an element difference in advance, the element difference does not appear in the correlation detection between the two histogram distributions, and the uniformity determination of the flash illumination becomes more accurate.

<Claim 6>
The electronic camera according to claim 6 detects the correlation between the two histogram distributions after rounding off the predetermined frequency value or less from the two histogram distributions.
For example, such a truncation operation can remove infrequent frequency values corresponding to noise from the histogram distribution. In this case, it is possible to avoid erroneous determination due to noise at the time of detecting correlation of the histogram distribution, and the uniformity of flash illumination can be determined more accurately.

  Further, for example, a small degree value or less can be cut off from the histogram distribution so as to leave only a high frequency value in the screen. In this case, only high-frequency regions, that is, large-area regions in the same gradation region are extracted and the histogram distributions are compared. The image area extracted in this way is likely to be a conspicuous area even on the screen, and by determining the uniformity of the flash illumination for this part, it is possible to determine the uniformity of the flash illumination with an emphasis on the conspicuous area. Become. As a result, white balance adjustment that matches human vision is realized.

<Claim 7>
By executing the program of claim 7 on a computer, the computer functions as a white balance adjusting device.
In this case, it is possible to perform the white balance adjustment according to any one of claims 1 to 6 by inputting a test image and a main image captured by an electronic camera or the like to a computer.

<< Description of Configuration of this Embodiment >>
FIG. 1 is a diagram for explaining a component arrangement of the electronic camera 11 in the present embodiment.
In FIG. 1, the electronic camera 11 is provided with an electronic flash device 12 and a photographing lens 13. A quick return type mirror 15 is provided on the image space side of the photographic lens 13. A diffusion plate 16 is disposed at the imaging position of the subject light beam reflected by the mirror 15. The user observes the subject image formed on the diffusion plate 16 via the finder optical system 17. The light beam from the diffusion plate 16 is guided to the imaging surface of the photometric image sensor 18 provided at the corner of the finder optical system 17 and re-images the subject image on the imaging surface. Further, behind the mirror 15, a shutter 19a, a recording image sensor 19 and the like are arranged.

FIG. 2 is a block diagram relating to image processing of the electronic camera 11.
In FIG. 2, the output of the photometric image sensor 18 is input to the image memory 22 via the A / D converter 20. The output of the recording image sensor 19 is input to the image memory 22 via the A / D converter 21. The image memory 22 is connected to the bus 23. Connected to the bus 23 are an image processing unit 24, a compression recording unit 25, an imaging control unit 27, a flash emission control unit 28, a microprocessor 29, and the like.
Among these, the compression recording unit 25 compresses and records image data on a removable memory card 26. The microprocessor 29 is given an operation input from a release button 29a or the like.

<< Correspondence with Invention >>
Hereinafter, the correspondence between the description items of the claims and the present embodiment will be described. Note that the correspondence relationship here illustrates one interpretation for reference, and does not limit the present invention.
The imaging unit described in claims corresponds to the photometric imaging element 18 and the recording imaging element 19.
The imaging control unit described in the claims corresponds to the imaging control unit 27.
The histogram calculation unit described in the claims corresponds to “a function for obtaining histogram distribution by analyzing histogram of image data” by the microprocessor 29.
The histogram correlation calculation unit described in the claims corresponds to “a function of comparing two histogram distributions and detecting a correlation” by the microprocessor 29.
The white balance calculation unit described in the claims corresponds to “a function for determining a white balance adjustment value according to the correlation of the histogram distribution” by the microprocessor 29.
The white balance adjustment unit described in the claims corresponds to the “function for white balance adjustment of the main image” by the image processing unit 24.

<< Explanation of operation of this embodiment >>
FIG. 3 is a flowchart for explaining the operation of the electronic camera 11.
Hereinafter, the operation of the electronic camera 11 will be described along the step numbers shown in FIG.

[Step S1] When the main power supply of the electronic camera 11 is turned on, the microprocessor 29 starts an initialization process when the power is turned on. In this initialization process, the microprocessor 29 acquires information on the color temperature of the flash from the electronic flash device 12 side, and obtains a first white balance adjustment value that matches the color temperature.

[Step S2] The microprocessor 29 determines whether or not the electronic camera 11 is set to the flash photography mode.
When the flash photographing mode is not set, the microprocessor 29 shifts the operation to a normal photographing routine (not shown) in order to carry out known normal photographing. In this normal photographing routine, the image captured by the photometry image sensor 18 is used for divided photometry applications and the like.
On the other hand, in the flash photography mode, the microprocessor 29 shifts the operation to step S3.

[Step S3] Since the mirror 15 is down at this point, the subject luminous flux that has passed through the photographing lens 13 passes through the diffusing plate 16 and the finder optical system 17 to the subject image on the imaging surface of the photometric image sensor 18. Form.
In this state, the microprocessor 29 uses the imaging control unit 27 to give a drive signal to the photometric image sensor 18. A test image captured without flashing is output from the photometric image sensor 18. The test image is digitized in units of pixels via the A / D converter 20 and then temporarily stored in the image memory 22.

[Step S4] The microprocessor 29 accesses the image memory 22 to obtain a luminance component in a small area (for example, each pixel) of the test image.
It should be noted that the luminance components are generated by weighted addition of each color component constituting the test image according to the component ratio of the luminance component (for example, R: G: B = 0.29: 0.587: 0.114). Is preferred. Further, a color component (G color component or the like) containing a large amount of luminance component in the test image may be regarded as a luminance component as it is.
The microprocessor 29 classifies the luminance component of the test image obtained in this way into a rough luminance range, and obtains the frequency for each luminance range. In this way, the histogram distribution A of the test image illustrated in FIG. 4 is obtained.
It should be noted that data processing such as noise removal and large area extraction may be performed by removing a predetermined frequency value or less from the histogram distribution A.

[Step S5] The microprocessor 29 determines whether or not the release button 29a is fully pressed.
If the release button 29a is fully pressed, the microprocessor 29 proceeds to step S6.
On the other hand, if the release button 29a is not fully pressed (half pressed or not pressed), the microprocessor 29 returns the operation to step S3.

[Step S6] The microprocessor 29 performs known preliminary light emission as follows. First, the microprocessor 29 raises the mirror 15 and projects a subject image on the shutter curtain of the shutter 19a. In this state, the microprocessor 29 causes the electronic flash device 12 to perform minute preliminary light emission using the flash light emission control unit 28. The microprocessor 29 measures the brightness of the shutter curtain during preliminary light emission using a photometric element (not shown) provided at a position where the shutter 19a is viewed. The microprocessor 29 determines the target light amount for the main light emission in accordance with the brightness of the shutter curtain during the preliminary light emission. Information on the target light amount of the main light emission is transmitted to the electronic flash device 12 side.

[Step S7] Subsequently, the microprocessor 29 opens the front curtain of the shutter 19a and starts projecting a subject image on the imaging surface of the recording imaging device 19.
When single-shot main light emission is performed, the microprocessor 29 causes the electronic flash device 12 to start main light emission when the shutter 19a is fully opened. The electronic flash device 12 monitors the light emission amount of the main light emission, and stops the main light emission when reaching the target light amount for which information has been transmitted in advance.
On the other hand, when the set shutter time has elapsed, the microprocessor 29 closes the rear curtain of the shutter 19a. When the shutter 19a is completely closed in this way, the microprocessor 29 lowers the mirror 15 downward.

[Step S <b> 8] The microprocessor 29 supplies a drive signal to the recording image sensor 19 using the imaging control unit 27. The recording image sensor 19 outputs a main image captured with a flash. The main image is digitized in units of pixels via the A / D converter 21 and then temporarily stored in the image memory 22.

[Step S9] The microprocessor 29 accesses the image memory 22, reads the main image, and creates a processed image suitable for comparison with the test image.
In creating the processed image, it is preferable to perform image processing for suppressing element differences as described below.

(1) Processing to cut out a processed image from the main image in accordance with the imaging field angle of the test image.
(2) The main image is divided into pixel blocks in accordance with the positions of the small areas (see step S4) of the photometric image sensor 18, and luminance components (including pseudo luminance components such as G color) are divided in pixel blocks. The requested process.
(3) A process of normalizing the maximum gradation value and the minimum gradation value of the main image in accordance with the maximum gradation value and the minimum gradation value of the test image.
(4) A process of converting the number of gradation steps and gradation curve of the main image in accordance with the gradation characteristics of the test image.

[Step S10] The microprocessor 29 analyzes the luminance component of the processed image created from the main image in the same manner as the test image, and creates a histogram distribution B as illustrated in FIG.
It should be noted that data processing such as noise removal and large area extraction may be performed by removing a predetermined frequency value or less from the histogram distribution B.

[Step S <b> 11] The microprocessor 29 calculates whiteness and blackout frequency values from the histogram distribution B.
If this power value is greater than a predetermined threshold value, the signal level of the main image changes non-linearly, for example, it is saturated, so it can be determined that the main image and the test image are not suitable for comparison. In this case, the microprocessor 29 shifts the operation to step S12.
On the other hand, when the frequency value is equal to or smaller than a predetermined threshold value, the microprocessor 29 shifts the operation to step S13.

[Step S12] Here, the weight distribution of the second white balance adjustment value is set higher without performing the histogram comparison between the main image and the test image. After this operation, the operation proceeds to step S15.

ただし、ＮｉとＭｉは、同一の輝度範囲ｉにおけるヒストグラム分布Ａ，Ｂの度数値である。
また例えば、ヒストグラム分布Ａ，Ｂの分布形状をパターンマッチングして、両者間の類似度から相関を検出してもよい。
また例えば、ヒストグラム分布Ａ、Ｂのピーク数、ピークの間隔、複数のピークの大小関係、またはピーク形状などについて一致度を判定することにより、相関を求めてもよい。 [Step S13] The microprocessor 29 compares the distribution shapes of the two histogram distributions A and B and obtains a correlation between them.
For example, the Euclidean distance D of the histogram distributions A and B may be calculated by the following equation to obtain the correlation between the two. In this case, it can be determined that the smaller the Euclidean distance D, the higher the degree of correlation.

However, Ni and Mi are frequency values of the histogram distributions A and B in the same luminance range i.
Further, for example, the distribution shapes of the histogram distributions A and B may be pattern-matched, and the correlation may be detected from the similarity between the two.
Further, for example, the correlation may be obtained by determining the degree of coincidence with respect to the number of peaks of the histogram distributions A and B, the interval between peaks, the magnitude relationship of a plurality of peaks, or the peak shape.

[Step S14] The microprocessor 29 increases the weighted distribution of the second white balance adjustment value obtained in Step S14 as the degree of correlation obtained is higher. Conversely, the lower the degree of correlation, the higher the weighted distribution of the first white balance adjustment value.
FIG. 5 is a diagram illustrating a correspondence relationship between the Euclidean distance D obtained by the equation (1) and the weighted distribution.

[Step S15] The microprocessor 29 estimates a light source color temperature from the main image in the image memory 22, and obtains a second white balance adjustment value suitable for the light source color temperature.
For example, the light source color temperature can be estimated by obtaining an average value for each color component for all or part of the main image. In this case, the second white balance adjustment value may be determined so that the average color thus obtained becomes an achromatic color.

Further, for example, pixels in a range close to an achromatic color can be extracted from the main image, and the light source color temperature can be estimated from the average color of the extracted pixels. In this case, the second white balance adjustment value may be determined so that the average color of the extracted pixels is an achromatic color.
In step S14, when the weighted distribution of the second white balance adjustment value is zero, it is preferable to omit the process in step S15.

[Step S16] The microprocessor 29 weights and synthesizes the first and second white balance adjustment values according to the obtained weighted distribution. The microprocessor 29 transmits the obtained weighted composite value to the image processing unit 24 as a white balance adjustment value.
The image processing unit 24 performs white balance adjustment on the main image in the image memory 22 according to the white balance adjustment value.

[Step S17] The main image for which the white balance adjustment in step S16 has been completed is compressed by the compression recording unit 25 and recorded and stored in the memory card 26.

<< Effects of this embodiment >>
With the operation described above, in this embodiment, the histogram distribution is compared between the test image without flash and the main image with flash. At this time, it can be determined that flash illumination is more uniformly applied as the distribution shape of the histogram distribution is more similar. Conversely, it can be determined that the flash illumination is more unevenly applied as the distribution shape of the histogram distribution is different.

  In accordance with such a determination, in the present embodiment, switching to white balance adjustment that places importance on the estimated color temperature of the main image in a situation where flash illumination is highly uniform. The estimated color temperature in this case accurately reflects the color temperature of the mixed light at the time of flash photography. Therefore, by performing the white balance adjustment that places importance on the estimated color temperature of the main image, the entire screen is appropriately white balance adjusted.

  Further, in the present embodiment, in a situation where the uniformity of the flash illumination is low, the white balance adjustment is made with an emphasis on the color temperature of the flash. In this case, it is possible to appropriately adjust the white balance in an image area that is well lit, that is, an area that is close to the camera and has a high probability of being a main subject.

  In particular, in such comparison of histogram distributions, subtle changes in the pattern between the test image and the main image can be ignored. Therefore, unlike the conventional device of Patent Document 1, appropriate white balance adjustment can be performed with almost no influence of subject movement, camera shake, camera shake, and the like.

  By the way, there are cases where the flash illumination is too bright and the signal level of the main image changes nonlinearly. In such a situation, the test image and the main image cannot be accurately compared, and the uniformity of the flash illumination cannot be determined correctly. Therefore, in the present embodiment, if there are many overexposures or blackouts in the main image, the determination of the uniformity of the flash illumination is abandoned and the white balance adjustment is performed with emphasis on the color temperature of the main image. The main image at this time includes a whiteout area or a blackout area. However, these non-linear regions are originally low in saturation, and thus are not likely to be an error factor when estimating the light source color temperature from the main image. Therefore, even if the white balance adjustment is performed with reference to the main image including the whiteout or the blackout, it is possible to obtain a main image having a somewhat good color reproducibility.

  Further, in the present embodiment, a test image is captured using the photometric imaging element 18 for use in split photometry. Since the photometric image sensor 18 has a smaller number of pixels than the recording image sensor 19, it takes less time to capture a test image and to perform image processing (histogram creation or the like). As a result, even if the above-described white balance adjustment value determination operation is taken into account, the processing load is not increased and the lightness of the imaging operation of the electronic camera is not lost.

  In step S9 of the present embodiment, the test image and the main image are processed so as to be suitable for comparison. As a result, although the image pickup devices are different, a good comparison is possible, and the uniformity of flash illumination can be determined more accurately.

  Furthermore, in this embodiment, the two histogram distributions A and B can be compared after the predetermined frequency values are rounded down from the histogram distributions A and B. By such a truncation process, it becomes possible to reduce the influence of noise, and the uniformity of flash illumination can be determined more accurately. It is also possible to cut off a small degree value or less from the histogram distribution so that only a particularly high frequency value in the screen remains. In this case, it is possible to determine the uniformity of the flash illumination only for the high-frequency portion that stands out in the screen. As a result, it is possible to achieve white balance adjustment focusing on visually noticeable high-frequency portions.

<< Additional items of embodiment >>
In the above-described embodiment, the single-lens electronic camera 11 has been described. However, the present invention is not limited to this. For example, the present invention can be applied to a compact type electronic camera. In this case, it is preferable to capture the test image with a recording image sensor. In such a configuration, the photometric image sensor 18 is not required, and the configuration of the imaging unit can be further simplified.

  In the above-described embodiment, one white balance adjustment value is determined for the entire image. However, the present invention is not limited to this. For example, the screen may be divided into a plurality of areas, and the white balance adjustment value of the present invention may be determined for each area. In this case, the uniformity of flash illumination can be determined for each region, and an appropriate white balance adjustment value can be determined for each region according to the uniformity determination.

In the above-described embodiment, both whiteout and blackout are detected. However, the present invention is not limited to this. The determination of the uniformity of the flash illumination may be stopped by detecting an increase in either whiteout or blackout.
Further, the gradation range corresponding to the whiteout or blackout degree value of the main image may be detected from the white side or the black side of the histogram of the test image. In this case, the detected gradation area is removed from the histogram distribution of the test image (removal of the uppermost and / or lowermost gradation areas), or a plurality of detected gradation areas are changed to a single gradation area. It may be summarized. Such histogram processing on the test image side makes histogram comparison more accurate. Such an operation makes it possible to accurately determine the uniformity of the flash illumination regardless of whether the main image is whiteout or blackout.

  In the above-described embodiment, the case where the built-in type electronic flash device 12 is used has been described. However, the present invention is not limited to this. The present invention may be applied to a case in which an external type electronic flash device is used.

  Furthermore, in the above-described embodiment, the case where the white balance adjustment is completed in the electronic camera has been described. However, the present invention is not limited to this. For example, a program for executing part or all of the processing from the uniformity determination of the flash illumination shown in FIG. 3 to the white balance adjustment may be created. By executing this program on an external computer, the above-described operation can be performed on the computer.

  As described above, the present invention is a technique that can be used for an electronic camera capable of performing flash photography.

It is a figure explaining component arrangement | positioning of the electronic camera 11 in this embodiment. 2 is a block diagram relating to image processing of the electronic camera 11. FIG. 3 is a flowchart for explaining the operation of the electronic camera 11. It is a figure which shows an example of two histogram distribution A and B. FIG. It is a figure which illustrates the correspondence of the correlation value of histogram distribution A and B, and the weight distribution of the white balance adjustment value.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Electronic camera 12 Electronic flash device 13 Shooting lens 15 Mirror 16 Diffuser 17 Finder optical system 18 Photometric image sensor 19 Recording image sensor 19a Shutter 20 A / D converter 22 Image memory 23 Bus 24 Image processor 25 Compression recording unit 27 Imaging Control Unit 28 Flash Light Emission Control Unit 29 Microprocessor 29a Release Button

Claims (7)

An imaging unit that captures a subject image;
An imaging control unit that controls the imaging unit to generate a test image that is captured without flashing and a main image that is captured while flashing;
A histogram calculation unit for obtaining a histogram distribution for each of the test image and the main image;
A histogram correlation detector that detects a correlation between two histogram distributions calculated by the histogram calculator;
When the degree of correlation is low, it is determined that the flash illumination is non-uniform, and the white balance adjustment value is given with an emphasis on the color temperature of the flash. When the degree of correlation is high, the flash illumination is A white balance calculating unit that determines that the color is uniform (including a case where illumination is not yet achieved), and changes the white balance adjustment value with emphasis on the color temperature of the main image;
An electronic camera comprising: a white balance adjustment unit that performs white balance adjustment on the main image according to a white balance adjustment value obtained by the white balance calculation unit. The electronic camera according to claim 1,
The white balance calculation unit
A weighted composite value of a first white balance adjustment value determined based on the color temperature of the flash and a second white balance adjustment value determined based on the color temperature of the main image is obtained, and the white balance adjustment value When the degree of correlation is low, the weighted distribution of the first white balance adjustment value is increased, and when the degree of correlation is high, the weighted distribution of the second white balance adjustment value is increased. An electronic camera characterized by The electronic camera according to claim 1 or 2,
The white balance calculation unit
An electronic camera characterized in that, with respect to the main image, the frequency of overexposure and / or blackout is obtained, and when the frequency exceeds a predetermined threshold, the determination of the uniformity of the flash illumination is stopped. The electronic camera according to any one of claims 1 to 3,
The imaging unit
A photometric image sensor for performing an image-capturing operation for split photometry and a recording image sensor for performing an image-capturing image-recording operation
The imaging control unit
The photometric image sensor is controlled to capture the test image in the absence of flashing, and the recording image sensor is controlled to capture the main image in the flashing state. Electronic camera. The electronic camera according to claim 4,
The histogram calculation unit
An electronic camera that suppresses a difference between the two histogram distributions caused by an element difference between the photometric image sensor and the recording image sensor. The electronic camera according to any one of claims 1 to 5,
The histogram correlation detection unit
An electronic camera, wherein a correlation between two histogram distributions is detected after removing a predetermined frequency value or less from the two histogram distributions calculated by the histogram calculation unit.   A program that uses a test image captured without flash and a main image captured with flash emission,
  A histogram calculation process for obtaining a histogram distribution for each of the test image and the main image;
  A histogram correlation detection process for detecting a correlation between two histogram distributions calculated by the histogram calculation process;
  When the degree of correlation is low, it is determined that the flash illumination is non-uniform, and the white balance adjustment value is given with an emphasis on the color temperature of the flash. When the degree of correlation is high, the flash illumination is A white balance calculation process for determining that the color is uniform (including a case where illumination is not yet achieved), and changing the white balance adjustment value with an emphasis on the color temperature of the main image;
  A white balance adjustment process for performing a white balance adjustment on the main image in accordance with a white balance adjustment value obtained in the white balance calculation process;
  A program that causes a computer to execute.

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