JP2004222160A - Digital camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a digital camera capable of performing proper exposure correction at the time of photographing a back-lighted scene. <P>SOLUTION: A figure 1 shows outline flow of exposure correction processing. In step 101, a photometric value Evi calculated when a release button (not shown) is half depressed is fetched. In step 102, the photometric value Evi is used to perform a calculation of a parameter for back-lighted scene discrimination. In step 103, the parameter obtained in step 102 is used to discriminate whether or not it is a back-lighted scene. When the scene is discriminated to be a back-lighted scene, in step 104, an exposure correction value is determined. The exposure correction value is calculated on the basis of a weighted average photometric value EVa, a maximum value EVmax of the photometric value and a target value EVs to the maximum value EVmax of the photometric value. In step 105, an exposure value calculated by a normal method is corrected with the exposure correction value dEV, and the result is outputted. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a digital camera having an exposure correction function when shooting a backlight scene.
[0002]
[Prior art]
The dynamic range of a solid-state image sensor such as a CCD image sensor is generally narrow, and when shooting a high-contrast subject, the amount of light received by the solid-state image sensor exceeds the dynamic range and the output of the solid-state image sensor saturates. Information may be missing.
[0003]
In order to solve such a problem, the first image data with relatively high sensitivity (hereinafter sometimes simply referred to as “high sensitivity image data”) and the second image data with relatively low sensitivity. (Hereinafter, it may be simply described as “low-sensitivity image data”), and a technique for expanding the dynamic range by performing a synthesis process has been proposed (see Patent Documents 1 and 2).
[0004]
The technique described in Patent Document 1 is a standard luminance video signal (high-sensitivity image) obtained by setting an object with standard brightness to an appropriate level under at least one of different exposure conditions and different gain control. Data) and a high-luminance video signal (low-sensitivity image data) obtained so that an object brighter than a predetermined value has an appropriate level is added to obtain a wide dynamic range video signal. At that time, the peak value of the high-luminance video signal is detected, and based on this peak value, a multiplication coefficient for multiplying the standard luminance video signal and the high-luminance video signal is calculated. It is.
[0005]
The technique described in Japanese Patent Laid-Open No. 2004-228561 is an image capturing unit that captures an image of a scene and generates a high-sensitivity video signal and a low-sensitivity video signal representing the scene. First signal conversion means for quantizing with a quantization resolution and outputting corresponding high-sensitivity video signal data; and quantizing a low-sensitivity video signal with a second quantization resolution lower than the first quantization resolution And a second signal converting means for outputting the corresponding low-sensitivity video signal data, and adding and combining the high-sensitivity video signal data and the corresponding low-sensitivity video signal data to produce a wide dynamic range video signal Is formed.
[0006]
On the other hand, when shooting in a backlight state where the background of the main subject is very bright, the image of the main subject tends to be a dark image due to the influence of the brightness of the background. Therefore, when it is determined that the backlight is in the backlit state, the exposure is corrected on the over side (see Patent Documents 3 and 4).
[0007]
However, when the exposure is corrected to the over side in order to adjust the exposure to the main subject, there is a problem that the background portion is overexposed and the overexposure is likely to occur. This phenomenon is particularly noticeable in digital cameras that cannot form video signals with a wide dynamic range.
[0008]
[Patent Document 1]
JP 2001-94870 A [Patent Document 2]
JP 2001-8104 A [Patent Document 3]
Japanese Patent Laid-Open No. 2001-242504 [Patent Document 4]
Japanese Patent Laid-Open No. 2001-242504
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a digital camera capable of performing appropriate exposure correction when shooting a backlight scene.
[0010]
[Means for Solving the Problems]
The digital camera of the present invention is a digital camera having an exposure correction function at the time of shooting a backlight scene, a split photometry section for obtaining photometric values of a plurality of areas obtained by dividing an imaging surface, and a backlight scene based on the photometry values. Exposure compensation for determining an exposure correction amount based on a backlight determination unit for determining, a weighted average photometric value of the screen obtained based on the photometric value, a maximum value of the photometric value, and a target value for the maximum value of the photometric value The target value indicates a range of luminance that can be reproduced by the digital camera by deviation from a luminance value used as a reference for exposure setting at the time of shooting, and the exposure correction unit, When the backlight determination unit determines that the scene is a backlight scene, the difference between the maximum photometric value and the weighted average photometric value is compared with the target value, and the target value is greater than the difference. Dew the difference It is an amount of correction.
[0011]
The digital camera of the present invention includes a digital camera in which the exposure correction unit sets an exposure correction amount to 0 when the target value is smaller than the difference.
[0012]
The digital camera of the present invention includes one in which the exposure correction unit corrects the exposure correction amount according to a shooting mode.
[0013]
The digital camera of the present invention includes one in which the target value is changed according to a shooting scene.
[0014]
The digital camera of the present invention includes one in which the target value is changed according to a difference between a maximum value and a minimum value of the photometric values by the divided photometric unit.
[0015]
The digital camera of the present invention includes one in which the target value is changed according to a distribution of photometric values by the divided photometric unit.
[0016]
The digital camera of the present invention further has a function of performing composite processing of the relatively high sensitivity first image data and the relatively low sensitivity second image data to obtain composite image data, Based on the ratio x, a step of obtaining the ratio x between the maximum value of the subject brightness indicated by the second image data and the brightness value indicated by the maximum value that can be taken by the first image data, Determining a gamma correction parameter of the second image data and the synthesis processing parameter; correcting the second image data based on the gamma correction parameter; and based on the synthesis processing parameter. Including a step of performing a process of combining the first image data and the second image data.
[0017]
In the digital camera of the present invention, the composition processing further includes a step of determining a gamma correction parameter of the second image data based on the ratio x, and the second image data based on the gamma correction parameter. Including the step of correcting the above.
[0018]
In the digital camera of the present invention, the composition processing further includes a step of determining a saturation correction parameter of the composite image data based on the ratio x, and a saturation of the composite image data based on the saturation correction parameter. Including the step of correcting the degree.
[0019]
The digital camera of the present invention includes one in which the first image data and the second image data are acquired based on an imaging signal from an imaging device having photoelectric conversion units having different sensitivities.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 11 is a diagram showing a schematic configuration of the digital camera according to the embodiment of the present invention. The digital camera of FIG. 11 includes an imaging unit 1, an analog signal processing unit 2, an A / D conversion unit 3, a driving unit 4, a strobe 5, a digital signal processing unit 6, a compression / expansion processing unit 7, a display unit 8, and system control. A unit 9, an internal memory 10, a media interface 11, a recording medium 12, and an operation unit 13. The digital signal processing unit 6, compression / decompression processing unit 7, display unit 8, system control unit 9, internal memory 10, and media interface 11 are connected to a system bus 20.
[0021]
The imaging unit 1 includes an optical system such as a photographic lens and an imaging element such as a CCD image sensor, and shoots a subject, and outputs an analog imaging signal. An imaging signal obtained by the imaging unit 1 is sent to the analog signal processing unit 2, subjected to predetermined analog signal processing, converted into a digital signal by the A / D conversion unit 3, and then digitally converted as so-called RAW image data. It is sent to the signal processing unit 6. The RAW image data is digital image data that has been digitized in the form of the imaging signal from the imaging unit 1.
[0022]
When shooting, the imaging unit 1 is controlled via the drive unit 4. A solid-state image pickup device such as a CCD image sensor used as an image pickup device has a plurality of photoelectric conversion regions (for example, photodiodes) arranged on a semiconductor substrate surface in a row direction and a column direction orthogonal thereto, and incident light. An analog voltage signal based on the accumulated signal charge is output. The solid-state imaging device is a timing generator (not shown) included in the drive unit 4 at a predetermined timing when a release switch (not shown) is turned on by operating a release button (not shown) which is a part of the operation unit 13. 11 is written by a drive signal from TG). The drive unit 4 outputs a predetermined drive signal based on the control of the system control unit 9, and also outputs drive signals for the analog signal processing unit 2 and the A / D conversion unit 3.
[0023]
The imaging unit 1 outputs a relatively high-sensitivity imaging signal and a relatively low-sensitivity imaging signal. Both of the imaging signals pass through an analog signal processing unit 2 and an A / D conversion unit 3 to form a digital image. It is converted into data and sent to the digital signal processor 6. The imaging device included in the imaging unit 1 has photoelectric conversion units with different sensitivities. As an imaging device having photoelectric conversion units with different sensitivities, a solid-state imaging device having a high-sensitivity imaging cell and a low-sensitivity imaging cell may be used as shown in Patent Document 2, or a solid-state imaging as shown in FIG. An element may be used.
[0024]
FIG. 12 is a diagram illustrating a schematic configuration of a solid-state imaging device capable of outputting a high-sensitivity imaging signal and a low-sensitivity imaging signal. FIG. 12 is a partially enlarged plan view of a so-called honeycomb-structured solid-state imaging device, which is arranged on the surface of a semiconductor substrate in a row direction (a direction indicated by an arrow X) and a column direction (a direction indicated by an arrow Y) perpendicular thereto. The plurality of photoelectric conversion regions 111 to 157 (only some are numbered in the figure), the vertical transfer units 201 to 208, the horizontal transfer unit 300, and the output unit 400 are included. Among the plurality of photoelectric conversion regions 111 to 157, the odd-numbered columns are shifted in the column direction by about ½ of the column-direction pitch between the photoelectric conversion regions with respect to the even-numbered columns. The photoelectric conversion regions are arranged so as to be shifted in the row direction by about ½ of the row direction pitch between the photoelectric conversion regions with respect to even-numbered photoelectric conversion regions. In FIG. 12, a photoelectric conversion region of 5 rows and 8 columns is shown, but in reality, more photoelectric conversion regions are provided.
[0025]
The photoelectric conversion regions 111 to 157 generate and accumulate signal charges corresponding to the amount of incident light, and are, for example, photodiodes. The photoelectric conversion regions 111 to 157 are divided into a main region m having a relatively large light receiving area and a sub region s having a relatively small light receiving area (in FIG. 12, only the photoelectric conversion region 151 is given a reference numeral. A signal charge corresponding to light having a predetermined spectral sensitivity is generated and stored. In the solid-state imaging device of FIG. 12, red (indicated by R), green (indicated by G), or blue (indicated by B) filters (not shown) have respective photoelectric conversion regions 111 to 157. The signal charge corresponding to the light of each color is generated and accumulated.
[0026]
The vertical transfer units 201 to 208 read signal charges from the photoelectric conversion regions 111 to 157 and transfer them in the column direction. The vertical transfer units 201 to 208 are provided on the sides corresponding to the columns of the photoelectric conversion regions 111 to 157. The transfer in the column direction is performed by independently reading the signal charges in the main region m and the signal charges in the sub region s to the vertical transfer units 201 to 208, respectively. The horizontal transfer unit 300 transfers signal charges from the plurality of vertical transfer units 201 to 208 and transfers the transferred signal charges in the row direction. The output unit 400 outputs a voltage signal corresponding to the transferred signal charge amount.
[0027]
The strobe 5 operates when the brightness of the subject is equal to or lower than a predetermined value, and is controlled by the system control unit 9.
[0028]
The digital signal processing unit 6 performs digital signal processing on the digital image data from the A / D conversion unit 3 in accordance with the operation mode set by the operation unit 13. The processing performed by the digital signal processing unit 6 includes black level correction processing (OB processing) and linear matrix correction processing (removing mixed components due to the photoelectric conversion characteristics of the image sensor with respect to the primary color signal from the imaging unit. Processing for performing correction (by 3 × 3 matrix operation for RBG input), white balance adjustment processing (gain adjustment), gamma correction processing, image synthesis processing, synchronization processing, Y / C conversion processing, and the like. The image composition processing of the digital signal processing unit 6 will be described later.
[0029]
The digital signal processing unit 6 is configured by a DSP, for example. The compression / decompression processing unit 7 performs compression processing on the Y / C data obtained by the digital signal processing unit 6 and also performs decompression processing on the compressed image data obtained from the recording medium 12. .
[0030]
The display unit 8 includes, for example, an LCD display device, and displays an image based on image data that has been shot and has undergone digital signal processing. An image is also displayed based on the image data obtained by decompressing the compressed image data recorded on the recording medium. It is also possible to display a through image at the time of shooting, various states of the digital camera, information on operations, and the like.
[0031]
The internal memory 10 is, for example, a DRAM and is used as a work memory for the digital signal processing unit 6 and the system processing unit 9, as well as a buffer memory that temporarily stores captured image data recorded on the recording medium 12, and a display It is also used as a buffer memory for display image data to the unit 8. The media interface 11 inputs / outputs data to / from a recording medium 12 such as a memory card.
[0032]
The system control unit 9 controls the entire digital camera including shooting operations. Specifically, the system control unit 9 is mainly configured by a processor that operates according to a predetermined program.
[0033]
The operation unit 13 performs various operations when using the digital camera, and performs an operation mode (shooting mode, playback mode, etc.) of the digital camera, a shooting method at the time of shooting, shooting conditions, settings, and the like. The photographing conditions set by the operation unit 13 include photographing sensitivity (ISO sensitivity) setting, electronic zoom operation setting, and strobe operation setting. The operation unit 13 may be provided with an operation member corresponding to each function, but may share the operation member in conjunction with the display of the display unit 8. The operation unit 13 also includes a release button for starting a shooting operation.
[0034]
Next, a photometric function at the time of photographing and exposure control based on the photometric value will be briefly described. The photometric function is executed when a release button (not shown) included in the operation unit 13 is half-pressed. Specifically, under the control of the system control unit 9, a predetermined calculation is performed on the image data when the imaging unit 1 is set to a plurality of predetermined exposure values (for example, 7 EV, 11 EV, and 15 EV) and exposed. Is done by. The arithmetic processing is performed by the system control unit 6 and the digital signal processing unit 6. The procedure is as follows.
[0035]
(1) A plurality of captured image data (3 images in this example) are averaged over a plurality of divided areas (for example, 64 divided areas) of the photographing screen, and a photometric value Evi (i = 0 to 63) of each area is obtained. Ask for. The photometric value Evi is obtained by selecting an image obtained from an image that is not overflowed or underflowed from a plurality of accumulated calculation data, multiplying the accumulated value for each color (RGB) by a predetermined white balance gain, and It is the sum of them.
(2) The following calculation is performed on the photometric value Evi to calculate the weighted average photometric value EVa = Σ (Evi × Wi) / ΣWi. Here, Wi is a predetermined weight of each region, and has a large value in the central part and a small value in the peripheral part.
[0036]
The exposure at the time of photographing is determined based on the weighted average photometric value EVa obtained here. Then, each parameter of the imaging unit 1 is set from the system control unit 9 via the drive unit 4 based on the determined exposure, and capturing of the imaging signal and signal processing are performed by fully pressing a release button (not shown). Is started.
[0037]
Next, detection of a retrograde scene and exposure correction at the time of shooting a backlight scene will be described. FIG. 1 is a diagram showing a schematic flow of exposure correction processing in the digital camera according to the embodiment of the present invention. In step 101, the photometric value Evi obtained when the release button (not shown) is half-pressed is captured.
[0038]
In step 102, a parameter for backlight scene discrimination is calculated using the photometric value Evi. Here, the following three parameters are calculated. The first is a difference EVdiff between the maximum value EVmax of the photometric value Evi and the minimum value EVmin of the photometric value Evi. The difference EVdiff corresponds to the size of the luminance range of the subject and increases during backlighting, so that it can be used to determine backlighting. The second is a difference between the weighted average photometric value EVa and the peripheral photometric value EVb which is an average value of the photometric values in the peripheral portion. This parameter indicates the luminance distribution of the shooting screen, and it can be determined that the scene is backlit as it becomes negative. The third is the maximum value EVmax of the photometric value Evi. In a backlight scene, the maximum value EVmax is large, and can be used to determine a backlight scene.
[0039]
In step 103, it is determined whether the scene is a backlight scene using the parameters obtained in step 102. The backlight scene is preferably determined based on the size and combination of these parameters. If it is not a backlight scene, the process ends.
[0040]
If it is determined that the scene is a backlight scene, an exposure correction amount is determined in step 104. The exposure correction amount is calculated based on the weighted average photometric value EVa, the photometric value maximum value EVmax, and the target value EVs for the photometric value maximum value EVmax. Here, the target value EVs indicates a range of luminance that can be reproduced by the digital camera by deviation from a luminance value (such as a weighted average photometric value EVa) used as a reference for exposure setting at the time of photographing.
[0041]
The exposure correction amount dEV is obtained by calculating dEV = (EVs− (EVmax−EVa)) K. However, dEV = 0 when dEV <0. Here, K is a coefficient that varies depending on the shooting mode, and K = 1 during normal shooting. By changing the strobe on / off, the strobe operating mode (daytime sync mode, etc.), etc., the exposure correction amount can be corrected. For example, when the strobe is on, K = 0.7.
[0042]
When the exposure correction amount dEV is obtained in step 104, the exposure value obtained by a normal method (for example, the weighted average photometric value EVa is used as the exposure value) is corrected in step 105 by the exposure correction amount dEV. Output as exposure EVo (= EVa-dEV).
[0043]
Next, a method for determining the target value EVs will be described. The target value EVs indicates the luminance range that can be reproduced by the digital camera as a deviation max_dif from a luminance value (weighted average photometric value EVa, etc.) used as a reference for exposure setting at the time of shooting. It is changed according to EVdiff (difference between the maximum value EVmax and the minimum value EVmin of the photometric value Evi) corresponding to the size of the range. FIG. 2 shows a setting example of the target value EVs (= max_dif) according to EVdiff. 2A shows the relationship between the deviation max_dif and EVdiff, and FIG. 2B shows the relationship between the variable l and EVmax in FIG. 2A. According to the setting example in FIG. 2, the target value EVs (= max_dif) is 2+ (EVdiff−4.5) × (4-2) / (7.5−4.5) × l.
[0044]
The deviation max_dif that becomes the target value EVs is clipped according to the luminance distribution. FIG. 3 shows an example of the clip function dif_clip. Since the horizontal axis of FIG. 3 is the difference between the weighted average photometric value EVa and the peripheral photometric value EVb, the positive direction of the horizontal axis is the luminance distribution of the forward light, and the negative direction is the backlight. When such clipping is performed, even if EVdiff is large and EVmax is large, the target value EVs does not increase if the luminance distribution is not on the backlight side. Therefore, excessive backlight correction can be avoided.
[0045]
The imaging signal obtained by performing such exposure correction is digitized by the A / D conversion unit 3 and sent to the digital signal processing unit 6. The function of the digital signal processing unit 6 will be described focusing on image synthesis processing. FIG. 4 is a schematic functional block diagram of the digital signal processing unit 6. The digital signal processing unit 6 includes an OB (optical black) processing unit 61a, 61b, an LMTX (linear matrix) 62a, 62b, a WB (white balance) processing unit 63a, 63b, a gamma correction unit 64a, 64b, a combined gain LUT (look). (Uptable) 65, multiplication units 66 a and 66 b, addition unit 67, limiter 68, synchronization and Y / C processing unit 69, CMTX (color difference matrix) 70, and synthesis range calculation unit 71.
[0046]
The OB processing units 61a and 61b correct the black level of the image data, and the LMTXs 62a and 62b adjust the hue by performing, for example, a 3 × 3 matrix operation. The WB processing units 63a and 63b multiply the image data indicating the respective colors by the gain for white balance correction, and the gamma correction units 64a and 64b make the respective image data have desired gamma characteristics. The input / output characteristics are changed.
[0047]
The composite gain LUT 65 outputs the respective gain data when the high-sensitivity image data and the low-sensitivity image data are added and combined in accordance with the high-sensitivity image data. The gain data to be output is selected based on the luminance range information to be synthesized from the synthesis range calculation unit 71. A method for obtaining the composite gain will be described later.
[0048]
High-sensitivity image data High input to the digital processing unit 6 is input to the multiplication unit 66a through processing by the OB processing unit 61a, the LMTX 62a, the WB processing unit 63a, and the gamma correction unit 64a. Similarly, the low-sensitivity image data Low is input to the multiplication unit 66b after being processed by the OB processing unit 61b, the LMTX 62b, the WB processing unit 63b, and the gamma correction unit 64b. Then, the gain data from the combined gain LUT 65 is multiplied, and the multiplication result is added by the adder 67.
[0049]
The image data from the adder 67 is limited to be within a range that can be expressed by a limiter 68 in a predetermined number of bits (for example, 8 bits), and is input to the synchronization and Y / C processing unit 69. The synchronization and Y / C processing unit 69 interpolates point-sequential RGB signals, obtains RGB signals for each imaging position, and further converts the RGB signals into luminance signals Y and color difference signals Cr and Cb. Then, saturation correction is performed by the CMTX 70 and output as composite image data.
[0050]
The integration processing unit 71 integrates the low-sensitivity image data that has passed through the LMTX 62b. Specifically, the integration processing unit 71 divides the entire region of the image or the region excluding the peripheral portion of the image into 16 × 16, and the green in each divided region. Data G is integrated. The composition range calculation unit 72 obtains a composition range when performing image composition. The composite range is determined based on a ratio x between the maximum value of the subject luminance indicated by the low-sensitivity image data and the luminance value indicated by the maximum value that can be taken by the high-sensitivity image data. A method for obtaining the synthesis range will be described later.
[0051]
The output of the synthesis range calculation unit 72 is sent to the synthesis gain LUT 65 to output gain data corresponding to the synthesis range. In addition, the gamma correction characteristic of the low-sensitivity image data is sent to the gamma correction unit 64b according to the synthesis range. Further, the color difference data sent to the CMTX 70 and obtained by the synchronization and Y / C processing unit 69 is corrected with a gain corresponding to the synthesis range, and the saturation of the output synthesized image data is corrected.
[0052]
Note that the correction of the gamma characteristic and the saturation correction of the low-sensitivity image data based on the synthesis range from the synthesis range calculation unit 72 are not essential and may be omitted as appropriate.
[0053]
FIG. 5 shows a schematic sequence of digital signal processing up to the synthesis calculation processing at the time of photographing. After exposure is performed in the period t1, high-sensitivity image data is captured in the period t2. In the period t3, signal processing is performed on the high-sensitivity image data simultaneously with the capture of the low-sensitivity image data. Here, white balance gain determination processing (AWB), scratch correction processing (scratches), and preprocessing are performed. The preprocessing includes OB processing, LMTX calculation, WB processing, and gamma correction processing.
[0054]
When the capturing of the low-sensitivity image data is completed, the composite range calculation process is performed in the period t4. The calculation processing of the synthesis range here includes OB processing of low-sensitivity image data, LMTX calculation, and integration processing. When the composite range is determined, low-sensitivity image data preprocessing (including WB processing and gamma correction processing) is performed in a period t5. Then, in the period t6, a combination operation of the high sensitivity image data and the low sensitivity image data after the gamma correction is performed.
[0055]
Next, gain data stored in the combined gain LUT 65 will be described. The gain data is created based on the formula (1) for obtaining the composite data data.
[0056]
data = [high + MIN (high / th, 1) × low] × MAX [(− lg × high / th) +1, p] (1)
here,
high is the high-sensitivity image data low after the gamma correction, the low-sensitivity image data th after the gamma correction is the threshold p for changing the addition ratio of the low-sensitivity image data, the total gain lg is the same as the high-sensitivity image data and low The value depends on the ratio of the saturation output of the sensitivity image data (for example, if the saturation ratio is 4: 1, p = 0.2).
[0057]
Expand equation (1),
h_gain = MAX [(− lg × high / th) +1, p] (2)
l_gain = MAX [(− lg × high / th) +1, p] × MIN (high / th, 1) = h_gain × wl (3)
Then,
data = h_gain × high + l_gain × low
It becomes.
[0058]
h_gain is a gain for high-sensitivity image data and has the characteristics shown in FIG. Also, l_gain is a gain for low-sensitivity image data and is h_gain × wl, and wl has the characteristics shown in FIG.
[0059]
In the composite gain LUT 65, the gain h_gain for high-sensitivity image data and the gain l_gain for low-sensitivity image data obtained by the above equations (2) and (3) are tabular data corresponding to the high-sensitivity image data high. Is remembered as The stored gain data is stored in a number corresponding to the synthesis range output from the synthesis range calculation unit 72.
[0060]
Next, how to determine the synthesis range by the synthesis range calculation unit 72 will be described. First, a ratio x between the maximum value of the subject luminance indicated by the low-sensitivity image data and the luminance value indicated by the maximum value that can be taken by the high-sensitivity image data is obtained based on the integration data from the integration processing unit 71. The value obtained by dividing the integrated data by the number of integrated images is the average value of the G data for each divided region of the subject, and the subject brightness can be considered as 1.25 (80/64) times the G data. The maximum average value Gmax corresponds to the maximum value of the subject luminance indicated by the low-sensitivity image data.
[0061]
As shown in FIG. 5, the maximum value that the high-sensitivity image data can take is 4095, the maximum value that the low-sensitivity image data can take is 1023, and the luminance range of the low-sensitivity image data is relative to the luminance range of the high-sensitivity image data. If the luminance is 1.25 (80/64) times that of the green data G, the maximum value of the subject luminance indicated by the low-sensitivity image data and the luminance value indicated by the maximum value that can be taken by the high-sensitivity image data The ratio x is expressed by the following equation (4).
[0062]
x = Gmax × 80 × 400/1023 × 64 (%) (4)
[0063]
The synthesis range calculation unit 72 obtains the synthesis range based on the ratio x thus obtained. The synthesis range is determined by the size of the ratio x, for example, as shown in FIG.
[0064]
Based on the synthesis range thus determined, gain data to be multiplied with the high-sensitivity image data and the low-sensitivity image data is selected. FIG. 10 shows an example of the gradation characteristics corresponding to the composite range of the composite image data. As shown in FIG. 10, the reproduction range of the composite image data can be widened as the composite range is increased (the luminance range of the subject is increased). The example of FIG. 10 shows an example in which the gamma characteristic of the low-sensitivity image data is not changed. However, the gamma characteristic of the low-sensitivity image data can be made a more preferable gradation characteristic according to the synthesis range.
[0065]
In this way, by combining the composition parameters and the gamma characteristics of the low-sensitivity image according to the captured image data, a more preferable image can be obtained even when overexposure is performed during backlight scene shooting.
[0066]
【The invention's effect】
As is clear from the above description, according to the present invention, a digital camera capable of performing appropriate exposure correction at the time of shooting a backlight scene is provided.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic flow of exposure correction processing in a digital camera according to an embodiment of the present invention. FIG. 2 is a diagram showing a setting example of a target value EVs (= max_dif) corresponding to a photometric value range EVdiff. 3 is a diagram showing an example of a clip function for clipping the deviation max_dif according to the luminance distribution. FIG. 4 is a schematic functional block diagram of a digital signal processing unit. FIG. FIG. 6 is a diagram showing a schematic sequence. FIG. 6 is a diagram showing a characteristic of a gain for high-sensitivity image data. FIG. 7 is a diagram showing a characteristic of a ratio wl between a gain for low-sensitivity image data and a gain for high-sensitivity image data. FIG. 9 is a diagram showing an example of the relationship between high-sensitivity image data and low-sensitivity image data. FIG. 9 shows the maximum value of subject brightness indicated by low-sensitivity image data, and the brightness value indicated by the maximum value that can be taken by high-sensitivity image data. FIG. 10 is a diagram showing an example of the relationship between the ratio x and the synthesis range. FIG. 10 is a diagram showing how the tone characteristics of the synthesized image data change according to the synthesis range. FIG. 11 is a diagram of the digital camera according to the embodiment of the present invention. FIG. 12 is a diagram showing a schematic configuration. FIG. 12 is a diagram showing a schematic configuration of a solid-state imaging device capable of outputting a high-sensitivity imaging signal and a low-sensitivity imaging signal.
DESCRIPTION OF SYMBOLS 1 ... Imaging part 2 ... Analog signal processing part 3 ... A / D conversion part 4 ... Drive part 5 ... Strobe 6 ... Digital signal processing part 7 ... Compression / decompression process Unit 8 ... Display unit 9 ... System control unit 10 ... Internal memory 11 ... Media interface 12 ... Recording medium 13 ... Operation unit 20 ... System bus 61a, 61b ... OB (optical black) processing units 62a, 62b... LMTX (linear matrix)
63a, 63b ... WB (white balance) processing units 64a, 64b ... gamma correction unit 65 ... composite gain LUT (look-up table)
66a, 66b ... Multiplier 67 ... Adder 68 ... Limiter 69 ... Synchronization and Y / C processor 70 ... CMTX (color difference matrix)
71 ... Integration processing unit 72 ... Composite range calculation unit

Claims (10)

A digital camera having an exposure correction function when shooting a backlight scene,
A divided photometry unit for obtaining photometric values of a plurality of areas obtained by dividing the imaging surface;
A backlight determination unit that determines a backlight scene based on the photometric value;
An exposure correction unit that calculates an exposure correction amount based on a weighted average photometric value of the screen obtained based on the photometric value, a maximum value of the photometric value, and a target value for the maximum value of the photometric value;
The target value indicates a range of luminance that can be reproduced by the digital camera by deviation from a luminance value used as a reference for exposure setting at the time of shooting,
When the backlight determining unit determines that the backlight scene is a backlight scene, the exposure correction unit compares the difference between the maximum photometric value and the weighted average photometric value with the target value, and the target value is A digital camera that, when larger than the difference, uses the difference as the exposure compensation amount. The digital camera according to claim 1,
The exposure correction unit is a digital camera that sets an exposure correction amount to 0 when the target value is smaller than the difference. The digital camera according to claim 1 or 2,
The exposure correction unit is a digital camera that corrects the exposure correction amount according to a shooting mode. The digital camera according to any one of claims 1 to 3,
The digital camera in which the target value is changed according to a shooting scene. The digital camera according to claim 4,
The target value is a digital camera that is changed according to a difference between a maximum value and a minimum value of photometric values by the divided photometry unit. The digital camera according to claim 4 or 5,
The digital camera in which the target value is changed according to a distribution of photometric values by the divided photometric unit. The digital camera according to any one of claims 1 to 6,
Furthermore, it has a function of performing composite processing of the relatively high sensitivity first image data and the relatively low sensitivity second image data to obtain composite image data,
The synthesis process includes
Obtaining a ratio x between the maximum value of the subject brightness indicated by the second image data and the brightness value indicated by the maximum value that can be taken by the first image data;
Determining a gamma correction parameter of the second image data and the synthesis processing parameter based on the ratio x;
Correcting the second image data based on the gamma correction parameter;
A digital camera that includes a step of performing a combining process of the first image data and the second image data based on the combining processing parameter. The digital camera according to claim 7,
The synthesis process includes
And determining a gamma correction parameter of the second image data based on the ratio x;
And a step of correcting the second image data based on the gamma correction parameter. The digital camera according to claim 7 or 8,
The synthesis process includes
A step of determining a saturation correction parameter of the composite image data based on the ratio x;
And a step of correcting the saturation of the composite image data based on the saturation correction parameter. A digital camera according to any one of claims 7 to 9,
The digital camera in which the first image data and the second image data are acquired based on an imaging signal from an imaging device having photoelectric conversion units having different sensitivities.

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