JP5149055B2 - Imaging device - Google Patents

Description

   The present invention relates to an imaging apparatus, and more particularly to gradation correction.

  In order to correct an image captured under illumination conditions such as backlight, gradation correction for correcting luminance and contrast is known. Gamma correction and histogram correction are typical examples, but since correction is performed with a constant coefficient, there is a problem that overexposure and blackout occur.

On the other hand, adaptive tone correction has been proposed in which a correction coefficient is determined by using not only the tone value of image data but also information on neighboring pixels of the pixel to be corrected, and correction according to the content of the image is possible. It has become. Non-Patent Document 1 discloses adaptive tone correction. The adaptive tone correction is based on the Retinex theory. In the Retinex theory, the input image is represented by the product of illumination light and reflectance, and the reflectance image is corrected by separating the illumination light from the input image. What you get as Now, assuming that the input image I = illumination light L × reflectance R (corrected image), R (x, y) = exp {log (I (x, y)) − log (L (x, y))} is there. A variational method is used for estimating the illumination light, and a plurality of layers k having a resolution of 1/2 ^k of the original image are created, and the calculation of the illumination light update is repeated from the low resolution layer. The calculation of the illumination light update is L (x, y) = L (x, y) −μNSD × G (x, y). Here, G (x, y) is a gradient of the cost function, and μNSD is a learning coefficient. Specifically, first, G (x, y) is calculated, then μNSD (x, y) is calculated, and L (x, y) is calculated based on these.

  Further, in Patent Document 1, when the person photographing mode is selected, the exposure value is set lower than the appropriate exposure value calculated by the AE (automatic exposure) detection unit, and an image is photographed and photographed. By applying tone correction to the image data obtained using the γ conversion table that widens the dynamic range of the image data after the tone conversion processing, the luminance value of the portion where the luminance is insufficient near the subject center is obtained. It is disclosed to correct.

“Comparative evaluation of Retinex theory in hardware implementation of adaptive tone correction”, Nora et al., IEICE Tech. 19-24 (June 2005) JP 2007-27967 A

  In this way, the subject is photographed with an exposure value set under the optimal exposure value set in AE, and gradation correction is adaptively performed on the obtained image data, so that whiteout or blackout occurs. However, if the underexposure value is fixedly set, various images cannot be handled, and there is a problem that an image intended by the user cannot be obtained.

  An object of the present invention is intended by the user by adaptively performing gradation correction on image data taken by taking an exposure correction value corresponding to a shooting state into consideration for the derived appropriate exposure of AE. An object of the present invention is to provide an imaging apparatus capable of obtaining an image.

The present invention is an image pickup apparatus for picking up an image of a subject, exposure control means for calculating an appropriate exposure value, and image data obtained by setting an exposure value under or over the appropriate exposure value. On the other hand, correction means for performing gradation correction, and adjustment means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correction means satisfies a specific condition. And the adjustment unit supplies data related to adjustment to the image correction unit, the correction unit adjusts a correction amount using the data related to the adjustment, and the adjustment unit adjusts an appropriate exposure in the luminance histogram. The exposure value to be set to under is adjusted using at least one of a ratio of a high luminance pixel to a value and a ratio of a low luminance pixel .

In addition, the present invention is an imaging apparatus that captures a subject image, an exposure control unit that calculates an appropriate exposure value, and an image that is captured with an exposure value set to under or over than the appropriate exposure value. Correction means for performing gradation correction on data, and adjustment means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correction means satisfies a specific condition The adjustment means supplies data relating to adjustment to the image correction means, the correction means adjusts a correction amount using the data relating to the adjustment, and the adjustment means further includes the adjustment of the subject. It is characterized in that the exposure value set to under is adjusted by determining whether or not the luminance is a luminance level that requires the strobe to emit light .

In addition, the present invention is an imaging apparatus that captures a subject image, an exposure control unit that calculates an appropriate exposure value, and an image that is captured with an exposure value set to under or over than the appropriate exposure value. Correction means for performing gradation correction on data, and adjustment means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correction means satisfies a specific condition The adjustment means supplies data relating to adjustment to the image correction means, the correction means adjusts a correction amount using the data relating to the adjustment, and the adjustment means further includes: The exposure value set to under is adjusted by determining whether or not the scene is a specific scene .

In addition, the present invention is an imaging apparatus that captures a subject image, an exposure control unit that calculates an appropriate exposure value, and an image that is captured with an exposure value set to under or over than the appropriate exposure value. Correction means for performing gradation correction on data, and adjustment means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correction means satisfies a specific condition The adjustment means supplies data related to adjustment to the image correction means, the correction means adjusts a correction amount using the data related to the adjustment, and the adjustment means further has an ISO value. The exposure value set to under is adjusted by determining whether or not the threshold value is exceeded .

  According to the present invention, the user's intention is to adaptively perform tone correction on image data that is taken by taking an exposure correction value according to the shooting state into consideration for the derived appropriate exposure of AE. An image can be obtained.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings, taking a digital camera as an example of an imaging apparatus.

  FIG. 1 shows a functional block diagram of a digital camera in the present embodiment. An optical system for forming a subject image on an image sensor, a means for performing correlated double sampling on an image signal from the image sensor, a means for converting to a digital signal, a means for compressing / decompressing image data, and generating a preview image Means for displaying captured images or preview images on a display device, means for recording image data on a recording medium, autofocus (AF) means, white balance (WB) processing means, and the like are the same as those of known digital cameras. Therefore, it is omitted.

  In FIG. 1, a CCD as an image sensor outputs R, G, and B image signals. The image sensor is not limited to a CCD, and may be a CMOS.

  The area division average unit 12 divides the image data from the CCD 10, more specifically, the preview image data into a plurality of areas, and calculates and outputs an average of pixel values for each area. That is, the entire image data is divided into n × m rectangular areas, and a simple average of R, G, and B pixel values is calculated for each area.

  The AE control unit 14 performs exposure evaluation for preview based on the preview image data, and calculates an appropriate exposure value. There are two methods of exposure evaluation for preview: photometry with emphasis on the center and multipoint photometry. If a multipoint photometric method is employed, it can also be used for exposure evaluation for photography. Further, the AE control unit 14 creates a luminance histogram using the image data of the preview image, and supplies the luminance histogram to the exposure correction amount calculation unit 16.

The exposure correction amount calculation unit 16 calculates an exposure correction amount for proper exposure by determining whether or not the luminance histogram satisfies a specific condition, and supplies the exposure correction amount to the AE control unit 14 and the gradation correction unit 18. . The exposure correction amount calculation unit 16 calculates the exposure correction amount on the assumption that the captured image data is subjected to gradation correction by the gradation correction unit 18. In the present embodiment, the exposure correction amount is set in six stages with respect to the appropriate exposure value. That is, when the exposure correction amount is ΔEV, ΔEV = 0 EV, −Δ ₁ EV, −Δ ₂ EV, −Δ ₃ EV, −Δ ₄ EV, and + Δ ₅ EV are set. Here, a positive exposure correction amount means overexposure with respect to the proper exposure value, and a negative exposure correction amount means underexposure with respect to the proper exposure value. Further, Δ ₁ <Δ ₂ <Δ ₃ <Δ ₄ is set.

  The gradation correction unit 18 adaptively performs gradation correction on image data that has been subjected to exposure correction based on the exposure correction amount and the appropriate exposure value (exposure reference) from the exposure correction amount calculation unit 16. . The gradation correction method is arbitrary, but the gradation correction is performed by performing γ conversion on the luminance data using an adaptive γ function and correcting the color difference data. The gradation correction unit 18 compensates the exposure of the image data shot with underexposure and corrects the gradation so as to obtain an appropriate exposure. Therefore, when the exposure correction amount is small, the gradation correction correction amount correspondingly. If the exposure correction amount is large, the gradation correction amount increases accordingly, and the effect of gradation correction appears remarkably (in other words, the gradation correction distortion also appears remarkably). Specifically, the exposure correction amount calculation unit 16 in FIG. 1 is configured by a CPU. The gradation correction unit 18 is configured by a dedicated DSP.

  FIG. 2 shows a detailed functional block diagram of the AE control unit 14 and the exposure correction amount calculation unit 16 in FIG. A preview image 20 obtained by processing the image signal obtained by the CCD 10 is supplied to the area division averaging unit 12. The area division average unit 12 creates AE evaluation data. For example, the preview image is divided into 12 × 8 blocks, and an average value of R, G, and B is calculated for each block.

  The preview exposure evaluation unit 22 of the AE control unit 14 calculates an evaluation value for controlling the exposure of the preview image. The calculated evaluation value is supplied to the preview exposure control unit 32 and controlled to an appropriate exposure value. Based on the data from the preview exposure evaluation unit 22, the shooting exposure prediction histogram creation unit 28 creates a luminance histogram that will be obtained from the image data after shooting when shooting with the current exposure value. Specifically, the photographic exposure prediction histogram creation unit 28 converts the average value of R, G, and B for each block into luminance Y, and converts it into EV units using log2 (luminance Y / appropriate exposure value). After calculating the luminance Y for each block in EV units, a histogram is created with these 12 × 8 = 96 data. The horizontal axis of the histogram is the amount of deviation from the appropriate exposure (153). The calculated histogram is supplied to the exposure correction amount calculation unit 16.

The exposure correction amount determination unit 30 of the exposure correction amount calculation unit 18 determines the exposure correction amount based on the histogram and other determination data. The other determination data is determination data such as whether the flash is ON, whether a preview image exists, face information (presence / absence, position, size, etc.). The exposure correction amount is, for example, exposure correction when x ₄ EV or more exists in the histogram at 25% or more, x ₅ EV or more does not exist, and x ₂ EV or less exists at 10% or less of the whole. and the like that the amount and - [delta _3. A specific determination method based on the histogram will be described later. The determined exposure correction amount is supplied to the gradation correction unit 18 and to the still photographing exposure determination unit 26 of the AE control unit 22. The still shooting exposure determination unit 26 determines an exposure value at the time of shooting based on the evaluation result from the still shooting exposure evaluation unit 24 and the exposure correction amount.

  FIG. 3 shows a processing flowchart of the present embodiment. First, the CPU of the digital camera determines whether the brightness level of the preview image is darker than the level for turning on the strobe (flash) (S101). This determination is because when the flash is emitted, the shooting scene situation is dark, so that it is not further underexposed. Accordingly, when the brightness level is darker than the flash ON level, the exposure correction amount ΔEV with respect to the appropriate exposure value is kept at 0, that is, the appropriate exposure (S114).

If the brightness level of the preview image is brighter than the level for turning on the flash, it is next determined whether or not there is preview data (S102). There is no preview image data immediately after the power is turned on or immediately after the return from the reproduction mode. When the user presses the shutter at once in this state, the preview image data does not exist and exposure correction cannot be performed. Therefore, in such a case, priority is given to photographing, and the exposure correction amount calculation unit 16 sets the exposure correction amount as a default underexposure value (S115). That is, the underexposure value is set by ΔEV = −Δ ₁ EV with respect to the appropriate exposure value.

If there is a preview image, it is next determined whether or not it is in a backlight state (S103). Since the flash is forcibly emitted in the backlight state, in this case, the exposure correction amount calculation unit 16 sets the exposure correction amount as a default underexposure value (S115). That is, the underexposure value is set by ΔEV = −Δ ₁ EV with respect to the appropriate exposure value.

If it is not backlit, it is next determined whether or not the shooting scene is a specific scene, specifically, whether or not the shooting scene is a beach (coast) scene or a text (document) scene (S104). . This is because when the shooting scenes are those scenes, it may be better to shoot with overexposure than with underexposure. Therefore, when the shooting scenes are these scenes, the exposure correction amount calculation unit 16 sets the exposure correction amount to an overexposure value rather than the appropriate exposure value (S119). That is, the overexposure value is set by ΔEV = + Δ ₅ EV with respect to the appropriate exposure value. A known scene discrimination technique can be used to determine whether the shooting scene is a beach scene or a text shooting scene. For example, Japanese Patent Application Laid-Open No. 2007-121654 discloses a technique for discriminating a scene by combining temperature features and image motion, hue, saturation, luminance, and the like detected by an image feature extraction circuit.

If the shooting scene is not a specific scene, it is next determined whether or not ISO is equal to or greater than a threshold value (S105). When the shooting scene is dark, the shutter speed is fixed to increase the ISO by increasing the shutter speed in order to suppress camera shake. However, since the noise increases as the ISO increases, exposure compensation is performed when the ISO is above the threshold value. Is not executed. Therefore, when the ISO is equal to or greater than the threshold value, the exposure correction amount calculation unit 16 basically determines the exposure correction amount so that the proper exposure is maintained. However, if the ISO is high,
(1) When the shooting scene itself is dark (2) The shooting scene is not dark, but when using a high-speed shutter for the purpose of shooting while stopping a moving subject, the amount of light is insufficient, resulting in a high ISO. There are two. Of these, exposure correction is not performed in the case of (1), and exposure correction is performed in the case of (2). Therefore, it is determined whether or not the luminance of the shooting scene is lower than the luminance for the moving subject (S106), and when the luminance is lower than the luminance for the moving subject, the exposure correction amount calculation unit 16 keeps the appropriate exposure. Then, an exposure correction amount is determined (S115). That is, ΔEV = 0. Here, the threshold value of the EV value for determination represents a value on the horizontal axis of the histogram, and x ₀ <x ₁ <x ₂ <x ₃ <x ₄ <x ₅ , y ₁ <y ₂ <y. ₃ .

  If the ISO is smaller than the threshold value, or if the above condition (2) is met even if the ISO is equal to or greater than the threshold value, then the luminance difference is determined using the histogram (S107). Then, it is sequentially determined whether the luminance difference meets a specific condition, and an exposure correction amount is set according to the determination result.

  Specifically, first, it is determined whether or not a face exists in the shooting scene (S108). A technique for detecting a face is also known. For example, a skin color part is extracted from image data, a matching degree between a human face template and a skin color part image is obtained, and a part with a high matching degree is used as a face candidate. is there. Further, an eye part may be extracted from the face candidate part to determine whether or not the face is a face.

If a face exists, it is next determined whether or not the luminance histogram satisfies a condition corresponding to the exposure correction amount ΔEV = −Δ ₃ EV (S109). The conditions set for ΔEV = −Δ ₃ EV are as follows.
<Condition to be −Δ ₃ EV: With Face>
x ₄ EV or more is 25% or more, x ₅ EV or more does not exist, x ₂ EV or less is 10% or less, and the face area is y ₂ EV or more and y ₃ EV or less.
If the condition of −Δ ₃ EV is not satisfied, it is next determined whether or not the luminance histogram satisfies a condition corresponding to the exposure correction amount ΔEV = −Δ ₂ EV (S110). The conditions set for ΔEV = −Δ ₂ EV are as follows.
<Condition to be −Δ ₂ EV: with face>
x ₃ EV or more is 60% or more, and the face area is y ₁ EV or more.

When the luminance histogram does not satisfy these conditions, the exposure correction amount calculation unit 16 sets the exposure correction amount ΔEV to the default −Δ ₁ EV (S115), and satisfies the condition of −Δ ₂ EV. In step S116, the exposure correction amount ΔEV is set to −Δ ₂ EV. This means that shooting is performed with the exposure value set to an under value further than the default under exposure value. If the condition of -Δ ₃ EV is satisfied, the exposure correction amount ΔEV is set to -Δ ₃ EV (S117). This means that shooting is performed with the exposure value set to an under value further than the default under exposure value.

On the other hand, if the photographic scene no face, successively, determine if they satisfy the condition histogram of luminance corresponding to _{-Δ 4 EV (S111), -} Δ 3 determine if they satisfy the condition corresponding to EV (S112) , −Δ ₂ EV is determined (S113). These conditions are as follows.
<Condition to be −Δ ₄ EV: No Face>
x ₄ EV or more is 20% or more, x ₅ EV or more exists, and x ₁ EV or less is 10% or less, or x ₀ EV or less is 20% or more.
<Condition to be −Δ ₃ EV: No Face>
x ₄ EV or more is 25% or more, x ₅ EV or more is not present, and x ₂ EV or less is 10% or less.
<Condition to be −Δ ₂ EV: No Face>
x ₃ EV or more is 50% or more.

These conditions are collectively shown in FIG. The reason why the determination criterion is changed depending on the presence / absence of the face is to make the correction amount in the gradation correction relatively small when the face is present compared to when the face is not present. That is, the exposure correction amount ΔEV calculated by the exposure correction amount calculation unit 16 is used for exposure control at the time of photographing and is also supplied to the gradation correction unit 18 to adjust the correction amount at the time of gradation correction. Used. Since the gradation correction unit 32 performs gradation correction on the image data photographed with the underexposure value to correct the appropriate exposure value, if the underexposure value is small, the correction amount at the time of gradation correction becomes small. Therefore, adjusting the exposure correction amount means simultaneously adjusting the correction amount of gradation correction in the gradation correction unit 32. As shown in FIG. 4, when the face exists, the exposure correction amount ΔEV is set smaller than when the face does not exist. In FIG. 4, the maximum underexposure value when the face is present is the exposure correction amount ΔEV = −Δ ₃ EV, whereas the maximum underexposure value when the face is not present is the exposure correction amount. it should be noted that ΔEV = -Δ is _4. Lowering the underexposure value when there is a face makes it easier for noise to enter if the exposure value is set to underexposure. This is to prevent it.

  As described above, in the present embodiment, the exposure correction amount is dynamically adjusted based on the luminance histogram of the preview image, and the gradation correction is performed on the image data obtained by the shooting. Compared to the case where the gradation correction is performed by photographing with a fixed underexposure value, it is possible to cope with various photographing scenes. The exposure correction amount in the present embodiment is set on the assumption that gradation correction is adaptively performed in subsequent processing, and functions as a parameter for adjusting the correction amount of adaptive gradation correction. However, the exposure value is not simply set based on the luminance histogram.

It is a functional block diagram of an embodiment. It is a detailed functional block diagram of an embodiment. It is a processing flowchart of an embodiment. It is a table figure which shows the relationship between the luminance histogram of embodiment, and exposure correction amount.

Explanation of symbols

  10 CCD, 12 area division average part, 14 AE control part, 16 exposure correction amount calculation part, 18 gradation correction part.

Claims (4)

An imaging device that captures a subject image,
Exposure control means for calculating an appropriate exposure value;
Correction means for performing gradation correction on image data captured with the exposure value set to under or over than the appropriate exposure value;
Adjusting means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correcting means satisfies a specific condition;
Have
The adjustment means supplies data relating to adjustment to the image correction means,
The correction means adjusts a correction amount using data relating to the adjustment,
The adjustment unit adjusts an exposure value to be set to under using at least one of a ratio of a high luminance pixel and a ratio of a low luminance pixel to an appropriate exposure value in the luminance histogram. . An imaging device that captures a subject image,
Exposure control means for calculating an appropriate exposure value;
Correction means for performing gradation correction on image data captured with the exposure value set to under or over than the appropriate exposure value;
Adjusting means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correcting means satisfies a specific condition;
Have
The adjustment means supplies data relating to adjustment to the image correction means,
The correction means adjusts a correction amount using data relating to the adjustment,
The adjustment device further adjusts an exposure value to be set to under by determining whether or not the luminance of the subject is a luminance level that requires the strobe to emit light. An imaging device that captures a subject image,
Exposure control means for calculating an appropriate exposure value;
Correction means for performing gradation correction on image data captured with the exposure value set to under or over than the appropriate exposure value;
Adjusting means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correcting means satisfies a specific condition;
Have
The adjustment means supplies data relating to adjustment to the image correction means,
The correction means adjusts a correction amount using data relating to the adjustment,
The adjustment apparatus further adjusts an exposure value set to under by determining whether or not the subject is a specific scene. An imaging device that captures a subject image,
Exposure control means for calculating an appropriate exposure value;
Correction means for performing gradation correction on image data captured with the exposure value set to under or over than the appropriate exposure value;
Adjusting means for adjusting an exposure value by determining whether or not a luminance histogram of the image data before gradation correction by the correcting means satisfies a specific condition;
Have
The adjustment means supplies data relating to adjustment to the image correction means,
The correction means adjusts a correction amount using data relating to the adjustment,
The adjustment unit further adjusts an exposure value set to under by determining whether or not the ISO value is equal to or greater than a threshold value.

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