Imaging apparatus and control method for the same

Abstract

To provide a technique for appropriately correcting the tone of a high-luminance area.SOLUTION: An imaging apparatus is for controlling exposure by metering a metering area in accordance with a metering mode. The imaging apparatus has acquisition means for acquiring a target luminance level in a high luminance area of an image and tone correction means for correcting the tone of the image on the basis of a target luminance level. In a predetermined metering mode where the high luminance area is intensively metered as a metering area, the tone correction means suppresses the tone correction more than in other metering modes.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an imaging device and a control method for an imaging device. [Background technology]

[0002] Conventionally, techniques for correcting the gradation of an image to obtain an image with desirable brightness and contrast have been known. The lower the luminance of a high-luminance area in an image, the lower the contrast of the image. In this case, by correcting the gradation of the high-luminance area in the image to make it brighter, the contrast is increased and the visual impression is improved. Patent Document 1 discloses a technique for adjusting contrast and correcting color cast. Furthermore, Patent Document 2 discloses a technique for controlling exposure more stably while prioritizing highlight areas (high-luminance areas) within the image. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-36043 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-166767 Summary of the Invention [Problem to be solved by the invention]

[0004] If the gradation correction is performed to make high-brightness areas brighter in order to increase contrast, the brightness of the image may differ from the user's intention. Also, even if the exposure is controlled by giving priority to high-brightness areas, if the gradation correction is not performed appropriately, the brightness of the high-brightness areas may end up being different from the user's intention.

[0005] An object of the present invention is to provide a technique for appropriately correcting the gradation of luminance in a high-luminance area. [Means for solving the problem]

[0006] The imaging device according to the present invention comprises: An imaging device that controls exposure by measuring a photometric area according to a photometric mode, an acquisition means for acquiring a target luminance level of a high luminance area of ​​an image; a gradation correction means for performing gradation correction of the image based on the target luminance level; and The gradation correction means suppresses the gradation correction in a predetermined photometry mode in which the high-luminance region is used as a photometry region and photometry is focused on the high-luminance region, more than in other photometry modes. It is characterized by: [Effects of the Invention]

[0007] According to the present invention, it is possible to appropriately correct the gradation of the luminance in a high-luminance area. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a block diagram illustrating a configuration of a digital camera. [Figure 2] 10 is an example of a screen on which a user selects a target luminance of a high-luminance area. [Figure 3] 10 is a flowchart illustrating a basic tone correction process. [Figure 4] 10A and 10B are diagrams illustrating an example of a luminance histogram and feature amounts; [Figure 5] FIG. 10 is a diagram showing an example of a tone curve used for tone correction. [Figure 6] 10 is a flowchart illustrating a first tone correction process. [Figure 7] FIG. 10 is a diagram illustrating an example of a linear tone curve. [Figure 8] 10 is a flowchart illustrating a second tone correction process. [Figure 9] 10 is a flowchart illustrating a third tone correction process. [Figure 10] FIG. 10 is a diagram illustrating a target brightness TH. [Figure 11] 10 is an example of a screen on which a user selects a target brightness TH. [Figure 12]FIG. 10 is a diagram showing an example of a tone curve when the feature amount HL>target luminance TH. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0010] <Digital camera configuration> 1 is a block diagram illustrating the configuration of a digital camera 100 as an imaging device according to an embodiment of the present invention. The digital camera 100 includes an imaging mechanism including a taking lens 101, an aperture and shutter 102, an auto-exposure (AE) processor 103, a focus lens 104, an auto-focus (AF) processor 105, an image sensor 106, and an A / D converter 107.

[0011] The photographing lens 101 has a zoom mechanism. The aperture and shutter 102 controls the amount of incident light, which is reflected light from a subject, incident on the image sensor 106 and the charge accumulation time in accordance with instructions from the AE processing unit 103. The AE processing unit 103 measures the photometry area according to the metering mode and controls the operation of the aperture and shutter 102 to control exposure. The AE processing unit 103 also controls the A / D conversion unit 107. The focus lens 104 focuses on the light receiving surface of the image sensor 106 in accordance with a control signal from the AF processing unit 105 to form an optical image. The AF processing unit 105 also calculates distance information from the digital camera 100 to the subject.

[0012] The image sensor 106 converts the optical image formed on the light receiving surface into an electrical signal using a photoelectric conversion means such as a CCD element or a CMOS element, and outputs the signal to the A / D conversion unit 107. The A / D conversion unit 107 converts the received electrical signal (analog signal) into a digital signal. The A / D conversion unit 107 includes a CDS circuit that removes noise from the received electrical signal, and a nonlinear amplification circuit that nonlinearly amplifies the received electrical signal before converting it into a digital signal.

[0013] The digital camera 100 also includes an image processing unit 108, an image recognition unit 109, a format conversion unit 110, and a DRAM (Dynamic RAM) 111. The image processing unit 108 performs predetermined pixel interpolation, resizing such as image reduction, and color conversion processing on the digital signal input from the A / D conversion unit 107, and then performs development processing to output image data.

[0014] Image processing unit 108 adjusts the image quality of the captured image by adjusting the white balance (WB) of the digital signal input from A / D conversion unit 107 and performing gradation correction by increasing or decreasing the brightness level of the image. For example, image processing unit 108 has a function for increasing or decreasing the brightness level of the entire image at a uniform increase or decrease rate with respect to the brightness level of the image data, and a tone curve (gamma) function for converting the signal level according to the magnitude of the original signal level. Image processing unit 108 realizes gradation correction processing using these functions.

[0015] The image recognition unit 109 receives input of image data that has been appropriately processed by the image processing unit 108. The image recognition unit 109 can recognize the brightness of the input image as a photometry process. The image recognition unit 109 divides the image data into multiple regions and measures the light. Therefore, the image recognition unit 109 can determine high-brightness regions within the image and obtain photometry results for the high-brightness regions.

[0016] The high-luminance region may be a region (luminance range) having a luminance equal to or greater than a predetermined threshold, such as a region in an 8-bit image where the luminance is 210 or greater. Also, the high-luminance region may be a region in an image where the cumulative frequency from the high-luminance side in a luminance histogram is 10% or less. The brightness may be determined based on the relative brightness at

[0017] The image recognition unit 109 can recognize a scene using known technology. For example, when the image recognition unit 109 detects a face, it recognizes that the scene is one in which a person is to be photographed. The photometry results and scene recognition information by the image recognition unit 109 are output to the AE processing unit 103.

[0018] The user can select a metering mode using the operation unit 116. The operation unit 116 outputs the metering mode selected by the user to the AE processing unit 103. The AE processing unit 103 performs automatic exposure based on the metering results and scene recognition information output by the image recognition unit 109, and information such as the metering mode output by the operation unit 116.

[0019] The image recognition unit 109 can recognize the focus state of the input image. The recognition result of the focus state is output to the AF processing unit 105. The AF processing unit 105 realizes AF control based on the recognition result of the focus state.

[0020] The format conversion unit 110 converts the format of the image data generated by the image processing unit 108 in order to store the image data in the DRAM 111. The DRAM 111 is an internal memory that is used as a buffer for temporarily storing image data, or as a working memory for compressing / decompressing the image data.

[0021] Digital camera 100 includes an image recording unit 112, a system control unit 113, a VRAM (Video RAM) 114, a display unit 115, an operation unit 116, a main switch (main SW) 117, and a shooting switch (shooting SW) 118. Image recording unit 112 has a recording medium such as a memory card for recording shot images (still images and videos) and an interface therefor.

[0022] The system control unit 113 has a CPU (processor), ROM, and RAM. The CPU controls the overall operation of the digital camera 100 by expanding a program stored in the ROM into a work area of ​​the RAM and executing it. The system control unit 113 can realize the processing of each component of the digital camera 100 by executing a program stored in the ROM. The system control unit 113 controls which of the multiple imaging drive modes of the image sensor 106 is to be used. The VRAM 114 is a memory for displaying images.

[0023] Display unit 115 is, for example, an LCD (Liquid Crystal Display), etc. Display unit 115 displays images, operation assistance, and the status of digital camera 100, and also displays the shooting screen and ranging area during shooting. Display unit 115 also displays a screen for the user to select a target value or upper limit threshold for the luminance of the high-luminance area, as exemplified in Fig. 2.

[0024] The operation unit 116 is a member that allows the user to externally operate the digital camera 100. The user can use the operation unit 116 to perform various settings, such as exposure compensation, aperture setting, and image playback settings. The operation unit 116 has a menu switch, a zoom lever that commands the zoom operation of the photographing lens, an operation mode changeover switch that switches between the photographing mode and the playback mode, and the like.

[0025] The user can select a metering mode by operating the operation unit 116. The metering modes that the user can select are, for example, evaluative metering mode, partial metering mode, and highlight-weighted metering mode.

[0026] Evaluative metering mode divides the screen into multiple metering areas and measures the brightness of each area to determine the brightness of the subject. This mode determines the final exposure based on information such as distribution, color, distance, and composition. Evaluative metering mode is suitable for general photography, including backlit photography. Partial metering mode is a mode that measures the range in the center of the screen. Partial metering mode is effective when there is strong light around the subject, such as in backlit situations. Highlight-weighted metering mode is a mode that determines the exposure so that the high-brightness areas within the screen are primarily at the appropriate brightness. Highlight-weighted metering mode is a mode that focuses metering on the high-brightness areas as the metering area. Generally, when controlling exposure to ensure the high-brightness areas within the screen are at the appropriate brightness, the exposure will be darker in highlight-weighted metering mode than in evaluative metering mode.

[0027] The user can select a target value or upper threshold value for the brightness of a high-brightness area by operating the operation unit 116 on the screen shown in Fig. 2. The target value or upper threshold value for the brightness of a high-brightness area selected by the user is output to the AE processing unit 103 and used for exposure control, and is output to the image processing unit 108 and used for image processing control.

[0028] The main switch 117 is a switch for turning on the power to the digital camera 100. The shooting switch 118 is a switch that operates in two stages depending on how deeply it is pressed. When the shooting switch 118 is pressed halfway (SW1 operation), the system control unit 113 executes shooting preparation operations such as AE processing and AF processing. When the shooting switch 118 is pressed all the way (SW2 operation), the system control unit 113 executes shooting processing.

[0029] A series of processes executed by the digital camera 100 will be described. When the main switch 117 is pressed to turn on the power, the system control unit 113 of the digital camera 100 causes the image sensor 106 to perform image capture processing at a predetermined cycle (for example, every 33 ms). The digital camera 100 enters a standby state for actual photography, in which captured images are displayed sequentially on the display unit 115. When a capture instruction is received by pressing the capture switch 118 (SW2 operation), the system control unit 113 executes the main capture processing using the image sensor 106. The system control unit 113 executes image processing on the captured image using the image processing unit 108, and records the image data after image processing in the image recording unit 112. The digital camera 100 returns to the standby state for actual photography again. When the main switch 117 is pressed again, the power to the digital camera 100 is turned off.

[0030] 2 is an example of a screen for selecting a target value or upper threshold for the luminance of a high-luminance region. Hereinafter, the target value or upper threshold for the luminance of a high-luminance region will be collectively referred to as "target luminance of a high-luminance region." For example, the user can select the target luminance of a high-luminance region by operating the directional keys of the operation unit 116 to select one of TH1, TH2, or TH3, and then pressing the OK button to confirm various settings.

[0031] Here, we will explain tone correction when the user changes the target luminance of the high-brightness area. When shooting in highlight-weighted metering mode, the user can adjust the brightness of the image by selecting the target luminance of the high-brightness area. In the example of Figure 2, the user selects one of TH1, TH2, and TH3 as the target luminance of the high-brightness area. For example, in an 8-bit image, TH1 can be 120, TH2 can be 150, and TH3 can be 200.

[0032] The digital camera 100 controls the exposure so that the actual brightness of the high-brightness area approaches the selected target brightness TH. The high-brightness area may be an area having a brightness equal to or greater than a predetermined threshold, or an area having a brightness where the cumulative frequency from the high-brightness side in a brightness histogram is equal to or less than a predetermined percentage (e.g., 10% or less). The brightness that represents the high-brightness area (hereinafter also referred to as representative brightness) may be, for example, the average, maximum, minimum, median, or mode of the brightness of the pixels included in the high-brightness area.

[0033] If a user sets the target brightness TH to TH1 and the image is captured darker than intended, the user changes the target brightness TH from TH1 to TH2 to TH3 and re-captures the image. On the other hand, if a user sets the target brightness TH to TH3 and the image is captured brighter than intended, the user changes the target brightness TH from TH3 to TH2 to TH1 and re-captures the image.

[0034] When gradation correction is performed so that high-luminance areas become brighter, the contrast increases, but the image luminance may differ from what the user intended. We will explain how image luminance changes due to gradation correction, dividing it into cases where the target luminance TH is increased and where the target luminance TH is decreased.

[0035] If a user sets the target brightness TH to TH1 and the image is captured darker than intended, they may try to capture a brighter image by increasing the target brightness TH from TH1 to TH2 to TH3. When the target brightness TH is set to TH1, the image will be darker and the brightness of high-brightness areas will be lower than when the target brightness TH is TH2. For this reason, when the target brightness TH is TH1, gradation correction will be stronger than when the target brightness TH is TH2, and the high-brightness areas will be brighter.

[0036] In contrast, when the target brightness TH is set to TH2, the image becomes brighter and the brightness of the high-brightness areas becomes higher compared to when the target brightness TH is TH1. Therefore, when the target brightness TH is TH2, the gradation correction is suppressed more than when it is TH1. Therefore, the image after gradation correction does not give the impression of being as bright as when the target brightness TH is set to TH1.

[0037] The luminance of the high-luminance area before and after tone correction will be explained using relational expressions when the target luminance TH is set to TH1 and when it is set to TH2. When the target luminance TH is set to TH1, the luminance of the high-luminance area before tone correction is Y1, and the luminance of the high-luminance area after tone correction is Y'1. When the target luminance TH is set to TH2, the luminance of the high-luminance area before tone correction is Y2, and the luminance of the high-luminance area after tone correction is Y'2. Y1 approaches TH1 and becomes Y'1, and Y2 approaches TH2 and becomes Y'2.

[0038] By changing the target luminance TH from TH1 to TH2, a luminance increase of (Y2 - Y1) is expected, but the actual luminance increase is (Y'2 - Y'1). The relationship between the expected luminance increase and the actual luminance increase is (Y2 - Y1) > (Y'2 - Y'1). This is because the brighter the image, the weaker the tone correction. Furthermore, if the degree of tone correction is strong, Y'1 may become higher than Y'2, and the brightness of the image will be the opposite of the target luminance TH intended by the user. The same relationship holds when the target luminance TH is changed from TH2 to TH3.

[0039] If a user sets the target brightness TH to TH3 and the image is captured brighter than intended, they will attempt to capture it darker by lowering the target brightness TH from TH3 to TH2 to TH1. As the target brightness TH decreases, the image becomes darker, and the gradation correction becomes stronger. Therefore, even if the target brightness TH is lowered, the brightness of high-luminance areas does not decrease as much as the user expects, and the image does not become dark. Also, if the gradation correction of high-luminance areas becomes too strong, the image will be corrected to be brighter than the target brightness TH of the high-luminance areas selected by the user, and the screen will become brighter than the user expects.

[0040] The following describes a tone correction method executed by the digital camera 100 having the configuration described in Fig. 1. The tone correction is executed by the image processing unit 108.

[0041] <Basic tone correction method> The basic tone correction method will be described with reference to Fig. 3. Fig. 3 is a flowchart illustrating the basic tone correction process. The basic tone correction process is performed by a command from the system control unit 113. The image processing unit 108 receives the instruction and executes the process. Figure 3 shows an example of tone correction using tone curve correction. In the basic tone correction method, a target brightness set in advance is used, rather than the user setting the target brightness.

[0042] In step S301, the image processing unit 108 detects the luminance of the image data and generates a luminance histogram. The range for detecting luminance is the entire image data, but may be a region excluding the periphery (edges) of the image.

[0043] The reason for generating a brightness histogram from an area excluding the periphery of the image is explained below. The brightness tends to be low in the peripheral areas of the image due to peripheral light falloff caused by the lens. For this reason, the histogram for the peripheral areas of the image will be shifted to the low brightness side, and will not be detected properly.

[0044] In addition, it is preferable not to include the area around the image in the detection range because the subject may not be present and the user will not focus on it if the subject is not present. Furthermore, there are cases where you want to capture still images and videos of the same scene and achieve similar gradation correction effects. Since the aspect ratio of images is 3:2 or 4:3 for still images and 16:9 for videos, excluding the top and bottom edges of still images makes it possible to achieve similar gradation correction effects for still images and videos.

[0045] In step S302, the image processing unit 108 detects a feature amount of the generated brightness histogram. The feature amount detected in step S302 is an example of a representative brightness that represents the brightness of a high brightness area. The brightness histogram generated in step S301 and the feature amount detected in step S302 will be described with reference to FIG. 4. FIG. 4 shows the brightness histogram generated in step S301. The horizontal axis represents the brightness value, and the vertical axis represents the count number (number of pixels).

[0046] As shown in Fig. 4, the image processing unit 108 detects, as the feature amount HL, the level (luminance value) to which pixels whose cumulative frequency (shaded area) from the high luminance side in the luminance histogram belongs. Note that the cumulative frequency for detecting the feature amount HL is not limited to 1% and may be other percentages. For example, the cumulative frequency for detecting the feature amount HL is preferably 1% or more and 10% or less.

[0047] If the cumulative frequency ratio is set lower than 1%, the feature amount HL may be determined based on the brightness of high-luminance noise. In order to determine the feature amount HL based on the brightness of the subject, it is preferable that the cumulative frequency ratio be 1% or higher. Furthermore, since high-luminance noise increases as the ISO sensitivity increases, the cumulative frequency ratio for detecting the feature amount HL may be controlled to increase as the ISO sensitivity increases.

[0048] In step S303, the image processing unit 108 acquires a target luminance (target luminance level) for gradation correction. The target luminance level is a preset value (e.g., 210) and is stored in, for example, a ROM. The image processing unit 108 determines a tone curve and performs gradation correction so that the feature amount HL approaches the target luminance level.

[0049] If the target brightness level is set to a preset value, the image processing unit 108 performs gradation correction so that the image becomes brighter up to the target brightness level regardless of the brightness of the image, which may result in the image contrast becoming too strong.

[0050] To prevent the contrast from becoming too strong, the image processing unit 108 may set a value between the predetermined brightness Yt and the feature amount HL as the target brightness level. For example, the image processing unit 108 sets Yt-α×(Yt-HL) as the target brightness level, where α is a value in the range of 0≦α≦1. For example, the image processing unit 108 sets α to 0.5 or less to obtain the intended gradation. Even for dark images that are difficult to adjust, tone correction can be performed so that the image does not become brighter than the specified brightness Yt, preventing the contrast from increasing excessively. For example, when Yt=220 and α=0.5, if the feature amount HL=160, the target brightness level is 190, and if the feature amount HL=100, the target brightness level is 160. The lower the feature amount HL, the lower the target brightness level becomes.

[0051] In step S304, the image processing unit 108 determines a curve for tone curve correction. FIG. 5 is a diagram showing an example of a tone curve used for gradation correction. Assume that the feature amount HL detected in step S302 is 200, and the target brightness level acquired in step S303 is 210. In this case, the tone curve is a broken line tone curve connecting the points (0,0), (200,210), and (255,255), as shown in FIG. 5(A). That is, the brightness of the feature amount HL (200) is corrected to the brightness of the target brightness level (210). The input axis (horizontal axis) of the tone curve in FIG. 5(A) represents the brightness before correction, and the output axis (vertical axis) represents the brightness after correction.

[0052] The tone curve is not limited to a broken line type, and may be one of the examples shown in Figures 5(B) to 5(D). The input axis (horizontal axis) of the tone curves in Figures 5(B) to 5(D) is the pre-correction luminance, and the output axis (vertical axis) is the post-correction luminance.

[0053] The tone curve shown in FIG. 5(B) is an example of a curved line curve using spline interpolation or the like. The tone curve shown in FIG. 5(C) is an example in which a vertex is added to the broken-line tone curve of FIG. 5(A). The number of vertices excluding the endpoints is not limited to one as in FIG. 5(A), but may be two as in FIG. 5(C), or three or more. The tone curve of FIG. 5(C) is an example in which a vertex (Ymin, Ymin) is added when the minimum luminance in the image is Ymin. By adding the vertex (Ymin, Ymin), the pixel with the minimum luminance is not corrected, and floating black is reduced. The tone curve shown in FIG. 5(D) is an example of a tone curve generalized from FIG. 5(C). The vertex (200, 210) in FIG. 5(C) is written as (HL, target luminance level) in FIG. 5(D).

[0054] In step S305, the image processing unit 108 performs gradation correction on the image using the tone curve determined in step S304. Tone curve correction is a correction process that converts the gradation of the image by applying a function f:x→y (0≦x, y≦255 for 8-bit images) that associates an input luminance value x with an output luminance value y. The function f is the tone curve determined in step S304.

[0055] Three types of tone correction methods that are modifications of the basic tone correction method are described below. In highlight-weighted metering mode, if the user changes the target luminance TH of the high-brightness area, the brightness of the image may not be as intended by the user. The following first to third tone correction methods are methods for performing tone correction so that an image with the brightness intended by the user can be obtained according to the target luminance TH set by the user.

[0056] <First tone correction method> The first tone correction method is a method of switching between enabling and disabling tone correction depending on the metering mode. When the metering mode is not a predetermined metering mode, such as evaluative metering mode or partial metering mode, the basic tone correction described in FIG. 3 is enabled to enhance contrast. An example of a predetermined metering mode is highlight-weighted metering mode. When the metering mode is highlight-weighted metering mode, enabling basic tone correction can result in the brightness of the image not being as intended by the user. Therefore, in highlight-weighted metering mode, tone correction is disabled to prevent the contrast from becoming too strong.

[0057] The first gradation correction method will be described with reference to Fig. 6. Fig. 6(A) and Fig. 6(B) show 10A and 10B are flowcharts showing two examples of the first gradation correction process. The first gradation correction process is executed by the image processing unit 108 upon receiving an instruction from the system control unit 113.

[0058] 6A shows a control method for not executing tone correction when the metering mode is the highlight-weighted metering mode. In step S601, the image processing unit 108 determines whether the metering mode is the highlight-weighted metering mode.

[0059] If the metering mode is highlight-weighted metering mode, the image processing unit 108 disables tone correction, does not perform tone correction using a tone curve, and ends the processing of FIG. 6A. If the metering mode is not highlight-weighted metering mode, the image processing unit 108 performs basic tone correction in step S602. The processing of step S602 is the same as steps S301 to S305 in FIG. 3.

[0060] Fig. 6(B) shows a method of performing gradation correction so that brightness does not change when the metering mode is highlight-weighted metering mode. In the flowchart of Fig. 6(B), the same processes as those in Fig. 3 are assigned the same reference numerals, and detailed descriptions thereof will be omitted. In step S301 of Fig. 6(B), the image processing unit 108 generates a brightness histogram. In step S302, the image processing unit 108 detects feature amounts of the generated brightness histogram. In step S303, the image processing unit 108 acquires a target brightness level.

[0061] In step S603, the image processing unit 108 determines whether the metering mode is the highlight-weighted metering mode. If the metering mode is the highlight-weighted metering mode, in step S604, the image processing unit 108 determines the tone curve to be used for gradation correction to be the linear tone curve.

[0062] FIG. 7 is a diagram illustrating a linear tone curve. The horizontal axis of the tone curve in FIG. 7 is pre-correction luminance, and the vertical axis is post-correction luminance. A linear tone curve is a straight line connecting (0,0) and (255,255). Because the input luminance (pre-correction luminance) and the output luminance (post-correction luminance) are equal, the luminance does not change even if tone correction is performed in step S604 in FIG. 6. In other words, tone correction using a linear tone curve produces the same results as when tone correction is disabled.

[0063] The gradation correction process in Figure 6(A) is suitable for cases where you want to speed up processing by avoiding unnecessary processing. The gradation correction process in Figure 6(B) is suitable for cases where you do not want to change the processing time depending on the metering mode. This is because users may find it difficult to use if the response time when performing some operation changes. If the system is designed to move on to the next operation after completing the process corresponding to the user's operation, the time until moving on to the next operation in the gradation correction process in Figure 6(A) will change depending on the metering mode. For example, the timing from when the shutter is released in still image shooting to when the next operation can be performed, the timing for live view to return, and the timing for previewing the captured image will be faster in highlight-weighted metering mode than in other metering modes.

[0064] Furthermore, if the processing time differs depending on the metering mode, a different control pattern is used in the highlight-weighted metering mode than in the other metering modes in order to achieve real-time control similar to that in the other metering modes. On the other hand, if the processing flow is made common between the highlight-weighted metering mode and the other metering modes, as in the tone correction process of FIG. 6(B), the tone curve can be changed according to the metering mode, simplifying the design. For example, in the highlight-weighted metering mode, the image processing unit 108 can change the tone curve to a linear tone curve as shown in FIG. 7.

[0065] <Second tone correction method> The first gradation correction method disables gradation correction when the metering mode is highlight-weighted metering. In contrast, the second gradation correction method weakens the degree of gradation correction when the metering mode is highlight-weighted metering. The second gradation correction method suppresses gradation correction, thereby increasing contrast while reducing the user's discomfort with the brightness of the image after gradation correction.

[0066] Referring to FIG. 8, the second tone correction method will be described. FIG. 8 is a flowchart illustrating the second tone correction process. In the flowchart of FIG. 8, the same processes as those in FIG. 3 are denoted by the same reference numerals, and detailed descriptions thereof are omitted. In step S301 of FIG. 8, the image processing unit 108 generates a luminance histogram. In step S302, the image processing unit 108 detects a feature amount of the generated luminance histogram. In step S303, the image processing unit 108 acquires a target luminance level.

[0067] In step S801, the image processing unit 108 determines whether the photometry mode is the highlight priority photometry mode. If the photometry mode is the highlight priority photometry mode, the image processing unit 108 lowers the target luminance level in step S802.

[0068] The target luminance level can be lowered, for example, by using a predetermined offset Yo (0 < Yo < Yt). Assuming that the target luminance level acquired in S803 is Yt, the image processing unit 108 sets Yt - Yo as the new target luminance level. If Yt = 220 and Yo = 10, the target luminance level is Yt = 220 in the evaluation photometry mode and Yt - Yo = 220 - 10 = 210 in the highlight priority photometry mode.

[0069] As another method of lowering the target luminance level, the image processing unit 108 may multiply by a predetermined gain Gy (0 < Gy < 1) and set Gy × Yt as the new target luminance level. In either method, the target luminance level becomes smaller than Yt acquired in S803.

[0070] The image processing unit 108 determines a tone curve in step S304 and performs tone correction in step S305 using the new target luminance level lowered in step S802. Thus, by lowering the target luminance level, the image processing unit 108 can consequently suppress tone correction.

[0071] <The Third Tone Correction Method> The third gradation correction method is a method in which the strength of gradation correction is changed according to the target luminance of the high-luminance area selected by the user. With the third gradation correction method, it is possible to increase the contrast while realizing the image brightness intended by the user.

[0072] The image processing unit 108 acquires the target brightness TH of the high-brightness area selected by the user on the screen in FIG. 2 as the target brightness level. The automatic exposure (AE) processing unit 103 controls the exposure so that the brightness of the high-brightness area approaches the target brightness TH. However, if the scene is darker than the target brightness TH or if the lighting environment changes during shooting, the brightness of the image captured by automatic exposure control may be darker than the target brightness TH. In such cases, the image processing unit 108 can brighten the high-brightness area to the target brightness TH by performing gradation correction, thereby increasing the contrast.

[0073] The third gradation correction method will be described with reference to Fig. 9. Fig. 9 shows a flowchart illustrating the third gradation correction process. In the flowchart of Fig. 9, the same processes as those in Fig. 3 are denoted by the same reference numerals, and detailed description thereof will be omitted. In step S301 of Fig. 9, the image processing unit 108 generates a luminance histogram. In step S302, the image processing unit 108 detects feature amounts of the generated luminance histogram.

[0074] In step S901, the image processing unit 108 determines whether the metering mode is the highlight-weighted metering mode. If the metering mode is the highlight-weighted metering mode, the image processing unit 108 acquires the target luminance TH of the high-luminance area selected by the user via the operation unit 116 in step S902.

[0075] In step S903, the image processing unit 108 sets the target brightness TH of the high brightness area selected by the user as the target brightness level. The image processing unit 108 can increase the contrast within a range in which the brightness of the high brightness area does not exceed the target brightness TH.

[0076] As the difference between the feature amount HL detected in step S302 of FIG. 9 and the target brightness TH selected by the user increases, the contrast may become too strong. Therefore, the target brightness level may be a brightness value between the feature amount HL and the target brightness TH selected by the user. For example, the image processing unit 108 sets the target brightness level as HL+β×(TH−HL), where β is a value in the range of 0≦β≦1. As β approaches 0, the correction becomes weaker, and as β approaches 1, the correction becomes stronger. When β=1, the brightness of HL after correction becomes the target brightness TH.

[0077] In step S304 of FIG. 9, the image processing unit 108 determines a curve for tone curve correction based on the target luminance level set in step S303 or step S903.

[0078] 9, the image processing unit 108 performs gradation correction on the image using the tone curve determined in step S304. If the feature amount HL is determined in step S302 with a cumulative frequency of 1%, the area where the corrected brightness exceeds the feature amount HL is the area where the cumulative frequency is 1% or less at most. In other words, when the user switches the target brightness to TH1, TH2, or TH3, the maximum brightness of the image changes to approach the target brightness TH selected by the user, allowing the user to change the brightness as intended.

[0079] The target brightness TH used as the target brightness level has two meanings: an upper threshold and a target value. FIG. 10 is a diagram illustrating the target brightness TH. FIG. 10(A) shows an example of a brightness histogram when the target brightness TH is the upper threshold. The target brightness TH is the upper limit of brightness, and the image processing unit 108 performs gradation correction so that the target brightness TH (upper threshold) is not exceeded. However, even when the target brightness TH is set as the upper threshold, the image processing unit 108 may allow the target brightness TH to be exceeded as long as the cumulative frequency is within, for example, 1% from the high brightness side of the brightness histogram.

[0080] 10B shows an example of a luminance histogram when the target luminance TH is the target value. When the target luminance TH is the target value, it is desirable to perform gradation correction so that the luminance of the high luminance region approaches the target luminance TH (target value). The luminance of part of the high luminance region may exceed the target luminance TH.

[0081] A high-luminance region can be, for example, a region having a luminance equal to or greater than a predetermined threshold (e.g., 210), or a region having a luminance where the cumulative frequency from the high-luminance side in a luminance histogram is equal to or less than a predetermined percentage (e.g., 10%), but is not limited to this. FIG. 10B shows an example in which the shape of a luminance histogram is detected and a luminance range included in the peak on the high-luminance side is determined to be a high-luminance region. For example, the image recognition unit 109 can determine that the peak on the high-luminance side is the region in the luminance histogram where the count number (number of pixels) from the high-luminance side starts to increase, then decreases, and then changes from decrease to increase. A high-luminance region is a region of a group of pixels that corresponds to a luminance range brighter than the change point where the count number changes from decrease to increase.

[0082] 11A to 11C are examples of screens on which the user selects the target brightness TH. The screen shown in FIG. 11A is an example of a screen for selecting the target brightness TH as the upper limit threshold. FIG. 11B is an example of a screen for selecting the target brightness TH as the target value. The user may be able to switch between using the target brightness TH as the upper limit threshold and the target value. FIG. 11C is an example of a screen for the user to select between using the target brightness TH as the upper limit threshold and the target value.

[0083] Depending on whether the target brightness TH is an upper limit threshold or a target value, some of the processing in Fig. 9 may be changed. The example described above in Fig. 9 is an example in which it is assumed that the brightness after correction exceeds the feature amount HL in an area where the cumulative frequency is 1% or less, and the target brightness TH is the upper limit threshold.

[0084] On the other hand, a gradation correction method when the target luminance TH is set as the target value will be described. When the target luminance TH is set as the target value, the image processing unit 108 performs gradation correction so that the luminance (representative luminance) representing the high luminance region approaches the target luminance TH. The representative luminance is, for example, the average luminance (of pixels) in a region determined to be a high luminance region by the image recognition unit 109. The representative luminance may be the average luminance in the high luminance region, or a luminance corresponding to the maximum, minimum, median, or mode of the luminance in the high luminance region. Furthermore, the representative luminance may be the luminance to which pixels whose cumulative frequency is a predetermined percentage belong from the high luminance side or low luminance side of the luminance histogram. Furthermore, the representative luminance may be a value calculated by combining these.

[0085] The image processing unit 108 acquires the representative luminance and performs gradation correction using the acquired representative luminance as the feature amount HL. By performing gradation correction using the representative luminance as the feature amount HL, the image processing unit 108 can perform gradation correction so that the representative luminance approaches the target luminance TH.

[0086] The gradation correction process when the target brightness TH is set as the target value will be described. The differences from the gradation correction process when the target brightness TH in Fig. 9 is set as the upper limit threshold will be described. Instead of steps S301 and S302, the image processing unit 108 acquires an average value of brightness in a high brightness area, and determines the acquired average value as the feature amount HL.

[0087] In step S903, the image processing unit 108 calculates the target brightness level as HL+β×(TH−HL) based on the determined feature amount HL and the target brightness TH selected by the user in step S902. The image processing unit 108 determines a tone curve in step S304 and performs gradation correction in step S305. The average value of the high-brightness region approaches the target brightness TH, and the brightness of the entire high-brightness region approaches the target brightness TH. Since the user can make adjustments by viewing the entire high-brightness region rather than the brightest local region, the image can easily approach the desired brightness.

[0088] If the scene is brighter than the target brightness TH, or if the lighting environment changed during shooting, the brightness of the high-brightness area may already be brighter than the target brightness TH before correction. In such cases, the third gradation correction process in FIG. 9 can also be applied.

[0089] 9, the feature amount HL is greater than the target brightness TH. In step S903, the image processing unit 108 calculates the target brightness level by HL+β×(TH−HL). Because the feature amount HL is greater than the target brightness TH, the target brightness level is smaller than the feature amount HL.

[0090] Fig. 12 shows an example of a tone curve when the feature amount HL is greater than the target brightness TH. As shown in Fig. 12, the tone curve determined in step S304 has a peak (HL, target brightness level) located below the line connecting the peak (Ymin, Ymin) and the peak (255, 255). Therefore, the tone correction in S305 in Fig. 9 is a correction to darken the brightness. As a result, the luminance of the high luminance area after the gradation correction approaches the target luminance TH.

[0091] According to the above-described first to third tone correction methods, when the user changes the target brightness TH of the high-brightness area in highlight-weighted metering mode, the digital camera 100 can provide tone correction of the brightness of the high-brightness area so that the brightness of the image is as intended.

[0092] Although the present invention has been described in detail above based on preferred embodiments thereof, the present invention is not limited to these specific embodiments, and various forms within the scope of the gist of the present invention are also included in the present invention. Parts of the above-described embodiments may be combined as appropriate.

[0093] <Other embodiments> The present invention can also be realized by supplying a program that realizes one or more functions of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program.The present invention can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0094] The disclosure of this embodiment includes the following configurations and methods. (Configuration 1) An imaging device that controls exposure by measuring a photometric area according to a photometric mode, an acquisition means for acquiring a target luminance level of a high luminance area of ​​an image; a gradation correction means for performing gradation correction of the image based on the target luminance level; and The gradation correction means suppresses the gradation correction in a predetermined photometry mode in which the high-luminance region is used as a photometry region and photometry is focused on the high-luminance region, more than in other photometry modes. An imaging device characterized by: (Configuration 2) The gradation correction means disables the gradation correction in the predetermined photometric mode. 2. The imaging device according to claim 1, (Configuration 3) The tone correction means changes the tone curve used for the tone correction to a linear tone curve in the predetermined photometric mode, thereby invalidating the tone correction. 3. The imaging device according to configuration 2. (Configuration 4) The acquisition means acquires a target luminance selected by a user as the target luminance level. 2. The imaging device according to claim 1, (Configuration 5) The gradation correction means detects a representative luminance that represents the luminance of the high luminance region, and performs the gradation correction so that the representative luminance approaches the target luminance level. 2. The imaging device according to claim 1, (Configuration 6) The representative luminance is Any of the average value, maximum value, minimum value, median value, and mode value of the luminance in the high luminance region, or The luminance is the luminance of pixels to which the cumulative frequency from the high luminance side or the low luminance side of the luminance histogram of the image belongs at a predetermined rate. 6. The imaging device according to configuration 5. (Configuration 7) The luminance histogram is generated from a region of the image excluding the periphery. 7. The imaging device according to configuration 6, (Configuration 8) The predetermined percentage is equal to or greater than 1% and equal to or less than 10%. 8. The imaging device according to configuration 6 or 7, (Configuration 9) The predetermined ratio is controlled to be higher as the ISO sensitivity increases. 9. The imaging device according to any one of configurations 6 to 8, wherein: (Configuration 10) The acquisition means acquires a value between a predetermined luminance and the representative luminance as the target luminance level. 10. The imaging device according to any one of configurations 5 to 9, wherein: (Configuration 11) The acquisition means acquires the target luminance level based on the representative luminance and a target luminance selected by a user. 10. The imaging device according to any one of configurations 5 to 9, wherein: (Configuration 12) the target luminance level is an upper threshold of the luminance of the high luminance region, The gradation correction means performs the gradation correction in the predetermined photometric mode so that the luminance of the high luminance area does not exceed the upper limit threshold. 12. The imaging device according to any one of configurations 1 to 11, (Configuration 13) the target luminance level is a target value of luminance in the high luminance region, The gradation correction means performs the gradation correction in the predetermined photometric mode so that the luminance of the high-luminance area approaches the target value. 12. The imaging device according to any one of configurations 1 to 11, (Configuration 14) The high-luminance region is a region having a luminance equal to or greater than a predetermined threshold, or a region having a luminance where the cumulative frequency from the high-luminance side in a luminance histogram is equal to or less than a predetermined percentage. 14. The imaging device according to any one of configurations 1 to 13, (method) A control method for an imaging device that controls exposure by measuring a photometric area according to a photometric mode, comprising: an acquisition step of acquiring a target luminance level for a high luminance region of an image; a gradation correction step of performing gradation correction on the image based on the target luminance level; and In the gradation correction step, in a predetermined photometry mode in which the high luminance region is used as a photometry region and photometry is focused, the gradation correction is suppressed more than in other photometry modes. 10. A method for controlling an imaging device, comprising: (program) A program for causing a computer to function as each of the means of the imaging device according to any one of configurations 1 to 14. (medium) 15. A computer-readable storage medium storing a program for causing a computer to function as each of the means of the imaging device according to any one of configurations 1 to 14. [Explanation of symbols]

[0095] 100: imaging device, 103: AE processing unit, 108: image processing unit, 109: image recognition unit, 113: system control unit

Claims

[

1. ] A setting means for setting a target luminance level in a high-luminance region of an imaging scene, an exposure control means for performing photometry and controlling exposure, and a tone correction means for performing a predetermined tone correction on an image obtained by imaging and having, The mode of controlling exposure includes a first photometry mode in which photometry is performed with the high-luminance region as a photometry region and exposure is controlled based on the target luminance level set by the setting means, and a second photometry mode different from the first photometry mode, When the image is obtained by imaging using the first photometry mode, the tone correction means suppresses the predetermined tone correction more than when the image is obtained by imaging using the second photometry mode An imaging apparatus characterized by this. [

2. ] In the first photometry mode, the tone correction means invalidates the predetermined tone correction The imaging apparatus according to claim 1, characterized by this. [

3. ] The imaging apparatus according to claim 2, characterized in that in the first photometry mode, the tone correction means invalidates the predetermined tone correction by changing a tone curve used for the predetermined tone correction to a linear tone curve. [

4. ] The setting means sets a target luminance selected by the user as the target luminance level The imaging apparatus according to claim 1, characterized by this. [

5. ] The imaging apparatus according to claim 4, further comprising a presenting means for presenting a plurality of candidates for the target luminance that can be selected by the user characterized by this. [

6. ] The tone correction means detects a representative luminance representing the luminance of the high-luminance region, and performs the predetermined tone correction so that the representative luminance approaches the target luminance level The imaging apparatus according to claim 1, characterized by this. [

7. ] The representative luminance is any one of an average value, a maximum value, a minimum value, a median value, and a mode value of the luminance in the high-luminance region, or is the luminance to which pixels belong whose cumulative frequency from the high-luminance side or the low-luminance side of the luminance histogram of the image reaches a predetermined ratio The imaging apparatus according to claim 6, characterized by this. [

8. ] The luminance histogram is generated from a region excluding the periphery of the image The imaging apparatus according to claim 7, characterized by this. [

9. ] The predetermined ratio is 1% or more and 10% or less The imaging apparatus according to claim 7, characterized by this. [

10. ] The predetermined ratio is controlled to increase as the ISO sensitivity increases The imaging apparatus according to claim 7, characterized by this. [

11. ] The setting means sets a value between the predetermined luminance and the representative luminance as the target luminance level. The imaging device according to claim 6, characterized in that.

12. The setting means sets the target luminance level based on the representative luminance and the target luminance selected by the user. The imaging device according to claim 6, characterized in that.

13. The target luminance level is the upper limit threshold of the luminance in the high-luminance area, In the first photometry mode, the gradation correction means performs the predetermined gradation correction so that the luminance in the high-luminance area does not exceed the upper limit threshold. The imaging device according to claim 1, characterized in that.

14. The target luminance level is the target value of the luminance in the high-luminance area, In the first photometry mode, the gradation correction means performs the predetermined gradation correction so that the luminance in the high-luminance area approaches the target value. The imaging device according to claim 1, characterized in that.

15. The high-luminance area is an area having a luminance equal to or higher than a predetermined threshold, or an area having a luminance such that the cumulative frequency from the high-luminance side in the luminance histogram is equal to or less than a predetermined ratio. The imaging device according to claim 1, characterized in that.

16. A setting step of setting a target luminance level for a high-luminance area of an imaging scene, An exposure control step of performing photometry and controlling exposure, A gradation correction step of performing predetermined gradation correction on the image obtained by imaging, And having, The mode for controlling exposure includes a first photometry mode in which photometry is performed with the high-luminance area as the photometry area and exposure is controlled based on the target luminance level set in the setting step, and a second photometry mode different from the first photometry mode. In the gradation correction step, when the image is obtained by imaging using the first photometry mode, the predetermined gradation correction is suppressed more than when the image is obtained by imaging using the second photometry mode. A control method for an imaging device, characterized in that.

17. A program for causing a computer to function as each means of the imaging device according to any one of claims 1 to 15.

18. A computer-readable storage medium storing a program for causing a computer to function as each means of the imaging device according to any one of claims 1 to 15.

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