JP2011049737A - Imaging apparatus and exposure control method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus which achieves stabilization of the luminance of a face and can achieve exposure control for quickly making the exposure of the entire screen proper even when a state occurs in which the luminance value of the entire screen, such as switching of imaged scenes changes large. <P>SOLUTION: The imaging apparatus includes: an imaging element 8 for imaging an object; a face detection circuit 16 for detecting a face in an image of the object imaged by the imaging element 8; and a luminance information detection circuit 17 for detecting the luminance value of the position of the face detected by the face detection circuit 16 and the luminance value of the entire screen. Further, the imaging apparatus includes a microcomputer 12 for calculating a correction amount from a compensation value obtained by applying a time constant to the luminance value of the face detected by the luminance information detection circuit 17 and the luminance value of the entire screen. The microcomputer 12 clears the compensation value and determines a correction amount when the luminance value of the entire screen between frames exceeds a threshold. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an imaging apparatus capable of detecting a human face from captured image information and appropriately correcting exposure of the detected face, and an exposure control method thereof.

  In today's imaging devices such as digital cameras and digital camcorders, an imaging device having an AE (automatic exposure control) function that automatically determines exposure based on acquired image information at the time of imaging (photographing) is common. is there. In automatic exposure control, brightness value information is obtained by photometry from image information of the entire screen being imaged by using various types of photometry methods in order to perform appropriate exposure control.

  Here, as the metering method used mainly, the metering method that averages the metering values acquired from the entire screen to control exposure, and the screen is divided into multiple areas, and the metering values in each area are weighted Then, there is a photometry method for performing exposure control. By switching the photometry method according to the situation, it is possible to appropriately determine the exposure of the entire screen even when the luminance value of the entire screen is bright or when some luminance values are bright. In the dark state, the exposure can be properly converged by performing the same exposure control.

  Further, a face detection function for detecting a person's face from an image is provided. As shown in FIG. 4, a face photometry frame for obtaining a photometric value of a subject's face separately from a photometry value for obtaining a photometric value of the entire screen. There is a way to set and use. The face metering frame is set according to the position and the face size of the face of the subject detected on the screen.

  There has been proposed a method of controlling the brightness value of a face to an appropriate exposure by performing exposure control by weighting the face metering value acquired from the face metering frame larger than the metering value of the entire screen ( (See Patent Document 1). As a result, when the brightness value of the face is brighter than the entire screen, exposure control is performed based on the photometric value of the face. Therefore, it is possible to suppress overexposure of the face of the subject and control the exposure appropriately.

JP 2008-070562 A

  However, as shown in FIG. 5, when the face photometric value is obtained, the person is not always facing the front, so the luminance value other than the portion of the human skin that is originally desired to be photometrically measured, such as hair, is measured. It is conceivable that the face photometric value acquired from the frame is not stable. If this face photometric value is used as it is, the overall luminance value considering the face is not stabilized by exposure control, and the luminance value does not converge, resulting in an image with a flickering brightness.

  Therefore, in order to reduce the amount of change in the brightness of the face, the time constant is taken into consideration as in the exposure control of the entire screen. This is because even if the photometric value of the face is not stable and fluctuates due to the orientation of the face and the light source state, it absorbs a sudden change in brightness of the face due to the time constant and changes the exposure of the entire screen slowly. is there. Here, since the brightness change in the face is more noticeable than the brightness change in the background, the time constant used for gently changing the face brightness is larger than the time constant used for the exposure control of the entire screen (change Use the one that reduces the width of

  However, in the exposure control method using time constants having different sizes as described above, when the luminance difference of the entire screen is large due to scene switching or the like, the luminance of the entire screen is targeted due to the influence of the size of the face time constant. It will take a lot of time to converge to the value.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an imaging apparatus and its exposure control method that can achieve exposure control that stabilizes the brightness of the face and quickly and appropriately exposes the entire screen.

  In order to achieve the above object, an image pickup apparatus according to claim 1 includes an image pickup device, a face detection unit that detects a face in an image of a subject imaged by the image pickup device, and a face detected by the face detection unit. Obtained by applying a first time constant to the first luminance value and luminance information detecting means for detecting the first luminance value at the position and the second luminance value in a wider range than the face From the compensation value and the second luminance value, there is provided a correction amount calculating means for calculating a correction amount for making the luminance of the image appropriate, and the correction amount calculating means includes the correction amount calculating means between the plurality of images. When the difference between the second luminance values exceeds a threshold value, the compensation value is cleared to 0 or the compensation value is reduced.

  According to the present invention, the brightness of the face is stabilized, and even when a state in which the brightness value of the entire screen changes greatly, such as switching of an imaging scene, exposure control is realized to quickly and appropriately adjust the exposure of the entire screen. can do.

It is a figure which shows an example of schematic structure of the imaging device which concerns on embodiment of this invention. (A)-(e) is a figure which shows the exposure control method using the face information of this invention. 3 is a flowchart showing a procedure of exposure control processing using face information executed by the imaging apparatus of FIG. 1. It is a figure which shows the relationship between the face photometry frame used by this invention, and the photometry frame of the whole screen. It is a figure which shows the state of the face where the face photometric value becomes unstable.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a diagram illustrating an example of a schematic configuration of an imaging apparatus 100 according to an embodiment of the present invention.

  In FIG. 1, a single lens 1 that guides light incident from the outside of the imaging apparatus 100 to the inside of the imaging apparatus 100 is illustrated in a simplified manner, but is normally configured by a plurality of lenses. The diaphragm 2 adjusts the amount of incident light. A diaphragm drive motor (M) 3 drives the diaphragm 2. A diaphragm state detection circuit (S) 4 detects the driving state of the diaphragm 2.

  The ND filter 5 attenuates light incident from the lens 1. The ND filter drive motor (M) 6 drives the ND filter 5. The ND filter drive detection circuit (S) 7 detects the drive state of the ND filter 5.

  The type of the image sensor 8 that captures an image of a subject is not limited, such as a CCD or a CMOS sensor. A CDS (Correlated Double Sampling) / AGC (Auto Gain Control) circuit 9 samples and amplifies image information based on charges (image signals) stored in each pixel of the image sensor 8. . It is also possible to perform exposure control simply by eliminating the aperture 2 and the ND filter 5 and setting the charge accumulation time and ISO sensitivity of the image sensor 8.

  The A / D conversion circuit 10 converts the image information output from the CDS / AGC circuit 9 into a digital signal. The digital signal processing circuit 11 performs various signal processing on the digital image information output from the A / D conversion circuit 10.

  The microcomputer 12 controls the overall operation of the imaging apparatus 100 according to the present embodiment. For example, the microcomputer 12 receives information such as luminance and color from the digital signal processing circuit 11 and performs various arithmetic processes. . The aperture mechanism drive unit 13 supplies power to the aperture drive motor 3 based on control by the microcomputer 12.

  The image sensor driving unit 14 supplies drive pulses and the like for driving the image sensor 8 based on control by the microcomputer 12. The ND filter mechanism drive unit 15 supplies power to the ND filter drive motor 6 based on control by the microcomputer 12.

  The face detection circuit 16 detects the face position and the face size of the subject in the image (in the screen) from information on the entire image that has been subjected to various signal processing by the digital signal processing circuit 11. The luminance information detection circuit 17 detects a luminance value (luminance information) based on digital image data sent from the microcomputer 12.

  The microcomputer 12 functions as a correction amount calculation means for calculating a correction amount for making the exposure of the face appropriate from the luminance value detected by the luminance information detection circuit 17.

  The storage device 19 temporarily stores (stores) the captured image. The recording medium 20 records image information picked up by the image pickup apparatus 100.

  An overview of processing when performing exposure control for appropriately correcting the luminance value of a face in the imaging apparatus 100 of the present embodiment will be described. First, the image sensor 8 receives a subject image to be imaged, and the electric charge stored in each pixel of the image sensor 8 is output as an image signal, which is sampled and amplified by the CDS / AGC circuit 9. Then, it is converted into a digital signal by the A / D conversion circuit 10 and sent to the digital signal processing circuit 11.

  The digital signal processing circuit 11 performs various signal processing on the digital image signal output from the A / D conversion circuit 10. Then, the microcomputer 12 sends the result to the face detection circuit 16, and the face detection circuit 16 detects the face position and the face size of the subject in the screen from the image information.

  Thereafter, when the face detection circuit 16 detects the face position and the face size of the subject, the brightness information detection circuit 17 acquires the face photometric value (first brightness) in order to obtain the brightness value of the face position in the screen. Value). At this time, the luminance information detection circuit 17 also acquires the entire photometric value (second luminance value) of the entire screen at the same time.

  Then, the microcomputer 12 calculates a face difference value that is a difference between the face photometric value and the face target value set so that the face converges properly. Thereafter, the microcomputer 12 calculates a face compensation value to which the first time constant is applied from the calculated face difference value, and adds the face compensation value to the photometric value of the entire screen to obtain a final photometric value. The microcomputer 12 determines a final correction value for converging the final photometric value to the target value of the entire screen, and based on the final correction value, the diaphragm mechanism driving unit 13, the image sensor driving unit 14, and the ND filter The mechanism drive unit 15 is controlled.

  In the present embodiment, in order to express in an easy-to-understand manner, FIG. 1 shows a configuration of all independent circuits. However, all or a part of the configuration may be configured in the microcomputer 12. .

  A method for determining the final correction value will be described in detail with reference to FIG. FIG. 2 is a diagram for explaining a method for obtaining the final correction value from the face photometric value and the photometric value (overall photometric value) of the entire screen. 2A to 2E are time axes, and FIGS. 2A to 2E show respective values at the same time. In the example of FIG. 2, it is assumed that the brightness of the entire screen does not change so much that a scene in the process of a person approaching a sunny place from the shade is photographed.

  FIG. 2A shows the face photometric value in each frame image obtained by the luminance information detection circuit 17, and the microcomputer 12 uses the face photometric value of each frame in excess or deficiency with respect to the face target value as a face difference value. Ask.

  The graph on the left side of FIG. 2B shows the face difference value in each frame image. In this example, since the face photometric value is smaller than the face target value, the face difference value is obtained as a negative value. The microcomputer 12 obtains the face compensation value shown in the graph on the right side of FIG. 2B by applying the first time constant to the obtained face difference value in order to avoid a sudden change in the brightness of the face. . The face compensation value is set so that the difference between the frame images is smaller than the face difference value. As described above, since the change in the brightness of the face in the face is more conspicuous than the change in the brightness in the background, the first time constant for obtaining this face compensation value is the final correction value shown in FIG. It is set to a value larger than the second time constant applied when obtaining.

  FIG. 2C shows the total photometric value in each frame image obtained by the luminance information detection circuit 17. Here, a value obtained by averaging the photometric values in the entire screen will be described as the total photometric value. However, a value obtained by weighting and averaging each area of the screen may be used as the total photometric value. Or it is good also considering not only the whole screen but the photometric value obtained from the area | region of about 80% of the center part of a screen as a whole photometric value. That is, the total photometric value may be a photometric value obtained from a wider range than the detected face area.

  The microcomputer 12 adds the face compensation value shown in FIG. 2B to the total photometric value to obtain the final photometric value. In this example, since the face compensation value is a negative value, the final photometric value is smaller than the overall photometric value.

  Then, the microcomputer 12 obtains a value obtained by applying the second time constant to the final photometric value in order to suppress the luminance change of the final photometric value, and cancels the difference between the value and the target value of the entire screen. A value for this is obtained as a final correction value shown in FIG.

  The microcomputer performs exposure control by controlling the diaphragm mechanism driving unit 13, the image sensor driving unit 14, and the ND filter mechanism driving unit 15 according to the final correction value. By doing so, it is possible to perform exposure control that suppresses a rapid change in luminance of the face and brings the luminance of the face closer to the target value.

  However, with this method alone, when the brightness difference of the entire screen is large due to scene switching or the like, a problem may occur due to the slow change of the face compensation value due to the magnitude of the first time constant. This will be described by taking as an example a case where a scene having a low overall brightness such as indoors is suddenly switched to a scene having a high brightness such as outdoors under sunlight.

  Under such circumstances, the face photometric value is lower than the face target value indoors, but suddenly becomes higher than the face target value due to a scene change. However, since the value of the first time constant is set to be larger than that of the second time constant, the face compensation value is greatly influenced by the face photometric value before the scene change, and even if the scene changes, Is negative. Therefore, despite the fact that the actual face photometric value after the scene change exceeds the face target value, a negative face compensation value is added to the total photometric value for a while after the scene change, The final photometric value is obtained. In other words, after the scene changes, although the total photometric value and the face photometric value are both higher than the target value, exposure control to reduce these photometric values by the face compensation value is suppressed, and the total photometric value is also reduced. A situation occurs in which the face photometric value also prolongs the period exceeding each target value.

  Further, when no face is detected from the image, it is not necessary to consider the face compensation value when obtaining the final photometric value. Therefore, even if the same sudden change in luminance occurs between when the face is detected and when it is not detected, a large difference occurs in the time until the luminance converges to the target value. .

  Therefore, in the present embodiment, during exposure control, when the luminance information detection circuit 17 detects a state in which the luminance difference is large between the previous frame and the current frame, the microcomputer 12 uses the face compensation value currently held. Set to 0 (clear). That is, the exposure control is switched to the final photometric value obtained from only the total photometric value without considering the face compensation value. The final correction value is a value obtained by applying the second time constant to the final photometric value. As described above, the second time constant is greater than the first time constant obtained when the face compensation value is obtained. Is also small. Therefore, it is possible to converge to the target value faster than when the face compensation value is not set to 0. In addition, the final photometric value can be converged to the target value in the same period as compared with the case where no face is detected.

  In addition, when the microcomputer 12 detects a state in which the luminance difference between the frames is large, the microcomputer 12 not only clears the face compensation value but also reduces the face compensation value to reduce the time required for convergence. It can be shortened.

  Thereafter, when the final photometric value converges to the target value, the face detection circuit 16 again detects the face position and the face size of the subject, and the luminance information detection circuit 17 measures the luminance value of the face position.

  Then, the face compensation value is calculated by the microcomputer 12 using the photometry value acquired by the luminance information detection circuit 17 and added to the overall photometry value, thereby obtaining the final photometry value obtained by taking the face compensation value into consideration. The process of converging to the target value of the entire screen and making the exposure of the face appropriate is resumed.

  Next, an exposure control method by the imaging apparatus 100 according to the present embodiment will be described.

  FIG. 3 is a flowchart showing the procedure of exposure control processing using face information executed by the imaging apparatus 100 of FIG.

  This process is executed under the control of the microcomputer 12 of FIG.

  When performing exposure control using face information, in step S101, it is determined whether or not the face of the subject exists in the screen imaged by the face detection circuit 16. As a result of the determination in step S101, if it is determined that no face exists in the screen, the entire photometric value is regarded as the final photometric value without calculating the face compensation value, and the process proceeds to step S108.

  On the other hand, if it is determined in step S101 that the subject's face is present on the screen, the process proceeds to step S102. In step S <b> 102, the microcomputer 12 sets a face frame at the face position detected in step S <b> 101, and acquires a face photometric value by the luminance information detection circuit 17. At this time, if a plurality of faces are detected, the face photometric value to be obtained may be obtained from the main face of one person or from the faces of all persons.

  In step S103, the microcomputer 12 calculates a face difference value that is a difference value between the face photometric value and the face target value. Thereafter, in step S104, the microcomputer 12 calculates a face compensation value to which the first time constant is applied from the calculated face difference value.

  Subsequently, the luminance information detection circuit 17 calculates the difference between the luminance values (total photometric values) of the entire screen between the previous frame and the current frame. In step S105, the microcomputer 12 calculates the difference between the calculated total photometric values. It is determined whether or not it is larger than the threshold value.

  If the difference between frames of the total photometric value becomes larger than the threshold during exposure control, the microcomputer 12 clears the currently held face compensation value to 0 in step S106. That is, the exposure control is switched to the final photometry value obtained from only the total photometry value without considering the face compensation value. On the other hand, if it is determined in step S105 that the difference between the frames of the total photometric value is equal to or less than the threshold, the microcomputer 12 proceeds to step S107 without performing the process in step S106. In step S107, the microcomputer 12 calculates the final photometric value by adding the face compensation value to the overall photometric value. In step S108, a final correction value for converging the final photometric value to the target value of the entire screen is determined.

  In the present embodiment, the microcomputer 12 determines whether or not the overall photometric value has changed abruptly in step S105. If only the face photometric value has changed abruptly, the microcomputer 12 determines in step S106. Does not proceed to processing. This is because the change in the brightness of the entire screen is small, so the possibility of a change in the scene is low, and even if only the brightness value of the face changes suddenly, it is a temporary change due to changes in the face orientation, light source state, etc. This is because it is considered that there is a high possibility of a change in luminance. Therefore, in such a state, it is considered that the influence is small if exposure control is performed by considering the time constant in the normal face correction value, reducing the change amount of the correction value, and performing exposure control. Therefore, this exposure control is performed only when the difference in the luminance values of the entire screen is large.

  By the series of processes in steps S101 to S108, exposure control using the face compensation value by the imaging apparatus 100 according to the present embodiment is executed.

  Finally, a characteristic operation in the imaging apparatus 100 of the present embodiment will be described.

  First, a face compensation value for converging the face to appropriate exposure is obtained from the face photometric value. Then, the final photometric value is obtained by adding the face compensation value to the total photometric value. If a brightness difference that exceeds the threshold is detected due to scene switching, etc., the currently retained face compensation value is cleared to 0, thereby overcorrecting the exposure of the entire screen with the retained face compensation value. To prevent it.

  As a result, even when a state in which the luminance value of the entire screen changes greatly, such as switching of an imaging scene, exposure control can be performed to quickly and appropriately adjust the exposure of the entire screen as compared with the conventional method.

8 Image sensor 11 Digital signal processing circuit 12 Microcomputer 13 Aperture mechanism drive unit 16 Face detection circuit 17 Luminance information detection circuit 100 Imaging device

Claims (5)

An image sensor;
Face detection means for detecting a face in an image of a subject imaged by the imaging element;
Luminance information detecting means for detecting a first luminance value at the face position detected by the face detecting means and a second luminance value in a wider range than the face;
A correction amount for calculating a correction amount for making the luminance of the image appropriate from the compensation value obtained by applying the first time constant to the first luminance value and the second luminance value. Having a calculation means,
The correction amount calculating means clears the compensation value to 0 or reduces the compensation value when a difference in the second luminance value between a plurality of images exceeds a threshold value. An imaging device.   The imaging apparatus according to claim 1, wherein the correction amount calculation unit calculates the correction amount from a value obtained by adding the compensation value and the second luminance value.   The correction amount calculating means includes a value obtained by applying a second time constant smaller than the first time constant to a value obtained by adding the compensation value and the second luminance value, and a target value The imaging apparatus according to claim 2, wherein the correction amount is calculated from a difference between the imaging device and the correction amount.   The correction amount calculation means, when the face detection means cannot detect a face, a value obtained by applying the second time constant to the second luminance value, the target value, The imaging apparatus according to claim 3, wherein the correction amount is calculated from a difference between the two. A face detection step of detecting a face in the image of the subject imaged by the image sensor;
A luminance information detecting step for detecting a first luminance value at the position of the face detected in the face detecting step and a second luminance value in a range wider than the face;
A correction amount for calculating a correction amount for making the luminance of the image appropriate from the compensation value obtained by applying the first time constant to the first luminance value and the second luminance value. A calculation step,
In the correction amount calculating step, when the difference in the second luminance value between a plurality of images exceeds a threshold value, the compensation value is cleared to 0, or the compensation value is reduced. An exposure control method for an imaging apparatus.