JP2005109999A - White balance adjustment method and camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent unnatural change in colors of an image on the border between low-brightness light emission and daylight synchronization during shooting using stroboscope. <P>SOLUTION: A region of target luminosity is divided into a low-brightness light emission region, a daylight synchronization region and a border region between the low-brightness light emission region and the daylight synchronization region. A target luminosity before stroboscopic light emission is obtained (S10). If it is in the low-brightness region, white balance of an image signal is adjusted by a white balance gain corresponding to stroboscope light (S14 and S16). If it is in the daylight synchronization region, the white balance of the image signal is adjusted by a white balance gain for the daylight synchronization (S20). If it is in the border region, the white balance of the image signal is adjusted by an intermediate white balance gain between a white balance gain corresponding to color temperature of the stroboscope light and the white balance for the daylight synchronization (S22 and S26). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a white balance adjustment method and a camera, and more particularly, to a white balance adjustment method and a camera for adjusting a white balance of an image signal captured by irradiating a subject with strobe light.

  The subject image obtained by irradiating the stroboscopic light and picking up an image with the color image pickup device generally has a bluish color as compared with an image taken with only the daylight light source. In addition, even if the flash is emitted with the same amount of light, the flash is emitted in a low-brightness state (strobe light is dominant), and the day is synchronized (the ambient light and the strobe light are mixed). In general, the color temperature is different. Accordingly, various white balance adjustment methods have been proposed in consideration of flash photography such as low-luminance light emission and daytime synchros (see, for example, Patent Documents 1 and 2).

  Patent Document 1 describes a color temperature that depends on strobe light and a color that depends on ambient light for the purpose of obtaining an optimum white balance even when shooting with strobe light in a bright state such as daytime synchro. It is described that white balance is adjusted by measuring temperature.

Japanese Patent Application Laid-Open No. 2004-228688 describes white balance control performed according to a light source type by determining a light source type, determining whether or not to emit strobe light based on subject luminance, and a case where it is determined that strobe light is emitted and strobe light emission. It is described that white balance control is performed differently when it is determined not to be performed.
Japanese Patent No. 2698578 JP 2000-224608 A

  When the gain value applied to the image signal for white balance adjustment is switched between the case of low-brightness light emission where the strobe light is dominant and the case of daytime synchro in which ambient light and strobe light are mixed, the image suddenly changes. The color of will change, making it unnatural.

  Specifically, strobe light generally has a bluish color than a daylight light source, and uses a gain value corresponding to the color temperature depending on the strobe light for low-intensity light emission, and ambient light for daytime synchro. When the gain value of the color temperature depending on the daylight light source is used, overcorrection is caused depending on the environment at the boundary between low-luminance light emission and daytime synchro. In other words, if a gain value for low-intensity light emission for correcting the effect of strobe light is given to the image signal, it becomes a reddish color than the original color, and the daylight light source that considers ambient light is also taken into account When the sync gain value is given to the image signal, the color becomes more bluish than the original color. The gain value for low-luminance light emission and the gain value for daytime synchronization are suddenly switched, and in some cases, the original color is not reproduced.

  In the method of switching the gain value based on the color temperature, the actual ambient light when taking a stroboscopic image outdoors can take the color temperature in a wide range from about 5500 K to about 10000 K depending on the weather (for example, sunny or cloudy). On the other hand, the strobe light generally has a color temperature in a range from about 5500K to about 6000K, and since it overlaps on the color temperature, there is a problem that determination is difficult. In addition, the actual ambient light when taking a strobe image in a room is difficult to determine because the color temperature can be taken in a wide range depending on a specific type of indoor lighting (for example, a fluorescent lamp or a light bulb).

  With the method of switching the gain value based on the subject brightness (specifically, the EV value), it is particularly difficult to handle light cloudy. Specifically, in light cloudy (that is, dark external light), it is difficult to switch to the gain value for daytime synchro, so the gain value that depends on the color temperature of the strobe light for low-intensity light emission works. The background that does not reach becomes a reddish color. This is not a problem that can be solved by simply lowering the threshold for discrimination. When the threshold value is lowered, the bright indoor light source is also identified as daytime synchro, and an appropriate gain value does not work, and conversely, the hue becomes bluish.

  The present invention has been made in view of such circumstances, and a white balance adjustment method and a camera capable of preventing the color of an image from switching unnaturally at the boundary between low-intensity light emission and daytime synchronization at the time of flash photography. The purpose is to provide.

  In order to achieve the object, the invention according to claim 1 is a white balance adjustment method for adjusting a white balance of an image signal of each color obtained by irradiating a subject with strobe light and imaging the subject. The subject brightness area is divided into a low brightness light emitting area, a daytime synchro area, and a boundary area between the low brightness light emission area and the daytime synchro area, and the subject brightness before the strobe light emission is acquired, When the acquired subject brightness is in the low brightness region, the white balance of the image signal of each color is adjusted by a first white balance correction value corresponding to strobe light, and the acquired subject brightness is in the daytime sync region. In this case, the white balance of the image signal of each color is adjusted by the second white balance correction value corresponding to daytime synchronization, and the acquired subject brightness is adjusted. Is the boundary region, the white balance of the image signal of each color is adjusted by a third white balance correction value intermediate between the first white balance correction value and the second white balance correction value. It is a feature.

  With this configuration, the white balance of the image signal is adjusted based on the subject brightness with the low brightness light emission, the daytime synchro, and the white balance correction value suitable for each of these boundaries. Moreover, it is possible to prevent the background from becoming a reddish color due to overcorrection by strobe emission under dark external light.

  According to a second aspect of the invention, in the first aspect of the invention, the second white balance correction value is a first white balance correction value set corresponding to a color temperature of the strobe light. The white balance correction value set corresponding to the color temperature of the predetermined ambient light is a value mixed at a predetermined ratio.

  Here, the ambient light includes an outdoor light source and an indoor light source, and a white balance correction value suitable for each photographing environment may be mixed.

  According to a third aspect of the present invention, the third white balance correction value is continuously from the first white balance correction value to the second white balance correction value in accordance with the acquired subject luminance level. It is a characteristic that the value changes.

  For example, a membership function representing the likelihood of daytime synchronization is introduced using the subject brightness as a variable, and continuously based on this function value (an evaluation value representing the likelihood of daytime synchronization) with respect to changes in the subject brightness. Find the changing white balance correction value.

  According to a fourth aspect of the present invention, there is provided a camera capable of adjusting a white balance of an image signal of each color obtained by irradiating a subject with strobe light and picking up the subject. Means for acquiring a subject brightness before strobe emission, and dividing into a light emitting area, a daytime synchro area, and a boundary area between the low brightness light emitting area and the daytime synchro area; When the luminance is in the low luminance area, the white balance of the image signal of each color is adjusted by the first white balance correction value corresponding to the strobe light, and when the acquired subject luminance is in the daytime synchronized area. When the white balance of the image signal of each color is adjusted by the second white balance correction value corresponding to daytime synchronization, and the acquired subject luminance is the boundary region Comprises means for adjusting the white balance of the image signal of each color by a third white balance correction value intermediate between the first white balance correction value and the second white balance correction value. Yes.

  According to the present invention, it is possible to prevent the color of an image from switching unnaturally at the boundary between low-luminance light emission and daytime synchronization during flash photography.

  The best mode for carrying out the present invention will be described below in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram showing a schematic configuration of a camera to which a white balance adjustment method according to the present invention is applied.

  The camera 10 is a digital camera having a still image and moving image shooting function. Further, at the time of shooting, the strobe light 70 can be emitted to irradiate the subject with strobe light.

  The overall operation of the camera 10 is centrally controlled by a central processing unit (CPU) 12. The CPU 12 functions as a control unit that controls the camera 10 according to a predetermined program, and as a calculation unit that performs various calculations such as automatic exposure (AE) calculation, automatic focus adjustment (AF), and white balance (WB) calculation. Function.

  The CPU 12 performs automatic strobe control on the strobe 70 when the strobe 70 is caused to emit light. Specifically, when the strobe 70 emits light and the subject brightness reaches a predetermined brightness, a light emission stop signal is output to the strobe 70 to stop the light emission of the strobe 70. On the other hand, if the subject luminance does not reach a predetermined luminance even when the strobe 70 emits light, a light emission stop signal is not output and the strobe 70 performs full light emission.

  In addition, when shooting with the flash 70 (flash shooting), there are light emission when the subject is entirely dark (low-intensity light emission) and when a part of the subject is darker than the surroundings due to backlight or the like. There is luminescence (daytime synchro). In low-luminance light emission, the subject is photographed mainly by reflection of strobe light. On the other hand, in daytime synchronization, a subject is photographed by reflection of ambient light and strobe light.

  In the white balance calculation, the CPU 12 performs white balance adjustment on image signals of red (R), green (G), and blue (B) colors obtained by imaging with a color imaging device (CCD) 38 described later. Calculate the white balance correction value.

  In the case of flash photography, as shown in FIG. 2, the subject brightness (EV value) indicating the brightness of the subject before the flash emission is within the white balance adjustment range 200, the low brightness light emitting area 201, the daytime synchro area. 202, and a boundary area 203 between the low-luminance light emitting area 201 and the daytime synchro area 202. Then, the CPU 12 determines whether the actual subject brightness before the flash emission is in the low brightness light emitting area 201, the daytime synchro area 202, or the boundary area 203, and the white balance correction corresponding to the determined area. The value is calculated. At the time of white balance calculation at the time of such flash photography, the CPU 12 uses a membership function f representing the synchronicity during the day using the subject brightness (EV value) before flash emission as a variable as shown in FIG. Based on the membership function value SY = f (EV) representing the synchronicity of daytime, a white balance value that continuously changes in response to a change in subject brightness in the boundary region 203 is calculated. The white balance adjustment at the time of flash photography will be described in detail later.

  The ROM 16 connected to the CPU 12 via the bus 14 stores programs executed by the CPU 12 and various fixed data necessary for the operation of the programs, and the EEPROM 17 stores various setting data relating to the operation of the camera 10. Stored. The memory (SDRAM) 18 is used as an area for various operations performed by the CPU 12 and is used as a temporary storage area for image data. The VRAM 20 is a temporary storage memory dedicated to image data when displaying an image and includes an A area 20A and a B area 20B.

  The camera 10 is provided with a mode selection switch 22, a shooting button 24, and other screen operation keys 26 including a menu key, an OK key, a cross key, a cancel key, and the like. Signals from these various operation means (22, 24, 26) are input to the CPU 12, and the CPU 12 controls each part of the camera 10 based on the input signals, and controls lens driving, imaging control, strobe control, and image signal processing. Control such as control, image recording control, and image reproduction control is performed. The mode selection switch 22 is an operation for switching between a photographing mode for photographing a subject and recording image data on a predetermined recording medium 32 and a reproduction mode for reproducing image data recorded on the recording medium 32. Means. The shooting button 24 is an operation means for inputting a shooting preparation instruction and a shooting start instruction, and is composed of a two-stroke switch having an S1 switch that is turned on when half-pressed and an S2 switch that is turned on when fully pressed. ing. The menu key is an operation means for inputting an instruction to display a menu on the screen of the image display device 52. The OK key is an operation means for inputting an instruction to confirm and execute the selected content. The cross key is an operation means for inputting instructions in four directions, up, down, left, and right for selecting items in the menu. The cancel key is an operation means for inputting a cancellation instruction such as selection contents and instruction contents.

  The image display device 28 is composed of a liquid crystal display (LCD) capable of color display. The image display device 28 can be used as an electronic viewfinder for checking the angle of view at the time of shooting, and is used as a means for reproducing and displaying a recorded image. The image display device 28 is also used as a user interface display device, and displays information such as menus, selection items, and setting contents as necessary.

  The media controller 34 performs predetermined signal conversion in order to exchange input / output signals suitable for the recording medium 32 mounted in the media socket 30.

  The USB interface unit 36 is a communication unit for connecting a personal computer or other external device to the camera 10. By connecting the camera 10 and an external device using a USB cable (not shown), it is possible to exchange data with the external device. Of course, the communication method is not limited to USB, and IEE1394 or other communication methods may be applied.

  Next, the shooting function of the camera 10 will be described.

  When the shooting mode is selected by the mode selection switch 22, power is supplied to the shooting-related portion (shooting unit) including the CCD 38, and shooting is ready.

  The lens unit 40 is an optical unit including a photographic lens 42 including a focus lens and a diaphragm shutter mechanical shutter 44. The lens unit 40 is electrically driven by a lens driving unit 46 and a diaphragm driving unit 48 controlled by the CPU 12 to perform zoom control, focus control, and iris control. The light that has passed through the lens unit 40 is imaged on the light receiving surface of the CCD 38. A large number of photodiodes (light receiving elements) are two-dimensionally arranged on the light receiving surface of the CCD 38, and primary color filters of red (R), green (G), and blue (B) are provided for each photodiode. They are arranged in a predetermined arrangement structure (Bayer, G stripe, etc.). The CCD 38 has a so-called electronic shutter function for controlling the charge accumulation time (shutter speed). The CPU 12 controls the charge accumulation time in the CCD 38 via the timing generator 50. The subject image formed on the light receiving surface of the CCD 38 is converted into a signal charge of an amount corresponding to the amount of incident light by each photodiode. The signal charges accumulated in the respective photodiodes are sequentially read out as voltage signals (R, G, and B color image signals) corresponding to the signal charges based on the drive pulse given from the timing generator 50 in accordance with instructions from the CPU 12.

  The R, G, B color image signals extracted from the CCD 38 are sent to an analog processing unit (CDS / AMP circuit) 52, where they are sampled and held (correlated double sampling processing) and amplified for each pixel. The A / D converter 54 converts the analog signal into a digital signal. The R, G, and B image signals output from the A / D converter 54 are temporarily stored in the memory 18 via the image input controller 56. The image signal processing circuit 58 processes the R, G, B image signals stored in the memory 18 in accordance with instructions from the CPU 12. Specifically, the image signal processing circuit 58 includes a synchronization circuit (a circuit that corrects a spatial shift associated with the color filter array of the CCD 38 and converts the color signal into a simultaneous expression), a white balance correction circuit, It functions as an image signal processing means including a gamma correction circuit, a luminance signal Y and a color difference signal generation circuit, etc., and performs predetermined signal processing using the memory 18 in accordance with commands from the CPU 12. The image data obtained by the processing of the image signal processing circuit 58 is stored in the VRAM 20.

  As for image output to the image display device 28, image data is read from the VRAM 20 and sent to the video encoder 60 via the bus 14. The video encoder 60 converts the input image data into a signal of a predetermined display system (for example, NTSC system) and outputs it to the image display device 28. Specifically, the image signal output from the CCD 38 is alternately rewritten into the A area 20A and the B area 20B of the VRAM 20 as image data for one frame. Of these A area 22A and B area 22B, image data is read from an area other than the area where the image data is rewritten. Then, by periodically rewriting the image data in the VRAM 20, the photographer can check the shooting angle of view by the video (through movie image) displayed on the image display device 28.

  When the shooting button 24 is half-pressed by the photographer who has confirmed the shooting angle of view and S1 is turned on, the CPU 12 starts AE and AF processing as shooting preparation processing. The R, G, and B image signals output from the CCD 38 are input to the AF detection circuit 62 and the AE / AWB detection circuit 64 via the CDS / AMP circuit 52, the A / D converter 54, and the image input controller 56. The

  The AE / AWB detection circuit 64 includes a circuit that divides one screen into a plurality of areas (for example, 16 × 16) and integrates image signals of R, G, and B colors for each divided area. provide. The CPU 12 obtains subject brightness (EV value) indicating the brightness of the subject based on the integrated value obtained from the AE / AWB detection circuit 64. The obtained EV value is stored in the memory 18. The CPU 12 determines the aperture value and shutter speed according to the EV value stored in the memory 18 and a predetermined program diagram, and controls the electronic shutter of the CCD 38 and the iris of the lens unit 40 according to these values to obtain an appropriate exposure amount. . Further, as will be described later, the CPU 12 performs white balance calculation based on the subject brightness before the strobe emission at the start of shooting (when the shooting button 24 is fully pressed).

  The AF control in the camera 10 is, for example, a contrast AF that moves a focusing lens (a moving lens that contributes to focus adjustment among the lens optical systems constituting the photographing lens 42) so that the high-frequency component of the G signal of the video signal is maximized. Applies. That is, the AF detection circuit 62 cuts out a signal in a focus-to-image area preset in a high-pass filter that passes only a high-frequency component of the G signal, an absolute value processing unit, and a screen (for example, the center of the screen). An area extraction unit and an integration unit that integrates absolute value data in the AF area are configured. The integrated value data obtained by the AF detection circuit 62 is notified to the CPU 12. The CPU 12 calculates a focus evaluation value (AF evaluation value) at a plurality of AF detection points while moving the focusing lens by controlling the lens driving unit 46, and determines a lens position where the evaluation value is a maximum as a focus position. To do. Then, the lens driving unit 46 is controlled so as to move the focusing lens to the obtained in-focus position. The calculation of the AF evaluation value is not limited to a mode using the G signal, and a luminance signal (Y signal) may be used.

  After shooting preparation processing such as AE processing and AF processing is performed by half-pressing the shooting button 24 (S1 on), when the shooting button 24 is fully pressed (S2 on), a shooting operation for recording an image is performed. Start.

  The AE / AWB detection circuit 64 outputs the R, G, and B colors output from the CCD 38 and input to the AE / AWB detection circuit 64 via the CDS / AMP circuit 52, the A / D converter 54, and the image input controller 56. Information necessary for white balance adjustment is detected from the image signal. Specifically, one screen is divided into a plurality of areas (for example, 16 × 16), the RGB color image signals are integrated for each divided area, and the integrated value is provided to the CPU 12. The CPU 12 obtains the integrated value of R, the integrated value of B, and the integrated value of G, and for each divided area, the ratio R / G of the integrated value of R and the integrated value of G, and the integrated value of B and G The ratio B / G of the integrated values is obtained. In other words, the integrated value for each divided area is converted from a color space of (R, G, B) coordinates to a color space of (R / G, B / G) coordinates.

  When the strobe 70 is not emitting light, the CPU 12 determines the light source type based on the distribution state of the integrated value of each divided area in the color space of (R / G, B / G) coordinates, and is suitable for the determined light source type. According to the white balance correction value, for example, the value of each ratio (ratio R / G, ratio B / G) is approximately 1 (that is, the RGB integration ratio R: G: B is approximately 1: 1: 1 in one screen). ), The gain value (white balance correction value) for the R, G, and B image signals in the white balance adjustment circuit in the image signal processing circuit 58 is controlled. Further, when the gain value in the white balance adjustment circuit is adjusted so that the value of each ratio (ratio R / G, ratio B / G) is a value other than 1, an image signal in which a certain color remains is generated. Can do.

  When the strobe 70 emits light, the CPU 12 performs white balance adjustment processing shown in FIG.

  First, the subject brightness (EV value) before the flash 70 emits light is acquired (S10). Specifically, the EV value obtained when the shooting button 24 is half-pressed is read from the memory 18.

  It is determined whether or not the acquired subject luminance (EV value) before the flash emission is smaller than a predetermined first threshold value (S12). Here, the first threshold value is a threshold value (for example, “9”) for determining whether or not the low-luminance light emitting region 201 shown in FIGS. When it is smaller than the first threshold value, that is, when it is regarded as low-luminance shooting by the strobe light 70, an auto white balance calculation for calculating a gain depending on the color temperature of the strobe light is performed (S14). The corrected white balance correction value is output to the image signal processing circuit 58 (S16). Here, when the strobe 70 emits full light, the strobe light may not reach the subject or the amount of irradiation to the subject may not be sufficient. Therefore, the light source type of ambient light is determined based on the information obtained from the AE / AWB detection circuit 64 (integrated value for each divided area), the white balance correction value is calculated based on the determined light source type, and the image signal Output to the processing circuit 58. When the strobe light is sufficiently irradiated to the subject, specifically, when the current subject brightness (EV value) at the time of flash emission reaches a predetermined brightness, a predetermined white balance that depends on the color temperature of the strobe light A correction value (referred to as “strobe WB gain”) is selected and output to the image signal processing circuit 58.

  Further, it is determined whether or not the subject brightness (EV value) before the strobe light emission is larger than a predetermined second threshold value (S18). Here, the second threshold value is a threshold value (for example, “11”) for determining whether or not it is the daytime synchro region 202 shown in FIGS. 2 and 3. When the value is larger than the second threshold value, that is, when it is considered that daytime synchro photography is performed by the flash 70, a predetermined white balance correction value (referred to as “daytime synchro WB gain”) depending on the color temperature of daytime synchro. Is output to the image signal processing circuit 58 (S20). Here, the daytime synchro WB gain is, for example, a predetermined white balance correction value for non-flashing (referred to as “sunny WB gain”) corresponding to the color temperature of the daylight light source in fine weather, and a strobe WB gain. Are used at a predetermined ratio (for example, 7: 3).

When the subject luminance (EV value) before the strobe light emission is between the first threshold value and the second threshold value, that is, in the boundary region 203 between the low luminance light emitting region 201 and the daytime synchro region 202. In some cases, an intermediate white balance correction value (referred to as “boundary WB gain”) between the strobe WB gain and the daytime synchro WB gain is calculated (S22). Specifically, as shown in FIG. 3, a membership function f representing the daytime synchronicity is used by using a membership function f representing the daytime synchronicity using the subject luminance (EV value) before the flash emission as a variable. A white balance correction value is calculated based on the value SY = f (EV). This membership function f is stored in advance in the ROM 16. The membership function f shown in FIG. 3 is assumed to be daytime synchromatic as the function value SY is low, and the case where it can be completely regarded as daytime synchro is “0.3”, and it is completely regarded as low-luminance light emission. The case where this is possible is set to “1”, and the calculation of the boundary WB gain is simplified. In this case, as shown in [Formula 1], the boundary WB gain (Glw, Rlw, Blw) is calculated based on the strobe WB gain (Gst, Rst, Bst) and the clear WB gain (Gd, Rd, Bd). Is done.
[Formula 1]
Glw = Gst
Rlw = (Rst-Rd) × SY + Rd
Blw = (Bst-Bd) × SY + Bd
Here, Glw, Gst, and Gd are gains applied to the G signal. Rlw, Rst, and Rd are gains applied to the R signal. Blw, Bst, and Bd are gains applied to the B signal. The membership function value SY changes smoothly and continuously with respect to the change in the subject brightness EV, and the boundary WB gain (Glw, Rlw, Blw) also changes with the change in the subject brightness EV according to the membership function value SY. Will change smoothly and continuously.

  Further, auto white balance calculation is performed (S24). Here, the light source type of ambient light is determined based on the information obtained from the AE / AWB detection circuit 64 (integrated value for each divided area), and a white balance correction value is calculated based on the determined light source type. For example, when the ambient light is an indoor light source, the boundary WB gain calculated based on the clear WB gain may not be appropriate. Therefore, when it is determined that the ambient light is an indoor light source, a white balance correction value obtained by auto white balance calculation based on the light source type is selected. That is, the white balance correction value based on the MIX light ratio of the light source color is selected. Further, when the light source type is tungsten or the like, it may be determined that it is not daytime synchronization, and the membership function value SY may not be used. The selected white balance correction value (WB gain) is output to the image signal processing circuit 58 (S26).

  The image signal processing circuit 58 performs white balance adjustment by multiplying the R, G, B color white balance correction values (gains) output from the CPU 12 by the R, G, B color image signals. The image signal subjected to the white balance adjustment is subjected to image processing such as gamma correction by the image signal processing circuit 58, converted into a YC signal (luminance signal Y and color difference signal C), and compressed by the compression / decompression circuit 66. After that, it is recorded as image data on the recording medium 32 via the media controller 34 and the media socket 30.

  When the playback mode is selected by the mode selection switch 22, the compressed image data recorded on the recording medium 32 is read out via the media controller 34. The read image data is decompressed by the compression / decompression circuit 66 and displayed on the image display device 28 via the VRAM 20 and the video encoder 60. The user can reproduce and enjoy an image that has been subjected to appropriate white balance adjustment.

  In addition, although the case where the value which mixed the fine WB gain and the strobe WB gain was used as the white balance correction value (daytime synchronization WB gain) for daytime synchronization was described as an example, the present invention is not limited to this. is not. For example, the light source type of ambient light may be determined, and a value depending on the color temperature of the determined light source type may be mixed with the strobe WB gain. The white balance correction value may be calculated indoors using a membership function.

  Further, the use of the membership function may be restricted depending on the light source type. In the case of tungsten, it is possible to always set SY = 1 because it is clearly not daytime synchronized photography.

  The use of the membership function is not limited to the case where the function value is obtained by calculation, but the function value is obtained by referring to the table information indicating the relationship between the subject brightness and the function value stored in advance in the ROM 16 or the like. Also good.

  Further, the present invention includes a case where the white balance correction value is obtained directly from the subject luminance without using the membership function.

  Further, although the case where the white balance correction value continuously changes with respect to the change in the subject brightness before the strobe emission in the boundary region has been described as an example, the present invention is not limited to this. The present invention includes a case where the white balance correction value changes stepwise with respect to a change in subject brightness before flash emission in the boundary region.

1 is a block diagram showing a schematic configuration of a camera to which a white balance adjustment method according to the present invention is applied. Explanatory drawing used to explain low-luminance light emitting area, daytime synchro area, and border area Explanatory drawing used to explain the membership function introduced to smoothly switch the white balance correction value in the boundary region The flowchart which shows the outline of an example of the white balance adjustment method which concerns on this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Camera, 12 ... CPU, 16 ... ROM, 17 ... EEPROM, 18 ... Memory (RAM), 24 ... Shooting button, 32 ... Recording medium, 34 ... Media controller, 38 ... Color image sensor (CCD), 40 ... Lens Unit: 42 ... Shooting lens, 58 ... Image signal processing circuit, 70 ... Strobe, 201 ... Low-luminance light emitting area, 202 ... Daytime synchro area, 203 ... Border area

Claims (4)

In a white balance adjustment method for adjusting the white balance of each color image signal obtained by irradiating a subject with strobe light and imaging the subject,
The area of the subject brightness is divided into a low brightness light emitting area, a daytime synchro area, and a boundary area between the low brightness light emitting area and the daytime synchro area,
Obtain the subject brightness before the flash fires,
When the acquired subject luminance is the low luminance region, the white balance of the image signal of each color is adjusted by a first white balance correction value corresponding to strobe light,
When the acquired subject brightness is the daytime sync region, the white balance of the image signal of each color is adjusted by a second white balance correction value corresponding to daytime synchro;
When the acquired subject luminance is the boundary region, the white balance of the image signal of each color is determined by a third white balance correction value intermediate between the first white balance correction value and the second white balance correction value. Adjust the
The white balance adjustment method characterized by the above-mentioned.   The second white balance correction value includes a first white balance correction value set corresponding to the color temperature of the strobe light and a white balance correction value set corresponding to the color temperature of a predetermined ambient light. 2. The white balance adjustment method according to claim 1, wherein the white balance is a value mixed at a predetermined ratio.   The third white balance correction value is a value that continuously changes from the first white balance correction value to the second white balance correction value according to the level of the acquired subject luminance. The white balance adjustment method according to claim 1 or 2. In a camera capable of adjusting the white balance of each color image signal obtained by illuminating a subject with flash light and imaging the subject,
The subject brightness area is divided into a low brightness light emission area, a daytime synchro area, and a boundary area between the low brightness light emission area and the daytime synchro area, and the subject brightness before strobe emission is acquired. Means to
When the acquired subject brightness is in the low brightness region, the white balance of the image signal of each color is adjusted by a first white balance correction value corresponding to strobe light, and the acquired subject brightness is in the daytime sync region. In this case, the white balance of the image signal of each color is adjusted by a second white balance correction value corresponding to daytime synchronization, and the first white balance correction is performed when the acquired subject luminance is the boundary region. Means for adjusting the white balance of the image signal of each color by a third white balance correction value intermediate between the value and the second white balance correction value;
A camera characterized by comprising

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