JP2935116B2 - Imaging device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to an imaging device such as an electronic camera or a VTR camera, and more particularly, to a scene having a large luminance difference or a main subject having a high luminance difference in a luminance distribution. The present invention relates to a circuit technique for enabling good imaging to be performed even when the image is unevenly distributed on one of a luminance side and a low luminance side.

[Conventional technology]

2. Description of the Related Art In recent years, video equipment including an image pickup apparatus has become widespread, and the general public has taken a picture using an electronic camera or a VTR camera.

However, ordinary users usually do not have a special photographing technique, and photographing conditions vary widely.

In consideration of such a background, it is desirable that the imaging device be designed so that a general user can always take good images.

For example, when photographing a person who is the main subject, if the sun is in the background, the person becomes dark due to backlight, and the luminance difference between the person who is the main subject and the background becomes large.

On the other hand, if the person who is the main subject is unevenly distributed on the high luminance side in the luminance distribution, or conversely, is unevenly distributed on the low luminance side, the white or black portion collapses, and the shooting scene is reduced. Cannot be faithfully reproduced.

Therefore, conventionally, an exposure correction circuit is provided in an image pickup apparatus, and exposure correction is performed in accordance with the main subject even under the above-described shooting conditions, that is, exposure correction is performed on the plus side in backlight and on the negative side in highlight. , So that the photographing can be performed with an appropriate exposure value.

[Problems to be solved by the invention]

However, although the conventional imaging apparatus includes an exposure correction circuit, the exposure correction amount is fixed.

For this reason, in the case of a scene having a large luminance difference as described above, and in the case where the main subject is biased in the luminance distribution as described above, if the main subject is set to an appropriate exposure value by exposure correction, the entire screen becomes bright. As a result, a bright portion becomes entirely white, that is, a so-called broken state, whereas a dark portion as a whole becomes black, that is, a so-called crushed state.

In addition, since the gamma correction amount cannot be changed by automatically determining the shooting state in the image pickup apparatus, the above-described shooting error may be made at a photo opportunity at an angle.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an image pickup apparatus capable of performing good photographing by automatically determining a photographing state and performing gamma correction.

[Means for solving the problem]

In order to achieve such an object, the present invention divides a shooting screen into a plurality of areas, performs a comparison operation of a photometric signal or an image signal corresponding to the luminance of the center part and the peripheral part of the screen, and makes the central part a peripheral part. On the other hand, when it is determined that the image is darker than a certain value, the backlight is determined.When the image is brighter than a certain value, it is determined that the image is a highlight. A luminance detection unit that detects the maximum luminance value or the minimum luminance value, calculates an average luminance value from these luminance values and outputs an output signal, a gamma correction circuit unit including a plurality of selectable gamma correction curves,
Control means for selecting, from the plurality of gamma correction curves, a gamma curve for performing correction corresponding to the degree of backlight or highlight based on the output signals of the photographing situation determination unit and the luminance detection unit. It is assumed that.

[Action]

In the present invention having such a configuration, the gamma correction curve for setting the optimum exposure value is automatically selected, so that even if the luminance difference between the main subject and its background is large, a good gamma correction curve is always obtained. Shooting can be performed.

In addition, since the selection of the gamma correction curve is automatically performed, excellent effects such as improvement in usability of the imaging device are achieved.

〔Example〕

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

FIG. 1 is a circuit diagram showing an example of an electronic still camera to which the present invention is applied, FIG. 2 is an explanatory diagram showing a shooting situation of a subject, FIG. 3 is an explanatory diagram showing multi-division of a subject,
FIG. 4 is an explanatory diagram showing a shooting situation of a subject and division of shooting data, and FIG. 5 is a characteristic diagram showing a gamma correction curve and selection of gamma correction at the time of reproduction.

First, the image pickup optical system 1 will be described. The image pickup optical system 1 includes an autofocus lens 2, an aperture mechanism 3, a condenser lens 4, a half mirror 5, and an optical low-pass filter 6, and the optical system 1 is an image sensor 7 such as a solid-state image sensor. A subject image is optically formed on the top.

An optical color filter (not shown) is provided on the front surface of the image sensor 7, that is, on the image forming surface of the subject image.

The photometric optical system 11 includes lenses 12 and 13, and a half mirror 14, monitors an image being photographed, and forms a subject image serving as a photometric signal on the light receiving element 15.

Next, the signal processing system will be described. The image sensor 7 generates a color image signal Va corresponding to the brightness and color of the subject image by a photoelectric conversion effect. The image signal Va has a very small voltage level, and is amplified by the preamplifier 21 to a desired voltage level.

The color separation circuit 22 obtains R (red), G (roku), and B (blue) color signals, and supplies the color signals R, G, and B to the gamma correction circuit unit 23 and the luminance detection system 51. Is done. Then, a gamma correction curve is selected based on an output signal obtained from the luminance detection system 51 and a shooting situation determination unit including the AE photometry unit 16, which will be described later.
Details will be described later with reference to FIGS.

The matrix circuit 24 includes a gamma-corrected color signal R,
Predetermined signal processing is performed on G and B to obtain a luminance signal Y and color difference signals RY and BY.

In addition, the color encoder 25 outputs a total color video signal (NTSC color video signal), Y + S, RY,
Various signals such as BY are obtained. These signals are supplied to a display device or a recording circuit (not shown) and are provided for a desired purpose.

Next, the photometric system will be described.
The subject image reflected by the camera is projected on the half mirror 14 via the lens 12, and the subject image can be viewed as a monitor image from the finder F via the lens 13. Further, the brightness of the subject image transmitted through the half mirror 14 is detected by the light receiving element 15 which forms a part of the photographing situation determination unit. The light receiving element 15 is constituted by a photoelectric conversion element having excellent photoelectric conversion efficiency, such as a silicon photocell.

In this embodiment, not only the photometric signal corresponding to the brightness of the subject image is obtained from the light receiving element 15, but also the light receiving element 15 is configured to divide the subject image into multiple segments and perform photoelectric conversion.

That is, the light receiving element 15 is configured to divide a subject of one frame into a plurality of areas, and individually output photometric signals corresponding to the luminance of each area.

Here, an example of a subject with abnormal light will be described with reference to FIG. 2. Since the sun B is located behind the person A, which is the main subject, not only the face of the person A but also the whole person is shaded and dark, It is a typical backlight state.

On the other hand, as shown in FIG. 3, for the light receiving element 15, for example, areas P1, P2, P3, and P4 divided into four parts are configured as follows.
When photographing the subject shown in FIG. 2, the person A, which is the main subject, is located in the area P1 and the bright background portion is located in the areas P2, P3, and P4 as indicated by the dotted line.

In such a backlight state, the person A in the area P1,
In other words, the brightness of the main subject is at a low level, and the brightness of the areas P2 to P4 divided and formed around the main subject is at a high level.

Therefore, a photometric signal Vb corresponding to the luminance of each area is obtained from each of the areas P1 to P4 of the light receiving element 15. The obtained photometric signal Vb is supplied to the AE photometric section 16, and the AE photometric section 16 performs a remarkable operation as described below.

That is, the photometric signals Vb obtained from the respective areas P1 to P4 are set to Vb1, Vb2, Vb3, and Vb4, respectively. And for example Vb4 / Vb1
The value k is obtained by the calculation of and backlight or highlight is determined. This determination is automatically performed when the photometric signals Vb1 to Vb4 of each area are input.

The value k is obtained by the above calculation, and the value k is k> k 'with respect to a value k' determined in advance to determine backlight.
At this time, it is determined that the subject is backlight. Similarly, if k <k ″ with respect to a value k ″ determined to determine a highlight, the highlight is determined. In the determination, the calculation may be logarithmic calculation, and may be performed by logVb4−logVb1.

When it is determined that the image is backlight or highlight, an output signal Vc for controlling the amount of gamma correction is supplied to the camera control circuit 31, and the camera control circuit 31 drives the gamma correction circuit 23 to output a gamma corresponding to the output signal Vc. Select a correction curve. The operation of the photometric unit may be configured to perform a variety of operations other than those described above, and other examples of the operation will be described later.

In the present embodiment, in order to improve the accuracy of gamma correction, a luminance detection system 51 is provided in addition to the AE photometer, and is configured to superimpose on the control signal Vc to perform selection control of a gamma correction curve.

That is, the R, G, and B color signals are output from the color separation circuit 22, and each of the color signals includes a luminance component.
Then, the color signals R, G, and B containing the luminance component are supplied to the luminance detection unit 55 via the buffer circuits 52, 53, and 54 constituting the luminance detection system 51.

The luminance detecting unit 55 divides the image signal of one frame into a plurality of areas as shown by a dashed line in FIG. 4, and detects a maximum luminance value and a minimum luminance value from the image signal of each divided area. The average luminance value is calculated, and a second output signal Ve based on the calculation result is supplied to the camera control unit 31.

Here, taking the subject shown in FIGS. 2 and 4 as an example, when the maximum value of the luminance is the area of the sun B and the minimum value of the luminance is the area of the person A as the main subject, the output signal The gamma correction circuit 23 is driven by Vc to select an optimal gamma correction curve. However, in this embodiment, although the person who is the main subject happens to have the same minimum luminance value, the minimum luminance value is not always located at the center. In such a case, an average value is calculated based on the maximum luminance value and the minimum luminance value in the luminance detecting section 55, and the gamma correction circuit section 23 is driven by the second output signal Ve to select an optimal gamma curve.

As a result, the camera control unit 31 is supplied with the two types of control signals Vc and Ve obtained as described above, and drives the gamma correction circuit unit 23 based on the information to select an optimal gamma curve. The characteristics of the various gamma correction curves can be created in a software or hardware manner in the same manner as the characteristic of γ = 0.45 conventionally created in the gamma correction circuit section.

The camera control circuit 31 is constituted by a microprocessor, and the gamma correction circuit unit 23 is provided in accordance with the output signals Vc and Ve.
In the gamma correction circuit section 23, a number of gamma correction curves are formed as shown in FIG.

That is, the normal gamma correction circuit uses γ = 0.
The gamma correction is performed by the 45 gamma correction curves. In this embodiment, in addition to c, a, b, d,
A number of gamma correction curves are formed as shown as e,
These gamma correction curves are automatically selected by the camera control unit 31.

The selection of the gamma correction curve is performed simultaneously based on the output signals Vc and Ve. First, the selection operation based on the output signal Vc and then the selection operation based on the output signal Ve will be described.

If the AE metering unit 16 determines that the light is backlit by the above calculation, the AE metering unit supplies the output signal Vc to the camera control unit. To the unit 31. In other words, it is determined that the subject is backlit when the value k> k 'of the calculation, and a gamma correction curve of d or e is selected according to the size of the value k. The camera controller outputs the output signal Vc that selects this gamma correction curve.
, The gamma-corrected R, G, and B color signals are obtained and supplied to the matrix circuit 24 at the next stage.

It is also assumed that the AE metering section 16 determines the highlight as a result of the calculation and supplies an output signal Vc corresponding to the degree of highlight to the camera control section 31.

The camera controller 31 drives the gamma correction circuit 23 based on the output signal Vc, and selects one of the gamma correction curves a and b according to the degree of highlight.

As a result, gamma-corrected R, G, and B color signals are obtained from the gamma correction circuit 23, and the matrix circuit 24 at the next stage is obtained.
Supplied to

By performing the above-described gamma correction, a portion that has been darkened by backlight is corrected to be bright, and in the example of FIG. 2, the brightness of the person A becomes appropriate and the image becomes very easy to see. Conversely, when the person A is highlighted, the portion is corrected in the dark direction, so that the brightness becomes appropriate and the image becomes easy to see.

Next, an operation of selecting a gamma correction curve based on the output signal Ve will be described.

That is, in a scene such as a backlight, in the case of normal (γ curve of 0.45 in FIG. 5A), the main subject becomes dark when average photometry is performed. I do. For this reason, the image is not reproduced in a state where the high luminance side is cut off as shown by oblique lines in the lower part of FIG. 5 (A). Now, assuming that an appropriate output level of a main subject such as a person's face is 70%, in the case of the above-described backlight, the main subject is on the dark side like d 'and e' in FIG. By selecting the gamma curve of d and e to expand the side and compress the high luminance side, it can be said that the dark main subject is reproduced at an appropriate output level and the high luminance side is compressed but not reproduced. Disappears. Therefore, the output signal Ve selects the gamma correction curves d and e according to the intensity of the backlight, and the range from the minimum luminance Min to the maximum luminance Max including the main subject (the x mark) falls within the latitude as indicated by the arrow. Tobi on the high luminance side is eliminated.

Therefore, the gamma correction curve is selected with high accuracy based on the output signals Vc and Ve, and it is possible to eliminate the above-mentioned torsion on the high luminance side and to maintain a good luminance balance of the captured image.

FIG. 5 shows a gamma correction curve at the time of reproduction. A gamma curve can be arbitrarily selected at the time of reproduction.
Although the detailed explanation is omitted because it is different from the object, when the exposure correction is performed, the entire characteristic is shifted up as shown by c ', so that the overexposure and underexposure occur as described above.

Next, another control system related to the camera control unit 31 will be described. The camera control unit 31 has a central function of the camera, and is configured to perform various kinds of control in addition to the control of the gamma correction. The lens 32, the light receiving element 33, and the AF distance measurement unit 34 obtain distance data for auto focus, and supply data Vd obtained by measuring the distance to the main subject to the camera control unit 31. Camera control unit
The 31 supplies a control signal for focusing to the lens driving unit 35 based on the distance data Vd. Lens Drive County 35
In response to the control signal, the lens drive mechanism 36 composed of a pinion, a rack, and the like is driven to move the lens 2 in the left and right directions in FIG. 1 to perform automatic focus adjustment.

Further, the camera control unit 31 drives the aperture driving unit 37 to control the aperture mechanism 3 and performs optimal exposure control.

Further, the camera control section 31 drives the CCD drive section 38 to control the image sensor 7 to perform various operations such as performing a predetermined photographing operation.

Although one embodiment of the present invention has been described above, the present invention is not limited to the above, and various modifications can be made.

For example, the calculation in the AE metering section 16 is
Vb1 + Vb2-Vb4, Vb1 + Vb2 + Vb3-Vb4, Vb2-Vb4, V
An arrangement may be made wherein the output signal Vc is obtained after discrimination based on the operation value obtained by performing b3−Vb4.

The output signal Ve may be a signal obtained by calculating an average value from luminance values of all blocks in one scene.

In the above embodiment, the average luminance value is obtained for the color signals R, G, and B obtained from the color separation circuit 22, and the photographing state is determined from the photometric signal Vb. However, the present invention is not limited to this configuration. Absent.

That is, the photometric signal Vb may be divided into multiple parts as shown in FIG. 4 and the average luminance value may be detected, while the photographing state may be determined for the color signals R, G and B.

 In this case, the same effect as described above can be obtained.

Further, one or both of the output signals Vc and Ve may be directly supplied to the gamma correction circuit section 23 to select a gamma correction curve.

Further, the aperture mechanism 3 may be configured to control based on the output signals Vc and Ve.

It should be noted that the following modifications are possible for forming the gamma correction curve.

That is, when the gamma correction curve is changed as described above, it is necessary to balance the gamma correction curve. When a large number of gamma correction curves are provided, the circuit load of the camera becomes large. However, even if the number of gamma correction curves is reduced, it may be desired to reduce the circuit load.

And there are many situations where gamma correction is required during backlighting,
There are few highlight situations, so only when backlighting
The output signal Vc may be obtained from the AE metering unit 16 to perform the gamma correction. Even with such a configuration, even when the main subject is on the high-luminance side, that is, in the position corresponding to the area P4 or the like according to the above-described embodiment, it is possible to enter the latitude and perform good shooting. Although the above-described embodiment shows an example of application to an analog electronic still camera, the present invention can be used over a wide range.

That is, it can be used for gamma correction of a digital electronic still camera.

Some digital electronic still cameras are configured to apply gamma correction or the like to an image signal obtained from an image sensor in a preprocessing circuit, and are suitable for this preprocessing circuit.

Furthermore, it goes without saying that the present invention can be used for a gamma correction circuit of a VTR camera known as a video movie.

〔The invention's effect〕

As described above, according to the imaging apparatus of the present invention, the shooting screen or a screen equivalent to the shooting screen is divided into a plurality of areas, and the side light signal or the image signal corresponding to the luminance at the center and the periphery of the screen is divided. A comparison operation is performed to determine backlight or highlight, an average luminance value is calculated from the image signal or the photometric signal, and a plurality of gammas formed in a gamma correction circuit unit based on the determination result and the average luminance value are calculated. The optimal gamma correction curve is automatically selected from the correction curves.

Therefore, even when photographing under abnormal light such as backlight or highlight, photographing errors such as the main subject becoming dark or becoming too bright are reduced, and the gradation of the entire screen of one frame is reduced. This makes it possible to perform shooting with good image quality.

Further, since the selection of the gamma correction curve is performed completely automatically, the usability of the imaging device is improved, and the added value of the imaging device is further improved.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an electronic still camera showing one embodiment of the present invention, FIG. 2 is an explanatory diagram showing a shooting situation of a subject, FIG. 3 is an explanatory diagram showing multi-division of a subject, and FIG. FIG. 5 shows a gamma correction curve and a gamma characteristic diagram at the time of reproduction. Reference numeral 1 in the figure 1 shooting optical system 7 image sensor 15 light receiving element 16 AE metering group 22 color separation circuit 23 gamma correction circuit 24 matrix circuit 25 encoder 31 camera control Unit 51: Luminance detection system 52 to 54 Buffer amplifier 55: Luminance detection unit Va: Color video signal Vb: Photometric signal Vc, Ve: Output signal P1 to P4: Predetermined area a -E: Gamma correction curve A: Main subject

Claims (1)

(57) [Claims] 1. A photographing screen is divided into a plurality of areas, and a photometric signal or an image signal corresponding to the luminance of a central portion and a peripheral portion of the screen is compared to determine that the central portion is darker than a peripheral portion by a certain value. When the backlight, when the brightness is more than a certain value is determined to be a highlight, the backlight, the shooting situation determination unit that outputs an output signal indicating the degree of highlight, the maximum luminance value or the minimum luminance value from the image signal or the photometric signal A luminance detection unit that detects and calculates an average luminance value from these luminance values and outputs an output signal; a gamma correction circuit unit having a plurality of selectable gamma correction curves; and a photographing state determination unit and the luminance detection unit. Control means for selecting, from the plurality of gamma correction curves, a gamma curve for performing correction corresponding to the degree of backlight or highlight based on an output signal.