JPH02268081A - Exposure controller - Google Patents

Abstract

PURPOSE:To attain reproduction of proper brightness level of a face and stable exposure control by applying exposure control in response to the proper photometric value based on scene discrimination through the comparison with an initial threshold level and revising the threshold level at the discrimination of the succeeding scene based on the result of the discrimination of the preceding scene. CONSTITUTION:A brightness data from a CCD 6 of a video movie camera 2 is formed into a brightness data of a photometric region split into plural numbers by a pattern split circuit 17 of a CPU 8 via an A/D converter circuit 15 and a clip circuit 16 and the mean brightness of each region is compared with the initial threshold level of a threshold level setting section 21 by a photometric value arithmetic circuit 20 to discriminate the scene and the photometric value in response tc the scene is detected and proper exposure control is applied. At the discrimination of the succeeding scene, the threshold level of the setting section 21 is revised based on the preceding scene to prevent the revision of the undesired scene. Through the constitution above, the proper brightness of a face is reproduced without being affected by clothing or the like and when the brightness of an object is fluctuated minutely, the photometric value is made stable and stable exposure control is applied.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an exposure control method, and more particularly to an exposure control method for a video camera in which a solid-state imaging device is also used for photometry of subject brightness.

[Conventional technology]

As an exposure control method employed in a video camera, for example, the method described in Japanese Patent Application Laid-Open No. 62-203141 is known. When it is determined that the size of the main subject corresponds to the center photometry area based on the distance to the main subject such as a person and the focal length information of the photographing lens, the image obtained by each output from the center photometry area and the adjacent photometry area is calculated. The exposure compensation amount is determined from the relationship between the brightness of the main subject and the background, and the exposure value is determined by adding the above exposure compensation amount to the center-weighted average photometric value determined by each output from the entire photometry area of the photometer. It has been devised so that appropriate exposure can be obtained even in various scenes.

[Problems to be Solved by the Invention] However, in the above exposure control method, if the person is wearing clothes that have a high (for example, white) or low (for example, black) reflectance on the face, the appropriate exposure may not be achieved. The disadvantage is that it cannot be obtained. In addition, when determining the amount of exposure compensation, the scene is determined by comparing the brightness difference between each metering area with a predetermined threshold. In changing circumstances,
The amount of exposure compensation also changes frequently, which may result in unnatural exposure control.

[Purpose of the invention]

The present invention was made to solve the above-mentioned problems, and it is possible to reproduce the brightness of the face at an appropriate level regardless of the reflectance level of the clothes worn by the main subject (for example, a person). Another object of the present invention is to provide an exposure control method that allows natural and stable exposure control.

[Means for Solving the Problem] In order to achieve the above object, the exposure control method of the present invention includes the following:
The photometric area of the solid-state imaging device is divided into multiple areas, and the difference in average brightness value of each area is compared with an initial threshold to determine the scene.Based on this scene, the photometric value is calculated and exposure is adjusted. At the time of next scene discrimination, the initial threshold value is changed based on the previously discriminated scene.

[Effect]

According to the above means, first, the average brightness value of each photometric area of the solid-state imaging device is calculated and compared with the initial threshold value. As a result, the brightness distribution of the subject is detected and scene discrimination is performed. A photometric value is calculated according to the selected scene, and exposure control is performed based on this photometric value. Then, in the next scene determination, the initial threshold value is changed based on the previously determined scene so that the same scene is likely to be selected. As a result, even if the subject brightness slightly fluctuates, the scene selection is not changed unnecessarily, and stable exposure control is performed.

Embodiments of the present invention will be described in detail below with reference to the drawings.

[Example] In FIG. 1 showing a block diagram of a video movie camera 2 employing the exposure control method of the present invention, a diaphragm 5 is arranged behind the photographing lens 3, and further behind the diaphragm 5,
A CCD 6, which is a solid-state imaging device, is arranged. This C
A timing generator 7 that generates clock pulses is connected to the CD 6, and a CPU that performs overall control.
It is synchronized with 8. The CCD 6 also receives the read image signal at a predetermined sampling frequency (for example, 3
．． 58M1(z) is connected to an amplifier 10 that samples and holds the sample and amplifies it.

This amplifier 10 includes a video signal processing section 12 that converts the video signal read out from the CCD 6 into an NTSC signal. N.T.
Recording units 13 for writing SC signals onto video tape are connected in sequence. Further, a clip circuit 16 is connected to the amplifier 10 via an A/D conversion circuit 15. This clip circuit 16 is connected to the amplifier 10 as shown in FIG.
A predetermined level (for example, 100I) of the luminance signal output from
A high-brightness clip that makes the brightness above the RE) the same and a low-brightness clip that makes the brightness the same below a predetermined level (for example, 3IRE) are performed.

The CPU 8 includes a screen division circuit 17. Frame memory 18. Adder 19. Photometric value calculation circuit 20° threshold value setting section 2
1 is provided. The screen dividing circuit 17 reads one screen worth of image signals into the frame memory 18, divides the imaged screen 27 into five regions 1 to 5, and sends the divided regions to the adder 19, as shown in FIG. However, since area 5 is an area where the main subject is rarely located, it is excluded. The adder 19 calculates average luminance values 31 to S4 by averaging the image signals of each region 1 to 4, and
Average brightness value of the area combining multiple areas 5 = Jk
(t+j, = 1 to 4) is calculated.

The photometric value calculation circuit 20 calculates the average brightness values S1 to 54.
1Sijk to determine the scene, select equations #0 to #6 corresponding to each scene, and calculate the appropriate photometric value. These calculation formulas #0 to #6 are shown below.

#O: C, -3, □.

#6: Cb = S 1zs4 In addition, weights W++, W, z, WZI, W31. Wzz, W
The values of 4++ and W5+ are, for example, Wz=10. W
+. -i, w2. -i o, w3. = t o, w, □
=1, W41-10, WsI=IO.

The threshold value setting unit 21 sets the threshold values Th, ~Thj used when performing scene discrimination using the previously selected arithmetic expressions #O~6.
Set by. The photometric value calculation circuit 20 is connected with an aperture driver 23 for driving the aperture 5 via a D/A conversion circuit 22 based on the calculated photometric value, and an A/D conversion circuit 24 A Hall element 25 for measuring the aperture diameter of the diaphragm 5 is connected thereto.

Note that, as shown in FIG. 3, Ll, L2, I(+, Hz) indicating the sizes of the regions 1 to 4 are set as follows, for example.

0.076≦L, /L≦0.305.0.305≦L
2/ L≦0.4370.110≦H, /L≦0.36
8.0.522≦Hz/L≦0.772 Next, the operation of the exposure control method of the present invention will be explained. First, a video tape cassette is loaded into the video movie camera 2, the power is turned on, and the photographic lens 3 is directed toward the subject. The object light that has passed through the photographic lens 3 is incident on the CCD 6 via the aperture 5.

The CCD 6 converts the subject light into an image signal according to the clock pulses generated by the timing generator 7 and sends it to the amplifier 10. The image signal input to the amplifier 10 is sampled and held at a predetermined sampling frequency, and sent to the clip circuit 16 via the A/D conversion circuit 15.

The image signal input to the clipping circuit 16 is clipped with high and low brightness and sent to the CPU 8.

As shown in the flowchart of FIG. 4, the image signal sent to the CPU 8 is first read into the frame memory 18 by the screen division circuit 17 and divided into five right page areas ( (See Figure 3). The image signals of each area are added by an adder 19, and the average luminance value S, ~S4 and S, a(i,
j, k·1 to 4) are calculated. These average brightness values are written to a memory within the CPUB and read by the photometric value calculation circuit 20 as necessary.

The photometric value calculation circuit 20 first calculates the brightness difference between each area. If the target value of the screen brightness level is set so that the video output is, for example, 601RE (γ = 4.5 system) and this is REF, then this target value REF and areas 1, 2, .
The brightness difference D0 from the average brightness value S1□3 of 3 is Do = log (SIX:l / REF) /lo
It becomes g2. Furthermore, the brightness difference D1 between area 1 and area 3 is
D+ = j2og (Sl / S3) / lo
g2 Similarly, the luminance difference D2 between area 1 and area 2 is Dz
= fog (Sl/3!)/log2 Also, the brightness difference D3 between the average brightness value Sl of area 1.3 and area 4 is Ds -1og (Sl:l/34)/1
It becomes 0g2.

The photometric value calculation circuit 20 receives the threshold value Th0 from the threshold value setting section 21.
~Th3 is read and compared with the absolute values ID01~1D31 of the brightness differences D0~D3 to determine the scene, and equations #0~6 for calculating the photometric value according to each scene are selected.

Incidentally, the luminance differences D0 to D3 vary slightly due to slight movement of the subject, noise, etc., and this causes the inconvenience that the selected arithmetic expression is frequently changed. In order to prevent this, the threshold values The to Th3 are changed according to the previously selected arithmetic expressions #0 to #6. This conversion table 1 is shown below.

In addition, for example, dTho=o, I EV, dTh+-0°05 EV, aTh,=o, l 5 EV,
dThs==o.

It is an IEV.

The photometric value calculation circuit 20 uses the threshold 4fi set in this way.
Read the threshold values Tho to Th from the threshold value setting section 21, and
Compare D01 with a threshold Th, (for example, 3EV). I
If D01≧The, the current screen brightness and its target value ref are significantly different from each other, for example, immediately after the video movie camera 2 is turned on,
This is the case when you suddenly move from inside a dark room to a bright outdoors. In such a case, if an attempt is made to perform scene discrimination and calculate a photometric value, it will take time for the diaphragm 5 to be stabilized by feedback control. In order to shorten this time, it is assumed that the main subject is located in the shaded area in Fig. 5 (A), and #0 is selected as the formula for calculating the photometric value.
Calculate photometric value C0.

If DOI≧T h o, ID, 1 and threshold T
h, (for example, 0.5EV). ID≧Th
, it is assumed that the main subject, for example, the face, is approximately located in area 1. In this case, further I
D21 is compared with a threshold Thz (for example, 0.75EV). When ID21≧'r'h2, there is a difference between the brightness of the face and the brightness of the clothes, so as shown in FIG. 5(B), the area where the face is likely to be located is 1 is selected to calculate the photometric value, and the photometric value CI is calculated. In addition, if ID21≧Th2, there is almost no difference in brightness between the subject's face and clothes, so as shown in Figure 5(C), the average brightness value of area 1 and area 2 is Equation #2, which calculates the photometric value with emphasis, is selected, and the photometric value C2 is calculated.

If D, l≧Th, there is no difference in brightness between region 1 and region 3. At this time, D, l and threshold T
Compare h t. If ID21≧Tht, there is no difference in brightness between area 1 and area 3, and there is a difference in brightness between area 1 and area 2.
It is determined whether or not ≧Th (for example, 0.5EV). If ID31≧Thz, there is a difference between the average brightness value of area 1.3 and the average brightness value of area 4, so the fifth
As shown in Figure (D), the average brightness value SI3 of area 1.3
Equation #3 with emphasis on is selected, and the photometric value C1 is calculated.

If Dff l≧Th, there is no difference between the average brightness values of areas 1 and 3 and the average brightness value of area 4, as shown in FIG. 5(E). Equation #4, which places emphasis on the average luminance value of .4, is selected to calculate the photometric value C4.

D, l≧The and IDII≧Th,
And if ID21≧Th, then area 1.2.3
There is no brightness difference between and the average brightness value S of areas 1 and 2.3,
2. is a value close to the target value ref, so ID31
It is determined whether or not ≧Th3. ID, 1≧T h
i, as shown in Figure 5 CF), s+
! Equation #5 with emphasis on s is selected to calculate the photometric value C3. Further, if ID, 1≧Thx, as shown in FIG. 5 (G), since there is no brightness difference in areas 1, 2, and 3.4, formula #6 is selected, and the photometric value C6 Calculate.

When the photometric value is calculated in this way, the CPU 8 drives the aperture driver 23 via the D/A conversion circuit 22 based on this photometric value, and sets the aperture 5 to an appropriate aperture value. The aperture diameter of the diaphragm 5 is measured by the Hall element 25, and A/
The signal is sent to the CPU 8 via the D conversion circuit 24. C.P.
U8 thereby detects whether the aperture diameter of the aperture 5 is based on the photometric value.

In this way, the aperture can be adjusted accurately based on the calculated photometric value.
is controlled and exposure is performed. The video signal processed as described above by the video signal processing section 12 is sent to the recording section 13, and the video signal read out from the CCD 6 is recorded on a video tape in this recording section 13.

The photometric value calculation sequence described above is executed, for example, at intervals of 1 second, and the threshold value setting unit 21 determines whether the calculation formula selected 4 degrees earlier is one of formulas #0 to #6 for the next photometry value. Threshold values Th0 to Th used for value calculation,
is set according to the conversion table 1 above.

Note that the exposure control method of the present invention is effective not only for video movie cameras but also for the snapshot mode of electronic still video cameras.

〔Effect of the invention〕

As explained above, according to the exposure control method of the present invention,
The photometric area of the solid-state imaging device is divided into multiple areas, and the difference in average brightness value of each area is compared with an initial threshold to determine the scene.Based on this scene, the photometric value is calculated and exposure is adjusted. When determining the next scene, the initial threshold value is changed based on the previously determined scene, so the brightness of the face can be adjusted to an appropriate level regardless of the reflectance of the clothes worn by the person. In addition, even if the brightness of the subject changes slightly, the photometric value will not change unnecessarily, making it possible to perform natural and stable exposure control.

[Brief explanation of drawings]

FIG. 1 is a block diagram schematically showing the structure of a video movie camera employing an embodiment of the present invention. FIG. 2 is a graph showing an example of high/low brightness clipping of an image signal. FIG. 3 is a schematic diagram showing a state in which the imaging screen is divided. FIG. 4 is a flowchart showing the main part of the sequence program of this embodiment. FIGS. 5A to 5G are schematic diagrams showing scene patterns selected by scene discrimination. 2...Video movie camera 5...Aperture 6...CCD 8...CPU 16...Clip circuit 17...Screen division circuit 19...Adder 20...Photometering Value calculation circuit 21...threshold value setting section. U (mV) Figure (B) (E) (C) (F)

Claims (1)

[Claims] (1) In an exposure control method for a video camera in which a solid-state imaging device placed on an imaging plane and converting and outputting an optical image into an electrical signal is also used for photometry of subject brightness, the solid-state imaging device has a plurality of photometric areas. The scene is determined by comparing the difference in the average brightness value of each area with the initial threshold value.Based on this scene, the photometric value is calculated and exposure is adjusted.The next time the scene is determined,
An exposure control method for a video camera, characterized in that unnecessary changes in scene selection are prevented by changing the initial threshold value based on a previously determined scene.