JPH04340875A - Image pickup device - Google Patents

Abstract

PURPOSE:To obtain an image pickup device capable of correcting so that the black collapse may be prevented when the main object exists in the surroundings of center part of the picture at the time of the counter light state and even at the time of the excess luminance state. CONSTITUTION:The average luminance distribution of the respective video signal in the plural areas divided on the picture is detected as the exposure evaluation value by an area division circuit 12. The luminance distribution within each area is determined, the correlation between the picture center part and the other areas is determined, the area having a correlation with the picture center part is defined as the main object area and the areas other than it are defined as non-main object areas. By the ratio of the main object area and non-main object area, the counter light and excess luminance are decided and the gain of the video signal is controlled according to the degree. When the gain of the video signal is controlled, the gain in the lower part of the luminance level of the video signal is constituted so that it may be higher as compared with in the higher part of the luminance level.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging apparatus, and more particularly to gradation correction for a video camera or the like.

0002

2. Description of the Related Art FIG. 6 is a block diagram showing the configuration of a conventional imaging device. In the figure, 1 is a lens, 2 is an aperture mechanism,
3 is an image sensor, 4 is a preamplifier that amplifies the output of the image sensor 3 to an appropriate size, 5 is an integration circuit, 6 is an aperture control circuit, 7 is a process circuit consisting of a gamma correction circuit, a white balance circuit, etc., 8 2 is an automatic gain control circuit (hereinafter referred to as an AGC circuit); 9 is an integrating circuit; 10 is an AGC control circuit that generates a signal to control the gain of the AGC circuit 8;
2 is a gate circuit that gates the center part of the screen of the video signal;
3 is a voltage comparator, 24 is an integration circuit, 18 is an integration circuit 24
19 is a gain control circuit that controls the gain of the video signal according to the control signal output from the control signal generation circuit 18; 20;
2 is a signal processing circuit of the camera, and 21 is a signal output terminal.

Next, the operation will be explained. The amount of light passing through the lens 1 is limited by an aperture mechanism 2, converted into an electrical signal by an image sensor 3, and then amplified by a preamplifier 4. The output of this preamplifier 4 is integrated by an integrating circuit 5, and becomes a DC signal corresponding to the output signal level of the preamplifier 4, which is input to an aperture control circuit 6. The aperture control circuit 6 compares the input DC signal level with a reference voltage and outputs a control signal to operate the aperture mechanism 2 so that the output signal level of the preamplifier 4 is constant.

On the other hand, the output of the preamplifier 4 passes through a process circuit 7 that performs gamma correction and white balance.
It is input to the C circuit 8. This AGC circuit 8 integrates the output of the AGC circuit 8 in an integrating circuit 9 to obtain a DC signal corresponding to the output level of the AGC circuit 8, and then outputs the signal to the AGC control circuit 1.
The output signal level of the AGC circuit 8 is kept constant by the AGC control signal generated by comparing it with the reference voltage at 0.

The output of the AGC circuit 8 is input to a voltage comparator 23 after being extracted from the center of the screen by a gate circuit 22 as shown in FIG. The voltage comparator 23 outputs Hi when the input is below a specific level, and outputs Lo when it is above the specific level. By integrating the output of the voltage comparator 23 in the integrating circuit 24, the AGC circuit 8
Of the outputs, a signal corresponding to the area of the part below a specific level in the center of the screen is obtained. That is, when the main subject in the center becomes dark because the background is bright, such as in the case of backlighting, the output of the integrating circuit 24 increases depending on the degree of backlighting. This signal is then input to the control signal generation circuit 18. On the other hand, control signal generation circuit 1
8, the output of the AGC circuit 8 is also input. The control signal generating circuit 18 generates a control signal that increases the gain of the low signal level portion of the luminance signal output from the AGC circuit 8 when the output signal level of the integrating circuit 24 is high as shown in FIG. Then, the level of the video signal is changed by the gain control circuit 19. Therefore, the gain control circuit 19 increases the gain according to the area of the dark part when there is a lot of dark part in the center of the screen, such as when there is backlight, and the gradation characteristics of the dark part are corrected to increase the contrast. The signal is output, and after various processing is performed by the signal processing circuit 20, a video signal is output from the output terminal 21.

[0006]

[Problems to be Solved by the Invention] Conventional imaging devices are configured as described above, so if there is no main subject in the center of the screen, the correction is not effective even in backlighting, and the correction is not effective even in backlighting. The problem is that when the main subject is bright, but the background is dark and the shadows are crushed, there is no correction effect at all.

[0007] This invention was made in order to solve the above-mentioned problems, and the correlation value between the central area of the screen and each other area is determined from the luminance distribution of each area, and The main subject area and non-main subject areas are determined based on the correlation value. Then, by comparing the determined evaluation value of the main subject area and the evaluation value of the non-main subject area, backlighting and excessive front lighting are determined, and the gain of the video signal is controlled.
To provide an imaging device that performs gradation correction so as to eliminate blocked up shadows even when the main subject is not in the center of the screen during backlighting or when there is excessive forward lighting.

[0008]

[Means for Solving the Problems] An imaging device according to the present invention includes an imaging means for outputting a video signal, and an average luminance component of each video signal in a plurality of divided regions on a screen. an evaluation value detection means for outputting an exposure evaluation value; a brightness frequency distribution calculation means for dividing the brightness component into a plurality of brightness level stages for each of the plurality of regions and counting the number of times the brightness appears in each stage; a correlation calculating means for calculating a correlation value between the area at the center of the screen and each area from the frequency distribution of the luminance; a discriminating means for discriminating between a main subject area and a non-main subject area based on the correlation value; and a luminance of the video signal. gain control means capable of varying the gain according to the level, and by comparing the exposure evaluation value of the main subject area and the exposure evaluation value of the non-main subject area,
The degree of backlighting and excessive forwardlighting is detected, and the relationship between the brightness level and the gain of the gain control means is controlled based on the detection results.

[0009]

[Operation] The imaging device according to the present invention compares the evaluation values of the main subject area and the non-main subject area, and if the ratio of the evaluation value of the main subject area to the evaluation value of the non-main subject area is sufficiently larger than 1. When the gain is sufficiently small, the relationship between the input luminance level and the gain of a gain control circuit that controls the gain of the video signal is controlled.

0010

【Example】

Example 1. FIG. 1 is a block diagram of an imaging apparatus showing an embodiment of the present invention, and 1 to 10 and 18 to 21 are the same as those in the conventional example, and their explanation will be omitted. 11 is an A/D converter that A/D converts the video signal output of the AGC circuit 8; 12 is an A/D converter;
An area dividing circuit divides the converted signal into a plurality of areas; 13 is an integration circuit that calculates an evaluation value corresponding to the brightness of each area from the signal divided into each area; 14 is an integration circuit for calculating the evaluation value corresponding to the brightness of each area; 15 is an interface circuit for inputting the output of the integration circuit 13 and the output of the frequency distribution calculation circuit 14 to the microcomputer; 16 is the microcomputer;
7 is a D/A converter that converts the digital signal output of the microcomputer 16 into an analog signal.

Similar to the conventional example, the amount of light passing through the lens 1 is limited by the diaphragm mechanism 2, imaged by the image sensor 3, converted into an electrical signal, and then amplified by the preamplifier 4, and the signal level is determined by the integrating circuit. 5. The aperture control circuit 6 and the aperture control mechanism 2 keep the aperture constant until the aperture is opened. The output of the preamplifier 4 is input to the AGC circuit 8 after being subjected to processing such as gamma correction and white balance in the process circuit 7 . Similarly to the conventional example, the AGC circuit 8 is also controlled by control signals generated from the integrating circuit 9 and the AGC control circuit 10 so that the output signal level of the AGC circuit 8 is constant.

The output of the AGC circuit 8 is sent to an A/D converter 11.
The signal is converted into a digital signal by the area dividing circuit 12 and distributed into signals corresponding to each area divided into screens as shown in FIG. The distributed digital signals are integrated by an integration circuit 13, and an evaluation value corresponding to the brightness of each area is detected. On the other hand, frequency distribution calculation circuit 1
4, a frequency distribution as shown in FIG. 3 is detected by counting the distributed digital signals according to their magnitudes. After these detection signals are held in the interface circuit 15, data is selected by a control signal from the microcomputer 16 and input to the microcomputer 16. Then, the microcomputer 16 performs the following processing.

First, a correlation value is calculated. Since the main subject is often located at the center of the screen, it is assumed that the main subject is always included in the central area (area 5 in the case of screen division as shown in FIG. 2). The correlation between this area and other areas is determined from the brightness distribution. In order to obtain the correlation value, for example, it is calculated using the following arithmetic expression. In this case, if the correlation is strong, the correlation value will be small, and if the correlation is weak, the correlation value will be large. Correlation value = |M1|+|M2|+|M3|+...+|M
n|Mi=Xi-Yi+Mi-1 M0=0 Xi: frequency of i stage in the luminance frequency distribution of the central region. Yi: i in the luminance frequency distribution of the area where you want to find the correlation value
Number of steps. n: Number of stages of luminance frequency distribution.

Next, the main subject area and the non-main subject area are discriminated based on the previously determined correlation value. That is, if the correlation is stronger than a predetermined value (the correlation value according to the above-mentioned correlation value calculation formula becomes smaller), it is determined as a main subject region, and if not, it is determined as a non-main subject region. Then, a ratio of evaluation values according to the brightness of the main subject area and the non-main subject area (evaluation value of the non-main subject area/evaluation value of the main subject area) is calculated. When the calculated ratio is greater than 1, it indicates the degree of backlighting, and when it is smaller than 1, it indicates the degree of over-frontlighting. Finally, a control voltage is output depending on the degree of backlighting and excessive forwardlighting. For example, as shown in Figure 4, if the ratio of the evaluation values according to the brightness of the main subject area and non-main subject area is close to 1, it is considered that the exposure is appropriate and a constant value is output. , if it is sufficiently larger than 1 or if it is sufficiently smaller than 1, each correction is required, so a control voltage corresponding to the ratio of the evaluation values is output.

Therefore, for a typical backlit subject with a person in front of a high-luminance background as shown in FIG. 2, the probability that the main subject exists is low, as is clear from FIG. The frequency distribution of the central area with the highest value (area 5 in the division method shown in Figure 2) is closer to the dark side, and the frequency distribution of the area that contains more background (areas 1, 2, 3, 6,
In the frequency distribution of 9), the frequency is biased toward the bright side, so if we calculate the correlation between each region and the central region, the correlation in regions 1, 2, 3, 6, and 9, which contains a lot of background, is weak (as mentioned above) (The correlation value calculated by the correlation value calculation formula becomes larger), and the correlation of regions 4, 5, 7, and 8 including the main subject becomes stronger. (The correlation value calculated by the above-mentioned correlation value calculation formula becomes small.) Therefore, areas 4, 5, 7, and 8 are determined to be the main subject areas, and area 1
, 2, 3, 6, and 9 are determined to be non-main subject areas. The evaluation value of the main subject area is calculated from the evaluation value according to the brightness of each area in the main subject area. Non-main subject areas are also calculated in the same way. And these ratios (
Evaluation value of non-main subject area/Evaluation value of main subject area)
seek. In the case of FIG. 2, as is clear from FIG. 3, the evaluation value of the main subject area becomes small, and conversely, the evaluation value of the non-main subject area becomes large. Therefore, the ratio of the evaluation value of the main subject area to the evaluation value of the non-main subject area is 1.
When the light becomes sufficiently large, it is determined that there is backlight, and the microcomputer outputs a control voltage for correction. Similarly,
When the main subject is located outside the center of the screen due to backlighting or when the subject is excessively frontlit, the evaluation value of the main subject area becomes large and the evaluation value of the non-main subject area becomes small. Therefore, the ratio of the evaluation value of the main subject area to the evaluation value of the non-main subject area is 1.
When the light becomes sufficiently smaller, it is determined that there is excessive brightness, and the microcomputer 16 outputs a control voltage for correction. The relationship between the evaluation value ratio and the control voltage output from the microcomputer 16 is shown in FIG.

The control voltage output from the microcomputer 16 is D
The signal is converted into an analog signal by the /A converter 17 and input to the control signal generation circuit 18. On the other hand, the brightness signal output from the AGC circuit 8 is also input to the control signal generation circuit 18 . As in the conventional example, the control signal generation circuit 18 performs AGC control in the case of backlighting and excessive forwardlighting, that is, when the output voltage of the D/A converter 17 is high, as shown in FIG.
A control signal is generated to increase the gain of the low luminance level portion of the luminance signal output from the circuit 8, and the gain control circuit 19 changes the level of the video signal. Therefore, the gain control circuit 19 increases the gain depending on the degree of backlighting or excessive forward lighting, corrects the gradation characteristics of dark areas, and outputs a signal with contrast, which is then subjected to various processing by the signal processing circuit 20. Thereafter, it is output from the output terminal 21.

Example 2. In the first embodiment described above, the screen is divided into nine equal parts, but the present invention can be applied to any imaging device having a divided area in the center of the screen, regardless of the number of screen divisions. Same effect.

[0018]

As described above, according to the present invention, the main subject area is determined, and if the ratio of the evaluation values of the main subject area and the non-main subject area is sufficiently larger than 1 or sufficiently smaller than 1, The configuration is configured to perform gradation correction, so when the main subject is outside the center of the screen when backlit, or
This is effective in correcting situations where the main subject is bright, but the background is dark and the shadows are crushed, as in the case of excessive direct lighting.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an imaging device according to an embodiment of the present invention.

FIG. 2 is a diagram showing an example of screen division in the present invention.

FIG. 3 is a diagram showing the frequency distribution of each region in the case shown in FIG. 2 in the present invention.

FIG. 4 is a graph showing the relationship between the evaluation value ratio and the control voltage output from the D/A converter in the present invention.

FIG. 5 is a graph showing the relationship between the output voltage of the D/A converter and the gain of the gain control circuit in the present invention.

FIG. 6 is a block diagram showing a conventional imaging device.

FIG. 7 is an explanatory diagram showing an example of the central part of the screen extracted by a gate circuit in a conventional example.

FIG. 8 shows the relationship between the output voltage of the integrating circuit and the gain of the gain control circuit in the conventional example when the brightness level of the signal input to the gain control circuit is high, low, and in between.
FIG.

[Explanation of symbols]

3 Imaging device 12 Area division circuit 13 Integration circuit 14 Frequency distribution calculation circuit 16 Microcomputer 19 Gain control circuit

Claims (1)

[Claims] 1. An imaging means for outputting a video signal; an evaluation value detection means for outputting an average luminance component of each video signal in a plurality of regions divided on a screen as an exposure evaluation value for each region; Luminance frequency distribution calculating means for dividing the luminance component into a plurality of luminance level stages for each of the plurality of regions and counting the number of luminance appearances of each stage, and a correlation value between the region at the center of the screen and each region. a correlation calculation means for calculating from the frequency distribution of the luminance, a discrimination means for discriminating between a main subject area and a non-main subject area based on the correlation value, and a gain whose gain is variable according to the luminance level of the video signal. control means, which detects the degree of backlighting and excess front lighting by comparing the exposure evaluation value of the main subject area with the exposure evaluation value of the non-main subject area, and based on the detection result, the gain control means An imaging device characterized by controlling a relationship between a brightness level and a gain.