JP2005109579A - Imaging apparatus employing solid-state imaging device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus employing a solid-state imaging device capable of yielding a video image without gradation deficiency in black and white parts even in a scene with a great contrast difference. <P>SOLUTION: The imaging apparatus employing the solid-state imaging device for controlling the exposure so as to bring the luminance integral value of a photometry region closer to a target luminance integral value includes: an exposure control circuit for decreasing the exposure by decreasing the target luminance integral value of the luminance integral value of the photometry region when the proportion of pixels with a prescribed output value or over is greater than a prescribed proportion; and a gradation correction circuit for carrying out gradation correction so that a signal processing section assigns greater gradations to a low luminance part when the exposure control circuit decreases the exposure. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an imaging apparatus using a solid-state imaging device such as a CCD.

  In a conventional imaging device using a solid-state imaging device such as a CCD, the amount of light (exposure amount) input to the CCD is adjusted by a diaphragm or an electronic shutter speed. That is, the exposure amount is adjusted so that the CCD signal is not saturated when a bright scene is imaged, and conversely, in the dark scene, the exposure amount is increased so that black collapse does not occur.

However, when shooting a scene with a large contrast between light and dark (backlighting, indoor / outdoor simultaneous shooting), due to insufficient dynamic range of the solid-state image sensor used, adjusting the exposure amount causes saturation of bright areas and whiteout. There is a problem that both portions cannot be reproduced properly because black crushing occurs in dark portions.
JP-A-6-141229

  Conventional techniques have been devised to obtain a dynamic range expansion effect by combining images taken with different exposures. However, these techniques produce double images in fast-moving scenes and are extremely difficult to see. It becomes an image. In order to reduce this double image, it may be possible to use an image sensor capable of high-speed shooting, but this increases the cost.

  An object of the present invention is to provide an image pickup apparatus using a solid-state image pickup device that can obtain an image without black crushing and whiteout without generating a double image even in a scene with a large contrast. .

  According to the first aspect of the present invention, in the imaging apparatus using the solid-state imaging device that controls the exposure amount so that the luminance integrated value in the photometric area is close to the target luminance integrated value, the ratio of pixels that are equal to or greater than the predetermined output value is a predetermined value. If the exposure amount is larger, the exposure amount control circuit for reducing the exposure amount by reducing the target luminance integration value of the luminance integration value in the photometry area, and the exposure amount control circuit when the exposure amount is controlled to be reduced. The signal processing unit includes a gradation correction circuit that performs gradation correction so that more gradations are assigned to the low luminance part.

  According to a second aspect of the present invention, in the first aspect of the present invention, a noise reduction processing circuit for reducing noise in the low luminance portion is further provided before performing the gradation correction by the gradation correction circuit. It is characterized by that.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the low luminance portion contour extraction circuit generates a low luminance portion contour correction signal from the luminance signal subjected to gradation correction by the gradation correction circuit. , A high-brightness-portion contour extraction circuit that generates a high-brightness-portion contour correction signal from a luminance signal before gradation correction by the gradation correction circuit, a low-brightness-portion contour correction signal, A weighted addition circuit for performing weighted addition based on the luminance signal, and a contour compensation circuit for adding the contour correction signal obtained by the weighted addition circuit to the luminance signal subjected to gradation correction by the gradation correction circuit. And

  According to the present invention, even in a scene with a large difference in contrast, an image without black crushing and whiteout can be obtained. In addition, according to the present invention, unlike a method of obtaining a dynamic range expansion effect by a process of compositing multiple exposed images, an image with reduced black crushing and overexposure does not become a double image even in a fast-moving scene. Can be obtained.

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

[1] Description of configuration of imaging apparatus

  FIG. 1 shows a configuration of an imaging apparatus using a CCD.

  The imaging unit includes a lens 1a, a diaphragm (not shown), and a CCD 1. The signal output from the CCD 1 is sent to the CDS / AGC / ADC circuit 2. The CDS / AGC / ADC circuit 2 performs AGC (automatic gain control) and CDS (correlated double sampling) processing on the output signal of the CCD 1 and then performs AD conversion. The output of the CDS / AGC / ADC circuit 2 is sent to the digital process circuit.

  The output of the CDS / AGC / ADC circuit 2 is sent to the Y noise reduction processing circuit 11, the C noise reduction circuit 21, and the screen information detection / luminance signal human sgram generation / AE / AF evaluation value generation unit 31. The noise reduction processing circuit for Y 11 gains up the low luminance part of the luminance signal in the gradation correction circuit 13 in the subsequent stage, so that the increase in noise in the low level part of the gain up signal is reduced. It is provided for the purpose of suppressing noise in the low level portion in advance. Similarly, since the color signal is gained up by the gain adjustment circuit 23 at the subsequent stage, the noise reduction processing circuit 21 for C suppresses noise in advance in order to reduce an increase in noise of the gain-up signal. It is provided for the purpose.

  The output of the Y noise reduction processing circuit 11 is sent to the luminance signal filter 12 to generate the luminance signal Y. The luminance signal Y generated by the luminance signal filter 12 is output as a Y signal via the gradation correction circuit 13, the first Y process circuit 14, the γ correction circuit 15, and the second Y process circuit 16.

  The output of the C noise reduction processing circuit 21 is sent to the color separation circuit 22 to generate an RGB signal. The RGB signal generated by the color separation circuit 22 is gain-adjusted by a gain adjustment circuit 23 according to a wide dynamic range processing gain g, which will be described later, via the first C process circuit 24, the γ correction circuit 25, and the second C process circuit 26. And output as a C signal. The first C process circuit 24 performs processing such as white balance adjustment. The second C process circuit 26 generates color difference signals (R−Y) and (B−Y) from the RGB signals, color encodes the color difference signals (R−Y) and (B−Y), and generates the carrier color signal C. Processing such as generation is performed.

  Each circuit in the digital process circuit is controlled by the controller 40. The controller 40 is controlled by the system controller 50 via the interface 41. Information output from the screen information detection / luminance signal human sgram generation / AE / AF evaluation value generation unit 31 is sent to the system controller 50 via the interface 41, and the system controller 50 controls the controller 40 based on the information. It controls, the drive control part 51 of the aperture of an imaging part, and the electronic shutter speed of CCD1 are controlled.

[2] Explanation of iris control (exposure amount control)

  FIG. 2 shows the relationship between the brightness during normal exposure and the CCD output (broken line a), and the relationship between the brightness during 1/4 exposure and the CCD output (broken line b).

  During normal exposure, in the range of 0 to L, the CCD output increases as the brightness increases. When the brightness exceeds L, the CCD output is saturated and constant. During 1/4 exposure, the CCD output increases as the brightness increases within the range of 0 to 4L. When the brightness exceeds 4L, the CCD output becomes saturated and constant.

  Therefore, when the luminance distribution is as shown by the solid line c in FIG. 2, the luminance histogram of the CCD output during normal exposure is as shown by the broken line a ′ in FIGS. The output luminance histogram is as shown by a broken line b 'in FIGS. In FIGS. 3 and 4, the solid line indicates the cumulative ratio.

  In normal iris control, the iris is controlled so that the integrated luminance value in the photometric area becomes the set value A1 of the target integrated luminance value. At the time of wide dynamic range processing, normal iris control is performed, and the ratio of the frequency equal to or higher than the saturation determination threshold Thsat (see FIGS. 3 and 4) in the histogram to the total histogram frequency is equal to or higher than a set value (k%, for example, 5%). In this case, the target integrated luminance setting value A1 is gradually decreased, and the iris control is performed so that the ratio equal to or higher than the saturation determination threshold Thsat is equal to or lower than the set value. If the final target integrated luminance setting value is A2, the wide dynamic range processing gain g is g = A1 / A2.

[3] Explanation of the gradation correction circuit 13

  The gradation correction circuit 13 performs gradation correction so that the relationship between brightness and output gradation becomes a solid line d in FIG. That is, the gain is increased in the low luminance part to reproduce the same brightness as before adjusting the light amount, and an image without whiteout due to saturation is realized even in the high luminance part. Hereinafter, an example of controlling the input / output characteristics of the gradation correction circuit 13 will be described.

[3-1] Description of first control example

The first control example will be described by taking the case where the human sgram for the CCD output is as shown in FIG. FIG. 5 shows a luminance histogram and a cumulative ratio at the time of 1 / g exposure (g is a wide dynamic range processing gain). In FIG. 5, a solid line q indicates a luminance histogram of CCD output, and a solid line r indicates a cumulative ratio. Th ₁ , Th ₅₀ , and Th ₈₀ indicate CCD output values corresponding to cumulative ratios of 1%, 50%, and 80%, respectively.

  FIG. 6 shows the input / output characteristics of the gradation correction circuit 13 when the human sgram is as shown in FIG.

  The broken line representing the input / output characteristics of the gradation correction circuit 13 is composed of a straight line portion L1, a straight line portion L2, a straight line portion L3, and a straight line portion L4. The range of each straight line portion and the determination method will be described. The input of the gradation correction circuit 13 is represented by IN and the output is represented by OUT.

(1) Straight line portion L1
The straight line portion L1 represents the characteristics in the range of IN <Th ₁ . If the slope of the straight line portion L1 is α ₁ and the wide dynamic range processing gain is g, the slope α ₁ of the straight line portion L1 is set to a value in the range of α ₁ ≦ g. In the case of 1/4 exposure, g = 4.

(2) Straight line portion L2
The straight line portion L2 represents the characteristics in the range of Th ₁ ≦ IN <Th ₅₀ and OUT ≦ k1 × max _OUT . In this example, k1 = 0.5 is set. The inclination α ₂ of the straight line portion L2 is set to α ₂ = g.

(3) Straight line portion L3
The straight line portion L3 represents the characteristics in the range of Th ₅₀ ≦ IN <Th ₈₀ , OUT ≦ k2 × max _OUT , and OUT ≦ straight line L4. In this example, k2 = 0.9 is set. The inclination α ₃ of the straight line portion L3 is set to a value in the range of the inclination of the straight line L5 (see FIG. 6) ≦ α ₃ ≦ g. However, the straight line L5 is a straight line connecting the end of the straight line L2 and the point (max _IN , max _OUT ).

(4) Straight line portion L4
The straight line portion L4 represents the characteristics in the range of Th ₈₀ ≦ IN. Inclination alpha ₄ linear portion L4 is set to a value in the range of tilt of β ≦ α ₄ ≦ linear L5. β is set in advance according to the gain g so that the gradation of the high luminance portion is not extremely reduced.

[3-2] Description of second control example

The second control example will be described by taking as an example the case where the human sgram for the CCD output is as shown in FIG. FIG. 7 shows a luminance histogram and a cumulative ratio at the time of 1 / g exposure (g is a wide dynamic range processing gain). In FIG. 7, a solid line q represents a luminance histogram of CCD output, and a solid line r represents a cumulative ratio. Th ₁ , Th ₅₀ , and Th ₈₀ indicate CCD output values corresponding to cumulative ratios of 1%, 50%, and 80%, respectively.

  FIG. 8 shows the input / output characteristics of the gradation correction circuit 13 when the human sgram is as shown in FIG.

  The broken line representing the input / output characteristics of the gradation correction circuit 13 is composed of a straight line portion L1, a straight line portion L2, a straight line portion L3, and a straight line portion L4. The range of each straight line portion and the determination method will be described. The input of the gradation correction circuit 13 is represented by IN and the output is represented by OUT.

(1) Straight line portion L1
The straight line portion L1 represents the characteristics in the range of IN <Th ₁ . The slope α ₁ of the straight line portion L1 is set to a value in the range of α ₁ ≦ g. In the case of 1/4 exposure, g = 4.

(2) Straight line portion L2
The straight line portion L2 represents the characteristics in the range of Th ₁ ≦ IN <Th ₅₀ and OUT ≦ k1 × max _OUT . In this example, k1 = 0.5 is set. In this example, when IN = nTh ₅₀ (nTh ₅₀ <Th ₅₀ ), OUT = k1 × max _OUT , and the actual range of the straight line portion L2 is Th ₁ ≦ IN <nTh ₅₀ . The inclination α ₂ of the straight line portion L2 is set to α ₂ = g.

(3) Straight line portion L3
The straight line portion L3 represents the characteristics in the range of nTh ₅₀ ≦ IN <Th ₈₀ , OUT ≦ k2 × max _OUT , and OUT ≦ straight line L4. In this example, k2 = 0.9 is set. In this example, when IN> nTh ₈₀ (nTh ₈₀ <Th ₈₀ ), OUT> straight line L4, so the actual range of the straight line portion L2 is nTh ₅₀ ≦ IN <nTh ₈₀ . The slope α ₃ of the straight line portion L3 is set to a value in the range of the slope of the straight line L5 (see FIG. 8) ≦ α ₃ ≦ g. However, the straight line L5 is a straight line connecting the end of the straight line L2 and the point (max _IN , max _OUT ).

(4) Straight line portion L4
The straight line portion L4 represents the characteristic in the range of nTh ₈₀ ≦ IN. Inclination alpha ₄ linear portion L4 is set to a value in the range of tilt of β ≦ α ₄ ≦ linear L5. β is set in advance according to the gain g so that the gradation of the high luminance portion is not extremely reduced.

[3-3] Description of third control example

A third control example will be described by taking the case where the human sgram for the CCD output is as shown in FIG. FIG. 9 shows a luminance histogram and a cumulative ratio at the time of 1 / g exposure (g is a wide dynamic range processing gain). In FIG. 9, a solid line q indicates a luminance histogram of CCD output, and a solid line r indicates a cumulative ratio. Th ₁ , Th ₅₀ , and Th ₈₀ indicate CCD output values corresponding to cumulative ratios of 1%, 50%, and 80%, respectively.

  FIG. 10 shows the input / output characteristics of the gradation correction circuit 13 when the human sgram is as shown in FIG.

  The broken line representing the input / output characteristics of the gradation correction circuit 13 is composed of a straight line portion L1, a straight line portion L2, and a curved line portion L3. The range of each part and the determination method will be described. The input of the gradation correction circuit 13 is represented by IN and the output is represented by OUT.

(1) Straight line portion L1
The straight line portion L1 represents the characteristics in the range of IN <Th ₁ . The slope α ₁ of the straight line portion L1 is set to a value in the range of α ₁ ≦ g. In the case of 1/4 exposure, g = 4.

(2) Straight line portion L2
The straight line portion L2 represents the characteristics in the range of Th ₁ ≦ IN <Th ₅₀ and OUT ≦ k1 × max _OUT . In this example, k1 = 0.5 is set. The inclination α ₂ of the straight line portion L2 is set to α ₂ = g.

(3) Curve portion L3
A curved line portion L3 represents the characteristics in the range of Th ₅₀ ≦ IN. The curved line portion L3 is calculated from a lookup table (LUT) prepared in advance. Note that the look-up table is variable depending on the gain g. FIG. 11 shows an example of a lookup table.

[4] Contour Compensation Processing FIG. 12 shows a processing circuit for the luminance signal Y in the digital process circuit.

  The first Y process circuit 14 includes a low luminance portion contour extraction circuit 71, a high luminance portion contour extraction circuit 72, a weighted addition circuit 73, a gain adjustment circuit 74, and a contour compensation circuit 75. The second Y process circuit 16 includes a gain adjustment / knee correction / synchronization adding circuit 81.

  The luminance signal Y generated by the luminance signal filter 12 is applied to the gradation correction circuit 13, the high luminance portion contour extraction circuit 72 in the first Y process circuit 14, and the weighted addition circuit 73 in the first Y process circuit 14, respectively. . The luminance signal that has been subjected to gradation correction by the gradation correction circuit 13 is applied to the contour extraction circuit 71 for the low luminance part in the first Y process circuit 14 and the contour compensation circuit 75 in the first Y process circuit 14, respectively.

  The low luminance portion contour extraction circuit 71 is based on the luminance signal (the luminance signal obtained by gaining up the low luminance portion) whose gradation has been corrected by the gradation correction circuit 13, and has a contour correction signal (low level) suitable for the low luminance portion. (Brightness contour correction signal) is generated. Based on the luminance signal Y before gradation correction, the high luminance portion contour extraction circuit 72 generates a contour correction signal (high luminance portion contour correction signal) suitable for the high luminance portion.

  The low luminance portion contour correction signal and the high luminance portion contour correction signal are applied to the weighted addition circuit 73. The weighted addition circuit 73 performs weighted addition of the low luminance portion contour correction signal and the high luminance portion contour correction signal based on the luminance signal Y generated by the luminance signal filter 12. At this time, the weighting for the contour correction signal for the high luminance portion is increased in the high luminance portion, and the weighting for the contour correction signal for the low luminance portion is increased in the low luminance portion.

  The contour correction signal obtained by the weighted addition circuit 73 is given to the contour compensation circuit 75 via the gain adjustment circuit 74. The contour compensation circuit 75 performs contour compensation by adding the contour correction signal supplied from the gain adjustment circuit 74 to the luminance signal subjected to gradation correction.

  By performing such contour compensation processing, appropriate contour compensation can be performed in both the low luminance portion and the high luminance portion.

  It is preferable that the number of bits of the AD converter in the CDS / AGC / ADC circuit 2 is larger than that used in the conventional processing. In this way, even if the gain of the low luminance part is increased in the gradation correction circuit 13, the gradation property equivalent to or higher than that of the conventional processing can be reproduced.

  In the above embodiment, a histogram is created to determine whether the ratio of pixels equal to or greater than the predetermined output value (saturation determination threshold Thsat) is greater than the predetermined value. However, the predetermined output value is not generated without generating the histogram. The above pixels may be counted and the above determination may be performed based on the obtained count value. Further, when a histogram is not created, as a method for performing gradation correction, for example, it is conceivable to perform gradation correction using an LUT with adjustable gain as shown in FIG.

It is a block diagram which shows the structure of the imaging device using CCD. It is a graph which shows the relationship between the brightness at the time of normal exposure, and a CCD output (broken line a), and the relationship between the brightness at the time of 1/4 exposure, and a CCD output (broken line b). It is a graph which shows the brightness | luminance histogram of CCD output at the time of normal exposure. It is a graph which shows the brightness | luminance histogram of CCD output at the time of 1/4 exposure. It is a graph which shows an example of the human sgram with respect to CCD output. 6 is a graph illustrating a first control example of input / output characteristics of the gradation correction circuit 13 and illustrating input / output characteristics of the gradation correction circuit 13 when a human sgram is illustrated in FIG. 5. It is a graph which shows an example of the human sgram with respect to CCD output. It is a graph which shows the 2nd example of an input / output characteristic of the gradation correction circuit 13, and shows the input / output characteristic of the gradation correction circuit 13 when a human sgram is as shown in FIG. It is a graph which shows an example of the human sgram with respect to CCD output. FIG. 10 is a graph illustrating a third control example of input / output characteristics of the gradation correction circuit 13 and illustrating input / output characteristics of the gradation correction circuit 13 when a human sgram is illustrated in FIG. 9. doing. It is a graph showing an example of a lookup table It is a block diagram which shows the processing circuit with respect to the luminance signal Y in a digital process circuit.

Explanation of symbols

1 CCD
2 CDS / AGC / ADC circuit 11 Y noise reduction processing circuit 12 luminance signal filter 13 gradation correction circuit 14 first Y process circuit 15 γ correction circuit 16 second Y process circuit 40 controller 31 screen information detection / luminance signal human sgram generation / AE AF evaluation value generator 50 System controller 51 Drive controller

Claims (3)

In an imaging apparatus using a solid-state imaging device that controls the exposure amount so that the luminance integrated value of the photometric area is close to the target luminance integrated value,
When the ratio of pixels equal to or greater than the predetermined output value is greater than the predetermined value, the exposure amount control circuit that reduces the exposure amount by reducing the target luminance integration value of the luminance integration value in the photometry area, and the exposure amount control circuit A gradation correction circuit that performs gradation correction so that a large amount of gradation is allocated to the low-luminance part in the signal processing unit when the exposure amount is controlled to be small;
An image pickup apparatus using a solid-state image pickup device. 2. The imaging using the solid-state imaging device according to claim 1, further comprising a noise reduction processing circuit for reducing noise in a low luminance part before performing gradation correction by the gradation correction circuit. apparatus. A low-brightness-portion contour extraction circuit that generates a low-brightness-portion contour correction signal from a luminance signal that has been tone-corrected by the tone correction circuit. Obtained by a contour extraction circuit for a high luminance part for generating a contour correction signal, a weighted addition circuit for weighted addition of a contour correction signal for a low luminance part and a contour correction signal for a high luminance part based on the luminance signal, and a weighted addition circuit The imaging using the solid-state imaging device according to claim 1, further comprising a contour compensation circuit that adds the contour correction signal to the luminance signal subjected to gradation correction by the gradation correction circuit. apparatus.

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