JP2005341243A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus in which the low-luminance part of a video signal can be corrected, while suppressing the amplification of noise in the vicinity of black without generating a false profile. <P>SOLUTION: A low-luminance region where the luminance of an input signal is between Xa and Xe, four regions in the low-luminance region, low-luminance side and high-luminance side regions in the low-luminance region, and a quadratic function representative of relation between the input signal and the output signal in each of four regions in the low-luminance region are set previously. A comparator 18 decides, to which of six regions including the four regions in the low-luminance region the luminance of an inputted video signal belongs. Based on the decision result, a selector 19 output a factor selectively. Output from an adder 14 represents a correction value and multipliers 15r, 15g and 15b multiply the input signal by the correction value to extend the luminance of the input signal and delivers the multiplication results as an output signal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an imaging apparatus that changes an amplification factor for each luminance of a video signal, and more particularly to an imaging apparatus that improves a black stretch function that increases an amplification factor of a low luminance part.

  In a conventional imaging device, for example, in the case of backlight, there is a problem that the main subject becomes dark because the background is captured so that the background is not completely saturated. Also, when the main subject is very bright, there is a problem that the surrounding image is crushed darkly. As a countermeasure against such a problem, for example, as disclosed in Patent Documents 1 to 3, a method of increasing the contrast of the low luminance part by correcting the gradation of the low luminance part has been proposed.

Patent Document 1 discloses a technique for correcting a low luminance part by nonlinear amplification using a broken line. Patent Document 2 discloses a technique for calculating a screen ratio that occupies a dark portion and controlling black expansion of a luminance signal in accordance with the screen ratio. Patent Document 3 discloses a method for correcting the gradation of a low luminance part by digital processing.
Japanese Patent Laid-Open No. 2-140088 JP-A-7-75115 JP-A-8-107519

  As described above, various methods for increasing the contrast of the low luminance part by correcting the gradation of the low luminance part have been proposed. FIG. 18 shows an example of the relationship between the level of the input signal and the level of the output signal in the conventional gradation correction. When gradation correction is performed using a polygonal line as shown in FIG. 18, the slope changes suddenly at the crease point, so that when an image is displayed, a pseudo contour is generated in an object that changes smoothly near its brightness. Resulting in. In FIG. 18, a pseudo contour is generated at each luminance portion of Xa, Xb, and Xc.

  Also, as disclosed in Patent Document 2, when tone correction is performed using a nonlinear element such as a diode, the amplification factor is increased from the black level. Near black, noise is conspicuous. If the amplification factor is increased from the black part, the noise becomes conspicuous. In the technique described in Patent Document 3, non-linear characteristics are realized by digital calculation. However, since the gain is similarly increased from the black level, noise becomes conspicuous. By performing digital processing to shift this characteristic in the luminance direction and to apply gain correction from the middle, as shown in FIG. 19, the correction start part becomes a discontinuous characteristic. A pseudo contour is generated.

  The present invention has been made in view of the above-described problems, and can correct a low-luminance portion of a video signal without generating pseudo contours and suppressing amplification of noise near black. An object is to provide an imaging device.

  SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and the invention according to claim 1 captures a subject, generates a video signal, and expands the luminance of a low luminance region of the video signal. In an imaging apparatus that performs expansion processing, in a graph showing a relationship between an input signal that is the video signal before luminance expansion processing and an output signal that is the video signal after luminance expansion processing, the luminance of the input signal is first Setting the low luminance area that is greater than or equal to the luminance value and less than the second luminance value, and setting area setting means for setting four areas for the low luminance area, and each of the four areas of the low luminance area In a region, function setting means for setting a relationship between the input signal and the output signal by a quadratic function, and determining whether the luminance of the input signal belongs to any one of the four regions of the low luminance region Luminance When the determination unit and the luminance determination unit determine that the luminance of the input signal belongs to one of the four regions, the function setting unit sets the luminance according to the region to which the luminance of the input signal belongs. An imaging apparatus comprising: a luminance expansion unit that expands the luminance of the input signal based on the quadratic function.

  According to a second aspect of the present invention, in the imaging apparatus according to the first aspect, when the low luminance region is a first region, a second region, a third region, and a fourth region in order from the low luminance side, The region width at the level of the input signal in the region composed of the first region and the second region is different from the region width at the level of the input signal in the region composed of the third region and the fourth region, To do.

According to a third aspect of the present invention, in the imaging apparatus according to the first or second aspect, the region setting means includes a first region (Xa ≦ X <Xb where X represents the luminance of the input signal. ), The second region (Xb ≦ X <Xc), the third region (Xc ≦ X <Xd), and the fourth region (Xd ≦ X <Xe) are set as the low luminance region, and the function setting means includes: When A is a positive number and a = A / (Xb−Xa) ² and b = A / (Xd−Xc) ² , the correction function f for the luminance of the input signal in each region of the low luminance region F = a (X−Xa) ² +1 for the first region, f = −a (Xc−X) ² + 2A + 1 for the second region, and f for the third region. ^{= -b (X-Xc) 2} + 2A + 1, with respect to the fourth ^{region, f = b (Xe-X} ) 2 +1, set by comprising the formula When the luminance determining unit determines that the luminance of the input signal belongs to the low luminance region, the luminance expansion unit calculates the value of the correction function f in the luminance of the input signal. The luminance of the input signal is expanded by multiplying the luminance.

According to a fourth aspect of the present invention, in the imaging apparatus according to the first or second aspect, the region setting means includes a first region (where Xa ≦ X <Xb where the luminance of the input signal is X). ), The second region (Xb ≦ X <Xc), the third region (Xc ≦ X <Xd), and the fourth region (Xd ≦ X <Xe) are set as the low luminance region, and the function setting means includes: When A is a positive number and a = A / (Xb−Xa) ² and b = A / (Xd−Xc) ² , the correction function f for the luminance of the input signal in each region of the low luminance region For the first region, f = a (X−Xa) ² , for the second region, f = −a (Xc−X) ² + 2A, for the third region, f = ^{-b (X-Xc) 2 +} 2A, with respect to the fourth region, f = b (Xe-X ) 2, set by comprising wherein said luminance Shin The means adds the value of the correction function f in the luminance of the input signal to the luminance of the input signal when the luminance determining means determines that the luminance of the input signal belongs to the low luminance region. Thus, the luminance of the video signal is expanded.

  According to a fifth aspect of the present invention, in the imaging apparatus that captures an image of a subject, generates a video signal, and performs a luminance expansion process for expanding a luminance of a low luminance area of the video signal, the video signal before the luminance expansion process A low luminance region in which the luminance of the input signal is greater than or equal to the first luminance value and less than the second luminance value in the graph showing the relationship between the input signal that is and the output signal that is the video signal after luminance expansion processing Area setting means for setting the relationship, function setting means for setting the relationship between the input signal and the output signal by a quartic function in the low luminance area, and the luminance of the video signal generated by the imaging means When the luminance determining means for determining whether the luminance signal belongs to the low luminance area and the luminance determining means determines that the luminance of the video signal belongs to the low luminance area, the function setting means And a luminance expansion unit that expands the luminance of the video signal based on the quartic function set in the above.

  According to a sixth aspect of the present invention, in the imaging apparatus according to the fifth aspect, when the luminance of the input signal is X, the region setting means sets a region where Xa ≦ X <Xc as the low luminance region. The function setting means sets a correction function f for the luminance of the input signal in the low luminance region when α is a positive number.

  The brightness expansion means sets the value of the correction function f in the brightness of the input signal when the brightness discrimination means determines that the brightness of the input signal belongs to the low brightness area. The luminance of the video signal is expanded by multiplying the luminance of the input signal.

  According to a seventh aspect of the present invention, in the imaging device according to the fifth aspect, when the luminance of the input signal is X, the region setting means sets a region where Xa ≦ X <Xc as the low luminance region. The function setting means sets a correction function f for the luminance of the input signal in the low luminance region when α is a positive number.

  The brightness expansion means sets the value of the correction function f in the brightness of the input signal when the brightness discrimination means determines that the brightness of the input signal belongs to the low brightness area. The luminance of the video signal is expanded by adding to the luminance of the input signal.

  According to an eighth aspect of the present invention, in the imaging apparatus according to the first or fifth aspect, the luminance expansion unit further includes the video signal according to a luminance expansion amount in the low luminance region of the video signal. The color difference component in the low luminance region is expanded.

  According to a ninth aspect of the present invention, in the imaging device according to the eighth aspect, the expansion amount of the chrominance component in the low luminance region of the video signal, which is expanded by the luminance expansion unit, is the value of the video signal. It is characterized by being larger than the amount of luminance expansion in the low luminance region.

  According to a tenth aspect of the present invention, in the imaging apparatus according to any one of the first to ninth aspects, the luminance expansion unit performs the blanking period of the video signal generated by the imaging unit. A signal generating means for generating a confirmation signal for confirming luminance expansion, outputting the confirmation signal to the luminance expansion means, the confirmation signal after processing processed by the luminance expansion means, and the signal generating means The apparatus further comprises inspection means for inspecting whether or not there is a problem in the expansion of the luminance of the confirmation signal by the luminance expansion means based on the generated confirmation signal before processing.

  According to an eleventh aspect of the present invention, in the imaging device according to the tenth aspect, the function setting means is a function indicating a relationship between the input signal and the output signal when a defect is detected by the inspection means. The signal generation means generates the confirmation signal during the blanking period of the video signal and outputs the confirmation signal to the luminance expansion means. The inspection means outputs the luminance of the confirmation signal by the luminance expansion means. The present invention is characterized in that the presence or absence of a defect in the extension of the image is checked again.

  According to the present invention, it is possible to correct the low luminance portion of the video signal without generating pseudo contours and suppressing amplification of noise near black.

  The best mode for carrying out the present invention (first embodiment) will be described below with reference to the drawings. First, a method for correcting a low luminance portion of a video signal according to the present embodiment will be described. In the present embodiment, the input / output characteristics of the low luminance part are obtained by calculation.

As shown in FIG. 3B, the low luminance region (region where the luminance is Xa or more and Xe or less at the level of the input signal) for which the amplification factor is corrected is divided into four areas, and each is divided into quadratic functions ( Amplification factor is changed non-linearly by calculation using a correction function. The function of area II in the figure is a quadratic function in which the slope is 0 at the point where the input X = Xa and the correction gain f at X = Xb is A + 1 (A is a positive number). This functional equation can be expressed by the following equation.
f = a (X−Xa) ² +1 where a = A / (Xb−Xa) ² , Xa <X ≦ Xb (1)

The function of area III is a function obtained by rotating the function of area II by 180 ° at the input point X = Xb. Since the slope of the function of area II is the same as that of area II at X = Xb, the function of area II is smooth. Connect to. If the condition Xc−Xb = Xb−Xa is satisfied, the slope of the area III function at the point where X = Xc is 0, and the correction gain f is 2A + 1. This function formula is expressed by the following formula.
f = −a (Xc−X) ² + 2A + 1 where a = A / (Xb−Xa) ² , Xb <X ≦ Xc (2)

The area IV function is a quadratic function having a vertex at the point where the correction gain f is 2A + 1 at X = Xc, and a quadratic function at which the correction gain f is A + 1 at X = Xd. This functional equation can be expressed by the following equation.
f = −b (X−Xc) ² + 2A + 1 where b = A / (Xd−Xc) ² , Xc <X ≦ Xd (3)

The function of area V is a function obtained by rotating the function of area IV by 180 ° at the point of X = Xd and correction gain f = A + 1. Since the slope is the same as the function of area IV at X = Xd, Connect smoothly with functions. If the condition of Xe−Xd = Xd−Xc is satisfied, the slope of this function at the point X = Xe is 0 and the correction gain f is 1. This function formula is expressed by the following formula.
f = b (Xe−X) ² +1 where b = A / (Xd−Xc) ² , Xd <X ≦ Xe (4)

  The remaining functions of the areas I and VI are functions that are not corrected, that is, the gain is 1. As described above, a smooth gradation correction curve as shown in FIG. 3A (a graph showing input / output characteristics of an input signal and an output signal of a video signal) can be obtained by combining the quadratic functions. In the control of this curve, the video signal has a large degree of freedom by adjusting the start point Xa and end point Xe of the nonlinear characteristic, the correction peak position Xc, and the gain correction strength A. It is possible to correct the gradation in the low luminance part.

  Next, the configuration of the imaging apparatus that realizes the above concept will be described. FIG. 1 is a block diagram illustrating the configuration of the imaging apparatus according to the present embodiment. Hereinafter, each component in the figure will be described. The lens 1 forms an image on the sensor 2. The image formed by the lens 1 is converted into an electrical signal by the sensor 2 and converted into digital data (video signal) by the ADC 3. The gain adjustment circuit 4 performs processing such as shading correction and white balance on the video signal output from the ADC 3 and outputs the processed signal to the black stretch circuit 5.

  The black stretch circuit 5 corrects the gradation of the low luminance portion of the input video signal (luminance expansion processing), and outputs the corrected video signal to the signal processing circuit 6. The detailed configuration and function of the black stretch circuit 5 will be described later. The signal processing circuit 6 performs contour emphasis, γ conversion, and output format conversion on the video signal whose gradation has been corrected by the black stretch circuit 5, and outputs the result to the DAC 7. The DAC 7 converts the digital video signal into an analog signal and outputs it.

  The controller 8 controls each part described above. In particular, the controller 8 sets the above-described low luminance region in the input / output characteristics of the video signal, and includes the four regions for the low luminance region, the low luminance side of the low luminance region, and the high luminance side region. Set one area. Further, the controller 8 sets the relationship between the input signal and the output signal of the video signal by a quadratic function in each of the four areas of the low luminance area in the input / output characteristics. The user I / F 9 is operated by the user and outputs a signal indicating an operation result by the user to the controller 8.

FIG. 2 is a block diagram showing the configuration of the black stretch circuit 5. Hereinafter, each component in the figure will be described. The luminance generation circuit 10 calculates luminance components from the input R, G, and B signals. The adder 11 adds the output of the luminance generation circuit 10 and the output D _{1 of the} selector 19. The square circuit 12 squares the signal value of the signal output from the adder 11. The multiplier 13 multiplies the output of the squaring circuit 12 and the output D _{1 of the} selector 19. The adder 14 adds the output of the multiplier 13 and the output D _{3 of the} selector 19.

The multiplier 15r multiplies the output of the adder 14 and the R signal and outputs the result as an R output signal. The multiplier 15g multiplies the output of the adder 14 and the G signal and outputs the result as a G output signal. The multiplier 15b multiplies the output of the adder 14 and the B signal and outputs the result as a B output signal. The comparator 18 compares Xa, Xb, Xc, Xd, and Xe set by the controller 8 with the luminance of the signal input from the luminance generation circuit 10, and the luminance of the input signal is compared with the above-described areas I to I. Which area of the area VI belongs to is determined, and the determination result is notified to the selector 19. The selector 19 selects a coefficient from among Xa, Xc, Xe, A, a, and b set by the controller 8 based on the result of determination by the comparator 18 and outputs it as outputs D _{1 to} D _3. To do.

Next, the operation of the black stretch circuit 5 configured as described above will be described. The black stretch circuit 5 receives R, G, and B signals constituting the video signal from the gain adjustment circuit 4. The luminance generation circuit 10 calculates a luminance component from the input R, G, and B signals. For example,
Y = 0.2R + 0.7G + 0.1B (5)
The luminance component may be calculated as

The comparator 18 compares the output of the luminance generation circuit 10 with the set values Xa, Xb, Xc, Xd, and Xe set by the controller 8 to determine which area the input luminance corresponds to. The area information indicating the determination result is sent to the selector 19. Here, Xb and Xd can be obtained as Xb = Xa + (Xc−Xa) / 2 and Xd = Xc + (Xe−Xc) / 2, respectively. Regarding this calculation, hardware can be easily realized by an adder / subtracter and a bit shift, and therefore hardware may be used. Selector 19, controller - set value set by La 8 Xa, Xc, Xe, A , a, and a necessary setting from among b, the output _D 1 ~ selected based on the result of the comparator 18 and outputs it as a D _3.

In this example, A is the intensity of gain correction, and the larger A is, the higher the contrast in the low luminance region. a and b are numerical values necessary for calculation of each function, and a = A / (Xb−Xa) ² , b = A / (Xd−Xc) ² (6)
Can be calculated with This calculation requires division, and the scale becomes large if it is configured with hardware. Therefore, the controller 8 is configured to calculate and set.

The output of the selector 19 with respect to the output of the comparator 18 is as follows.
When a region where the brightness of the input signal belongs is an area I, and outputs a 1 to _{D 1} in 0, _{D 2} to 0, _{D 3.}
When a region where the brightness of the input signal belongs is an area II, -Xa to _{D 1,} and outputs a 1 a, the _{D 3} to _{D 2.}
When a region where the brightness of the input signal belongs is an area III, -Xc to _{D 1,} and outputs -a, a 2A + 1 to _{D 3} in _{D 2.}
When a region where the brightness of the input signal belongs area IV, -Xc to _{D 1,} and outputs -b, a 2A + 1 to _{D 3} in _{D 2.}
When a region where the brightness of the input signal belongs areas V, -Xe to _{D 1,} to _{D 2} b, and outputs a 1 to _{D 3.}
When a region where the brightness of the input signal belongs areas VI, outputs 1 to _{D 1} in 0, _{D 2} to 0, _{D 3.}

For example, when the luminance signal X belongs to the area III, the difference between X and Xc is calculated by the adder 11, the addition result is squared by the squaring circuit 12, and -a is multiplied by the multiplier 13. The adder 14 adds 2A + 1. From the above calculation, the correction gain f = −a (Xc−X) ² + 2A + 1 where a = A / (Xb−Xa) ²
Is obtained. The same applies to other areas, and the gain correction curve shown in FIG. The multipliers 15r, 15g, and 15b multiply the calculation results by the R, G, and B signals, respectively, so that the characteristic of smoothly amplifying the low luminance portion in a non-linear manner as shown in FIG.

  4 to 7 show changes in gradation characteristics when the control values Xa, Xc, Xe, and A from the controller 8 are adjusted. FIG. 4 shows changes in gradation characteristics when A is changed, and the correction amount can be adjusted. FIG. 5 shows changes in gradation characteristics when Xe is changed with Xc being the midpoint of Xe, and the range on the high luminance side where gradation correction is applied can be adjusted. FIG. 6 shows the change in gradation characteristics when the difference between Xa and Xc (the absolute value thereof) and the difference between Xc and Xe (the absolute value thereof) are fixed and the value of Xc is changed to the left and right. Thus, the position where the gradation correction is applied can be adjusted. FIG. 7 shows the change in gradation characteristics when the position of Xc is changed. The input level of the area consisting of areas II and III in FIG. 3 and the input level of the area consisting of areas IV and V are shown. Thus, the rising portion of the gradation characteristic of Xc can be shifted to the left and right.

  The combination of these adjustments can perform gradation correction in a low luminance region with a large degree of freedom. However, when the user makes adjustments, it becomes difficult to understand if the degree of freedom is too large. Based on the above, the position of Xa is set to a fixed value, the upper limit luminance Xe to be corrected is fixed to, for example, about 50%, and the user adjusts only the central luminance Xc to be corrected and the correction amount A. May be.

  However, for example, if the correction amount A is too large, as shown in FIG. 8, a reverse phenomenon occurs in which the luminance of the subject decreases but the luminance decreases on the display. If the adjustment items are limited to some extent, the adjustment range may be limited so that such a failure does not occur, but if there are many adjustment items, the adjustment range is divided into various cases. Setting is complicated and leaks are likely to occur. Therefore, as shown in FIG. 9, a signal generator 20 for generating a test signal (confirmation signal) is provided at the input stage, and test signals are output for both RGB via the selector 19 during the blanking period (blank period). The check circuit 22 confirms the correction result.

  Hereinafter, a specific operation of the black stretch circuit 5 will be described. In this case, it is assumed that the gradation correction start position (Xa), end position (Xe or width Xe-Xa), black stretch peak position (Xc), or gain (A) is changed in advance by the user. During the blank period, the test signal generator 20 outputs a so-called ramp waveform that increases by 1 from 0. The check circuit 22 compares the signal before correction with the signal after correction, and checks whether the luminance of the current pixel is lower than the luminance of the previous pixel. That is, the check circuit 22 checks whether there is a problem in the luminance expansion processing.

  However, when the test signal is 0, the check is not performed because the data of the previous pixel is unknown. If a correction failure is found, the check circuit 22 notifies the controller 8 of the correction failure. Upon receiving this notification, the controller 8 restores the correction data (Xa etc.) changed by the user and resets the output value of the selector 19. The timing for restoring the correction data may be the moment when a failure during the test is found or after the test is completed.

  Alternatively, when a failure in correction is found, the controller 8 gradually brings the correction amount closer to the original data, finds a value that does not fail and sets it to that value, thereby outputting an image without failure. For example, when a failure occurs due to the change of Xa to Xa ′, if Xa is smaller than Xa ′, the controller 8 sets Xa′-1, Xa′-2, Xa′-3,. The quadratic function is set again while changing the value, and the test is instructed to the test signal generator 20 and the check circuit 22.

  The test signal generator 20 generates the test signal again during the blanking period of the video signal, and the check circuit 22 executes the test for the luminance expansion processing of the test signal again. If no failure is found by the check circuit 22, the controller 8 sets the value at that time to the value of Xa and resets the output value of the selector 19. Regarding how to change the above set value, for example, as b = (Xa′−Xa) / 16, the set value is changed as Xa′−b, Xa′−2b, Xa′−3b. You may let them.

  Next, another configuration example of the black stretch circuit 5 according to the present embodiment will be described. FIG. 10 is a block diagram showing another configuration example of the black stretch circuit 5. Hereinafter, each component in the figure will be described. The same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted. In this configuration, adders 26r, 26g, and 26b are provided instead of the multipliers 15r, 15g, and 15b of FIG. The adder 26r adds the output of the adder 14 and the R signal, and outputs the result as an R output signal. The adder 26g adds the output of the adder 14 and the G signal, and outputs the result as a G output signal. The adder 26b adds the output of the adder 14 and the B signal and outputs the result as a B output signal. With the above configuration, a smooth correction curve can be obtained.

Hereinafter, the correction method will be described. As for the correction coefficient in FIG. 2, a correction function is created by combining a curve of a quadratic function on gain 1, but in the black stretch circuit 5 of FIG. 10, as shown in FIG. A correction function combining a quadratic function is created on the top. The relationship between the luminance values that divide each area is the same as in the example of FIG.
Xb = Xa + (Xc−Xa) / 2 (7)
Xd = Xc + (Xe−Xc) / 2 (8)
It is.

The function formula in each area is as follows in area I (X ≦ Xa):
f = 0
In Area II (Xa <X ≦ Xb)
f = a (X−Xa) ² where a = A / (Xb−Xa) ² (9)
In Area III (Xb <X ≦ Xc)
f = −a (Xc−X) ² + 2A where a = A / (Xb−Xa) ² (10)
In area IV (Xc <X ≦ Xd)
f = −b (X−Xc) ² + 2A where b = A / (Xd−Xc) ² (11)
In area V (Xd <X ≦ Xe)
f = b (Xe−X) ² where b = A / (Xd−Xc) ² (12)
In area VI (Xe <X)
f = 0
It is. Since the correction method is different from that in the case of FIG. 2, the shape of the curve is different with the same coefficient, but similarly smooth correction characteristics can be obtained.

  In this embodiment, the black stretch circuit 5 is disposed immediately after the gain adjustment circuit 4, but is disposed immediately after the signal processing circuit 6 as shown in FIG. You may make it correct | amend a low-intensity part later. When outputting in Y, Cb, Cr format as video output, if the black stretch circuit 5 is inserted after conversion to Y, Cb, Cr, the luminance is already present when the signal is input to the black stretch circuit 5 Since the signal is calculated, the circuit scale can be reduced.

  Next, another configuration example of the black stretch circuit 5 according to the present embodiment will be described. FIG. 13 is a block diagram showing another configuration example of the black stretch circuit 5. The same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted. The black stretch circuit 5 has a configuration in which a Y signal (luminance signal) is input and a multiplier 37 multiplies the correction value and the input signal to add black stretch only to the Y signal. In this way, if the gain of the low luminance part is increased only for the Y signal, the circuit scale can be reduced. However, in this case, only the luminance is increased, so that the displayed image looks slightly whitish.

  Therefore, by configuring the black stretch circuit 5 as shown in FIG. 14, gain correction can be performed even if the color difference signals Cb and Cr are increased. Hereinafter, each component in the figure will be described. The same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted. The adders 33 and 36 add 1 to the input signal and output it. The multiplier 34 multiplies the Y signal by the correction value and outputs the multiplication result. The multiplier 35 multiplies the output of the adder 14 and the coefficient Cgain. The multiplier 37cb multiplies the output of the adder 36 and the Cb signal and outputs the multiplication result. The multiplier 37cr multiplies the output of the adder 36 and the Cr signal and outputs the multiplication result.

Next, the operation of the black stretch circuit 5 configured as described above will be described. Here, the output _{D 1} and _{D 2} of the selector 19 is similar to FIG. 2, the output _{D 3} is different from the value _{D 3} in FIG. 13. The output of the selector 19 with respect to the output of the comparator 18 is as follows.
If the region where the luminance belongs inputted Y signal is an area I outputs 0 to D ₁ to 0, D ₂ to 0, D _3.
If the region where the luminance of the input Y signal belongs an area II outputs 0 a, a _{D 3} to _{D 1} -Xa, the _{D 2.}
If the region where the luminance belongs inputted Y signal is an area III is, -Xc to _{D 1,} and outputs the 2A -a, a _{D 3} to _{D 2.}
If the region where the luminance belongs inputted Y signal is an area IV is, -Xc to _{D 1,} and outputs the 2A -b, the _{D 3} to _{D 2.}
If the region where the luminance belongs inputted Y signal is an area V outputs 0 b, the _{D 3} to _{D 1} -Xe, the _{D 2.}
If the region where the luminance belongs inputted Y signal is area VI outputs 0 to _{D 1} to 0, _{D 2} to 0, _{D 3.}

  The operations of the adders 11 and 14, the square circuit 12, and the multiplier 13 are the same as in FIG. 2, but the value of the coefficient D3 output from the selector 19 to the adder 14 is one less. As a result, the output of the adder 14 has characteristics as indicated by the solid line fy in FIG. When the adder 33 adds +1 to the output result of the adder 14, the output of the adder 33 has characteristics as indicated by the solid line Fy in FIG. 15, and a correction gain is obtained. The multiplier 34 multiplies the Y signal by this correction value to correct the luminance component.

  The multiplier 35 multiplies the correction gain difference output from the adder 14 by the coefficient Cgain set by the controller 8 for the color difference signal. The output of the multiplier 35 has characteristics as indicated by the broken line fc in FIG. 15, and the correction amount can be increased in this way. The adder 36 adds +1 to the output of the multiplier 35. The output of the adder 36 has characteristics as indicated by a broken line Fc in FIG. As a result, a correction coefficient for the chrominance component expressed by the correction characteristic Fc larger than the luminance correction value Fy is obtained, and the multipliers 37cb and 37cr multiply the chrominance signals Cb and Cr by the correction coefficient, respectively. Can be raised.

  When the coefficient Cgain input to the multiplier 35 is made larger than 1, the color difference component can be corrected more strongly than the luminance component. In a dark part of the human eye, a rod-shaped body that detects only light and dark is more dominant than a cone that detects color, and the sensitivity to the color is reduced, so the color appears to fade. Therefore, the black stretch circuit 5 in FIG. 14 is configured such that the amplification factor (expansion factor) of the color difference component can be made higher than the amplification factor of the luminance component in the low luminance region. With this configuration, it is possible to further enhance the saturation at the luminance to be corrected and improve the visibility.

  As described above, in the present embodiment, in the input / output characteristics indicating the relationship between the input signal and the output signal of the video signal, the controller 8 sets a low luminance region where the luminance of the input signal is Xa or more and Xe or less. In addition, four regions (area II, area III, area IV, and area V) are set for the low luminance region, and the low luminance side and high luminance side regions (area I and area) of the low luminance region are set. Six areas including VI) are set. Further, the controller 8 sets the relationship between the input signal and the output signal by a quadratic function in each of the four areas of the low luminance area, and sets the coefficient of the quadratic function.

  The comparator 18 determines which of the six regions including the four regions of the low luminance region the luminance of the input video signal belongs, and the selector 19 selects a coefficient based on the determination result. Output. In FIG. 2, multipliers 15r, 15g, and 15b multiply the input signal by a correction value to expand the luminance of the input signal and output the multiplication result as an output signal. This makes it possible to obtain a correction curve that smoothly connects to the straight line of the region that is not corrected on the left and right sides of the low luminance region in the input / output characteristics, so that correction of the low luminance region of the video signal can be performed without generating a pseudo contour. It can be performed. Further, by appropriately setting the low luminance region, it is possible to suppress amplification of noise near black.

  Further, as shown in FIG. 13, the luminance signal may be corrected in the low luminance region. In this case, the circuit scale can be kept small. Further, as shown in FIG. 14, the low luminance region can be corrected separately for the luminance component and the color difference component. Furthermore, in the black stretch circuit 5 shown in FIG. 14, visibility at low luminance can be improved by making the correction for the color difference component in the low luminance region stronger than the correction for the luminance component.

  Further, instead of multiplying the input signal by the correction coefficient, a configuration in which the correction coefficient is added to the input signal as shown in FIG.

  In addition, the test signal generator 20 generates a test signal during the blanking period of the video signal, and the check circuit 22 checks whether there is a defect in the extension of the brightness of the test signal, thereby increasing the brightness of the subject. However, it is possible to detect the occurrence of a reversal phenomenon in which the brightness decreases on the display. Further, when a failure is found by the check circuit 22, the controller 8 changes the set value to set the quadratic function again, and the check circuit 22 checks again whether there is a failure until the reverse phenomenon does not occur. By repeating this operation, it is possible to prevent the reverse phenomenon from occurring.

  Next, a second embodiment of the present invention will be described. In the present embodiment, a correction curve is created using a function of fourth order or higher. First, a method for correcting a low luminance portion of a video signal according to the present embodiment will be described. Consider the characteristics of the curve to which correction is applied. FIG. 17A shows correction characteristics (relationship between an input signal before correction and an output signal after correction), and FIG. 17B shows a correction coefficient f (gain, correction function, correction) for determining a correction amount. Value). FIG. 3C shows the differential characteristic of the correction coefficient f.

As shown in the graph of FIG. 17B, the slope of the correction coefficient f is 0 when X = Xa, and f increases as X increases. However, when X = Xb, the slope becomes 0 again and X increases. As f decreases, the slope of f becomes 0 again when X = Xc. Therefore, a graph obtained by differentiating the correction coefficient f has a form as shown in FIG. Here, when the differential coefficient f ′ of f in the range from X = Xa to X = Xc in the graph of FIG.
f ′ = α (X−Xa) (X−Xb) (X−Xc) (13)
Here, α is an arbitrary expression other than 0. Further, since the value of the correction coefficient f at X = Xa and the value at X = Xc are equal to 1, the integral value of f ′ between X = Xa and X = Xc is 0. That is,

  Here, when this integral is solved with α as a constant,

  Thus, it can be seen that Xb should be the midpoint (average value) of Xa and Xc. Therefore, if Xa and Xc for determining the black stretch range are determined, the equation (13) is integrated, and the correction intensity α is determined so that f becomes 1 when X = Xa, the input luminance X of 4 The correction coefficient f can be obtained by the following equation. The correction coefficient f is expressed by the following equation. Here, α is a positive number.

Next, the configuration of the black stretch circuit 5 that realizes the above concept will be described. FIG. 16 is a block diagram showing the configuration of the black stretch circuit 5 according to the present embodiment. Hereinafter, each component in the figure will be described. Since the functions of the test signal generator 20 and the check circuit 22 are the same as those in the first embodiment, description thereof will be omitted. The multiplier 51 multiplies the outputs of _{D 4} and the selector 63 of the selector 62. The adder 52 adds the output of the output _{D 3} of the selector 62 multiplier 51.

The multiplier 53 multiplies the output of the adder 52 and the output of the selector 63. The adder 54 adds the outputs of _{D 2} and the multiplier 53 of the selector 62. The multiplier 55 multiplies the output of the adder 54 and the output of the selector 63. The adder 56 adds the output D _{1 of the} selector 62 and the output of the multiplier 55. Multiplier 57 multiplies the output of adder 56 and the output of selector 63. The adder 58 multiplies the output D _{0 of the} selector 62 and the output of the multiplier 57. The multiplier 59 multiplies the output of the output _{D 5} and the adder 58 of the selector 62. The multiplier 60 multiplies the output of the multiplier 59 and the output of the selector 63 and outputs the multiplication result as an output signal.

The comparator 61 compares Xa and Xc set by the controller 8 with the luminance of the signal output from the selector 63, and the luminance of the signal is a low luminance region (luminance is equal to or higher than Xa) in a correction range. Xc or lower area), and the determination result is notified to the selector 62. The selector 62 outputs the set values A _{0 to} A ₄ and α set by the controller 8 as outputs D _{0 to} D ₅ based on the determination result by the comparator 61. The selector 63 outputs a signal input from the test signal generator 20 during the blanking period of the video signal, and outputs a luminance signal (Y signal) otherwise. The controller 8 sets the values of the set values A _{0 to} A ₄ according to the following [Equation 6] to [Equation 10] based on the result of the input by the user via the user I / F 9, the initial value set in advance, or the like. Set as follows.

Next, the operation of the black stretch circuit 5 configured as described above will be described. In the following description, the operation when it is not the blanking period will be described. Since the operation during the blanking period is the same as in the first embodiment, a description thereof will be omitted. The controller 8 sets the values of Xa, Xc and α in advance and sets the values of A _{0 to} A ₄ . The selector 63 outputs the input luminance signal. The comparator 61 compares Xa and Xc, which are values indicating the correction range (low luminance region) set by the controller 8, with the luminance of the signal to determine whether or not the luminance of the signal belongs to the low luminance region. The result is output to the selector 62. Result of the determination by the comparator 61, when the luminance of the luminance signal belongs to a low luminance area, the selector 62 outputs the set value A ₀ to A _4, which is set by the controller 8 as the output D ₀ to D ₄ respectively , Α is output as output D ₅ . As a result of the determination by the comparator 61, when the luminance of the luminance signal does not belong to the low luminance area, the selector 62 outputs D ₀ and D ₅ and 1, all other outputs D ₁ to D ₄ 0 and To do.

  The multipliers 51, 53, 55, 57, 59, 60 and the adders 52, 54, 56, 58 perform the operation described above. In FIG. 16, the correction strength α is finally multiplied so that calculation of the coefficient is reduced as much as possible even if the correction strength α is changed. With such a configuration, as can be seen from [Equation 5], only the constant term needs to be recalculated.

With the above configuration, an imaging apparatus that can smoothly correct the gradation characteristics of an arbitrary range without dividing the correction range into a plurality of areas can be obtained. In the present embodiment, only the luminance component is corrected. However, similarly to the first embodiment, the color difference component may be corrected in the same manner. Further, although the black stretch function is realized by multiplying the input coefficient by the correction coefficient, the correction coefficient may be added to the input signal as in the first embodiment. In this case, the correction coefficient f is the following [Equation 11], the settings of A _{1 to} A ₄ are the above [Equation 6] to [Equation 9], and the setting of A ₀ is the following [Equation 12]. And Further, the multiplier 60 may be changed to an adder.

  As described above, in the present embodiment, in the input / output characteristics indicating the relationship between the input signal and the output signal of the video signal, the controller 8 sets a low luminance region where the luminance of the input signal is Xa or more and Xc or less. To do. Further, the controller 8 sets the relationship between the input signal and the output signal by a quartic function in the low luminance region, and sets the coefficient of the quartic function.

  The comparator 61 determines whether the luminance of the input video signal belongs to the low luminance region, and the selector 62 selects and outputs a coefficient based on the determination result. In FIG. 16, a multiplier 60 multiplies the input signal by a correction value to expand the luminance of the input signal and outputs the multiplication result as an output signal. As described above, in the input / output characteristics, it is possible to obtain a correction curve that smoothly connects with a straight line of a region where correction is not performed on the left and right of the low luminance region, so that noise near black can be amplified without generating a pseudo contour. The low luminance region of the video signal can be corrected while holding down.

  As described above, the embodiments of the present invention have been described in detail with reference to the drawings, but the specific configuration is not limited to these embodiments, and includes design changes and the like within a scope not departing from the gist of the present invention. It is.

1 is a block diagram illustrating a configuration of an imaging apparatus according to a first embodiment of the present invention. It is a block diagram which shows the structure of the black stretch circuit by the embodiment. FIG. 5 is a reference diagram for explaining a method for correcting a low luminance region of a video signal according to the embodiment. In the same embodiment, it is a reference figure which shows the change of the gradation characteristic of the video signal at the time of adjusting a control value. In the same embodiment, it is a reference figure which shows the change of the gradation characteristic of the video signal at the time of adjusting a control value. In the same embodiment, it is a reference figure which shows the change of the gradation characteristic of the video signal at the time of adjusting a control value. In the same embodiment, it is a reference figure which shows the change of the gradation characteristic of the video signal when a control value is adjusted. FIG. 5 is a reference diagram for explaining a tone reversal phenomenon in the embodiment. It is a block diagram which shows the other structural example of the black stretch circuit by the embodiment. It is a block diagram which shows the other structural example of the black stretch circuit by the embodiment. It is a reference diagram for explaining another correction method of the low luminance region of the video signal according to the embodiment. It is a block diagram which shows the other structural example of the imaging device by the embodiment. It is a block diagram which shows the other structural example of the black stretch circuit by the embodiment. It is a block diagram which shows the other structural example of the black stretch circuit by the embodiment. 5 is a reference diagram illustrating correction coefficients for luminance components and color difference components in the embodiment. FIG. It is a block diagram which shows the structure of the black stretch circuit by the 2nd Embodiment of this invention. FIG. 5 is a reference diagram for explaining a method for correcting a low luminance region of a video signal according to the embodiment. It is a reference figure which shows an example of the relationship between the level of the input signal in the conventional gradation correction | amendment, and the level of an output signal. It is a reference figure which shows an example of the relationship between the level of the input signal in the conventional gradation correction | amendment, and the level of an output signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Lens, 2 ... Sensor, 3 ... ADC, 4 ... Gain adjustment circuit, 5 ... Black stretch circuit (luminance expansion means), 6 ... Signal processing circuit, 7 ... DAC, 8... Controller (area setting means, function setting means), 9... User I / F, 10... Luminance generation circuit, 11, 14, 26 r, 26 g, 26 b, 33, 36, 52 , 54, 56, 58... Adder, 12... Square circuit, 13, 15r, 15g, 15b, 26r, 26g, 26b, 34, 35, 37, 37cb, 37cr, 51, 53, 55, 57, 59, 60... Multiplier, 18, 61... Comparator (luminance discrimination means), 19, 62, 63... Selector, 20 .. test signal generator (signal generation means), 22 ... Check circuit (inspection means).

Claims (11)

In an imaging apparatus that captures an image of a subject, generates a video signal, and performs a luminance expansion process that expands the luminance of a low-luminance region of the video signal.
In the graph showing the relationship between the input signal that is the video signal before the luminance expansion processing and the output signal that is the video signal after the luminance expansion processing, the luminance of the input signal is equal to or higher than the first luminance value and the second An area setting means for setting the low luminance area that is less than the luminance value and setting four areas for the low luminance area;
Function setting means for setting a relationship between the input signal and the output signal by a quadratic function in each of the four regions of the low luminance region;
Luminance determining means for determining whether the luminance of the input signal belongs to any one of the four regions of the low luminance region;
When the luminance determining unit determines that the luminance of the input signal belongs to one of the four regions, the 2 set by the function setting unit according to the region to which the luminance of the input signal belongs A luminance expansion means for expanding the luminance of the input signal based on a next function;
An imaging apparatus comprising:   When the low luminance area is a first area, a second area, a third area, and a fourth area in order from the low luminance side, the area at the level of the input signal of the area composed of the first area and the second area The imaging apparatus according to claim 1, wherein a width differs from a region width at a level of the input signal in a region including the third region and the fourth region. The region setting means includes a first region (Xa ≦ X <Xb when the luminance of the input signal is X), a second region (Xb ≦ X <Xc), and a third region (Xc ≦ X <Xd). , And the fourth region (Xd ≦ X <Xe) are set as the low luminance region,
The function setting means, when A is a positive number and a = A / (Xb−Xa) ² and b = A / (Xd−Xc) ² , the input signal of each region of the low luminance region The correction function f for the brightness of
For the first region, f = a (X−Xa) ² +1,
For the second region, f = −a (Xc−X) ² + 2A + 1,
For the third region, f = −b (X−Xc) ² + 2A + 1,
For the fourth region, f = b (Xe−X) ² +1,
Set by the formula
The luminance expansion unit converts the value of the correction function f in the luminance of the input signal to the luminance of the input signal when the luminance determining unit determines that the luminance of the input signal belongs to the low luminance region. The imaging apparatus according to claim 1, wherein the luminance of the input signal is expanded by multiplication. The region setting means includes a first region (Xa ≦ X <Xb when the luminance of the input signal is X), a second region (Xb ≦ X <Xc), and a third region (Xc ≦ X <Xd). , And the fourth region (Xd ≦ X <Xe) are set as the low luminance region,
The function setting means, when A is a positive number and a = A / (Xb−Xa) ² and b = A / (Xd−Xc) ² , the input signal of each region of the low luminance region The correction function f for the brightness of
For the first region, f = a (X−Xa) ² ,
For the second region, f = −a (Xc−X) ² + 2A,
For the third region, f = −b (X−Xc) ² + 2A,
For the fourth region, f = b (Xe−X) ² ,
Set by the formula
The luminance expansion unit converts the value of the correction function f in the luminance of the input signal to the luminance of the input signal when the luminance determining unit determines that the luminance of the input signal belongs to the low luminance region. The imaging apparatus according to claim 1, wherein the luminance of the video signal is expanded by adding. In an imaging apparatus that captures an image of a subject, generates a video signal, and performs a luminance expansion process that expands the luminance of a low-luminance region of the video signal.
In the graph showing the relationship between the input signal that is the video signal before the luminance expansion processing and the output signal that is the video signal after the luminance expansion processing, the luminance of the input signal is equal to or higher than the first luminance value and the second Area setting means for setting a low luminance area that is less than the luminance value;
Function setting means for setting a relationship between the input signal and the output signal by a quartic function in the low luminance region;
Luminance determining means for determining whether the luminance of the video signal generated by the imaging means belongs to the low luminance region;
A luminance that expands the luminance of the video signal based on the quartic function set by the function setting unit when the luminance determination unit determines that the luminance of the video signal belongs to the low luminance region Stretching means;
An imaging apparatus comprising: The region setting means sets a region where Xa ≦ X <Xc as the low luminance region when the luminance of the input signal is X,
The function setting means sets a correction function f for the luminance of the input signal in the low luminance region when α is a positive number,

Set by the formula
The luminance expansion unit converts the value of the correction function f in the luminance of the input signal to the luminance of the input signal when the luminance determining unit determines that the luminance of the input signal belongs to the low luminance region. The image pickup apparatus according to claim 5, wherein the luminance of the video signal is expanded by multiplication. The region setting means sets a region where Xa ≦ X <Xc as the low luminance region when the luminance of the input signal is X,
The function setting means sets a correction function f for the luminance of the input signal in the low luminance region when α is a positive number,

Set by the formula
The luminance expansion unit converts the value of the correction function f in the luminance of the input signal to the luminance of the input signal when the luminance determining unit determines that the luminance of the input signal belongs to the low luminance region. The imaging apparatus according to claim 5, wherein the luminance of the video signal is expanded by adding.   6. The luminance expansion means further expands a color difference component in the low luminance region of the video signal in accordance with a luminance expansion amount in the low luminance region of the video signal. The imaging device described in 1.   9. The expansion amount of the color difference component in the low luminance region of the video signal, which is expanded by the luminance expansion unit, is larger than the luminance expansion amount in the low luminance region of the video signal. The imaging device described in 1. A signal generation means for generating a confirmation signal for confirming the luminance expansion by the luminance expansion means during the blanking period of the video signal generated by the imaging means, and outputting the confirmation signal to the luminance expansion means; ,
Based on the confirmation signal after processing performed by the luminance expansion unit and the confirmation signal before processing generated by the signal generation unit, a problem in the expansion of the luminance of the confirmation signal by the luminance expansion unit occurs. An inspection means for inspecting presence or absence;
The image pickup apparatus according to claim 1, further comprising: When a malfunction is detected by the inspection unit, the function setting unit sets again a function indicating a relationship between the input signal and the output signal,
The signal generating means generates the confirmation signal during a blanking period of the video signal and outputs the confirmation signal to the luminance expansion means,
The imaging apparatus according to claim 10, wherein the inspecting unit inspects again whether there is a defect in the luminance expansion of the confirmation signal by the luminance expansion unit.

Images