JPH07322282A - Gamma correction method - Google Patents

Abstract

PURPOSE:To avoid a defect in which noise of an image pickup system or a granular state of a film is remarkable even in the case of gamma correction by setting a differentiation value of a curve representing an input output characteristic in a gamma correction circuit to be a prescribed value or below. CONSTITUTION:A CCD line sensor 14 reads an image of a color photo film to be lighted, an A/D converter 18 converts the image into a digital signal and the converted signal is given to a signal processing circuit 20. The circuit 20 conducts white balance, black balance, negative/positive inversion, and gamma correction or the like for each of R, G, B signals. A lookup table storing base correction value for each gradation is prepared for gamma correction. A correction value is obtained by multiplying a gamma gain for each of R, G, B with a gamma correction value read based on point sequential R, G, B digital image signals and gamma correction is applied to each color data by subtracting the correction value. Furthermore, a variable range of the gamma gain to be multiplied is limited to avoid loss of monotonous increasing performance by preventing adjacent differences (differentiation value) from being too high as a lookup table content.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma correction method,
In particular, the present invention relates to a gamma correction method for gamma-correcting a digital image signal using a look-up table.

[0002]

2. Description of the Related Art Conventionally, in a signal processing circuit disclosed in Japanese Patent Laid-Open No. 4-107083, a variable gamma is used in order to compensate for differences in gamma characteristics of a scene and three types of R, G and B color signals. The image signal processing was performed. This signal processing has a power function curve up to a certain input level, and a gamma correction circuit having an input / output characteristic represented by a knee curve having a small slope when the level exceeds a certain level, and the front and rear stages of this gamma correction circuit. A variable gain amplifier circuit is provided for each, and the gamma is changed by adjusting the gain of these variable gain amplifier circuits.

[0003]

However, when the input signal is gamma-corrected by the conventional variable gamma correction circuit, the gain of the variable gain amplification circuit in the preceding stage is increased,
When the gain of the variable gain amplifier circuit in the subsequent stage is reduced, the differential value (slope or difference value between adjacent curves) of the function of the power of the curve showing the final input / output characteristics may be significantly increased. In this case, for gamma correction,
The grain of the negative film and the noise of the signal processing system may be noticeable.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a gamma correction method capable of preventing deterioration of the S / N of a gamma-corrected image signal.

[0005]

In order to achieve the above object, the present invention sets a gamma of an image signal obtained by picking up an original image by setting a gamma correction circuit having a predetermined input / output characteristic in advance. In the gamma correction method for correcting by the gamma correction circuit, the differential value of the curve showing the input / output characteristics of the gamma correction circuit or the difference value between adjacent curves does not become noise with respect to the input image signal. It is characterized in that the input / output characteristics are set.

[0006]

According to the present invention, the curve showing the input / output characteristics in the gamma correction circuit is set so as not to become noise with respect to the input image signal. That is, the input / output characteristics are set so that the differential value of the curve indicating the input / output characteristics or the difference value between adjacent curves is equal to or less than a constant value (maximum value that does not cause noise with respect to the input image signal). There is. As a result, even if the image signal input by the gamma correction circuit having the above input / output characteristics is gamma corrected,
The noise of the image pickup system and the defects that make the grain of the negative film stand out are eliminated.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the gamma correction method according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 is a block diagram of essential parts showing an embodiment of a film scanner to which the present invention is applied. This film scanner mainly includes a light source 10 for illumination, a photographing lens 12, and a CCD line sensor 1.
4, analog amplifier 16, A / D converter 18, digital signal processing circuit 20, motor 31, capstan 32
And a film driving device including a pinch roller 33, a central processing unit (CPU) 40, and the like.

The light source 10 illuminates the developed negative film 52 drawn out from the film cartridge 50 through an infrared cut filter (not shown), and the film 52 is illuminated.
The transmitted light transmitted through is imaged on the light receiving surface of the CCD line sensor 14 via the taking lens 12. The CCD line sensor 14 is placed in the direction 102 orthogonal to the film transport direction.
A light receiving portion for four pixels is provided, and the image light formed on the light receiving surface of the CCD line sensor 14 is provided with R, G, and B filters to accumulate charges in each light receiving portion, and the intensity of the light is increased. It is converted into R, G, and B signal charges in an amount corresponding to the level. The R, G, and B charges thus accumulated are transferred to the shift register when a read gate pulse of one line period applied from the CCD drive circuit 15 is applied, and then sequentially converted into voltage signals by register transfer pulses. Is output. Further, the CCD line sensor 14 is provided with a shutter gate and a shutter drain adjacent to each light receiving portion. By driving this shutter gate with a shutter gate pulse, the electric charge accumulated in the light receiving portion is shutter drained. Can be swept up to. That is, the CCD line sensor 14 has a so-called electronic shutter function capable of controlling the electric charge accumulated in the light receiving portion according to the shutter gate pulse applied from the CCD drive circuit 15.

The R, G, B voltage signals read from the CCD line sensor 14 are clamped by a CDS clamp (not shown) and added to the analog amplifier 16, and the gain is controlled as described later. The R, G, B voltage signals for one frame output from the analog amplifier 16 are dot-sequential R, G, B by the A / D converter 18.
After being converted into a B digital image signal, the digital signal processing circuit 20 performs white balance, black balance, and
Negative / positive inversion, gamma correction, and the like are performed, and the YCC conversion circuit 35 converts the luminance signal Y into the chroma signals C _{r and} C _b . Then, the luminance signal Y and the chroma signals C _{r and} C _b passed through the chroma suppression circuit 36 are stored in an image memory (not shown).

The luminance signal Y and the chroma signals C _r, C _{b for} one frame stored in the image memory are repeatedly read and converted into analog signals by the D / A converter, and then the NTSC system is used by the encoder. Is converted into a composite video signal of and output to the monitor TV. As a result, the film image can be viewed on the monitor TV.

The film drive device engages with the spool 50A of the film cartridge 50, and the spool 50A
A film supply unit that drives the film in the normal / reverse direction, a film winding unit that winds the film 52 sent from the film supply unit, and a capstan that is disposed in the film transport path and that drives the film 52 by a motor 31. It is constituted by a means for sandwiching the film 32 and a pinch roller 33 and conveying the film 32 at a desired speed. The film supply section is a spool 50A of the film cartridge 50.
1 is driven clockwise in FIG. 1 to feed the film 52 from the film cartridge 50 until the leading end of the film is wound by the film winding section.
Further, the CPU 40 controls the motor speed / direction control circuit 34.
Through this, the forward / reverse rotation of the motor 31, the start / stop, and the film transport speed by pulse width modulation can be controlled.

Now, when the film cartridge 50 is set in the cartridge storing portion (not shown), the film 52 is sent out from the film cartridge 50 and the leading end of the film is wound around the winding shaft of the film winding portion (the film loading is When completed), the film 52 is transported at a constant speed. As a result, the film image is scanned, and the dot-sequential R, G, B digital image signals are captured by the integrating block 41 via the CCD line sensor 14, the analog amplifier 16 and the A / D converter 18.

The integration block 41 integrates the gradations (9-bit (0 to 511) gradations in this embodiment) of the digital image signals in a predetermined integration area for each R, G, B digital image signal, The average gradation of the integrated area is obtained, and each gradation data of the integrated area of 5000 to 10000 points is created per screen. Furthermore, the integration block 41 counts the frequency for each gradation based on the gradation data that is sequentially created, and this frequency is a threshold value TH (the total number of points in this embodiment) set for the total number of points of the gradation data. 1%) of the above, the counting is stopped. That is, the integration block 41 creates a simple histogram (histogram shown by diagonal lines in FIG. 2) that counts up to the maximum threshold value TH for all gradations from 0 to 511 as shown in FIG. To do. The number of bits of the counter can be greatly reduced by not counting the frequency that exceeds the threshold TH. The two-dot chain line in FIG. 2 is the original histogram when the total number of points is counted.

The CPU 40 sequentially accumulates frequencies from the smallest gradation of the simple histogram shown in FIG. 2, and the gradation when the accumulated frequency matches or first exceeds the threshold value TH is a reference minimum value. As the R, G, and B values, the frequencies are sequentially accumulated from the one with the highest gradation in the simple histogram, and the gradation when the accumulated frequency matches or first exceeds the threshold TH is the reference maximum value. As R, G, B.

Next, the digital signal processing circuit 20 for performing white balance, black balance, negative / positive inversion, gamma correction, etc. will be described. First, a method of calculating an offset value and a gain amount used to match the white balance and the black balance will be described. The CPU 40 calculates the offset value for each R, G, B based on the reference maximum value obtained for each R, G, B, and also for each R, G, B based on the reference maximum value and the reference minimum value. Calculate the amount of gain. That is, R
_Where R _{ref max is} the reference maximum value and R _{ref min} is the reference minimum value, the offset value and the gain amount are as follows: offset value = 511−R _{ref max} (1) gain amount = 511 / (R _{ref max-} R _{ref min} ) ... Calculated by (2).

The equations (1) and (2) relate to R, but other color channels are calculated in the same manner. Also, here, the R, G, B digital image signals are represented as 9 bits, and 511 is the maximum value thereof. Then, the black point is offset by adding the offset value of R to the original R _org output from the A / D converter 18 at the time of scanning as shown in the following equation: R1 = R _org + offset value (3) The digital image signal R1 can be obtained. By performing the same processing on the G and B originals, the peak values of the R, G, and B digital image signals (black of the positive image) are matched (see FIG. 3A).

Next, the negative / positive inversion is performed on the offset digital image signal R1 by executing the following equation, R2 = 511-R1 (4) (see FIG. 3B). ). Next, the negative-positive inverted digital image signal R2 is multiplied by the gain amount obtained by the equation (2) as shown in the following equation to obtain R3 = R2 × gain amount (5) R, G , B of the B digital image signal are matched with each other (white of the positive image) (see FIG. 3C).

Finally, R, G, B multiplied by the gain amount
Grays are matched by performing different gamma corrections on the digital image signals (see FIG. 3D).
Next, the gamma correction will be described in more detail. First, as shown in FIG. 4, a look-up table (hereinafter referred to as a base LUT) serving as a reference when performing gamma correction is prepared.

In this base LUT, the gamma correction value indicating the difference between the gamma curve of the negative film and the gamma curve (generally γ = 0.45) of the video signal output to the cathode ray tube has each gradation. It is stored for each.
An actual look-up table showing input / output characteristics (hereinafter referred to as an actual LUT) is obtained by subtracting the base LUT (gamma correction value) from the function y = x as shown in FIG.

The base LUT can be changed by multiplying the base LUT by a gamma gain (see FIG. 4B). Thus, by multiplying an appropriate gamma gain from one base LUT, it is possible to obtain an LUT in which the gamma correction value is expanded or compressed for each of R, G, and B. Note that FIG. 4C shows the function y
= X obtained by subtracting the LUT in which the gamma correction values are expanded or compressed for R, G, and B, respectively,
It is an actual LUT for each of G and B.

Therefore, when the white balance and the black balance are adjusted by the above equations (3) to (5) and the gamma correction is performed on the negative-positive inverted dot-sequential R, G, B digital image signals, Gamma correction values are sequentially read out from the base LUT based on dot-sequential R, G, B digital image signals, and the gamma correction values are multiplied by R, G, B gamma gains and expanded or compressed as appropriate. Is obtained, and the gamma correction value expanded or compressed for each color is subtracted from the dot-sequential R, G, B digital image signal to perform the gamma correction for each color in the dot sequence.

By the way, the characteristic of the base LUT is that the gamma correction value has only a single sign (a positive value in this embodiment) as shown in FIG. As a result, when the gamma gain by which the base LUT is multiplied is changed, the actual LUT changes over all gradations as shown in FIG. On the other hand, FIG. 5 (B)
When the gamma correction value has both positive and negative signs as in the base LUT shown in FIG.
This zero-cross point becomes a fixed point (FIG. 5A) that does not change even if any gamma gain is multiplied, and the tone cannot be controlled well.

Further, FIG. 6 shows that from the function y = x to the base LU
It shows the actual LUT from which T is subtracted and the frequency distribution of adjacent difference values of the actual LUT. As shown in the figure, the adjacent difference values of the actual LUT are suppressed below a predetermined value (4 in this embodiment) smaller than the noise level. That is, if the difference values (differential values, slopes) adjacent to each other in the actual LUT are made too large, the noise acts on the imaging system and the graininess of the negative is conspicuous. Therefore, in order to avoid this, the difference as described above is used. The value is limited to improve S / N.

The curve of the actual LUT changes depending on the size of the gamma gain as shown in FIG. 4C, but as described above, the difference value between adjacent LUTs of the actual LUT becomes less than or equal to the predetermined value. It is necessary to limit the variable range of the gamma gain so that Furthermore, as shown in FIG. 7, the actual LUT
The input / output characteristic changes depending on the size of the gamma gain by which the base LUT is multiplied. When the gamma gain exceeds a predetermined value, the monotonic increase property is lost. For example, when the gamma gain is 1.2, the actual LUT shown by the solid line is as shown by the broken line, and the monotonic increase property is maintained, but when the gamma gain is 1.4, the actual LUT shown by the solid line. LUT becomes as shown by the alternate long and short dash line, and monotonicity is lost.

Therefore, in the present invention, the variable range of the gamma gain is set within the range in which the actual LUT monotonically increases, whereby the gamma correction is satisfactorily performed. still,
The value of gamma gain at which monotonic increase is lost is the base LU.
It depends on T. FIG. 8 is a block diagram including the internal configuration of the digital signal processing circuit 20 shown in FIG. The digital signal processing circuit 20 performs the digital signal processing described above, and is mainly composed of adders 21, 22, 24, multipliers 23, 26, and a base LUT 25. The dot-sequential R, G, B digital image signal CMPAD from the A / D converter 18 is input to the adder 21.
The digital image signal CMPAD flows in time series R, G, B, G according to a predetermined clock.

On the other hand, the CPU 40 uses the equations (1) and (2).
As shown in, the offset value (R
_offset, G _offset , B _offset ) and gain amount (R
_wbgain, G _wbgain , B _wbgain ) is calculated and stored, and gamma gain (R _{ga mgain,} G) is calculated for each R, G, B.
_gamgain, B _gamgain ) is memorized. Further, these offset values and the like are stored for each frame. Then, the offset value or the like corresponding to the frame to be scanned is selected by the address decoder 42.
According to INTDATA, the offset values of R, G, B are stored in the registers 43R, 43G, 43B, the gain amounts of R, G, B are stored in the registers 44R, 44G, 44B,
R, G, and B gamma gains are registered in registers 45R, 45G,
45B. Incidentally, these registers hold the R, G, B digital image signals for one frame until they are processed.

The offset values (R _offset, G _{of fset} , B _offset ) stored in the registers 43R, 43G, 43B are added to the multiplexer 46, and the predetermined clock is _distributed to the other input of the multiplexer 46. Timing signals INTCOLSL0, 1 created by making a circle are added. The multiplexer 46 uses the timing signal INTCOLSL
One of the three offset values is selected by 0 and 1, and the selected offset value is output to the other input of the adder 21 of the digital signal processing circuit 20.

Similarly, the multiplexer 47 _selects one gain amount from the three gain amounts (R _wbgain, G _wbgain , B _wbgain ) input from the registers 44R, 44G, 44B, and selects the selected gain amount. Is output to the multiplier 23, and the multiplexer 48
Any one gamma gain is selected from the three gamma _gains (R _gamgain, G _gamgain, B _gamgain ) input from R, 45G, and 45B, and the selected gamma gain is output to the multiplier 26.

On the other hand, as described above, the digital image signal CMPAD is input to the adder 21, and the adder 21 adds the digital image signal CMPAD and the offset value. As a result, a digital image signal with black point offset is obtained (see equation (4), FIG. 3A). The black dot offset digital image signal output from the adder 21 is added to the negative input of the adder 22, and a value (511) indicating the white peak level is added to the positive input of the adder 22,
The adder 22 subtracts the black dot offset digital image signal from 511. As a result, a negative-positive inverted digital image signal is obtained (see formula (5) and FIG. 3 (B)).

Subsequently, the negative-positive inverted digital image signal is applied to the multiplier 23. The gain amount from the multiplexer 47 is added to the other input of the multiplier 23, and the multiplier 23 multiplies the two inputs to obtain R,
The whites of the positive images of the G and B digital image signals are matched (see formula (6), FIG. 3 (C)). Next, the digital image signal output from the multiplier 23 is added to the adder 24 and the base LU.
Added to T25. The base LUT 25 is shown in FIG.
As shown in FIG. 3, it has a gamma correction value corresponding to the gradation of the input signal, reads the gamma correction value corresponding to the gradation of the input digital image signal, and uses this gamma correction value in the multiplier 2
Output to 6. The gamma gain is applied to the other input of the multiplier 26 from the multiplexer 48, and the multiplier 23
Generates a gamma correction value for each color of the R, G, B digital image signals by multiplying the two inputs, and this is added by the adder 2
Output to the negative input of 4.

The adder 24 subtracts the expanded or compressed gamma correction value for each color from the input R, G, B digital image signals. With this, gamma-corrected regular R,
G, B digital image signals RGBG _gam are obtained. still,
Negatives with different exposures have different gradation characteristics with respect to subject brightness and also different gamma characteristics. Therefore, it is necessary to change the base LUT according to the negative exposure amount. Therefore, depending on the exposure amount of the negative, the gamma gain (R
_gamgain, G _{ga mgain,} if _B gamgain) to vary the, it is possible to perform gamma correction corresponding to the exposure amount of the negative.

In the gamma correction method described above, one base LUT is used, and the gamma gain is changed for each of R, G, B to match the gray of the halftone of the R, G, B signals. In the shadow portion, it is difficult to match gray with only gamma gain. This is because the gamma gain is determined so as to match the halftone. As a result, the highlight portion and the shadow portion do not completely coincide with each other, and the coloring caused by the disagreement in the highlight portion gives a bad impression.

Therefore, in the present invention, the following signal processing is performed to reduce coloring in the highlight portion and the shadow portion. That is, the gamma-corrected R output from the digital signal processing circuit 20 of FIG.
The G and B digital image signals are applied to the YCC conversion circuit 35. YCC conversion circuit 35, R to enter, G, based on B digital image signals, R by performing the following calculation, G, luminance signal from the B signal Y and the chroma signal C _r,
Perform YCC conversion of C _b .

Y = {(R / 2 + R / 8) + (G + G / 8) + B / 4} / 2 (6) C _r = K1 (R−G) + K2 (B−G) (7) C _b = K3 (R−G) + K4 (B−G) (8) Note that K1, K2, K3, and K4 are constants. The luminance signal Y and the chroma signals C _r and C _b YCC-converted by the YCC conversion circuit 35 are added to the chroma suppression circuit 36. The chroma suppression circuit 36 is composed of a variable amplifier, and varies the chroma gain according to the gradation of the luminance signal as shown in FIG. That is, in the highlight portion where the luminance signal is large, the chroma gain is smaller than 1, and similarly in the shadow portion where the luminance signal is small, the chroma gain is also smaller than 1, whereby the chroma signal C _r in the highlight portion and the shadow portion is reduced. It _tries to suppress C _b . still,
Except for the highlight portion and the shadow portion, the chroma gain is set to 1 and the input chroma signals C _r and C _b are output as they are.

By suppressing the chroma signals C _r and C _b in this way, it is possible to reduce coloring in the highlight portion and the shadow portion. Next, a method of creating a LUT for a reversal film by using the above-described base LUT for a negative film will be described. In this case, when reading the gamma correction value from the base LUT for the negative film, the gamma correction value is read from the opposite side. For example, when gamma correction values are stored in the base LUT for each gradation of 0 to 511 and the gradation of the image signal of the reversal film is 0, 1, 2, ...
At, the gamma correction values stored corresponding to the gradations of 511, 510, 509 ... Are read out. By reading out in this manner, it is practically possible to read FIG.
It will be read from the base LUT as shown in FIG. The actual LUT of the reversal film is shown in FIG.
As shown in (A), the function y = x is added with the base LUT shown in FIG. 10 (B).

Next, signal processing when a reversal film image is picked up will be described with reference to the digital signal processing circuit 20 of FIG. First, the adder 21 subtracts the R, G, and B offset values from the digital image signal CMPAD to match the black levels. The offset value is a reference minimum value for each of R, G, and B. Then, the black-point-offset digital image signal is passed through the adder 22 for negative-positive inversion as it is. Subsequently, the digital image signal that has passed through the adder 22 is multiplied by the gain amounts of R, G, and B in the multiplier 23, and the white peak is adjusted.

Next, the digital image signal output from the multiplier 23 is added to the adder 24 and the base LUT 25. The base LUT 25 reads out a gamma correction value corresponding to the grayscale of the input digital image signal that has been inverted in black and white, and outputs this gamma correction value to the multiplier 26.
To the other input of the multiplier 26, a gamma gain for adjusting the gray of halftone is added from the multiplexer 48, and the multiplier 26 multiplies two inputs to obtain R,
A gamma correction value for each color of the G and B digital image signals is generated and output to the adder 24.

The adder 24 adds the input R, G, B digital image signals and the expanded or compressed gamma correction value for each color. With this, gamma-corrected regular R,
G, B digital image signals RGBG _gam are obtained. still,
In the above embodiment, the case where the negative film base LUT is applied to the reversal film has been described, but the present invention is not limited to this, and the reversal film base LUT can also be applied to the negative film.

[0039]

As described above, according to the gamma correction method of the present invention, the curve showing the input / output characteristics in the gamma correction circuit is set so as not to be noise with respect to the input image signal, and therefore the input is made. Even if the image signal is gamma-corrected, there is no problem that the noise of the imaging system or the graininess of the negative film becomes conspicuous, and the S / N of the gamma-corrected image signal can be prevented from deteriorating.

[Brief description of drawings]

FIG. 1 is a principal block diagram showing an embodiment of a film scanner to which the present invention is applied.

FIG. 2 is a histogram used to explain how to obtain a reference maximum value and a reference minimum value.

3A to 3D are graphs showing processing contents in respective units of the digital signal processing circuit of FIG.

4A to 4C are graphs used to describe a gamma correction method.

5A and 5B are graphs showing an actual LUT having a fixed point and a base LUT, respectively.

FIG. 6 is a graph showing an actual LUT and a frequency distribution of adjacent difference values of the actual LUT.

FIG. 7 is a graph showing an actual L depending on the magnitude of gamma gain.
It is a graph used for explaining the variation range in which the UT varies.

FIG. 8 is a block diagram showing a detailed configuration of the digital signal processing circuit of FIG.

FIG. 9 is a diagram showing a chroma gain with respect to a luminance level used for explaining the chroma suppressing circuit of FIG. 1;

10A and 10B are an actual LUT and a base L for an image of a reversal film, respectively.
It is a graph which shows UT.

[Explanation of symbols]

 10 ... Light source 12 ... Photographing lens 14 ... CCD line sensor 15 ... CCD drive circuit 18 ... A / D converter 20 ... Digital signal processing circuit 21, 22, 24 ... Adder 23, 26 ... Multiplier 25 ... Base LUT 31 ... Motor 40 ... Central processing unit (CPU) 41 ... Accumulation block 42 ... Address decoder 43R-45B ... Registers 46, 47, 48 ... Multiplexer 50 ... Film cartridge 52 ... Negative film

Claims (4)

[Claims] 1. A gamma correction method in which a gamma correction circuit having a predetermined input / output characteristic is set in advance and the gamma of an image signal obtained by capturing an original image is corrected by the gamma correction circuit. A gamma correction method characterized in that the input / output characteristic is set so that the differential value of the curve showing the input / output characteristic of the gamma correction circuit or the difference value between adjacent curves does not become noise with respect to the input image signal. . 2. The gamma correction circuit comprises a look-up table having the input / output characteristics, and reads output data from the look-up table based on the gradation of a digital image signal obtained by picking up an original image. The gamma correction method according to claim 1, characterized in that. 3. The gamma correction circuit stores a gamma correction value according to the gradation of an input signal in advance, and reads the gamma correction value based on the gradation of a digital image signal obtained by capturing an original image. A lookup table and a circuit for adding or subtracting the gamma correction value read from the base lookup table to the digital image signal, and the input / output for the digital image signal input / output to / from the addition or subtraction circuit. 2. The gamma correction method according to claim 1, wherein the gamma correction method has characteristics. 4. A base for storing the gamma correction value according to the gradation of an input signal in advance, and reading the gamma correction value based on the gradation of a digital image signal obtained by capturing an original image. A look-up table, a multiplication circuit that multiplies a gamma correction value read from the base look-up table by a preset gamma gain, and a gamma correction value that is appropriately expanded or compressed by multiplication with respect to the digital image signal is added or subtracted. 2. A gamma correction method according to claim 1, further comprising a circuit, wherein the digital image signal input to and output from the adding or subtracting circuit has the input / output characteristic.