JP3575113B2 - Film image processing method - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a film image processing method, and more particularly to a film image processing method for performing image processing according to the exposure state of a film image.
[0002]
[Prior art]
Conventionally, as a film image processing method for adjusting the white balance and black balance of an image signal obtained by capturing a film image, a reference maximum value and a reference minimum value are obtained from the image signal, and these reference maximum value and reference minimum value are obtained. There is a method in which an offset and a white balance gain are obtained so that each has a predetermined gradation, and the offset and the gain of the image signal are controlled by the offset and the white balance gain.
[0003]
The reference maximum value and the reference minimum value are obtained by obtaining a histogram for each gradation of the image signal, and determining the reference maximum value and the reference minimum value from the histogram.
[0004]
[Problems to be solved by the invention]
By the way, according to the above-mentioned film image processing method, there is a problem that tone reproduction is inferior in a film image of extreme underexposure and the image quality becomes dark as a whole.
That is, a film image (negative image) of extreme underexposure has a narrow dynamic range and a small gamma due to the characteristics of the negative. When such a negative image is subjected to negative-positive conversion, the level of the image signal becomes extremely low, and the image has a dark finish due to the negative gamma characteristic. Even if the image signal of a low level is corrected to a good value by the white balance gain, the finish is dark and tone reproduction is poor due to the gamma characteristic of the negative.
[0005]
The present invention has been made in view of such circumstances, and when the exposure state of a film image is underexposure, a film image processing method that reproduces lighter than normal image processing and can reproduce good tone. The purpose is to provide.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention obtains a reference maximum value and a reference minimum value from an image signal obtained by capturing a film image, and sets the reference maximum value and the reference minimum value to predetermined gradations. In a film image processing method for determining an offset and a white balance gain, and processing the image signal based on the determined offset and white balance gain, an exposure state of the film image is determined, and based on the determination result of the exposure state. When the under exposure is determined, the offset and the white balance gain are determined so that the gradation range of the image signal of the film image is narrower than the normal gradation range and deviates toward a brighter one. And
[0007]
Further, the present invention obtains a reference maximum value and a reference minimum value from an image signal obtained by capturing a film image, and sets an offset and a white balance gain such that the reference maximum value and the reference minimum value have predetermined gradations. In the film image processing method for processing the image signal based on the determined offset and white balance gain, the exposure state of the film image is determined, and underexposure is determined based on the determination result of the exposure state. Then, the offset is set to zero so that the gradation range of the image signal of the film image is narrower than the normal gradation range and deviates toward a brighter one, the exposure at the time of capturing the film image is controlled, and the white balance is adjusted. It is characterized in that the gain is determined.
[0008]
[Action]
According to the present invention, the exposure state of the film image is determined, and if it is determined that the film image is underexposed, the gradation range of the image signal of the film image is offset so as to be narrower than the normal gradation range and biased toward a brighter one. And the white balance gain are determined, and the image signal is processed based on the determined offset and white balance gain. As a result, dark portions are reproduced brighter, and good tone reproduction can be realized.
[0009]
Further, according to another aspect of the present invention, the exposure at the time of imaging is controlled without performing the correction by the offset (that is, by setting the offset to zero), so that the film image is reproduced brightly. .
[0010]
【Example】
Hereinafter, preferred embodiments of a film image processing method according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a main block diagram 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, a CCD line sensor 14, an analog amplifier 16, an A / D converter 18, a digital signal processing circuit 20, a motor 31, a capstan 32, and a pinch roller 33. A driving device, a central processing unit (CPU) 40, an integrating block 41, and the like are provided.
[0011]
The light source 10 illuminates the developed negative film 52 drawn out of the film cartridge 50 through an infrared cut filter (not shown), and the transmitted light transmitted through the film 52 passes through the photographing lens 12 to the CCD line sensor 14. An image is formed on the light receiving surface.
The CCD line sensor 14 is provided with a light receiving portion for 1024 pixels in a direction orthogonal to the film transport direction. The image light formed on the light receiving surface of the CCD line sensor 14 is filtered by R, G, B filters. Charges are accumulated in each provided light receiving unit, and are converted into R, G, and B signal charges in an amount corresponding to the intensity of light. The R, G, and B electric charges thus accumulated are transferred to a shift register when a read gate pulse of one line cycle is applied from the CCD drive circuit 15, and then sequentially converted into a voltage signal by a register transfer pulse. Is output. Further, the CCD line sensor 14 is provided with a shutter gate and a shutter drain adjacent to each light receiving section. By driving the shutter gate by a shutter gate pulse, charges accumulated in the light receiving section are discharged to the shutter drain. Can be swept out. That is, the CCD line sensor 14 has a so-called electronic shutter function that can control the electric charge accumulated in the light receiving section in accordance with the shutter gate pulse applied from the CCD drive circuit 15.
[0012]
The R, G, and B voltage signals read from the CCD line sensor 14 are clamped by a CDS clamp (not shown) and applied to the analog amplifier 16, where the gain is controlled as described later. The R, G, and B voltage signals for one frame output from the analog amplifier 16 are converted into dot-sequential R, G, and B digital image signals by the A / D converter 18, and the digital signal processing circuit 20 described later. After performing white balance, black balance, negative / positive inversion, gamma correction, and the like, the image data is stored in an image memory (not shown).
[0013]
The image signal for one frame stored in the image memory is repeatedly read out, converted to an analog signal by a D / A converter, converted to an NTSC composite video signal by an encoder, and output to a monitor TV. Is done. As a result, the film image can be viewed on the monitor TV.
The film drive unit engages with the spool 50A of the film cartridge 50, and drives a forward / reverse rotation of the spool 50A, a film winding unit that winds up the film 52 sent from the film supply unit, Means are provided in the film transport path, and means for transporting the film 52 at a desired speed by sandwiching the film 52 between the capstan 32 driven by the motor 31 and the pinch roller 33. The film supply unit drives the spool 50A of the film cartridge 50 clockwise in FIG. 1 so as to send out the film 52 from the film cartridge 50 until the leading end of the film is wound by the film winding unit. . Further, the CPU 40 can control the forward / reverse rotation, start / stop, and control of the film transport speed by pulse width modulation of the motor 31 through the motor speed / direction control circuit 34.
[0014]
Next, the procedure of image processing will be described with reference to the flowchart shown in FIG. First, when the film cartridge 50 is set in a cartridge storage section (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 section (when the film loading is completed). Then, the following calibration is executed (step S10).
[0015]
That is, the electronic shutter value T of the CCD line sensor 14 ₁ Is set to a specified value (for example, 40%), and an unexposed portion of the film 52 (for example, a negative base at the leading end of the film) is imaged. Then, the gain of the analog amplifier 16 is determined so that each of the R, G, and B signals has a predetermined maximum value (that is, the input range of the A / D converter 18 is maximum).
[0016]
After the gain of the analog amplifier 16 is adjusted in this manner, the film 52 is transported at a constant speed, and a scan (pre-scan) of the film image is executed (step S12). During this pre-scan, dot-sequential R, G, B digital image signals are taken into the integrating block 41 via the CCD line sensor 14, the analog amplifier 16, and the A / D converter 18.
[0017]
The integration block 41 integrates the gradation (8-bit (0-255) gradation in this embodiment) of the digital image signal in a predetermined integration area for each of the R, G, and B digital image signals. Is obtained, and each gradation data of an accumulation area of 5000 to 10000 points per screen is created. Further, the integrating block 41 counts the frequency for each gradation based on the gradation data sequentially created, and the frequency is set to a threshold value TH (total points in this embodiment) set with respect to the total points of the gradation data. If it exceeds 1%), the counting is stopped.
[0018]
That is, the accumulation block 41 creates a simple histogram (a histogram indicated by oblique lines in FIG. 3) in which all the gradations from 0 to 255 are counted up to the maximum threshold TH as shown in FIG. 3 (step S14). . By not counting the frequency exceeding the threshold value TH, the number of bits of the counter can be greatly reduced. Further, a two-dot chain line in FIG. 3 is an original histogram when the total number of points is counted.
[0019]
Then, the integration block 41 sequentially accumulates the frequencies from the smaller gradation of the simple histogram shown in FIG. 3, and determines the gradation when the accumulated frequency coincides with or exceeds the threshold value TH first as a reference minimum. The values are obtained for each of R, G, and B, and the frequencies are sequentially accumulated from the larger gradation of the simple histogram, and the gradation when the accumulated frequency coincides with or exceeds the threshold value TH for the first time is set as a reference maximum. Values are obtained for each of R, G, and B. Further, the maximum value Dmax among R, G, and B is obtained from the simple histogram (step S16).
[0020]
Next, an exposure state such as underexposure or overexposure of the film image is determined based on the reference maximum value and the reference minimum value obtained as described above, and the electronic shutter value T is determined based on the maximum value Dmax. ₂ Is determined (step S18).
That is, the determination of the exposure state uses the reference maximum value, and when this reference maximum value is equal to or less than the reference value for overexposure determination, it is regarded as overexposure. , Underexposure. The determination of the exposure state is not limited to the reference maximum value, and may be performed using, for example, both the reference minimum value and the reference maximum value or the reference minimum value. Further, the exposure state may be determined from the difference or ratio between the maximum value at the time of calibration in step S10 and the reference maximum value or reference minimum value. According to this, even if the maximum value at the time of calibration has a slight variation width (width for determining the analog gain), it is possible to accurately determine the exposure state.
[0021]
Also, the electronic shutter value T ₂ Is determined using the maximum value Dmax obtained from the simple histogram, and the electronic shutter value Tmax is set so that the maximum value Dmax at the time of fine scanning, which will be described later, becomes 255 in the case of an 8-bit system, for example. ₂ To determine. That is, the electronic shutter value T ₂ With the following equation:
T ₂ = 255 * T ₁ / Dmax
Determined by Where T ₁ Is an electronic shutter value at the time of prescan.
[0022]
Next, the reference maximum value R obtained for each of R, G, and B _max, G _max , B _max And the reference minimum value R _min, G _min , B _min Is used to determine the exposure state determination result and the electronic shutter value T ₂ (Step S20).
First, if it is determined that the exposure is not underexposure according to the determination result of the exposure state, the reference maximum value R _max, G _max , B _max And the reference minimum value R _min, G _min , B _min With the electronic shutter value T ₂ Is corrected based on That is, T is smaller during fine scan than during prescan. ₂ / T ₁ Since the exposure amount is increased by a factor of two, the reference maximum value and the reference minimum value are corrected by the amount of change in the electronic shutter value between prescan and fine scan.
[0023]
Then, based on the corrected reference maximum value and reference minimum value, the offset A and the white balance gain (WB gain) A used for adjusting the white balance and the black balance are determined (step S22).
That is, the CPU 40 calculates the offset A for each of R, G, and B based on the finally obtained reference maximum value as follows:
Offset A = 255-G _max … (1)
, And the gain for each of R, G, and B based on the reference maximum value and the reference minimum value,
WB gain A = 255 / (G _max -G _min …… (2)
It is calculated by: Expressions (1) and (2) relate to G, but other color channels are similarly calculated.
[0024]
On the other hand, if it is determined that the exposure is underexposure based on the determination result of the exposure state in step S18, the offset B for correcting the image signal in the bright direction is determined. That is, in the case of a negative image signal, the offset B is a value to be subtracted from the image signal. As shown in the graph of FIG. 4, the larger the degree of underexposure, the larger the offset B is determined. .
[0025]
The reference minimum value is calculated by the following equation:
G _min '= G _min -Offset B ... (3)
And the reference minimum value R _min ′ _, G _min ', B _min ′ With the electronic shutter value T ₂ Is corrected based on
Then, based on the corrected reference minimum value, a WB gain B for each of R, G, and B is calculated by the following equation:
WB gain B = 255 / (255-G _min ') ... (4)
It is calculated by: Expressions (3) and (4) relate to G, but other color channels are similarly calculated.
[0026]
Next, in the case of a negative image which is not underexposed, the electronic shutter value T determined as described above is used. ₂ , The fine scan of the negative image is performed based on the offset A and the WB gain A. In the case of the underexposed negative image, the determined electronic shutter value T ₂ The fine scan of the negative image is executed based on the offset B and the WB gain B (step S24).
[0027]
That is, during the fine scan, the electronic shutter value of the CCD line sensor 14 is set to T ₂ , The film 52 is transported at a constant speed, and a film image scan (fine scan) is executed. During this fine scan, dot-sequential R, G, B digital image signals are output to the digital signal processing circuit 20 via the CCD line sensor 14, analog amplifier 16, and A / D converter 18.
[0028]
Next, signal processing in the digital signal processing circuit 20 will be described.
In the case of a negative image that is not underexposed, the original G output from the A / D converter 18 during the fine scan _org For the following equation:
G1 = G _org + Offset A ... (5)
The digital image signal G1 with the black point offset can be obtained by adding the G offset as shown in FIG. By performing the same processing for the R and B originals, the peak values (black of the positive image) of the R, G and B digital image signals are matched (see FIG. 5A).
[0029]
Subsequently, for the offset digital image signal G1,
G2 = 255-G1 (6)
By performing the calculation of (1), negative / positive inversion is performed (see FIG. 5B).
Next, by multiplying the negative-positive inverted digital image signal G2 by the WB gain A obtained by the equation (2) as shown in the following equation,
G3 = G2 × WB gain A (7)
The other peak values (white of the positive image) of the R, G, and B digital image signals are matched (see FIG. 5C).
[0030]
Finally, gray is adjusted by performing different gamma correction on the R, G, and B digital image signals multiplied by the WB gain, respectively (see FIG. 5D).
On the other hand, in the case of an underexposed negative image, the original G output from the A / D converter 18 during the fine scan. _org For the following equation:
G1 '= G _org -Offset B ... (5) '
The image signal is corrected in the bright direction by subtracting the offset B of G as shown in FIG.
[0031]
Subsequently, the following equation is applied to the offset digital image signal G1 '.
G2 '= 255-G1' (6) '
, Negative-positive inversion is performed.
Next, the negative-positive inverted digital image signal G2 'is multiplied by the WB gain B obtained by the equation (4) as shown in the following equation, whereby
G3 ′ = G2 ′ × WB gain B (7) ′
The other peak values (white of the positive image) of the R, G, B digital image signals are matched.
[0032]
Finally, gray is adjusted by performing different gamma corrections on the R, G, and B digital image signals multiplied by the WB gain.
Next, the gamma correction will be described in more detail.
First, a lookup table (hereinafter, referred to as a base LUT) is prepared as a reference for performing gamma correction (see FIG. 6A).
[0033]
This base LUT has a gamma correction value indicating a difference between a gamma curve of a negative film and a gamma curve (generally γ = 0.45) of a video signal output to a cathode ray tube for each gradation. Is stored in An actual look-up table indicating input / output characteristics (hereinafter referred to as an actual LUT) is obtained by subtracting a base LUT (gamma correction value) from a function y = x as shown in FIG.
[0034]
Further, the base LUT can be changed by multiplying the base LUT by a gamma gain (see FIG. 6B). Thus, by multiplying one base LUT by an appropriate gamma gain, it is possible to obtain an LUT in which a gamma correction value is expanded or compressed for each of R, G, and B. FIG. 6C is an actual LUT for each of R, G, and B obtained by subtracting an LUT whose gamma correction value has been expanded or compressed for each of R, G, and B from the function y = x. .
[0035]
Therefore, when gamma correction is performed on the dot-sequential R, G, B digital image signals subjected to the processing shown in the above-described equations (5) to (7) or (5) ′ to (7) ′, Is a gamma correction unit that sequentially reads gamma correction values from the base LUT based on dot-sequential R, G, B digital image signals, and multiplies the gamma correction values by gamma gains of R, G, and B to appropriately expand or compress gamma. By calculating the correction value and subtracting the gamma correction value expanded or compressed for each color from the point-sequential R, G, B digital image signal, gamma correction can be performed for each color in a point-sequential manner.
[0036]
FIG. 7 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 above-described digital signal processing, and mainly includes adders 21, 22, 23, 25, multipliers 24, 27, and a base LUT 26. The adder 21 receives the dot-sequential R, G, B digital image signal CMPAD from the A / D converter 18. Note that the digital image signal CMPAD flows through R, G, B, and G in a time series according to a predetermined clock.
[0037]
Here, processing of a negative image that is not under-exposure will be described first.
The CPU 40 calculates the offset A (R) for each of R, G, and B as shown in Expressions (1) and (2). _offset, G _offset , B _offset ) And WB gain A (R _wbgain, G _wbgain , B _wbgain ) Is calculated and stored, and the gamma gain (R _gammagain, G _gammagain, B _gammagain ) Is remembered. These offsets and the like are stored for each frame. Then, an offset or the like corresponding to the frame to be scanned is selected by the address decoder 42, and the offsets of R, G, and B are stored in registers 43R, 43G, and 43B by INTDATA in FIG. The gains are stored in registers 44R, 44G, 44B, and the gamma gains of R, G, B are stored in registers 45R, 45G, 45B. These registers hold the R, G, B digital image signals for one frame until they are processed.
[0038]
The offset (R) stored in the registers 43R, 43G, 43B _offset, G _offset , B _offset ) Is applied to the multiplexer 46, and the other input of the multiplexer 46 is applied with a timing signal INTCOLSL0,1 generated by dividing the predetermined clock. The multiplexer 46 selects any one of the three offsets according to the timing signals INTCOLSL0,1 and outputs the selected offset to another input of the adder 22 of the digital signal processing circuit 20.
[0039]
Similarly, the multiplexer 47 outputs three WB gains (R) input from the registers 44R, 44G, and 44B. _wbgain, G _wbgain , B _wbgain ), And outputs the selected WB gain to the multiplier 24. The multiplexer 48 outputs three gamma gains (R) input from the registers 45R, 45G, and 45B. _gammagain, G _gammagain, B _gammagain ) Is selected, and the selected gamma gain is output to the multiplier 27.
[0040]
As described above, the digital image signal CMPAD is applied to the positive input of the adder 21, and the signal indicating the dark current of the CCD line sensor 14 is applied to the negative input. The adder 21 includes the dark current. A dark current component is subtracted from the digital image signal CMPAD, and the digital image signal in which the dark current is corrected is output to the adder 22.
The adder 22 adds the digital image signal whose dark current has been corrected and the offset. As a result, a digital image signal with a black point offset is obtained (see equation (5) and FIG. 5A).
[0041]
The digital image signal with the black point offset output from the adder 22 is applied to the negative input of the adder 23, and the value (255) indicating the white peak level is applied to the positive input of the adder 22. The subtractor 22 subtracts the black point offset digital image signal from 255. As a result, a negative-positive inverted digital image signal is obtained (see Equation (6) and FIG. 5B).
[0042]
Subsequently, the negative-positive inverted digital image signal is applied to the multiplier 24. The WB gain A is added to the other input of the multiplier 24 from the multiplexer 47, and the multiplier 24 multiplies the two inputs to adjust the white of the positive image of the R, G, B digital image signal (Equation 4). (7), see FIG. 5 (C)).
Next, the digital image signal output from the multiplier 24 is applied to the adder 25 and the base LUT 26. The base LUT 26 has a gamma correction value corresponding to the gradation of the input signal as shown in FIG. 6A, reads out the gamma correction value corresponding to the gradation of the input digital image signal, and reads the gamma correction value. The correction value is output to the multiplier 27. A gamma gain is added to the other input of the multiplier 27 from the multiplexer 48. The multiplier 27 generates a gamma correction value for each color of the R, G, and B digital image signals by multiplying the two inputs. This is output to the negative input of the adder 25.
[0043]
The adder 25 subtracts the expanded or compressed gamma correction value for each color from the input R, G, B digital image signal. Thus, the gamma-corrected regular R, G, B digital image signal RGBG _gam Is obtained.
Next, the processing of an underexposed negative image will be described. Only the processing different from the image processing of the negative image which is not under exposure will be described.
[0044]
When processing an underexposed negative image, the subtraction of the offset B shown in Expression (5) 'is performed in the adder 21 for subtracting the dark current. That is, the adder 21 subtracts the signal indicating the dark current component from the digital image signal CMPAD including the dark current component and also subtracts the offset B. On the other hand, the offset added to the adder 22 is set to zero. The reason for this is that the adder 22 can only add an offset. If the adder 22 can add / subtract the offset, the subtraction of the offset B may be performed by the adder 22 instead of the adder 21.
[0045]
The digital image signal output from the adder 22 is subjected to a negative / positive inversion via an adder 23 and then applied to a multiplier 24. A WB gain B (see equation (7) ') is added to the other input of the multiplier 24 from the multiplexer 47. The multiplier 24 multiplies the two inputs to obtain the R, G, B digital image signal. Adjust the white of the positive image.
[0046]
FIG. 8 is a conceptual diagram illustrating image processing of an underexposed negative image, in which a solid line indicates image processing according to the present invention and a dotted line indicates conventional image processing for an underexposed negative image.
As shown in the drawing, the finally obtained image signal is processed so that the gradation range is narrower and brighter than the normal gradation range as shown by the solid line. That is, a dark portion is reproduced brighter, thereby achieving good tone reproduction.
[0047]
In the present embodiment, the offset B is subtracted from the image signal. However, the present invention is not limited to this. The correction may be made. In this case, the offset is set to zero, and the WB gain is obtained based on the reference minimum value.
[0048]
【The invention's effect】
As described above, according to the film image processing method of the present invention, when the exposure state of the film image is underexposed, the gradation range of the image signal of the film image is narrower and brighter than the normal gradation range. Since the offset and the white balance gain are determined so as to deviate, and the image signal is processed based on the determined offset and white balance gain, dark portions are reproduced brighter, and good tone reproduction can be realized.
[Brief description of the drawings]
FIG. 1 is a main block diagram showing an embodiment of a film scanner to which the present invention is applied.
FIG. 2 is a flowchart used to explain an image processing procedure in the film scanner shown in FIG.
FIG. 3 is a histogram used for explaining how to obtain a reference maximum value and a reference minimum value.
FIG. 4 is a graph showing a relationship between a degree of underexposure and an offset B;
5 (A) to 5 (D) are graphs each showing processing contents in each section of the digital signal processing circuit of FIG. 1;
FIGS. 6A to 6C are graphs used to explain a gamma correction method.
FIG. 7 is a block diagram showing a detailed configuration of the digital signal processing circuit of FIG. 1;
FIG. 8 is a conceptual diagram showing image processing of an underexposed negative image.
[Explanation of symbols]
10. Light source
12 ... photographic lens
14 ... CCD line sensor
15 ... CCD drive circuit
18 A / D converter
20 Digital signal processing circuit
21, 22, 23, 25 ... adder
24, 27 ... multiplier
26 ... Base LUT
40 ... Central processing unit (CPU)
41 ... Integration block
50 ... Film cartridge
52 ... Negative film

Claims (4)

A reference maximum value and a reference minimum value are obtained from an image signal obtained by capturing a film image, and an offset and a white balance gain are determined so that the reference maximum value and the reference minimum value have predetermined gradations, respectively. In the film image processing method for processing the image signal based on the offset and white balance gain obtained,
Determine the exposure state of the film image,
When the underexposure is determined based on the determination result of the exposure state, the offset and the white balance gain are adjusted so that the gradation range of the image signal of the film image is narrower and brighter than the normal gradation range. A film image processing method characterized in that it is determined. 2. The film image processing method according to claim 1, wherein the offset for correcting the image signal in a brighter direction is increased as the degree of the determined underexposure increases. A reference maximum value and a reference minimum value are obtained from an image signal obtained by capturing a film image, and an offset and a white balance gain are determined so that the reference maximum value and the reference minimum value have predetermined gradations, respectively. In the film image processing method for processing the image signal based on the offset and white balance gain obtained,
Determine the exposure state of the film image,
If the underexposure is determined based on the determination result of the exposure state, the offset is set to zero so that the gradation range of the image signal of the film image is narrower than a normal gradation range and is shifted toward a brighter one. A film image processing method, wherein exposure at the time of capturing a film image is controlled and a white balance gain is determined. 4. The film image processing method according to claim 1, wherein the determination of the exposure state of the film image is performed based on at least one of the reference maximum value and the reference minimum value.

Images