JP3490832B2 - Imaging device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gamma correction circuit used in an image pickup device such as a video camera or an electronic still camera.

[0002]

2. Description of the Related Art In order to correctly reproduce the gradation of an object, the gamma of the overall characteristics of the entire system from the image pickup device such as a camera to the image output device such as a monitor must be set to 1. However, the photoelectric conversion characteristics of the image sensor,
That is, there are various gamma characteristics, the brightness of the fluorescent screen of the monitor is not linear, and the gamma characteristics are generally about 2.2.

Therefore, in order to set the total gamma characteristic of the entire system to 1, it is necessary to perform gamma correction on a circuit basis. Usually, this gamma correction circuit is disclosed in Japanese Patent Laid-Open No. 1-2.
As disclosed in Japanese Patent Publication No. 92971 (H04N5 / 202), a method of controlling an input / output characteristic based on a gamma correction curve by passing an image pickup output from an image pickup element through a gamma correction circuit is widely used.

As a concrete process of this gamma correction,
For example, if the output values for all input values are stored in the storage means in advance and the output value is determined according to each input value, optimum gamma correction can be performed. Requires a huge amount of capacity. Therefore, FIG.
As shown in, several gamma correction curves (10 in Fig. 2)
By approximating the gamma correction value using any one of the straight lines according to the input level and by approximating the gamma correction value by using the straight line of Will be possible.

By the way, in the gamma correction curve of FIG. 2, in the low input level region where the level of the image pickup signal such as the black level is low, the amplification factor by the gamma correction is set to be large, so that the brightness is close to black and brightness close to black. In this case, the S / N in the area is deteriorated, and the image quality is deteriorated accordingly. Also,
In recent years, as a characteristic of the camera for video camera users,
There is a strong demand for obtaining an imaged screen with a bright and dark effect, that is, a high contrast, and in order to realize this, it is necessary to perform processing that emphasizes suppression of the black level rather than gradation reproducibility.

Therefore, the S / N in the dark portion as described above
In order to prevent the deterioration of the image and to realize a high-contrast screen, it is necessary to modify the gamma curve so that the gamma is intentionally shifted from 1 only in the low input level portion. Specifically, it is ideal that the gamma curve is S-shaped as shown in FIG.

By the way, the S-shaped curve as shown in FIG.
Since the curve is more complicated than the normal gamma curve as shown in FIG. 2, a large number of approximation straight lines are required to approximate the curve by a polygonal line, which complicates the processing.

On the other hand, in recent years, in order to improve the resolution,
As disclosed in Japanese Patent Laid-Open No. 62-92587, color filters for primary colors are arranged in a mosaic pattern and adjacent to each other.
A 2-line simultaneous read-out type CCD imager for simultaneously taking out photoelectric conversion outputs of lines has been favored.

[0009]

When performing the S-curve gamma correction on the above-described 2-line simultaneous readout type CCD imager, an S-curve gamma correction circuit is provided for each photoelectric conversion output of the two lines read simultaneously. It must be added, and complicated processing for realizing the characteristic of the S-curve is required at least at two places.

[0010]

SUMMARY OF THE INVENTION The present invention gamma-corrects each output of two channels of a dual-channel CCD imager with a predetermined characteristic, and outputs a luminance signal and R-Y from both channel outputs after the gamma correction processing. It is characterized in that a BY color difference signal is created, and the obtained luminance signal is amplified based on the characteristic for low luminance suppression that the amplification degree is reduced in the low luminance portion and increased in the high luminance portion. And In particular, R, G, and B color filters are arranged in a mosaic pattern on the light-receiving surface side of the CCD imager, and the B and G signals are obtained for each pixel as one channel output and the other channel output is obtained. The G and R signals are obtained for each pixel. Further, the gamma correction processing is characterized in that it is executed based on the input / output characteristics that suppress the amplification degree as the input level increases.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An apparatus according to an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is an overall block diagram of a camera unit of a video camera which is the device of this embodiment, and FIG. 5 is a block diagram showing a CCD imager. In the figure, reference numeral 12 is a two-line simultaneous readout type CCD imager, which includes M photoelectric conversion photodiodes 14, vertical transfer CCDs 16 and horizontal transfer CCDs 20a and 20b. Vertical transfer CC
D16 is driven by the vertical drive circuit 18, and the horizontal transfer C
The CDs 20a and 20b are driven by the horizontal drive circuit 22.

On the light receiving surface side of the CCD imager 12, color filters 80 for the primary colors of R, G and B shown in FIG.
Are mounted in a mosaic pattern (Bayer array) and horizontally transferred CC
From D20a and 20b, signals with the same number of lines as the vertical pixel number 480 are read out. That is, the CCD imager 12 has a dual channel structure for improving the resolution by reading the data of all pixels once in one field without mixing the charges of two vertical pixels.

The operation of this CCD imager operates according to the field storage mode. That is, the charges accumulated in the photodiode 14 are read out to the vertical transfer CCD 16 once in one field. After that, vertical transfer CC
Two vertical transfer pulses are continuously supplied to D16, and
Once for H, charges for 2 lines are transferred horizontally for each CCD
20a20b are simultaneously transferred. Horizontal transfer CCD 20
a and 20b are each 1H according to the horizontal transfer clock.
The electric charge for one line is output to. That is, horizontal transfer CCD
The output of the odd-numbered lines such as 1, 3, 5, ... Is output as the first channel output from the 20a, and the horizontal transfer CCD 2
From 0b, outputs of even lines such as 2, 4, 6, ... Are output as the second channel output.

Here, since the color filters are arranged in the Bayer array, B as the first channel output,
G, B, G ..., B (blue) signal and G (green) signal are alternately output for each pixel. Similarly, as the second channel output, G, R, G, R ... And G signals and R (red) signals are alternately output for each pixel.

The thus-output image pickup output for each channel is subjected to noise removal by the first and second CDS circuits 1 and 2, respectively, and is made to have an optimum amplitude by the well-known AGC circuits 3 and 4, and then A / The signals are converted into digital signals by the D converters 5 and 6, and are input to the first and second gamma correction circuits 7 and 8, respectively.

The first gamma correction circuit 7 has a characteristic shown by a well-known gamma curve. As a concrete process, the gamma curve is approximated by ten straight lines Q1 to Q10 as shown in FIG. , The input imaging signal level is 0 to e1
Output level is determined on the basis of the straight line Q1 during the period between,
In addition, the output level is determined based on the straight line Q3 and the ten straight lines between e2 and e3. By approximating the gamma curve with several polygonal lines in this way, the B and G signals alternately input for each pixel as the input image pickup signal are amplified by the amplification degree based on the gamma curve and output.

The second gamma correction circuit 8 also has the same gamma curve characteristic as the first gamma correction circuit 7, and the G and R signals alternately input for each pixel have the same characteristics as shown in FIG. It will be amplified and output depending on the characteristics.

As described above, each color signal subjected to gamma correction with the same characteristics by the first and second gamma correction circuits 7 and 8 is input to the color separation circuit 9 shown in FIG. Used for signal generation. That is, the first channel output is supplied to the selection circuit 48 as D0 and D2 directly or via the 1H delay circuit 47, and the second channel output is directly or via the 1H delay circuit 46 to D1 and D3, respectively. Is supplied to the selection circuit 48.
The 1H delay circuits 46 and 47 change the input imaging output to 1
It is a memory capable of storing for H period, and an imaging output delayed by 1H is obtained by passing through this circuit. The writing and reading of the signal to and from the 1H delay circuit are executed in synchronization with the horizontal transfer by the horizontal transfer CCDs 20a and 20b.

The selection circuit 48 selects a digital signal for three lines from digital signals for four adjacent lines of D0 to D3 according to whether the field to be processed is an odd field or an even field. In the odd field, each of D1 to D3 is output as L0 to L2, and in the even field, each of D0 to D2 is L.
It is output as 0 to L2.

The three outputs L0 to L2 of the selection circuit 48 are directly input to the interpolation calculation circuit 51 and the delay circuit 4
9 and the output of the delay circuit 49 is further input to the delay circuit 50.
Entered in. Here, the delay circuits 49 and 50 both have a delay time of one pixel, and the outputs of both delay circuits 49 and 50 are input to the interpolation calculation circuit 51. Therefore, the interpolation calculation circuit 51 has three consecutive pixels of three adjacent lines,
That is, signals for a total of 9 pixels are input.

In the CCD imager 12, color filters of three primary colors are arranged in a mosaic pattern, and any one of R, G, and B is selected from any pixel.
Since only one signal can be obtained, the interpolation calculation circuit 51 interpolates the other two color signals from surrounding pixels.
At this time, the relationship between the array of pixels on the CCD imager 12 and the selected pixel is expressed as shown in FIG. As described above, since the line signals D1 to D3 are selected in the odd field, the pattern of the odd-numbered pixels is expressed as shown in FIG. 7B. The pattern of even-numbered pixels is as shown in FIG. On the other hand, since the line signals D0 to D2 are selected in the even field, the pattern of the odd-numbered pixels is as shown in FIG.
The pattern of even-numbered pixels is represented as shown in FIG. It should be noted that FIG. 7A schematically shows an arrangement of some pixels on the CCD imager 12.

As is apparent from FIG. 7, whether the field to be processed is an odd field or an even field, and whether the pixel to be processed is an odd number or an even number based on various timing signals. Is determined, one of the pixel patterns shown in FIGS. 7B to 7E is determined. Therefore, in the case of FIG. 7B, for example, the interpolation calculation circuit 51 outputs the G signal as it is because the G signal is obtained from the central pixel, and the R signal is obtained from the upper and lower two pixels of the central column. Therefore, the signals of these two pixels are averaged and output as the R signal, and since the B signal is obtained from the left and right two pixels of the central row, the signals of these two pixels are averaged and output as the B signal. In the case of FIG. 7C, the R and G signals are obtained by averaging the same color signals of four adjacent pixels. In this manner, the color signals of the two colors in which the pixel to be processed is missing are generated from the signals of the pixels of the same color in the periphery and interpolated, so that the R, G, and B signals of each pixel are output.

In this way, the three primary color signals of each pixel output from the color separation circuit 9 are input to the matrix circuit 10 and the luminance signals Y and RY, BY color difference signals of the corresponding pixel are calculated by a predetermined arithmetic expression. Is created. Specifically, when the signal levels of the R, G, and B primary color signals are denoted by r, g, and b, Y =
0.6g + 0.3b + 0.1r, RY = 0.9r-
0.6g-0.3b, BY = 0.7b-0.6g-
It is created by an arithmetic expression of 0.1r.

Only the luminance signal Y output from the matrix circuit 10 is input to the low input suppression circuit 11 and is amplified based on the amplification degree changed according to the input level. The low input suppression circuit 11 has a characteristic shown by a polygonal line composed of five straight lines Q21, Q22, Q23, Q24, Q25 as shown in FIG. 8, and as the input level decreases, Q25 → Q24 → It becomes possible to suppress the low level input by reducing the amplification degree from Q23 → Q22 → Q21. Thus, with respect to only the luminance signal, noise near the black level is suppressed to suppress the S / N deterioration, and the black level itself is submerged, so that an image with high contrast can be formed.

The luminance signal thus passed through the low input suppression circuit 11 is recorded on the recording medium by the recording circuit (not shown) together with the color difference signals.

With the above-mentioned configuration, the level of the luminance signal is controlled by both the gamma correction circuits 7 and 8 and the low input suppression circuit 11 with a characteristic equivalent to the S-shaped curve of FIG. The gamma correction is executed so that the luminance level is suppressed in the low luminance area to prevent the S / N deterioration in the dark portion and the screen contrast can be increased, and gamma becomes 1 in the areas other than the low luminance area. And again
The normal gamma correction is executed by the gamma correction circuits 7 and 8 for both color difference signals. Therefore, it is possible to increase the size of a circuit that realizes complicated processing for enabling the input / output characteristics of the S-shaped curve or to reduce the load of software.

In the above embodiment, the low input suppression circuit 11
Although the amplification degree is approximated by using the polygonal line in FIG. 8, the present invention is not limited to this and, for example, Q21 to Q24 in which the amplification degree slightly changes.
It is needless to say that in the area (1), the output value corresponding to each input value may be stored in the memory in advance and the output value may be read from the memory and determined for each input.

[0028]

As described above, according to the present invention, in addition to the normal gamma correction processing and the processing for suppressing the level of the low luminance region separately from the normal gamma correction to the 2-channel CCD imager output, in the 2-channel stage, By performing only the gamma correction process and performing the process of suppressing the low level only on the luminance signal created from the two-channel output, the ideal input / output characteristic of the S-shaped curve is substantially given only to the luminance signal. S, each channel of CCD imager
It is not necessary to provide each of the complicated processing means for realizing the input / output characteristics of the curved line, and it is possible to suppress an increase in circuit scale or an increase in software load.

Further, since the optimum gamma correction is applied at the stage of the R, G, B signals, the optimum gamma correction is applied to the color difference signals created from these color signals. Further, the color difference signal can be subjected to optimum gamma correction.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is an input / output characteristic diagram for gamma correction of first and second gamma correction circuits according to an embodiment of the present invention.

FIG. 3 is a diagram showing characteristics of an S-shaped curve according to an embodiment of the present invention.

FIG. 4 is an explanatory diagram of a color filter according to an embodiment of the present invention.

FIG. 5 is an explanatory diagram of a CCD imager 12 according to an embodiment of the present invention.

FIG. 6 is a diagram illustrating a color separation circuit 9 according to an embodiment of the present invention.

FIG. 7 is a diagram illustrating processing in the color separation circuit 9 according to the embodiment of the present invention.

FIG. 8 is a diagram showing input / output characteristics of the low-level suppression circuit 11 according to an embodiment of the present invention.

[Explanation of symbols]

12 CCD imager 7 First gamma correction circuit 8 Second gamma correction circuit 9 color separation circuit 10 Matrix circuit 11 Low input suppression circuit

─────────────────────────────────────────────────── ─── Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H04N 5/14-5/217 H04N 5/30-5/335 H04N 9/04-9/11 H04N 9 / 44-9/78

Claims (3)

(57) [Claims] 1. A CCD imager in which photoelectric conversion elements are arranged in an array and simultaneously outputs adjacent photoelectric conversion element outputs of two lines as first and second channel outputs, and the first channel output with a predetermined characteristic. 1st gamma correction
Gamma correction means, second gamma correction means for gamma-correcting the second channel output with a predetermined characteristic, and a signal for producing a luminance signal and both RY and BY color difference signals from the outputs of both gamma correction means. An image pickup apparatus comprising: a processing unit; and an amplification unit that amplifies a luminance signal obtained from the signal processing unit based on a characteristic for low luminance suppression that reduces an amplification degree in a low luminance portion and increases an amplification degree in a high luminance portion. . 2. A color filter of R, G, and B arranged in a mosaic pattern is mounted on the light receiving surface side of the CCD imager, and the B signal and the G signal are 1 as the first channel output.
The image pickup apparatus according to claim 1, wherein the G signal and the R signal alternately appear for each pixel as the second channel output, and the G signal and the R signal alternately appear for each pixel. 3. The image pickup apparatus according to claim 1, wherein the predetermined characteristic of the first and second gamma correction means is an input / output characteristic that suppresses the amplification degree as the input level increases.