JPS58186291A - Gradation converting method of color video signal - Google Patents

Abstract

PURPOSE:To make the contrast of a reproduced picture suitable automatically even under the image pickup conditions easily causing excess or deficiency of contrast, such as rear light, home illumination, and field, by controlling gamma of converting function of X<gamma> or an equivalent value so that the frequency of generation of data in a prescribed amplitude of a color video signal is a prescribed value. CONSTITUTION:A data conversion circuit 3 includes a data conversion table 4 of a conversion function (X<gamma>) for contrast correction and a data conversion table 5 of conversion function (Y<gamma>0) (generally, gamma0=0.45) for the substantial gamma correction of nonlinearity of a color picture tube, and an output signal V is obtained. In case of two-gradation equally, the most significant bit of the prescribed sample of an output signal being 0 (1st gradation) is counted at a counter 6 and the count value h0 is applied to a detection control circuit 7. To the circuit 7, the 1st gradation value of the amplitude distribution to be an object and its corresponding value d0 are applied from a terminal 8 for the comparison of h0 and d0. The value Y of the data conversion table 4 is changed gradually so that the both are coincident. Through the negative feedback control system like this, the convergence of h0=d0 is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a tone conversion method for a color video signal, which is applied to improving the contrast of a color video signal.

This invention automatically improves contrast using the amplitude distribution of an input video signal.

General contrast correction will be explained with reference to FIG.

Figure 1 A shows the brightness of the subject L = (a, +m, house ωt)
When this subject is photographed with a video camera, the imaging output shown in FIG. 1B (x = 24 squares ωt) is obtained.
occurs. This is because the video camera has an average value type automatic diaphragm device having the characteristics shown in FIG. 1C. Since this imaging output lacks contrast, the AC component is level-adjusted using the gradation conversion function (y = f(x)) shown in Figure 1, resulting in the converted output as shown in Figure 1E. A video signal is obtained.

In this way, it is possible to automatically correct when there is haze or flare in the shooting location and the blacks appear floating (lack of contrast). Also, there are many shadow areas under home lighting (
When the subject is backlit (excessive contrast), the gradation conversion function shown in FIG. 1F is used.

Figure 2 is a book that explains tone conversion from the frequency distribution of the amplitude of the imaging signal (hereinafter simply referred to as amplitude distribution), so in the figure, φ1(x) (1=o, 1.2) is the imaging signal shows the amplitude distribution of The flat amplitude distribution φ shown in Figure A.

If (X) is assumed to be under proper illumination, the amplitude distribution φI(x) shown in Figure λB is the one when the contrast is insufficient, and the amplitude distribution φ2(x) shown in Figure 2C is the one when the contrast is excessive. It is a matter of the case.

The target value of gradation correction (amplitude distribution under proper illumination ^ (X) is determined, and as shown in Figure Ω F and GK of the same figure, the amplitude distribution after correction is f (z), h (IC). and when,
The conversion functions 7x(x) and /''z(x) shown in the second diagram and E in the same figure, where (?-center=φ0), are the optimum ones to be found.

As mentioned above, in order to easily convert the amplitude distribution of the imaging output to the target amplitude distribution when the cosilast is appropriate, the amplitude distribution of the imaging signal is once changed to a flat distribution. Alternatively, it is conceivable to convert this flat distribution into a target amplitude distribution. This conversion to a flat distribution will be explained.

It is assumed that the amplitude axis of the imaging signal is (0≦t≦/), and the amplitude axis y after conversion is also within the same range.

Further, the amplitude distribution of the imaging signal is assumed to be φ(X), the one after conversion is assumed to be ψ(y), and the target value is assumed to be ψd(y). ψ(y)−ψd
If the conversion function necessary to obtain (y) is y = f (x), then φ (1) dx = ψ d (y) ay
...Song (1) However, it is generally difficult to obtain the gradation conversion function f(x) from equation 1), but it is easy in the case of ψd(y) -/. Tsumashi Y=f(X)−fφ(x)+dx−
-(3) This will be called histogram equalization. Amplitude distribution φ shown in FIGS. 3A and 3B, respectively
(X) becomes K when converted into a flat distribution by the gradation conversion functions shown in C and the same figure, respectively.

Next, let us consider converting the amplitude distribution φ(χ) of the flat distribution shown in Figure A to the amplitude distribution ψd(y)K shown in Figure BK. Here, ψd(y) is expressed as a linear equation,
It is approximated by a square.

ψd(y) = at yi≦y≦yi+1 (1
;Q, 1. "-" m) However-
・Concave (4) T! jdx (y i+1
y engineering) −/ ・Concave curve (5) 1 φ (x) dx = d + Ndy
River − (6) Therefore, dy = di−′
・φ(1)−d(1) −・・−・−(7) In this case, the gradation conversion function f(X)#′i is shown in Fig. C,
It is expressed by the following formula.

f(x) = di-J”φ(x)dx + yl
yi≦y≦y++, (8) In the case of a television camera, complicated gradation correction is not necessary, and as described above, the target amplitude distribution ψd (y) is roughened, etc. Approximating the interval to a rectangular shape is sufficient for practical purposes.

Furthermore, the amplitude distribution φ(1) of the imaging signal may be similarly approximated to a rectangular shape.

In this way, by approximating the amplitude distribution of the imaging signal and the target amplitude distribution to a rectangular shape and using the above-described histogram equalization, a practical method for deriving a tone conversion function can be obtained.

Derivation of the conversion function in the case of (ψd=/) will be explained. First, the amplitude distribution φ(X) of the imaging signal is approximated as follows. However, 1. , ht is the frequency of the equally spaced histogram.

φ(X)±bi xIWx≦xt++ −
--(9) If it is a rectangular approximation with equal intervals, 1m is the number of classes in the histogram, and Jy and hi are (
14) Satisfies the formula.

Next, ψd(y) has some degree of Hakusan (ψd(y)ki/)
I will explain the tricks to prevent this from happening. At this time, BX
)? , it is too troublesome to obtain it directly, so we break it down into the following three steps.

(1) As mentioned above, the imaging output φ(X) is approximated as a rectangle as shown in FIG. 5A, and the S
The conversion function /E (X) shown in Figure B is determined by equation (13).

(2) The amplitude distribution of φ(u)=/ is transformed into the target amplitude distribution ψd (y) by rectangular approximation as shown in Figure S.
A conversion function f9(u) shown in the S-th diagram to be converted is determined in advance.

(3) The conversion function J(1) for converting the imaging output φ(X) to ψd(y) is y = , /'(x) = fD(u) = fD(/
, (x) ) (15) J'(X), which is the product of the functions shown in Figures B and S, is as shown in Figure 5E.

Note that fD(x) can be obtained by substituting φ(x)=/ into equation (7), or by returning to equation (1) and substituting φ(x)=/. In the case of Tawaraya, the conversion formula from y to
...(16). Since Equation (16) is a function given ψd(y), it can be calculated in advance and converted into a data chisol.

The above-described gradation conversion method has a problem in that no particular consideration is given to color reproduction. The object of the present invention is to realize a gradation conversion method that satisfies the following requirements for one-color reproduction. ' The first requirement is that the hues of the primary and complementary color axes do not change due to gradation conversion.

The first requirement is that subjects with the same hue and saturation that differ only in brightness (such as colored grayscale) have the same hue even after gradation conversion.

The saturation level is the same.

For the first requirement, R (red) of the imaging output.

The same gradation correction is applied to each color signal of O (green) and B (f).

The first necessary condition is that J・<
This is satisfied by using (17). To explain this point,
The three primary color components of any color are each expressed as a ratio to the red component as shown below.

Each color signal is converted by a gradation conversion function of f(x)-Xγ. Also, a color that differs only in brightness from the above will be converted into a single-color gradation. As is clear from this equation (19) and (21), the ratio of the three primary color signals is the same regardless of k, that is, the same hue, and saturation 1IL (b) (however, CI (absolute value of color signal level) is also the same.

The conversion function of the polygonal line approximation is a necessary condition for color reproduction (1
Consider approximation with 7 wipes. (In order to approximate a polygonal line with 17 wipes, the seams of the polygonal line are limited to 7 points. In other words, when the amplitude distribution φ(1) of the imaging signal is approximated as a rectangle, it becomes - class. , shows a square approximation of the amplitude distribution φ(x) divided into one class, and on the other hand,
As shown in FIG. 3B, one class of rectangular approximations are made to the unbalanced intervals. And (lγ) is shown by a broken line in FIG. 6C.
The transformation function of is approximated by a polygonal line. This FIG. 6C is an example of the conversion function, and by changing the value of γ, the 6th
It is also possible to obtain a conversion function as shown in the figure. Also,
The class end XI when divided into classes is arbitrary.

FIG. 7 shows an example of the basic configuration for performing gradation conversion according to the present invention. In FIG. 7, 1 is an input terminal, 2 is an output terminal, and 3 is a data conversion circuit. This data conversion circuit 3 includes a data conversion table 4 of the above-mentioned conversion function (Xγ) for contrast correction and a conversion function (yrG) (generally ro = 0.IIL5)
, and an output signal V is obtained.

When dividing into one class, if it is divided at equal intervals, the counter 6 counts the samples (first class value) whose most significant bit is θ out of a predetermined number of samples of the output signal, and this count value ho is used for detection control. Supplied to circuit I. A predetermined value dO corresponding to the first class value of the target amplitude distribution is supplied to the detection control circuit 7 from a terminal 8, and ho and do
A comparison is made with The value of γ in the data conversion table 4 is gradually changed so that these two values match. With such a negative feedback control system, it is possible to converge to (ho=do).

Note that the configuration is open Luso, and h
The value of O may also be detected. Further, the data conversion tables 4 and 5 may not be separate tables, but a combined data conversion table may be used.

FIG. 5 shows the configuration of an embodiment of a color imaging device to which tone conversion according to the present invention is applied.

In Figure 5, 9.10.11 is C as an image sensor.
Imaging light is incident on the CCD 9.10.11 through the iris 12, and red, edge, and blue color signals are generated by a color separation filter (not shown). The iris 12 is configured to be variable by an iris drive device 13, and automatic iris operation is performed at any given time.

C0D9.10. The color signal generated from the A10 converter 17.11 passes through the A10 converter 17.16.
18.19, is digitized, and is supplied to address selectors 20, 21.22.

Address from address bus 31 is also address selector 2
0, 21.22, and the selected address is supplied to RAM 23, 24.25. Also, 26.
27 and 28 indicate a data selector, and in the case of a write operation, data conversion data from the data bus 30 is sent to RA.
Supplied to M23, 24.25, in case of continuous operation,
Data from RAM 23.24°25 is sent out as output.

The writing and reading operations of the RAMs 23, 24, and 25 are controlled by a common memory control circuit 29. The gearless selectors 20 , 21 , 22 and the data selectors 26 , 27 , 28 are also controlled by the control circuit 29 .

CPU in connection with data bus 30 and address bus 31
32. l (A microcomputer consisting of OM 33 and RAM 34 is provided. ROM 33
The data for generating a plurality of data conversion tables is stored together with a contrast correction program.

In this embodiment, a conversion function of (f(x) - x') is used to perform the order X conversion, and a plurality of types of conversion functions with different values of r are required. If all of these conversion functions are stored in memory, the amount of memory will increase, so in order to reduce this amount, the following arithmetic processing is used. First, let us generate n types of conversion functions between f(X) and g(x).

■+cn, a 33 has 1.7'(X) and (J'(x)
-g(x)). and f(1)
and the intermediate function y of g(x) is ye − yyo−s + 4 gb)
−f(χ) ・・・・・・・・・(
The target conversion function can be obtained by increasing/decreasing l by using the difference equation (22). This calculation of equation (22) is performed by the CPU 32.

Then, the number of O's in a predetermined number of samples of the most significant bit MSB of the color data appearing from the data selectors 26, 27 and 2817) is counted by the counters 35 and 36°3T. This count value hor, b. g.

hob is supplied to the data bus 3o via four circuits 38. The CPU 32 performs calculations and judgments using this count value. Further, a data retrieval circuit 39 is provided,
Data for each color is supplied to the data bus 3o, and the CPU 32
In step 5, for example, the average value of data for each color is produced, and the iris 12 is controlled so that this becomes a predetermined value. Control data for this auto-iris control is supplied to a 4-co/verter 41 via a 4-circuit 40, and is supplied to the iris drive circuit 13 as an analog signal.

The operation of the embodiment of the present invention described above will be explained with reference to the flowchart of FIG.

First, it is checked whether the iris control is appropriate, and if it is appropriate, contrast correction is performed. When performing contrast correction, the number of MSBs of θ is counted by the counters 35, 36, and 37 among the predetermined number of samples of data of each color, and the counted value ho
r, hog, bob, connect to the CPU via the data bus 30.
It is supplied to 32.

The CPU 32 calculates the absolute value of the difference between the target value do and each of the counted values by 1hor dol, +110gdol,
1hobdol is calculated and this is the minimum hoq
(ha, ha, hgg or hob) is selected.

The average value of three values may be determined instead of the minimum value.

Depending on the polarity of this (h(1a-do)), (22X
The CPU 32 generates a contrast correction data table (this data table).
(Write to AM23, 24.25. Next, these RA
With M in the starting state, the output data is output. Since it takes some time to change this data table, it is possible to use a data missing period such as a vertical blanking period, or to configure each RAM 23.24°25 to shoulder two memory punctures. , when one memory puncture is in the writing state M (data table change state), the other memory puncture is in the writing state M (contrast correction state)
It is controlled so that

When contrast correction is performed, the average level of the imaging signal may change, so it is checked whether the iris is appropriate. h if this iris is appropriate
It is checked whether 1hoa-dol≦ξ holds true. This ε means an error within a permissible range. The conversion function is gradually changed until this relationship is established.

As can be understood from the description of the above-mentioned embodiment, according to the present invention, the contrast of the reproduced image can be improved even under shooting conditions where excess or deficiency of contrast is likely to occur, such as backlighting, under home lighting, or outdoors. It can be automatically set to an appropriate value. Further, according to the present invention, even if gradation correction is performed,
This can prevent the hue and saturation from differing from the original. Furthermore, by using a microcomputer and a data table, the tone correction device according to the present invention can be realized relatively easily.

[Brief explanation of drawings]

Figures 1 and 2 are route maps used to explain gradation conversion, Figures 3, 9 and S are route maps used to explain an example of the gradation conversion method, and Figure 6 is a route map according to the present invention. The route map used to explain the floor 14 conversion method, FIG. 7 is a block diagram showing the basic configuration of the gradation conversion device according to the present invention, and FIGS.
The figure is a block diagram showing the configuration of an example of a gradation conversion device to which this steam light is applied, and a flowchart used to explain its operation. １・・・・・・・・・Input terminal, ２・・・・・・・・・
Output terminal, 3... Data conversion circuit, 9,
10.11... CCD. 23.24.25... RAM, 30.
・・・・・・・・・Data bus, 31・・・・・・・・・
Address bus, 32...tCPU. Agent Tadashi Sugiura Tomo Figure 3 φ (X) φ (X) Figure 4 AV'a ('j) ” C □□ φ (X) y3 −−−−−− ■ + y, =11 Y+ l '11 Figure 5 Figure 6 Figure 1

Claims (1)

[Claims] gradation converting the color video signal using xr or its approximate folding 111% property as a conversion function, and detecting the frequency of occurrence of data within a certain amplitude of the color video signal;
r of the conversion function so that this frequency of occurrence becomes a predetermined value.
A method for converting a color video signal by controlling a value corresponding to the above.