KR0176602B1 - Picture quality improving method & circuit using mean-matching histogram equalization having brightness compensation function - Google Patents

Abstract

According to the present invention, the image quality improvement method using the average-matching histogram equalization with brightness compensation function calculates the gray level distribution of the input image signal in units of screens, and calculates a probability density function based on the gray level distributions of the screen units. Calculating a cumulative density function on a screen basis based on the obtained probability density function, calculating an average level of the input video signal on a screen basis, and brightness according to a predetermined correction function based on the average brightness of the input image. Adding a compensation value to the average level to obtain a compensated average level and mapping the input image signal to a gray level according to the obtained cumulative density function, but converting the average function of the input image to a compensated average level. The average level of the input image depends on the average brightness of the given image, including the step of adjusting it. By mapping to a mean level of brightness can be compensated and the contrast enhancement at the same time.

Description

Image Quality Improvement Method Using Mean-Matched Histogram Equalization with Brightness Compensation and Its Circuit

1 is a block diagram according to an embodiment of an image quality improvement circuit using mean-matching histogram equalization with brightness compensation according to the present invention.

2A and 2B are diagrams for explaining an example of a brightness correction function applied to the present invention.

3A and 3B show examples of the relationship between the average level compensated by the brightness correction functions shown in FIGS. 2A and 2B and the average level of the input image.

4 is a block diagram according to another embodiment of an image quality improvement circuit using mean-matching histogram equalization with brightness compensation according to the present invention.

The present invention relates to a method for improving image quality using a mean-matching histogram equalization with a brightness compensation function and a circuit thereof. In particular, the brightness of an input image is adjusted by adjusting a conversion function such that the average gray level of a given image is mapped to the compensated average level. A method of improving image quality by compensating and a circuit thereof.

Histograms of gray levels provide an overall depiction of the appearance of the image. Properly adjusted gray levels for a given image improve the appearance or contrast of the image.

Among many methods for improving contrast, histogram equalization, which is a method of improving the contrast of a given image according to the sample distribution of the image, is the most widely known and described in [1], [2]: [1] J.S. Lim, Two-Dimensional Signal and Image Processing, Prentice Hall, Englewood Cliffs, New Jersey, 1990, [2] R. C. Gonzalez and P. Wints, Digital Image Processing, Addison-wesley, Reading, Massachusetts 1977.

In addition, useful applications of histogram equalization methods, including medical image processing and radar image processing, are disclosed in [3], [4]: [3] J. Zimmerman, S, Pizer, E. Staab, E. Perry, W. Mccartney, and B. Brenton, Evaluation of the effectiveness of adaptive histogram equalization for contrast enhancement, IEEE Tr. on Medical Imaging, pp. 304-312, Dec. 1988, (4) Y. Li, W. Wang, and D. Y. Yu, Application of adaptive histogram equalization to x-ray chest image, Proc. of the SPIE, pp. 513-514, vol. 2321,1994.

In general, because histogram equalization has the effect of stretching the dynamic range, histogram equalization flattens the gray distribution of the resulting image, and as a result improves the contrast of the image.

This characteristic of the well-known histogram equalization can be a drawback in practical cases. That is, since the output density of the histogram equalization is constant, the average brightness of the output image is close to the intermediate gray level. In practice, for histogram equalization of an analog image, the average brightness of the output image in histogram equalization is exactly intermediate gray level regardless of the average brightness of the input image. Clearly, this property is undesirable in practical applications. For example, a scene taken at night may appear to look like a scene taken during the day after histogram equalization. In addition, an excessively dark or bright video signal also results in low contrast.

In order to solve the above problems, an object of the present invention is to compensate for the brightness by adjusting the conversion function so that the average gray level of the input image is mapped to the compensated average level by putting the brightness compensation value at the average level according to the average brightness of the input image. And an image quality improving method for simultaneously improving contrast.

Another object of the present invention is to place brightness compensation values at average levels according to the average brightness of the input image, and to adjust the conversion function so that the average gray level of the input image is mapped to the compensated average level while simultaneously compensating for brightness and contrast. It is to provide an improvement circuit.

In order to achieve the above object, the image quality improving method according to the present invention is a method for improving image quality by histogram equalization of a video signal represented by a predetermined number of gray levels:

(a) obtaining a gray level distribution chart of the input video signal in units of screens, and obtaining a probability density function based on the obtained gray level distribution chart in screen units; (b) calculating a cumulative density function in units of screens based on the obtained probability density function; (c) calculating an average level of the input video signal in units of screens; obtaining a compensated average level by adding a brightness compensation value according to a predetermined correction function to the average level based on the average brightness of the input image; And (e) adjusting a conversion function such that the input image signal is mapped to the gray level according to the obtained cumulative density function, but the average level of the input image is mapped to the compensated average level.

In order to achieve the above another object, the image quality improvement circuit according to the present invention comprises a circuit for improving image quality by histogram equalization of video signals represented by a predetermined number of gray levels;

First calculation means for calculating a probability density function based on the gray level distribution in the screen unit calculated by calculating the gray level distribution in the unit of the input image signal; Second calculation means for calculating a cumulative density function on the basis of the probability density function calculated by the first calculation means; Third calculating means for calculating an average level of the input video signal in units of screens; Brightness compensation means for outputting a compensated average level by adding a brightness compensation value according to a predetermined correction function to the average level based on the average brightness of an input image; And an output means for mapping the gray level according to the cumulative density function, and outputting an equalized signal by adjusting a conversion function so that the average level of the input image is mapped to the compensated average level.

Hereinafter, a method of improving image quality using a mean-matching histogram equalization and a circuit thereof according to the present invention will be described with reference to the accompanying drawings.

First, a method of improving image quality using mean-matching histogram equalization proposed by the present invention will be described.

{X} represents a given image, and X _m represents an average level of a given image {X}.

A given image {X} consists of L discrete gray levels {X ₀ , X ₁ , ....., X _L-1 }, where X ₀ = 0 represents a black level and X _{L -1} = 1 indicates white level. In addition, Xm ₀ {X ₀ , X ₁ , ....., X _L-1 }.

Probability density funtion (PDF) is defined as

Here, n _k represents the number of times the gray level X _k appears in the image {X}, and n represents the total number of samples in the image {X}. At that time, the cumulative density function (CDF) is defined as follows.

Based on the cumulative density function, the output (Y) of a typical histogram equalization for a given input sample (X _k ) is given by

The biggest problem of histogram equalization is that the average brightness between input and output signals can vary significantly depending on the cumulative density function used as the conversion function.

In order to solve this problem, the following mapping operation based on the average level of the input image in combination with the cumulative density function is proposed.

Accordingly, the present invention proposes the following mapping operation that combines brightness compensation when the average brightness of a given input video signal is too dark or too bright.

here,

That is, B _m is called a compensated average level, and Δ is a brightness compensation value preset by a predetermined correction function according to brightness. The compensated average level B _m is the result of adding the brightness compensation value Δ to the average level X _m . At this time, B _m ∈ {X ₀ , X ₁ , ....., X _L-1 }.

Thus, if the brightness compensation value is greater than zero (Δ 0), the equalized output Y _H will be bright, and if the brightness compensation value is smaller than zero (Δ 0), the equalized output Y _H will be dark. . As Δ increases, the dynamic range of the lower region will improve, and as Δ decreases, the dynamic range of the upper region will improve. The average level B _m , which is properly compensated according to the average level X _m of a given image signal, ie, bright and dark, greatly improves the image quality of the input image with the average-matching histogram equalization technique.

In conclusion, Equation (5) is the transform function for the input sample below average (X _m ). _Maps to gray levels from X ₀ to B _m , and transforms input samples that are larger than the mean (X _m ) Maps to gray levels from B _m to X _L-1 . In equation (5), it can be seen that X _m is mapped to B _m .

Therefore, when the histogram is equalized according to the calculated cumulative density function, the average of the given image is adjusted by adjusting the transform function based on the probability density function as shown in Equation (5) so that the average level of the given image is mapped to the compensated average level. The method of the present invention which improves contrast by histogram equalization while compensating for brightness is called mean-matching histogram equalization with brightness compensation.

Next, an embodiment of an image quality improvement circuit using mean-matching histogram equalization will be described with reference to FIGS. 1 and 2.

1 is a block diagram according to an embodiment of an image quality improvement circuit using mean-matching histogram equalization according to the present invention.

In FIG. 1, the frame histogram calculator 102 calculates the gray level distribution in one screen unit for the input image X _k and calculates the probability density function p (X _k ) using Equation (1). Calculate Here, the screen unit may be a field, but here, it is a frame.

The CDF calculator 104 calculates the cumulative density function c (X _k ) using Equation (2) based on the probability density function p (X _k ) of one frame calculated by the frame histogram calculator 102. Calculate

The frame average calculator 106 outputs an average level X _m of the input image X _{k in} units of frames, and calculates an average level X _m calculated according to a synchronization signal (here, frame synchronization signal SYNC). .

The frame memory 108 stores the input image signal X _{k in} units of one frame. The calculated accumulation in the CDF calculator 104, the density function (c (X _k)) because it is one-frame cumulative density function of the delayed video signal relative to the video signal (L _k) is the current input is a cumulative density function (c (X _k The frame memory 108 delays the input image signal X _k for inputting the image signal having the same frame to the CDF memory 110 and the first and second mappers 112 and 114 by one frame). .

The CDF memory 110 updates the cumulative density function c (X _k ) calculated by the CDF calculator 104 in units of frames in accordance with a synchronization signal (here, frame synchronization signal SYNC), and before a frame stored during the update. In the cumulative density function, the cumulative density function value c (X _k ) corresponding to the sample (X _k ) input from the frame memory 108 and the average level (X _m ) output from the frame average calculator 106 of the cumulative density function. Outputs the cumulative density function value (c (X _m )) for. The CDF memory 110 is used as a buffer.

Meanwhile, the brightness compensator 112 inputs an average level X _m output from the frame average calculator 106 to average the brightness compensation value Δ according to the average brightness of the input image as shown in Equation (6). The compensated average level B _m is output by adding to the level X _m .

This brightness compensation value? Is determined by a correction function as shown in Figs. 2A and 2B. There may be other applications besides the correction function shown in FIGS. 2A and 2B.

The brightness of the equalized output is adjusted by the brightness compensation value according to the correction function shown in FIGS. 2A and 2B. That is, if the average level (X _m ) of the input image is very small, that is, the average separation histogram proposed by the present invention by adding a brightness compensation value (△) greater than 0 to the average level (X _m ) for a very dark image Processed by the equalization method, the equalized output becomes brighter.

In addition, if the average level (X _m ) of the input image is very large, that is, a very bright image, the average histogram equalization method proposed by the present invention by adding a brightness compensation value (△) smaller than 0 to the average level (X _m ). The equalized output becomes dark when processed by. Therefore, the average level B _m compensated by a predetermined appropriate brightness compensation value Δ according to the average level X _m greatly improves the image quality of the input image.

3a and 3b show the compensated average level B _m given the brightness compensation value Δ according to the brightness correction function shown in FIGS. 2a and 2b and the average level X _m of the input image. The figure shows the relationship.

The first mapper 114 outputs the cumulative density function value c (X _k ) output from the CDF memory 110, the cumulative density function value c (X _m ) for the average level, and the brightness compensator 112. the mean level (B _m), a frame memory 108, a frame average level by entering the delayed input image by using the expression (5) (X _m) or less one-frame delayed input image (X _k) output from the compensation that the Outputs an equalized signal Y _H by mapping to gray levels from X ₀ to B _m .

The second mapper 116 outputs the cumulative density function value c (X _k ) output from the CDF memory 110, the cumulative density function value c (X _m ) with respect to the average level, and the brightness compensator 112. the average level compensation (B _m), a frame memory 108, one-frame delayed input image (X _k), a large one-frame delayed input image (X _k) than the mean level (X _m) by entering the output from the formula ( 5) is used to map the gray level from B _m to X _L-1 to output the equalized signal Y _H.

The comparator 118 compares the input image signal output from the frame memory 108 with the average level X _m output from the frame average calculator 104 and outputs a selection control signal.

The selector 120 selects the first mapper 114 according to the selection control signal, i.e., if the video signal output from the frame memory 108 is equal to or less than the average level X _m , otherwise, the selector 120 selects the second mapper 116. And outputs the output image Y _H shown in equation (5).

4 is a block diagram according to another embodiment of an image quality improvement circuit using mean-matching histogram equalization with brightness compensation according to the present invention.

In FIG. 4, the frame histogram calculator 202 calculates a gray level distribution on a frame-by-frame basis for the input image X _k and calculates the probability density function p (X _k ) using Equation (1). do.

The CDF calculator 204 calculates the cumulative density function c (X _k ) using Equation (2) based on the probability density function p (X _k ) of one frame calculated by the frame histogram calculator 202. Calculate

The frame average calculator 206 calculates an average level X _m of the input image X _{k in} units of frames. At this time, the frame average calculator 206 outputs an average level X _m calculated according to the synchronization signal SYNC.

The CDF memory 208 updates the cumulative density function c (X _k ) calculated by the CDF calculator 204 in units of frames according to the synchronization signal SYNC, and inputs the cumulative density function before one frame stored during the update. Cumulative density function value (c (X _k )) corresponding to the sample (X _k ), and the mean cumulative density function value (c (X _m ) for the average level (X _m ) output from the frame average calculator 206. )

The brightness compensator 210 inputs an average level (X _m ) output from the frame average calculator 206 to convert the brightness compensation value? According to the average brightness of the input image as shown in Equation (6) to an average level ( X _m ) is added to output the compensated average level B _m .

The first mapper 212 outputs the cumulative density function value c (X _k ) output from the CDF memory 208, the cumulative density function value c (X _m ) with respect to the average level, and the brightness compensator 210. a compensated mean level (B _m), the input image average level by entering the (X _k) (X _m) or less input image (X _k) using equation (5) is a gray level of from X ₀ to X _m Mapping outputs the equalized signal Y _H.

The second mapper 214 outputs the cumulative density function value c (X _k ) output from the CDF memory 208, the cumulative density function value c (X _m ) with respect to the average level, and the brightness compensator 210. the average level compensation (B _m), the input image large input image than the mean level (X _m) by entering the (X _k) using equation (5) (X _k) from B _m to X _L-1 that is The equalized signal Y _H is output by mapping to the gray level.

At this time, the video signal X _k input to the first and second mappers 212 and 214 is a signal of the next frame compared to the output signal of the CDF memory 208.

Therefore, the frame memory for delaying the input image signal by one frame to input the cumulative density function output from the CDF memory 208 and the same image signal to the first and second mappers 212 and 214 is configured as shown in FIG. Can be. However, hardware is reduced by omitting the frame memory in FIG. 2 by utilizing the property of having high correlation between adjacent frames.

The comparator 216 compares the input image signal X _k with the average level X _m output from the frame average calculator 206 and outputs a selection control signal.

The selector 218 selects the first mapper 212 according to the selection control signal, i.e., if the input image signal X _k is equal to or less than the average level X _m , otherwise selects the second mapper 214. Output a control signal.

As described above, the present invention adjusts the conversion function so that when the histogram equalization according to the cumulative density function of a given video signal is mapped to the compensated average level compensated according to the brightness, brightness compensation and contrast improvement You can do it at the same time. In addition, the image quality is greatly improved by improving the contrast of an excessively dark or light input video signal.

Claims (9)

A method of improving image quality by histogram equalizing a video signal represented by a predetermined number of gray levels, the method comprising: (a) obtaining a gray level distribution chart for each input video signal based on a gray level distribution for the screen unit; Obtaining a density function; (b) calculating a cumulative density function in units of screens based on the obtained probability density function; (c) calculating an average level of the input video signal in units of screens; And (d) obtaining a compensated average level by adding a brightness compensation value according to a predetermined correction function to the average level based on the average brightness of the input image. And (e) adjusting the conversion function to map the input image signal to the gray level according to the obtained cumulative density function, wherein the average level of the input image is mapped to the compensated average level. Way. The image quality improving method according to claim 1, further comprising the step (f) of delaying the input video signal in units of screens and outputting it in the step (e). A method for improving image quality by histogram equalization of video signals represented by a predetermined number of gray levels; (a) obtaining a gray level distribution chart of the input video signal in units of screens, and obtaining a probability density function based on the obtained gray level distribution chart in screen units; (b) calculating an average level of the input video signal in units of screens; (c) calculating a cumulative density function for the average level and a cumulative density function value for the average level based on the obtained probability density function; obtaining a compensated average level by adding a brightness compensation value according to a predetermined correction function to the average level based on the average brightness of the input image; And (e) mapping the gray level according to the cumulative density function value corresponding to the image signal input from the cumulative density function of the screen unit and the cumulative density function value for the average level, wherein the average level of the input image is compensated for. And adjusting the conversion function to be mapped to the level. 4. The method of claim 3, further comprising the step (f) of delaying the input video signal in units of screens and outputting it in the step (e). A circuit for improving image quality by histogram equalization of video signals represented by a predetermined number of gray levels; First calculation means for calculating a probability density function based on the gray level distribution in the screen unit calculated by calculating the gray level distribution in the unit of the input image signal; Second calculation means for calculating a cumulative density function on the basis of the probability density function calculated by the first calculation means; Third calculating means for calculating an average level of the input video signal in units of screens; Brightness compensation means for outputting a compensated average level by adding a brightness compensation value according to a predetermined correction function to the average level based on the average brightness of an input image; And a gray level according to the cumulative density function value corresponding to the input image signal and the cumulative density function value for the average level, wherein the average level of the input image is mapped to the compensated average level. And an output means for outputting an equalized signal by adjusting a conversion function. 6. The apparatus of claim 5, further comprising a screen memory for delaying the input video signal by one screen unit in order to input the video signal of the same frame and the cumulative density function calculated by the second calculation means to the mapping means. An image quality improvement circuit. 6. The apparatus of claim 5, further comprising a buffer for updating the cumulative density function calculated by the second calculating means in units of screens, and outputting a cumulative density function stored in the updating unit and a cumulative density function value for the average level. Image quality improvement circuit characterized in that. The method of claim 5, wherein the output means maps an average level of the input image to a compensated average level, and an input image signal below the average level is a cumulative density function value and a cumulative density function value for the average level. A first mapper that maps to a gray level of the first range according to the first mapper; A second mapper for mapping the input image signal larger than the average level to a gray level in a second range according to a corresponding cumulative density function value and a cumulative density function value for the average level; A comparator for generating a selection control signal by comparing the input video signal with the average level; And a selector for selecting a first mapper according to the selection control signal, that is, selecting a first mapper if the input image signal is less than the average level, and otherwise selecting a second mapper. The method of claim 6, wherein the output means maps an average level of the input image to a compensated average level, and an image signal output from the screen memory below the average level is corresponding to a corresponding cumulative density function value and the average level. A first mapper that maps to a gray level of the first range according to a cumulative density function value of the first range; A second mapper which maps an image signal output from the screen memory larger than the average level to a gray level in a second range according to a corresponding cumulative density function value and a cumulative density function value for the average level; A comparator for generating a selection control signal by comparing the video signal output from the screen memory with the average level; And a selector for selecting a first mapper according to the selection control signal, that is, selecting a first mapper if the video signal output from the screen memory is less than the average level, and otherwise selecting the second mapper.