Image enhancement circuit and method using mean matching/quantized mean matching histogram equalization and color compensation

• US 6,507,372 B1
• Filed: 10/19/1999
• Issued: 01/14/2003
• Est. Priority Date: 10/09/1996
• Status: Expired due to Term

• Current Assignee: Samsung Electronics Co. Ltd.
• Original Assignee: Samsung Electronics Co. Ltd.
• Inventors: Kim, Yeong-taeg
• Primary Examiner(s): Miller, John
• Assistant Examiner(s): YENKE, BRIAN P
• Application Number:
  US09/420,583
• Time in Patent Office: 1,183 Days
• Field of Search: H04/N.521, 348/630, 348/631, 348/659, 348/661, 348/672, 348/712, 348/713, 348/914, 382/167, 382/168, 382/172, 358/448, 358/461, 358/515, 358/516, 358/517, 358/518, 358/519, 358/520, 358/521, 358/522, 358/523
• US Class Current: 348/630
• CPC Class Codes: G06T 2207/10024 Color image
• CPC Class Codes: G06T 2207/20021 Dividing image into blocks, subimages or windows
• CPC Class Codes: G06T 5/40 using histogram techniques
• CPC Class Codes: G06T 5/90 Dynamic range modification of images or parts thereof
• CPC Class Codes: H04N 1/6027 Correction or control of colour gradation or colour contrast H04N1/6058 takes precedence

Abstract

In an image enhancement method and circuit, a luminance signal is extracted from input color signals. Histogram equalization is performed using a cumulative density function of the extracted luminance signal which is input in a screen unit. A transform function is controlled to map a mean level of the extracted luminance signal to itself, so that an adjusted luminance signal is output. The color signals are varied according to the variation of the luminance signals to output compensated color signals. Thus, image contrast is enhanced, and an undistorted color signal is provided at the same time. In order to reduce the hardware of the circuit, an interpolated cumulative density function is obtained by interpolating a quantized cumulative density function of an input luminance image and is used as a transform function. The transform function is controlled to map a mean level of the input luminance image to itself.

Claims

1. An image enhancement method comprising the steps of:

(a) converting input color signals R, G and B into a luminance signal and system-defined color signals;

(b) performing histogram equalization using a cumulative density function of said luminance signal as a transform function, and outputting an adjusted luminance signal by controlling said transform function to map a mean level of said luminance signal to itself;

(c) varying said system-defined color signals based on said adjusted luminance signal, and outputting compensated system-defined color signals; and

(d) converting said adjusted luminance signal and said compensated system-defined color signals into compensated color signals R′

, G′ and

B′

.

2. An image enhancement method as claimed in claim 1, wherein in said step (a), said input signal (R,G,B) is converted into one of system-defined color signals (Y,U,V), (Y,R-Y,B-Y) and (Y,I,Q).

3. An image enhancement method as claimed in claim 2, wherein said adjusted luminance signal forms and adjusted luminance plane in a space of said input color signals and wherein said compensated color signals are obtained by an intersection point between a) a compensation line, said compensation line being extended in a direction of a line of said input color signals R, G and B. and b) said adjusted luminance plane.

4. An image enhancement method as claimed in claim 3, wherein, when a ratio between said luminance signal and said adjusted luminance signal is expressed as a factor of k, said compensation line is expressed as follows:

5. An image enhancement method as claimed in claim 3, wherein, when a ratio between said luminance signal and said adjusted luminance signal is expressed as a factor of k, said compensation line is expressed as follows:

6. An image enhancement method as claimed in claim 1, wherein said step (b) comprises:

(b1) calculating said cumulative density function of said luminance signal, said luminance signal being provided in screen units;

(b2) calculating said mean level of said luminance signal; and

(b3) transforming a gray level of said luminance signal using said calculated cumulative density function as said transform function, and outputting said adjusted luminance signal by controlling said transform function so that said mean level is mapped to itself.

7. An image enhancement method as claimed in claim 1, wherein said step (b) comprises:

(b1) quantizing gray levels of said luminance signal, said luminance signal being provided in screen units, and outputting a quantized signal;

(b2) calculating a cumulative density function based on a gray level distribution of said quantized signal, and outputting a quantized cumulative density function;

(b3) calculating a mean level of gray levels of said luminance signal in a screen unit;

(b4) outputting an interpolated cumulative density function value obtained by an interpolation of said quantized cumulative density function and a cumulative density function value with respect to said mean gray level; and

(b5) mapping said luminance signal to an adjusted gray level according to said interpolated cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean gray level, using said interpolated cumulative density function as said transform function, and outputting said adjusted luminance signal by controlling said transform function so that said mean gray level obtained in step (b3) is mapped to itself.

8. An image enhancement method as claimed in claim 7, wherein said interpolation in step (b4) is a linear interpolation.

9. An image enhancement circuit comprising:

first converting means for converting input color signals R, G and B into a luminance signal and system-defined color signals;

histogram equalizing means for performing histogram equalization using a cumulative density function of said luminance signal as a transform function, and outputting an adjusted luminance signal by controlling said transform function to map a mean level of said luminance signal to itself;

color compensating means for outputting compensated system-defined color signals by varying said system-defined color signals based on said adjusted luminance signal; and

second converting means for converting said adjusted luminance signal and said compensated system-defined color signals into compensated color signals R′

, G′ and

B′

.

10. An image enhancement circuit as claimed in claim 9, wherein said first converting means converts said input signal (R,G,B) into one of system-defined color signals (Y,U,V), (Y,R-Y,B-Y) and (Y,I,Q).

11. An image enhancement circuit as claimed in claim 9, wherein said color compensating means comprises:

an operator for producing a ratio between said luminance signal and said adjusted luminance signal; and

a plurality of multipliers for multiplying the ratio output by said operator with each system-defined color signal and outputting compensated system-defined color signals.

12. An image enhancement circuit as claimed in claim 11, wherein said color compensating means comprises:

an operator for producing a ratio between said luminance signal and said adjusted luminance signal; and

a plurality of controllers for receiving the ratio output by said operator and a parameter (α

), for varying the value of each system-defined color signal using a compensation line based on said parameter, and outputting said compensated system-defined color signals.

13. An image enhancement circuit as claimed in claim 12, wherein, when a ratio between said luminance signal and said adjusted luminance signal is expressed as a factor of k, said compensation line is expressed as follows:

14. An image enhancement circuit as claimed in claim 12, wherein, when a ratio between said luminance signal and said adjusted luminance signal is expressed as a factor of k, said compensation line is expressed as follows:

15. An image enhancement circuit as claimed in claim 9, wherein said histogram equalizing means comprises:

a first calculator for calculating said mean level of said luminance signal, said luminance signal being provided in screen units;

a second calculator for calculating said cumulative density function based on a gray level distribution of a screen unit of said luminance signal; and

a mapper for mapping said luminance signal to an adjusted gray level according to a cumulative density function value corresponding to said luminance signal and a cumulative density function value with respect to said mean level, and outputting said adjusted luminance signal by mapping said mean level to itself.

16. An image enhancement circuit as claimed in claim 15, wherein said histogram equalizing means further comprises a screen memory for delaying said luminance signal by a screen unit to input a luminance signal having the same frame as that of said cumulative density function calculated by said second calculator, to said mapper.

17. An image enhancement circuit as claimed in claim 15, wherein said mapper comprises:

a first mapper for mapping said mean level calculated by said first calculator to itself by controlling said cumulative density function as said transform function, and for mapping said luminance signal having a level smaller than or equal to said mean level to an adjusted gray level having a first range, according to said cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean level;

a second mapper for mapping said luminance signal having a level greater than said mean level to an adjusted gray level having a second range, according to said cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean level;

a comparator for outputting a selection control signal by comparing said luminance signal with said mean level; and

a selector for selecting said first mapper when said luminance signal is smaller than or equal to said mean level, and otherwise, selecting said second mapper, according to said selection control signal.

18. An image enhancement circuit as claimed in claim 16, wherein said mapper comprises:

a first mapper for mapping said mean level calculated by said first calculator to itself by controlling said cumulative density function as said transform function, and for mapping said luminance signal having a level smaller than or equal to said mean level, output by said screen memory, to an adjusted gray level having a first range, according to said cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean level;

a second mapper for mapping said luminance signal, output by said screen memory, having a level greater than or said mean level to an adjusted gray level having a second range, according to said cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean level;

a comparator for outputting a selection control signal by comparing said luminance signal output by said screen memory with said mean level; and

a selector for selecting said first mapper when said luminance signal output by said screen memory is smaller than or equal to said mean level, and otherwise, selecting said second mapper, according to said selection control signal.

19. An image enhancement circuit as claimed in claim 9, wherein said histogram equalizing means comprises:

a quantizer for quantizing a level of said luminance signal and outputting a quantized signal;

a first calculator for calculating a cumulative density function based on a gray level distribution of a screen unit with respect to said quantized signal and outputting a quantized cumulative density function value;

a second calculator for calculating a mean level of said luminance signal in a screen unit;

an interpolator for interpolating said quantized cumulative density function value and outputting an interpolated cumulative density function value; and

a mapper for mapping said luminance signal to an adjusted gray level using said interpolated cumulative density function as a transform function, and outputting said adjusted luminance signal by controlling said transform function to map said mean level obtained by said second calculator to itself.

20. An image enhancement circuit as claimed in claim 19, wherein said histogram equalizing means further comprises a screen memory for delaying said luminance signal by a screen unit to input a luminance signal having the same frame as that of said interpolated cumulative density function value, to said mapper.

21. An image enhancement circuit as claimed in claim 19, wherein said mapper comprises:

a first mapper for mapping said mean level calculated by said second calculator to itself, and for mapping said luminance signal having a level smaller than or equal to said mean level to an adjusted gray level having a first range, according to said interpolated cumulative density function value corresponding to said luminance signal and a cumulative density function value with respect to said mean level;

a second mapper for mapping said luminance signal having a level greater than said mean level to an adjusted gray level having a second range, according to said interpolated cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean level;

a comparator for outputting a selection control signal by comparing said luminance signal with said mean level; and

a selector for selecting said first mapper when said luminance signal is smaller than or equal to said mean level, and otherwise, selecting said second mapper, according to said selection control signal.

22. An image enhancement circuit as claimed in claim 20, wherein said mapper comprises:

a first mapper for mapping said mean level calculated by said second calculator to itself, and for mapping said luminance signal having a level smaller than or equal to said mean level, output by said screen memory, to an adjusted gray level having a first range, according to said interpolated cumulative density function value corresponding to said luminance signal and a cumulative density function value with respect to said mean level;

a second mapper for mapping said luminance signal, output by said screen memory, having a level greater than said mean level to an adjusted gray level having a second range, according to said interpolated cumulative density function value corresponding to said luminance signal and said cumulative density function value with respect to said mean level;

a comparator for outputting a selection control signal by comparing said luminance signal output by said screen memory with said mean level; and

a selector for selecting said first mapper when said luminance signal output by said screen memory is smaller than or equal to said mean level, and otherwise, selecting said second mapper, according to said selection control signal.