Image Enhancing Method Using Mean-Separate Histogram Equalization and a Circuit Therefor

1. A method for enhancing an image by histogram-equalizing an input image signal represented by a predetermined number of gray levels, said method comprising: calculating a mean level of the input image; grouping a plurality of pixels of said input image signal into a plurality of groups of image in accordance with the gray level of each of said pixels, wherein division of the input image signal into a plurality of groups of image occurs in accordance with the mean level; calculating a cumulative density function for each group of image; and histogram-equalizing each group of image independent of the other groups of image by mapping an input sample of the group of image into a new gray level based on the cumulative density functions; wherein the number of groups of images per each frame of the input image signal is predetermined to be two; and wherein said histogram-equalizing comprises: mapping the input sample into a first gray level by using a first transform function, wherein the first transform function is based on one of the cumulative density functions and the first gray level is in a first range; mapping the input sample into a second gray level by using a second transform function, wherein the second transform function is based on another of the cumulative density functions and the second gray level is in a second range; comparing the input sample with the mean level; and selecting one of said first and second gray levels according to the comparison of he input sample with the mean level.

2. A method for enhancing an image by histogram-equalizing an input image signal represented by a predetermined number of gray levels, said method comprising: calculating a mean level in a picture unit of the input image signal; obtaining a gray level distribution in the picture unit of the input image signal; dividing the gray level distribution into a predetermined number of sub-images; calculating a cumulative density function for each sub-image based on the gray level distribution; and histogram-equalizing each sub-image independent of one another by mapping a sample of the sub-image into a new gray level based on the cumulative density functions.

3. A method for enhancing an image as claimed in claim 2, wherein the predetermined number of sub-images is two and the picture unit is a frame.

4. A method for enhancing an image as claimed in claim 3, wherein said histogram-equalizing comprises: mapping the sample into a first gray level by using a first transform function which is defined by use of a first one of the cumulative density functions, wherein the first gray level is in a first range of gray levels; mapping the sample into a second gray level by using a second transform function which is defined by use of a second one of the cumulative density functions, wherein the second gray level is in a second range of gray levels; comparing the sample with the mean level; and selecting one of said first and second gray levels according to a result of comparing the sample with the mean level.

5. A method for enhancing an image as claimed in claim 4, wherein the first range of the gray levels comprises gray levels from a minimum gray level X.sub.0 to a mean gray level X.sub.m, and the second range comprises gray levels from a gray level X.sub.m+1 adjacent to the mean level X.sub.m to a maximum level X.sub.L-1.

6. A method for enhancing an image by histogram-equalizing an input image signal represented by a predetermined number of grays levels, said method comprising: calculating a mean level in a picture unit of the input image signal; obtaining a gray level distribution in the picture unit of the input image signal; dividing the gray level distribution into a predetermined number of sub-images; calculating a cumulative density function for each sub-image based on the gray level distribution; and obtaining a compensated mean level by adding to said mean level a brightness compensation value determined according to a predetermined correction function based on a mean brightness of the input image signal; and histogram-equalizing each sub-image independent of one another by mapping an input sample of the sub-image into a new gray level based on the cumulative density function and the compensated mean level.

7. A method for enhancing an image as claimed in claim 6, wherein the predetermined number of sub-images is two and the picture unit is a frame.

8. A method for enhancing an image as claimed in claim 7, wherein said histogram-equalizing comprises: mapping the input sample to a gray level ranging from a minimum level X.sub.0 to a compensated mean level B.sub.m according to a corresponding cumulative density function value of the input sample if the input sample is equal to or less than the mean level; and mapping the input image signal to a gray level ranging from a gray level B.sub.m ' adjacent to B.sub.m to a maximum gray level X.sub.L-1 according to a corresponding cumulative density function value of the input sample if the input sample is greater than the mean level, wherein B.sub.m '=B.sub.m +X.sub.L-1 /(L-1) and L is the number of gray levels.

9. A method for enhancing an image as claimed in claim 6, wherein in obtaining a compensated mean value the brightness compensation value is greater than zero when the mean level is less than a first threshold, and the brightness compensation value is less than zero when the mean level is greater than a second threshold.

10. A circuit for enhancing an image by histogram-equalizing an image signal represented by a predetermined number of gray levels in a picture unit, said circuit comprising: first calculating means for calculating a gray level distribution of the image signal; second calculating means for calculating a mean level of the image signal; third calculating means for dividing the gray level distribution into a predetermined number of sub-images based on the mean level and calculating a cumulative density function for each sub-image; a picture memory for storing samples of the image signal and providing an input sample delayed by a picture unit; mapping means for mapping the input sample output by said picture memory into mapped gray levels based on the sub-images and the cumulative density functions, and outputting the mapped gray levels; and outputting means for comparing the input sample with the mean level, selecting one of the mapped gray levels output by said mapping means and outputting the selected level.

11. A circuit for enhancing an image as claimed in claim 10, wherein the picture unit is a frame and the predetermined number is two.

12. A circuit for enhancing an image as claimed in claim 11, wherein said mapping means comprises: a first mapper the input sample into a first gray level ranging from a minimum level X.sub.0 to the mean level X.sub.m by using a first transform function which is defined based on one of the cumulative density functions; and a second mapper for mapping the input sample into a second gray level ranging from a level X.sub.m+1 which is next to the mean level to a maximum level X.sub.L-1 by using a second transform function which is defined based on another one of the cumulative density functions.

13. A circuit for enhancing an image as claimed in claim 12, wherein said outputting means comprises: a comparator for comparing the mean level with the input sample from said picture memory and outputting a selection control signal; and a selector for selecting one of the first and second gray levels from said first and second mappers, respectively, according to the selection control signal.

14. A circuit for enhancing an image as claimed in claim 10, further comprising a buffer for storing the cumulative density functions calculated by said third calculating means, updating the stored cumulative density functions based on the picture unit, and providing a predetermined number of cumulative density function values to said mapping means.

15. A circuit for enhancing an image by histogram-equalizing an image signal represented by a predetermined number of gray levels in a picture unit, said circuit comprising: first calculating means for calculating a gray level distribution of the image signal; second calculating means for calculating a mean level of the image signal; third calculating means for dividing the gray level distribution into a predetermined number of sub-images based on the mean level and calculating a cumulative density function for each sub-image; mapping means for mapping an input sample of the image signal into mapped gray levels based on the sub-images and the cumulative density functions calculated for a picture just before a current picture, and outputting the mapped gray levels; and outputting means for comparing the input sample with the mean level, selecting one of the mapped gray levels output by said mapping means and outputting the selected gray level.

16. A circuit for enhancing an image as claimed in claim 15, wherein the picture unit is a frame and the predetermined number is two.

17. A circuit for enhancing an image as claimed in claim 16, wherein said mapping means comprises: a first mapper for mapping the input sample into a first gray level ranging from a minimum level X.sub.0 to the mean level X.sub.m by using a first transform function which is defined based on one of the cumulative density functions; and a second mapper for mapping the input sample into a second gray level ranging from a level X.sub.m+1 which is next to the mean level to a maximum level X.sub.L-1 by using a second transform which is defined based on another one of the cumulative density functions.

18. A circuit for enhancing an image as claimed in claim 17, wherein said outputting means comprises: a comparator for comparing the input sample with the mean level to output a selection control signal; and a selector for selecting one of the first and second gray levels from said first and second mappers, respectively, according to the selection control signal.

19. A circuit for enhancing an image as claimed in claim 15, further comprising a buffer for storing the cumulative density functions calculated by said third calculating means, updating the stored cumulative density functions based on the picture unit, and providing a predetermined number of cumulative density function values to said mapping means.

20. A circuit for enhancing an image by histogram-equalizing an image signal represented by a predetermined number of gray levels in a picture unit, said circuit comprising: first calculating means for calculating a gray level distribution of the image signal; second calculating means for calculating a mean level of the image signal; compensation means for calculating a compensated mean level by adding a brightness compensation value according to a predetermined correction function to the mean level based on the mean brightness of the input image; third calculating means for dividing the gray level distribution into a predetermined number of sub-images on the mean level and calculating a cumulative density function for each sub-image; a picture memory for delaying the image signal by a picture unit and providing an input sample; mapping means for mapping the input sample output by said picture memory into mapped gray levels based on the sub-images, the cumulative density functions and the compensated mean level, and outputting the mapped gray levels; and outputting means for comparing the input sample with the mean level and selecting one of the mapped gray levels output by said mapping means and outputting the selected level.

21. A circuit for enhancing an image as claimed in claim 20, wherein the picture unit is a frame and the predetermined number is two.

22. A circuit for enhancing an image as claimed in claim 21, wherein said mapping means comprises: a first mapper for mapping the input sample into a first gray level ranging from a minimum level X.sub.0 to the compensated mean level B.sub.m according to a first transform function which is defined based on one of the cumulative density functions; and a second mapper for mapping the input sample into a second gray level ranging from B.sub.m ' to a maximum level X.sub.L-1 according to a second transform function which is defined based on another one of the cumulative density functions, wherein B.sub.m '=B.sub.m +X.sub.L-1 /(L-1) and L is the number of gray levels.

23. A circuit for enhancing an image as claimed in claim 22, wherein said outputting means comprises: a comparator for comparing the mean level with the input sample from said picture memory to output a selection control signal; and a selector for selecting one of the first and second gray levels from said first and second mappers, respectively, according to the selection control signal.

24. A circuit for enhancing an image as claimed in claim 20, further comprising a buffer for storing the cumulative density functions calculated by said third calculating means, updating the stored cumulative density functions based on the picture unit, and providing a predetermined number of cumulative density function values to said mapping means.

25. A circuit for enhancing an image as claimed in claim 20, wherein said brightness compensation value is a positive brightness compensation value which is greater than zero when the mean level is less than a first threshold, and is a negative brightness compensation value which is less than zero when the mean level is greater than a second threshold.

26. A circuit for enhancing an image by histogram-equalizing an image signal represented by a predetermined number of gray levels in a picture unit, said circuit comprising: first calculating means for calculating a gray level distribution of the image signal; second calculating means for calculating a mean level of the image signal; compensation means for calculating a compensated mean level by adding a brightness compensation value according to a predetermined correction function to the mean level based on the mean brightness of the input image; third calculating means for dividing the gray level distribution into a predetermined number of sub-images based on the mean level and calculating a cumulative density function for each sub-image; mapping means for mapping an input sample of the image signal into mapped gray levels based on the sub-images, the cumulative density functions and the compensated mean level, and outputting the mapped gray levels; and outputting means for comparing the input sample with the mean level and selecting one of the mapped gray levels output by said mapping means and outputting the selected level.

27. A circuit for enhancing an image as claimed in claim 26, wherein the picture unit is a frame and the predetermined number is two.

28. A circuit for enhancing an image as claimed in claim 27, wherein said mapping means comprises: a first mapper for mapping the input sample into a first gray level ranging from a minimum level X.sub.0 to the compensated mean level B.sub.m according to a first transform function which is defined based on one of the cumulative density functions; and a second mapper for mapping the input sample into a second gray level ranging from B.sub.m ' to a maximum level X.sub.L-1 according to a second transform function which is defined based on another one of the cumulative density functions, wherein B.sub.m '=B.sub.m +X.sub.L-1 /(L-1) and L is the number of gray levels.

29. A circuit for enhancing an image as claimed in claim 28, wherein said outputting means comprises: a comparator for comparing the mean level with the input sample to output a selection control signal; and a selector for selecting one of the first and second gray levels from said first and second mappers, respectively, according to the selection control signal.

30. A circuit for enhancing an image as claimed in claim 26, further comprising a buffer for storing the cumulative density functions calculated by said third calculating means, updating the stored cumulative density functions based on the picture unit, and providing a predetermined number of cumulative density function values to said mapping means.

31. A circuit for enhancing an image as claimed in claim 26, wherein said brightness compensation value is a positive brightness compensation value which is greater than zero when the mean level is less than a first threshold, and a negative brightness compensation value which is less than zero when the mean level is greater than a second threshold.

32. An image enhancing circuit in which an input image signal expressed as a predetermined number (L) of gray levels is histogram-equalized, said circuit comprising: first calculating means for calculating a gray level distribution in a picture unit of the input image signal; second calculating means for calculating a mean level in the picture unit of the input image signal; compensation means for adding a brightness compensation value according to a predetermined correction function to the mean level based on a mean brightness of the input image, and outputting a compensated mean level; third calculating means for dividing the gray level distribution into a predetermined number of sub-images according to the mean level and calculating a cumulative density function for each sub-image; mapping means for mapping a sample of the input image to mapped gray levels according to the cumulative density functions and the compensated mean level, and outputting the mapped gray levels; and outputting means for selectively outputting one of the mapped gray levels output from said mapping means as an enhanced signal, according to a result obtained by comparing the input image signal and the mean level.

33. A circuit for enhancing an image as claimed in claim 32, wherein said mapping means comprises: a first mapper for mapping a sample of the input image signal which is equal to or less than the mean level to a gray level ranging from a minimum level X.sub.0 to a compensated mean level B.sub.m according to a corresponding cumulative density function value of the input image signal; and a second mapper for mapping a sample of the input image signal which is greater than the mean level to a gray level ranging from B.sub.m ' to a maximum level X.sub.L-1 according to a corresponding cumulative density function value of the input image signal, wherein B.sub.m '=B.sub.m +X.sub.L-1 /(L-1).

34. A circuit for enhancing an image as claimed in claim 33, wherein said outputting means comprises: a comparator for comparing the level of the input image signal with the mean level and outputting a selecting control signal; and a selector for selecting an output of one of said first and second mappers according to the selecting control signal.

35. A circuit for enhancing an image as claimed in claim 32, further comprising a buffer for updating said cumulative density functions and providing previously stored cumulative density functions to said mapping means.

36. A circuit for enhancing an image as claimed in claim 32, wherein said brightness compensation value is a positive brightness compensation value which is greater than zero when the mean level is very small corresponding to the input image being very dark, and is a negative brightness compensation value which is less than zero when the mean level is very large corresponding to the input image being very bright, said compensation means outputting a compensated mean level by which the brightness of the enhanced signal is controlled.

37. A circuit for enhancing an image as claimed in claim 32, wherein said picture unit is a frame and the predetermined number of sub-images is two.

38. A circuit for enhancing an image by histogram-equalizing an image signal represented by a predetermined number of gray levels in a picture unit, said circuit comprising: a first calculator receiving the image signal, calculating a gray level distribution of the image signal and outputting a gray level distribution signal; a second calculator receiving the image signal, calculating a mean level of the image signal and outputting a mean level signal; a third calculator which receives the gray level distribution signal and the mean level signal, divides the gray level distribution into a predetermined number of sub-images based on the mean level signal and calculates a cumulative density function for each sub-image and outputs cumulative density function signals representing the calculated cumulative density functions; a mapping unit receiving the mean level signal, the cumulative density function signals and the image signal, mapping an input sample of the image signal to a plurality of gray levels based on the cumulative density functions, and outputting mapped gray levels corresponding to the input sample; and an output unit receiving the mapped gray levels from said mapping unit, comparing the input sample with the mean level, selecting one of the mapped gray levels output by said mapping unit based on the comparison and outputting the selected gray level.

39. A circuit for enhancing an image as claimed in claim 38, wherein the picture unit is a frame and the predetermined number is two.

40. A circuit for enhancing an image as claimed in claim 39, wherein said mapping unit comprises: a first mapper for mapping the input sample into a first gray level ranging from a minimum level X.sub.0 to the mean level X.sub.m by using a first transform function which is defined based on one of the cumulative density functions; and a second mapper for mapping the input sample into a second gray level ranging from a level X.sub.m+1 which is next to the mean level to a maximum level X.sub.L-1 by using a second transform function which is defined based on another one of the cumulative density functions.

41. A circuit for enhancing an image as claimed in claim 40, wherein said outputting unit comprises: a comparator for comparing the input sample with the mean level to output a selection control signal; and a selector for selecting one of the first and second gray levels from said first and second mappers, respectively, according to the selection control signal.

42. A circuit for enhancing an image as claimed in claim 38, further comprising a buffer for storing the cumulative density functions calculated by said third calculator, updating the stored cumulative density functions based on the picture unit, and providing a predetermined number of cumulative density function values to said mapping unit.

43. A circuit for enhancing an image as claimed in claim 38, further comprising a brightness compensator which receives the mean value signal and calculates a compensated mean level by adding to the mean level a brightness compensation value determined according to a predetermined correction function and based on the mean brightness of the input image, and outputs a compensated mean level signal representing the compensated mean value, wherein said mapping unit receives the compensated mean value signal and operates based on the compensated mean level.

44. A circuit for enhancing an image as claimed in claim 38, further comprising a picture memory for delaying the image signal by a picture unit and outputting an input sample to said mapping unit and said outputting unit for use as the image signal.

45. A circuit for enhancing an image as claimed in claim 38, wherein said mapping unit performs mapping based on cumulative density functions and sub-images calculated for a picture prior to a current picture.