Image quality enhancing method using mean-matching histogram equalization and a circuit therefor

Claim 2

A method for enhancing image as claimed in claim 1, further comprising a step: (d) delaying the image signal by a unit of a picture; wherein, a delayed image signal is equalized in said step (c).

Claim 4

A method for enhancing image as claimed in claim 3, further comprising a step: (e) delaying the image signal by a unit of a picture; wherein, a delayed image signal is equalized in said step (c).

Claim 6

A method for enhancing image as claimed in claim 5, further comprising a step: (e) delaying the image signal by a unit of a picture; wherein, a delayed image signal is equalized in said step (d).

Claim 8

A method for enhancing image as claimed in claim 7, further comprising a step: (f) delaying the image signal by a unit of a picture; wherein, a delayed image signal is equalized in said step (e).

Claim 9

An image enhancing circuit for histogram-equalizing an image signal represented by a predetermined number of gray levels, said circuit comprising: first calculating means (106; 206; 306; 406) for calculating a mean level of an input image signal in a unit of a picture; second calculating means (102, 104; 202, 204; 302, 304; 402, 404) for calculating a gray level distribution of the image signal in a unit of a picture, calculating a cumulative density function in a unit of a picture based on the gray level distribution, and outputting a cumulative density function value corresponding to each input sample of the image signal and a cumulative density function value of the mean level; and outputting means (112, 114, 116, 118; 210 - 216; 314 - 320; 412 - 418) for mapping the input sample to a gray level according cumulative density function values corresponding to the input sample and the mean level and outputting a mapped level as an equalized signal, wherein the mean level is mapped into the same level.

Claim 10

An image enhancing circuit as claimed in claim 9, further comprising a picture memory (108; 308) for delaying the image signal by a unit of a picture to provide said outputting means (112 - 118; 314 - 320) with a image sample which belongs to a same frame as that from which the cumulative density function is calculated.

Claim 11

An image enhancing circuit as claimed in claim 9 or 10, further comprising a buffer (110; 208; 310; 408) for storing the cumulative density function calculated by said second calculating means (102, 104; 202, 204; 302, 304; 402, 404), updating by a unit of a picture, and outputting a cumulative density function values corresponding to the input sample and the mean level.

Claim 12

An image enhancing circuit as claimed in claim 9 or claim 11 as dependent thereon, wherein said outputting means comprises: a first mapper (210; 412) for mapping the input sample to a gray level of a first range according to a first transform function which is defined by use of the cumulative density function; a second mapper (212; 414) for mapping the input sample to a gray level of a first range according to a second transform function which is defined by use of the cumulative density function; a comparator (214; 416) for comparing the input sample with the mean level to generate a selection control signal; and a selector (216; 418) for selecting one of output signals from said first mapper (210; 412) and said second mapper (212; 414) according to the selection control signal, wherein said selector (216; 418) selects the output signal from said first mapper (210; 412) when the selection control signal indicates that the input sample is equal to or less than the mean level, and selects the output signal from said second mapper (212; 414) when the selection control signal indicates that the input sample is larger than the mean level.

Claim 13

An image enhancing circuit as claimed in claim 10 or 11 as dependent thereon, wherein said outputting means comprises: a first mapper (112; 314) for mapping the image sample output by said picture memory to a gray level of a first range according to a first transform function which is defined by use of the cumulative density function; a second mapper (114; 316) for mapping the image sample output by said picture memory (108; 308) to a gray level of a first range according to a second transform function which is defined by use of the cumulative density function; a comparator (116; 318) for comparing the image sample output by said picture memory (108; 308) with the mean level to generate a selection control signal; and a selector (118; 320) for selecting one of output signals from said first mapper and said second mapper according to the selection control signal, wherein said selector selects the output signal from said first mapper when the selection control signal indicates that the image sample output by said picture memory (108; 308) is equal to or less than the mean level, and selects the output signal from said second mapper when the selection control signal indicates that the image sample output by said picture memory (108; 308) is larger than the mean level.

Claim 14

An image enhancing circuit for histogram-equalizing an image signal represented by a predetermined number of gray levels, said circuit comprising: first calculating means (306; 406) for calculating a mean level of an input image signal in a unit of a picture; second calculating means (302, 304; 402, 404) for calculating a gray level distribution of the image signal in a unit of a picture, calculating a cumulative density function in a unit of a picture based on the gray level distribution, and outputting a cumulative density function value corresponding to each input sample of the image signal and a cumulative density function value of the mean level; brightness compensating means (312; 410) for adding to the mean level a brightness compensation value calculated by a predetermined compensation function according to the mean level of the input image signal to output a compensated mean level; and outputting means (314 - 320; 412 - 418) for mapping the input sample to a gray level according cumulative density function values corresponding to the input sample and the mean level and outputting a mapped level as an equalized signal, wherein the mean level is mapped into the compensated mean level.

Claim 15

An image enhancing circuit as claimed in claim 14, further comprising a picture memory (308) for delaying the image signal by a unit of a picture to provide said outputting means (314 - 320) with a image sample which belongs to a same frame as that from which the cumulative density function is calculated.

Claim 16

An image enhancing circuit as claimed in claim 14 or 15, further comprising a buffer (310; 408) for storing the cumulative density function calculated by said second calculating means (302, 304; 402, 404), updating by a unit of a picture, and outputting a cumulative density function values corresponding to the input sample and the mean level.

Claim 17

An image enhancing circuit as claimed in claim 14 or claim 16 as dependent thereon, wherein said outputting means comprises: a first mapper (412) for mapping the input sample to a gray level of a first range according to a first transform function which is defined by use of the cumulative density function; a second mapper (414) for mapping the input sample to a gray level of a first range according to a second transform function which is defined by use of the cumulative density function; a comparator (416) for comparing the input sample with the mean level to generate a selection control signal; and a selector (418) for selecting one of output signals from said first mapper (412) and said second mapper (414) according to the selection control signal, wherein said selector selects the output signal from said first mapper (412) when the selection control signal indicates that the input sample is equal to or less than the mean level, and selects the output signal from said second mapper (414) when the selection control signal indicates that the input sample is larger than the mean level.

Claim 18

An image enhancing circuit as claimed in claim 15, wherein said outputting means comprises: a first mapper (314) for mapping the image sample output by said picture memory (308) to a gray level of a first range according to a first transform function which is defined by use of the cumulative density function; a second mapper (316) for mapping the image sample output by said picture memory (308) to a gray level of a first range according to a second transform function which is defined by use of the cumulative density function; a comparator (318) for comparing the image sample output by said picture memory with the mean level to generate a selection control signal; and a selector (320) for selecting one of output signals from said first mapper and said second mapper according to the selection control signal, wherein said selector selects the output signal from said first mapper when the selection control signal indicates that the image sample output by said picture memory (308) is equal to or less than the mean level, and selects the output signal from said second mapper (316) when the selection control signal indicates that the image sample output by said picture memory (308) is larger than the mean level.