Image processing method and apparatus

Claim 2

An image conversion method of processing a source image in which each pixel has a luminance value, in order to compress the dynamic range of the image in terms of brightness, the method comprising: a low-frequency-image extraction step of extracting a low frequency image consisting of a low frequency component of the source image in terms of variation in luminance; a coefficient determination step of obtaining two coefficients C 1 and C 2 from luminance Y 2 of each pixel of the low frequency image corresponding to luminance Y 1 of a corresponding pixel of the source image; and a linear computation step of obtaining luminance Y 3 of a corresponding pixel of a converted image by use of an expression Y 3 =C 1 Y 1 +C 2 Y 2 and from the two coefficients C 1 and C 2 , the luminance Y 1 of the corresponding pixel of the source image, and the luminance Y 2 of the pixel of the low frequency image, wherein each of the coefficients C 1 and C 2 is a continuous function of luminance Y 2 of a relevant pixel of the low frequency image; the coefficient C 1 is positive or zero; and the coefficient C 2 is a monotonic decreasing function in a broad sense, in a continuous first range where the luminance Y 2 assumes a minimum value, such that the coefficient C 2 assumes a positive value, becomes zero, and then assumes a negative value, and is a monotonic increasing function in a broad sense, in a continuous second range where the luminance Y 2 assumes a maximum value, such that the coefficient C 2 assumes a negative value or zero.

Claim 3

An image conversion method according to claim 1 , wherein the sum of the two coefficients C 1 and C 2 assumes a constant positive value at least within a continuous third range within the entire range of the luminance Y 2 .

Claim 4

An image conversion method according to claim 2 , wherein the sum of the two coefficients C 1 and C 2 is constant over the entire range of the luminance Y 2 .

Claim 5

An image conversion method according to claim 2 , wherein within a continuous fourth range within the third range of the luminance Y 2 , the coefficient C 1 assumes a constant positive value, and the coefficient C 2 assumes a constant negative value.

Claim 6

An image conversion method according to claim 3 , wherein within a continuous fourth range within the third range of the luminance Y 2 , the coefficient C 1 assumes a constant positive value, and the coefficient C 2 assumes a constant negative value.

Claim 7

An image conversion method according to claim 1 , wherein C 2 /C 1 is a monotonic decreasing function of the luminance Y 2 in a broad sense in the first range of the luminance Y 2 and is a monotonic increasing function of the luminance Y 2 in a broad sense in the second range of the luminance Y 2 .

Claim 8

An image conversion method according to claim 2 , wherein C 2 /C 1 is a monotonic decreasing function of the luminance Y 2 in a broad sense in the first range of the luminance Y 2 and is a monotonic increasing function of the luminance Y 2 in a broad sense in the second range of the luminance Y 2 .

Claim 9

An image conversion method according to claim 5 , wherein C 2 /C 1 is a monotonic decreasing function of the luminance Y 2 in a broad sense in the first range of the luminance Y 2 and is a monotonic increasing function of the luminance Y 2 in a broad sense in the second range of the luminance Y 2 .

Claim 10

An image conversion method according to claim 4 , wherein the coefficient C 1 assumes a constant positive value in a range which includes the entire fourth range of the luminance Y 2 and covers a greater range than the fourth range.

Claim 11

An image conversion method according to claim 3 , wherein C 1 is zero in a continuous fifth range where the luminance Y 2 assumes the minimum value; C 2 is zero in a continuous sixth range where the luminance Y 2 assumes the maximum value; and C 1 is a constant positive value and C 2 is a constant negative value within at least a continuous portion between the fifth and sixth ranges.

Claim 12

An image conversion method according to claim 1 , wherein extraction of the low frequency image is performed based on one dimension frequency analysis along each pixel line.

Claim 13

An image conversion apparatus for processing a source image in which each pixel has a luminance value in order to compress the dynamic range of the image in terms of brightness, the apparatus comprising: low-frequency-image extraction means for extracting a low frequency image consisting of a low frequency component of the source image in terms of variation in luminance; first and second coefficient determination means for obtaining two coefficients C 1 and C 2 , respectively, from luminance Y 2 of each pixel of the low frequency image output from the low-frequency-image extraction means; linear computation means for obtaining luminance Y 3 of a corresponding pixel of a converted image by use of an expression Y 3 =C 1 Y i +C 2 Y 2 and from the two coefficients C 1 and C 2 obtained by the first and second coefficient determination means, the luminance Y 2 of the pixel of the low frequency image output from the low-frequency-image extraction means, the luminance Y 2 having been used for determination of the coefficients C 1 and C 2 , and luminance Y 1 of a corresponding pixel of the source image, wherein each of the coefficients C 1 and C 2 is a continuous function of luminance Y 2 of a relevant pixel of the low frequency image; the coefficient C 1 is positive or zero; and the coefficient C 2 is a monotonic decreasing function in a broad sense, in a continuous first range where the luminance Y 2 assumes a minimum value, such that the coefficient C 2 assumes a positive value, becomes zero, and then assumes a negative value, and is a monotonic increasing function in a broad sense, in a continuous second range where the luminance Y 2 assumes a maximum value, such that the coefficient C 2 assumes a negative value or zero.

Claim 14

An image conversion apparatus for processing a source image in which each pixel has a luminance value in order to compress the dynamic range of the image in terms of brightness, the apparatus comprising: low-frequency-image extraction means for extracting a low frequency image consisting of a low frequency component of the source image in terms of variation in luminance; first and second coefficient determination means for obtaining two coefficients C 1 and C 2 , respectively, from luminance Y 2 of each pixel of the low frequency image output from the low-frequency-image extraction means; linear computation means for obtaining luminance Y 3 of a corresponding pixel of a converted image by use of an expression Y 3 =C 1 Y i +C 2 Y 2 and from the two coefficients C 1 and C 2 obtained by the first and second coefficient determination means, the luminance Y 2 of the pixel of the low frequency image output from the low-frequency-image extraction means, the luminance Y 2 having been used for determination of the coefficients C 1 and C 2 , and luminance Y 1 of a corresponding pixel of the source image, wherein each of the coefficients C 1 and C 2 is a continuous function of luminance Y 2 of a relevant pixel of the low frequency image; the coefficient C 1 is positive or zero; and the coefficient C 2 is a monotonic decreasing function in a broad sense, in a continuous first range where the luminance Y 2 assumes a minimum value, such that the coefficient C 2 assumes a positive value, becomes zero, and then assumes a negative value, and is a monotonic increasing function in a broad sense, in a continuous second range where the luminance Y 2 assumes a maximum value, such that the coefficient C 2 assumes a negative value or zero.

Claim 15

An image conversion apparatus according to claim 12 , wherein the sum of the coefficients C 1 and C 2 assumes a constant positive value at least within a continuous third range within the entire range of the luminance Y 2 .

Claim 16

An image conversion apparatus according to claim 13 , wherein the sum of the coefficients C 1 and C 2 is constant over the entire range of the luminance Y 2 .

Claim 17

An image conversion apparatus according to claim 13 , wherein within a continuous fourth range within the third range of the luminance Y 2 , the coefficient C 1 assumes a constant positive value, and the coefficient C 2 assumes a constant negative value.

Claim 18

An image conversion apparatus according to claim 14 , wherein within a continuous fourth range within the third range of the luminance Y 2 , the coefficient C 1 assumes a constant positive value, and the coefficient C 2 assumes a constant negative value.

Claim 19

An image conversion apparatus according to claim 12 , wherein C 2 /C 1 is a monotonic decreasing function of the luminance Y 2 in a broad sense in the first range of the luminance Y 2 and is a monotonic increasing function of the luminance Y 2 in a broad sense in the second range of the luminance Y 2 .

Claim 20

An image conversion apparatus according to claim 13 , wherein C 2 /C 1 is a monotonic decreasing function of the luminance Y 2 in a broad sense in the first range of the luminance Y 2 and is a monotonic increasing function of the luminance Y 2 in a broad sense in the second range of the luminance Y 2 .

Claim 21

An image conversion apparatus according to claim 16 , wherein C 2 /C 1 is a monotonic decreasing function of the luminance Y 2 in a broad sense in the first range of the luminance Y 2 and is a monotonic increasing function of the luminance Y 2 in a broad sense in the second range of the luminance Y 2 .

Claim 22

An image conversion apparatus according to claim 15 , wherein the coefficient C 1 assumes a constant positive value in a range which includes the entire fourth range of the luminance Y 2 and covers a greater range than the fourth range.

Claim 23

An image conversion apparatus according to claim 14 , wherein C 1 is zero in a continuous fifth range where the luminance Y 2 assumes the minimum value; C 2 is zero in a continuous sixth range where the luminance Y 2 assumes the maximum value; and C 1 is a constant positive value and C 2 is a constant negative value within at least a continuous portion between the fifth and sixth ranges.

Claim 24

An image conversion apparatus according to claim 12 , wherein the low-frequency-image extraction means extracts the low frequency image based on one dimension frequency analysis along each pixel line.