Method and apparatus for adjusting brightness of image
A method and apparatus for adjusting brightness of an image are provided, in which number distribution information of pixel brightness is detected, and the brightness of a specific area of the image is adjusted using characteristic values according to a distribution characteristic of the detected number distribution information. Accordingly, a decrease in an image recognition characteristic of a human being is compensated for in a specific area, such as, low-brightness area, of an image.
1. A method of adjusting brightness of an image, the method comprising:
detecting number distribution information of pixel brightness;
adjusting the brightness of a specific area of the image using characteristic values according to a distribution characteristic of the detected number distribution information;
determining whether contrast adjustment of the image is required; and
adjusting contrast of the image in response to determining contrast adjustment of the image is required,
wherein at least one of the steps of adjusting the brightness of a specific area of the image, determining whether contrast adjustment of an image is require, and adjusting contrast of the image is performed by a microprocessor.
2. The method of claim 1 , wherein the adjusting of the contrast of the image comprises:
detecting a first reference brightness value indicating low-brightness for the image and a second reference brightness value indicating high-brightness for the image;
adjusting R, G, and B of the image using the detected first and second reference brightness values; and
detecting number distribution information of the RGB-adjusted image.
3. The method of claim 2 , wherein the adjusting of R, G, and B of the image, is adjusted using equation
R ′( r,c )=(2 n −1)*( R ( r,c )−low)/(high−low)
G ′( r,c )=(2 n −1)*( G ( r,c )−low)/(high−low)
B ′( r,c )=(2 n −1)*( B ( r,c )−low)/(high−low)
where r and c denote coordinates of each pixel of the image, n denotes a constant indicating the number of gradations of brightness, low denotes the first reference brightness value, and high denotes the second reference brightness value.
4. The method of claim 1 , wherein the adjusting of the brightness of the specific area of the image comprises:
detecting the characteristic values;
detecting a factor for adjusting the brightness of the specific area using the detected characteristic values;
transforming an RGB color coordinate system of the image to a color coordinate system presenting brightness;
performing edge preserving noise filtering of the transformed image;
detecting an image in which contrast of the specific area has been adjusted from the image in which the edge preserving noise filtering has been performed;
adjusting brightness of the specific area using the contrast-adjusted image; and
transforming the color coordinate system of the brightness-adjusted image to the RGB color coordinate system.
5. The method of claim 4 , wherein in the detecting of the characteristic values, the number distribution information presented as a histogram is divided into a plurality of areas based on the brightness, and characteristic values of the divided areas are detected.
6. The method of claim 5 , wherein the plurality of areas is divided into a first divided area, a second divided area, and a third divided area.
7. The method of claim 6 , wherein the detecting of the characteristic values is detected using the equations below:
g
m
=
max
(
H
(
i
)
)
i
∈
[
0
,
2
n
-
1
]
where H(i) denotes the number of pixels corresponding to a brightness value i, gm denotes the maximum number of pixels among the number of pixels corresponding to brightness values of the image, and n denotes a constant indicating the number of gradations of brightness;
S
1
=
∑
first
H
(
i
)
/
(
M
*
N
)
,
S
2
=
∑
second
H
(
i
)
/
(
M
*
N
)
,
S
3
=
∑
third
H
(
i
)
/
(
M
*
N
)
where S 1 denotes a pixel number ratio of brightness corresponding to the first divided area in the image, S 2 denotes a pixel number ratio of brightness corresponding to the second divided area in the image, S 3 denotes a pixel number ratio of brightness corresponding to the third divided area in the image, and M*N denotes the size (resolution) of the image;
S
11
=
∑
i
=
0
{
(
2
n
-
1
)
/
6
}
-
1
H
(
i
)
/
(
M
*
N
)
,
S
12
=
∑
i
=
(
2
n
-
1
)
/
6
(
2
n
-
1
)
/
3
H
(
i
)
/
(
M
*
N
)
where S 11 denotes a pixel number ratio of brightness corresponding to the first sub-area in the image when the first divided area is divided into a first sub-area and a second sub-area, and S 12 denotes a pixel number ratio of brightness corresponding to the second sub-area in the image;
R
1
=
max
(
H
(
i
)
)
first
/
g
m
,
R
2
=
max
(
H
(
i
)
)
second
/
g
m
,
R
3
=
max
(
H
(
i
)
)
third
/
g
m
where R 1 a ratio of the maximum number of pixels among the number of pixels corresponding to brightness values in the first divided area to the maximum number of pixels among the number of pixels corresponding to brightness values of the image, R 2 denotes a ratio of the maximum number of pixels among the number of pixels corresponding to brightness values in the second divided area to the maximum number of pixels among the number of pixels corresponding to brightness values of the image, and R 3 denotes a ratio of the maximum number of pixels among the number of pixels corresponding to brightness values in the third divided area to the maximum number of pixels among the number of pixels corresponding to brightness values of the image; and
P 1 =h /(2 n −1),P 2 =l /(2 n −1)
where h denotes a brightness value having the maximum number of pixels in the first divided area, P 1 denotes a ratio of h to 2 n gradations, l denotes a brightness value having the maximum number of pixels in the third divided area, and P 2 denotes a ratio of l to 2 n gradations.
8. The method of claim 4 , wherein in the transforming of the RGB color coordinate system of the image to the color coordinate system presenting brightness, the RGB color coordinate system of the image is transformed to one of an Hue, Saturation, Value (HSV) color coordinate system, a YCbGr color coordinate system, a Lab color coordinate system, and an XYZ color coordinate system.
9. The method of claim 4 , wherein in the performing of the edge preserving noise filtering, rows of the image are filtered and then columns of the image are filtered based on the row filtering result, or columns of the image filtered and then rows of the image filtered based on the column filtering result.
10. The method of claim 9 , wherein the performing of the edge preserving noise filtering is performed using the equations below:
V
f
*
(
r
,
c
)
=
∑
j
=
-
S
/
2
S
/
2
V
(
r
,
c
+
j
)
*
u
(
j
)
*
w
(
V
(
r
,
c
+
j
)
,
V
(
r
,
c
)
)
∑
j
=
-
S
/
2
S
/
2
u
(
j
)
*
w
(
V
(
r
,
c
+
j
)
,
V
(
r
,
c
)
)
where V(r,c) denotes brightness of the original image, V* f (r,c) denotes a result obtained by filtering one of rows and columns of the image V(r,c), S denotes a window size for the filtering, u(z) denotes the equation
u
(
z
)
=
exp
(
-
z
2
2
σ
D
2
)
,
w(x,y) denotes the equation
w
(
x
,
y
)
=
exp
(
-
(
x
-
y
)
2
2
σ
R
2
)
,
and σ D and σ R denote predetermined constants; and
V
f
(
r
,
c
)
=
∑
i
=
-
S
/
2
S
/
2
V
f
*
(
r
+
i
,
c
)
*
u
(
i
)
*
w
(
V
f
*
(
r
+
i
,
c
)
,
V
f
*
(
r
,
c
)
)
∑
i
=
-
S
/
2
S
/
2
u
(
i
)
*
w
(
V
f
*
(
r
+
i
,
c
)
,
V
f
*
(
r
,
c
)
)
where V(r,c) denotes a result obtained by filtering the unfiltered rows or columns using the filtering result V* f (r,c).
11. The method of claim 4 , wherein the detecting of the image in which contrast of the specific area has been adjusted is detected using the equation
D=f ( V f ,V)=(2 n −1) *k*V /( V f +b )
where D denotes the contrast-adjusted image, and k and b denote predetermined constants.
12. The method of claim 4 , wherein the adjusting of the brightness of the specific area is adjusted using the equation
V′=f w ( V+a *(2 n −1 −V f )* f h ( D−V ))
where V′ denotes the image in which the brightness of the specific area has been adjusted, a denotes the factor detected using the characteristic values, f w (x) denotes
f
w
(
x
)
=
{
x
:
x
≤
(
2
n
-
1
)
2
n
-
1
:
x
>
(
2
n
-
1
)
,
and
f
b
(
x
)
denotes
f
b
(
x
)
=
{
x
/
(
2
n
-
1
)
:
x
≥
0
0
:
x
<
0
.
13. A computer readable recording medium storing a computer readable program for executing the method of one of claims 1 and 4 .
14. An apparatus for adjusting brightness of an image, the apparatus comprising:
a distribution information detector for detecting number distribution information of pixel brightness;
a brightness adjuster for adjusting the brightness of a specific area of the image using characteristic values according to a distribution characteristic of the detected number distribution information;
an examiner for determining whether contrast adjustment of the image is required; and
a contrast adjuster for adjusting contrast of the image in response to determining that the contrast adjustment of the image is required.
15. The apparatus of claim 14 , wherein the contrast adjuster comprises:
a reference brightness value detector for detecting a first reference brightness value indicating low-brightness for the image and a second reference brightness value for indicating high-brightness for the image; and
an RGB adjuster for adjusting R, G, and B of the image using the detected first and second reference brightness values,
wherein the distribution information detector detects number distribution information of the RGB-adjusted image.
16. The apparatus of claim 14 , wherein the brightness adjuster comprises:
a characteristic value detector for detecting the characteristic values;
a factor detector for detecting a factor for adjusting the brightness of the specific area using the detected characteristic values;
a first color coordinate system transformer for transforming an RGB color coordinate system of the image to a color coordinate system presenting brightness;
a filtering unit for performing edge preserving noise filtering of the transformed image;
a specific area contrast adjuster for detecting an image in which contrast of the specific area has been adjusted from the image in which the edge preserving noise filtering has been performed;
a specific area brightness adjuster for adjusting brightness of the specific area using the contrast-adjusted image; and
a second color coordinate system transformer for transforming the color coordinate system of the brightness-adjusted image to the RGB color coordinate system.
17. The apparatus of claim 16 , wherein the characteristic value detector divides the number distribution information presented as a histogram into a plurality of areas based on the brightness and detects characteristic values of the divided areas.
18. The apparatus of claim 16 , wherein the filtering unit filters rows of the image and then filters columns of the image based on the row filtering result, or filters columns of the image and then filters rows of the image based on the column filtering result.