Image background enhancement for color images
View Patent ↗A technique is provided for enhancing the background of an original document image. In the case of a black-and-white image, the background color of the original document image is detected, the desired enhanced background color of the original document image is determined from a background pixel value P b that is in the center of the background color range, and the original document image is enhanced to the desired enhanced background color. However, if the background of the original document image is in color, the technique further includes obtaining color image histograms of red, blue and green colors of the original document image, smoothing the histograms, and comparing the histograms to determine if they have the same shape. If the histograms do not have the same shape, and hence the background is in color, the background color of the original document image is detected for each color channel and the background color is enhanced for each channel based on table values so that the hue of the background is not changed.
1. A computer-implemented method of enhancing the background of an original document image, comprising:
detecting the background color of the original document image by finding a pixel value corresponding to a peak in a histogram of the original document image;
determining the desired enhanced background color of the original document image from a background pixel value P b that is in the center of the background color range;
determining the factors F l and F r as follows:
F
l
=
{
β
if
P
b
≥
T
ghigh
α
Otherwise
F
r
=
{
α
if
P
b
≤
T
glow
β
Otherwise
where values α and β are decided based on experimental values and where T ghigh and T glow are high and low pixel thresholds, respectively, and said factors F l and F r define a range to distinguish the background pixel value P b from a foreground pixel value; and
enhancing pixels of the original document image in said range to the desired enhanced background color.
2. The method of claim 1 , wherein factor values F l and F r are fed to a function to find the left pixel value P lb of the background color range as a function of the histogram of P b divided by F l and the right pixel value P rb of the background color range as a function of the histogram of P b divided by F r , whereby pixel values in the color range defined by P lb and P rb are determined to be part of the image background.
3. The method of claim 1 , wherein determining the desired enhanced background color P eb of the original document image from P b comprises determining P eb as:
P
eb
=
{
N
if
P
b
≥
P
gw
min
P
gb
max
if
P
b
<
P
gb
max
P
b
Otherwise
where P gwmin is the minimum pixel value that can be safely set to white on the enhanced image and P gbmax is the maximum pixel value that can be safely considered as black on the enhanced image, and N is the pixel resolution of the original image.
4. The method of claim 3 , wherein enhancing the original document image to the desired enhanced background color comprises:
if P eb is N, generating a look-up table LT as follows, where i is the grayscale image pixel value in the range [0,N], and B is a small value that can always be safely set to pure black value 0:
LT
(
i
)
=
{
0
i
≤
B
N
i
≥
P
l
b
N
·
(
i
-
B
)
P
l
b
-
B
Otherwise
;
if P eb is not N, generating a look-up table LT as:
LT
(
i
)
=
{
P
eb
if
P
l
b
≤
i
≤
P
rb
i
Otherwise
;
and
enhancing the image background such that for each image pixel with value p on the original image the pixel value is changed from p to LT(p).
5. A system for enhancing the background of an original document image, comprising:
a memory that stores a binary representation of the original document image; and
a processor that detects the background color of the original document image by finding a pixel value corresponding to a peak in a histogram of the original document image, determines the desired enhanced background color of the original document image from a background pixel value P b that is in the center of the background color range, determines the factors F l and F r as follows:
F
l
=
{
β
if
P
b
≥
T
ghigh
α
Otherwise
F
r
=
{
α
if
P
b
≤
T
glow
β
Otherwise
where values α and β are decided based on ex erimental values and where T ghigh and T glow are high and low pixel thresholds, respectively, and said factors F l and F r define a range to distinguish the background pixel value P b from a foreground pixel value, and enhances pixels of the original document image in said range to the desired enhanced background color.
6. The system of claim 5 , wherein the processor provides the factor values F l and F r to a function to find the left pixel value P lb of the background color range as a function of the histogram of P b divided by F l and the right pixel value P rb of the background color range as a function of the histogram of P b divided by F r , whereby pixel values in the color range defined by P lb and P rb are determined to be part of the image background.
7. The system of claim 5 , wherein the processor determines the desired enhanced background color P eb of the original document image from P b by determining P eb as:
P
eb
=
{
N
if
P
b
≥
P
gw
min
P
gb
max
if
P
b
<
P
gb
max
P
b
Otherwise
where P gwmin is the minimum pixel value that can be safely set to white on the enhanced image and P gbmax is the maximum pixel value that can be safely considered as black on the enhanced image, and N is the pixel resolution of the original image.
8. The system of claim 7 , wherein the processor enhances the original document image to the desired enhanced background color by:
if P eb is N, generating a look-up table LT as follows, where i is the grayscale image pixel value in the range [0,N], and B is a small value that can always be safely set to pure black value 0:
LT
(
i
)
=
{
0
i
≤
B
N
i
≥
P
l
b
N
·
(
i
-
B
)
P
l
b
-
B
Otherwise
;
if P eb is not N, generating a look-up table LT as:
LT
(
i
)
=
{
P
eb
if
P
l
b
≤
i
≤
P
rb
i
Otherwise
;
and
enhancing the image background such that for each image pixel with value p on the original image the pixel value is changed from p to LT(p),
LT
(
i
)
=
{
P
eb
if
P
l
b
≤
i
≤
P
rb
i
Otherwise
;
and
enhancing the image background such that for each image pixel with value p in a specific color channel of the original document image the pixel value is changed from p to LT(p) in that channel.
9. A non-transitory computer readable storage medium containing instructions stored thereon that when processed by a processor cause said processor to implement a method of enhancing the background of an original document image, the instructions comprising instructions for:
detecting the background color of the original document image by finding a pixel value instructions for determining P eb as:
P
eb
=
{
N
if
P
b
≥
P
gw
min
P
gb
max
if
P
b
<
P
gb
max
P
b
Otherwise
where P gwmin is the minimum pixel value that can be safely set to white on the enhanced image and P gbmax is the maximum pixel value that can be safely considered as black on the enhanced image, and N is the pixel resolution of the original image.
10. The medium of claim 9 , further comprising instructions for providing factor values F l and F r to instructions for implementing a function to find the left pixel value P lb of the background color range as a function of the histogram of P b divided by F l and the right pixel value P rb of the background color range as a function of the histogram of P b divided by F r , whereby pixel values in the color range defined by P lb and P rb are determined to be part of the image background.
11. The medium of claim 9 , wherein the instructions for determining the desired enhanced background color P eb of the original document image from P b comprises corresponding to a peak in a histogram of the original document image;
determining the desired enhanced background color of the original document image from a background pixel value P b that is in the center of the background color range;
determining the factors F l and F r as follows:
F
l
=
{
β
if
P
b
≥
T
ghigh
α
Otherwise
F
r
=
{
α
if
P
b
≤
T
glow
β
Otherwise
where values α and β are decided based on experimental values and where T ghigh and T glow are high and low pixel thresholds, respectively, and said factors F l and F r define a range to distinguish the background pixel value P b from a foreground pixel value; and
enhancing pixels of the original document image in said range to the desired enhanced background color.
12. The medium of claim 11 , wherein the instructions for enhancing the original document image to the desired enhanced background color comprises instructions for:
if P eb is N, generating a look-up table LT as follows, where i is the grayscale image pixel value in the range [0,N], and B is a small value that can always be safely set to pure black value 0:
LT
(
i
)
=
{
0
i
≤
B
N
i
≥
P
l
b
N
·
(
i
-
B
)
P
l
b
-
B
Otherwise
;
if P eb is not N, generating a look-up table LT as:
LT
(
i
)
=
{
P
eb
if
P
l
b
≤
i
≤
P
rb
i
Otherwise
;
and
enhancing the image background such that for each image pixel with value p on the original image the pixel value is changed from p to LT(p).