Reducing signal overshoots and undershoots in demosaicking
An image processing method that demosaicks a mosaic input image to generate a full color output image. The image processing method calculates both vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image. Next, the image processing method calculates an enhanced version of both vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image. Then, the image processing method interpolates a G component for each of the original R and B components. Next, the image processing method detects a signal overshoot or undershoot in each interpolated G component and to clamps each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component. Next, the image processing method interpolates missing R and/or B components in each pixel location of the captured image.
1. An image processing apparatus that receives a mosaic input image and subjects the mosaic input image subject to a series of image processing to generate a full color output image, the image processing apparatus comprising:
a luminance-chrominance difference component computation module configured to calculate both vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image;
an enhanced luminance-chrominance difference component computation module configured to calculate an enhanced version of both vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image;
an edge orientation detection module configured to evaluate the variations in the enhanced luminance-chrominance difference components in order to determine the direction in which demosaicking should be performed;
a luminance component interpolation module configured to interpolate a G component for each of the original R and B components;
a clamping module configured to detect a signal overshoot or undershoot in each interpolated G component and to clamp each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component;
a chrominance component interpolation module configured to interpolating an R component for each of the original G and B components and a B component for each of the original R and G components.
2. The image processing apparatus as recited in claim 1 , wherein:
the luminance-chrominance difference component computation module is configured to calculate both vertical and horizontal luminance-chrominance difference components Δ v and Δ h for each pixel (r,s) of the mosaic input image z according to the following equations:
Δ
(
r
,
s
)
v
=
{
z
(
r
,
s
)
-
(
z
(
r
-
1
,
s
)
+
z
(
r
+
1
,
s
)
)
/
2
for
(
odd
r
,
odd
s
)
and
(
even
r
,
even
s
)
(
z
(
r
-
1
,
s
)
+
z
(
r
+
1
,
s
)
)
/
2
-
z
(
r
,
s
)
otherwise
;
Δ
(
r
,
s
)
h
=
{
z
(
r
,
s
)
-
(
z
(
r
,
s
-
1
)
+
z
(
r
,
s
+
1
)
)
/
2
for
(
odd
r
,
odd
s
)
and
(
even
r
,
even
s
)
(
z
(
r
,
s
-
1
)
+
z
(
r
,
s
+
1
)
)
/
2
-
z
(
r
,
s
)
otherwise
;
the enhanced luminance-chrominance difference component computation module is configured to calculate the enhanced versions {circumflex over (Δ)} v and {circumflex over (Δ)} h of the vertical and horizontal luminance-chrominance difference components Δ v and Δ h , respectively, for each pixel (r,s) of the mosaic input image z according to the following equations:
Δ
^
(
r
,
s
)
v
=
(
Δ
(
r
-
2
,
s
)
v
+
2
Δ
(
r
-
1
,
s
)
v
+
4
Δ
(
r
,
s
)
v
+
2
Δ
(
r
+
1
,
s
)
v
+
Δ
(
r
+
2
,
s
)
v
)
/
10
;
Δ
^
(
r
,
s
)
h
=
(
Δ
(
r
,
s
-
2
)
h
+
2
Δ
(
r
,
s
-
1
)
h
+
4
Δ
(
r
,
s
)
h
+
2
Δ
(
r
,
s
+
1
)
h
+
Δ
(
r
,
s
+
2
)
h
)
/
10
;
the edge orientation detection module is configured to evaluate the variations in the enhanced luminance-chrominance difference components {circumflex over (Δ)} v and {circumflex over (Δ)} h according to the following equations:
υ
(
r
,
s
)
v
=
∑
i
=
-
1
1
(
(
Δ
^
(
r
-
1
,
s
+
i
)
v
-
Δ
^
(
r
,
s
+
i
)
v
)
2
+
(
Δ
^
(
r
,
s
+
i
)
v
-
Δ
^
(
r
+
1
,
s
+
i
)
v
)
2
+
(
Δ
^
(
r
-
1
,
s
+
i
)
v
-
Δ
^
(
r
+
1
,
s
+
i
)
v
)
2
)
;
υ
(
r
,
s
)
h
=
∑
i
=
-
1
1
(
(
Δ
^
(
r
+
i
,
s
-
1
)
h
-
Δ
^
(
r
+
i
,
s
)
h
)
2
+
(
Δ
^
(
r
+
i
,
s
)
h
-
Δ
^
(
r
+
i
,
s
+
1
)
h
)
2
+
(
Δ
^
(
r
+
i
,
s
-
1
)
h
-
Δ
^
(
r
+
i
,
s
+
1
)
h
)
2
)
;
and
the luminance component interpolation module is configured to interpolate a luminance component for each of the original R and B components according to the following equations:
G
(
r
,
s
)
=
z
(
r
,
s
)
+
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
,
if
υ
(
r
,
s
)
v
≤
υ
(
r
,
s
)
h
,
or
G
(
r
,
s
)
=
z
(
r
,
s
)
+
(
Δ
^
(
r
,
s
-
1
)
h
+
Δ
^
(
r
,
s
+
1
)
h
)
/
2
,
if
υ
(
r
,
s
)
v
>
υ
(
r
,
s
)
h
.
3. The image processing apparatus as recited in claim 2 , wherein the clamping module is configured to detect a signal overshoot or undershoot in the interpolated G component and to clamp the interpolated G component to the closest neighboring original G component in pixel locations with a detected signal overshoot or undershoot according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
+
1
,
s
)
}
for
VD
if
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
>
Δ
(
r
,
s
)
v
-
Δ
^
(
r
,
s
)
v
f
C
{
G
(
r
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
}
for
HD
if
(
Δ
^
(
r
,
s
-
1
)
h
+
Δ
^
(
r
,
s
+
1
)
h
)
/
2
>
Δ
(
r
,
s
)
h
-
Δ
^
(
r
,
s
)
h
G
(
r
,
s
)
otherwise
,
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed; and
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the two original green neighbors in the demosaicking direction if G (r,s) is larger or smaller than the two original green neighbors in the demosaicking direction.
4. The image processing apparatus as recited in claim 2 , wherein the clamping module is configured to detect a signal overshoot or undershoot in the interpolated G component and to clamp the interpolated G component to the closest neighboring original G component in pixel locations with a detected signal overshoot or undershoot according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
+
1
,
s
)
}
for
VD
if
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
>
min
(
α
Δ
(
r
,
s
)
v
-
Δ
(
r
,
s
)
v
,
β
)
f
C
{
G
(
r
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
}
for
HD
if
(
Δ
^
(
r
,
s
-
1
)
h
+
Δ
^
(
r
,
s
+
1
)
h
)
/
2
>
min
(
α
Δ
(
r
,
s
)
h
-
Δ
(
r
,
s
)
h
,
β
)
G
(
r
,
s
)
otherwise
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed;
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the two original green neighbors in the demosaicking direction if G (r,s) is larger or smaller than the two original green neighbors in the demosaicking direction;
α is a positive scaling parameter;
β is a positive global luminance-chrominance difference; and
min(·) denotes the minimum operator.
5. The image processing apparatus as recited in claim 1 , wherein the clamping module is further configured to:
detect a signal overshoot or undershoot in each interpolated G component by comparing the luminance-chrominance difference components with one or more parameters and clamp each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component located in the direction in which demosaicking is being performed; and
in each interpolated G component pixel locations where no signal overshoot or undershoot was detected by comparing the luminance-chrominance difference components with the one or more parameters, the clamping module is further configured to:
estimate the missing chrominance component using the interpolated G component;
compare the original and estimated chrominance components with one or more parameters in order to detect a signal overshoot or undershoot in the interpolated G component; and
clamp the interpolated G component with a detected signal overshoot or undershoot to the closest original neighboring original G component.
6. The image processing apparatus as recited in claim 5 , wherein the clamping module is configured to estimate the missing chrominance component using the interpolated G component according to the following equations:
G
(
r
,
s
)
-
(
Δ
^
(
r
,
s
-
1
)
v
+
Δ
^
(
r
,
s
+
1
)
v
)
/
2
if
υ
(
r
,
s
)
v
>
υ
(
r
,
s
)
h
;
and
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
h
+
Δ
^
(
r
+
1
,
s
)
h
)
/
2
if
υ
(
r
,
s
)
v
≤
υ
(
r
,
s
)
h
.
7. The image processing apparatus as recited in claim 5 , wherein the clamping module is configured to compare the original and estimated chrominance components with one or more parameters in order to detect a signal overshoot or undershoot in the interpolated G component and clamp each interpolated G component with a signal overshoot or undershoot to the closest original neighboring original G component according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
for
VD
if
{
z
(
r
,
s
)
<
δ
or
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
h
+
Δ
^
(
r
+
1
,
s
)
h
)
/
2
<
δ
}
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
for
HD
if
{
z
(
r
,
s
)
<
δ
or
G
(
r
,
s
)
-
(
Δ
^
(
r
,
s
-
1
)
v
+
Δ
^
(
r
,
s
+
1
)
v
)
/
2
<
δ
}
G
(
r
,
s
)
otherwise
,
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed;
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the four original green neighbors if G (r,s) is larger or smaller than the four original green neighbors; and
δ denotes the predetermined global image level used to localize image areas with the reduced brightness in the chrominance channel.
8. The image processing apparatus as recited in claim 5 , wherein the clamping module is configured to compare the original and estimated chrominance components with one or more parameters in order to detect a signal overshoot or undershoot in the interpolated G component and clamp each interpolated G component with a signal overshoot or undershoot to the closest original neighboring original G component according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
for
VD
if
{
[
z
(
r
,
s
)
]
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
v
)
or
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
[
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
h
+
Δ
^
(
r
+
1
,
s
)
h
)
/
2
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
v
)
}
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
for
HD
if
{
[
z
(
r
,
s
)
]
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
v
)
or
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
[
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
h
)
}
G
(
r
,
s
)
otherwise
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed;
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the four original green neighbors if G (r,s) is larger or smaller than the four original green neighbors;
δ denotes the predetermined global image level used to localize image areas with the reduced brightness in the chrominance channel;
min(·) denotes the minimum operator; and
[·] denotes an inverse gamma correction operation used to map the relevant components to their original range if a gamma correction step is used before demosaicking.
9. A method for demosaicking a mosaic input image to generate a full color output image, the method comprising the steps of:
a) calculating both vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image;
b) calculating an enhanced version of both the vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image;
c) evaluating the variations in the enhanced luminance-chrominance difference components in order to determine the direction in which demosaicking should be performed;
d) interpolating a G component for each of the original R and B components;
e) detecting a signal overshoot or undershoot in each interpolated G component and clamping each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component; and
f) interpolating an R component for each of the original G and B components and a B component for each of the original R and G components.
10. The method as recited in claim 9 , wherein:
the step a) comprises calculating both vertical and horizontal luminance-chrominance difference components Δ v and Δ h for each pixel (r,s) of the mosaic input image z according to the following equations:
Δ
(
r
,
s
)
v
=
{
z
(
r
,
s
)
-
(
z
(
r
-
1
,
s
)
+
z
(
r
+
1
,
s
)
)
/
2
for
(
odd
r
,
odd
s
)
and
(
even
r
,
even
s
)
(
z
(
r
-
1
,
s
)
+
z
(
r
+
1
,
s
)
)
/
2
-
z
(
r
,
s
)
otherwise
;
and
Δ
(
r
,
s
)
h
=
{
z
(
r
,
s
)
-
(
z
(
r
,
s
-
1
)
+
z
(
r
,
s
+
1
)
)
/
2
for
(
odd
r
,
odd
s
)
and
(
even
r
,
even
s
)
(
z
(
r
,
s
-
1
)
+
z
(
r
,
s
+
1
)
)
/
2
-
z
(
r
,
s
)
otherwise
.
11. The method as recited in claim 10 , wherein:
the step b) comprises calculating an enhanced version {circumflex over (Δ)} v and {circumflex over (Δ)} h of both the vertical luminance-chrominance difference components Δ v and Δ h for each pixel (r,s) of the mosaic input image z according to the following equations:
Δ
^
(
r
,
s
)
v
=
(
Δ
(
r
-
2
,
s
)
v
+
2
Δ
(
r
-
1
,
s
)
v
+
4
Δ
(
r
,
s
)
v
+
2
Δ
(
r
+
1
,
s
)
v
+
Δ
(
r
+
2
,
s
)
v
)
/
10
:
and
Δ
^
(
r
,
s
)
h
=
(
Δ
(
r
,
s
-
2
)
h
+
2
Δ
(
r
,
s
-
1
)
h
+
4
Δ
(
r
,
s
)
h
+
2
Δ
(
r
,
s
+
1
)
h
+
Δ
(
r
,
s
+
2
)
h
)
/
10.
12. The method as recited in claim 11 , wherein:
the step c) comprising evaluating the variations in the enhanced luminance-chrominance difference components {circumflex over (Δ)} v and {circumflex over (Δ)} h in order to determine the direction in which demosaicking should be performed according to the following equations:
υ
(
r
,
s
)
v
=
∑
i
=
-
1
1
(
(
Δ
^
(
r
-
1
,
s
+
i
)
v
-
Δ
^
(
r
,
s
+
i
)
v
)
2
+
(
Δ
^
(
r
,
s
+
i
)
v
-
Δ
^
(
r
+
1
,
s
+
i
)
v
)
2
+
(
Δ
^
(
r
-
1
,
s
+
i
)
v
-
Δ
^
(
r
+
1
,
s
+
i
)
v
)
2
)
;
υ
(
r
,
s
)
h
=
∑
i
=
-
1
1
(
(
Δ
^
(
r
+
i
,
s
-
i
)
h
-
Δ
^
(
r
+
i
,
s
)
h
)
2
+
(
Δ
^
(
r
+
i
,
s
)
h
-
Δ
^
(
r
+
i
,
s
+
i
)
h
)
2
+
(
Δ
^
(
r
+
i
,
s
-
1
)
h
-
Δ
^
(
r
+
i
,
s
+
i
)
h
)
2
)
.
13. The method as recited in claim 12 , wherein:
the step d) comprises interpolating a G component for each of the original R and B components according to the following equations:
G
(
r
,
s
)
=
z
(
r
,
s
)
+
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
,
if
υ
(
r
,
s
)
v
≤
υ
(
r
,
s
)
h
,
or
G
(
r
,
s
)
=
z
(
r
,
s
)
+
(
Δ
^
(
r
,
s
-
1
)
h
+
Δ
^
(
r
,
s
+
1
)
h
)
/
2
,
if
υ
(
r
,
s
)
v
>
υ
(
r
,
s
)
h
.
14. The method as recited in claim 13 , wherein:
the step e) is accomplished according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
+
1
,
s
)
}
for
VD
if
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
>
Δ
(
r
,
s
)
v
-
Δ
^
(
r
,
s
)
v
f
C
{
G
(
r
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
}
for
HD
if
(
Δ
^
(
r
,
s
-
1
)
h
+
Δ
^
(
r
,
s
+
1
)
h
)
/
2
>
Δ
(
r
,
s
)
h
-
Δ
^
(
r
,
s
)
h
G
(
r
,
s
)
otherwise
,
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed; and
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the two original green neighbors in the demosaicking direction if G (r,s) is larger or smaller than the two original green neighbors in the demosaicking direction.
15. The method as recited in claim 13 , wherein:
the step e) is accomplished according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
for
VD
if
z
(
r
-
1
,
s
)
,
z
(
r
+
1
,
s
)
}
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
>
min
(
α
Δ
(
r
,
s
)
v
-
Δ
^
(
r
,
s
)
v
,
β
)
f
C
{
G
(
r
,
s
)
,
for
HD
if
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
}
(
Δ
^
(
r
,
s
-
1
)
h
+
Δ
^
(
r
,
s
+
1
)
h
)
/
2
>
min
(
α
Δ
(
r
,
s
)
h
-
Δ
^
(
r
,
s
)
h
,
β
)
G
(
r
,
s
)
otherwise
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed;
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the two original green neighbors in the demosaicking direction if G (r,s) is larger or smaller than the two original green neighbors in the demosaicking direction;
α is a positive scaling parameter;
β is a positive global luminance-chrominance difference; and
min(·) denotes the minimum operator.
16. The method as recited in claim 13 , wherein the step e) further comprises the steps of:
e1) detecting a signal overshoot or undershoot in each interpolated G component by comparing the luminance-chrominance difference components with one or more parameters and clamping each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component located in the direction in which demosaicking is being performed; and
e2) in each interpolated G component pixel locations where no signal overshoot or undershoot was detected in e1), performing the following steps:
estimating the missing chrominance component using the interpolated G component;
comparing the original and estimated chrominance components with one or more parameters in order to detect a low chrominance value; and
clamping the interpolated G component with a low chrominance value to the closest original neighboring original G component.
17. The method as recited in claim 16 , wherein estimating the missing chrominance component using the interpolated G component is accomplished according to the following equations:
G
(
r
,
s
)
-
(
Δ
^
(
r
,
s
-
1
)
v
+
Δ
^
(
r
,
s
+
1
)
v
)
/
2
if
υ
(
r
,
s
)
v
>
υ
(
r
,
s
)
h
;
and
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
h
+
Δ
^
(
r
+
1
,
s
)
h
)
/
2
if
υ
(
r
,
s
)
v
≤
υ
(
r
,
s
)
h
.
18. The method as recited in claim 16 , wherein the step e2) is accomplished according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
for
VD
if
{
z
(
r
,
s
)
<
δ
or
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
h
+
Δ
^
(
r
+
1
,
s
)
h
)
/
2
<
δ
}
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
for
HD
if
{
z
(
r
,
s
)
<
δ
or
G
(
r
,
s
)
-
(
Δ
^
(
r
,
s
-
1
)
v
+
Δ
^
(
r
,
s
+
1
)
v
)
/
2
<
δ
}
G
(
r
,
s
)
otherwise
,
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed;
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the four original green neighbors if G (r,s) is larger or smaller than the four original green neighbors; and
δ denotes the predetermined global image level used to localize image areas with the reduced brightness in the chrominance channel.
19. The method as recited in claim 16 , wherein the step e2) are accomplished according to the following equation:
G
(
r
,
s
)
=
{
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
for
VD
if
{
[
z
(
r
,
s
)
]
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
v
)
or
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
[
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
h
+
Δ
^
(
r
+
1
,
s
)
h
)
/
2
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
v
)
}
f
C
{
G
(
r
,
s
)
,
z
(
r
-
1
,
s
)
,
z
(
r
,
s
-
1
)
,
for
HD
if
{
[
z
(
r
,
s
)
]
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
v
)
or
z
(
r
,
s
+
1
)
,
z
(
r
+
1
,
s
)
[
G
(
r
,
s
)
-
(
Δ
^
(
r
-
1
,
s
)
v
+
Δ
^
(
r
+
1
,
s
)
v
)
/
2
<
min
(
δ
,
z
(
r
,
s
)
+
Δ
(
r
,
s
)
h
)
}
G
(
r
,
s
)
otherwise
where:
VD and HD denote the detected vertical and horizontal direction in which the demosaicking process is being performed;
f C {·} is a clamping function that replaces G (r,s) with the closest, in terms of the magnitude, of the four original green neighbors if G (r,s) is larger or smaller than the four original green neighbors;
δ denotes the predetermined global image level used to localize image areas with the reduced brightness in the chrominance channel;
min(·) denotes the minimum operator; and
[·] denotes an inverse gamma correction operation used to map the relevant components to their original range if a gamma correction step is used before demosaicking.
20. A non-transitory computer-readable medium having a program that causes a computer to actualize a method for demosaicking a mosaic input image subject to generate a full color output image, the program causing the computer to attain the functions of:
a) calculating both vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image;
b) calculating an enhanced version of both the vertical and horizontal luminance-chrominance difference components for each pixel of the mosaic input image;
c) evaluating the variations in the enhanced luminance-chrominance difference components in order to determine the direction in which demosaicking should be performed;
d) interpolating a G component for each of the original R and B components;
e) detecting a signal overshoot or undershoot in each interpolated G component and clamping each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component; and
f) interpolating an R component for each of the original G and B components and a B component for each of the original R and G components.
21. The non-transitory computer-readable medium as recited in claim 20 , the program further causing the computer to attain the functions of step e) by:
e1) detecting a signal overshoot or undershoot in each interpolated G component by comparing the luminance-chrominance difference components with one or more parameters and clamping each interpolated G component with a detected signal overshoot or undershoot to the closest neighboring original G component located in the direction in which demosaicking is being performed; and
e2) in each interpolated G component pixel locations where no signal overshoot or undershoot was detected in e1), performing the following steps:
estimating the missing chrominance component using the interpolated G component;
comparing the original and estimated chrominance components with one or more parameters in order to detect a low chrominance value; and
clamping the interpolated G component with a low chrominance value to the closest original neighboring original G component.