Method and system to reduce flicker artifacts in captured images
View Patent ↗In one embodiment, a method for controlling artifacts in an image due to a presence of flicker in ambient light is provided. The method comprises determining a power frequency for the ambient light; determining a magnitude and phase of flicker fluctuation in the image due to the flicker; and adjusting the image for the flicker fluctuation based on the power frequency, magnitude, and phase.
1. A method for controlling artifacts in an image due to a presence of flicker in ambient light, comprising:
determining a power frequency for the ambient light;
determining a magnitude and phase of flicker fluctuation in the image due to the flicker comprising:
capturing a first image at an exposure time T 1 =π/ω;
capturing a second image at an exposure time T 2 =π/(2ω);
determining a function Z(α) defined as
Z
(
α
)
=
Y
(
α
,
π
2
ω
)
-
1
2
Y
(
α
,
π
ω
)
,
where
Y
(
α
,
π
2
ω
)
and
Y
(
C
,
π
ω
)
are corresponding pixel values in the second and first images respectively, α is the phase of the flicker fluctuation, and
ω=2πf where f is the power frequency of the ambient light; and
deriving the magnitude and phase of the flicker fluctuation based on the Z function; and
adjusting the image for the flicker fluctuation based on the power frequency, magnitude, and phase.
2. The method of claim 1 , wherein determining the power frequency comprises comparing differences between images taken at exposure times matched to possible choices for the power frequency.
3. The method of claim 2 , wherein comparing the differences comprises calculating a first difference between two images taken at an exposure time matched to a 50 Hz power frequency, calculating a second difference between two images taken at an exposure time matched to a 60 Hz power frequency, and comparing the first difference to the second difference.
4. The method of claim 3 , wherein if the first difference is less than the second difference, then the power frequency is determined to be 50 Hz.
5. The method of claim 3 , wherein if the first difference is greater than the second difference, then the power frequency is determined to be 60 Hz.
6. The method of claim 1 , further comprising adjusting the images for motion compensation prior to comparing the differences in the images.
7. The method of claim 6 , wherein deriving the magnitude and phase of the flicker fluctuation comprises estimating the phase α of the flicker fluctuation using the equation
α
=
(
2
k
+
1
)
π
2
ω
=
(
n
+
u
)
β
+
α
0
if
Z
=
0
and is going from negative to positive; where n represents a pixel row number, β is a difference between read-outs of two consecutive pixel rows, and α 0 the initial phase for the first pixel row.
8. The method of claim 6 , wherein deriving the magnitude and phase of flicker fluctuation comprises estimating the phase α of the flicker fluctuation using the equation
α
=
(
2
k
+
2
)
π
2
ω
=
(
n
+
u
)
β
+
α
0
if
Z
=
0
and is going from positive to negative; where n represents a pixel row number, β is a difference between read-outs of two consecutive pixel rows, and α 0 is the initial phase for the first pixel row.
9. The method of claim 6 , wherein deriving the magnitude and phase of the flicker fluctuation comprises finding maximum and minimum points Z max and Z min , respectively in the Z function and solving for the amplitude A using the equations.
Z
ma
x
=
A
ω
(
2
-
1
)
Z
m
i
n
=
-
A
ω
(
2
-
1
)
Z
m
ax
=
-
Z
m
i
n
.
10. The method of claim 9 , wherein deriving the magnitude and phase of the flicker fluctuation determining α 0 ,the initial phase for the first pixel row as per claims 8 or claim 9 , and solving for the amplitude A using the equation
Z
(
α
)
=
Y
(
α
,
π
2
ω
)
-
1
2
Y
(
α
,
π
ω
)
=
A
ω
(
1
-
sin
(
ωα
)
-
cos
(
ωα
)
)
sign
[
sin
(
2
ωα
)
]
.
11. The method of claim 6 , wherein the determining the function Z(α) comprises averaging multiple periods of the function Z(α) to smooth out noise.
12. The method of claim 1 , wherein the determining the function Z(α) comprises averaging the values
Z
(
α
)
=
Y
(
α
,
π
2
ω
)
-
1
2
Y
(
α
,
π
ω
)
for each pixel row.
13. The method of claim 1 , wherein the determining the function Z(α) comprises determining the values
Z
(
α
)
=
Y
(
α
,
π
2
ω
)
-
1
2
Y
(
α
,
π
ω
)
for one period of the function Z(α).
14. The method of claim 1 , wherein adjusting the image for the flicker fluctuation comprises determining a flicker term based on the power frequency, magnitude, and phase of the ambient light for each pixel row in the image, and subtracting the flicker term from each pixel in the row.
15. The method of claim 1 , further comprising adding a constant to each pixel in the row to compensate for a loss in brightness.
16. The method of claim 1 , wherein determining the magnitude and phase of flicker fluctuation in the image due to the flicker is performed periodically.
17. An image processor, comprising:
an image buffer to store image data for a captured image; and
flicker reduction logic to determine a power frequency for ambient light; to determine a magnitude and phase of flicker fluctuation in the image data due to the flicker; and to adjust the image data for the flicker fluctuation based on the power frequency, magnitude, and phase; wherein determining the magnitude and phase of the flicker fluctuation in the image due to the flicker comprises capturing a first image at an exposure time T 1 =π/ω;
capturing a second image at an exposure time T 2 =π/(2ω);
determining a function Z(α) defined as
Z
(
α
)
=
Y
(
α
,
π
2
ω
)
-
1
2
Y
(
α
,
π
ω
)
,
where
Y
(
α
,
π
2
ω
)
and
Y
(
C
,
π
ω
)
are corresponding pixel values in the second and first images respectively, α is the phase of the flicker fluctuation, and ω=2πf where f is the power frequency of the ambient light; and
deriving the magnitude and phase of the flicker fluctuation based on the Z function.
18. The image processor of claim 17 , wherein the flicker reduction logic determines the power frequency by comparing differences between images taken at exposure times matched to possible choices for the power frequency.
19. A camera system, comprising:
camera optics;
an image sensor positioned so that light passing through the camera optics impinges on the image sensor; and
an image processor coupled to the image sensor to receive image data for a captured image therefrom, wherein the image processor comprises flicker reduction logic to determine a power frequency for the ambient light; to determine a magnitude and phase of flicker fluctuation in the image data due to the flicker; and to adjust the image data for the flicker fluctuation based on the power frequency, magnitude, and phase; wherein determining the magnitude and phase of the flicker fluctuation in the image due to the flicker comprises capturing a first image at an exposure time T 1 =π/ω;
capturing a second image at an exposure time T 2 =π/(2ω);
determining a function Z(α) defined as
Z
(
α
)
=
Y
(
α
,
π
2
ω
)
-
1
2
Y
(
α
,
π
ω
)
,
where
Y
(
α
,
π
2
ω
)
and
Y
(
C
,
π
ω
)
are corresponding pixel values in the second and first images respectively, α is the phase of the flicker fluctuation, and
ω=2πf where f is the power frequency of the ambient light; and
deriving the magnitude and phase of the flicker fluctuation based on the Z function.