Method for dynamically calibrating an image capture device
A method for dynamically calibrating an image capture device comprises: a) determining a distance (D CRT , D EST ) to an object within a scene; b) determining a first lens actuator setting (DAC INIT ) for the determined distance; c) determining a second lens actuator setting (DAC FOCUS ) providing maximum sharpness for the object in a captured image of the scene; and d) storing the determined distance (D CRT , D EST ) and the first and second lens actuator settings. These steps are repeated at a second determined distance separated from the first determined distance. A calibration correction (ERR NEAR PLP , ERR FAR PLP ) for stored calibrated lens actuator settings (DAC NEAR PLP , DAC FAR PLP ) is determined as a function of at least: respective differences between the second lens actuator setting (DAC FOCUS ) and the first lens actuator setting (DAC INIT ) for each of the first and second determined distances; and the stored calibrated lens actuator settings are adjusted according to the determined calibration corrections.
1. A method for dynamically calibrating an image capture device comprising:
a) determining a distance (D CRT , D EST ) to an object within a scene being imaged by said image capture device;
b) determining a first lens actuator setting (DAC INIT ) as a function of: a stored calibrated lens actuator setting (DAC NEAR PLP ) for a pre-determined near focus distance (D NEAR ), a stored calibrated lens actuator setting (DAC FAR PLP ) for a pre-determined far focus distance (D FAR ), a focal length (f) of the image capture device and said determined distance (D CRT , D EST );
c) determining a second lens actuator setting (DAC FOCUS ) providing maximum sharpness for said object in a captured image of said scene; and
d) storing said determined distance (D CRT , D EST ), first lens actuator setting (DAC INIT ) and second lens actuator setting (DAC FOCUS );
e) subsequently repeating steps a) to c) at a second determined distance separated from said first determined distance;
f) determining a calibration correction (ERR NEAR PLP , ERR FAR PLP ) for each of said calibrated lens actuator settings (DAC NEAR PLP , DAC FAR PLP ) as a function of at least: respective differences between said second lens actuator setting (DAC FOCUS ) and said first lens actuator setting (DAC INIT ) for each of said first and second determined distances; and
g) adjusting the stored calibrated lens actuator settings according to said determined calibration corrections.
2. A method according to claim 1 further comprising, prior to said step of determining a first lens actuator setting (DAC INIT ), adjusting said stored calibrated lens actuator settings (DAC NEAR PLP , DAC FAR PLP ) as a function of an operating temperature of the device and/or a function of an operating orientation of the device; and
storing said operating temperature and/or operating orientation of the device with said determined distance (D CRT , D EST ), first lens actuator setting (DAC INIT ) and second lens actuator setting (DAC FOCUS ).
3. A method according to claim 1 wherein determining said second lens actuator setting (DAC FOCUS ) comprises beginning a search using said first lens actuator setting.
4. A method according to claim 1 wherein said first and second determined distances are separated from one another by at least the respective depths of field at said first and second determined distances.
5. A method according to claim 1 comprising periodically repeating steps a) to g).
6. A method according to claim 1 wherein there is a linear relationship between a lens actuator setting and a lens position required to focus at a given distance.
7. A method according to claim 1 wherein determining said first lens actuator setting (DAC INIT ) is based on the following formula:
DAC
INIT
=
DAC
FAR
PLP
+
(
DAC
NEAR
PLP
-
DAC
FAR
PLP
)
*
(
D
FAR
-
D
CRT
,
EST
)
(
D
FAR
-
D
NEAR
)
*
(
D
NEAR
-
f
)
(
D
CRT
,
EST
-
f
)
where
DAC FAR PLP is said stored calibrated lens actuator setting for a pre-determined far focus distance;
DAC NEAR PLP stored calibrated lens actuator setting for a pre-determined near focus distance;
D FAR is said far focus distance
D NEAR is said near focus distance
D CRT,EST is said determined distance; and
f is said focal length.
8. The method of claim 1 wherein said step of determining said distance to an object comprises measuring said distance (D CRT ).
9. A method according to claim 1 comprising determining said calibration correction (ERR NEAR PLP , ERR FAR PLP ) for each of said calibrated lens actuator settings (DAC NEAR PLP , DAC FAR PLP ) as a function of: respective differences between said second lens actuator setting (DAC D CRT FOCUS ) and said first lens actuator setting (DAC D CRT INIT ) for each of said first and second determined distances; respective differences between said first and second determined distances and one of said pre-determined near or far focus distances; a difference between said pre-determined near and far focus distances; and respective differences between said first and second determined distances and said focal length (f).
10. A method according to claim 9 wherein said calibration correction for each of said calibrated lens actuator settings, ERR FAR PLP and ERR NEAR PLP , is calculated according to the following formulae:
{
ERR
FAR
PLP
=
∝
D
2
*
ERR
D
1
-
∝
D
1
*
ERR
D
2
∝
D
2
-
∝
D
1
ERR
NEAR
PLP
=
(
1
-
∝
D
2
)
*
ERR
D
1
-
(
1
-
∝
D
1
)
*
ERR
D
2
∝
D
1
-
∝
D
2
where D 1 and D 2 are said first and second determined distances respectively, ERR D 1 and ERR D 2 are computed using the following formula:
ERR D 1 =DAC D 1 FOCUS −DAC D 1 INIT
ERR D 2 =DAC D 2 FOCUS −DAC D 2 INIT
and ∝ D 1 and ∝ D 2 are computed using the following formulae:
{
∝
D
1
=
(
D
FAR
-
D
1
)
(
D
FAR
-
D
NEAR
)
*
(
D
NEAR
-
f
)
(
D
1
-
f
)
∝
D
2
=
(
D
FAR
-
D
2
)
(
D
FAR
-
D
NEAR
)
*
(
D
NEAR
-
f
)
(
D
2
-
f
)
.
11. A method according to claim 10 wherein said adjusting the stored calibrated lens actuator settings comprises providing new settings DAC FAR NEW and DAC NEAR NEW according to the formulae:
{
DAC
FAR
NEW
=
DAC
FAR
PLP
+
ERR
FAR
PLP
DAC
NEAR
NEW
=
DAC
NEAR
PLP
+
ERR
NEAR
PLP
.
12. The method of claim 1 wherein the object is a human face.
13. The method of claim 1 wherein said step of determining said distance to an object comprises estimating a distance (D EST ) to said object based on an imaged size of said object and an assumed dimension of said object.
14. The method of claim 13 comprising testing whether a potential correction resulting from an estimated distance exceeds a threshold value before either: storing said determined distance, first lens actuator setting and second lens actuator setting; or determining said calibration correction.
15. The method of claim 14 wherein said threshold is a function of an absolute difference (DAC STEP ) between the corresponding actuator settings at a far limit of focus (D F ) and a near limit of focus (D N ) at a given focus distance (D).
16. A non-transitory computer program product comprising a computer readable medium on computer readable instructions are stored and which when executed on an image capture device are arranged to perform the steps of claim 1 .
17. An image capture device configured to perform the steps of claim 1 .