Graphics Reference
In-Depth Information
Lens plane
Sensor plane
Virtual sensor plane
x
x - u
x - u
α
u
F
F
' = α
F
Figure 5.22
Digital refocusing involves moving the sensor plane with respect to the lens, just as a
real camera would be focused. (For clarity, the planes are shown in only one dimension.)
Refocusing on a virtual sensor plane a factor of α further away from the lens than the
original sensor plane moves the ray intersection a factor of α away from the perpendicular.
(After [Ng et al. 05] c
2005 ACM, Inc. Included here by permission.)
where
d
is the diameter of the camera aperture. The value
L
F
is known as the
scene radiance
of the pixel.
The cosine factor is sometimes absorbed into
L
F
, or, in the case of a suf-
ficiently small field of view, regarded as constant and then factored out of the
integral. The aperture function can likewise be absorbed into
L
F
, in which case
Equation (5.6) reduces to just
F
2
L
F
(
1
E
F
(
x
,
y
)=
x
,
y
,
u
,
v
)
dudv
,
(5.7)
and
E
F
thus becomes directly proportional to
L
F
.
Now suppose that the camera focus is changed, which amounts to changing
the distance
F
between the lens and sensor plane as illustrated in Figure 5.22.
The new position of the sensor plane, which is called the “virtual sensor plane” in
Figure 5.22, is placed at distance
F
=
α
F
. The new light field
L
F
on the sensor
plane at
F
becomes a “sheared” version of the original sensor plane light field:
L
F
1
u
1
v
v
1
α
x
α
,
1
α
y
α
,
L
F
(
u
,
v
,
x
,
y
)=
−
+
−
+
u
,
.
(5.8)
The photosite irradiance (pixel value) is then given by substituting Equation (5.8)
into Equation (5.7):
2
F
2
L
F
1
u
1
v
v
dudv
1
1
α
x
α
,
1
α
y
α
,
E
F
(
x
,
y
)=
−
+
−
+
u
,
.
(5.9)
α
