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Digital Camera Patent Abstract
A digital camera capable of performing more stable white balance
adjustment is provided. In a digital camera for adjusting white
balance of a video signal corresponding to an object and supplied
from an image sensor, a white balance adjustment circuit 34 changes
at least some of a plurality of light source regions predefined
on a color difference plane based on a result of detection of flicker
in a light source illuminating the object performed by a flicker
detection circuit 70. The white balance adjustment circuit 34 then
checks which of the plurality of light source regions containing
the changed region includes the color difference component of the
video signal, thereby estimating the light source of the object,
and adjusting white balance in accordance with the estimation result.
Digital Camera Patent Claims
1. A digital camera for performing white balance adjustment on a
video signal corresponding to an object and output from an image
sensor, comprising: a light source estimation circuit for estimating
a light source illuminating the object by checking, among a plurality
of light source regions predefined on a color difference plane,
which region includes a color difference component of the video
signal; an adjustment circuit for adjusting white balance of the
video signal in accordance with the estimated light source; and
a flicker detection circuit for detecting flicker of the light source
illuminating the object; wherein the light source estimation circuit
changes the light source region based on a result of flicker detection
performed by the flicker detection circuit.
2. A digital camera according to claim 1, wherein at least a fluorescent
light region and a daylight region are defined as the light source
regions, and the light source estimation circuit reduces an area
of the daylight region overlapping the fluorescent light region
when the result of flicker detection indicates that a flicker is
present.
3. A digital camera according to claim 1, wherein at least a fluorescent
light region and a daylight region are defined as the light source
regions, and the light source estimation circuit reduces an area
of the fluorescent light region overlapping the daylight light region
when the result of flicker detection indicates that no flicker is
present.
4. A white balance adjustment method for adjusting white balance
of a video signal corresponding to an object and output from an
image sensor, comprising: a flicker detection step for detecting
flicker of a light source illuminating the object; a region changing
step for changing at least part of a plurality of light source regions
predefined on a color difference plane based on a result of flicker
detection; a light source estimation step for estimating the light
source of the object by checking which of the plurality of light
source regions predefined on the color difference plane and containing
the changed region includes a color difference component of the
video signal; and an adjustment step for adjusting white balance
of the video signal in accordance with the estimated light source.
5. A white balance adjustment method according to claim 4, wherein
at least a fluorescent light region and a daylight region are defined
as the light source regions, and at the light source estimation
step, an area of the daylight region overlapping the fluorescent
light region is reduced when the result of flicker detection indicates
that flicker is present.
6. A white balance adjustment method according to claim 4, wherein
at least a fluorescent light region and a daylight region are defined
as the light source regions, and at the light source estimation
step, an area of the fluorescent light region overlapping the daylight
light region is reduced when the result of flicker detection indicates
that no flicker is present.
Digital Camera Patent Description
FIELD OF THE INVENTION
[0001] The present invention relates to a digital camera and a
white balance adjustment method for adjusting white balance of a
video signal output from an image sensor.
BACKGROUND OF THE INVENTION
[0002] In digital cameras, such as video cameras and digital still
cameras, white balance is automatically adjusted to reproduce a
white object in white. As a conventional automatic white balance
adjustment method, a method of adjusting the balance of RGB components
(three primary color components of red, green, and blue) of a signal
for each pixel so that the average of an entire image becomes achromatic
color, is well-known in the art. This method, however, tends to
result in incorrect white balance adjustment when chromatic colors
occupy a major portion of the image.
[0003] Such incorrect white balance adjustment is called color
failure. A technique disclosed in Japanese Patent Laid-Open Publication
No. Hei 5-292533 is known as an automatic white balance adjustment
method reducing such color failure. According to this technique,
an image is divided into a plurality of blocks, and an average of
RGB values in each block is calculated to extract only blocks having
an average that falls within a predetermined range. The RGB components
are each adjusted so that the average of RGB values in the extracted
group of blocks becomes achromatic.
[0004] Another automatic white balance adjustment method for reducing
color failure is disclosed in Japanese Patent Laid-Open Publication
No. Hei 5-7369. In this method, a range of values the white balance
adjustment signal can assume is limited, thereby avoiding excessive
white balance adjustment.
[0005] Further, automatic white balance adjustment methods disclosed
in Japanese Patent Laid-Open Publications No. Hei 8-289314 and No.
2000-92509, respectively, are also known. According to such methods,
an image is divided into a plurality of blocks, and for each block,
a representative value including luminance and color difference
representing the block is calculated based on each color value within
the block. A light source illuminating an object is estimated using
the calculated representative value, and white balance is adjusted
in accordance with the estimation result.
[0006] However, an image of a white object located under indoor
fluorescent lighting usually becomes greenish, and therefore it
is hard to distinguish it from an image of a green object, such
as plants, under an outdoor solar light source, leading to occasional
false estimation of the light source illuminating a white object.
As a result, appropriate white balance adjustment may not be performed.
SUMMARY OF THE INVENTION
[0007] The present invention aims to provide a digital camera capable
of performing more stable white balance adjustment.
[0008] The digital camera according to the present invention is
a digital camera for performing white balance adjustment on a video
signal corresponding to an object and output from an image sensor,
comprising a light source estimation circuit for estimating a light
source illuminating the object by checking, among a plurality of
light source regions predefined on a color difference plane, which
region includes a color difference component of the video signal,
an adjustment circuit for adjusting white balance of the video signal
in accordance with the estimated light source, and a flicker detection
circuit for detecting flicker of the light source illuminating the
object, wherein the light source estimation circuit changes the
light source region based on a result of flicker detection performed
by the flicker detection circuit.
[0009] According to the present invention, the light source estimation
circuit changes a light source region used for estimating the light
source illuminating the object based on a result of flicker detection
by the flicker detection circuit. Assuming that, for example, a
fluorescent light region and a daylight region are defined as the
light source regions, the light source estimation circuit reduces
an area of the daylight region overlapping the fluorescent light
region when the result of the flicker detection indicates that flicker
is present. On the other hand, when the flicker detection result
indicates that no flicker is present, the light source estimation
circuit reduces an area of the fluorescent light region overlapping
the daylight region. As a result, the light source can be more accurately
estimated by the light source estimation circuit, thereby achieving
more stable white balance adjustment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows functional blocks of a digital camera according
to an embodiment of the present invention;
[0011] FIG. 2 shows in detail functional blocks of an imaging unit
in the digital camera according to the embodiment of the present
invention;
[0012] FIG. 3 schematically shows a circuit configuration of a
CMOS image sensor according to the embodiment of the present invention;
[0013] FIG. 4 shows in detail the circuit configuration of the
CMOS image sensor according to the embodiment of the present invention;
[0014] FIG. 5 shows in detail a circuit configuration of a pixel
circuit forming part of the CMOS image sensor according to the embodiment
of the present invention;
[0015] FIG. 6 shows an example of a timing chart for a variety
of signals supplied to the CMOS image sensor upon flicker detection;
[0016] FIG. 7 is a chart for describing a fluctuation cycle of
a luminance level of a 50 Hz fluorescent light;
[0017] FIG. 8 shows a circuit configuration of the CMOS image sensor
having two output terminals for supplying a flicker detection video
signal;
[0018] FIG. 9 shows an example of a timing chart of a variety of
signals supplied to the CMOS image sensor having two output terminals
for separately supplying flicker detection video signals sampled
in different cycles;
[0019] FIG. 10 shows an example of a timing chart of a variety
of signals supplied to the CMOS image sensor upon taking a still
image;
[0020] FIG. 11 shows fluctuation of the luminance level when a
light source illuminating an object is a repeatedly blinking light
source, such as a fluorescent light;
[0021] FIG. 12 shows functional blocks of an image processing circuit
according to the embodiment of the present invention;
[0022] FIG. 13 shows functional blocks of a white balance adjustment
circuit according to the embodiment of the present invention;
[0023] FIG. 14 shows an example of light source regions of a fluorescent
light and daylight defined on a color difference plane;
[0024] FIG. 15A shows an example of light source regions used by
a white balance evaluation circuit to estimate the light source
illuminating the object when flicker is present according to the
embodiment of the present invention; and
[0025] FIG. 15B shows an example of light source regions used by
the white balance evaluation circuit to estimate the light source
illuminating the object when no flicker is present according to
the embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Preferred embodiments of the present invention (hereinafter
referred to as "embodiments") will be described with reference
to the accompanying drawings.
[0027] FIG. 1 is a functional block diagram of a digital camera
according to the present embodiment. An imaging unit 10 receives
light from an object under the control of a CPU 20, and supplies
a video signal in accordance with the received light. The CPU 20
is a central processing unit controlling the entire digital camera
for performing arithmetic operations for each circuit, controlling
each circuit, and the like. An image processing circuit 30 performs
predetermined image processing, such as white balance adjustment,
on a video signal, and provides the resulting image data. A display
device 40 sequentially displays a video image based on the image
data to function as a viewfinder for photographing. A storage unit
50 records image data. An operation unit 60 is a user interface
for a user to operate the digital camera when he/she takes a still
image or a moving image using the digital camera. A flicker detection
circuit 70 detects flicker of a light source, such as a fluorescent
light, having a cyclically fluctuating luminance level.
[0028] According to the present embodiment, the image processing
circuit 30 estimates the light source illuminating the object using
a result of flicker detection by the flicker detection circuit 70,
and adjusts the white balance in accordance with the detection result.
[0029] Next, the imaging unit 10 will be more specifically described.
FIG. 2 more specifically shows functional blocks of the imaging
unit 10 of the digital camera.
[0030] An optical system 110 includes a lens and an aperture diaphragm
for allowing light from the object to enter a CMOS image sensor
120 so that a desired video signal is obtained. The CMOS image sensor
120 includes a plurality of pixel circuits and the like for performing
photoelectric conversion on light received by each pixel circuit,
and supplying a video signal. The CMOS image sensor 120 is an image
sensor of an XY addressing type capable of controlling an output
of the video signal for each pixel circuit regardless of pixel circuit
arrangement. Further, according to the present embodiment, the CMOS
image sensor 120 includes two output terminals for the video signal.
When a video image is displayed on the display device 40, one of
the output terminals supplies a display video signal used for displaying
the video image on the display device 40, and the other supplies
a flicker detection video signal used by the flicker detection circuit
70 to perform flicker detection. When a still image is taken, each
output terminal supplies a recording video signal. A gain control
amplifier (AMP) 130 adjusts a gain of each video signal. An analog/digital
conversion circuit (A/D) 140 converts each video signal supplied
from the AMP 130 to a digital signal. A signal generator (SG) 160
generates a signal for synchronization between the CPU 20 and the
CMOS image sensor 120, between the CPU 20 and the AMP 130, and between
the CPU 20 and the A/D 140.
[0031] A first video memory 150 temporarily holds the display or
recording video signal supplied from the A/D 140. A second video
memory 152 temporarily holds the flicker detection or recording
video signal supplied from the A/D 140. A memory controller 154
controls output of each video signal held in the first and second
video memories 150 and 152. A switch 170 switches whether to supply
the flicker detection video signal held in the second video memory
152 to the flicker detection circuit 70 or to supply the recording
video signal to the image processing circuit 30.
[0032] When a video image is displayed on the display device 40,
the display video signal supplied from the first video memory 150
is input to the image processing circuit 30, and the flicker detection
video signal supplied from the second video memory 152 is input
to the flicker detection circuit 70. The image processing circuit
30 performs predetermined image processing on the display video
signal, and supplies the resulting data to the display device 40.
When a still image is taken, the image processing circuit 30 performs
predetermined image processing on each recording video signal supplied
from the first and second video memories 150 and 152, and produces
image data for the still image.
[0033] The flicker detection circuit 70 detects flicker based on
the flicker detection video signal, and supplies the detection result
to the CPU 20. The CPU 20 supplies the detection result to the image
processing circuit 30, which in turn estimates a light source illuminating
an object using the detection result, and performs white balance
adjustment of the input video signal.
[0034] Operation of the CMOS image sensor 120 will next be described
in further detail. FIG. 3 schematically shows a circuit configuration
of the CMOS image sensor 120. An imaging circuit 122 includes a
plurality of pixel circuits 200. The video signal is produced through
photoelectric conversion of light received in each pixel circuit
200. A first vertical scanning circuit 124 transfers to a horizontal
scanning circuit 126 the video signal supplied from each pixel circuit
assigned for video image display on the display device 40 among
a group of pixel circuits forming the imaging circuit 122. A second
vertical scanning circuit 125 transfers to the horizontal scanning
circuit 126 the video signal supplied from each pixel circuit assigned
for flicker detection in the flicker detection circuit 70 among
the group of pixel circuits forming the imaging circuit 122. The
horizontal scanning circuit 126 supplies the video signal transferred
from the first vertical scanning circuit 124 from a first output
terminal 128, and supplies the video signal transferred from the
second vertical scanning circuit 125 from a second output terminal
129.
[0035] FIG. 4 shows in detail the circuit configuration of the
CMOS image sensor 120. As illustrated in FIG. 4, the pixel circuits
200 forming the imaging circuit 122 are arranged in a lattice pattern,
and a total of four imaging circuits 200, i.e. two circuits in a
horizontal direction (from right to left in the figure) and two
in a vertical direction (from top to bottom in the figure), form
a pixel as a unit. Assuming that two rows of pixel circuits in the
vertical direction form one pixel column, the pixel columns of the
pixel circuits 200 are alternately connected to the first and second
vertical scanning circuits 124 and 125. Each video signal supplied
from each pixel circuit 200 connected to the first vertical scanning
circuit 124 is output from the first output terminal 128 through
the horizontal scanning circuit 126. On the other hand, each video
signal supplied from each pixel circuit 200 connected to the second
vertical scanning circuit 125 is output from the second output terminal
129 through the horizontal scanning circuit 126. Signals HD, VD1,
VD2, and CPU in FIG. 4 are instruction signals output from the CPU
20. The signal HD is a horizontal synchronization signal for the
horizontal scanning circuit 126, the signal VD1 is a vertical synchronization
signal for the first vertical scanning circuit 124, and the signal
VD2 is a vertical synchronization signal for the second vertical
scanning circuit 125. The signal CPU is a reset signal or a selection
signal for each pixel circuit. The reset and selection signals will
be described later. Note that assignment of the group of pixel circuits
connected to each vertical scanning circuit illustrated in FIG.
4 is illustrative only.
[0036] For example, the group of pixel circuits may be alternately
connected to each vertical scanning circuit with the pixel column
being composed of two columns of pixels as a unit.
[0037] FIG. 5 shows in detail the circuit configuration of each
pixel circuit 200 forming the imaging circuit 122. As illustrated
in FIG. 5, a cathode side terminal of a photodiode 210 is connected
to a voltage power source VDD through a reset switch 220, and to
a gate terminal of an amplifying transistor 230. An output terminal
of the amplifying transistor 230 is connected through a selection
switch 240 to a signal output line Xn.
[0038] The pixel configured as described above operates in the
following manner. The reset signal is applied to a gate electrode
of the reset switch 220 through a reset signal line Rn to turn on
the reset switch 220, thereby fixing a voltage of the photodiode
210 on the cathode side to a voltage VDD. Thereafter, when the reset
switch 220 turns off, the photodiode 210 starts accumulation of
photo charges. The potential of the photodiode 210 on the cathode
side changes in accordance with such photo charge accumulation.
The amount .DELTA.V of change can be expressed by the following
equation (1): .DELTA.V=Qph/(Cj+Cg) (1) wherein Qph denotes the accumulated
charges, Cj denotes the junction capacitance of the photodiode 210,
and Cg denotes the gate capacitance of the amplifying transistor
230. After the charge accumulation period, the selection signal
is applied to the gate electrode of the selection switch 240 through
a selection signal line Yn to turn on the selection switch 240,
and the video signal is supplied to the signal output line Xn. A
current lout of the video signal flowing at this moment depends
on the amount .DELTA.V, and an amount of change .DELTA.I approximately
satisfies the following equation (2): .DELTA.Iout=gm*.times..DELTA.V
(2) wherein gm* denotes a voltage-current conversion gain of an
electric charge reading circuit including an ON resistance Ron of
the selection switch 240 and the gain of the amplifying transistor
230, and is in the range of, for example, 1.times.10.sup.-3 (A/V)
to 1.times.10.sup.4 (A/V).
[0039] As described above, between the time when the reset switch
220 is turned on/off by the reset signal and the time when the selection
switch 240 is turned on by the selection signal, the photodiode
210 accumulates the photo charges, and the current lout corresponding
to the amount of the charges is supplied. In other words, the pixel
circuit 200 supplies the video signal in accordance with the amount
of light received during an exposure period, which is between the
turn-off of the reset switch 220 and the turn-on of the selection
switch 240.
[0040] Operation of the CMOS image sensor 120 upon display and
flicker detection will next be described.
[0041] FIG. 6 shows an example of a timing chart for signals input
to the CMOS image sensor 120. The pixel circuit 200 accepts a reset
signal input from the connected vertical scanning circuit through
the reset signal line Rn. Further, after a predetermined exposure
period has elapsed, the selection signal is supplied to the pixel
circuit 200 through the selection signal line Yn.
[0042] In accordance with the timing of each vertical synchronization
signal (VD1, VD2), the video signal is supplied from each pixel
circuit 200 through each vertical scanning circuit 124, 125, while
in accordance with the timing of the horizontal synchronization
signal (HD), the video signal is output from the corresponding output
terminal 128, 129 through the horizontal scanning circuit 126.
[0043] The cycles of the first and second vertical synchronization
signals correspond to each interval for reading out the video signal
for one frame from the pixel circuit, i.e. a sampling frequency
during sampling of the video signal for one frame output from the
pixel circuit. The sampling frequency for the second vertical scanning
circuit 125 (hereinafter referred to as a "second sampling
frequency") is preferably set taking into consideration a fluctuation
cycle of a luminance level of a light source for which a flicker
is to be detected because flicker detection is performed based on
the video signal supplied through the second vertical scanning circuit
125.
[0044] For example, the luminance level of a fluorescent light
having a power source frequency of 50 Hz indicates repetitive blinking
at the frequency of 100 Hz, as illustrated in FIG. 7. Accordingly,
when the exposure period of the pixel circuit is set as 1/100s or
an integral multiple thereof, the luminance level of the video signal
read out at this timing is averaged, and flicker may not be detected.
For accurate detection of a flicker in the 50 Hz fluorescent light,
exposure must be conducted at the timing (indicated by circles in
the figure) when the luminance marks the highest and lowest levels,
and the video signals based on such exposure must be sequentially
sampled. For example, in order to detect flicker in the 50 Hz fluorescent
light, the video signal is sequentially sampled from each pixel
circuit connected to the second vertical scanning circuit under
conditions of an exposure period of 1/400s and a sampling frequency
of 200 Hz, and flicker is detected based on such video signals.
For flicker detection in a light source of a high-speed inverter
type, such as a light source blinking repeatedly at 100 kHz, the
exposure period and the sampling frequency are set at, for example,
1/4000000s and 200 kHz, respectively.
[0045] When the exposure period and the sampling frequency are
set so as to detect flicker in a light source repeatedly blinking
at a relatively high speed, such as a light source of a high-speed
inverter type, flicker in a light source, such as a fluorescent
light having a power source frequency of 50 Hz or 60 Hz, repeatedly
blinking at a lower speed than the light source, such as a fluorescent
light of the high-speed inverter type, can also be detected.
[0046] Although the amount of the received light may be too small
to supply the appropriate video signal when the exposure period
for each pixel circuit is shortened as described above, adjustment
can be made to increase only the gain for the flicker detection
video signal because the gain for the video signal can be individually
adjusted in the CMOS image sensor 120 for each pixel circuit.
[0047] As described above, by setting the exposure period and the
sampling frequency for each pixel circuit connected to the second
vertical scanning circuit in accordance with the fluctuation cycle
of the luminance level of the light source subjected to flicker
detection, a flicker in that particular light source can be more
accurately detected.
[0048] In the above description, the second sampling frequency,
i.e. the cycle of the second vertical synchronization signal, is
set based on the fluctuation cycle of the luminance level of the
light source estimated as the light source illuminating the object,
and the cycle has a single fixed value. However, when a plurality
of light sources each having a different fluctuation cycle of the
luminance level are estimated as the light source, the second vertical
synchronization signals having different cycles for different fluctuation
cycles may be prearranged, so that the cycles of the second vertical
synchronization signals can be sequentially switched to sample the
video signal. By thus performing flicker detection based on the
video signal obtained through sampling in different cycles, flicker
can be more accurately detected for a plurality of light sources
with different fluctuation cycles of the luminance level.
[0049] The video signal may be sampled through the second vertical
synchronization signal having a different cycle for each pixel column.
In such a case, the CMOS image sensor 120 is provided with as many
output terminals supplying the flicker detection video signal as
there are second vertical synchronization signals with different
cycles. For example, when the video signal is supplied from different
pixel columns based on two second vertical synchronization signals
with different cycles, the CMOS image sensor 120 is provided with
a circuit configuration shown in FIG. 8. More specifically, a second
output-1 and a second output-2 are provided as the second output
terminals for supplying the video signal from the group of pixel
circuits connected to the second vertical scanning circuit. The
video signal supplied from the group of pixel circuits based on
the second vertical synchronization signal having one cycle is output
from the second output-1, while the video signal based on the second
vertical synchronization signal having the other cycle is output
from the second output-2. Such a configuration makes it possible
to supply the video signal from different pixel columns based on
two second vertical synchronization signals having different cycles.
FIG. 9 shows an example of a timing chart for the signals (the reset
signal, the selection signal, and the vertical synchronization signal)
in which the video signals are supplied from different pixel columns
based on the two second vertical synchronization signals with different
cycles.
[0050] Operation of the CMOS image sensor 120 when a still image
is captured will next be described.
[0051] FIG. 10 is a timing chart of signals supplied to each pixel
circuit 200 when a still image is captured. The operation differs
from that upon display and flicker detection in that each pixel
circuit 200 connected to the first and second vertical scanning
circuits are operated by a vertical synchronization signal having
the same cycle and the same recording exposure period.
[0052] By such operation of the CMOS image sensor 120, the recording
video signals are output from the first and second output terminals
128 and 129, and each video signal is temporarily held in the first
video memory 150 or the second video memory 152 through the AMP
130 and the A/D 140. The recording video signals temporarily held
in the first and second video image memories 150 and 152 are sequentially
supplied to the image processing circuit 30. The image processing
circuit 30 performs predetermined image processing on a group of
recording video signals for one frame, and records the processed
data in the storage unit 50 as image data.
[0053] A method of detecting flicker by the flicker detection circuit
70 will next be described. Flicker detection by the flicker detection
circuit can be performed by a general method, as in the following
example.
[0054] The flicker detection circuit 70 accepts input of the flicker
detection video signal temporarily held in the second video memory
152 through the switch 170. When the light source illuminating the
object is a repeatedly blinking light source, such as a fluorescent
light, the luminance level of the flicker detection video signal
fluctuates cyclically, as illustrated in FIG. 11. Therefore, the
flicker detection circuit 70 can detect the presence or absence
of a flicker based on whether or not the luminance level fluctuates
cyclically. Whether the luminance level fluctuates cyclically or
not can be determined based on, for example, the degree of variation
in luminance level of each video signal by referring to history
of the luminance level of each input video signal stored for a predetermined
period in the flicker detection circuit 70.
[0055] The flicker detection circuit 70 can sequentially compare
the luminance level of the previously input video signal and that
of the newly input video signal, and count the number of the video
signals whose luminance levels differ by a predetermined value,
and flicker detection is determined when the count exceeds a predetermined
value.
[0056] As described above, the flicker detection circuit 70 determines
whether or not the light source for the object causes flicker based
on the flicker detection video signal temporarily held in the second
video memory 152, and supplies the determination result to the CPU
20. The CPU 20 provides the determination result to the image processing
circuit 30, which estimates the light source for the object based
on the determination result, i.e. the presence or absence of flicker,
and performs white balance adjustment in accordance with the estimation
result.
[0057] According to the present embodiment, flicker can be accurately
detected based on the flicker detection video signal supplied by
the group of pixel circuits while the video image based on the display
video signal supplied by the group of pixel circuits is presented
on the display device 40 without providing a dedicated flicker detection
device, such as an external sensor for detecting flicker, in a digital
camera.
[0058] The image processing circuit 30 will next be described in
detail. FIG. 12 shows detailed functional blocks of the image processing
circuit 30.
[0059] An RGB separation circuit 32 separates an input video signal
into RGB components to be supplied as color signals. A white balance
adjustment circuit 34 estimates a light source of an object based
on luminance and color difference of the RGB color signals, and
adjusts white balance on the RGB color signals based on the estimation
result. The present embodiment is characterized in that the white
balance adjustment circuit 34 estimates the light source of the
object taking into consideration the flicker detection result provided
by the flicker detection circuit 70. A .gamma. correction circuit
36 performs .gamma. correction on the RGB color signals having adjusted
white balance, thereby performing tone correction. A color difference
matrix circuit 38 performs color difference matrix conversion on
the .gamma.-corrected RGB color signals, and supplies a luminance
signal (Y) and color difference signals (R-Y, B-Y).
[0060] The video signal input to the image processing circuit 30
is subjected to the above-described image processing, thereby causing
the processing result to be displayed on the display device 40 as
a video image, or to be recorded in the storage unit 50 as image
data.
[0061] The white balance adjustment circuit 34 will be further
described. FIG. 13 shows functional blocks of the white balance
adjustment circuit 34. The white balance adjustment circuit described
hereinafter is illustrative only, and alternative circuits may also
be used as long as they adjust white balance based on the result
of estimating the light source illuminating the object. For description
purposes, the RGB color signals for one frame will be defined as
a single image signal.
[0062] A block division circuit 310 obtains a single image signal
from the RGB color signals for one frame input from the RGB separation
circuit 32, and divides the image signal into a plurality of blocks.
Further, a representative value calculation circuit 320 calculates
for each block an average of the color signals (R, G, B) in the
block, and performs linear transformation on the calculated average
based on the following expression (3), thereby obtaining luminance
(L) and color difference (u, v) as values representing the block
(hereinafter referred to as representative values). ( L u v ) =
( 1 / 4 1 / 2 1 / 4 - 1 / 4 1 / 2 - 1 / 4 - 1 / 2 0 1 / 2 ) .times.
( R G B ) ( 3 )
[0063] A white balance evaluation circuit 330 estimates the light
source illuminating the object based on the calculated representative
value and the like for each block. A white balance gain calculation
circuit 340 calculates a gain for white balance adjustment based
on the estimation result, and a gain adjustment circuit 350 adjusts
white balance of the input RGB color signals based on the gain.
[0064] The gain for white balance adjustment is obtained as a value
correcting estimated color of light of the light source illuminating
the object to gray (achromatic color). Assuming that the estimated
color of illumination is denoted as (IL, Iu, Iv), the gain (Rgain,
Ggain, Bgain) for white balance adjustment can be derived from the
following expressions (4)-(6). ( IR IG IB ) = ( 1 - 1 - 1 1 1 0
1 - 1 1 ) .times. ( IL Iu Iv ) ( 4 ) Imax=max(IR,IG,IB) (5) Rgain=Imax/IR,Ggain=Imax/IG,Bgain=Imax/IB
(6) wherein (IR, IG, IB) is RGB expression of the color of the illumination.
[0065] The derived white balance gain (Rgain, Ggain, Bgain) is
a value correcting the color appearing when the illumination of
this color (i.e. (IR, IG, IB) itself) is reflected by a white object
to gray (i.e. R=G=B). The derived white balance gain is input to
the gain adjustment circuit 350.
[0066] The gain adjustment circuit 350 multiplies the RGB color
signals by the gain (Rgain, Ggain, Bgain) calculated by the white
balance gain calculation circuit 340, thereby adjusting white balance
of the image signal. Therefore, an output (Rout, Gout, Bout) derived
by the following equation (7) is supplied from the white balance
adjustment circuit 34: Rout=Rgain*R,Gout=Ggain*G,Bout=Bgain*B (7)
[0067] A method of estimating a light source illuminating an object
in the white balance evaluation circuit 330 will next be described.
For simplicity of description, the light source illuminating the
object is assumed as a fluorescent light and daylight.
[0068] The white balance evaluation circuit 330 checks whether
a color difference component of a representative value for each
block is included in a fluorescent light region 332 or a daylight
region 334 predefined on a color difference plane shown in FIG.
14, thereby estimating the light source for each block. Note that
the fluorescent light region 332 is a range of values that can be
taken by a color difference component of a white object under fluorescent
lighting, and that the daylight region 334 is a range of values
that can be taken by a color difference component of a white object
under daylight, i.e. solar light. Each region is predefined by experiments
and the like.
[0069] As illustrated in FIG. 14, the color difference component
of the white object under fluorescent lighting and that under daylight
are close to each other. As a result, the light source estimation
using color difference components have often been incorrect, thereby
preventing appropriate white balance adjustment. According to the
present embodiment, the fluorescent light region 332 and the daylight
region 334 defined on the color difference plane are modified in
accordance with the flicker detection result. More specifically,
the white balance evaluation circuit 330 estimates the light source
based on the fluorescent light region and the daylight region each
defined separately for the cases with and without flicker. FIG.
15A shows light source regions used when flicker is present, and
defined so that a smaller portion of the daylight region overlaps
the fluorescent light region. On the other hand, FIG. 15B shows
light source regions used when no flicker is present, and defined
so that a smaller portion of the fluorescent light region overlaps
the daylight region.
[0070] Thus, the light source regions on the color difference plane
used for light source estimation are changed in accordance with
presence or absence of flicker, achieving more appropriate light
source estimation. More specifically, when flicker is determined
as being present by the flicker detection circuit 70, the light
source is more likely to be a fluorescent light than daylight. Therefore,
the area where the daylight region and the fluorescent light region
overlap is shifted toward the fluorescent light region, thereby
making it easier for the white balance evaluation circuit 330 to
determine the light source of the object as the fluorescent light.
On the other hand, when it is determined that no flicker is present
by the flicker detection circuit 70, the light source is more likely
to be daylight light than a fluorescent light. Therefore, the area
where the daylight region and the fluorescent light region overlap
is shifted toward the daylight region, thereby making it easier
for the white balance evaluation circuit 330 to determine the light
source of the object as daylight. Consequently, the white balance
evaluation circuit 330 can estimate the light source more appropriately,
thereby reducing inappropriate white balance adjustment.
[0071] In the above description, the white balance evaluation circuit
330 changes the light source regions based on the result of flicker
detection performed based on the video signal. However, if a user
makes a selection prior to photographing as to whether a picture
is taken indoors or outdoors and the white balance evaluation circuit
330 judges based on the selection result that flicker is present
when a picture is taken indoors or that no flicker is present when
it is taken outdoors, the white balance evaluation circuit 330 can
simply determine the presence or absence of flicker without providing
a circuit, such as the flicker detection circuit 70. While a CMOS
image sensor is described above as an example of the image sensor,
any other image sensor can be used as long as the sensor can form
an image of an object on an imaging plane and extract an electrical
signal corresponding to intensity of the light as a video signal.
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