Digital Camera Patent Abstract
An auto-strobe apparatus and an auto-strobe method for the digital
camera are provided. There is provided a variable reference voltage
signal being inversely proportional to a product of an analog gain,
a digital gain, and a gamma value of the digital camera. When taking
a picture and generating flash, the brightness is detected and,
when a voltage signal representing the detected brightness is larger
than the variable reference voltage signal, the flash is stopped.
Digital Camera Patent Claims
What is claimed is:
1. An auto-strobe apparatus for a digital camera, comprising: an
image sensor for capturing external images; an image processing
circuit including an auto-exposure control circuit, an auto-white
balance circuit, and a gamma setting circuit for setting an analog
gain value, a digital gain value, and a gamma value, respectively,
based on a brightness of the image captured by the image sensor;
a flash module including a flash lamp for providing light source
while taking a picture, a photo detector for detecting brightness
when the flash lamp flashes and converting the detected brightness
into a brightness voltage signal, and a comparator for comparing
the brightness voltage signal with a variable reference voltage
signal, so as to stop flashing the flash lamp when the brightness
voltage signal is larger than the variable reference voltage signal;
and a comparison voltage generator for generating the variable reference
voltage signal based on the analog gain, the digital gain, and the
gamma value.
2. The auto-strobe apparatus as claimed in claim 1, wherein the
variable reference voltage signal generated by the comparison voltage
generator is inversely proportional to a product of the analog gain,
the digital gain, and the gamma value.
3. The auto-strobe apparatus as claimed in claim 2, wherein the
variable reference voltage signal generated by the comparison voltage
generator is equal to a reference voltage divided by the product
of the analog increment, the digital increment, and the gamma value.
4. An auto-strobe method for a digital camera, comprising the steps
of: (A) providing a variable reference voltage signal being inversely
proportional to a product of an analog gain, a digital gain, and
a gamma value of the digital camera; (B) flashing to take a picture;
and (C) detecting brightness and, when a voltage signal representing
the detected brightness is larger than the variable reference voltage
signal, stopping flashing.
5. The auto-strobe method as claimed in claim 4, wherein in step
(A) the variable reference voltage signal is equal to a reference
voltage divided by the product of the analog gain, the digital gain,
and the gamma value.
Digital Camera Patent Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an auto-strobe apparatus and an
auto-strobe method for the digital camera and, more particularly,
to an auto-strobe apparatus and an auto-strobe method capable of
automatically adjusting the flashing brightness based on different
gain settings.
2. Description of Related Art
In the conventional built-in flashing technique, a simple implementation
is to make the strobe apparatus complete flash or not (i.e., full
or null). Also, for avoiding an excessive amount of light from radiating
from a flash lamp 11, as shown in FIG. 1, an additional photo detector
12 and a comparator 13 are provided in a strobe apparatus. Hence,
the brightness sensed by the photo detector 12 is compared with
a predetermined reference voltage signal Vref by the comparator
13, and the flash lamp 11 is disabled when the photo detector 12
senses an appropriate amount of brightness. This can obtain a maximum
flashing effect of the strobe apparatus.
For precisely controlling the flash lamp 11, data about aperture,
distance from an object to be captured by the camera, etc. is also
taken into consideration for calculating an more accurate predetermined
reference voltage signal Vref.
However, it is known that the gain setting is critical to the brightness
and quality of image captured by a digital camera. The physical
meaning of the gain setting is to amplify or reduce the amount of
signal from an image sensor. When the photo detector of the strobe
apparatus senses an appropriate amount of light and stops flashing
the flash lamp, theoretically, the image sensor is well exposed.
However, because the physical meaning of the gain setting is to
amplify or reduce the amount of signal from an image sensor, this
image signal is amplified by the gain device, and thus the actually
captured image is over-light. Thus, it is desired for the above
conventional strobe apparatus to be improved to mitigate and/or
obviate the problems.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an auto-strobe
apparatus and an auto-strobe method for digital camera, which are
capable of automatically adjusting the flashing brightness based
on different gain settings so as to obtain the same flashing exposure
effect no matter what the gain setting is.
In one aspect of the present invention, an auto-strobe apparatus
for a digital camera is provided, which comprises: an image sensor
for capturing external images; an image processing circuit including
an auto-exposure control circuit, an auto-white balance circuit,
and a gamma setting circuit for setting an analog gain value, a
digital gain value, and a gamma value, respectively, based on a
brightness of the image captured by the image sensor; a flash module
including a flash lamp for providing light source while taking a
picture, a photo detector for detecting brightness when the flash
lamp flashes and converting the detected brightness into a brightness
voltage signal, and a comparator for comparing the brightness voltage
signal with a variable reference voltage signal, so as to stop flashing
the flash lamp when the brightness voltage signal is larger than
the variable reference voltage signal; and a comparison voltage
generator for generating the variable reference voltage signal based
on the analog gain, the digital gain, and the gamma value.
In another aspect of the present invention, there is provided an
auto-strobe method for a digital camera, which comprises the steps
of: (A) providing a variable reference voltage signal being inversely
proportional to a product of an analog gain, a digital gain, and
a gamma value of the digital camera; (B) flashing to take a picture;
and (C) detecting brightness and, when a voltage signal representing
the detected brightness is larger than the variable reference voltage
signal, stopping flashing.
Other objects, advantages, and novel features of the invention
will become more apparent from the detailed description when taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a conventional strobe apparatus for
a camera;
FIG. 2 is a block diagram of an auto-strobe apparatus for a digital
camera according to the present invention; and
FIG. 3 is a circuit diagram of a comparison voltage generator of
the auto-strobe apparatus for a digital camera according to the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 2, there is shown a auto-strobe apparatus
for digital camera in accordance with the present invention, which
includes an image sensor 21, an image processing circuit 22, a comparison
voltage generator 23, and a flash module 24. The image sensor 21
is, for example, a CCD device (charge coupled device) for sensing
external images. The flash module 24 includes a flash lamp 241,
a photo detector 242, and a comparator 243. The flash lamp 241 is
provided as a light source while taking a picture. The photo detector
242 is provided for detecting brightness when the flash lamp 241
flashes and converting the detected brightness into a brightness
voltage signal V. The comparator 243 compares the brightness voltage
signal V with a variable reference voltage signal Vx, so as to stop
flashing the flash lamp 241 when the brightness voltage signal V
is larger than the variable reference voltage signal Vx.
The image processing circuit 22 is provided to set parameters related
to image fetching, and process the image sensed by the image sensor
21. The image processing circuit 22 includes an auto-exposure control
circuit 221, an auto-white balance circuit 222, and a gamma setting
circuit 223. The auto-exposure control circuit 221 can set an analog
gain value based on the brightness of the image sensed by the image
sensor 21. The auto-white balance circuit 222 can set a digital
gain value based on the brightness of the image sensed by the image
sensor 21. The gamma setting circuit 223 can set a gamma value based
on the brightness of the image sensed by the image sensor 21.
The above analog gain value, digital gain value, and gamma value
are used to determine various gains of the digital camera including
the analog gain, the digital gain, and the gamma adjustment. The
analog gain refers to an amplification of the analog signal which
is generated by the image sensor 21 and processed by a CDS (correlated
double sampling). Therefore, the same image captured by the image
sensor 21 may exhibit different brightness if the setting of analog
gains is different. The digital gain refers to performing various
settings on the digital signal obtained from converting the amplified
analog signal. Therefore, the brightness of image is different if
the setting of digital gain is changed. The gamma adjustment is
used to re-adjust the brightness of each pixel of the image. Hence,
the brightness of the image is different if the gamma setting is
different.
From the above analysis, it is known that the brightness of the
image will be affected by the analog gain, the digital gain, and
the gamma setting of digital camera. For obtaining the same flashing
exposure effect in different gain settings, it is required to dynamically
adjust the illuminating time of the flash lamp 241 based on the
gain setting. In the case of analog gain, if the analog gain is
g and the brightness of the image sensed by the image sensor 21
is y, the mathematical model for obtaining the desired same flashing
exposure effect is: g.times.y=k, where k is a constant.
The brightness of the image sensed by the image sensor 21 can be
represented by brightness voltage signal V generated by photo detector
242 of the flash module 24. Furthermore, the analog gain, the digital
gain, and the gamma setting are determined before the flash lamp
241 turns on to flash. Therefore, the comparison voltage generator
23 can dynamically adjust the variable reference voltage signal
Vx based on the analog gain, the digital gain, and the gamma value
set by the image processing circuit 22. In other words, the variable
reference voltage signal Vx is varied as the analog gain, the digital
gain, and the gamma value. Accordingly, when the flash module 24
flashes in taking a picture, the brightness voltage signal V generated
by the photo detector 242 is transmitted to the comparator 243 for
being compared with the variable reference voltage signal Vx predetermined
by the analog gain, the digital gain, and the gamma value. When
V>Vx, the power supply to the flash lamp 241 is inhibited to
stop flashing. As such, it is possible of obtaining the same flashing
exposure effect no matter what the gain setting is.
The comparison voltage generator 23 acts to determine the variable
reference voltage signal Vx from the analog gain, the digital gain,
and the gamma value by calculation or referring to a table. Furthermore,
the variable reference voltage signal Vx is inversely proportional
to the product of the analog gain, digital gain, and gamma value.
Preferably, the variable reference voltage signal Vx is equal to
a reference voltage Vref divided by the product of analog gain,
digital gain, and gamma value. FIG. 3 shows a preferred embodiment
of the comparison voltage generator 23, which comprises four resistors
R serially connected to a reference voltage Vref to provide the
divided voltages including Vref, 3/4 Vref, 1/2 Vref, and 1/4 Vref,
and four switch transistors M0, M1, M2, and M3 which are selectively
turned on to select one of the divided voltages Vref, 3/4 Vref,
1/2 Vref, and 1/4 Vref for being used as the variable reference
voltage signal Vx. The switch control signals d0, d1, d2, and d3
of the switch transistors M0, M1, M2, and M3 is obtained by looking
up a table. As shown, when the products of the analog gain, digital
gain, and gamma value are 4, 2, 4/3, and 1, the signals d0, d1,
d2, and d3 are all 1, so as to respectively turn on the switch transistors
M0, M1, M2, and M3. As a result, the generated variable reference
voltage signals Vx are 1/4 Vref, 1/2 Vref, 3/4 Vref, and Vref, respectively.
Although the present invention has been explained in relation to
its preferred embodiment, it is to be understood that many other
possible modifications and variations can be made without departing
from the spirit and scope of the invention as hereinafter claimed. |