Digital Camera Patent Abstract
In accordance with the present invention, a new and improved digital
camera has a capture mode of operation where a scene to be captured
can be captured and stored in a removable memory device as a digital
still image and subsequently retrieved and displayed on a LCD in
a review mode of operation. Alternatively, the scene to be captured
can be captured repeatedly and buffered to the LCD where the buffering
rate is determined in a live view capture mode of operation allowing
a user to preview the scene to be captured in various ambient lighting
conditions. In one instance, under low ambient lighting conditions
a user is able to manually adjust the live view/preview apparent
image brightness level of tie displayed scene independently of any
LCD brightness or contrast adjustments. Digital Camera Patent Claims
We claim:
1. An electronic device for displaying a buffered image, comprising:
an image capture device having a range of exposure times for converting
light to an electrical signal; and a programmable amplifier coupled
to said image capture device and responsive to a manual indication
from a user that the image is sufficiently obfuscated due to lighting
conditions to lack discernible features for adjusting the strength
of said electrical signal; wherein a set of controls icons are automatically
displayed whenever the exposure time of said image capture device
is at about 13.33 milliseconds or greater to help facilitate user
adjustments to improve quality.
2. An electronic device for displaying a buffered image according
to claim 1, wherein said programmable amplifier is further responsive
to said manual indication from the user that the image is smeared
for further adjusting the strength of said electrical signal to
cause the display of an unsmeared image.
3. An electronic device for displaying a buffered image, according
to claim 1, further comprising: multiplying means responsive to
said programmable amplifier for further increasing the strength
of said electrical signal to cause the display of an image of sufficient
detail to facilitate live view observation.
4. An electronic device for displaying a buffered image, according
to claim 3, wherein said multiplying means includes: an analog to
digital converter for converting said electrical signal into a digital
signal; and digital multiplier means for increasing the strength
of said digital signal.
5. An electronic device for displaying a buffered image according
to claim 4, wherein said digital multiplier means is a digital multiplier.
6. An electronic device for displaying a buffered image according
to claim 4, wherein said digital multiplier means is a microprocessor.
Digital Camera Patent Description
FIELD OF INVENTION
The present invention relates generally to cameras, and more particularly,
to a digital camera having an adjustment feature for improving image
quality in a live view mode of operation.
BACKGROUND OF THE INVENTION
In general, analog cameras must give feedback to a user of what
area of a scene to be photographed will be captured when a picture
is taken. This has traditionally been accomplished using a built-in
device called an optical viewfinder or an optical window that enables
the user to visualize the scene and see approximately what area
of the scene that will be captured by the camera lens system.
In modern day digital cameras, the viewing of the scene that will
be captured is accomplished with either an optical viewfinder or
a liquid crystal display (LCD) unit in combination with an optical
viewfinder. The LCD in this instance displays the scene on a viewing
screen for preview by a user. The viewing of such a scene prior
to the image being captured and stored on a removable memory device
is called a preview or live view mode of operation. When the image
has been captured and stored on such a removable memory device,
the LCD unit may also be utilized to view the stored digital image
in what is called a post view or review mode of operation where
the captured image is retrieved from the removable memory device
and displayed on the LCD.
In the live view mode of operation, the scene to be previewed is
repeatedly captured by a charged coupled device (CCD) and then buffered
to the LCD to allow the image to be refreshed at some predetermined
refresh rate or frames per second rate without the benefit of any
long term memory storage device. The purpose of this mode of operation
is to provide the user with benefit of viewing the exact framing
of the image prior to the image being captured. In such a mode of
operation, as mentioned earlier, the image is captured at a predetermined
frame rate of N frames per second. Thus, for this example, the image
is captured at thirty frames per second. Since the camera is capturing
the image at a predetermined frame rate, when the camera is repositioned,
the image buffered previously disappears and a new image appears.
Based upon the limitations of the CCD and depending upon the brightness
of the scene to be captured, the range of image capture times is
limited to a fixed range of between t.sub.1, seconds and t.sub.2
seconds, where t.sub.1 >>t.sub.2. In short then, to maintain
the N frames per second viewing rate, the image capture time may
not fall below t.sub.2 seconds without causing a substantial degradation
in the quality of the buffered image.
In most, if not all digital cameras, the above-mentioned limitations
are recognized and thus, such cameras are designed to provide a
range of exposure times and aperture settings with a constant gain
factor for boosting the signal from the CCD to provide an optimum
image at a given brightness. While such an arrangement is generally
acceptable for a point, shoot and capture image operation, where
the image is captured and stored in a long term storage device,
such an arrangement is not optimum for a live view mode of operation
because of the wide variable lighting conditions that may be present
to the camera as the user moves from an indoor setting to an outdoor
setting, and from a bright sunny setting to a relative dark shadow
setting. Thus, unlike the point and shoot mode for capturing a scene
where a strobe or flash is a convenient way of adjusting for poor
lighting conditions, in a buffered image mode or live view mode,
a strobe or flash would not be a satisfactory solution.
Therefore it would be highly desirable to have a new and improved
digital camera that could provide a user with the ability to correct
for image degradation in a live view mode of operation resulting
from variable low lighting conditions.
SUMMARY OF THE INVENTION
In accordance with the present invention, a new and improved digital
camera has a capture mode of operation where a scene to be captured
can be captured and stored in a removable memory device as a digital
still image and subsequently retrieved and displayed on a LCD in
a review mode of operation. Alternatively, the scene to be captured
can be captured repeatedly and buffered to the LCD where the buffering
rate is determined in a live view capture mode of operation allowing
a user to preview the scene to be captured in a wider range of ambient
lighting conditions. In one instance, under low ambient lighting
conditions a user is able to manually adjust and increase the apparent
image brightness level of the displayed scene independently of any
LCD back light level or contrast adjustments.
BRIEF DESCRIPTION OF DRAWINGS
The above-mentioned features of the present invention and the manner
of attaining them will become apparent and the invention itself
will be best understood by reference to the following description
of the preferred embodiment of the invention, in conjunction with
the accompanying drawings, wherein:
FIG. 1 is a block diagrammatic representation of a digital camera
that is constructed in accordance with the preferred embodiment
of the present invention;
FIG. 2 is a high level flowchart illustrating a manual process
of adjusting a displayed image;
FIG. 3 is high level flowchart of an auto exposure algorithm;
FIG. 4 is graphical representation of an effective frame rate during
a live view mode of camera operation without adjustment;
FIG. 5 is graphical representation of auto exposure sequencing;
and
FIG. 6 is a flow chart of the auto exposure sequencing algorithm.
BEST MODE FOR CARRYING OUT THE INVENTION
Before discussing the present invention it may be appropriate to
review some of the problems associated with prior art digital cameras.
Cameras are portable products that users can take to various locations
to capture images of interest. With this portability and user range
of interest, the cameras must account for variable lighting conditions.
Film cameras accomplish this by having an adjustable lens aperture
and shutter speed to capture images of proper exposure. This combination
of shutter speed and aperture has a limited range of light capture.
The user can select different sensitivity films that can extend
the range of image capture of the camera under low light conditions,
in addition to using a built-in strobe as a light supplement.
With a digital camera, both aperture and shutter speed controls
are included with the camera. However, the third parameter for the
user to capture images under various lighting conditions is removed
since the CCD has a fixed sensitivity setting. Just like film cameras,
digital cameras have defined a range of apertures and shutter speeds
available to capture images. When the image light level drops below
this defined range, the image will be underexposed which will appear
dark to the user in the final image. The conventional solution for
low lighting conditions is the use of a flash or a strobe, however,
such a solution would not be satisfactory for a buffered live view
mode of operation since the light produced by such a strobe is not
continuous.
Digital cameras typically have different modes of operation. These
are capture, review and connect modes of operation. The capture
mode is where the user aims the camera at a scene to be recorded
and the camera in response to the user actuating the shutter, captures
an image of the scene. The image is then processed and stored on
a removable memory device.
There are two methods for the user to frame the image to be captured.
These are using an optical viewfinder and a through the lens viewing
of the image on a liquid crystal display unit (LCD). Viewing through
the lens with a LCD presentation of the image is commonly called
a preview or live view mode of operation. In this live view mode,
image data is captured from the CCD, processed and displayed on
the image LCD. This allows the user to see what the camera will
capture when the shutter button is actuated.
Since the camera is portable, it may experience a variety of lighting
conditions. The user can activate or start the live view/preview
mode at any time while in the capture mode. With these two factors
combined, the live view/preview mode should be able to display an
image under a wide range of scene lighting conditions. The aperture
setting and the electronic shutter speed or exposure time of the
CCD controls the amount of light that can be converted by the CCD
during a given period of time. Both of these variables, aperture
setting and exposure time of the CCD can be adjusted over certain
fixed ranges. For example, the aperture range in f-stops can be
adjusted between about 2.8 to about 9.8, while the exposure time
range of the CCD can be adjusted between about 2 seconds and about
1/750 of a second. When the light level of a scene is dark and the
aperture setting and exposure time of the CCD are set to maximum
setting for capturing and converting light, but yet relative to
the amount of available light entering the camera, a dark image
results in the live view/preview mode since an insufficient amount
of light is delivered to the CCD Another effect under low light
conditions in the live view/preview mode is that the exposure time
for the CCD increases up to the maximum value to capture as much
light as possible. However, in an exposure range below about 1/30.sup.th
of a second, the user will experience smearing of the displayed
image when moving the camera searching for an optimum framing. As
this CCD exposure time increases, so does the smearing seen by the
user in the displayed image. This is annoying to the user as it
is hard to see what is being framed when moving the camera relative
to a subject.
One proposed solution for solving the above mentioned image degradation
problems is to increase the sensitivity of the CCD. Such a solution
however, would be technologically difficult and would greatly increase
the cost of an improved camera due to the necessity of recovering
the R&D cost associated with developing such an improved CCD.
Another proposed solution would be to increase the aperture size
of the lens to allow more light to fall upon the CCD. Although this
solution could possibly solve the problem, increasing the size of
the aperture of the lens would be relatively expensive and would
thus, make the camera cost prohibitive to most users.
Therefore, there is a need for a new and improved digital camera
that provides a user with the ability to correct for image degradation
in a live view mode of operation resulting from variable lighting
conditions. Such a new and improved camera should be relatively
inexpensive and easy to operate under the variable lighting conditions.
Referring now to the drawings and more particularly to FIG. 1,
there is illustrated a digital camera 10, which is constructed in
accordance with the present invention. The digital camera 10 operates
in three modes of operation: 1) a capture mode of operation where
either in a capture still image operation, a framed image with the
use of an optical viewfinder is captured and stored or in a live
view operation, live action images are viewed without storing the
images for immediate viewing until the user presses the shutter
button, 2) a review mode where stored Images are retrieved and displayed
for review by the user; and 3) a connect mode of operation where
the camera may be coupled to a personal computer, stand alone printer,
or other type of communication device that accepts image data for
storage or printing purposes.
In accordance with the present invention, in the live view mode
of operation, a user is able to adjust the viewability of the captured
image by causing the strength of an analog or digital signal indicative
of the displayed image to be increased or decreased depending upon
ambient lighting conditions and user input as will be explained
hereinafter in greater detail.
Considering the camera 10 in greater detail with reference to FIG.
1, the camera 10, generally includes a housing 16 having mounted
therein an aperture/lens system or arrangement 13 that facilitates
zooming, aperture adjustments and image focusing for image reproduction
purposes. In this regard, the aperture/lens arrangement 13 generally
includes a primary lens system indicated generally at 14 for focusing
and optical zooming in and out of a scene to be captured, a shutter
19 for allowing the light indicative of the object to be captured
to enter the photosensitive plane of the camera and an aperture
18 that is adjusted for receiving the proper amount of light to
assure proper image formation.
In order to allow the user to interact with the camera electronics,
the camera 10 includes a switch control panel 17 mounted within
the housing 16 and includes a group of switches that help facilitate
camera operation. In this regard, the control panel 17 includes
a power on/off switch 26, a mode selection switch 28, a zoom in
switch 30, a zoom out switch 32, a view select switch 33, and a
shutter actuation switch 34. The function of each of the switches
mentioned-above will be described hereinafter in greater detail.
It should suffice however, to mention that the mode selection switch
28 permits the user to select a desired one of the different modes
of operation mentioned earlier; e.g., the connect mode, the capture
mode, and the review mode.
In order to allow the user to preview the scene to be captured,
the camera 10 also includes a microprocessor 25, a internal memory
storage unit 84 for control camera operations, a stepper motor 29
responsive to the microprocessor for controlling lens, aperture
and shutter operations, a removable memory device 52 for receiving
a flash memory card 56 to store captured digital images and a liquid
crystal display (LCD) unit 38 for displaying images and messages.
The microprocessor 25 is coupled electrically to a charged coupled
device or CCD 15 that converts light to an electrical signal. In
this regard, the CCD 15 is disposed in a primary light path LP for
converting light to an electrical signal that is ultimately processed
by the microprocessor 25 As best seen in FIG. 1, the primary light
path extends through the primary lens system 14, the aperture 18
and the shutter 19 to the charged couple device 15.
The camera 10 also includes a zoom effect viewfinder arrangement
43 having a secondary lens system 44 and viewfinder 45. The viewfinder
arrangement 43 operates in cooperation with a viewfinder zoom control
algorithm, to provide the user with, an optical preview mode of
operation where the user is able to see a visualization of the object
scene that will be captured. In this mode of operation, the user
is able to visualize the object scene with no power applied to the
LCD unit 38 to affect a significant increase in the operating period
of the camera. In short then, in the viewfinder preview mode of
operation, the LCD unit 38 does not exert a power drain on the electrical
system of the camera 10 as the LCD unit 38 is not placed in a power
on condition. The live view mode selection switch 31 allows the
user to switch the operating mode of the camera 10 between the two
different viewing modes of operation, a viewfinder only mode and
a LCD mode when in either the capture mode or the review mode of
operation. In order to facilitate zoom-in and zoom-out commands,
the camera 10 also includes a zoom in switch 30 and a zoom out switch
32 as best seen in FIG. 1.
Considering the digital camera 10 in greater detail, in order to
improve the live view/preview performance of the camera 10, the
camera 10 further includes a gain control circuit 47 and a digital
multiplier 49. The gain control circuit 47 amplifies the analog
signal coming from the CCD 15, while the digital multiplier 49 multiplies
the digital value derived from the output signal from the analog
to digital converter 48.
It should be understood by those skilled in the art that amplifying
the output signal from the charged coupled device 15, whether analog
or digitally boosts the image signal gain.
It should further be understood by those skilled in the art, that
the analog gain value is typically optimized for the stored picture
image quality is not optimized for live view/preview performance.
The analog gain value in the capture still image operation is set
to a preset constant fixed value. Conversely, when the camera is
operated in the capture live view operation, adjusting the gain
(analog or digitally) enables improved live view performance; thus,
improving the ease of the user framing the scene.
The downside of such amplification is that noise in the image is
also amplified. Therefore, under low light conditions, the signal
of the CCD and the noise can be a higher percentage. This results
in reduced image quality to the user. However, the user can see
what is being framed better than without this technique.
The analog gain is implemented in the gain control circuit 47 which
is a programmable amplifier disposed between the output of the charged
coupled device 15 and input to the A/D converter 48. A digital equivalent
is a multiplier 49 on the output of the A/D converter 48 or an internal
multiplying operation performed by the microprocessor 25.
It should be noted that in a live view buffered capture mode of
operation image quality is a second priority to displaying a viewable
image and thus, in the live view mode, noise in the displayed image
is acceptable. With this in mind, the programmable amplifier 47
boosts the analog/digital gain during live view only and the microprocessor
25 return the analog/digital gain to a preset constant value when
an image signal is captured and stored for later retrieval. The
programmable amplifier 47, which acts as the analog gain increase
for the live view mode has a range of between about G.sub.min at
-3.3 dB and about G.sub.max at 34.78 dB. The current default capture
mode value for the programmable amplifier 47 is fixed at a predetermined
dB level at the time of camera manufacture, for example the programmable
amplifier is set for example at G.sub.min of about 11.36 dB.
Considering now the operation of the camera 10 in greater detail,
the idea is to increase the gain of the image signal when both the
aperture 18 is at the maximum aperture setting (maximum light impinging
on the CCD) and exposure time for the CCD 15 is below 1/75th of
a second. At this point the CCD 15 is not sensitive enough to capture
a meaning image. This is the point where the LCD 38 would not be
able to display a viewable image. When in this condition, the user
can manually adjust the amplification of the image by pressing on
switches 35, 36 in accordance, with an associated overlay icon displayed
on the LCD 38, to increase or decrease the analog amplifier gain.
The placement of the icon is to one side of the LCD 38 and is controlled
with switches 35, 36. In this regard, each time the up arrow is
depressed the analog amplifier gain would be increased in incremental
steps (for example, in 3 dB steps) until a maximum value G.sub.max
is reached (for example, a G.sub.max value cf 34.78 dB). The down
arrow would decrease the analog amplifier gain by a similar value
( for example, a 3 dB step value) until the nominal gain setting
G.sub.min is achieved (for example, where a G.sub.min of -3.3 dB
is achieved).
It should be noted that the exact gain constants need to be determined
by experimentation since performance varies from camera to camera
due to differences between lens, CCD, and LCD components. It should
further be noted that soft-key actuation relative to switches 35,
36 is also contemplated in order to implement other camera functions.
As will be explained hereinafter in greater detail, the camera
10 is adapted to permit a user to adjust the camera 10 to the liking
of the user relative to the image displayed on the screen of the
LCD 38. Depending on CCD characteristics, the maximum gain G.sub.max
may or may not be 34.78 dB as a lower value may be selected since
the user is able to adjust the gain level from the nominal gain
level of G.sub.min to the maximum gain level G.sub.max in a "dial
up" manner as will be explained hereinafter in greater detail.
Considering now the operation of the camera 10 in still greater
detail with reference to FIG. 2, the camera begins operation when
the user initiates a start operation by advancing from a start step
102 to a power on step 104. In this regard, the user actuates the
power on switch 26.
Next the user selects a mode of operation at step 106 by actuating
the mode selection switch 28 to select one of the operation modes
of the camera 10. If the user selects a capture mode of operation,
the camera advances from step 106 to set gain to capture mode step
118 via a determination step 107. In a similar manner if the user
did not select the capture mode of operation but instead a review
mode of operation, the camera advances from the determination step
107 to a review mode of operation step 160 via a determination step
110. Finally, if the user did not select the review mode of operation
but instead a connect mode of operation, the camera proceeds from
the determination step 110 to a connect mode of operation step 180
via a determination step 111. If the user ha s not actuated the
mode selection switch, the camera returns to step 106 to wait for
the user to make a selection.
Considering now the capture mode of operation in greater detail
with reference to FIGS. 2A and 2B, when the user selects the capture
mode of operation, the microprocessor 25 causes the programmable
amplifier 47 to be set to a capture mode gain value by advancing
from step 107 to step 118. As mentioned earlier, the capture mode
gain is a preset gain value that is a factory determined setting
based upon image quality studies.
The camera 10 then goes to a determination step 119 to verify that
the memory card 56 has a sufficient amount of u unused memory storage
to receive another image. In this regard, if the memory card 56
is full and is no longer able to store additional images, the camera
advances from the determination step 119 to an exit step 125. The
exit step 125 also provides the user with a visual or sound indication
that the storage capacity of the card 56 has been reached.
If the memory card 56 is not fill, the camera 10 proceeds from
the determination step 119 to a determination step 120 to verify
that the user has not switched the camera mode of operation from
the still image capture mode to the live view capture mode. If the
user has not switched to the live view mode, the camera 10 proceeds
to a command step 121 to initiate a capture image process in response
to each time a user actuates the shutter switch 34 as will be explained
in greater detail.
When the user actuates the shutter switch 34, the camera 10 proceeds
to a call auto exposure step 122 that causes an auto exposure algorithm
200 (FIG. 3) to be executed. The auto exposure algorithm 200 will
be described hereinafter in greater detail.
Once the auto exposure algorithm 200 has been executed, the subroutine
goes to a capture image command step 123 that causes the camera
10 to capture an image and store the image on the removable memory
device flash memory card 56. The capture image command step 123
also causes the mechanical shutter 19 to close. The program then
returns to step 119 and proceeds as described previously.
Considering now the operation of the live view capture mode of
operation in greater detail with reference to FIGS. 2A and 2B, if
at step 120 a determination is made that the user has actuated a
live view mode selection switch 31 to select the live view mode
of operation, the camera advances a command step 127 that causes
the LCD 38 to be activated. The camera then proceeds to the command
step 109 that causes the gain of the programmable amplifier 47 to
be set to a nominal gain value anywhere between a G.sub.min value
and a G.sub.max value and the multiply value of the digital multiplier
49 to be set to unity. The firmware then advances through a series
of steps, where step 112 causes an image to be captured, step 113
calls the auto exposure subroutine 200, step 114 causes the image
to be processed, step 116 causes the image to be displayed on the
LCD 38.
From the display command 116, the subroutine advances to a decision
step 124 to determine whether the exposure time of the CCD 15 is
greater than one-eighth of a second which is indicative of a dark
image condition. If the exposure time is greater than one-eighth
of a second, the subroutine goes to a command step 130 that causes
an icon image to overlay the image displayed on the LCD 38. The
icon image is indicative of an up arrow icon and a down arrow icon
to prompt the user to actuate a boost/decrease gain switch 35 &
36 disposed on the switch control panel 17. In any event, after
displaying the up/down arrow icons, at a decision step 130, the
subroutine proceeds to a decision step 132 to make a determination
whether the user actuated an increase gain request by actuation
of the up switch 35.
If at the decision step 124 a determination was made that the exposure
time of the CCD 15 was not greater than about one-eighth of a second,
the subroutine proceeds to a determination step 126. At step 126
the subroutine determines whether the exposure time of the CCD 15
is greater than one seventy-fifth of a second which would be indicative
of a blurred or smeared image situation as best seen at 404 (FIG.4).
If the exposure time of the CCD 15 is not greater than about one
seventy-fifth of a second, the subroutine proceeds to step 108 and
continues as described previously. On the other hand, if the exposure
time of the CCD is greater than about one seventy-fifth of a second,
the subroutine advances to step 130 and continues as described previously.
Although in the preferred embodiment, a boost/decrease gain switch
35 & 36 respectively is illustrated, it should be understood
by those skilled in the art, that other control arrangements can
accomplish the same function. Thus for example, it is contemplated
that the LCD 38 could include a touch screen 300 to permit a soft-key
actuation. It is further contemplated that selection could be accomplished
by voice recognition of increase gain or decrease gain or simply
up or down. Therefore there is no intention of limiting the present
invention to a hard key switch to affect increasing or decreasing
the gain of the programmable amplifier 47 or the multiply factor
of the digital multiplier 49.
Auto Exposure Algorithm
Before discussing the auto exposure algorithm in detail, it would
be beneficial to briefly review the importance of an auto exposure
feature in a camera. In this regard, to accommodate a wide range
of lighting conditions, a typical camera has two major light control
mechanisms. The first mechanism is contained within the lens system
13, 14, the aperture 18. The aperture constricts the amount of light
reaching the light converting device, such as the CCD 15. The various
constriction positions are commonly called f-stops or f-numbers.
The numerically smaller the f-stop, the larger the amount of light
that reaches the light converting device. The larger the f-stop
number, the smaller amount of light.
Each f-stop is related by v 2=1.41421 (ex f2.8 to f4 is a factor
of v 2). The maximum aperture (smallest numerical f-stop) for a
lens offers no light loss. When the lens is at the minimum aperture,
the amount of light, which reaches the light converting device,
is a minimum. Typically, lenses will have multiple apertures to
be able to capture images under a wide range of lighting conditions.
The second light control mechanism is exposure time. This is the
amount of time that the light converting device is exposed to light.
Exposure time is typically expressed in fractions of a second. The
shorter the exposure time, the better the ability of the camera
to stop subject motion. Generally, exposure times less than 1/90
sec are needed to have photographer hand hold pictures with good
results (avoiding motion blur from the natural photographer hand
shaking).
An algorithm that controls (selects) these two light adjustment
mechanism is called auto exposure (AE). The purpose of such an algorithm
is to adjust the aperture and exposure time in accordance with the
photographed scene lighting conditions. It is desirable for a camera
to capture pictures over a wide range of lighting conditions, from
sunny beach to indoor at a party. The algorithm also determines
when to activate a strobe light source to provide additional artificial
lighting when needed.
Other factors that influence such an auto exposure algorithm are
the fight sensitivity of the sensor and the gain setting. The higher
the sensor sensitivity, the less light is needed to properly expose
a picture. This could be translated into shorter exposure times
or smaller apertures. This depends on how the auto exposure algorithm
is set to function. For digital cameras, the gain actor acts very
similar to increasing the sensor sensitivity. Increasing gain can
be translated into a shorter exposure time or smaller aperture.
As disclosed in the present specification there are two ways in
which to implement a gain factor. First, the variable gain analogy
amplifier 47 is added between the output of the CCD 15 and the input
to the A/D converter 48. The gain is adjusted digitally by the micro-processor
25 presenting a digital word, which represents the desired gain
value. The amplifier then uses analog amplification to modify the
analog signal from the CCD 15.
The second way is to multiply the digital data after going through
the A/D converter 48. The processor 25 or external digital multiplier
49 would multiply the binary integer number by another selected
binary integer gain value to adjust the data to the desired level.
The auto exposure process is activated in one of two ways. First
in the still image capture mode of operation the auto exposure algorithm
is executed when the user aims the camera at an object scene and
then depresses the shutter switch 34 to a half-way position. The
camera then measures the amount of light that is being reflected
from the object scene to be captured. Based upon this information,
the algorithm computes, using the unique combination of aperture
and exposure time to properly expose the scene.
The light may be out of range, however. This requires the algorithm
to be performed repetitively. In this regard, the algorithm adjusts
the settings to determine if the settings are within range. If the
settings are within range, the camera are set to these settings
and another light level is measured. This process is performed repeatedly
until the final setting of aperture and exposure time is determined.
When the exposure time drops below 1/90 sec, the auto exposure
algorithm will fire a strobe light (not shown) in order to provide
additional light, provided the strobe function is enabled. When
the user disables the strobe, the exposure times will extend down
to two seconds. In this case, the camera should be mounted on a
stationary object, like a tripod to avoid camera motion (this assumes
that the subjects are not moving also).
Considering now the auto exposure algorithm relative to the preview/live
view mode of operation, in order to be able to accurately frame
a scene to be captured, to know what is and what is not in the frame,
a preview/live view mode is implemented. The preview/live view mode
is where an image is captured and displayed on the LCD. This process
is rapidly repeated to form a "moving" image on the LCD.
This "moving" image changes in response to the user aiming
the camera at different portions of a desired object. Thus, live
view is similar to a video image capture without image storage.
However, the preview images are for the user to merely view the
object and not to store the object.
In the live view mode, to capture a frame the following process
is generated:
T.sub.a =Time to open the mechanical shutter
T.sub.b =Time to set aperture, focus and gain
T.sub.c =T.sub.ocd =exposure time of CCD
T.sub.d =Time to unload CCD data
T.sub.e =Time to reset CCD
T.sub.f =Time to process image
T.sub.g =Time to display image on LCD
General Process:
The mechanical shutter 19 is opened during the time period T.sub.a
to expose the CCD 15 to fight. Auto exposure/auto focus information
settings are set mechanically by the lens arrangement 13 operating
under the control of the microprocessor 25 during time T.sub.b.
The CCD15 is then exposed during time T.sub.c. Data is off loaded
from the CCD 15 during time T.sub.e, into the micro-processor 25
via the analog to digital converter 48. The image is processed by
the micro-processor 25 and auto exposure values are calculated from
the resulting image statistics during the time period T.sub.f. The
image is then ready for display on the LCD 38 at time period T.sub.g.
Due to mismatched resolutions between the CCD15 and the LCD 38,
the resolution of the CCD data must be reduced for display on the
LCD 38. When the data is ready for display the process can the be
repeated starting from T.sub.b.
Considering now the rive view frame rate, the frame rate is defined
as the number of individual frames displayed to the LCD per second.
This is determined by how long it takes the micro-processor 25 to
process the image, scene exposure time, how fast the LCD 38 can
be updated and how smooth thee image is perceived by the user. Typically,
thirty frames per second (fps) is considered by users as motion
quality (TV for example), and thus, most, if not all manufacturers
produced LCD units to accept a thirty fps transfer rate. Certain
systems put an artificial upper limit on the LCD frame rate due
to component limitations; and thus, the only other variable factors
in determine a maximum frame rate are CCD exposure time and micro-processor
overhead time.
If all the variables other than the exposure time are considered
to be system overhead constants, the sum of these variables are
expressed as a constant value K, and then the frame rate per second
is expressed as follows:
FIG. 4 is a graph of equation 1 or an effective frame rate using
a constant K value of 20 milliseconds.
It is possible to increase the frame rate by skipping frames that
the AE information is processed within. By calculating these parameters
twice to four times a second, instead of each frame, reduces the
processing overhead and helps to increase the frame rate.
As the exposure time increases beyond about 13 ms (1/75 sec), the
frame rate must reduce. The exposure time of the CCD 15 is beginning
to dominate the amount of total time for the frame. When the frame
rate lowers, there are two major affects that are visible to the
user. First, when the exposure time lengthens to approximately 1/15
sec or greater and if the camera is moving relative to the subject,
the resulting image will appear to smear as the camera pans across
a scene. This is because of the relative motion and long exposure
time. If no relative motion was to occur, then the image will be
easily recognizable. Once the light continues to diminish, the second
artifact of the system occurs, that is a dark or no image. This
occurs when there is not enough light to expose the CCD 15. In the
still image capture mode, the strobe would have been fired to illuminate
the scene. In live view mode, there is no equivalent to the strobe
in capture mode.
For both of these cases, the aperture is maximized, at the largest
setting and the exposure time is the only variable to adjust to
account for the low light conditions.
The gain factor, as will be explained hereinafter in greater detail
is utilized to reduce LCD image smearing, reduction in frame rate
and dark images.
Considering now the auto exposure subroutine 200 in greater detail
with reference to FIGS. 3 and 5, the auto exposure subroutine 200
is initiated by the call commands 113 and 121 for the live view
capture mode and the still view capture mode of operations. In this
regard, the camera goes to a start command 201 to a open mechanical
shutter command 202 that allows light to be converted by the CCD
15.
The subroutine then proceeds to an initialize command 204 that
initializes the variables utilized by the auto exposure subroutine.
In this regard, an initialize flag INITIAL is set to a logical one
that is indicative of a first pass through the algorithm, and EV.sub.Last
is set to zero. The subroutine then advances to a set exposure command
205 that causes the exposure value to be set to the default value,
that is about EV=EV.sub.Default =12 (which assumes an initial daylight
environment or lighting condition).
Next the subroutine advances to a call command 206 that causes
an auto exposure sequencing algorithm 600 (FIG. 6) to be called
to select the corresponding exposure time and aperture for the EV
value. After the algorithm 600 is executed, the program returns
to the auto exposure algorithm at a reset command 208. The reset
command 208 causes the CCD 15 to be reset allowing new image data
to be captured at a capture image data step 210. Once the image
data has been captured by the CCD at the selected lens aperture
and exposure time, the captured image data is off 15 loaded from
the CCD 15 to the microprocessor 25 via an off load command step
212.
The subroutine then proceeds to a calculate command 214 that uses
the off loaded captured image data and causes the microprocessor
25 to calculate image statistics (histogram). The micro-processor
25 then performs a series of determination steps to determine whether
the exposure time for the CCD 15 needs to be increased or decreased
based upon the histogram. In this regard, the camera advances from
the calculate step 214 to a determination step 216 to determine
whether greater than 10% of the captured data is less than 10% of
the CCD saturation level. If this determination is verified, the
subroutine advances to a determination step 216 to determine if
the initialize flag INITIAL is set to a logical one which is indicative
that this is the first pass through the algorithm. If this condition
is true, the program goes to an setup command 220 that reset the
INITIAL flag to a logical zero, set a DIRECTION flag to a negative
direction which is indicative that the exposure value needs to be
decreased (longer exposure time/larger aperture). The subroutine
then advances to a command step 228 that will be described hereinafter
in greater detail.
If at the determination step 216 it is determined that the INITIAL
flag is not equal to a logical one, the program proceeds to a determination
step 222. At determination step 222 a determination is made on the
DIRECTION flag to determine whether the DIRECTION flag is set to
a negative one. If this condition is true, the program goes to the
command step 228 which will be described hereinafter in greater
detail. Otherwise, the subroutine goes to a command step 224. The
command step 224 causes the limit value of the exposure time EV
to be set to EV.sub.LAST. The subroutine then proceeds to a command
step 228.
The command step 228 sets EV.sub.LAST to EV and STEP to (EV.sub.LIMIT
-EV) divided by two. The exposure value is then EV=EV+STEP, and
the DIRECTION is determined to be equal to STEP divided by the absolute
value of STEP. In this regard, the exposure value will be increased
by half the value from the current setting to the limit value.
Next a determination step 230 is executed to determine whether
STEP is less than 0.5EV which is indicative that the actual exposure
is within plus or minus 0.5 EV. If this condition is not true, the
subroutine goes to step 206 and iterates to get closer to the correct
exposure value. Otherwise the subroutine proceeds to a return command
232 that returns the program to the main control algorithm, that
is the camera exits the auto exposure routine 200 and returns to
either step 114 if the call command that caused the execution of
the subroutine 200 was call step 113, or the command step 123 if
the call command that caused the execution of the subroutine 200
was call step 122.
If the determination at step 216 is not verified, the camera advances
from the determination step 216 to a determination step 218 to determine
whether 10% of the captured image data is >90% of CCD.sub.MAX
(over exposure condition exists). If this determination is negative,
the camera returns the program to the main control algorithm, that
is the camera exits the auto exposure routine 200 and returns to
either step 114 if the call command that caused the execution of
the subroutine 200 was call step 113, or the command step 123 if
the call command that caused the execution of the subroutine 200
was call step 122. Otherwise the program goes to determination command
to determine whether the INITIAL flag is set to a logical one.
INITIAL is set to a logical one the subroutine advances to a command
step 223 that set the limit exposure value EVE.sub.LIMIT to a maximum
exposure value EVE.sub.MAX, that resets the INITIAL flag to zero,
and sets the DIRECTION flag to a positive direction. The program
then advances to the command step 228 and proceeds as described
previously.
Considering now if the user initiates an up actuation of switch
31, which is indicative of the displayed image being dark, the camera
10 proceeds to a decision step 134 to determine whether the gain
of the programmable amplifier 47 is set to G.sub.max. If the programmable
amplifier is set to G.sub.max, the image cannot be further corrected.
In this regard, the camera advances to an ignore request step 154
that causes the displayed icons to blink to provide the user with
an indication that the image may not be further corrected. In other
words, the ambient lighting conditions are so low, that the camera
will be incapable of displaying an acceptable image. After command
140 is executed the program advances to a command step 140 that
will be described hereinafter in greater detail.
If the programmable amplifier 47 is not set to G.sub.max the camera
advances to an increment gain by one step value step 136. The camera
firmware then proceeds to a determination step 138 to determine
whether the boosting of the gain of the programmable amplifier 47
by one incremental value step was sufficient to set the gain to
G.sub.max. If the programmable amplifier is set to G.sub.max, the
camera proceeds to an erase step 140 that causes the displayed up
arrow icon to be erased or no longer displayed. This then provides
the user with an alternative indication that the gain has been set
to G .sub.max and that no further image adjustment is possible.
The firmware of the camera 10 then returns to the step 112 where
the camera 10 causes another image to be captured. In this regard,
it should be understood by those skilled in the art, that the camera
10 operates in a series of repeated steps of capturing, processing
and displaying the image as seen through the lens arrangement 13.
This buffered capture mode continues repeatedly until the user switches
to the point and shoot capture mode.
Considering now step 132 once again with reference to FIG. 2B,
if the user has not actuated boost gain, the firmware determines
whether the user has initiated a decrease gain via the actuation
of switch 31. In this regard, the camera firmware returns to step
112 (FIG. 2A) and proceeds as described previously until the user
actuates switch 31.
If a determination is made that the user initiated a decrease actuation
of switch 36, the camera 10 proceeds to a determination step 144
to determine whether the gain of the programmable amplifier has
been set to G.sub.min. If so, the firmware of camera 10 ignores
the request of the user, but causes the displayed icons to blink
to provide the user with an indication that the image may not be
further corrected by decreasing the gain by executing a command
step 152. After executing the ignore command 152 the program proceeds
to an erase step 150 that will be described hereinafter in greater
detail.
If a determination is made at decision step 144 that the gain is
not set to G.sub.min the firmware advances to a decrease gain step
146 that causes the gain of the programmable amplifier 47 to be
decreased by one incremental value. The camera firmware then proceeds
to a determination step to verify whether the gain of programmable
amplifier 47 is now set to G.sub.min. If so, the firmware goes to
the erase step 150 that causes the displayed down arrow icon to
no longer be displayed. If the programmable amplifier 47 gain is
not set to G.sub.min the firmware returns to step 112 and proceeds
as described previously.
Considering now the review mode of operation relative to step 110
(FIG. 2A), if the user has selected the review mode of operation,
the firmware proceeds to a review mode of operation step 160. The
camera remains in the review mode operation, until the user exits
from the review mode. In this regard, a denomination step 162 is
performed by the firmware to verify when the user exits causing
the firmware to return to step 106 where the firmware proceeds as
described previously.
Considering now the connect mode of operation relative to step
111 (FIG. 2A), if the user has selected the connect mode of operation,
the firmware proceeds to a connect mode of operation step 180 (FIG.
2A). The camera remains in the connect mode operation, until the
user exits from the connect mode. In this regard, a determination
step 182 is performed by the firmware to verify when the user exits
causing the firmware to return to step 106 where the firmware proceeds
as described previously.
Considering now the digital camera 10 in still greater detail with
reference to FIG. 1, a stepper motor 29 coupled between the microprocessor
25 and the primary and secondary lens systems 14 and 44 respectively,
cause the respective lens systems 14 and 44 to be moved for focusing
and zooming purposes. The stepper motor 29 under the control of
the microprocessor 25 and a fixed focus control algorithm-adjusts,
for a given aperture setting the primary lens system 14 to a predetermined
focal point setting when the camera 10 is operating in the fixed
focus mode of operation.
The stepper motor 29 under the control of the microprocessor 25
and the zoom control algorithm also zooms the primary lens system
14 and the secondary lens system 44 over an optical zoom range between
1.times.magnification and 2.times.magnification. At the end of the
optical zoom range, when the primary lens system 14 can no longer
affect any greater zooming effects, the microprocessor 25 causes
the stepper motor 29 to continue to move the secondary lens system
44 in such a manner as to simulate digital zooming effects that
are directly viewable by the user without activating the LCD unit
38. That is, the secondary lens system 44 will continue to magnify
the object scene as viewed in the viewfinder 43, even though the
primary lens system 14 is no longer moving. In short then, the amount
of magnification the optical-viewfinder 43 will show is the product
of the optical and digital zoom factors.
Considering now the auto exposure sequencing algorithm 600 in greater
detail with reference to FIG. 6, the auto exposure algorithm 600
begins at a start command 602 whenever a call auto exposure command
is executed (see FIG.2A). The algorithm 600 set the aperture and
exposure time in accordance with a measure EV value. In this regard,
after executing the start command 602, the program proceeds to an
initialize command that causes a hystersis direction flag to be
reset to a zero value and an exposure hystersis value of the camera
10 to be set to about 0.5 EV (see FIG. 5) to minimize the auto exposure
sequencing process oscillations. The hystersis direction flag is
set to zero when increasing the exposure index (EV) and to a negative
one when decreasing the exposure index.
Next, the program proceeds to a determination step 606 to determine
when the new exposure value is less than the previous exposure value
(EV.sub.BEFORE). If so, the program goes to a set command 608 that
causes a hystersis feature to be enabled by setting a hystersis
direction flag. From the set command 608, the program advances to
an increment command 610 that will be described hereinafter in greater
detail.
If a determination is made at the determination step 606 that she
new exposure value is not less than the actual exposure value, the
program advances directly to the increment step 610. That is, if
the new exposure value is greater than the previous exposure value,
no hystersis feature is needed. Step 610 calculates the hystersis
value.
From the increment step 610, the program goes to a determination
step 612 to determine if the exposure value of the camera is less
than 11 plus the incremental step value. If so, the algorithm causes
the aperture of the camera to be set to f2.8 and the exposure time
to 0.5.sup.(EV-3) seconds. After setting the aperture and exposure
time, the algorithm returns to step 626 from the call step (FIG.
5) and proceeds as described previously.
Alternatively, if the exposure value of the camera is not less
than 11 plus the incremental step value, the algorithm proceeds
to another determination step 616. At step 616 the exposure value
is examined to determine whether the value is at least equal to
11 plus the incremental value and less than 12.5 plus the incremental
value.
If a determination is made as noted, the algorithm proceeds to
a set command 618 that sets the aperture to f4 and the exposure
time to 0.5.sup.(EV-4) seconds. After the exposure time and aperture
have been set the algorithm proceeds to the return command 618 and
continues as described previously. Alternatively, if the exposure
values are not at least equal 11 plus the incremental step value
and less than 12 plus the incremental value, the program goes to
a determination step 620.
At the determination command 620 the exposure value is examined
to determine whether if is at least equal to 12 plus the incremental
value and less than 13.5 plus the incremental value.
If this condition is true, the program advances to a set command
622 that causes the aperture of the camera to be set to f5.6 and
the exposure time to 0.5.sup.(EV-5) seconds. The program then proceeds
to the return command 618 and continues as described previously.
Otherwise, the algorithm goes to a set command 624 that causes the
aperture of the camera to be set to f9.8 and the exposure time to
0.5.sup.(EV-6.5) seconds.
After executing the command 624, the program then proceeds to a
set command 626 that stores the value of EV in EV.sub.BEFORE. Then
algorithm then advances to the return command 618 and proceeds as
described previously.
While particular embodiments of the present invention have been
disclosed, it is to be understood that various different modifications
are possible and are contemplated within the true spirit and scope
of the appended claims. For example, throughout the specification
reference has been made to a digital camera However, it is contemplated
that any electronic device having a display device that displays
a buffered image captured at a fixed frame rate would benefit from
the disclosed invention. There is no intention, therefore, of limitations
to the exact abstract or disclosure herein presented.
|