Note: Descriptions are shown in the official language in which they were submitted.
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LINE OBJECT SCENE GENERATION APPARATUS
This is a divisional of Canadian Patent Application Number 2,193,740
which is the national phase application of PCT International Application
Number
PCT/US95/08359 filed 28 June 1995.
Field of the Invention
This invention is directed generally to systems which monitor and record
motion events. More particularly, it relates to the accurate management and
control of
time-sequential imaging and display, with application in numerous fields of
science and
technology. Most particularly, the invention provides a total race-management
system
which has wide ranging utility in measuring timed sporting events.
The invention also relates to systems and methods for generating a scene by
compilating successively-scanned line objects. As such, the invention also
concerns
apparatus for compressing those scenes into manageable data files and for
colour coding
scene information efficiently.
Background of the Invention
Prior art systems which track and record a motion event over time are
overwhelmingly directed towards the support of racing events. Standard
photographic
techniques which monitor the finish line of a race are known. Typically,
cameras
equipped for high resolution imaging view the finish line and sequentially
capture pictures
at a high rate for later use by an interpreter. However, this process is
cumbersome,
wasteful, and time-consuming, requiring, for example, an apparatus of
photographic film
and paper, processing chemicals and image enlargers or projection optics.
Consequently,
most races rely on human judges and revert to "photo-finish" technology only
in
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extremely close races. Specialty Instrument Corporation provides a myriad of
such
electronic and photo-finish systems under the trademark AccutrackT'". U.S.
Patent No.
3,829,869 exemplifies one such Accutrack'~ system.
Because of the problems with the "photo-finish" technology, numerous other
systems for monitoring racing events have been developed. However, these other
methods
and systems for timing sporting events present new difficulties. Video systems
which
record and display races in a standard television or video format are popular,
but
regardless of the particular implementation of these systems, a portion of the
electronic
Image remains on an analog medium, e.g., video tape. Since analog data from
the
systems consists of a continuum of information over time, it is relatively
difficult to
accurately apportion to a unique time
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interval. It is even more difficult to access a particular moment in time in
the recorded
sequence because the associated system must search the storage medium,
typically having a
long physical length in a spooled format, e.g., a video cassette. This
presents both limitations
and difficulties for users wishing to simultaneously record, view the current
race, and review
earlier segments of the race (or even a previous race) because only one user
can have access
to any of the information stored and recorded at any one time.
A further difficulty in analog data is that it must be converted to a signal
usable for video, television, or a computer before it is displayed. For
example, after a
completed search, the selected video tape segment is typically sent to active
memory before it
can be processed by a computer and, quite possibly, by supplemental cumplex
graphics
generators. Altogether, the analog format and related processing adds to the
time required to
review a race and therefore lengthens the decision making process.
Another problem faced by race systems occurs in the management of extended
time events, like a marathon or bicycle race. which can last for hours or
until each entrant
finishes. The runners or cyclists cross the finish line in groups; and for
long periods. the
finish line is void of persons. The relevant information at the finish line is
thus sporadic, and
includes significant amounts of "dead" time. In analog systems, this dead time
is
nevertheless recorded and stored so that the system can retain time
synchronism with the
event, even though it is generally useless for other reasons and adds to the
time required for
processing and reviewing the race.
Several race systems have attempted to improve the management and
accessibility of data taken during a race by transforming the recorded
information to a digital
equivalent. But, these systems also revert to an analog format before
displaying the race on a
screen. As examples, U.S. Patent No. 4,797,751 shows a video recording system
having both
digital and analog sections to provide display on a common cathode ray tube
(CRT). U.S.
Patent No. 5,136,283 similarly describes another partially digital system
which displays races
on a standard television format. These analog/digital systems still have many
of the problems
inherent in all analog systems.
It is, accordingly, an object of the invention to provide a system for
recording
and displaying a time-sequential scene of bodies crossing a plane. In
particular, the system
provides improvements in managing and recording timed sporting events which
reference
bodies or entrants crossing a finish line relative to the start of an external
event.
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Still another object of the invention is to provide improvements in the
manipulation of timed information representative of bodies passing a plane in
space, such as
person crossing a finish line in a race.
Yet another object is to provide improved access, control and storage of a
scene comprising a sequence of discrete time images.
Most particularly, it is an object of the invention to provide a race
monitoring
l0 and recording system which can record and display a race in a digital
format.
Still another object of the invention is to store and display color scenes on
a
computer system with efFcient color coding.
15 These and other objects will become apparent in the description below.
$~rv of the Invention
The invention features, in one aspect, a system for recording and displaying a
ZO time sequential scene of bodies moving across a plane in space. The system
includes at least
one digital camera which views and images a line object in the plane of
interest. The camera
time-sequentially captures the line object by imaging it onto an array of
detector elements and
converts the sampled signal into a digital image, or frame, of the line
object. Each digital
image frame uniquely represents a slice of the moving scene at a moment in
time. The
25 system also includes an image timer, with a timer processor, that responds
to a preselected
digital value from the camera and marks each frame with a digital time
reference using a
preselected ntunber of bytes within the frame information. The image timer may
also store
the digital frames from the camera in an internal buffer. The system further
includes a main
control computer having an internal memory, a user console, and a graphics
display monitor.
30 The computer stores the frames from the image timer buffer as blocks of
information in its
internal memory, via an associated software pointer, and selectively displays
a portion of the
stored frames as a time-sequential scene on the monitor. A user at the
computer console can
command a variety of functions provided by the invention to manipulate and
analyze the
captured scene, most particularly to display any portion of the scene of
bodies moving across
35 the plane and access an associated time for any frame within.
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The system thus summarized is particularly useful in recording and managing
the official times of objects or entrants crossing a finish line, and at a
rate which is over fifty
times faster (and more accurate) than a related video race management system.
A user can
record and display the bodies crossing the plane of interest, i.e., the finish
line, with accuracy
while maintaining the ability to review and edit in real-time the stored
images. Accordingly,
it is useful that the time references associated with each frame are triggered
with a start
sensor and correlated to the start of an external event, e.g., a gun start
signal. The invention
thus provides, in another aspect, a timer clock that is synchronized to the
start of an event and
which provides the timer processor with an accurate reference to mark the
respective
to moments in time for each frame.
In other aspects, the digital camera can include a line scan charge coupled
device which forms the array of detector elements. The camera can also include
a greyscale
gain controller to adjust the digital output signal according to a preselected
gain level,
15 preferably selectable at the main control computer, and, preferably, to a
gain level
corresponding to the digital values in the captured frames. The gain
controller can function
in a real-time fashion by adjusting the gres~scale gain applied to each frame
as captured by the
camera during operation of the invention. The camera is completely computer
controlled
from a remote location. This computer control, which is achieved by passing
signals along
20 the coaxial cable, allows remote control of focus, zoom, pan and all other
camera functions.
In still other aspects, the resolution of the scene as captured by the camera,
or
as displayed on the screen, is selectable by a user. With respect to the
camera, the resolution
in the time-domain, i.e., in the direction of motion, is adjustable by
selecting the frame rate at
25 which the camera captures the digital image frames. The resolution in the
spatial-domain,
i.e., along the line object length, is adjustable by changing the camera
density control which
activates only particular detector elements in the array.
With respect to the resolution as displayed on the monitor, the user can, in
3o another aspect, zoom a particular scene in or out on the screen. For
example, by zooming the
scene out, the whole race can be viewed at once; and by zooming in, particular
areas of the
scene are enlarged on the screen, suitable, for example, to interpret the
number identifier of a
particular runner. The zoom capability is available to users at the main
control computer in
either screen dimension, i.e., in the time and spatial directions,
concurrently or independently.
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A user of the system thus described has a variety of controls at the main
control computer. Any frame of a displayed scene can be removed, either
temporarily or
permanently, according to other aspects of the invention by "cropping" the
frames. A list of
"cropped" frames is placed into a listing memory. A time-crop control allows a
user to cut-
out, or "crop", uninteresting portions of the scene - for example periods of
time containing no
activity or bodies crossing the plane - while retaining an absolute time
reference for the
remaining frames, which collectively display a continuous scene. The time-crop
control
further can restore the cropped frames, by deleting them from the listing
memory, or
permanently erasing them.
to
In another aspect, a user can also selectively reverse the time-sequential
order
of the displayed scene so that the scene as display;.d appears as thcugh the
motion of bodies
passing the plane in space occurred in the other direction. In addition, a
user can point to
particular bodies on the display to provide both an object identifier, if
selected. and a unique
IS time identification representative of the point selected.
The system constructed in accordance with the invention also provides, in
another aspect, a virtual memory subsystem, like a hard-disc drive. The main
control
computer stores blocks of information into the virtual memory subsystem to
free space in its
20 own internal memory and to provide a storage medium for previous scenes,
for example
previous races. Storage into the virtual memory subsystem is initiated by an
optional
command or can occur automatically when a predetermined selectable fraction of
the internal
memory is utilized. The storage arrangement on the virtual memory subsystem
is, in another
aspect, ordered so that the main control computer can access and selectively
retrieve a block
25 of information from the virtual memory subsystem for storage in its
internal memory by
computing an offset from the initial memory location where the blocks of data
are stored. In
this manner, the internal memory of the main control computer can function as
a cache for
the virtual memory subsystem, thereby storing only a few active blocks in
volatile RAM.
30 To aid the storage capability of the virtual memory subsystem, the
invention
accordingly provides a compression system to compress the blocks of
information into less
memory space. The compression system is selectively controlled, both in
initiating the
compression of certain data and in regulating the accuracy of the compressed
information.
35 The compression system takes advantage of the fact that each digital image
frame comprises a column of n-bit numbers, and a sequence of digital frames
thus forms an
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array of rows of n-bit numbers. In a preferred aspect, the compression occurs
by first
converting the array of digital image frames to an array of rows of m-bit
greyscale nurrtbers
(where the integer m is less than the integer n). The converted array of rows
of m-bit digital
image frames is then reformatted in a row-by-row fashion by collecting
adjacent and equal
m-bit ntunbers into a group and representing the collection as a "count" and a
greyscale
"value". These rows are reformatted again into a sequential memory string
thereby
compressing the data to a smaller amount. Preferably, the "count" is either a
byte or a 3-bit
ntuttber, and the m-bit "value" is formed by a 5-bit representation of each of
the original n-bit
nturtbers, although the accuracy of the compression is selectable by a user by
changing the
l0 number m. If the count is a 3-bit number, the count and value form one
byte. For example, a
4-bit number can be used to compress the data further.
In yet another aspect, the invention can include s plurality of digital
cameras,
each with an associated buffer within the image timer, to independently
capture a sequence of
13 digital image frames. Thus multiple scenes are generated, preferably of a
view containing
substantially the same line object, for display on the computer. At least two
scenes can be
shown simultaneously on a single monitor from nvo separate cameras in both a
real-time
display or from previously recorded segments. In another aspect, one or more
additional
computers are installed in communication with the virtual memory subsystem to
access and
20 separately display and manipulate data captured by any one of the connected
cameras. Thus,
a second user can analyze previously recorded motion segments while a first
user
concentrates on a current motion event.
The digital camera and image timer each have associated processing CPUs
25 which can selectively compress data before transmission along a signal
line. For example,
the digital camera can reduce the bandwidth requirements of the signal line or
cabling
between it and the image timer by commanding a first compression on the
digital data
transmitted from the camera. The image timer can reduce the bandwidth
requirements of the
cabling or signal line between it and the main control computer by commanding
a second
3o compression on the data transmitted between the two using a similar
compression scheme.
In a preferred aspect, a single cabling is used between the image timer and
camera. This cable preferably is in the form of a single coaxial cable that
functions as a
signal line to command various functions at the camera, a data transfer line
to transmit digital
35 information to the image timer, and a power line to supply power to the
camera. Similarly,
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the cabling between the image timer and main control computer or any
additional
computers can have like capability.
In still another aspect, the main control computer allows a user to
access an object identifier and an associated time corresponding to the
object, e.g., for
use in a race-management system. For example, prior to the start of a race,
the lanes
within the camera field of view can be spatially reference to a portion of the
displayed
image such that when a user points to that portion, both the lane number and
any
associated object, e.g., a race entrant, is available on the display monitor.
Further,
according to an additional aspect, the time and name of a particular object is
automatically
entered within a results window on the display monitor once a user so commands
it.
In a further aspect, the present invention comprises an event
recording video camera comprising: optical means for imaging a visual field, a
line
sensor for converting a fixed line portion of the imaged visual field into
electrical signals
representative of a line of pixels of the image of said field, the line of
pixels being a
frame representative of said fixed portion at an instant in time so that each
successive
frame is another line of pixels at a successive time, circuit means for
processing said
electrical signals to form a digital output stream containing digitized pixel
values of a
sequence of frames at a high scanning rate, a buffer for storing said digital
output stream
before transmission, and compression means for effecting frame-to-frame
compression of
data in said buffer to reduce bandwidth thereof before it is transmitted so
that the buffer
contains compressed data representing said sequence of lines of pixels.
Accordingly, in one aspect, the present invention provides a
compression system for compressing a sequence of digital data frames, wherein
each of
said digital data frames forms a column of n-bit greyscale numbers
representative of
luminance values detected at pixels of a line sensor on which a fixed line
portion of a
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scene is imaged in a video camera, and wherein said sequence forms an array of
rows of
n-bit numbers, such compression system comprising:
a. greyscale conversion means, for converting each of said digital
data frames into a column of m-bit greyscale numbers, wherein the integer m is
less than
n, thereby forming an array of rows of m-bit numbers;
b. counter means, for providing a count of the number of
occurrences of equivalent m-bit greyscale numbers in adjacent frames in each
row of said
array of rows of m-bit numbers;
c. raw reformatting means, for reformatting each row of said
array of rows of m-bit numbers such that every row sequence of equivalent m-
hit numbers
is represented by one m-bit number value and the count thereof; and
d. memory reformatting means, for staring each reformatted row
of Bald array of rows sequentially;
whereby said compression system reduces the memory required to stare said
sequence of
digital data frames representing the fiixed line portion of the imaged
visua~.l field.
These and other aspects will become apparent in the following
description, where the invention is described and illustrated in connection
with certain
preferred embodiments; however, it should be clear that various additions,
subtractions,
and modifications can be made by those skilled in the art without departing
from the
scape of the invention.
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Brief Description of the Drawings
A more complete understanding of the invention may be obtained by
reference to the drawings in which:
FIGURE 1 schematically illustrates a system constructed in
accordance with the invention for recording and displaying a time-sequential
scene of
bodies crossing a plane.
FIGURE 1 A illustrates adaptive signal processing in a preferred
camera used in the system of FIGURE 1.
FIGURE 2 illustrates how a system constructed according to the
invention sequentially constructs a scene from discreetly sampled line
objects.
FIGURE 3 illustrates a collection of digital image frames forming a
block of information, where each frame is marked with an associated time
reference.
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FIGURE 3A illustrates a preferred embodiment of the invention in which
particular frames are cropped from a sequence of frames.
FIGURE 4 illustrates a preferred use of the invention in which a system
constructed in accordance with the invention operates as a race-management
system which
views and records a race.
FIGURE 5 illustrates a typical display of a racing scene generated by a system
constructed in accordance with the invention.
to
FIGURE 6 illustrates a system constructed in accordance with the invention
having a virtual memory subsystem.
FIGURE 7 shows a sequence of data corresponding to an illustrative first step
in a compression of the sequence of digital image frames as provided by the
invention.
FIGURE 7A shows a second illustrative step in the compression of the data
sequence shown in FIGURE 7.
2o FIGURE 7B shows a third illustrative step in the compression of the data
sequence shown in FIGURE 7A.
FIGURE 8 schematically shows a first compression system constructed in
accordance with the invention.
FIGURE 8A schematically shows a second compression system constructed in
accordance with the invention.
FIGURE 8B shows representative data pixels generated in accord with the
3o invention and within successive frames of a black and white camera.
FIGURE 8C shows representative data pixels generated in accord with the
invention and within successive frames of a color camera.
FIGURES 8D-8G show successive and illustrative compression operations in
accord with further features of the invention.
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FIGURE 9 illustrates a preferred embodiment of the invention utilizing a
virtual memory subsystem, multiple cameras, and multiple buffers within an
image timer.
FIGURES 9A. 9B and 9C illustrate another multiple camera system with
tunable timers in the cameras to mark frames.
FIGURE 10 illustrates a display scene showing two scene sequences generated
by two separate cameras.
FIGURES 1 l and 1 !A illustrate adaptive color paletti~,ation in a preferred
embodiment of the system.
petailed De~criplion of the Invention
FIGURE 1 illustrates a system 10 constructed in accordance with the
invention for recording and displaying a sequence of bodies crossing a plane
in space. The
system includes a digital camera 12, an image timer 14, and a main control
computer 16.
With internal optics 15, the camera 12 views and images the line object 18
onto an array of
detector elements 20, preferably a Line Scan Charge Coupled Device (LS-CCD). A
camera
processor 22 time-sequentially samples the image at the detector elements 20
and amplifies
and digitizes the output signal at the gain controller 24 and A/D converter
26, respectively.
Each sampled image represents a frame of digital information at a unique
moment in time.
Each digital image frame is transmitted along a signal line 28, preferably a
coaxial cable, to the buffer memory 30 of the image timer 14. The timer
processor 32 marks
each frame as it enters the image timer 14 with a time reference, preferably
generated by the
timer clock 34, by storing the time reference within the digital information
of the frame.
Thus each digital image frame stored in the buffer contains both the stored
digital
3o representation of the line object 18 and a unique time associated with it.
In a preferred
embodiment, the time reference for each frame is indicative of the time the
camera 12
captured the picture relative to the start of an external event.
The buffer 30 stores the frames generated from the camera 12 until they
accumulate to a preselected memory allocation, called a "block", after which
the main control
computer 16 transfers the block to its own internal memory 36 via a signal
line 38.
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The main control computer 16 has a central processor 40 that processes the
blocks of information stored within the internal memory 36 so that the scene
and time
contents of a sequence of digital image frames are displayed on the display
monitor 42. The
central processor 40 also controls the automatic operation and memory
management of the
system 10, and responds to inputs at the keyboard 44 and mouse 45 so that a
user can
selectively command the display of any scene captured by the system 10,
including a real-
time display or previously recorded segments. More particularly, a user can
access the
unique times associated with any portion of the scene.
In a preferred embodiment, commands to the digital camera I'_' from the timer
14 and the main control computer 16 are transmitted within the signal line 28,
which is a
single coaxial cable. The coaxial cable 28 additionally acts as a power
control line to supply
energy to the camera 12 so that the camera 12 can operate without a remote
power source.
IS
With further reference and description of FIGURE 1, a three-dimensional
orientation chart 50 is provided to facilitate a better understanding of the
operation of the
invention. The system 10 operates by sequentially capturing the image of the
line object 18
as viewed by the camera 12 at discrete moments in time. The line object I 8 is
typically only
2o a fraction of a "scene" as observed by a person at the display monitor 42.
That is, each line
object 18 captured by the camera 12 sequentially forms part of a larger
picture, or "scene", of
the bodies moving by the field of view (FOV) of the camera 12. This FOV is
essentially a
plane in space, representable by the axes 52 and 54 of the chart 50, because
the detector array
20, and its conjugate line object 18, are practically one dimensional: the
line object 18 has its
25 long dimension along the axis 52 of the chart 50 and its short dimension
(not shown) along
the axis 56 perpendicular to the page of FIGURE 1. The camera 12 focuses on
the line object
18 at a distance directed along the axis 56 from the camera to the object 18.
Thus for example, FIGURE 2 illustrates an object 60 which is in motion along
3o the axis 56 of chart SO', a 90o rotation of chart 50. The camera (not
shown) is focused on the
object 60 with a FOV substantially in the plane of axes 52 and 54. As each
frame is captured,
a portion of the object 60, i.e.. a line object, is uniquely and spatially
represented as a digital
image frame. In FIGURE 2, the successive line objects captured by the system
are
illustratively shown on the object 60 as successive rectangles 62. For each of
the line-objects
35 62, the digital camera 12 correspondingly generates a frame by sampling the
image of the line
object according to the number of detector elements within the array 20. That
is, each of the
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line objecu 62 is digitally segmented along its length (i.e., along the axis
52) into a digital
image frame according to the sampling at the detector array 20 and transmitted
at a moment
in time to the image timer 14. In a real-time aspect, line object 64 represenu
the most
recently captured frame and the remainder of the object 60 to the left of line
object 64 has yet
to be captured by the system. A scene or composite image of an object
displayed on the
computer 16 can look very much like the actual object passing by the FOV of
the camera 12.
As mentioned earlier, each digital image frame captured by the system 10 of
FIGURE 1 is marked with a particular time reference from the camera and stored
into blocks
of information at the computer 16. The camera sends a special digital value to
the timer
which the timer recognizes as a start/end of a frame. The timer then marks the
received frame
with the associated time reference.
FIGURE 3 illustrates more fully these operations. Each digital image frame
70 captured by the system 10 includes an array of digital bytes corresponding
to the signal
detected by the activated elements of the array 20. When a frame enters the
image timer 14,
the timer processor stores the time associated with that frame in the last
four bytes 7?,
thereby permanently associating each frane with a unique time. In FIGURE 3,
time is shown
increasing with the arrow 74; thus frames towards the left of FIGURE 3 are
later in time than
2o those on the right.
FIGURE 3 also illustrates the collection of frames which form a block of
information 76 utilized by the main control computer 16. According to a
preferred
embodiment, frames are organized into blocks of infotTnation that are 16k-
bytes in size. The
number of frames which make up the block 76 therefore depends upon the amount
of
information within each frame - which is a variable dependent upon the further
features of the
invention discussed below.
FIGURE 3A illustrates a particular feature of the invention which is enabled
3o because of the unique time reference associated with each frame of FIGURE
3. In particular,
according to a preferred embodiment, a user at the display monitor 42 of
FIGURE 1 can
select and crop selected frames 73 from the scene displayed without disrupting
any of the
information available in any other frames. A cropped portion 73 can be within
a block 76, as
illustrated in FIGURE 3A, or some portion thereof (not shown). The address of
the cropped
frames are initially sent to a listing memory (a memory associated listing
table) which holds
their addresses until they are either erased permanently (which can occur by a
"save "
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operation) or re-inserted within the cropped sequence. This is particularly
useful in a scene
or race which has uninteresting segments that simply waste valuable memory. It
also helps in
the interpretation of a displayed scene because both the overall length of the
scene as viewed
from the monitor is decreased and the depth aspect of the displayed scene is
improved. If, for
instance, a first runner was several seconds ahead of a second runner, the
recorded sequence
between the runners can be cropped and the display on the screen appears as
though no
significant time between the numers exists. Of course, the time relationship
of the runners
remains accurate and when a user accesses the times associated with the
runners, the time-
discontinuity between the two will become apparent.
As discussed earlier, the most obvious use for a system constructed in
accordance with the invention is directed towards race management. FIGURE 4
illustrates a
system 10 in a configuration suitable to capture the motion of bodies crossing
the finish line
of a race. The system 10 is illustratively shown next to the race course 80
with the digital
camera 12 located to view the plane representative of the finish line. The
image timer 14
receives digital image frames from the camera 12 at a frame rate selectable
within the system
10 and marks each frame with its associated time reference. The main control
computer 16
retrieves and stores the frames from the image timer 14 as blocks of
information and displays
the recorded scene on the display monitor 42. The computer 16 also allows a
user, in the
illustrated embodiment, to control certain features of the invention described
below by the
keyboard 44 and a computer mouse 45.
Also shown in FIGURE 4 is a start sensor 84 which responds to the start of an
external event, for example the start gun which signals that the race has
begun, and which
signals this time to the image timer 14. The timer clock 34 of FIGURE 1 is
calibrated to this
start signal and the timer processor 32 marks each of the frames entering the
timer 14 with a
time reference that is relative to the detected start time.
A printer 86 can be installed with the system 10 to print selected scenes and
information about the event recorded.
FIGURE 5 illustrates a scene generated by the system 10 of a race containing
four participants, exemplifying a typical display available on the monitor 42
(FIGUREs 1 and
4) and printer 86 (FIGURE 4). In FIGURE 5, the arrow 90 is in the time
dimension and
points to increasing time; while the arrow 92 refers to the spatial dimension
corresponding to
the spatial information within the digital image frames. This image can be
zoomed in or out
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on the monitor 42 (described in more detail below) by a user operating the
computer 16. In
the preferred embodiment, a user of the system 10 can select the resolution or
image quality
of a scene generated by the system 10 by adjusting any of three separate
parameters. First, by
adjusting the frame rate, i.e., the rate at which the camera captures each
line object in the
scene, the amount of resolution information available along the axis 90
changes. Secondly,
by adjusting the camera density, i.e., by selectively activating or
deactivating certain elements
along the detector array 20, the resolution information available along the
axis 92 changes.
Third, by zooming in and out on the display monitor, the amount of displayed
resolution
changes in either or both of the axes 90 and 92. In general, the best display
detail or
resolution occurs when the user displays every frame recorded by the system 10
at a high
frame rate and every pixel is activated on the detector array 20.
However, other considerations must be made when adjusting these three
parameters. First, it may seem intuitive that the highest frame rate available
by the system is
preferable in all instances. But if the frame rate is too fast, the objects
can appear "fat" on the
monitor 42, unless zoomed out. For example, if the race consists of runners
traveling at
about 20mph, approximately 500 frames per second makes the participants appear
normal on
the display 42 without zooming. If the entrants were instead tortoises, a much
slower frame
rate would be preferable (if they are slower by 1/100, for example, a frame
rate of 5 Hz would
2o be ideal). Another factor influenced by the frame rate is the energy
available to the detectors
within the camera. If the frame rate is too fast, the image could appear dim
because the
detectors did not have sufficient time to integrate the available light energy
from the line
object. This depends, of course, on the sensitivity of the detectors and the
spectrum utilized
by the camera. In the preferred embodiment, the detector array 20 of FIGURE 1
responds to
visible light energy, and therefore requires more time to capture a particular
image at dusk or
twilight hours. If, on the other hand, the array 20 was constructed to respond
to infrared
energy, for example with HgCdTe material, the frame rate would be adjusted
according to the
temperature and speed of the objects.
3o The invention thus provides an automatic gain control (AGC) mechanism to
actively compensate for differing levels of light energy from the scene of
interest. The
camera processor 22 and gain controller 24 of FIGURE 1 programmably adjust the
gain
applied to the digital image frames transmitted from the camera to the image
timer in real-
time. The camera processor 22 responds to a command from the main control
computer to
raise or lower the gain in conjunction with the gain controller 24 by
quantifying the digital
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values within the digital image frames output from the A/D converter 26 and
modifying the
sensitivity of the camera.
For example, if the average digital value in a series of image frames is too
low, the scene at the computer would appear dim. A command by the user at the
main
control computer to increase the gain appropriately improves the displayed
screen contrast.
A user can also select how fast the AGC control of the camera operates by
adjusting the
bandwidth of the gain control. A user effectively changes the AGC bandwidth by
selecting
the amount of time the camera 12 takes to quantify the average digital value
used in setting
l0 the active gain. Although not generally practical for the majority of
motion events, like races,
the gain could theoretically vary for every frame generated by the camera by
increasing the
AGC bandwidth to a maximum setting.
In a preferred embodiment of this aspect of the invention, the camera 12
employs a novel signal conditioning circuit 24a, 2~ (FIGURE 1 A) to improve
the pixel data
generated by the line sensor 20. As shown, the sensor output 20 is amplified
by the gain
controller 24 which generally selects one of a number of possible gain levels
Li based on
camera operator or system operator commands as discussed above, and/or
programmed
microprocessor control based on current lighting conditions, scene light
levels and the like, or
'?0 a combination of program control and operator selection. The gain control
24 employs an
initial preamplifier 24i which compares the sensor output level of each pixel
to an offset
voltage. In the prior art this has been generally a constant threshold voltage
set at the factory
based on sensor characteristics. However, in accordance with this aspect of
the present
invention, rather than a preset threshold, preamplifier 24i receives an offset
voltage input
?5 from circuit (designated generally 24a) that is a variable function of the
noise or dark current
output of the CCD sensor array. The offset voltage is subtracted from the
video output of the
sensor by the preamplifier. The video signal is then amplified by one of
several selectable
gain circuits as before and the analog to digital converter 26 converts the
amplified video
signal into an 8 bit value.
As shown in FIGURE IA, the CCD sensor 20 has a line of sensing pixels and
also has one or several "dark pixels". These are regions of the array that
have their charge
accumulating and amplifying or gating structures fabricated identically to
those of the active
light sensing pixels, but they are masked so that no light reaches them. Their
output thus
represents the background noise level, due, for example, to penetrating or non-
visible
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radiation, heat, shot noise, and the like, and may thus be taken to represent
the component of
noise which appears in the output signal of each of the active sensing pixels.
A CCD timing module 24j provides the clock signals for sending the sensor
charge data to the initial preamplifier 24i, and also provides a dark pixel
flag on line 241 to a
decision circuit 24k during the time that the output from the dark pixels of
sensor 20 appear
in the data stream. Decision circuit 24k receives the stream of 8-bit digital
pixel values and
holds the flagged dark pixel data, which it compares to a desired dark level.
Decision module
24k then sends an UP or DOWN signal to a voltage integrator 24m, the analog
output of
which provides the offset voltage to the preamplifier 24i. The voltage
integrator stores a
voltage value that can be adjusted up or down in small increments by the UP
and DOWN
signals. As the offset voltage causes the dark pixel value to reach the
desired level, the
voltage integrator will hold the offset voltage constant if no UP or DOWN
signals are
received from the decision module 24k. The dark current offset module 24a thus
serves as a
t5 closed loop feedback from the noise component of the output pixel data
stream of A/D
converter 26, to the gain control 24, and it operates to zero out noise
contribution prior to the
high gain video signal amplification of gain control 2~1. This results in
enhanced image
quality over a much greater temperature range, and assures that the
preamplifier threshold
level remains effective despite drift or component aging. Notably, the
decision module 24k
operates on the digitized amplified dark current pixel value, and thus
provides a highly
accurate correction for current operating conditions.
The circuit works on all frames, even during video capture. If the gain that
is
selected is changed, then the offset voltage required may change also. The
circuit will react
to this and correct the offset voltage over the next few frames.
As also shown in FIGURE 1A, in addition to the amplified video signal with a
gain level L 1 . . . or Ln on line 26a, the A/D converter 26 also receives a
fine control
adjustment signal on line 26b from D/A converter 24n. This is an analog
voltage converted
from an 8-bit control word which sets the range of the A/D convener 26 used to
digitize the
amplified video. As the value of the 8 bit control word provided to D/A
converter 24n is
reduced, lowering the voltage on line 26b, the A/D converter uses a smaller
range. Thus,
smaller inputs on line 26b represent larger portions of the range, and the
image values are
larger. In this way, lowering the D/A voltage on line 26b increases the
apparent gain.
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In a prototype embodiment, the coarse gain ranges are related by a factor of
three, with relative gains of .5, 1.5, 4.5 and 13.5, and the switching between
gain levels is
coordinated with corresponding changes in the control word on line 26a to
avoid changes in
magnitude. When switching from one gain Li to another Li+1, a corresponding
change by a
factor of three in the D/A signal on line 26b will result in a nearly
identical image. For
example, a coarse gain of 1.5 and a fine gain of 85 (that is, 255/3)
corresponds to a coarse
gain of 4.5 and a fine gain of 255. Thus, changes in fine gain are coordinated
with operation
of the gain selector, to smooth transitions between the coarse gain levels L
1, L2 . . . That is,
as the module determines that light levels or operator commands will require a
jump in gain
level, the fine control word is decreased (if the gain is to go up) or
increased (if down) so that
when gain level is switched, the transition appears smooth.
As previously mentioned, the resolution is modified in the spatial dimension
92 of FIGURE 5 by changing the camera density which selectively activates
certain detector
elements on the detector array 20. Thus by decreasing the camera density by a
two or four-
factor, the resulting spatial resolution will similarly decrease by one-half
or one-fourth,
respectively. As more detectors are deactivated by decreasing the camera
density, the amount
of detail recorded along the axis 92 of FIGURE 5 decreases. This can be a
desirable feature
if high detail is not required for a particular scene since it significantly
reduces the amount of
information stored by the system 10.
More particularly, the camera density is adjustable in a programmable fashion
by a user at the computer 16 of FIGURE 1, which transmits a signal to the
camera processor
22. The processor 22 thereafter selects only the appropriate data to send to
the image timer
14, corresponding to the commanded camera density that activates the
particular detector
elements within the array 20.
With respect to displayed resolution, a user most readily adjusts the
displayed
image quality by zoom operations. Because each frame stored by the system
contains a
unique time reference, the process of zooming is easily attained by skipping
or duplicating
frames in the scene without compromising the critical time relationships
within the scene.
The mouse 45 of FIGURE 1 and 4 allows the user to point to particular objects
on the scene
and zoom either in or out by clicking the mouse, to thereby see more or less
detail of the
image, respectively. It is particularly useful when used in conjunction with
the time-crop
control discussed earlier whereby a user can zoom out and crop several
sequences of "dead"
time within a long race to shorten the stored file length . The zoom operation
is available
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along both directions of the displayed scene; that is, a zoom within the
spatial and time
dimensions may be made either concurrently or independently. Multiple zooms
are centered
on the display according to the selected point of interest. However, the
information available
during zoom operations is constrained to the amount of information captured
according to the
frame rate and density settings of the digital camera; it does not generate
new information. If
the display resolution exceeds the available captured resolution, the
displayed image can be
smoothed by an appropriate interpolation scheme.
The amount of information captured by the system 10 is itself an important
quantity. 'The LS-CCD detector element array 20 of FIGURE 1 is preferably 1024
elements
long, which is commercially available. Once the detector array is sampled and
digitized. each
detector element activated has 8-bits, or a "byte", of information associated
with the
particular frame. In this configuration, each frame has 1024 bytes of
information at the
highest camera density. In a preferred embodiment, a block of information
contains 16k-
bytes of memory, and therefore sixteen frames form a block of information if
every detector
on the LS-CCD array is activated.
However, if a user decreases the camera density by activating every other
pixel along the array 20, the data amount within one frame is reduced by one-
half, i.e.. to
512-bytes and the number of frames within a block of information increases to
32. This is an
important feature because most computers are limited by processing speed and
memory. If a
long event is permanently recorded, at some point the amount of memory is
exceeded. A
virtual memory subsystem, or hard-disc drive, as described below in a
preferred embodiment
of the invention greatly adds to the amount of memory available for the
system.
Nevertheless, by reducing the camera density and frame rate, as well as the
judicious
cropping and data compression such as described below, the amount of digital
information
representing each frame and the rate at which data is transferred between the
camera and the
image timer can be greatly reduced.
The data rate processed by the system 10 typically does not exceed l OMbits/s
(e.g., corresponding to 1000 frames per second and 1024 active detector
elements with 8-bits
per element). Thus the system 10 is generally not subject to noise problems,
time delays and
processing constraints so that the distance between the camera 12 and timer 14
of FIGURES 1
and 4 can be at least one thousand feet in length.
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Nevertheless, the amount of information stored and recorded can be large for a
given event or series of events. At l OMbits/s, for example, every second of
an event requires
approximately one Mbyte of storage. In a preferred embodiment of the system
10, a virtual
memory subsystem is included to accommodate the storage of data. FIGURE 6
illustrates a
system 100 constructed in accordance with the teachings herein which includes
a virtual
memory subsystem, or hard-disc drive 102.
The system 100 stores blocks of information into the virtual memory
subsystem 102 when the blocks stored within the internal memory 36 for the
main control
IO computer 16 exceed a predetermined memory threshold. A user can select the
threshold or
rely on a default setting, e.g., 2-Mbytes of internal memory which can hold at
least 125
blocks of information. Functionally, the main control computer accesses each
block in
internal memory by an associated software pointer which has a zero value when
the block is
transferred to the virtual memory subsystem.
IS
Accordingly, the virtual memory subsystem can operate as an integral
component of the system 100, and essentially transparent to a user. When a
block of
information is needed for processing by the main control computer 16, the
block is
transferred to internal memory 36 and an unneeded block transferred to the
virtual memory
20 subsystem 102. In this fashion, the system 100 can hold a scene of
information which greatly
exceeds the amount of RAM in the internal memory 36. In practical terms, the
main internal
memory 36 operates as a cache for the hard disc virtual memory 102.
The semantics for accessing frames from any associated storage memory is
25 straightforward. Since a determinable number of frames comprise each of the
blocks of
information, the frame number divided by the number of frames per block gives
the correct
block address, and the remainder gives the correct frame address within the
selected block. If
cropping is involved, memory is simply re-ordered into new blocks of frames,
with a
corresponding reallocation of frame addresses.
Even with a very large memory capacity within the subsystem 102, it too can
be exceeded when several motion events in a row are processed and stored, or
long events are
timed. T'he invention thus provides a compression system for reducing the
amount of
information needed in the blocks of information. The compression system is
preferably
available for use by a system constructed in accordance with the invention
utilizing the
virtual memory subsystem 102, which conveniently operates as a depository for
the
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compressed data. The data compression can be automatic or selectively chosen
by a user, for
example after recording a particular motion event.
The compression system relies on the fact that each frame within the scene has
the same background information within it. That is, if there is no motion
activity within the
FOV of the camera, each frame has practically the same information within it
since the
system constantly captures a single stationary line object; it is the motion
of bodies crossing
the FOV plane which generate a scene of interest. Thus, many frames stored for
a given
scene or motion event have redundant information.
to
More particularly, the camera 12 of FIGURE 1 generates an 8-bit greyscale
number, i.e., a number between a dynamic range having up to 25(i shades of
grey, for every
active detector element in every frame. This corresponds to the amount of
light energy
within the image of the line object captured at that detector for a given
frame. When a
sequence of frames contains redundant information, the 8-bit numbers between
successive
frames are approximately equal.
In a preferred embodiment, the first step taken by the compression system is
to
convert every 8-bit number in a selected sequence of digital image frames into
a 5-bit number
in the range 0-31. Thus, an 8-bit ntunber between the values 248-25~ would be
transformed
to a 31; and a 8-bit number between 240-247 would be transformed to a 30; and
so on. This
compression process sacrifices the ntunber of greyscales available within a
picture (i.e. how
"bright" a particular point on an object is represented by 32 numbers instead
of 256 numbers)
but saves a large amount of memory. It is worth noting that this is not a
significant loss as
?5 some common VGA monitors have only 16 shades of grey available.
FIGURE 7 illustratively shows a sequence of frames 112 containing seven 5-
bit numbers with a possible digital value of "A" or "B", and an associated
time reference 114
in the last four bytes of the frame (in reality, there are 32 different values
possible in this
3t) sequence, but "A" and "B" are used for ease of demonstration). Thus FIGURE
7 illustrates a
sequence of frames after the initial transformation by the compression system
from 8-bit
numbers to 5-bit nturtbers. In FIGURE 7 (including FIGURES 7A and 7B), each
square
represents a byte. As you can see, the redundant 5-bit numbers are very
apparent and
unnecessarily waste memory space.
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FIGURE 7A shows the next step of the compression system where each row
of 5-bit numbers is reformatted by accumulating numbers of equal value within
a row and
then representing these accumulations as a "count" and a "value". The count
corresponds to
the number of equivalent 5-bit numbers in a series, and the "value"
corresponds to the actual
5-bit digital number. Thus, for example, the first row of sixteen 5-bit
numbers "A" can be
represented by a count "16" and a value "A". The second row has a count "2"
having the
value "A", followed by a count "7" having the value "B", and followed by a
count "7" having
a value of "A". This process continues until the information within every row
of 5-bit
numbers contains a progression of "counts" and "values", as FIGURE 7A
illustrates. It is
Io worth noting that if the "count" is less than or equal to "7", it is
representable by a 3-bit
number (the number "8" is reserved for an indication that the count will be
represented by a
separate byte). Thus in the second row of FIGURE 7A, each of the "counts" plus
"values"
can occupy the space of one 8-bit number. But if the count exceeds 7, the
"count" and
"value" numbers are each represented by a separate byte.
IS
The last step of the compression system is shown in FIGURE 7B. The
significantly trimmed rows of FIGURE 7A are appended to each other to form the
final and
compressed representation of the original sequence of digital image frames,
which now
occupies a significantly smaller amount of memory. The time reference
information I 14 is
2o kept unchanged, however. Note that provided m is less than n in this
compression scheme,
the file cannot exceed its original memory size.
FIGURE 8 illustrates a compression system 120 constructed in accordance
with the invention. In particular, FIGURE 8 shows three process actuators 121,
122, and 123
25 which perform the steps of the operations described in FIGURE 7, 7A, and
7B. A sequence
of digital image frames enters the system 120 at a first data port 124.
Process actuator 121
converts each n-bit number within the sequence as a representative m-bit
number, to form, for
example, an array such as shown in FIGURE 7. Process actuator 122 reformats
the array of
rows of m-bit numbers into representative "counts" and "values" as for
instance shown in
30 FIGURE 7A. Process actuator 123 again reformats the data to a sequential
listing with the
time reference information appended, such as shown in FIGURE 7B. Data thus
compressed
exits the compression system from a second data port 126 for transmission to a
storage
memory, e.g., the virtual memory subsystem.
35 When a frame is required for processing by the main control computer, the
compressed information is similarly uncompressed into a block of information
before it is
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processed for display and manipulation by a user of the system. Upon
uncompression,
however, only 32 greyscale numbers are available, not the original 256.
As noted, there is a limit to the rate at which a computer, disk, or
communication system can handle data. Typically, for example, the invention
can generate
visual scene information in excess of this data rate. Tnis is particularly
true if the scene
information includes color information. Therefore, it may be necessary to
compress scene
information before it is stored by the computer, e.g., the main control
computer 16 of
FIGURE 1.
to
Accordingly, the invention provides for another compression system 120a,
FIGURE 8A, ~~hich achieves high image quality over the variable ,iata rates
generated by the
camera, e.g. the camera 12 of FIGURE 1, as well as over the acceptable data
rates handled by
the computer. System 120a has three process actuators 121 a. 122x, and 123a
which perform
the steps of the compression and quantizing operations described below with
respect to
FIGURES 8B-8G.
FIGURE 8B shows representative data pixels 140 from a black and white
camera. where each illustrated pixel has an associated 8-bit numerical value,
0-255,
2o representing the amount of light (light intensity) received by that pixel
during acquisition of
that picture. FIGURE 8C, on the other hand, shows representative data pixels
142 from a
color camera, wherein each pixel is represented by three numerical values
142x, 142b, and
142c, which are assigned, respectively, to red, green, and blue light
intensities.
In either of FIGURES 8B and 8C, pixels are displaced horizontally along the
time axis 144, and vertically along the spatial separation axis 146, such as
described in
connection with FIGURES 2 and 3. Each vertical array of data pixels is
representative of the
data acquired in one frame 148 of a scene.
3t) To compress the information in either of FIGURES 8B or 8C, system 120a
performs a series of compression steps on the data. First, actuator 121 a of
system 120a
subtracts pixels in one frame from the data pixels generated at the same
spatial location in the
previous frame, such as shown in FIGURE 8D. Accordingly, only those pixels
whose values
change will have non-zero values. This differencing operation of actuator 120a
causes the
relative distribution of pixel values to change: many more data pixels will be
zero, or near
zero, and those data pixels can be encoded in an efficient manner, such as
described below.
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The compression operation illustrated in FIGURE 8D can be performed
similarly on color images by treating each color type as a single black and
white image, such
as shown FIGURE 8E. In this compression, actuator 121a of system 120a
subtracts the red
values of each frame from the red value at the same spatial location within
adjacent frames.
Similarly, actuator 121a subtracts green values from the green values, and
blue values from
the blue values in adjacent frames. The encoded value of each resultant color
pixel is then
composed of three differences.
l0 The above-described subtraction operations can be thought of as a
"predictor"
function, namely that the next pixel in time is predicted to be the same as
the previous frame.
Therefore, actuator 121 a produces a "predictor" to reduce the amou:n of data
required to
represent the information. It is necessary to store, or transmit, more
information only when
the given prediction is incorrect, for example, only if at least one of the B-
A pixel entries is
15 non-zero.
This differencing predictor may be enhanced further to better predict the
pixel
values in adjacent frames. For example, FIGURE 8F illustrates a double
differencing
predictor technique in accord with the invention. Actuator 122a produces this
predictor by
2o computing a double difference whereby the difference of the preceding pixel
pair is used to
more accurately predict the current pixel.
More particularly, FIGURE 8F shows four representative pixels A-D, which
are sequential pixels in four adjacent frames. In this example, pixel D is the
pixel being
25 compressed by the actuator 122a. The predictor value of D, i.e.,
Dpredictor~ ~s calculated as
follows:
D=C+(B-A)
30 This value is generated by the compressor at the camera, and by the
decompressor when received from storage or along a communications line, from
the already
acquired values of A, B and C.
Thereafter, the predictor error is encoded by actuator 122a as:
Ep~edictor = D- ~prediao~
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As with the simple differencing compression, it is only necessary to send this
prediction error code if it is non-zero.
The double differencing predictor of FIGURE 8F is easily extended to color
images by treating each color portion of the pixel separately, similar to that
described in
FIGURES 8B and 8C. That is, actuator 122a performs three similar operations
described
above for each of the three color components of the pixel.
1o Actuator 123a of system 120a thereafter digitizes the image frames using an
8
bit conversion process to produce image pixel values in the range of 0 to 25~.
These pixel
values are preferably quantized by actuator 123a by converting the 8-bit
number to a number
with fewer bits, e.g., 5-bits, before displaying the information to a viewer.
One method for
quantizing the data is to truncate the pixel value and ignore any lower order
bits, such as
f5 illustrated in FIGURE 8G.
The invention also provides a more sophisticated quantizing process by
detecting only those pixels values which differ from the predictor by an
amount that is more
than an amount T which represents a noise value and Serves as the quantizing
resolution step.
2o Those pixels which oscillate across the quantizing step represent noise.
Therefore, in accord
with one embodiment of the invention, actuator 123a generates those pixels
influenced by
noise as identical 5-bit truncated values, i.e., as zero difference pixels. An
example of this
enhanced quantizing process is illustrated in FIGURE 8H. Note that an upper
quantizing step
T1 and a different lower quantizing step T2 are shown, but a single step T may
also be used.
Once the image data pixels are processed by the noise band quantizer and the
predictor differencing processes above, the resulting set of pixels represent
pixel differences.
Since after applying the quantizer threshold T, a very large number of
difference values will
be zero, or nearly so, the data will be compressed from its original size. The
differenced
3o pixels may be further encoded, i.e., compressed, according to standard
coding schemes, or
according to the compression system such as described in connection with
FIGURES 6 and 7.
In a color system, it is possible to encode the three color differences as a
single
symbol. Such an encoding may take advantage of correlated behaviour of the
colors.
3~
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The compression system 120a of FIGURE 8A may be augmented or
supplemented with structure of the process actuators 121, 122, and 123 of
FIGURE 8, as
appropriate, to perform additional or supplemental compression steps. Thus, as
in
compression system 120 of FIGURE 8, a sequence of digital image frames
entering the
system 120a at a first data port 124a may be converted by process actuator 121
a from n-bit
numbers to m-bit numbers, to form, for example, an array such as shown in
FIGURE 7.
Process actuator 122a thereafter reformats the array of rows of m-bit numbers
into
representative "counts" and "values" as for instance shown in FIGURE 7A.
Process actuator
123a again reformats the data to a sequential listing with the time reference
information
appended, such as shown in FIGURE 7B. Data thus compressed exits the
compression
system from a second data port 126a for transmission to a storage memory,
e.g., the virtual
memory subsystem.
One important feature or advantage of the aforementioned compression
system 1''0a is its real-time tunable compression capability. A single hard-
wired
compression circuit would force the system to pick a mid-point in image
quality, thereby
lowering the image quality even if the bandwidth were adequate for the image
stream; or
further limiting the speed even in cases where image quality is not a concern.
Tunable
compression, in accord with a preferred aspect of the invention, performs this
trade-off in
2o real-time and only as necessary.
This is accomplished as follows. A buffer is provided in the camera or in the
compression module 120a for receiving the input line image data stream 124a,
and a buffer
monitor indicates how full the buffer is, e.g. ten percent, fifty percent,
eighty percent. The
type of compression implemented in module 120a is then selected based on the
backlog of
uncompressed image data in the buffer. At one extreme, there is no
compression, or
compression by a method such as first described above, which is lossless, or
very nearly so.
At the other end is a compression method which allows a large "noise margin"
in the
quantizer, and reduces the bandwidth of the data stream enormously, at the
expense of image
3o quality. Since this noise margin can be set instantaneously in the
compression module, the
quality of the image can be adjusted such that it is maximized for a given
data rate. For
example, using the original wire interface of the prototype system which can
move
SOOKBytes per second, raw 16-bit-per-pixel image data would only go at a rate
of 250 lines
per second for a 1000-pixel line. In order to send lines faster than this, one
must compress
the imase data in real time. How much compression to use depends on the line
rate needed
and the inherent "compressibility" of the data (a scene that is not changing
often, for example,
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is much more compressible since the frame-to-frame differences are very
small). The size of
the buffer is selected so that the system can temporarily get "behind" in its
compression of the
data and only switch to a more drastic compression if the buffer continues to
fall behind.
The system sets the tunable parameters based on the expected continuous data
rate rather than overreacting to small spikes in the data rate caused by
changes in the scene.
One way of achieving this is for the system to implement a decision rule that
it will use no
compression until the data fills 10% of the buffer. At that point a small
amount of
compression is added. When the buffer reaches some critical state (say, 75%
full) the system
to may resort to drastic measures, such as dropping every other frame or every
other pixel.
«hen used as an event recording system, such as at the finish line of a race,
the line scan camera system of the present invention may be operated to record
and resolve
motions one thousandth of a second apart. However, in such a setting, it is of
the utmost
15 importance not to lose whole sections of the image. The goal is to always
completely utilize
the 500ICB/s bandwidth of the wire while keeping the image as high a quality
as possible.
The 500KB/s not only represents the bandwidth of a wire interface to the
camera, but is very
close to the best performance of modern disk drives. Thus, even in order to
capture the image
direct to disk, this data rate should be maintained. By locating the
compression module 120a
2o at or in the camera, none of the processing power of the system computer I
6 is diverted to the
compression process.
The foregoing "tunable" adaptive method of compression is highly effective
for applicant's line images, and it is unlike compression used in many other
video
25 applications in that it sacrifices the quality of regions of the image for
the quality of the
edges. By contrast, most video or image compression algorithms (including
Discrete Cosine
Transform (DCT) or JPEG tend to smooth the edges in favor of the regions. In
applicant's
event camera application, it is the edges which typically determine which
object is in front of
which. Thus, the compression retains or enhances frame to frame edge
discrimination at high
3o data rates. Experience with a simple prototype implementation of this
algorithm which has a
theoretical upper limit of compression of five has established that in a well
lit scene it is
common to get real-time data compression ratios in the range of 3.5 to 4.5,
with little
noticeable effect on image quality.
35 In accordance with yet another aspect of a preferred embodiment of the
present invention, a color line scan event camera as described above or, more
preferably, an
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operator controlled computer 16 at a monitoring site, generates a small color
palette to which
it fits the color values of the pixels in its sequence of frames, so that each
I 5-bit RGB pixel
value is represented by one of no more than 256 colors in the palette. In
accordance with this
aspect of the invention, rather than using a preset palette, or a two-pass
procedure wherein all
pixel data colors in every frame are reviewed and a palette is selected which
most accurately
represents the range of colors, the palette composition is adaptive, and its
entries are selected
as the frame data is being processed.
This aspect of the invention addresses the difficulties of color display in
the
1o uniquely time sensitive environment of the present invention. In general,
compressed video
data from the camera is stored, with time-marked information. so that it is
accessible modulo
an inherent latency time due to the network communications, storage access,
and generally
also decompression processing. To make these delays sequential with a two-scan
palettization process, or even to display retrieved and delayed excerpts with
a small fixed
15 palette. would be restrictive. In accordance with the present invention,
video data, either
directly from the camera or as it is retrieved from storage, is palettized for
256 color display
by generating a color assignment table "on the fly". The palettization process
introduces no
obsen~able delay, and each frame sequence begins with nearly perfect color
fideliy. Later
frames are then forced to fit the small color palette adaptively derived from
the initial frames.
FIGURE 1 1 illustrates this aspect of the invention. Each line or "frame" FI,
. .
. Fm generated by the color camera has n pixels specified by a five bit word
for each of the
red, green, and blue components, the I S source bits making 215 = 32K possible
colors. This
data can be directly used on a video display operating in the so-called "high
color" mode of a
video card capable of simultaneous display of 32K colors. However, to be
useful for
interactive examination of the image - e.g. for the operator to call up the
image, inspect
frames and time markings while viewing material on a display - it is generally
preferable that
the raw data be mapped to a destination image containing fewer bits per pixel.
Many laptops
and other PC's, for example, are operated in the 256 color mode.
It is desirable that the source image colors appear similar to the image
fotTned
with only the small palette of destination image colors. However the usual
processes for such
mapping involve either a time consuming run through the entire source image to
identify the
range of colors and pick a reduced set of representative colors that are
sufficiently close, or
involve mapping all colors to a predetermined, fixed palette. In the latter
case, while the
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transformation to the color format of the destination image is quick, the
colors may appear
quite different from the actual view, leading to a misleading or unsettling
appearance.
This problem is solved in accordance with a color palettization aspect of the
present invention by a palettizer 210, which may for example be implemented in
software in
the terminal 16 (FIGURE 1 ), that receives frames Fi of source image data and
converts their
RGB valued pixels Pi to pixel values P'i having a small set of color values Ci
in a destination
image frame F'i. The values Ci are adaptively determined and built into a
color table 220 as
the conversion proceeds.
to
FIGURE 1 1 A illustrates this pixel color processing. The palettizer 210 is
activated when an operator calls up a sequence of frames, typically specified
by the timing
markers which index all images. The color look up table '_'20 is initially
empty. Palettizer
210 inspects the pixel values Pl, P2 . . . Pn of the first frame Fl. initially
choosing the value
of P 1 as the first color C 1 to add to the table 220 and assign to
corresponding pixel P'i of the
destination frame. Next a determination is made whether the value of the
second pixel P? of
the frame is sufficiently close to C 1 to represent it by the same color.
Closeness may be
defined, for example, by a distance function operating on the fifteen bit RGB
coordinates,
such as a sum of the differences in RGB values, or sum of squares of
differences.
Other "distance" functions may be used which have been empirically found to
reflect cognitive judgments of "closeness" in human color perception, to
assure, for example,
that a low intensity pure blue is never mapped to a low intensity red.
Continuing with the description of FIGURE 1 !A, if P2 is close to P1, then the
color value P'2 is also set to C 1. Otherwise, a new color C2 is entered in
the table 220 and
the value of P'2 in the destination image frame is set equal to C2.
Thereafter, each pixel
value Pi is inspected in order to determine if it is close to one of the
already assigned colors
Ci-ko for some value ko<i, and, if so, Pi is mapped to the table color
3o Ci - 1o. Otherwise, that is, if Pi is not close to any color, then provided
the table 220 has not
already been filled, the value of pixel Pi is used as the next color entry C
in the table and is
assigned to the pixel P'i in the destination frame. Otherwise, that is, if the
table 220 is full
but Pi is not close to (e.g., within b of) any Ci, then a different mapping is
employed. The
pixel Pi is simply assigned that value Cj which is closer than the other
colors of the table. At
this stage in the processing, the palettizer operates on the pixels in
sequence. and moves on to
the next frame.
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In this manner, a set of up to 256 colors are established for a color mapping
palette, in which the palette initially matches the source colors very
closely. After a number
of frames have been processed and the table 220 is full, subsequently
appearing colors will be
"forced" into the existing palette and the image color will be less faithful.
However, in the
context of a fixed view line scanning camera, certain portions of the image,
such as the track
and landscape background of a race finish line, will remain unchanged
throughout all frames,
and will appear in accurate colors having been coded as initial entries in the
adaptive color
palette. Furthermore, when the color mapping is only used to call up and
examine a few
to seconds' duration of line scan image data, the 256 colors will suffice to
fully and accurately
represent all frames.
The palettization steps, involving simple "closeness" measures on an incoming
digital word pixel value against a small set of color value words from the
table, is extremely
13 fast. and palettization may proceed in real time as the frames are viewed.
With reference to FIGURE 9, a system 101 constructed in accordance with the
invention also shows an additional computer 104 and additional buffers 106 and
108 within
the image timer 110. These additions are not at all required for the operation
of the virtual
2o memory subsystem 102, or any other component described above, but rather
are useful in
further features of the invention.
In particular, the computer 104 allows an additional user to access and
analyze
portions of previously recorded segments that are stored onto the hard-disc
102. Similar to
2.5 the main computer 16 of FIGURE 1, blocks of information are loaded into
the internal
memory 116 before processing and displaying the information. All of the
features available
at the main control computer 16 are also available at the additional computer
104, thereby
providing a convenient forum for other management of the data processed during
a sequence
of motion events, e.g., races already recorded. Of course the main control
computer 16 can
3o also operate to review previously recorded segments of any prior motion
event - and even
during the activities of a current motion event - or operate in a real-time
mode and display the
current motion event as captured by the camera 12.
A plurality of computers, like the computer 104. are similarly attached to the
35 virtual memory subsystem if other users wish to simultaneously access the
data stored on the
hard disc. A computer suitable for use within the system described includes
common IBM
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personal computers or compatibles with an 8086 processor, a VGA video adapter,
and 640K
of RAM.
The buffers 106 and 108 within the image timer 110 of FIGURE 9 illustrate
another important feature of the invention, namely the addition of a plurality
of digital
cameras to the system 10 of FIGURE 1. In some circumstances, it is desirable
to view the
motion of bodies crossing the plane in space from two or more vantage points.
For example,
in a race it is very possible in a close heat that one race participant blocks
the view of a single
camera with respect to another race participant. To correct this potential
problem, one or
to more additional digital cameras, like the first one, can generate
additional sequences of
digital image frames of the line object of interest. Preferably, each
additional camera views
substantially the same plane in space, e.g., two cameras on either side of a
race track
exemplifies the use.
Since the buffers for every camera are within the image timer, a single clock
1 I? provides the time reference for all frames entering the timer. The timer
processor 114
can thus mark each frame with a calibrated time reference thereby permitting
each camera to
operate asynchronously. That is, any of the plurality of cameras can have any
selected frame
rate or density, and the image timer continues to accurately mark each frame
as it enters.
In a preferred system, rather than designing a multi-buffer timekeeping
module for receiving and marking frames from a plurality of different cameras,
the system
employs cameras having tunable timers that are maintained accurate enough to
mark each
frame with an accurate time before sending it to storage or the central
processor. The tunable
?5 timers are not themselves extremely accurate, and they may be implemented
with simple
oscillator circuits and dividers to form a local clock. However, each tunable
camera
periodically communicates with a precision timer and after initially
establishing synchronous
time, periodically re-tunes its clock rate to maintain synchronicity. Thus, in
a system
employing such cameras, each frame generated by a camera is marked with an
"absolute"
time marking.
One embodiment of such a system 200 is illustrated in FIGURES 9A and 9B.
The system includes a plurality of line scan cameras of which one is a primary
camera C 1 and
the rest are secondary or slave cameras Cj. The primary camera C I contains a
precision time
33 source TI, and an imaging portion V 1 consisting of a line scan sensor
assembly and
associated video components substantially as described above. Each slave
camera Cj
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contains a tunable time source T2 as described below, with an imaging portion
V 1 identical
to that of the first camera. All cameras connect over cabling, so that the
secondary cameras
may communicate, either directly or via a central control computer, with the
precision time
source of camera C 1. It will be understood in the discussion below that while
C I has been
indicated as containing a precision time source, to which each slave camera
refers, the system
may also consist entirely of slave cameras. In that event, camera C 1 is
simply another slave
camera. and a separate precision time source is provided, such as that of
image timer 14 of
FIGURE 1, to which all the slave cameras periodically synchronize their
clocks. The process
of synchronizing includes specifying a time offset for each camera C, in a
manner known in
to the art for dispersed ranging or time recording systems, to compensate for
the delays in
sending and receiving time check messages along the cabling referencing the
source timer T1
(i.e. conduction delays, switching delays, and network message handling
delays), and this
initial offset correction will accordingly be assumed in the discussion that
follows.
t5 The local timer T? of each slave camera is shown in FIGURE 98. and consists
of a local oscillator LO together with one or more intermediate frequency
stages S 1, S?, S3
and a clock synthesis or counter stage S4. The intermediate frequency stages
typically are
formed by simple divide-by-n circuits - e.g. special logic arrays - while the
clock synthesis
stage comprises a circuit of one or more adders, counters and the like which
constitute a timer
Zo using selected ones of the IF signals. The local oscillator LO may be any
locally available
frequency source, such as a 16 MHz clock of the camera's microprocessor
controller, a
separate timing chip or the like. Since the IF divider stages simply divide
the local oscillator
frequency, the resulting local timer is accurate to within a tolerance
corresponding to that of
the oscillator.
As shown in FIGURE 9B, one of the intermediate frequency clock stages,
illustratively S1, referred to herein as the "prescaler" stage, contains two
divider circuits with
closely related divisors m and m+k, where k is a small number such as 1 or ?,
so that
depending which divider is used, it puts out a clock pulse train of frequency
f/m or f/(m+k), f
3o being the local oscillator frequency. The divisors m and m+k are selected
so that one
frequency f/m results in a faster-than-actual time clock rate, while the other
fl(m+k) results in
a slower-than-actual clock operation.
For use in a line scan event timing camera. it is preferable that the timer
resolve imaged events with a resolution of approximately one millisecond or
better. One
representative embodiment having such resolution is implemented using a 16 MHz
local
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oscillator, a single frequency divider or prescaler stage SI, and a 32-bit
counter. The
prescaler for normal clock operation might divide the oscillator frequency by
a number n to
achieve an approximately accurate clock. In accordance with the present
invention the
numbers m and k referred to above are selected to be n-1 and 2, respectively,
so that the
prescaler contains a divide by (n-1 ) and a divide by (nt I ) circuit. One of
these (n-I ) results
in a fast IF clock "F", while the other (n+1 ) produces a slow IF clock "S". A
selector operates
at some rate controlled by the camera processing circuits, which exchange
timing information
with the precision time source and select one divisor or the other depending
whether the
clock time is presently early or late. The rate of these special cycles and
the direction of
timing error determines the apparent speed of the local oscillator at any
instant in time, and
once synchronicity is established, maintain the absolute timing error within
preset limits.
For example, a 16 MHz oscillator would nominally be divided by 125 to form
a 128KHz clock value. If the prescaler uses a divide by 124 cycle every
millisecond, then the
nominal 1'_'8 KHz clock rate will increase to I?8.00791?5 KHz, a 61 parts per
million
increase. Likewise, if the prescaler uses a divide by 1?6 cycle every
millisecond, the clock
rate will decrease by 61 parts per million. Thus, by changing the amount of
time during
which N+I or N-1 cycle prescaling is performed by the prescaler, the clock may
be advanced
or retarded by several milliseconds per minute, and by retuning at much
shorter intervals, the
2D clock time may be maintained with the necessary accuracy.
By using three divisors 124. 125 and 126, the clock may be run at a slow,
normal or fast rate. In this case. the normal divisor 125 may be used by
default, until the
comparator determines it is necessary to accelerate or retard the clock.
FIGURE 9C shows
such a local clock with three discrete timing rates for the phase-locked
tracking of the
precision source.
This capability allows the camera to adjust its clock until it matches the
clock
of the reference timer. Once the camera has adjusted its clock well enough, it
can use its own
local timer value to mark time on the image frames. Periodically, the camera
checks actual
time and sets the prescaler timing to advance or retard the local clock to
keep in
synchronicity.
In this manner, the non-precision local oscillator in each secondary camera
provides a continuously tuned local clock that, although apparently speeding
up or slowing
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down by as much as one tenth of one percent, changes its direction of error
frequently and
always tracks the precision timer within one, or even one-tenth of a,
millisecond.
FIGURE 10 illustrates a typical display of a race having two separate cameras
on opposing sides of the track. This image is available to any of the
computers discussed
herein, including the additional computer 104, provided at least two cameras
are integrated
into a system constructed in accordance with the invention.
In a preferred embodiment, the direction of any displayed sequence of digital
l0 image frames forming a scene is reversible by re-ordering the sequence of
frames. Thus for
example, both of the scenes displayed within FIGURE 10 can be reversed if
selected in
preference by a user of the computer. By operation of a mouse therefore, the
separate views
of the identical participants within the two scenes can appear to have a
motion in the same
direction.
The system described herein has other features available to users at any
connected computer. Prior to recording an motion event, for example, a
selectable part of the
displayed scene can be uniquely associated with an object identifier. Thus
along the spatial
(vertical) domain of the screen, one or more lanes on a race course - and in
particular the
2o runner within - can be associated with a particular portion of the screen.
A user can, for
example, point to that portion and acquire the information relating to that
lane, like the
entrant's name. Furthermore, because every portion displayed on the screen has
a unique
time reference associated with it, a user can similarly access and display the
time associated
with a selected portion on the screen and display its results. In operation,
for example, a user
can select the lane of the winner in a race to see the person's name; and,
more particularly, a
user can select the first portion of the winner's body crossing the finish
line to display the
associated win time. In a preferred embodiment, both the object identifier and
the time
associated with any portion of the scene on display are automatically entered
in a results table
if selected by the user.
Any display, showing a scene, race results, or any other information
selectable
by the user can be printed at any time by an appropriate print command
selected at that
computer.
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The term "greyscale" as used herein shall include not just classic greyscale
values but also can include an equivalent color representation (e.g., either a
;tingle value or a
series of values representing different colors).
It is accordingly intended that all matters contained in the above description
or
shown in the accompanying drawings be interpreted as illustrative rather than
in a limiting
way.
It is also understood that the following claims are intended to claim all of
the
specif c and generic features of the invention as described herein, and alI
the statements of the
scope of the invention.
