指导英国留学生作业：女子行径的阴影：My job wasto capture the Shadow Lady

#p#分页标题#e#http://www.ukassignment.org/   was ready to “suit up.” She’d dropped the overcoat to reveala formfitting pink leotard. The color didn’t matter, ofcourse. It was her moves we wanted. Our magnetic systemincluded 13 receivers/sensors, each with 6 degrees of freedom.
We attached them to the Shadow Lady’s leotard withVelcro straps (see Figure 1). This gave us pretty good coverage—
good enough for this assignment, anyway, an animated
commercial.
Magnetic systems work by generating three orthogonalelectro-magnetic fields from each transmitter. TheBen Delaney
CyberEdgeInformation
Services
0272-1716/98/$10.00 © 1998 IEEE
On the Trail of the Shadow Woman:
The Mystery of Motion Capture ______________________
Applications
Editor: Michael J. Potel
http://www.wildcrest.com
14 September/October 1998
1 The Shadow
Lady strikes a
pose. She’s
wearing the
Ascension
MotionStar
Wireless magnetic
tracking
system. Each
small cube is a
receiver. Her
waist pack holds
a battery, data
collection unit,
and radio transmitter.
Photo courtesy of Ascension Technology
.
host computer is aware of the timing of the signals.
When the receivers pick up the signals, the host knowsthe distance from the transmitter by the time elapsedand the orientation of the receiver by the changes in thesignal caused by tilting of the magnetic fields.Optical systems are preferred for high-res work. Anoptical system can collect hundreds of data points,though you seldom need more than about thirty. It worksby visually tracking small reflective markers attached tothe performer at key points. Optical systems are also firstchoice when you have lots of action. You can’t have wiresgetting in the way with your actors bounding around, soyou need optical tracking (see Figure 2

http://www.ukassignment.org/   The more data points you track, the less extrapolationthe software has to do. So for an animation of a
character that doesn’t look much like a human beingsuch as those dancing credit cards—you only need
enough points to get the basic motion into a file. The animatorsthen use that as a framework. They do plenty ofwork to make those points fit their character.The object in most MoCap sessions is simple—save a
data set that represents the subject’s motion with theoptimal level of detail and the least amount of noise. The
optimal detail varies according to the project. Forergonomic study, you want fine resolution of the motion#p#分页标题#e#
and highly repeatable measurements. For animation ofa fantasy character like Moxie, the animated MTV host,
you need relatively low resolution and repeatability.Sports games such as EA Sports Madden NFL 99, orKnockout Kings, where the movements are fast and accuracyis essential to realism, would fall somewhere inbetween, since the motion animates a relatively realistichuman figure. For example, a setup to capture a martialarts sequence might use 20 to 30 sensors, and afull-body ergonomic study could use 100 or more.
Another issue is the speed of the motion being captured.Optical systems can run at 240 Hz or more—criticalfor discerning very fine movements, or when themotion is quick. In addition, high-speed systems typicallymultiplex their data capture channel among thesensors. So an optical system with 100 tracking pointswould need to operate about four times faster than asystem tracking just 25 points to obtain the same temporalresolution. Magnetic systems operate at about 140Hz and can support fewer sensors than optical systems.Of course, there are other considerations as well.
IEEE Computer Graphics and Applications 15World Wide Web URLs for MoCapYou can find out more about the following motion capture
houses and equipment manufacturers by visiting their sites on theWorld Wide Web.
Adaptive Optics Associates: http://www.aoainc.com
Analogus: http://www.analogus.com
Ascension Technology: http://www.ascension-tech.com
BioVision: http://boris.biovision.com
Digital Domain: http://www.d2.com
5DT: http://www.5dt.com
GameTek: http://www.gametek.com
House of Moves: http:// www.moves.com
MediaLab: http://www.demon.co.uk/mlab
Motion Analysis: http://www.motionanalysis.com
Performance Animation Society: http://www.pasociety.org
Polhemus: http://www.polhemus.com
Protozoa: http://www.protozoa.com
Richard Cray’s Performance Animation Page:
http://www.pasociety.org/perfanim
Vicon Biomechanics (was Oxford Metrics):
http://www.metrics.co.uk
Virtual Technologies: http://www.virtex.com
Photo courtesy of Motion Analysis
2 This heavy action scene uses
optical tracking with a Motion
Analysis system. The little balls on
the actors are the reflective markers,
tracked by the cameras surrounding
them.
.

http://www.ukassignment.org/    16 September/October 1998
Applications
The big three
Probably the three biggest issues in MoCap for entertainment
are range, interference, and wires. Let’s
address them one by one.
Range refers to both the distance between the performer#p#分页标题#e#
and the capture equipment, and the size of the
area in which capture can happen. These are significant
and related factors. Obviously, you need to have room
for the performers to move without running into the
walls or equipment. This area is blocked out before the
capture session, typically with the director or producer,
the actor/dancer, and the capture technician. A simple
dance may only need 100 square feet, while a fight scene
could need 500 square feet or more. If you need to capture
a performer running for some distance, treadmills
sometimes can provide the required mobility. No matter
how it’s arranged, the working area is a big factor in setting
up the session.
The other range issue is the distance between the performer
and the capture equipment. The inverse square
law dictates that signals get weaker in proportion to the
square of the distance between the source and receiver.
For magnetic trackers, that limits the range to about a
10-foot radius per transmitter. You can gang up some systems
to use multiple transmitters, increasing the range
and capture area. Optical trackers are virtually unlimited
in range (see Figure 3), but the spatial resolution suffers
because the tracking targets look smaller as they
move away from the cameras. This reduces accuracy.
The second issue is interference. Both optical and magnetic
systems suffer from interference, though the
details completely differ for the two systems. Optical
interference is primarily occlusion. For example, an arm
moves in front of a thigh marker, or the performer turns
so the camera can’t see some of the markers, or one performer
steps in front of another. Work-arounds—typically
adding more cameras—help avoid the problem,
but occlusion is a fact of life and a continuing annoyance
with optical tracking.
Don’t forget, though, that magnetic systems have their
own problems (see Figure 4). They suffer from electrical
interference caused by induced magnetic currents,
eddy patterns caused by large metallic objects nearby, or
external sources of radiation, such as TVs and computer
monitors. The work-arounds for these problems
include sophisticated software filtering of the data.
Wires pose the third big concern. For quite a while
optical systems were preferred for any sort of athletic
activity because they didn’t need the annoying cables
that magnetic systems required. In addition, cables
limit range, and when you work with more than one
actor, they can get quite ridiculously tangled. It’s just
in the past two years that both Ascension (Burlington,
Vermont) and Polhemus (Colchester, Vermont) introduced
wireless magnetic systems.
I got down to work, attaching 12 sensors to the Shadow#p#分页标题#e#
Lady’s arms, legs, and shoulders. Then I had her put on a
headband with one more sensor sewn on the front. I
strapped the pack around her waist and plugged the sensor
wires into the receiver in the pack. I used Velcro straps
to hold the wires snugly to her limbs. I checked the battery
again. Then I sat down at the console.
I asked the Shadow Lady to assume a series of poses and
watched the stick figure on my monitor as it followed her
through the moves. “Number three’s a little jittery,” I
thought, but decided we could live with it. I dialed in zero
points for each receiver and nodded to the director. “We’re
ready.”
The director waved to the sound man, and as I hit the
“Start” button, loud Caribbean music started to play. The
Shadow Lady started to dance. I watched the monitor, not
her. My little stick figure looked pretty good. Everything
was humming. We captured about three and a half minutes,
then the director yelled “Cut!” As he walked over to
talk with the Shadow Lady, my mind wandered. I was
thinking about where this technology came from—the roots
of MoCap, so to speak.
The family tree
Motion capture is classified as “special effects,” a term
used to explain just about anything you see in the movies
that isn’t a straight camera shot. Special effects are just
about as old as film itself. The term was first used in a
movie credit in 1926, on the film
What Price Glory? Back then, special
effects involved some of the same
mechanical tricks done in stage productions.
Soon, double exposures
and other in-camera tricks, like stopping
the camera, moving a person or
prop, and re-starting, were added to
the toolkit. Early filmmakers quickly
reached the limits of model making
and in-camera effects, though,
and started developing new tools.
One of the most important was the
matte shot.
A matte shot is created by taking a
piece of film with some action on it,
like the heroine running frantically
from a villain, and creating a matte,
or mask, that eliminates everything
but her image from the film. This is
Image courtesy of CyberEdge InfoGraphics 3
An optical tracking system uses
two or more video cameras to find
the reflective markers attached to
the actors in key locations. With no
wires leading from the actors, they
have complete freedom of motion.
.
then composited photographically with a painted background,
so the shot of her running in a studio can
become her running through the streets of Shanghai or
through the desert. This simple trick is still used frequently,
though today most of the compositing is done
with computers. This technique was improved upon#p#分页标题#e#
with the development of the traveling matte shot, in
which the matte changes with the action. This permits
combining the foreground action with another piece of
film, which can include background action.
Television made it possible to do matte shots in real
time using a technique called blue-screen matte. This technique
makes it seem like your local weather person is
standing in front of an animated map when they’re really
standing in front of a blank blue screen. A mixing
device combines their image with the picture of the map.
In both film and TV, matte shots just weren’t satisfying
enough. Directors wanted to mix animated
characters and live people,
and they wanted animated characters
to move in a more natural fashion.
Walt Disney was among the first
to combine animation and live action
in a feature film, and his seminal
Song of the South was a big hit in
1946. However, viewers saw an obvious
gap between the animated characters
and the live actors.
Stop-action animationwas another
attempt to make animation more
realistic and less expensive. Unlike
traditional cell animation, which
relies on hand drawn and colored
frames that are then photographed
one by one, stop-action animation
uses models, which are moved in
small increments and photographed
after each move. Possibly
the greatest practitioner of this art
was Ray Harryhausen, whose films
include The Seventh Voyage of Sinbad and Jason and the
Argonauts. His painstaking technique included matting
the stop action footage with real-life backgrounds. The
effect was pretty good, but the action was obviously
artificial.
Rotoscoping was developed around 1915 by Max
Fleischer to help bridge the gap between natural motion
and animation. This technique requires a technician to
trace an actor’s motion by hand on each frame of the
filmed sequence. This tracing then serves as the basis
for inking the actual cartoon character. You can imagine
how long that takes. It works, but it’s hard to do well,
and the results leave something to be desired.
In the 70s, the US military began funding the development
of magnetic tracking devices. They were used
for following the head movement of pilots, among other
things. As virtual reality appeared on the scene in the
late 80s, these same trackers were adapted for use in
tracking heads and hands in virtual worlds. By the mid-
90s, some animators realized that they could use the
same tracking systems for animation.
Optical tracking has a much longer history. Almost as
soon as moving pictures were developed (even before,
if you consider Muybridge’s efforts), people were tracing
movements with grease pencils on negatives and analyzing#p#分页标题#e#
those tracks. As video became inexpensive and
ubiquitous, people realized they could draw on an overlay
on the video and create an animation. Then computer
systems were developed that could perform the process
automatically. The addition of optically bright markers
made the task even easier, and today optical tracking is
a mainstay of performance animation.
In addition to body tracking, many studios add input
gloves, such as Virtual Technology’s (Palo Alto,
California) CyberGlove, or 5DT’s (Persequor, South
Africa) 5thGlove, to capture hand articulation. Facial
expressions are captured with optical systems, such as
one from Analogus (San Francisco, California), or by puppeteering,
as demonstrated by MediaLab (Paris, France).
Using MoCap
Until recently, MoCap was very touchy stuff. It still
demands careful attention from manufacturers to make
sure everything works right. Ascension Technology
offers an example.
Jack Scully is vice president of Ascension Technology,
manufacturers of the MotionStar and Bird tracking systems.
He explained that “It used to be, five years ago,
you needed somebody with a degree in electrical engineering
and a programmer to get this stuff working.
Now it’s much easier. It’s pretty much plug and play on
the hardware end. We have drivers to connect our hardware
to 3D Studio, Alias|Wavefront, SoftImage, all the
common programs.”
Scully said that Kaydara’s Filmbox makes one of the
best plug-ins for use with Ascension’s equipment.
FilmBox sits between the MotionStar and the animation
software. It controls the data capture, then cleans up
the data, edits it, and passes it on to the animation package.
“This, more than anything else, makes the process
plug and play,” Scully explained.
Another step is necessary to assure that Ascension’s
IEEE Computer Graphics and Applications 17
4 A schematic of a typical magnetic
tracking system setup. The two
large boxes marked with T are
transmitters. The small boxes on
the dancers are receivers. This
system is hardwired; newer units
use radio transmission to eliminate
wires between the actors and the
host computer.
Image courtesy of CyberEdge InfoGraphics
.
systems work right the first time. The company sends
prospective customers a detailed survey of the equipment
and software that the customer wants to use with
the tracking hardware. When Ascension knows what
computer, software, and environment the client intends
to use, they fine-tune the system and makes recommendations
to the client regarding optimal setup. The
company even pays a visit with a “sniffer” that maps the
electrical and magnetic characteristics of the stage. Prior#p#分页标题#e#
to shipping a tracking system, Ascension provides recommendations
regarding placement and operating conditions
that will make the system more foolproof. They
recommend a wooden stage, at least 18 inches away
from a metal or concrete and steel floor, and isolated
from power mains, large metallic objects, and RFI emitters
such as monitors.
Ayes and nays
Using motion capture in animation is not without critics.
Traditional animators rightfully take a lot of pride
in their craft. Some of them see rotoscoping and MoCap
as cheating. In a 1997 Siggraph panel on MoCap, Craig
Hayes—a director, animator, and MoCap developer at
Tippet Studios (Berkeley, California)—said, “Motion
capture tends to be used as a crutch, or even worse, to
create performances/images that could have been created
with filmed, live actors.” Steph Greenberg, an independent
animator who has worked at Disney and many
other studios, added, “An animated character has capabilities
that no human can replicate without possible
injury. Characters can snap into position, their movement
deliberate and uncompromising—their athletic
abilities simply can’t be matched.”
Even the vice president of marketing at a leading
MoCap equipment manufacturer has reservations. Chris
Welch, who holds that position at Motion Analysis, said,
“MoCap is a tool the animator uses. You can’t use motion
capture to make Bugs Bunny walk like Bugs Bunny. But
MoCap gives the animator the time to do good work.
Instead of dealing with animation one frame at a time,
they can spend time on painting, backgrounds, and
other stuff.”
According to Welch, the bottom line is, “If you want a
character that dances like Baryshnikov but looks like an
elephant, MoCap makes that look good, without spending
hours on every frame. MoCap will provide the capability
to put high-quality animation out again.” See
Figure 5.
Gary Roberts of Centroid Studios in London, England
is a fan of MoCap, which he recently used on the movie
Lost in Space. As he explained in a press release issued by
the studio,
I was tasked with capturing actor Gary Oldman
for over 10 minutes of on-screen time. We elected
to use the Motion Analysis system with 8 cameras
. . . . We used between 35 and 42 markers for
the facial capture. The character Gary played had
metal plates over his face, so we positioned markers
to accurately replicate the movement of these
plates as closely as possible. . . . We required a volume
of 1.5m by 1.5m by 1m and a 200-degree
field of movement in both the X- and Y-axis for
Gary’s face. Using 8 cameras and a selection of
1mm to 2mm markers, we were able to achieve#p#分页标题#e#
this without any problems.
Roberts isn’t alone in his appreciation of MoCap as a
way to translate the characteristic movements of real people
to animated characters. From the molten metal robot
in Terminator 2 to the strolling passengers on the deck in
Titanic, Hollywood has taken to MoCap in a big way.
The bottom line
What really made this all practical, though, is the constant
downward spiral of computer prices. It still takes a
18 September/October 1998
5 An elephant might dance like
Baryshnikov, or Baryshnikov might
prance like a horse. Using a large
set and an optical tracking system
lets this horse provide the moves
for an animated character.
Photo courtesy of Motion Analysis
Applications
.
lot of computer power to create performance animation
from tracking data, especially in real time, but today that
power costs a fraction of what it did even five years ago.
While a good performance animation system still costs
$50,000 and up, it would have cost millions just a few
years back.
Of course, filmmakers don’t do motion capture just
because they can. Combining captured motion data and
3D animation gives them a lot of advantages the cell animators
will never have, even if cost is no object. For
example, it is inconsequential to rotate a 3D character,
or change the lighting, because the software treats the
3D model like a real object. When you have a 3D model,
it’s easy to map motion data to control points on the
model. So, at a basic level, using MoCap for animation
is really simple. If you need to mix live action and realtime
or canned animation, 3D figures permit a greater
range of interaction.
However, if cost is an object, MoCap animation overwhelms
traditional cell work. Traditional animation can
cost $20,000 per minute of finished footage, or even
more. A good MoCap house, working with journeymen
animators, can cut that cost to as little as $500 per
minute. For certain types of work, the savings could be
even greater. For example, if you wanted to include an
animated Michael Jackson in a piece using traditional
cell animation, someone would need to become an
expert in animating Jackson’s moon walk, gestures, and
physical style. It would be obvious to anyone familiar
with Jackson if the moves were off. With MoCap, this
issue just doesn’t exist. Simply wire Michael, as he was
for his Ghosts music video, and have him do his thing.
When you see that footage, you immediately know that
it is Michael Jackson dancing, not an imitation.
These economies and fidelities drive the MoCap business
today. A recent study conducted by CyberEdge
Information Services (Sausalito, California) found that
about 30 to 40 MoCap houses provide motion capture#p#分页标题#e#
services for hire. Those service bureaus bring in, on average,
around $675,600 each (in 1998). Many more studios
are divisions of movie studios, special effects
houses, and game publishers. They contribute savings,
rather than revenue, to the bottom line. Business is very
good for the service bureaus—the study respondents
anticipated average growth of more than 75 percent in
1999.
That growth rate, if accurate, bodes well for equipment
manufacturers. MoCap users spend, on average,
more than $90,000 per system. Ascension Technology
and Polhemus, who both make magnetic systems,
control the lion’s share of the MoCap market, with
around 40 percent between them. Optical system manufacturers
Vicon (Oxford, UK) and Motion Analysis
(Santa Rosa, California) follow them in market share,
and a flock of smaller companies divvy up the rest of the
market.
Magnetic MoCap systems cost less than optical systems,
starting at around $20,000 and rising quickly as
you add sensors and range. Optical systems require an
initial investment of more than $100,000. Of course,
these costs do not include the computers, software, and
miscellany required to actually do any work.
Whither MoCap?
Though MoCap’s future looks assured, a few concerns
still exist. The CyberEdge study revealed that while most
users of MoCap systems are pretty satisfied with their
equipment, several areas bother them. The single biggest
issue for them is what they categorized as the general
difficulty of using the gear, plus its lack of robustness.
This is still new technology, and it shows. Wires drag
about everywhere, connectors break, and interference
pops up constantly. Magnetic systems can be difficult to
calibrate, and optical systems often require very precise
set-ups. Also, MoCap users want greater range, fewer
wires, and greater accuracy in the measurements provided.
While price isn’t a major issue—end customers
say they’re willing to pay what it takes—users, especially
users of optical systems, want systems that cost less.
Systems will certainly continue to improve, and the
problems will be solved. MoCap will get less expensive,
easier to use, and more common. The big question is,
where will MoCap show up next?
Within a few years, and perhaps even sooner, we will
see the first non-cartoon virtual actors. Non-cartoon
means that you may have to look twice to know if you’re
seeing a live person or a digital duplicate. Libraries of
MoCap data will make it possible for these synthetic
thespians to talk the talk as they walk the walk—
Groucho’s goofy prance, John Wayne’s swagger, or
Marilyn Monroe’s seductive swirl. Michael Jackson may
dance on for a hundred years, now that his moves are#p#分页标题#e#
recorded. Perhaps you’ve seen the dancing baby that
opens the “Ally McBeal” TV show. In a few years, the
whole cast may have originated in 3D Studio.
With virtual actors will come interesting new legal
discussions. What makes up an actor’s personality? If
Arnold Schwarzenegger has his face and body scanned,
his motion captured, and his voice recorded, does a synthetic
Arnold have the same rights as the flesh and blood
model? Only time (at $500 and up per hour of legal fees)
will tell.
The real performers left, such as those reading the
news on TV, will largely work in virtual sets. While not
a MoCap application, virtual sets use very similar technology
for tracking camera movement. With a virtual
set, the camera angle is a vital factor in rendering the
proper view of the set. Cameras are tracked with the
same systems used to track actors. We’ll undoubtedly
see virtual actors, virtual sets, and live actors all in the
same show, and probably soon.
Dawn was breaking over the mountains when I finally
left the studio. Our session had been a success. I had bagged
the prize and, as usual, had earned my money. As I waited
for the Red Car, I wanted nothing more than a tall, cold
carrot juice, an aspirin, a shower, and a bed. And maybe指导英国留学生作业some scrambled eggs. And sausage. With home fries. I definitely
needed some rest. I knew that next week I’d be seeing
the Shadow Lady every time I turned on my TV—as a
dancing banana. n
Delaney may be captured at [email protected].
Contact Potel at [email protected].
IEEE Computer Graphics and Applications 19
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