2022essay网站指导英国留学生作业：女子行径的阴影：My job wasto capture the Shadow Lady
It was a muggy summer night in one of Santa Monica’sseedier neighborhoods. I was on assignment. My job wasto capture the Shadow Lady. I didn’t expect it to be easy.But I guess that’s why I get the big bucks.I loitered in a dark doorway, watching and waiting. As Ifinished my Lucky Strike, I saw her. The Shadow Lady pulledup in her little red sports car and, without 指导英国ESSAY looking around,walked through an unmarked door into a big warehouse.She was wearing an overcoat and a beret. Odd, that overcoat.This July night in Santa Monica, the evening temperaturewas still in the 80s. I took one last drag, threw downthe butt, and followed her in. It was time for some action.I’m a motion capture technician. I handle the hard stuff—set-up, calibration, cleaning up data—all the stuff nobodyelse wants to do. We need lots of room, lots of electrical power,and the freedom to work any hours we need to. That’s whywe work in what looks like a run-down warehouse. Inside,we have nearly a million dollars worth of gear.The Shadow Lady is a professional dancer and actress.She’s lithe, graceful, expressive, and perhaps most importantly,really patient. She gave herself the name: “All youkeep is my shadow,” she said at the end of a session onemorning. Basically, she was right.Catching shadowsMotion capture houses throw away the person andkeep the shadow—the essence of their motion—andapply that motion to animate all kinds of characters.Perhaps you saw those dancing cars and credit cards inthe Shell ad on TV. That was done with MoCap. Or youhttp://www.ukassignment.org/ may have admired those folks strolling on the sun deckin Titanic. That was MoCap, too. In the past few years,as the technology has become less expensive and at thesame time more accurate, MoCap has helped lessen therigors of traditional cell animation—especially in highlycost-conscious projects like TV commercials.A MoCap studio can use both magnetic and opticalcapture systems. Each has certain advantages. The magneticsystem handles shoots that don’t need really highaccuracy, and the director wants to be able to apply thedata to his character in real time. Magnetic systems generallyare faster at providing data to use in animation,though optical systems are generally more accurate andcan track more points. That may be changing, though.The first real-time optical system was shown at Siggraph98 by Motion Analysis (Santa Rosa, California).When I walked onto the capture stage, 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 animatedcommercial.Magnetic systems work by generating three orthogonalelectro-magnetic fields from each transmitter. TheBen DelaneyCyberEdgeInformationServices0272-1716/98/$10.00 © 1998 IEEEOn the Trail of the Shadow Woman:The Mystery of Motion Capture ______________________ApplicationsEditor: Michael J. Potelhttp://www.wildcrest.com14 September/October 19981 The ShadowLady strikes apose. She’swearing theAscensionMotionStarWireless magnetictrackingsystem. Eachsmall cube is areceiver. Herwaist pack holdsa battery, datacollection 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 2http://www.ukassignment.org/ The more data points you track, the less extrapolationthe software has to do. So for an animation of acharacter that doesn’t look much like a human beingsuch as those dancing credit cards—you only needenough 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 adata set that represents the subject’s motion with theoptimal level of detail and the least amount of noise. Theoptimal 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 capturehouses and equipment manufacturers by visiting their sites on theWorld Wide Web.Adaptive Optics Associates: http://www.aoainc.comAnalogus: http://www.analogus.comAscension Technology: http://www.ascension-tech.comBioVision: http://boris.biovision.comDigital Domain: http://www.d2.com5DT: http://www.5dt.comGameTek: http://www.gametek.comHouse of Moves: http:// www.moves.comMediaLab: http://www.demon.co.uk/mlabMotion Analysis: http://www.motionanalysis.comPerformance Animation Society: http://www.pasociety.orgPolhemus: http://www.polhemus.comProtozoa: http://www.protozoa.comRichard Cray’s Performance Animation Page:http://www.pasociety.org/perfanimVicon Biomechanics (was Oxford Metrics):http://www.metrics.co.ukVirtual Technologies: http://www.virtex.comPhoto courtesy of Motion Analysis2 This heavy action scene usesoptical tracking with a MotionAnalysis system. The little balls onthe actors are the reflective markers,tracked by the cameras surroundingthem..http://www.ukassignment.org/ 16 September/October 1998ApplicationsThe big threeProbably the three biggest issues in MoCap for entertainmentare range, interference, and wires. Let’saddress 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 thearea in which capture can happen. These are significantand related factors. Obviously, you need to have roomfor the performers to move without running into thewalls or equipment. This area is blocked out before thecapture session, typically with the director or producer,the actor/dancer, and the capture technician. A simpledance may only need 100 square feet, while a fight scenecould need 500 square feet or more. If you need to capturea performer running for some distance, treadmillssometimes can provide the required mobility. No matterhow it’s arranged, the working area is a big factor in settingup the session.The other range issue is the distance between the performerand the capture equipment. The inverse squarelaw dictates that signals get weaker in proportion to thesquare of the distance between the source and receiver.For magnetic trackers, that limits the range to about a10-foot radius per transmitter. You can gang up some systemsto use multiple transmitters, increasing the rangeand capture area. Optical trackers are virtually unlimitedin range (see Figure 3), but the spatial resolution suffersbecause the tracking targets look smaller as theymove away from the cameras. This reduces accuracy.The second issue is interference. Both optical and magneticsystems suffer from interference, though thedetails completely differ for the two systems. Opticalinterference is primarily occlusion. For example, an armmoves in front of a thigh marker, or the performer turnsso the camera can’t see some of the markers, or one performersteps in front of another. Work-arounds—typicallyadding more cameras—help avoid the problem,but occlusion is a fact of life and a continuing annoyancewith optical tracking.Don’t forget, though, that magnetic systems have theirown problems (see Figure 4). They suffer from electricalinterference caused by induced magnetic currents,eddy patterns caused by large metallic objects nearby, orexternal sources of radiation, such as TVs and computermonitors. The work-arounds for these problemsinclude sophisticated software filtering of the data.Wires pose the third big concern. For quite a whileoptical systems were preferred for any sort of athleticactivity because they didn’t need the annoying cablesthat magnetic systems required. In addition, cableslimit range, and when you work with more than oneactor, they can get quite ridiculously tangled. It’s justin the past two years that both Ascension (Burlington,Vermont) and Polhemus (Colchester, Vermont) introducedwireless 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 aheadband with one more sensor sewn on the front. Istrapped the pack around her waist and plugged the sensorwires into the receiver in the pack. I used Velcro strapsto hold the wires snugly to her limbs. I checked the batteryagain. Then I sat down at the console.I asked the Shadow Lady to assume a series of poses andwatched the stick figure on my monitor as it followed herthrough the moves. “Number three’s a little jittery,” Ithought, but decided we could live with it. I dialed in zeropoints for each receiver and nodded to the director. “We’reready.”The director waved to the sound man, and as I hit the“Start” button, loud Caribbean music started to play. TheShadow Lady started to dance. I watched the monitor, nother. My little stick figure looked pretty good. Everythingwas humming. We captured about three and a half minutes,then the director yelled “Cut!” As he walked over totalk with the Shadow Lady, my mind wandered. I wasthinking about where this technology came from—the rootsof MoCap, so to speak.The family treeMotion capture is classified as “special effects,” a termused to explain just about anything you see in the moviesthat isn’t a straight camera shot. Special effects are justabout as old as film itself. The term was first used in amovie credit in 1926, on the filmWhat Price Glory? Back then, specialeffects involved some of the samemechanical tricks done in stage productions.Soon, double exposuresand other in-camera tricks, like stoppingthe camera, moving a person orprop, and re-starting, were added tothe toolkit. Early filmmakers quicklyreached the limits of model makingand in-camera effects, though,and started developing new tools.One of the most important was thematte shot.A matte shot is created by taking apiece of film with some action on it,like the heroine running franticallyfrom a villain, and creating a matte,or mask, that eliminates everythingbut her image from the film. This isImage courtesy of CyberEdge InfoGraphics 3An optical tracking system usestwo or more video cameras to findthe reflective markers attached tothe actors in key locations. With nowires leading from the actors, theyhave complete freedom of motion..then composited photographically with a painted background,so the shot of her running in a studio canbecome her running through the streets of Shanghai orthrough the desert. This simple trick is still used frequently,though today most of the compositing is donewith computers. This technique was improved upon#p#分页标题#e#with the development of the traveling matte shot, inwhich the matte changes with the action. This permitscombining the foreground action with another piece offilm, which can include background action.Television made it possible to do matte shots in realtime using a technique called blue-screen matte. This techniquemakes it seem like your local weather person isstanding in front of an animated map when they’re reallystanding in front of a blank blue screen. A mixingdevice combines their image with the picture of the map.In both film and TV, matte shots just weren’t satisfyingenough. Directors wanted to mix animatedcharacters and live people,and they wanted animated charactersto move in a more natural fashion.Walt Disney was among the firstto combine animation and live actionin a feature film, and his seminalSong of the South was a big hit in1946. However, viewers saw an obviousgap between the animated charactersand the live actors.Stop-action animationwas anotherattempt to make animation morerealistic and less expensive. Unliketraditional cell animation, whichrelies on hand drawn and coloredframes that are then photographedone by one, stop-action animationuses models, which are moved insmall increments and photographedafter each move. Possiblythe greatest practitioner of this artwas Ray Harryhausen, whose filmsinclude The Seventh Voyage of Sinbad and Jason and theArgonauts. His painstaking technique included mattingthe stop action footage with real-life backgrounds. Theeffect was pretty good, but the action was obviouslyartificial.Rotoscoping was developed around 1915 by MaxFleischer to help bridge the gap between natural motionand animation. This technique requires a technician totrace an actor’s motion by hand on each frame of thefilmed sequence. This tracing then serves as the basisfor inking the actual cartoon character. You can imaginehow 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 developmentof magnetic tracking devices. They were usedfor following the head movement of pilots, among otherthings. As virtual reality appeared on the scene in thelate 80s, these same trackers were adapted for use intracking heads and hands in virtual worlds. By the mid-90s, some animators realized that they could use thesame tracking systems for animation.Optical tracking has a much longer history. Almost assoon as moving pictures were developed (even before,if you consider Muybridge’s efforts), people were tracingmovements with grease pencils on negatives and analyzing#p#分页标题#e#those tracks. As video became inexpensive andubiquitous, people realized they could draw on an overlayon the video and create an animation. Then computersystems were developed that could perform the processautomatically. The addition of optically bright markersmade the task even easier, and today optical tracking isa mainstay of performance animation.In addition to body tracking, many studios add inputgloves, such as Virtual Technology’s (Palo Alto,California) CyberGlove, or 5DT’s (Persequor, SouthAfrica) 5thGlove, to capture hand articulation. Facialexpressions are captured with optical systems, such asone from Analogus (San Francisco, California), or by puppeteering,as demonstrated by MediaLab (Paris, France).Using MoCapUntil recently, MoCap was very touchy stuff. It stilldemands careful attention from manufacturers to makesure everything works right. Ascension Technologyoffers 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 engineeringand a programmer to get this stuff working.Now it’s much easier. It’s pretty much plug and play onthe hardware end. We have drivers to connect our hardwareto 3D Studio, Alias|Wavefront, SoftImage, all thecommon programs.”Scully said that Kaydara’s Filmbox makes one of thebest plug-ins for use with Ascension’s equipment.FilmBox sits between the MotionStar and the animationsoftware. It controls the data capture, then cleans upthe data, edits it, and passes it on to the animation package.“This, more than anything else, makes the processplug and play,” Scully explained.Another step is necessary to assure that Ascension’sIEEE Computer Graphics and Applications 174 A schematic of a typical magnetictracking system setup. The twolarge boxes marked with T aretransmitters. The small boxes onthe dancers are receivers. Thissystem is hardwired; newer unitsuse radio transmission to eliminatewires between the actors and thehost computer.Image courtesy of CyberEdge InfoGraphics.systems work right the first time. The company sendsprospective customers a detailed survey of the equipmentand software that the customer wants to use withthe tracking hardware. When Ascension knows whatcomputer, software, and environment the client intendsto use, they fine-tune the system and makes recommendationsto the client regarding optimal setup. Thecompany even pays a visit with a “sniffer” that maps theelectrical and magnetic characteristics of the stage. Prior#p#分页标题#e#to shipping a tracking system, Ascension provides recommendationsregarding placement and operating conditionsthat will make the system more foolproof. Theyrecommend a wooden stage, at least 18 inches awayfrom a metal or concrete and steel floor, and isolatedfrom power mains, large metallic objects, and RFI emitterssuch as monitors.Ayes and naysUsing motion capture in animation is not without critics.Traditional animators rightfully take a lot of pridein their craft. Some of them see rotoscoping and MoCapas cheating. In a 1997 Siggraph panel on MoCap, CraigHayes—a director, animator, and MoCap developer atTippet Studios (Berkeley, California)—said, “Motioncapture tends to be used as a crutch, or even worse, tocreate performances/images that could have been createdwith filmed, live actors.” Steph Greenberg, an independentanimator who has worked at Disney and manyother studios, added, “An animated character has capabilitiesthat no human can replicate without possibleinjury. Characters can snap into position, their movementdeliberate and uncompromising—their athleticabilities simply can’t be matched.”Even the vice president of marketing at a leadingMoCap equipment manufacturer has reservations. ChrisWelch, who holds that position at Motion Analysis, said,“MoCap is a tool the animator uses. You can’t use motioncapture to make Bugs Bunny walk like Bugs Bunny. ButMoCap 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, andother stuff.”According to Welch, the bottom line is, “If you want acharacter that dances like Baryshnikov but looks like anelephant, MoCap makes that look good, without spendinghours on every frame. MoCap will provide the capabilityto put high-quality animation out again.” SeeFigure 5.Gary Roberts of Centroid Studios in London, Englandis a fan of MoCap, which he recently used on the movieLost in Space. As he explained in a press release issued bythe studio,I was tasked with capturing actor Gary Oldmanfor over 10 minutes of on-screen time. We electedto use the Motion Analysis system with 8 cameras. . . . We used between 35 and 42 markers forthe facial capture. The character Gary played hadmetal plates over his face, so we positioned markersto accurately replicate the movement of theseplates as closely as possible. . . . We required a volumeof 1.5m by 1.5m by 1m and a 200-degreefield of movement in both the X- and Y-axis forGary’s face. Using 8 cameras and a selection of1mm 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 away to translate the characteristic movements of real peopleto animated characters. From the molten metal robotin Terminator 2 to the strolling passengers on the deck inTitanic, Hollywood has taken to MoCap in a big way.The bottom lineWhat really made this all practical, though, is the constantdownward spiral of computer prices. It still takes a18 September/October 19985 An elephant might dance likeBaryshnikov, or Baryshnikov mightprance like a horse. Using a largeset and an optical tracking systemlets this horse provide the movesfor an animated character.Photo courtesy of Motion AnalysisApplications.lot of computer power to create performance animationfrom tracking data, especially in real time, but today thatpower 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 fewyears back.Of course, filmmakers don’t do motion capture justbecause they can. Combining captured motion data and3D animation gives them a lot of advantages the cell animatorswill never have, even if cost is no object. Forexample, it is inconsequential to rotate a 3D character,or change the lighting, because the software treats the3D model like a real object. When you have a 3D model,it’s easy to map motion data to control points on themodel. So, at a basic level, using MoCap for animationis really simple. If you need to mix live action and realtimeor canned animation, 3D figures permit a greaterrange of interaction.However, if cost is an object, MoCap animation overwhelmstraditional cell work. Traditional animation cancost $20,000 per minute of finished footage, or evenmore. A good MoCap house, working with journeymenanimators, can cut that cost to as little as $500 perminute. For certain types of work, the savings could beeven greater. For example, if you wanted to include ananimated Michael Jackson in a piece using traditionalcell animation, someone would need to become anexpert in animating Jackson’s moon walk, gestures, andphysical style. It would be obvious to anyone familiarwith Jackson if the moves were off. With MoCap, thisissue just doesn’t exist. Simply wire Michael, as he wasfor his Ghosts music video, and have him do his thing.When you see that footage, you immediately know thatit is Michael Jackson dancing, not an imitation.These economies and fidelities drive the MoCap businesstoday. A recent study conducted by CyberEdgeInformation Services (Sausalito, California) found thatabout 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 studiosare divisions of movie studios, special effectshouses, and game publishers. They contribute savings,rather than revenue, to the bottom line. Business is verygood for the service bureaus—the study respondentsanticipated average growth of more than 75 percent in1999.That growth rate, if accurate, bodes well for equipmentmanufacturers. MoCap users spend, on average,more than $90,000 per system. Ascension Technologyand Polhemus, who both make magnetic systems,control the lion’s share of the MoCap market, witharound 40 percent between them. Optical system manufacturersVicon (Oxford, UK) and Motion Analysis(Santa Rosa, California) follow them in market share,and a flock of smaller companies divvy up the rest of themarket.Magnetic MoCap systems cost less than optical systems,starting at around $20,000 and rising quickly asyou add sensors and range. Optical systems require aninitial investment of more than $100,000. Of course,these costs do not include the computers, software, andmiscellany required to actually do any work.Whither MoCap?Though MoCap’s future looks assured, a few concernsstill exist. The CyberEdge study revealed that while mostusers of MoCap systems are pretty satisfied with theirequipment, several areas bother them. The single biggestissue for them is what they categorized as the generaldifficulty of using the gear, plus its lack of robustness.This is still new technology, and it shows. Wires dragabout everywhere, connectors break, and interferencepops up constantly. Magnetic systems can be difficult tocalibrate, and optical systems often require very preciseset-ups. Also, MoCap users want greater range, fewerwires, and greater accuracy in the measurements provided.While price isn’t a major issue—end customerssay they’re willing to pay what it takes—users, especiallyusers of optical systems, want systems that cost less.Systems will certainly continue to improve, and theproblems 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 willsee the first non-cartoon virtual actors. Non-cartoonmeans that you may have to look twice to know if you’reseeing a live person or a digital duplicate. Libraries ofMoCap data will make it possible for these syntheticthespians to talk the talk as they walk the walk—Groucho’s goofy prance, John Wayne’s swagger, orMarilyn Monroe’s seductive swirl. Michael Jackson maydance on for a hundred years, now that his moves are#p#分页标题#e#recorded. Perhaps you’ve seen the dancing baby thatopens the “Ally McBeal” TV show. In a few years, thewhole cast may have originated in 3D Studio.With virtual actors will come interesting new legaldiscussions. What makes up an actor’s personality? IfArnold Schwarzenegger has his face and body scanned,his motion captured, and his voice recorded, does a syntheticArnold have the same rights as the flesh and bloodmodel? Only time (at $500 and up per hour of legal fees)will tell.The real performers left, such as those reading thenews on TV, will largely work in virtual sets. While nota MoCap application, virtual sets use very similar technologyfor tracking camera movement. With a virtualset, the camera angle is a vital factor in rendering theproper view of the set. Cameras are tracked with thesame systems used to track actors. We’ll undoubtedlysee virtual actors, virtual sets, and live actors all in thesame show, and probably soon.Dawn was breaking over the mountains when I finallyleft the studio. Our session had been a success. I had baggedthe prize and, as usual, had earned my money. As I waitedfor the Red Car, I wanted nothing more than a tall, coldcarrot 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 definitelyneeded some rest. I knew that next week I’d be seeingthe Shadow Lady every time I turned on my TV—as adancing banana. nDelaney may be captured at [email protected] Potel at [email protected] Computer Graphics and Applications 19x
