CS-184: Computer GraphicsLecture #17: Introduction to AnimationProf. James O’BrienUniversity of California, BerkeleyV2008-S-17-1.012Introduction to AnimationGenerate perception of motion with sequence of image shown in rapid successionReal-time generation (e.g. video game)Off-line generation (e.g. movie or television)23Introduction to AnimationKey technical problem is how to generate and manipulate motionHuman motionInanimate objectsAmorphous objectsControl34Introduction to AnimationTechnical issues often dominated by aesthetic onesViolation of realism desirable in some contextsAnimation is a communication toolShould support desired communicationThere should be something to communicate45StoryAnimationLightingCameraShadingLayoutModelingFrom Parent, p.15Computer Animation PipelineIntroduction to Animation56StoryAnimationLightingCameraShadingLayoutModelingComputer Animation Pipeline (v 1.1)VoicePostprocessingFor more detailed diagram, see Kerlow p.54FoleyIntroduction to Animation67Introduction to AnimationKey-frame animationSpecification by handMotion captureRecording motionProcedural / simulationAutomatically generatedCombinationse.g. mocap + simulation78Key-framing (manual)Requires a highly skilled userPoorly suited for interactive applicationsHigh quality / high expenseLimited applicability From Learning Maya 2.089Motion Capture (recorded)Markers/sensors placed on subjectTime-consuming clean-upReasonable quality / reasonable priceManipulation algorithms an active research area3Figure 1.1: Optical motion capture systems work by tracking retroreflective markers placed on thebody. Since these markers appear as bright dots to the cameras filming them, they are easy to detect.Multiple cameras are then used to triangulate the position of these markers.as bright dots on the infrared cameras looking at them (see figure 1.1). Optical systems use anumber of such infrared cameras to detect the two dimensional positions of every marker. Becausethe cameras can be calibrated to a known common coordinate system, a marker’s three dimensionalposition can be triangulated from its two dimensional projections on the cameras that see it. Themaximum magnetic field that can be generated by the source and the minimum amount of field thatcan be measured by the markers mean that the markers can not get too far away from the source.Magnetic systems are also prone to magnetic interference generated by metallic objects, which arealmost impossible to avoid. Due to these reasons, optical motion capture systems are more commonthan magnetic ones.Typical optical motion capture systems are quite expensive. To obtain high accuracy,many cameras need to be employed. This is especially true if markers are to move in a large area.This is due to the perspective effect which means projections of father away points move less asthey move in space.In order for a marker to be triangulated, its projection on several cameras need to be found.Since all a camera sees is a cloud of bright dots, the markers corresponding to each other in all cam-era views must be identified. Some motion capture systems accomplish this correspondence using“active” markers that emit an infrared pulse which uniquely identifies every marker. Since theseMotionAnalysis / Performance Capture StudioOkan Arikan910Motion EditingArikan, Forsyth, O’Brien, SIGGRAPH 20021011Motion EditingArikan, Forsyth, O’Brien, SIGGRAPH 20021112Model ConstructionKirk, O’Brien, Forsyth, CVPR 20051213SimulationGenerate motion of objects using numerical simulation methodsxt+Δt= xt+ Δt vt+12Δt2atgv1314SimulationPerceptual accuracy requiredStability, easy of use, speed, robustness all importantPredictive accuracy less soControl desirable1415SimulationFeldman, Arikan, O’Brien, SIGGRAPH 20031516What to do with animations?Video tapeDigital videoPrint it on yellow sticky notes1617Video TapeAnalog tape formatsVHS/SVHSBeta SP3/4” U-maticDigital tape formats Digi BetaDV TapeDVD (yes, I know DVDs are not tapes)1718NTSC StandardUsed by DVD, DV, and VHS720x486 resolution (sort of)1.33 aspect ratioLimited color range30 frames per second (sort of 29.97)Interlaced videoOverscan regions1819Digital VideoWide range of file formatsQuickTimeMS Audio/Visual Interleaved (AVI) DV StreamBunch ‘o imagesSome formats accommodate different CODECsQuicktime: Cinepak, DV, Sorenson, DivX, etc.AVI: Cinepak, Indeo, DV, MPEG4, etc.Some formats imply a given CODECMPEGDV Streams1920Digital VideoNearly all CODECs are lossyParameter setting importantDifferent type of video work with different CODECsCompressors not all equally smartCompression artifacts are cumulative in a very bad wayPlayback issuesBandwidth and CPU limitationsHardware accelerationMissing CODECs (avoid MS CODECs and formats)2021Path toTapeNot much of an issue any longerCheap ( < $100 ) devices can give good amateur quality outputPro quality also cheap ( < $5000 )Beware many cheap solutions over use compressionGood analog tape decks still expensive2122EditingOld way:Multiple expensive tape decksSlowDifficultError proneNew way:Non-linear editing softwarePremiere, Final Cut Pro, others...Beware compressed solutionsMay take a long time for final encoding2223Motion BlurFast moving things look blurryHuman eyeFinite exposure time in camerasWithout blur: strobing and aliasingBlur over part of frame interval Measured in degrees (0..360)30 tends to often look good2324Motion BlurEasy to do in a sampling frameworkInterpolation is an issueInterpolation is an issueMotion Blur ! Cont.2425Motion BlurVelocity based blur often works
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