Animation CS 551 / 651 Perception of Physically Simulated Humans Hodgins et al., 1998

Perception How should we render our objects? To what end?To what end? –Verisimilitude –Mechanical accuracy –Impressionism What about motion?What about motion? How should we render our objects? To what end?To what end? –Verisimilitude –Mechanical accuracy –Impressionism What about motion?What about motion?

Is simplicity better? Advantages Abstraction is easierAbstraction is easier Obfuscations are removedObfuscations are removedDisadvantages Complementary features are removedComplementary features are removed –Edges, critical features May look “wrong”May look “wrong”Advantages Abstraction is easierAbstraction is easier Obfuscations are removedObfuscations are removedDisadvantages Complementary features are removedComplementary features are removed –Edges, critical features May look “wrong”May look “wrong”

Is complexity better? Advantages Details provide perceptual cuesDetails provide perceptual cues This is the way we perceive things in real worldThis is the way we perceive things in real worldDisadvantages Difficult to get the details rightDifficult to get the details right May distract from basic motionMay distract from basic motionAdvantages Details provide perceptual cuesDetails provide perceptual cues This is the way we perceive things in real worldThis is the way we perceive things in real worldDisadvantages Difficult to get the details rightDifficult to get the details right May distract from basic motionMay distract from basic motion

We have no idea… We turn to different experts PsychologistsPsychologists Automated computer visionAutomated computer vision We turn to different experts PsychologistsPsychologists Automated computer visionAutomated computer vision

Psychologists Kubovy and UVa Perception of patterned dot animationsPerception of patterned dot animations –Models of perception Perception as it relates to actionPerception as it relates to action –We perceive because it helps us to act Attacks the perception question within well- defined psychological models Kubovy and UVa Perception of patterned dot animationsPerception of patterned dot animations –Models of perception Perception as it relates to actionPerception as it relates to action –We perceive because it helps us to act Attacks the perception question within well- defined psychological models

Computer Vision Martin and Low-level visionLow-level vision –How do we detect edges, shadows, primitives High-level visionHigh-level vision –How do we compose “things” from primitives Martin and Low-level visionLow-level vision –How do we detect edges, shadows, primitives High-level visionHigh-level vision –How do we compose “things” from primitives

Still no solid answers Vision and psychology provide models of perception that influence graphics Graphics permits isolated experimentation with perception models The three fields move forward together Vision and psychology provide models of perception that influence graphics Graphics permits isolated experimentation with perception models The three fields move forward together

What’s amazing about us? Perceiving friends Just two moving lights on ankles is enoughJust two moving lights on ankles is enough Just two seconds is requiredJust two seconds is required –Johansson (1973) Vanrie & Verfaillie (2004) Perceiving pendula Humans thought moving dots were connected via flexible bar, not rigid pendulumHumans thought moving dots were connected via flexible bar, not rigid pendulum Perceiving friends Just two moving lights on ankles is enoughJust two moving lights on ankles is enough Just two seconds is requiredJust two seconds is required –Johansson (1973) Vanrie & Verfaillie (2004) Perceiving pendula Humans thought moving dots were connected via flexible bar, not rigid pendulumHumans thought moving dots were connected via flexible bar, not rigid pendulum

Hodgins’ comparison Is there a difference?

Observational tests Torso rotation Keep head looking forward, but rotate torso and armsKeep head looking forward, but rotate torso and arms Arm Motion Make arm swing more forward / backwardMake arm swing more forward / backward –Adjust dynamics accordingly –How much? Noise Randomly perturb joint angles (waist, shoulders, neck)Randomly perturb joint angles (waist, shoulders, neck) –No dynamics –How much? Torso rotation Keep head looking forward, but rotate torso and armsKeep head looking forward, but rotate torso and arms Arm Motion Make arm swing more forward / backwardMake arm swing more forward / backward –Adjust dynamics accordingly –How much? Noise Randomly perturb joint angles (waist, shoulders, neck)Randomly perturb joint angles (waist, shoulders, neck) –No dynamics –How much?

How the simulation works

Experimental protocol Watch animations in pairs 4 seconds of one then 4 seconds of a mate4 seconds of one then 4 seconds of a mate Indicate similarity or difference within pair forced choiceforced choice Could you forget what first looked like?Could you forget what first looked like? Approx. 25 people per condition Varied the order Avoids ordering effects (learning during experiment)Avoids ordering effects (learning during experiment) Watch animations in pairs 4 seconds of one then 4 seconds of a mate4 seconds of one then 4 seconds of a mate Indicate similarity or difference within pair forced choiceforced choice Could you forget what first looked like?Could you forget what first looked like? Approx. 25 people per condition Varied the order Avoids ordering effects (learning during experiment)Avoids ordering effects (learning during experiment)

Experimental protocol Animations rendered in same way Could this have made a difference?Could this have made a difference? –Is there a rendering that is conducive to stick figures? –What tricks would people use to identify motions? Played from VHS at 30 fps Can’t have any effects from rendering blipsCan’t have any effects from rendering blips Animations rendered in same way Could this have made a difference?Could this have made a difference? –Is there a rendering that is conducive to stick figures? –What tricks would people use to identify motions? Played from VHS at 30 fps Can’t have any effects from rendering blipsCan’t have any effects from rendering blips

Results On average, people were better with manH

Results But how did rendering affect each person’s ability? There’s a trick!

Take-away messages Don’t read too much into these results Each experiment may be differentEach experiment may be different More detailed model was also more human-likeMore detailed model was also more human-like Standardization of animation environments might be good for comparison Difficult to compare improvements from year to yearDifficult to compare improvements from year to year Don’t read too much into these results Each experiment may be differentEach experiment may be different More detailed model was also more human-likeMore detailed model was also more human-like Standardization of animation environments might be good for comparison Difficult to compare improvements from year to yearDifficult to compare improvements from year to year

What else matters? Camera movement Ground plane Motion blur Secondary motion (clothing / hair) Shadows Camera movement Ground plane Motion blur Secondary motion (clothing / hair) Shadows

Additional commentary Experiments are essential for graphics Yet rarely conductedYet rarely conducted How is graphics evaluated?How is graphics evaluated? –The SIGGRAPH “aaahhhh” factor Experiments are essential for graphics Yet rarely conductedYet rarely conducted How is graphics evaluated?How is graphics evaluated? –The SIGGRAPH “aaahhhh” factor

Additional commentary Creating experiments is dicey business Have to include psychologists who are experts of experiment designHave to include psychologists who are experts of experiment design Make sure enough subjects are includedMake sure enough subjects are included You need to understand the domain so well that you know the answer before the experiments are completeYou need to understand the domain so well that you know the answer before the experiments are complete –Many pretrials were conducted to refine amounts of noise to add (to avoid making it too easy or hard) Creating experiments is dicey business Have to include psychologists who are experts of experiment designHave to include psychologists who are experts of experiment design Make sure enough subjects are includedMake sure enough subjects are included You need to understand the domain so well that you know the answer before the experiments are completeYou need to understand the domain so well that you know the answer before the experiments are complete –Many pretrials were conducted to refine amounts of noise to add (to avoid making it too easy or hard)

Follow-up paper Bodenheimer et al., 1999 Eurographics Animation Workshop How does noise influence perception? Bodenheimer et al., 1999 Eurographics Animation Workshop How does noise influence perception?

How to add noise to simulation? Sensors When the arm reaches angle , trigger reactionWhen the arm reaches angle , trigger reaction Control gains How stiff/strong are the musclesHow stiff/strong are the muscles Output torques How regular and well-behaved are the musclesHow regular and well-behaved are the muscles Control parameters When does the arm swing backwardsWhen does the arm swing backwardsSensors When the arm reaches angle , trigger reactionWhen the arm reaches angle , trigger reaction Control gains How stiff/strong are the musclesHow stiff/strong are the muscles Output torques How regular and well-behaved are the musclesHow regular and well-behaved are the muscles Control parameters When does the arm swing backwardsWhen does the arm swing backwards

Output torques Noise inserted here didn’t work well Instantaneous noise was quickly corrected with subsequent countertorquesInstantaneous noise was quickly corrected with subsequent countertorques Noise inserted here didn’t work well Instantaneous noise was quickly corrected with subsequent countertorquesInstantaneous noise was quickly corrected with subsequent countertorques

What kind of noise? Variability of human motion is tied to large movements of the body Not a random sinusoidal noise functionNot a random sinusoidal noise function Not a white noiseNot a white noise Variability of human motion is tied to large movements of the body Not a random sinusoidal noise functionNot a random sinusoidal noise function Not a white noiseNot a white noise

Experimental scenario Watch 10 movies of varying noise and select the one that looks most “natural”