Perceiving Motion and Events

Transcription

1 Perceiving Motion and Events Chienchih Chen Yutian Chen The computational problem of motion space-time diagrams: image structure as it changes over time 1

2 The computational problem of motion space-time diagrams for some simple motion of a single dot p Stationary dot 2. Constant velocity motion 3. Harmonic motion 4. Apparent motion(stroboscopic motion) Continuous Motion An environmental object is considered to be moving if its position changes over time. p469 Motion perception threshold ex. The blades of a window fan The moon The movement of clock hands 2

3 Adaptation and Aftereffects Adaptation Motion Aftereffects o Waterfall illusion o Paradoxical motion o Spiral aftereffect Where are they occurred in the visual system? There are both monocular and binocular components to the motion perception system. Simultaneous Motion Contrast If a stationary path of random dots is surrounded by a field of dots moving steadily downward, the central patch will appear to drift upward. 3

4 Apparent Motion Human visual system can be fooled into perceiving continuous motion from a sequence of "snapshots" or "frames" presented at the proper rate. Early Gestalt Investigations Two lights across a fixed distance turn on and off alternatively from very fast to very slow: p Simultaneous flickering 2. Phi motion 3. Beta motion 4. Sequential alternation 4

5 Korte's laws Korte examined the quality of apparent motion as a function of the relations among three parameters: the alternation rate, the separation between the two points of light, and the intensity of the two lights. Separation vs. intensity: Larger separations require higher intensityes Rate vs. intensity: Slower rates require higher intensities Separation vs. rate: Larger separations require slower rates Motion picture technology 24 frames per second lies squarely within the range that produces beta motion, the most convincing perception of smooth continuous movement A flicker rate of at least 60 times per second is needed for flicker fusion--in which a flickering light appears to be on continuously. Thus, 72 flashes and 24 frames per second to produce good perception of apparent motion. As for TV and videotape, each video frame is effectively presented twice by interlacing the odd- and even- numbered lines of the video image. 30 frames per second. 5

6 The Correspondence Problem of Apparent Motion Motion is perceived between corresponding objects, but which objects are the "corresponding" ones? Wagon wheel illusion 6

7 Do other factors affect correspondence problem? Short-range VS. Long-range Apparent Motion Random-dot Kinematogram Classical phenomena of apparent motion 7

8 Short-range VS. Long-range Apparent Motion Short-range motion system is responsible for performance on Random-dot Kinematogram. And it is thought to occur fairly early in visual processing, before information from the two eyes has been integrated and before shape and color have been extensively analyzed. Long-range motion system is responsible for classical phenomena of apparent motion with large-scale, individual figures. It is thought to occur much later in visual processing, after information from the two eyes has been integrated. The Aperture Problem 8

9 The Barberpole illusion Plaid grating 9

10 Physiological Mechanisms The Magno and Parvo System Cortical Analysis of Motion Neuropsychology of Motion Perception The Magno and Parvo System In comparison to P cells, M cells respond rapidly to changes in stimulation, are highly sensitive to luminance contrast, have large receptive fields, and are low in spatial resolution. M cells are the first step in the visual system's analysis of image motion. 10

11 Cortical Analysis of Motion Selective response to different directions o component cells o pattern cells Neuropsychology of Motion Perception A patient was admitted to the hospital complaining of severe headaches, vertigo, nausea, and the inability to perceive motion. She reported that the world appeared to her as a series of frozen snapshots. Brain image tests indicated that the lesion in her brain was located in the border region between occipital and temporal cortex, clearly outside of primary visual cortex. 11

12 Computational Theories Directionally selective cells play a crucial role in our ability to perceive motion. But, how? Delay-and-Compare Networks 12

13 Edge-Based Models Analyze the change in illumination over time in conjunction with an edge detector Spatial-Frequency-Based Models 13

14 Integrating Local Motion Integrating Local Motion 14

15 next... Object Motion Depth and Motion Long-Range Apparent Motion Dynamic Perceptual Organization Self-Motion and Optic Flow 15

16 Image Motion Vs. Object Motion Image motion must be interpreted to provide information about the motion in environmental objects Image motion + movement of eye, head, body... => motion constancy Velocity Constancy Object speed, rather than image speed Good depth information is needed Adjust speed of moving objects at different depth (Rock, Hill, & Fineman, 1968) 16

17 Depth and Motion When only motion information is present, does the visual system interpret differential motion in terms of depth? Rigid Motion in Depth (Gunnar Johansson 1950) Rigidity heuristic The Kinetic Depth Effect (KDE) Experiment by Wallach and O Connell (1953) Using a shadow-casting technique to present 2-D images of angular 3-D wire objects to observers 17

18 Rigidity Heuristic All else being equal, if there is an interpretation in which rigid motion can be perceived, it will be. Correspondence Problem Unique points, such as vertices and endpoints (but not cross-points), appear to play an important role in the conditions required for veridical perception of rigid motion What if the correspondence problem is solved incorrectly? 18

19 The Stereo-Kinetic Effect Perception of Nonrigid Motion 19

20 Long-Range Apparent Motion Long-range apparent motion is fundamentally a 3-D, object-based perception Experiment by Shepard and Judd (1976) Two sets of displays for picture-plane rotation (A) and 3-D rotation in depth (B) Determine the fastest alternation rate for smooth rigid motion The process underlying apparent rotation is a continuous process that requires time to go through intermediate orientations, just as real objects do. The linear increase was virtually identical for the pairs related by picture-plane rotations and depth rotations. 20

21 Geometrical Model Map object motions to geometrical paths in a high-dimensional space Motion hyperspace Any continuous motion can be represented as a continuous path within the higher-dimensional space. Path impletion The visual system interprets apparent motion by traversing (impleting) one of many possible paths between corresponded points of the two displays. Maximum speed Rigid apparent motion will be perceived if and only if the alternation rate is sufficiently slow that the shortest path along the rigid-rotation surface can be traversed at or below a maximum speed in the time between two displays. Curved Apparent Motion 1. A translation along a straightline path from left to right with a concomitant clockwise rotation about the center of the line minimize the work required to move the object 2. A global rotation about a point below the two lines in which the points on the line follow curved paths minimize motion in kinematical geometry 21

22 Path-guided apparent motion The visual system has internalized the structure of motions in the real world to a remarkable degree Allowing it to fill in the most likely motion in apparent motion displays Dynamic Perceptual Organization What regions are perceived to be part of the same object or group of objects? How do we organize the motions of different objects? How they move with respect to each other? 22

23 Grouping by Movement Common fate: the tendency to group together units that move with the same velocity (direction and speed) Put back two or more parts of a single moving object separated by occluding objects 23

24 Common fate can destroy even the best natural camouflage Configural Motion 24

25 The visual system performs a vector analysis of the motion of objects moving synchronously. Motion vectors are decomposed in to two components: common motion and relative motion Induced Motion The moon is moving through the cloud rather than the cloud passes in front of the moon Karl Duncker (1929/1937) Two assumptions: 1. Sensitivity to relative motion 2. Stationarity of the surrounding object 25

26 Induced self-motion Self-Motion and Optic Flow Two differences between induced self-motion and induced motion 1. The motion of the environment is well above the threshold of absolute motion 2. There is an initial period of veridical perception: You first perceive the drum as rotating and yourself as stationary Perceiving Self-Motion Optic flow: the global patterns of retinal motion Direction 26

27 Speed of Self-Motion Information about absolute speed is not available solely from optic flow absolute distance is not available A A a b B B Time to contact: the length of time it will take the observer to reach the surface toward which he or she is heading under present motion conditions t=distance/speed 27

28 Understanding Events How balls move when one collides with another? How objects behave under gravitational free-fall when dropped from a moving object? Biological Motion Meaningless points when seated motionlessly Perceived as a person immediately and unmistakably when began to move Even walking versus jogging were easly discriminated 28

29 Biological Motion Biological Motion 29

30 Perceiving Causation Launching effect Triggering effect Entraining effect Perceiving Mass Relations Ricochet heuristic: When the incoming ball ricochets at a higher velocity than the forward motion of the initially stationary ball, the stationary ball is heavier than the incoming one Clobbering heuristic: When the stationary ball moves off with high velocity, the incoming ball is heavier than the stationary one The two heuristic conflict when the ricocheting ball moved more slowly than the ball it struck. Thus people appear to be recovering information about mass from visual information, but not with the quantitative precision and sophistication his analysis implied 30

31 Perceiving Mass Relations Intuitive Physics 31

32 Intuitive Physics Intuitive Physics One possibility is that the high-level cognitive system that is responsible for generating solutions to statically stated problems simply does not have access to the perceptual system's expertise in these domains reason: many of the same people who favored the curved trajectory when presented with the static version of the C-tube problem chose the correct, straight trajectory in the dynamic version 32