Capturing Movement. Foul or no foul! Why study human movement? Bio-39 October 5, Maturation and development.

Transcription

1 Capturing Movement Bio-39 October 5, 2017 Foul or no foul! Why study human movement? Maturation and development Child learning to walk Skill development Mastering a tennis serve, hitting a golf ball Activities of daily living Walking or rising from a chair following a stroke rehab interventions Gait training for an individual with a below knee amputation 1

2 Muybridge s horse Humans in motion 2

3 Marey s images Rotating shutters: Stroboscopic 3

4 Videography Old days of Bio-39 The present Charge-coupled-devices (CCD) A 2-D array of dots, called pixels A frame is made up of two halves One half (field) is odd numbered rows One half (field) is even numbered rows Interlaced: takes two fields to make one frame US cameras are 60 Hz: Full frame rate is 30 Hz Resolution of CCD device More pixels and more lines per frame means higher the resolution Vertical resolution (i.e. number of lines) is fixed by the format of the video Horizontal resolution (i.e. number of pixels per line) depends on how the signal is sampled If the advertisement claims of 400 lines of resolution actually means 400 pixels per line 4

5 Resolution of system Not the same as resolution of the camera Is determined by component (e.g. VCR, monitor, etc) with lowest resolution Resolution of screen on most cell phones far exceeds that of CCD Means that CCD is usually the limiting component Aspect ratio Because of CCD pictures are physically bigger horizontally than vertically Aspect ratio = field width divided by height Usually 4:3 Sampling frequency Faster the movement faster the sampling rate Wagon wheels go backwards in old movies Even High speed camcorders sample at 30 Hz Only means that the cameras have high shutter speeds 5

6 Shutter speeds Determines the time that the CCD is exposed to the image Faster the shutter (shorter the time) and hence more light is needed to obtain a good exposure Shutter speeds prevent image blurring, BUT DOES NOT ALLOW THE CAMERA TO CAPTURE MORE IMAGES PER SECOND Shutter speeds However, with present day motion capture and high-speed video cameras the main issue is frames per sec (fps). So to film a soccer kick, for example, one would want to film at 1000 fps or better High-speed video cameras Go-Pro camera systems Consequence of improvements in the film and computer design CMOS replaces CCD High speed, high capacity memory buffering systems Phantom video camera 6

7 spagetti Quantitative motion capture Goal is to collect data to quantify aspects of the movements The data collected is related to the movement of markers attached to the body Two types of marker systems: Active markers Passive markers Electromagnetic ( active system ) 7

8 Electromagnetic systems Advantages Elimination of marker drop-out Real time 6 DOF data Accuracy Insensitive to environmental conditions Disadvantages Time consuming to set up Cabling that connects sensors can inhibit movement Sensors can slip during movement Interference from metal objects Cost: $ 50,000 for one in lab Opto-electronic Opto-electronic systems ( active system ) Advantages Automated marker tracing Hence no marker misidentification Disadvantages Wire connections to the LEDs Cumbersome May inhibit movement Use in laboratory setting is limited 8

9 Passive systems Most commonly used Cameras emit infrared light that is reflected of markers that are attached to subject Each camera determines (x,y) coordinates; two together determine (x,y,z) by a process referred to as triangulation Wand calibration One motion capture cameras On board computer determines x and y coordinate of markers (up to 1024 markers in modern systems) Rate of capture up to Hz is available 9

10 Two+ motion capture cameras Two cameras together triangulate to determine the z coordinate Means at least two cameras must be able to see every marker at the same time Triangulation d d tan tan d sin sin sin( ) Passive systems Advantages No cables attached to subject Precision Disadvantages Expensive Large space These problems stem from requirement that at least two cameras must be able to detect a marker at any instance in time Typically cameras to measure a golf swing Each camera costs $ 25,000! BUT price is now dropping very fast (even camcorders can be incorporated into a motion capture system) 10

11 The Future: Wearable devices Wearable devices will revolutionize medicine Can wearable devices also be trained to do this? Dogs can be trained to anticipate the occurrence of a seizure. If seizure occurrence can be predicted, can we develop strategies to abort the seizure? Choosing a location to do your experiment Is the location appropriate? Can you control access to and from it? Do you have access to electrical power if needed? Availability of washrooms, water? How to I calibrate my measurements? Is camera oriented square to subject? How can measurements be recorded? 11

12 Example: courtyard outside this lecture room Conclusion: easy to control access Example: courtyard outside this lecture room Conclusion: water and washroom available Camera orientation can be checked Can use ledge to assure that camera is square to subject. Black tape on ground indicates where subject will be 12

13 Subject is positioned Calibrate movements Vertical line is of known length. Measure it on picture and then use this calibration to determine length of movements. Other considerations Get permission to use space that you have chosen (likely it will not be the courtyard) Reserve time Make sure that everything is set up and ready to go before your subject arrives Easier to recruit subjects if word gets out that you don t waste their time 13