L4 - Video Analysis

• Quantitative = analysis of measurement/quantity of movement
• Qualitative = (usually visual) analysis of the form/quality of the movement
• Frame Rate:

  • Number of pictures (frames) per second (fps or Hz)
  • TV shows film at 25 fps
  • If a fast event needed to be captured clearly, high frame rate & fast shutter are needed
  • Fps: to see specific event & orientation
  • For batting, see impact/when hit ball
  • Fast shutter: for clear picture
• 3 Components of Exposure:

  • Trade off in between them

  1. Aperture
  2. Shutter
  3. ISO

  • Need lots of light = best thing for all = best quality
• Components of Exposure - Aperture:

  • Size of the opening in the lens
  • Larger aperture allows more light in, but reduces depth of field
  • What focused on is more clear, but behind is more out of focus
  • Can change across all if have enough light
  • Measured in f-stops
• Components of Exposure - Shutter:

  • Opens & closes to expose sensor to light
  • Measured in time (s)
  • Also have rolling shutters, aren’t accurate pictures → cause image distortion
  • Slower shutter, more light, but can cause motion blur
  • Fast movements need fast shutter (& lots of light)
  • Fast shutter, less light available so need good light (if slow not as important)
  • Sunlight is best, indoors may need flood lighting
• Components of Exposure - ISO:

  • Sensitivity to light
  • Can lighten dark pictures, but adds graininess
  • Best image quality at low ISO values, but need lots of light
  • Higher ISO = more grainy
  • Electronically done
• Other things to consider:

  • Light behind person, not good, want it on person = backlighting?
  • Wide angle lengths distort image
  • Resolution? Low = not good, makes pixels bigger instead of adding more
• Camera Setup:

  • Think about camera settings, movement being filmed & lighting conditions to get proper exposure
  • Most likely 2D analysis, but are multiple 2D views necessary?
• Camera Setup:

  • Camera should be level, lens height at centre of image & perpendicular to plane of motion
  • Video what interested in - centre thing you’re interested in
  • Don't want to much empty space → want a lot of pixels on things you are interested in
• Perspective Error:

  • Decide on the main plane of motion
  • Setup camera on a tripod perpendicular to plane
  • Avoid panning & tilting if using calibration
  • Even when properly setup, out of plane is a problem
• Perspective Error:

  • Wide lens makes near objects appear large (eg on phones)
  • Large errors when out of plane
  • Position camera as far from subject as practically possible
  • Zoom in using telephoto lens
  • Focus
  • Zoom out so size of performer is maximised
• Video Calibration:

  • To make linear measurements you need a calibration object
  • Must be known size & in plane of motion
  • Relates pixels to real world units
  • Any camera movement ruins calibration
• Digitising Accuracy:

  • Estimating joint angles can have an effect
  • Issue in reliability & validity