How do I determine DISTANCE on a video?

For many biomechanical and physics calculations, you need to be able to determine a distance measure from a video recording.

To calculate accurate distance measurements from video footage, you need to have collected the video footage in the correct manner, otherwise inaccurate measurements will be made.

  1. The camera should be placed so that you get a side on view of the bowler.
  2. The camera should not be moving (no panning). That means it is better not to hold the camera in you hands, but to have it set up on a tripod.
  3. The zoom function should not be used.
  4. You must have in the view of the video a standard distance measure for calibration. This can be a measurement stick that you hold up in the video frame, or markings on the ground a known distance apart.

Calibrate for distance by determining the length of the calibration measure on the screen. It is just like using a scale measure on a map. If a three foot (36 inches) distance is one inch on the screen, the calibration ratio is 1:36. Everything in the same plane as the calibration measure will be 36 times bigger than what you can measure on the screen. If the bowler is either closer or further away from the camera than the calibration measure, then the scale will be different and the distance calculated is inaccurate.

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Now simply measure how far an object moves on the screen, and multiply that by the calibration ratio. This method can be used to measure how far each step is in the approach, and how fat the ball travels from one frame to the next. If the frame rate of the video is known, you can then calculate the velocity.

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