CH4: Data Collection - Digital Video Flashcards
What does optical refer to?
light
What does capture refer to?
recording light
What are the two types of optical capture?
still and motion
What is still capture?
Taking a single image (e.g. photography)
What is motion capture?
Taking several successive images (e.g. filmography and videography)
What is the principle of optical capture?
Light reflected off an object (which can be a person) is focussed through a lens and recorded on a medium or sensor
Factors that influence image quality
Sensory type. sensor resolution, frame rate, exposure time, light intensity
What is film defined as?
Continuous spatial
What is video tape [e.g. VHS] defined as?
Discrete spatial
What is digital defined as?
Discrete spatial
What are the two types of commonly used electromagnetic sensors to record image data?
- Charged Couple Device (CCD)
- Complementary Metal-Oxide Semiconductor (CMOS)
Most cameras used a CCD
What is resolution?
- Number of pixels recorded per image
- Increasing the number of pixels
increases the image quality
What is frame rate?
Number of images taken per second or the number of times that the sensor is exposed to light each second
What is a progressive scan?
It includes all horizontal lines
What does interlaced mean?
Horizontal lines are split into “odd” and “even” fields
What are the two purposes that deinterlacing can be used for?
- Showings on LCD screens
- Doubles the number of images therefore twice the sampling rate (but half the information for each sample)
LCD Screens
- CRT and plasma screens can play interlaced video
- LCD screens cannot play interlaced video properly
- Cannot show odd and even fields at different times
- Frames are deinterlaced, then recombined to play at the same time
Biomechanics
- Odd and even fields are separated
- Fields are analyzed in sequence
- Odd 1, even 1, odd 2, even 2, etc.
- Doubles the sampling rate
- Trade-off: ½ resolution
What is NTSC Standard video?
640 vertical lines and 480 horizontal lines, so 307,200 pixels. NTSC video recorded 30 images each second ( 30 frames per second or 30 Hz). Each of these frames could be split in half, resulting in 60 fields/second.
What is PAL Standard video?
PAL standard video had more pixels, but a lower frame rate. 768 vertical lines, 576 horizontal lines, so 442,368 pixels. 25 frames/second and 50 fields/second
Standard resolution nomenclature for resolution: 720p30
- The first value describes the number of horizontal lines. The number of vertical lines is inferred by the form factor, which we can then use to calculate the number of pixels.
- The second value, either a “p” or an “i”, describes whether the video is progressive scan or interlaced.
- The third value describes the frame rate.
What does the form factor or aspect ratio mean? 4:3
That means for every 3 horizontal lines, there are 4 vertical lines
What does frame rate mean?
Determines the number of frames per second, and each frame contains information for that time interval.
When a movement is very ___________, exposing the sensor for the entire time interval will result in __________.
fast, blurring
Recomended: for fast movements, we will decrease exposure time so we have minimal blurring
What is the relationship between shutter speed and exposure time.
Increasing shutter speed decreases the exposure time.
What is exposure time?
Film/sensor exposed to image for brief time. Exposure time depends on movement speed
What is the relationship between exposure time and the amount of light on a sensor
High shutter speed (short exposure time): decreased light
- The aperture is opened to maximize the light allowed in.
Low shutter speed (long exposure time): increased light
- The aperture is closed to reduce the light allowed in.
What is adjusted to increase/decrease light to sensor
aperature (iris)
What is parallax error?
If the sensor is not parallel to the motion, this will result in parallax error. The amount of parallax error increases the greater the deviation. For example, if you were recording sagittal plane motion, the sensor would be parallel to the sagittal plane (of the person you are recording).
ex. foot progression angle, foot width, camera placed incorrectly
What is field of view?
The extent of the observable world that is seen at any given moment. Determined by focal length; lens closer to the sensor (smaller focal length) would increase FOV.
What is Perspective Error?
Identical objects at different distances from the lens appear to be different sizes
1. Near object appears larger
2. Far object appears smaller
Distance between objects should be small versus distance from lens to object
- Rule of thumb: minimum 10 metres from lens to object
Lens Distortion Error
- No lens is perfectly shaped
- Irregularities in lens shape misdirect light towards sensor - Distortion is greatest towards edges of lens (Glass “ground” away from cylinder to form a lens)
The more glass that needs to be ground away, the more irregularities may be present, so distortion is greatest towards the edges of the lens.
What is the process of digitilization
- Assigning Cartesian coordinates to important landmarks
- Required for all image-based data
- Manual or “automatic tracking” – “Automatic tracking” uses pattern recognition
How can we use linear digital data?
- Position coordinates
- Calculate velocity
- Calculate acceleration
- Calculate force
How can we use angular digital data
- Position coordinates
- Define rigid body
- Calculate angular kinematics and kinetics
