2D-Filming Flashcards
Principles of digitising
- Camera remains steady on a tripod
- Computer screen therefore remains fixed in space = steady ref
- Movs on computer screen reflect movt w/ respect to the world (global coordinate system)
- Providing a “Scaling” object enables us to convert a distance measured from the computer to a distance in the real world = process called calibration
Camera Position
- Field of View must be appropriate (zoomed in as much as possible w/o losing any of the movt)
- Zoom + Focus must be locked during both movt + scaling = recommend manual focusing
- Manual Focus is essential to prevent changes in focus during filming
- Camera must be perpendicular to the plane of motion = generally looking at filming in the sagittal plane
What is focal length?
- Introduction to focal length
○ Focal length is the distance b/w the point of convergence in your lens + the camera measured in millimeters (mm)
○ Lens focal length relates to
§ Angle of view: how much of the scene will be captured
§ Magnification: how large individual elements will be - Shorter focal length = wider fields of view
- The longer the lens is, essentially longer focal lengths = narrower field of view
Sources of error
* Perspective Error
- Perspective Error
○ An apparent change in length of objects as they move away from the plane of movt
○ Can be minimised by positioning camera as far away as possible + zooming in (longer focal length)
Sources of error
* Parallax Error
○ Images towards the edge of the field of view are not viewed from exactly side on
- The greater you move to the edge = greater error in angles
- Move the camera further away = still have parallax error but it is less
Both perspective + parallax errors can affect measurement of angles
- Tilting part of an object away from the camera will affect angles measured (perspective error)
- Moving towards the edge of the field of view can have a small effect on angles through parallax error
- Both effects are minimised by positioning the camera far away + zooming in.
- Note: Movt towards or away from camera does not affect angles if the plane of motion is not changed
Camera setup
Frame Rate
* The no. of images exposed per sec
* Frame rate must be fast enough to catch the movt being analysed
* Fast actions have more movt b/w frames, so need a higher frame rate
* If frame rate too low, will miss key events in the movt
Shutter Speed = dependent on light conditions + speed of movt
* The duration of exposure of each image (how long the shutter is open)
* Slow shutter (long duration) allows more time for light to enter, therefore brighter image
* Slow shutter (long duration) allows movt within each frame which causes blurring of fast movts
Shutter speed vs frame rate
- Frame rate is how often the images occur
- Frame rate determines the amount of movt b/w images
- Shutter speed is the duration of exposure for each image
- Shutter speed determines the amount of movt within each image
Light Settings = mainly for depth of view
- Aperture controls the brightness of the image that passes through the lens expressed as an f-number such as f/1.4, f/2, f/2.8, /f4, f/5.6, f/8, f/11, f/16, f/22, or f/32.
- Lower apertures create depth of field by blurring the background.
- Shutter Speed
- Still photographer adjusts aperture to affect depth of field, then adjusts light w/ shutter speed
- Video photographer adjusts shutter speed to affect still image quality, then adjusts light w/ aperture
- Artificial light important for quality filming indoors
Scaling
- For quantitative analysis, an object of known dimensions must be recorded in the plane of motion
- The scaling object should fill the whole range of motion of the movt
- Camera position, zoom + focus must not be adjusted b/w filming movt + scaling object
Subject Markers
- Markers used to identify body landmarks
- For 2D filming, usually mark joint centres
- Markers must be high contrast so they are clear in low light
- Markers on loose clothing will move w/ the clothes
- Direct placement on skin is preferred. Modern, tight clothing can be acceptable
Critical Processes
- Camera orthogonal to plane of motion to subject
- Place camera as far as possible + then Zoom in
- Place calibration frame
- Participant remains in sagittal plane of motion throughout the trial
