Kinematic Motion 2 Flashcards
Briefly explain the difference between Optoelectric, Automatic and Real-time automatic motion
analysis systems.
Optoelectric
- Active markers: infrared signals
- Passive markers: reflective signals
Automatic
- Automatic marker tracking
- Automatic prediction and marker search
Real-time
- Use either Video or IR-sensitive cameras
- Can generate capture and reconstructed output in real-time (very slight delay)
Explain the main differences between 2D and 3D data collection procedures.
2D
- Generally requires the use of only one camera
- Data usually represented in an x and y direction
3D
- A minimum of two cameras
- Data is represented in x, y and z directions (rotation)
- Gen-locked for synchronization (shutter, frames)
Outline the advantages and disadvantages of using a 3D data collection system versus a 2D
system.
Advantages
- An accurate representation of the movement with all of directional movement components (i.e., x, y, z) to be captured is allowed
- Rotational movements to be analysed
- Can extrapolate data to obtain rotational kinematic and kinetic data
- Allows more complete and accurate analysis and refinement of movement patterns
Disadvantages
- The cameras required for motion capture are relatively expensive
- The computer hardware and software is also relatively expensive
- The complex nature of the set-up of data collection and use of associated software limits 3D data collection to technical specialists
- Because of the type and amount of equipment involved 3D data collection usually only occurs in a laboratory setting, thus the range of skills that can be accurately captured is limited
List several points of consideration when performing 2D data collection for cameras
- On tripod, spirit leveled, no panning
- Position far away, zoom in to focus then zoom out to the required image size
- Make image as large as possible
- Ensure plane of motion is perpendicular to the optical axis of camera
- Use sufficient frame rate and shutter speed to capture motion
- Ensure images are recorded
List several points of consideration when performing 2D data collection for calibration
- Scale device of known length must be filmed in plane of motion, same distance from camera as performer
- Horizontal device may remain throughout performance
- If video, must scale vertically also due to aspect ratio
- Scale device must be large enough
- Distortion error
List several points of consideration when performing 2D data collection for background
- Clear, uncluttered, non-reflective
- Avoid filming into the sun
- Ensure adequate light for correct exposure (beware of glare)
List several points of consideration when performing 2D data collection for the subjects
- Written informed consent
- Form fitting clothing (if possible)
- Land marks should contrast
- Reflective tape or balls
- Draw on skin
- Allow performance to be unobstructed
Explain what an analogue to digital converter does
Conversion of a physiological signal (Voltage) to a computer signal (binary digital code)
- An electrical signal fed into an A-D converter
- Signal is sampled (series of snapshots)
- Fed to computer memory
- Analysed by us
Explain the “Sampling Theorem” using video collection as an example.
- Digital signals consist of a series of snapshots of a physiological signal taken at regular intervals
- How often in 1 second these snapshots are taken -> sampling rate
- “The process signal must be sampled at a frequency at least twice as high as the highest frequency present in the signal itself”
- Too low: analysis errors & false frequencies
- Too high: more expensive equipment to sample at high frequency as well as analyse
Video collection
- Image capture rate must be fast enough to provide sufficient information
- e.g. freeze leg in running
- High speed cameras are available but expensive
What is digitising?
Process of converting film/video images into computer images with known co-ordinate geometry
List and explain several sources of error when digitising and how these errors might be minimised.
- Grain / pixels - definition of video
- Marker shift - because of clothing or skin movement
- Segmental data assumptions
- Motion outside plane perpendicular to camera axis (2D)
- Motion outside calibration volume
- Data filtering/smoothing technique
- Calculation of derived quantities
- Timing calibration
What minimum number of reference control points is required for the solution of Direct Linear Transformation
A min number of six control points on a reference structure with known X, Y and Z co-ordinates is needed for the solution of the DLT.
Explain the statement: Accurate co-ordinate reconstruction can only be guaranteed within the
calibration volume
Accurate co-ordination reconstruction can only be guaranteed when there is an object with a known reference point
There are several forms of data smoothing and filtering that can be applied to kinematic motion
raw data. Be able to name and explain several of these processes.
ANSWER
Explain the problems associated with over and under smoothing.
- If the choice of cut off frequency is too low or the data is over smoothed then valuable data will be lost
- If the cut off frequency is too high or the data is not smoothed enough then unwanted noise will remain.
- The degree of smoothing from digitized data is often subjective – visual observation decision
