video analysis/set up Flashcards
Annotate the following on the vertical force data and shade the areas of negative and positive impulse presented in Figure 1.
• Start of squat down –
• Bottom of squat –
• Instant of take-off –
• Instant of landing –
• Mid-flight –
Bottom of squat – where negative and positive impulse appear equal
Instant of take-off – when acceleration starts to flat-line (~1.25 )
Negative impulse areas = less than 0 – Positive impulse areas = greater than 0
Explain what is meant by the terms systematic error and random error and give an example of how they might both be introduced during manual digitising of a video clip (4 marks).
Systematic error is where a set value is consistently out by a set amount, from frame to frame, this affects accuracy. e.g continuously saying a joint center is 1cm lower than it is, so one is always 1 cm off for each data entry.
Random errors occur when one makes a small bit of error that changes from frame to frame is, i.e when marking the joint center, the center could be marked as 2mm to the right then in the next frame 2mm to low. it has no pattern, but affects precision.
What are two disadvantages of using markers during 2D video analysis?
Can only film and see one plane of motion of movement due to 2D analysis, and marker placement must be correct for accurate analysis, there is a lack of ecologically validity, easily skewed by human error (marker placement and camera set up).
What are two advantages of using markers during 2D video analysis?
manual 2d analysis is reliable and quick/efficient to set up as external factors like reflective gear don’t affect the video capturing,
it is a cheaper alternative to 3d motion capture,
keeps original/raw data
Explain why shutter speed is important?
a suitable shutter speed is needed, otherwise, you run the risk of missing vital/relevant information.
What happens to the quality of the image when the shutter speed is increased, and how can it be reduced?
when shutter speed increases, there is less time shutter is open for, so less light strikes the sensors, therefore the image appears darker. THis can be counted by opening the iris wider to allow more light in.
What is meant by parallax error in 2d video anlysis, and how can it be prevented?
Parallax errors are introduced when the camera is not completely perpendicular to the plane of motion and the person/object has moved away from the optical axis of the camera. This means when measuring or quantifying an image, the angle of view is not perpendicular and measurements can be skewed.
This can be minimised by aligning the optical axis of the camera with the central part of the movement and zooming in to only record the required movement.
Describe the process required to ensure that a camera is perpendicular to the plane of motion of a sporting motion. (2marks)
Triangulation is where we find the center of the motion we are capturing by using two tape measures and creating a triangle with equal sides to find the mid point to ensure the camera is perfectly perpendicular. To minimise any further errors we adjust the pan, roll and tilt of the camera.
What is a perspective error?
The perspective error occurs when one is looking at an image at too close of a distance, this distorts the image and its proportions.
What is pan?
Pan is when one side of the field of view is more in focus than the other when we want the center to be filmed giving equal distances on either side. this can be checked by making sure both cones/markers leave the field of view on the camera at the same time, meaning the cones are at equidistant from the field of view.
What is roll,
Is ensuring the field of view is level on flat horizontal line. This can be chcked by making sure cones/markers are level with each other, and one is not higher or lower than the other.
What is tilt?
Tilit is weather the front of the camera is tilted up or down, this is important so movement is in the center of the field of view, this can be checked by placing an object right infornt of the camera and then moving into the field of view, the object should remain in the center, if incorrect the object will look lower or higher than the center.
State four important considerations when setting up a 2D video camera to capture accurate data (4 marks) and state why they are important to collect accurate data (4 marks).
Triangulation to ensure camera is perpendiulcar to the plane of motion, and by claibrating the pan, roll and tilt we can minimise errors like perspective errors occuring.
By adjusting the aperture/iris we can ensure the video is clear by ensuring there is enough light entering the camera so motion is visible.
By adjusting the shutter speed we ensure we do not miss value information by taking more images within the recording time so there is more to analyse as well as making the images more clear.
