An object whose dimensions are accurately known must be recorded in the plane of motion. This is to enable image co-ordinates to be transformed to object-space (real world) co-ordinates following the digitising stage. Recording of the scaling object(s) must be done only after the camera set-up is complete.
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The use of both horizontal and vertical scaling objects is essential, because the computer may display the image with an aspect ratio (ratio of the width to the height) that distorts it in one dimension. To minimise the error in the scaling process, the dimensions of the scaling objects should be such that they occupy a good proportion of the width and height of the field of view. For a given digitising error, the scaling error will be inversely proportional to the length of the scaling object.
For field widths greater than 2–3 m, scaling is usually done using the known distance between two or more reference markers or control points, positioned in the plane of motion. In some circumstances it is not possible to align the camera optical axis correctly with the plane of motion, for example when filming in a competition. Here, digitisation of a grid of control points, placed in the plane of motion, can be used to correct for the camera misalignment.
This method is called 2D-DLT and has been shown to provide significantly more accurate reconstruction of two-dimensional co-ordinate data than the more commonly used scaling techniques, particularly when the optical axis of the camera is In activities such as running, jumping, throwing and kicking, it is the most distal body segments, the hands and feet, which move the quickest. A shutter speed should be selected that is sufficient to provide a non-blurred image of the fastest moving body segments (or sports implements).
The choice of shutter speed depends on the type of activity being recorded. For slow movements, such as a grande plié in ballet or walking, shutter speeds of 1/150–1/250 of a second should be adequate; for moderately fast activities, such as running or a swimming start, shutter speeds of 1/350–1/750 of a second are more appropriate; for fast activities such as a golf swing or a tennis serve, a shutter speed of 1/1000 of a second or above may be needed.