Biomedical Engineering Reference
In-Depth Information
can be determined. Thus, with the movie camera set at the right frame rate,
f -stop, and range, the filming is ready to commence.
3.3.3.1 Movie Conversion Techniques. As 16-mm movie cameras are the
most common form of data collection, it is important to be aware of various
coordinate extraction techniques. Each system that has evolved requires the
projection of each movie frame on some form of screen. The most common
type requires the operator to move a mechanical xy coordinate system until a
point, a light, or cross hairs lies over the desired anatomical landmark. Then
the x and y coordinates can be read off or transferred to a computer at the
push of a button. Figure 3.11 shows the component parts of such a conversion
system.
A second type of system involves the projection of the film image onto a
special grid system. When the operator touches the grid with a special pen,
the coordinates are automatically transferred into a computer. Both systems
are limited to the speed and accuracy of the human operator. Our experience
indicates that an experienced operator can convert an average of 15 coordinate
pairs per minute. Thus a 3-s film record filmed at 50 frames per second could
have five markers converted in 30 min.
Figure 3.11 Typical arrangement for the microcomputer digitization of data coordi-
nates from movie film. Foot pedal allows operator to transfer coordinate data to the
computer at a rate of about 10 coordinate pairs per minute. Digitizing error is about
1 mm rms with the camera located 4 m from the subject.
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