Cartesian analysis: A computer-video interface for measuring motion without physical contact

In this paper, we describe a computer-video interface capable of measuring videotaped motion without physical contact. It constructs imaginaryx-, y-, z-coordinates around a moving object by juxtaposing front and side views on one video record. A special effects generator (SEG) superimposes this two-part image onto the microcomputer’s graphic output. The user watches the SEG’s output and traces the motion of interest in real-time with a computer-generated cursor, a process that assesses the onset time, termination time, duration, linear displacement (i.e., length), and velocity of each traced movement. Reliability coefficients between two independent users were .99 for onset time, .85 for duration, .89 for displacement, and .90 for velocity. In 80 gestures, the correlation between estimated displacement and measured displacement was .73; the correlation between estimated duration (determined from frame-by-frame inspection of the videotape) and measured duration was .80. Twenty-five measurements of displacement for a motorized target traveling 75.4 in. differed from the true value by | .06 |, | .06 |, and | .30 | in. for thex, y, andz dimensions, respectively.