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View Full Version : Calculation of 2D Angles ('CAST' problem)



Dr. Chris Kirtley
12-18-2002, 06:00 AM
Dear all,

Many years ago, Ray Smith and I developed a software package for
digitizing marker locations from digital movies of patients. We then
calculated the segment angles and derived the joint angles. It was
published here: Kirtley C & Smith RA (2001) Application of Multimedia to
the study of Human Movement. Multimedia Tools and Applications: 14 (3):
259-268.

I recall that at the time, Ray and I were fairly new to motion analysis
and we agonized for quite some time about how to cope with the
sectorization ('CAST') problem. For example, say you have a shank
segment that's almost vertical. The ATAN function of (Yknee -
Yankle)/(Xknee - Xankle) returns the correct angle of the shank when the
angle with the floor is less than 90 degrees, but returns a negative
value when it's at more than 90 degrees. For example, 120 degrees would
come out -30 degrees.

Eventually, Ray came up with a method of laboriously checking which
sector (quadrant) the segment is in and thereby correcting the angle.
This works fine, but I have always thought that there must be a more
elegant solution. I wonder if anyone can enlighten me?

I may as well take the opportunity to wish everyone Happy holidays...

Chris
--
Dr. Chris Kirtley MD PhD
Associate Professor
Dept. of Biomedical Engineering
Catholic University of America
620 Michigan Ave NE, Washington, DC 20064
Tel. 202-319-6247, fax 202-319-4287
Email: kirtley@cua.edu
http://engineering.cua.edu/biomedical/faculty/kirtley

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