Math Forum / Math Software / MATLAB / October 2008

Does anyone know of good code/algorithm for tracking the
coordinates of multiple points in a video file? I work in a
microrobotics lab where we take lots of high-speed video of
very tiny parts. Most of the parts are dark, navy blue or
black, so we can put spots of retroreflective paint at the
points of interest, then use very bright lets to get an
extremely high-contrast video. The algorithm to track ONE
dot is simple enough, just use the threshold function to
convert the grayscale image to black and white, then use the
regionprops function to find the white areas and their
centroids, and track the centroid of the largest area
(assuming your video isn't perfect and there might be some
small white spots that don't get filtered out by threshold).

So how do you extend that to track multiple dots at once? If
you want to track point A and point B (up to an arbitrary
number of points, within reason), you have to somehow make
sure that you are consistent as to which point is which from
frame to frame; i.e., if you start out with point A on the
left and point B on the right, and in the next frame each
point has moved up a tiny bit, odds are that A is still on
the left and B is still on the right, they didn't both jump
all the way across the screen. Using different colors to
track different dots isn't an option since the high-speed
camera only records in grayscale.

Any ideas, suggestions, or links to useful code if it's
already out there would be appreciated.