View Full Version : Re: Surface markers in 3D analysis

Tochi Omenukor
12-09-1995, 12:13 PM
On Sat, 9 Dec 1995, Jesus Dapena wrote:

> Tochi:
> > the motion of surface markers can be recorded directly by
> > data acquisition equiment....time savings over other
> > methods, yes?
> Yes, surface markers may permit "automatic" digitizing, and that
> could be a very good time savings. But often you can't put anything on your
> subjects -- can you imagine being at the Olympic final and asking all the
> high jumpers to stop by you so that you can stick these things on them??
> The idea of surface markers is good for a lab situation, where you
> have control over your subjects. You have to consider, though, that you
> will need to put a lot of markers on your subjects in order to predict
> internal landmark positions from external landmarks. Remember that some of
> these markers are going to be hidden in some images from certain points of
> view, so one marker will be visible to one camera but not to the other, and
> in that case that marker will not be good for anything during the time that
> only one camera sees it. That means that you are going to need to put extra
> surface markers, to ensure that at all times you see three surface markers
> to predict the position of an internal landmark.
> ---
> Jesus Dapena

dear jesus:

thanx for your response.

i can agree with you there; however, i believe that extrapolation
techniques are used for missing surface marker data, for a certain number
of trials [five?]...should the missing marker data be excessive, then the
investigators will have to repeat the trials, employ more markers, adjust
the camera[s] angles, adjust the lighting [leds or reflective surfaces] or
just plain reposition everything.

i suppose that similar problems could occur with filming the athletic
event. if the subject is involved in a sport that requires some twisting,
somersaulting, etc in conjunction with translation, ie motion in more than
one plane and quadrant, what happens? will alll segments of interest be
actually filmed appropriately for analysis? how many cameras will then be
required to get the complete picture? would it not be better to get the same
subject to perform under controlled conditions [eg in a laboratory], and
thus use surface markers and the associated equipment?

in situations like pathological gait analyses [eg for the elderly and
infirm], i think that surface markers would be oe of the major ways to
go. disagree? ;-)