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xwang98
03-13-2004, 03:02 AM
hello everyone,
I have a simple and basic question to ask. anybody who will answer me as soon as possbile will be very very appreciated!
my confusion is about the joint rotation description based on skin-markers.
I have read a paper by Ralf Schmidt et al----- a maker-based measurement proceduce for unconstrainted wrist and elbow motion. I tried to email prof. Ralf Schmidt , but the email address can't accessed. so I trouble you,if you can correct or clear my confusion,I will be appreictted very much!
In that paper,the center of shoulder is assumed to 7cm inferior to the acromion maker which is the average of visually deterimined distance.
question:
Are there some better ways to determine the rotation center of shoulder. are they easy to be used in actual calculation ? has relative programme code of them has been present?
Mr. Ralf Schmidt use a static refernce measurement (at time t1) of "joint marker" to maker sure the original center coordinates. then use marker on segment to calculate the homogenous transformation matrix at time t1, t2 respectively,where t2 stands for another time instant , and then use these matrixes and the origianl center coordiante to get the rotation center coordiante at time t2.
my confusion:
to get a tranformation matix using the least squares algorithm presented by veldpaus et al (1988). two group of coordinates of 3 maker at least on segment at two position are needed. from my understanding, the corrdinates of makers at time t1,t2 are with respect to a gobal reference frame. how to get the transformation matrix at time t1 and t2.
I think we can calculate a homogenous tranformation matrix describe the mapping relation of coordinates at time t1 and t2.
I think that the basic realtion to calculate the transformation is :
X2=RX1+d, =TX1---(1)
where R is rotation matrix ,d is translation vector.T is the homogenous matrix.
my confusion in detail is:
1 the homogenous tansformation matrix T should include both R and d. right?I think so!
2 take the time t1 as example, (the static reference time), how to get the homogenous transformation matix according to (1) using the least square algorithm?
I am very confused at this point.
any noe who ever read this paper can answer me will be appreciated!
finally ,i would see if anyone know the eamil contact ways of Prof. Ralf Schmidt ,please tell me.

thanks in advance!
xinting wang
Tianjin ,China



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