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Topic: Vector Coding
Replies: 6   Last Post: Nov 26, 2013 11:57 PM

 Messages: [ Previous | Next ]
 Derek Goring Posts: 3,922 Registered: 12/7/04
Re: Vector Coding
Posted: Nov 26, 2013 7:43 PM

On Wednesday, November 27, 2013 8:36:08 AM UTC+13, Douglas wrote:
> I apologize for the inaccuracy in my typing. Gamma was a matrix (gamma(i)) and the atan2 argument had a comma rather than the division symbol.
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> After making the suggested changes, the issue of vertical asymptotes still exists. The revised code is listed below.
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> gamma=zeros(m-1,1);
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> for row=1:m-2
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> gamma(row)=atan2((y(row+1)-y(row)),(x(row+1)-x(row)));
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> end
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> TideMan <mulgor@gmail.com> wrote in message <b265bf79-061a-4ef4-98ec-6ff04a8bfdf6@googlegroups.com>...
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> > On Wednesday, November 27, 2013 4:00:20 AM UTC+13, Douglas wrote:
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> > > Roger,
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> > >
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> > > I am trying to conduct an analysis of coordination between two adjacent segments called Vector coding (Chang et al., 2008; Journal of Biomechanics). The coupling angle is defined as the inverse tangent of the change in y divided by the change in x (see code below). However, I cannot condition my data such that I avoid the vertical asymptotes of the atan2 function. Can you please offer some suggestions.
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> > >
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> > > x and y are single column arrays with length m.
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> > >
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> > > [m,n]=size(x);
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> > >
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> > >
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> > > for i=1:m-1
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> > >
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> > > gamma=atan2((y(i+1)-y(i))/(x(i+1)-x(i)))
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> > >
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> > > end
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> >
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> > There are several things wrong with your code:
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> > 1. gamma gets over-written each time through the loop, you should use gamma(i)
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> > 2. By default, i and j are sqrt(-1). You should not use i or j as indexes, especially for this particular task which is often easier using complex numbers.
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> > 3. atan2 takes two arguments. You have only one. You will find that if you use atan2 properly, it handles atan2(1,0) OK.
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> > 4. if you use gamma(i), you need to preallocate storage with gamma=zeros(m-1,1); before the loop.

Please do not top post, it makes the thread hard to follow.

I don't understand why you have asymptote problems with atan2.
In my version of Matlab, it doesn't matter what I put in atan2, it works.
atan2(0,1) is OK
atan2(1,0) is OK
atan2(inf,inf) is OK
atan2(0,0) is OK
Can you be more specific as to when atan2 fails?

BTW, you don't need a loop for your calculations, try:
gamma=angle(diff(x) + i*diff(y));
or
gamma=atan2(diff(y),diff(x));

Date Subject Author
11/26/13 Douglas
11/26/13 Derek Goring
11/26/13 Douglas
11/26/13 Derek Goring
11/26/13 Douglas
11/26/13 Derek Goring
11/26/13 Roger Stafford