How can I measure/track the angle/polar coordinates of a circular movement?

[Trottolotti]

Hi all,
I have some difficulties finding a way to track the polar coordinates of an object that performs a circular movement.
In my case the "object" is the tip of a flame in a circular channel, I hope this picture makes it easy to understand:



Does anybody know a way to manually track the polar coordinates of the tip of flame (~100 different experiments, each one with a Stack of 50-200 images), or do I have to track the x-y-coordinates and transfer them into polar coordinates in a second step?
Many thanks in advance!
Jan

[dscho]

Hi Jan,

On Tue, 4 Oct 2011, Trottolotti wrote:

> I have some difficulties finding a way to track the polar coordinates of
> an object that performs a circular movement. In my case the "object" is
> the tip of a flame in a circular channel, I hope this picture makes it
> easy to understand:
>
> http://imagej.588099.n2.nabble.com/file/n6859324/circle.jpg 
>
> Does anybody know a way to manually track the polar coordinates of the
> tip of flame (~100 different experiments, each one with a Stack of
> 50-200 images), or do I have to track the x-y-coordinates and transfer
> them into polar coordinates in a second step?

I think the image is good enough for some rudimentary pre-processing and
then segmentation by threshold. A simple macro should be able to find the
tip of the flame (I assume you are looking for the point you marked by an
X?). Unfortunately, this will have to be done in x,y coordinates, but your
circle probably does not move, so it should be relatively easy to
calculate the polar coordinates using the atan2() function after
subtracting the center.

Ciao,
Johannes