Re: Loosing Particles when Rotating Image

Posted by Gluender on Nov 07, 2007; 6:27pm
URL: http://imagej.273.s1.nabble.com/Loosing-Particles-when-Rotating-Image-tp3696797p3696804.html

Dear Brendan Hermalyn,

you may first try to imagine how images are rotated by arbitrary angles.
Of course this operation involves some kind of interpolation, the
result of which you are observing.

If you like to stick with the very method, then you are to always
rotate from the original image _once_. Doing so will at least not
accumulate interpolation artefacts. However, you must accept
artefacts from a single rotation.

>I have a series of images where a few thousand tiny particles form a
>circular ring, and I need a
>way to count the number of particles in a discrete fashion; i.e.
>form a 5 degree selection wedge
>(using the center of the ring as the vertex, and half the image
>width, which is slightly larger than
>the ring diameter, as the radii) that includes 5 degree's worth of
>particles, count them, then rotate
>the wedge by 5 degrees, and iterate.
>
>I'm having some troubles getting this to work however.  My first
>idea was to make the wedge,
>count the particles, rotate the image, and reiterate.  This keeps
>the wedge stationary, but "sweeps"
>the image past the selection, eventually counting all the particles
>in discrete steps.  I got the
>macro to work fine, but I noticed a few things:
>
>1) When I rotate the image arbitrarily without iterating, I loose a
>bunch of particles each time.  E.g.
>when I do a total particle count before rotation, I get 1669
>particles, but after one arbitrary
>rotation (43degrees in this case) the particle count drops to 1557.
>My ring is centered in the
>square window, so the particles are not leaving the field of view;
>also 90 degree rotations do not
>yield a flux in the number of particles.  I noticed this problem
>because as I ran the macro, the
>particles got more "clumpy" with each small rotation; it's clearly
>an additive issue.
>
>2) If I iterate when rotating, I have to re-threshold every
>rotation.  This has the same clumping
>effect (although much more severe) as the simple rotation.
>
>Anybody have any ideas?  I'm pretty stuck on this!!
>
>Thanks for your help,
>Brendan Hermalyn

HTH
--


                   Herbie

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