Re: Translate and pad image

On May 15, 12:56 am, "Rajesh Acharya" <rv_acha...@xxxxxxxxx> wrote:
Dear ImageAnalyst:
on second thoughts I think your idea of image registration
is fine and workable...
what i was thinking though was something different ...
I think i am not stating my problem properly and adequately.
let me start from begining...
I am taking images of an object(specimen/phantom) placed on
a turn table at various angles from 0deg to 360 deg. (like
they do in CT/MRI etc.) The necessary condition here is to
match the center of rotation of the turn table (axis of
rotation) with the verticle center line of the image. this
condition is difficult to fulfill manually while grabbing
image....
however, we can computerise this. how? well, irrespective
of the shape and size of the object we are sure that the
images at 0 deg and 180 deg are mirror images of each other.
so what I do is I flip the image at 180 deg. and subtract
it from 0 deg ref. image. ideally the Background and
specimen being same i should get zeros if the aforesaid
condition matches...practically speaking there would be
temporal statistical variations (ignorable) and variations
if there's mismatch in the two axis (ie if the object is
rotating eccentrically). therefore, I simply translate the
image matrix of 180 deg image one column at a time and find
the difference with ref image as stated in the original
post...the point where the diff is minimum gives me the
mismatch between the two axes....

 the advantage is i am not involving the user in this
process to feed control points ...

 coming back to my point.
Firstly, here i see that if i move my 180 deg image matrix
by 13 columns to left i get minimum difference bet ref
image (0 deg) and this one. Can i refine this difference
(13 columns) even further at subpixel level...
just to elaborate, this difference of 13 col will have to
be appropriately distributed in all the images between 0deg
and 180 deg images to allign all of them...

Secondly, there may be a chance that the axis of rotation
is not exactly verticle, this induces angular misalignment
in all the images... how do i correct this misalignment in
all the images
thanks yet again..

-------------------------------------
Rajesh:
The way to "correct" for angular misalignment is to just eliminate it
in the first place, or, as you know, you'll get artifacts. To do
this, you construct a "bead" phantom. Get a tube (like a rigid
drinking or something better) and fill it with ball bearings. Mount
it vertically on yoru platform sticking up perpendicular to the
platform. Then spin it and snap pictures. You'll see a series of
crossing horizontal lines (like X's) if it's not perfectly
perpendicular to your platform. It doesn't even have to be in the
center of the axis of rotation. Being off-center just affects how
long the lines are. If it is perfectly aligned (using tip/tilt on
your platform), then you will see no crossing X's and just perfectly
straight lines (with the lines being longer the farther they are away
from the axis of rotation).
This is the alignment method used by Hytec to align their industrial
CT systems.
http://www.hytecinc.com/category.php?action=expand&id=17
I think your method finds the center column of pixels - the one that
corresponds to the vertical axis of rotation - but doesn't help you
otherwise align your system. And it doesn't help you find the center
of your image, just the vertical axis. You'd be better off fastening
a bed of nails to your sensor and looking at the shadows (where they
all point to) to find the center of your x-ray image in both vertical
and horizontal directions rather than just the vertical direction like
you're doing.
Regards,
ImageAnalyst

By the way, there is a nice microCT short course at SPIE Symposium in
San Diego this August 2008.
http://spie.org//app/program/index.cfm?fuseaction=COURSE&export_id=x13102&ID=x12770&redir=x12770.xml&course_id=E0865949&event_id=840091&programtrack_id=844382
.

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