Determine approximate shapes of detected objects in a binary image

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Hi, i'm doing a project on void detection in concrete structures. in this i am using thermal images, converted them into gray scale. later i need to convert it to binary, but one problem here is i need to see few objects which are brighter than background and sometimes i'll need to see dark objects.
i need a way to choose the threshold interactively, like a sliding button that allows me to select threshold. can any one help me how can i include that in code with example.
second problem is(temporarily i kept threshold at 0.67) in my binary image i need to determine shapes of objects approximately, circle, square,triangle or rectangle. i know a relationship to detect circular shape in terms of area and perimeter."perimeter^2/(4*pi*area)" which will be close to one for near circular shape, close to zero otherwise. also for square it is"perimeter^2/(16*area)" will be between zero and one.
can any one help me with other shapes(triangular and rectangular shapes)??? or even relations including other parameters in 'regionprops'(centroid... etc).
my gray scale image is <http://tinypic.com/view.php?pic=k4z0x5&s=5> thanks in advance...

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