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>
> what is the morphological opening operator?
>

A word of background -- the basic operations in morphological processing
are erosion and dilation. These are similar to adding a layer or peeling
a layer off of an object. Which does which depends on whether your
objects are bright on a dark background or dark on a bright background.

Opening or closing are complementary operations which involve doing
a dilation followed by an erosion or vice versa. It is called opening
when it tends to pinch off narrow structures and therefore separate
objects, while closing tends to do the opposite (eg, closing may eliminate
holes in an object by completely filling them in). You should really
see it to understand it.

In the case of the top-hat filter, the opening operation essentially
removes the peaks which are narrowen that the kernel used in the
opening operation. This is then subtracted from the original image to
remove the background and leave the peaks.

I do not have my books handy right now, but I think that
you can find a very readable treatment of the subject in an SPIE
tutorial series book called "Morphological Image Processing" by
ER Dougherty.

Although binary morphology (ie, morphological operations on binary images)
is much easier to grasp, gray scale morphology is often very useful
in microscopy. This is a bit harder to implement, but the ideas carry
over. I do not know if ImageJ has a gray scale morphology plug-in (yet),
but I use Image Pro Plus, which has a nice set of morphological operators
implemented. IPP is expensive, but the demo is free, and it will let you
do everything it can do, but only  on the demo images (there are a
lot of them). This means that it is great for learning what these operations
do, and I highly recommend this for learning about image processing.

Disclaimer -- I get no kickbacks from Media Cybernetics (who sell IPP).
I simply use it a lot, so that is what I know.

--aryeh
Aryeh Weiss                          | email: [log in to unmask]
Department of Electronics            | URL:   http://optics.jct.ac.il/~aryeh
Jerusalem College of Technology      | phone: 972-2-6751146
POB 16031                            | FAX:   972-2-6751275
Jerusalem, Israel                    | ham radio: 4X1PB/KA1PB