[Insight-users] Sigmoid Filter

[Luis Ibanez]

Hi Maria,

The Sigmod filter may not be the best mechanism for altering the
signal to noise ratio of an image. It is a pixel-wise intensity
transformation, it does not take into account neighbors when
computing the values of the output pixels, and therefore it will
be very limited for eliminating noise.

You can interpret this filter as a way of selecting an window of
intensities in the image. In this context, Beta is the intensity
value of the center of the window, Alpha is related to the width
of the window. E.g. roughly speaking, you are going to select the
values of intensity in the range of [Beta-Alpha : Beta+Alpha] and
map them to intensities in the range of Maximum to Minimum.

Note that the range is not really a hard window. The precise function
is showed in the ITK Software Guide.

As a general rule, set Beta to the intensity of the object that you
are interested in the image, and set Alpha to 2 or 3 times the
standard deviation of the intensities in your object of interest.



   Regards,


      Luis



--------------------------
Maria Cira Avvinto wrote:
> Thank you Daniel;
> my problem is experimentation. I have red section in
> ITK manual, but i don't understood the wheight of this parameters in
> the filter; for example: if i choose a big  Alpha (or Beta) value how
> change the output? Is there a range of specific value? What's the
> better choise? How can i find the priori knowledge of my image? How
> can i maximize my signal to noise ratio? Tnx.
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