Re: Size of Gaussian filter for best subsampling

Jonathon wrote: 
I have a fairly large image that I need to subsample for analysis.  I
believe that, rather than subsample the original image, it is better
to low-pass filter the image and subsample the filtered image.  First
question, is this correct?


Yes.  You must filter to avoid aliasing.

I would prefer to use a Gaussian filter, as I can implement this with
two 1-D passes, rather than a full 2-D convolution.  Second question,
how do I choose the proper standard deviation and extent of the
Gaussian filter if I want to subsample by any given factor?


To work correctly, your anti-aliasing filter must
eliminate high frequencies above half your new
sampling frequency.  What you are trying to do
is to satisfy Nyquist.  When your original image
was created this was (hopefully) done using a
low-pass analogue filter.

So, if you are sampling by 2 then you need to
halve the frequency content.  If you are sampling
by 5 then you need to reduce the frequency
content to one fifth of the original.

Look at how your Gaussian filter behaves in the
frequency domain to choose a sensible value for
the width in the spatial domain.

--
Regards,
Martin Leese
E-mail: please@xxxxxxxxxxxxxxxxxxxxxxxxxx
Web: http://members.tripod.com/martin_leese/

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