http://imagej.273.s1.nabble.com/downsampling-methods-tp3690444p3690448.html
signal theory.
precede any subsampling.
>On Mon, 2009-11-16 at 11:52 +0100, Gluender wrote:
>> Just a note:
>>
>> Downsampling at least in theory definitely requires a _real_ bandlimit.
>>
>> That said, Gaussian lowpass filtering surely is not optimum because
>> per definition it doesn't really zero frequency components above the
>> Nyquist limit.
>>
>
>This is correct for functions that are created through a spectral
>generator which we assume is the case for an image when transferring it
>to the frequency domain from it's intensity samples but in fact is not.
>For an image captured with a camera it is just better to re-sample by
>diffusion, that is through the focus of a Gaussian.
>
>Best,
>Stephan
>
>> >Hi Aviva,
>> >
>> >you should try Gaussian downsampling:
>> >
>> >In Fiji, there is a Javascript in PLugins->Examples->downsample
>> >explained at
>> >
>> >
http://pacific.mpi-cbg.de/wiki/index.php/Downsample>> >
>> >the same Javascript is at the ImageJ website
>> >
>> >
http://rsb.info.nih.gov/ij/macros/js/Downsample.js>> >
>> >or, alternatively a public API method in the packages of mpicbg_.jar
>> >that are part of Fiji, such that you can call it for a nopen Image, say,
>> >from Plugins->Scripting->BeanshellInterpreter:
>> >
>> >import ij.*;
>> >import mpicbg.ij.util.Filter;
>> >new ImagePlus( "downsampled", Filter.scale( IJ.getImage().getProcessor(),
>> >0.2f ) ).show();
>> >
>> >Lanczos being just an approximation of a bandpass filter in the
>> >frequency domain, you could also try to do the downsampling directly
>> >through that bandpass-filter, ImageJ has the respective functions.
>> >Depends very much on what you want. For most cases, the Gaussian thing
>> >is just optimal.
>> >
>> >Best,
>> >Stephan
>> >
>> >
>> >
>> >On Mon, 2009-11-16 at 08:07 +0200, Aviva Vaknin wrote:
>> >> Hello all!
>> >>
>> >> I have implemented a Lanczos8 kernel convolution to downsample images
>> >> which works well for 30-40 iterations.
>> >>
>> >> After that ringing artifacts enter, even if I reduce a
>>1600x1200 image one
>> >> pixel
>> >> each iteration.
>> >>
>> >> Should this be occurring ? Is there a way around it ?
>> >> Is there a better down sampling method for this type of problem ?
>> >>
>> >> thanks,
>> >> aviva
>>
> > Best