Posted by
Jan Stuehmer on
Apr 06, 2008; 3:22pm
URL: http://imagej.273.s1.nabble.com/Visualize-orthogonal-views-3D-reconstruction-tp3696642p3696647.html
Hi Mathieu,
have you tried a bigger convolution kernel than 3x3 ?
A higher sigma than half the size of the convolution kernel makes not much
sense if you want to have a reasonable precision.
To adjust the kernel size ( m x m ) to the sigma value you can compute
the value
of the normal distribution (normalized gaussian) for x = 0.5 * m
This gives you a hint of the error you get if you discretize the normal
distribution
for that filter size.
What is a normal distribution with sigma=0 ? It should be the Dirac measure
which should result in a non-modified image, you are right. But this is
a special
case of the normal distribution.
In the end it depends on how ImageJ computes it's filter kernel because
you get
a division by zero if sigma is zero.
Cheers,
Jan
Mathieu Goeminne schrieb:
> Hi all,
>
> I try to use ImageJ to blur an image using the API (not the GUI) with a
> 3x3 convolution matrix gaussian blur. So I use a GaussianBlur object,
> and call the blur(ImagePlus, double) method. Comparing the result with
> that is obtained using the ImageJ GUI or the GIMP, it appears my call
> make an "excessive" blur : for a given sigma (the standard deviation),
> "my" blurring is stronger that the Gimp or the ImageJ GUI blurring.
>
> An other problem seems to be an artificial minimum on the sigma : for
> every sigma < 1, the result seems to be
> Approximately the same as a sigma=1 blurring. For instance, a sigma=0
> blurring blurs really my image, but it should return a non-modified
> image.
>
> The documentation (and the API) of ImageJ says :
>
>
>> 'Radius' means the radius of decay to exp(-0.5) ~ 61%, i.e. the
>>
> standard deviation sigma of the Gaussian (this is the same as in
> Photoshop, but different from the previous ImageJ function 'Gaussian
> Blur', where a value 2.5 times as much has to be entered)
>
> I tried to divide my sigma by 2.5, but there is no visible difference,
> and the sigma=0 case is not resolved.
>
> I use ImageJ 1.38
>
> Any idea?
>
> Thanks.
>
>