# Meaning of Radius in a Filter? Imagej/Fiji

Hi there,

when we using a filter (mean,median etc) in imagej/fiji the parameter we need to pass is radius. I hope this means imagej/Fiji always use circular filters? If so is there a way us to implement a square filter.

Thanks…!

Good day!

Have a look at “Process >> Filters >> Convolve…” but be aware that, by this free definition of kernels, you are only able to implement linear operators, i.e. for example not median-operators and the like!

Have success

Herbie

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Thank you very much for the quick response…!

so a radius of 1 means sa @imagejan pointed out a 3x3 kernel, a radius of 2 a 5x5 kernel…

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Wow…thank you very much. I have misunderstood. I thought since it is radius it should corresponds to a circle.

Have a good day…!

Of course the operators are circular!

Why don’t you just test it?

Here are input and output of the mean-filter (lowpass) for radius 4:

The point-response (operator size) measures 9 x 9 pixels, i.e. (2 * 4 + 1) pixels^2 as Emanuele tried to explain.

Regards

Herbie

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Great thank you very much…!

On this topic, I don’t get why Median with radius Zero actually has any effect ? Because if it’s a kernel encompassing 2n+1 , that is just 1 and the median of 1 pixel is just its own value. And yet …

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Good question.

Check the code here imagej1/RankFilters.java at 8c753f646e0c2250254dd0cb1ecb84b2e1522b4a · imagej/imagej1 · GitHub
to see how the kernel is created.