Filtering with 1 pixel radius

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Dear forum,
just a generic question about filtering with a radius of 1 pixel the the Fiji->Process->Filtering

In my mind most of them should not do nothing with a radius of 1 pixel, instead they still does something.

Does someone know what they do with a pixel radius of 1?

I am just collaborating with a researcher and we are submitting a paper: in this paper I developed a method to identify nucleosomes in Atomic Force Microscope images where I use a Maximum filter with a radius of 1 pixel and I would like to write something about this step and I am facing this question about use a filter with 1 pixel radius.
I’ve implemented it just because “it works fine” but honestly I am not getting why a Maximum filter of 1 pixel radius will not return me the same image in input.

Thank you,
Emanuele Martini


A radius of 1 will result in a 3x3 kernel for filtering, including all pixels that are 1 pixel away from the center pixel.

For details, have a look at the RankFilters class (the radius handling in these filters is quite special):


Thank you very much @imagejan
since now I always thought that the number on the filter dialog was the kernel size like 3 -> 3x3 kernel.

have a nice day,


It says “radius” for a reason :wink:


may I join this conversation?
I am a newbie in filters and imageJ universe and this conversation was very useful, but I am not sure I have understood correctly. I have been using the minimum filter, instead of the maximum, and I have been trying different radius values starting from 2, down to 1, 0.5 and even lower, to see if there were differences, and actually I could still observe differences by eye at 0.1. But I do not know if it is correct and allowed theoretically to use such values. Looking at the rank filters class that @imagejan has posted there is written that 0.5 radius includes the 4 neighbors of the pixel in the center. So,if I understood correctly, I can still use 0.5… what about 0.1? Is there like a proportion that the program does that allows to use any kind of value?
I am sorry for such a long and possibly stupid question, but it´s really difficult for me to understand this new “language”.
Many thanks!