Blur command comparison

AFNI version info (afni -ver): 22.3.03


I have just a quick question regarding blurring of 4D fMRI data sets.

It seems there are at least two commands to achieve this:

3dmerge -1blur_fwhm

I was wondering what the differences are and which one should be preferred.


Hi, Philipp-

  • 3dmerge: apply a constant kernel-size blur across a dataset.
  • 3dBlurToFWHM: Blurs a 'master' dataset until it reaches a specified FWHM smoothness (approximately). The same blurring schedule is applied to the input dataset to produce the output. The goal is to make the output dataset have the given smoothness, no
    matter what smoothness it had on input (however, the program cannot 'unsmooth' a dataset!). See below for the METHOD used.

The 3dBlurToFWHM program applies nonconstant blur to get to a constant smoothness everywhere it works; it likely requires using a mask. 3dmerge applies a constant blur kernel everywhere, so places with different smoothness stay differently smooth; masking would be optional and not necessary.

Typically, 3dBlurToFWHM is used when merging data with very different properties into a single study. Datasets might come from different sites, or different scanners, or use different protocols, etc. 3dmerge gets used in most other cases.


I see. Very sophisticated for smoothing!

Thanks, Paul!