3dPVmap [-prefix XXX] [-mask MMM] [-automask] inputdataset

Computes the first 2 principal component vectors of a
time series datasets, then outputs the R-squared coefficient
of each voxel time series with these first 2 components.

Each voxel times series from the input dataset is minimally pre-processed
before the PCA is computed:
  Legendre polynomial detrending
  L2 normalizing (sum-of-squares = 1)
If you want more impressive pre-processing, you'll have to do that
before running 3dPVmap (e.g., use the errts dataset from afni_proc.py).

Program also outputs the first 2 principal component time series
vectors into a 1D file, for fun and profit.

The fractions of total-sum-of-squares allocable to the first 2
principal components are written to stdout at the end of the program.
along with a 3rd number that is a measure of the spatial concentration
or dispersion of the PVmap.

These values can be captured into a file by Unix shell redirection
or into a shell variable by assignment:
  3dPVmap -mask AUTO Fred.nii > Fred.sval.1D
  set sval = ( `3dPVmap -mask AUTO Fred.nii` )  # csh syntax
If the first value is very large, for example, this might indicate
the widespread presence of some artifact in the dataset.

If the 3rd number is bigger than 1, it indicates that the PVmap
is more concentrated in space; if it is less than one, it indicates
that it is more dispersed in space (relative to a uniform density).
  3dPVmap -mask AUTO Zork.nii
  ++ mask has 21300 voxels
  ++ Output dataset ./PVmap+orig.BRIK
  0.095960 0.074847 1.356635
The first principal component accounted for 9.6% of the total sum-of-squares,
the second component for 7.5%, and the PVmap is fairly concentrated in space.
These % values are not very unusual, but the concentration is fairly high
and the dataset should be further investigated.

A concentration value below 1 indicates the PVmap is fairly dispersed; this
often means the larger PVmap values are found near the edges of the brain
and can be caused by motion or respiration artifacts.

The goal is to visualize any widespread time series artifacts.
For example, if a 'significant' part of the brain shows R-squared > 0.25,
that could be a subject for concern -- look at your data!

Author: Zhark the Unprincipaled