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tickbox var: $MRIflag -- make shrink consider "im:" data
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default: 0 (FALSE)

The "ckMRI" tickbox (F3 panel, lower right) toggles
whether 3D MRI data is considered during a SHRINK.
 
If ticked, *and* a 3D data set has been loaded
using the "im:" entry (upper left of the F3
panel), then the value of 3D image data near each
vertex is considered during deformation of the
surface by the C/tcl function shrink (SHRINK
button).

There are 4 different ways of calculating the
'force' generated from the local image data (see
R-click help for $forcemode).

If no MRI data is loaded, shrink procedes anyway,
ignoring the image data term.
