-------------------------------------------
tickbox var: $chkthirdflag -- consider im3 data
entry var: $chkthird_lolim
entry var: $chkthird_hilim
-------------------------------------------


The "ck3rd" tickbox (F3 panel, lower middle)
toggles whether a loaded "im3:" volume (e.g., T2
image) is checked during shrink operations.

One use is ignoring T1 voxels that are detected
to be blood vessels in a corresponding T2 image.

This will zero the shrink normal force according
to a comparison between the current "im3:" voxel
value and the current settings of $chkthird_lolim
and $chkthird_hilim ("ck3:lo" and "ck3:hi"
entries on next line).

If the current vertex finds itself within an
"im3:" voxel that is above $chkthird_hilim, the
normal force is zeroed ("vessel").  The normal
force is also zeroed if the "im3:" voxel is
*below* $chkthird_lolim ("CSF" for "T2" with
T1-weighting).
