-------------------------------------------
entry variable: $white_lolim -- 1st entry right of TRUNC
entry variable: $white_hilim -- 2nd entry right of TRUNC
entry variable: $gray_hilim -- 1st entry right of PF
-------------------------------------------
   [next appear on popup from R-click TRUNC/entries]
entry variable: $im2boundsflag -- bounds test on im2 (vs. im)
entry variable: $edit_lolim2 -- structure-saving im low limit
entry variable: $edit_hilim2 -- structure-saving im hi limit
-------------------------------------------

The pixel values in these three entries mainly
control the behavior of the TRUNC (truncate) and
PF (plane filter) buttons (see help for TRUNC and
PF).

These three entries also have an overloaded use
to set bounds and pixel value targets for
pixel-range-dependent editing (either batch or
mouse-based).  A right-click on the TRUNC button
or any of these entries makes a more convenient
popup.

The two entries to the right of the TRUNC button
set the lower and upper bounds of the pixel
values that indicate that the pixel should be
edited to the target value, which is set in the
entry to the right of the PF button.

The bounds test can either be done on the current
editable image (im) or on the second image (im2).
There are also whole-data-set and mouse cursor
controlled versions of these functions.  Here are
the 5 different, slightly asymmetric,
possibilities:

Bounds-dependent edit (all) on im
  -----------------------------------
  (shift-right-click-TRUNC)
  tclfunct: cp_im_to_im2_replacing_cols
  -----------------------------------
  Copy im -> im2, in the process editing pixels
  that are within the bounds ($white_lolim,
  $white_hilim) to the target value
  ($gray_hilim).  To save the result, swap
  editable and im2 using middle-click-COMPARE,
  and save images.  The copy to im2 allows blink
  comparison of the result with the original
  before committing.

Bounds-dependent edit (all) on im using im2
  -----------------------------------
  (ctrl-middle-click-TRUNC)
  tclfunct: crop_im_using_im2cols
  -----------------------------------
  Edit pixels in im for which im2 has pixels
  within the specified bounds ($white_lolim,
  $white_hilim) to the target value
  ($gray_hilim).  This requires first loading
  im2.  N.B.: edits appled to im, not im2!

Bounds-dependent edit (all) on im using im2, save struct
  -----------------------------------
  (ctrl-right-click-TRUNC)
  tclfunct: crop_im_using_im2cols_save
  -----------------------------------
  Same as previous, except edits are only applied
  if the im (editable) voxel is outside a second
  set of bounds ($edit_lolim2, $edit_hilim2).  To
  control the second set of bounds, R-click on
  TRUNC to get a popup (IMAGE BOUNDS DEP EDITING)
  with the second bounds.

Bounds-dependent edit (mouse) on im
  -----------------------------------
  (shift-left-click edit in image window)
  -----------------------------------
  Edit pixels in the editable image (im) that are
  within specified bounds to the target value
  controlled by mouse and brush diameter and
  brush 2D/3D (like first batch function).

Bounds-dependent edit (mouse) on im using im2
  -----------------------------------
  (shift-left-click edit in image window)
  -----------------------------------
  If $im2boundsflag is ticked (on the IMAGE
  BOUNDS DEP EDITING popup), edit voxels in the
  editable image to the target value
  ($gray_hilim) if the corresponding im2 voxel is
  within the bounds ($white_lolim, $white_hilim),
  but only apply the edit if the im voxel is
  outside the second set of bounds ($edit_lolim2,
  $edit_hilim2).  This can be used to preserve
  structure in the editable image.  To ignore the
  structure-preserving bounds, set both
  $edit_lolim2 and $edit_hilim2 to the same value
  (e.g., 0).

