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entry variable: $fs -- fieldsign (fsmask implied)
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def: */fs/$hemi.fs  (implies */fs/$hemi.fm)

The "fs:" entry shows the current fieldsign file
(and implied fieldsign mask file, both usu.
unloaded at initial startup).  Click "R" to load
fieldsign and fieldsign mask.

Session directory can be abbreviated as an
asterisk (*).  Absolute name OK, too.  Relative
name appended to session.

The fieldsign file (*.fs, variable: $fs) is the
dot product of the eccentricity and polar angle
gradients, which are estimated by fitting a plane
to the data around each vertex.  The color scale
usually effectively only uses the sign (but see
help for tickbox just to left for nonbinary
display).

The name of the fsmask (fieldsign mask) file
(*.fm, variable: $fm) is generated upon clicking
READ or WRITE by stripping the .fs suffix and
appending an .fm suffix.  It is the geometric
mean of the r,th power/significance.

Binary fieldsign file format:
  [no magic number]
  4-byte int patch vertex count (must be full)
  4-byte float fieldsign value
  ...
  [implicit vertex nums]

Binary fsmask (fieldsign mask) file format:
  [no magic number]
  4-byte int patch vertex count (must be full)
  4-byte float fsmask value
  ...
  [implicit vertex nums]
