-------------------------------------------
variable: $isocontourflag -- sphere.reg coords
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default: 0 (FALSE)

The "isocont" tickbox (lower middle) toggles
view of sphere-derived contour lines.

When $isocontourflag is 1 (ticked), x,y,z
coordinates from $hemi.sphere.reg are read
(causes small delay first time checkbutton
clicked).  Isocontours from these canonical
coordinates are then displayed on top of any
data.

The sphere has a radius of 100 mm (10 cm).  The
contours will appear as perfect circles on
sphere.reg, but on any other surface or patch,
they will be variously distorted.

Setting isocontourflag has no effect if the
registered sphere surface, sphere.reg, doesn't
(yet) exist.

The tcl parameters affecting the coordinate
isocontour display can be controlled on a popup
available from a R-click help on the "isocont"
tickbox.  They include:

  contourspacingRL    (deg/mm: def=10.0 -> deg/mm)
  contourspacingAP    (deg/mm: def=10.0 -> deg/mm)
  contourspacingSI    (deg/mm: def=10.0 -> deg/mm) 
  contourlinewidthRL  (pixels: def=2.0 -> pix)
  contourlinewidthAP  (pixels: def=2.0 -> pix)
  contourlinewidthSI  (pixels: def=2.0 -> pix)
  isoconttype         (0,1,2,3,4: def=4=latlong)

Setting a contourspacing to 0.0 turns off display
of that set of contours.

The default isocontour display (isoconttype=4) is
latitude and longitude with a superior/inferior
north/south pole, spaced at 10 degree intervals.
The isoconttype options 0-4 are:

  0 -- x,y,z isocontours at 10 mm spacing
  1 -- 3 orthogonal latitudes at 10 deg spacing
  2 -- latitude/longitude with right/left poles
  3 -- latitude/longitude with ant/post poles
  4 -- latitude/longitude with sup/inf poles

For x,y,z and latitudes display, it is less
confusing to look at if only two directions are
displayed, for example:

  set isoconttype 1    ;# latitudes
  set contourspacingRL 0.0
  redraw

In the GLX display window, the main contour lines
can be incrementally thickened with cmd-shift-L.
