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switch variable: $surfcolor -- tick toggles curv visibility
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default: 1 (TRUE)

The "curv" tickbox under the REDRAW button
controls whether curvature is diplayed.

If ticked ($surfcolor is 1), curvature is shown
as the saturation of red and green, or as two
binary shades of gray (if overlay):

  no overlay: red (concave) vs. green (convex)
  overlay: dark-gray (concave) vs. light gray (convex)

If not ticked ($surfcolor is 0), the background
is uniform.  This is useful for overlay data that
has smooth, low amplitude variations.

The default curvature data set shown is
$hemi.curv -- local curvature measured to
vertices immediately neighboring each single
vertex.  Gyral crowns are convex and sulcal fundi
are concave.  But the banks of sulci typically
contain an even mix of convex and concave
regions.

To see a contrast that is closer to gyrus vs.
sulcus, load the $hemi.sulc data set using the
"curv:" dropdown.  This displays the magnitude of
surface-perpendicular movement that occurred
during unfolding (sulci move further than gyri),
normalized around the mean movement distance.

(The "curv" tickbox is a shortcut to the rarely
used full radiobutton set of none/curv/area/shear
on the big F3 interface).
