-------------------------------------------
toggle variable (left-click):
  $surfcolor -- toggles curv visibility
-------------------------------------------
toggle variable (middle-click):
  $nobicurvfadeoverfthresh -- toggle separate fade to sulc/gyr
-------------------------------------------

The upper middle right "curv" tickbox controls
two different variables controlling the display
of curvature.

-------------------------------------------
toggle variable (left-click):
  $surfcolor -- toggles curv visibility
-------------------------------------------
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).

-------------------------------------------
toggle variable (middle-click):
  $nobicurvfadeoverfthresh -- toggle separate fade to sulc/gyr
-------------------------------------------
default: 0 (FALSE)

By default, overlay colors fade smoothly into the
two different background grays (gyrus light,
sulcus dark) when overlay amplitude decreases
less than fmid.  Ths goal is to see near 
threshold data without the distracting sharp
edges of a hard truncation (by fthresh).  The
differential fading is not normally visible for
overlay amplitudes greater than fmid unless
fslope is extremely shallow (less than 0.5).

To block this behavior, middle-click the "curv"
tick.  This will prevent any modulation of the
overlay color by the underlying sulcus vs. gyrus
color.  It does this by re-setting the underlay
color to sulcus dark, as long as the overlay
amplitude is larger than $fthresh, else both
sulcus dark and gyrus light will be displayed.

N.B.: if $fthresh is less than $fmid, this will
disturb the sulc/gyrus pattern (creating more
dark gray) in regions where overlay amplitude is
above $fthresh (which will cause the background
to be set to sulcus dark), but where a higher
fmid will have already rolled off the color to
background.

