-------------------------------------------
switch variable: $surfcolor -- background color
-------------------------------------------
default switch value: 1 => display curvature

Display curvature (if it has been read in
with function read_binary_curv).

 curv only:  convex = red      concave = green
   overlay:  convex = litegray concave = darkgray


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

An alternate (rarely used) middle-click on the
"cv" radio button toggles $nobicurvfadeoverfthresh,
which controls how overlay colors fade into the
two background grays:

  0: (default) fade seperately to two grays
  1: no differential fade if overlay above $fthresh

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.

