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variables: $light0, $light1, $light2, $light3
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defaults: 0.4, 0.0, 0.8, 0.2

The four "light{0,1,2,3}:" entries (F3 panel,
middle right) control the brightness of the 4
fixed lights that illuminate the brain.

On both F2 and F3 panels, shift-R-click REDRAW to
get a popup with all of the lighting parameters.

The lights don't change position as the pose of
the brain is changed.  The default brightnesses
and positions are:

  $light0 (0.4) -- straight from viewer into screen
  $light1 (0.0) -- from behind brain (i.e., turned off)
  $light2 (0.8) -- from upper right
  $light3 (0.2) -- from direct left

The x,y,z position variables look like this:

  $light0x
  $light0y
  $light0z
  $light1x
  $light1y
  [etc]

All 16 parameters can be interactively controlled
from the shift-R-click REDRAW popup.  A <Return>
in any parameter field will redraw the brain.

Non-default brightnesses and positions can be
saved on a per-subject basis in csurf -> Expert
Preferences -> Lights.

Brightness values over 1.0 work, but are
increasingly clipped to white.

The behind-the-brain light (brightness off, and
w/negative z-coord, so no effect as is) can be
re-positioned (with a positive z-coord so it's on
the same side of the brain as the viewer!) as
desired.  Positive x is to the right and positive
y is up.

In a script, use:

  set light0 <0-1>
  set light0x <-1.0 to 1.0>
  [etc]
  . . .
  do_lighting_model
  redraw

The redrawbutton proc includes the last two
commands (do_lighting_model, redraw).

Finally, the last parameter, $insidesurffact
(default: 0.6), controls the brightness of the
inside surface relative to the front surface
(over 1.0 OK).

No visible effect of $insidesurffact unless
$lighttwosidedflag is set (toggle with
ctrl-middle-click on REDRAW) *and* some part of
inside surface is visible.
