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variable: $xtrans -- translate brain in x-axis
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keyboard equivalent: [Cmd/Alt]-Shift-right/left arrow
direct manip. equiv.: Shift-middle-click-drag left/right

The "x:" entry (lower middle) translates brain
surface in the left/right direction in mm divided
by the current scale factor.

That is, translating +10 at the initial default
scale of 1.0 moves the brain 10 mm to the right,
while at a scale of 2.0 (200%), the brain is
moved 5 mm (same proportional distance relative
to the window).

Clicking in the slider troughs translates 1 unit.

In a tcl rendering script, this translation can
be achieved with:

  translate_brain_x <mm/scalefactor>
  redraw

If focus is on the brain surface window, then
Shift-right/left arrow translates the brain -2/+2
mm.  In the interface window, add a command key
(Mac=Cmd, Linux=Alt) to Shift-arrow (this avoids
instead moving the cursor in an entry).
