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variable: $ytrans -- translate brain in y-axis
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keyboard equivalent: [Cmd/Alt]-Shift-up/down arrow
direct manip. equiv.: Shift-middle-click-drag up/down

The "y:" entry (lower middle) translates brain
surface in the up/down 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 up, 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_y <mm/scalefactor>
  redraw

If focus is on the brain surface window, then
Shift-up/down 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).
