Rotate geometry by moving mouse inside viewer window and drag while left
mouse button is pressed. The rotation mechanism simulates a virtual track
ball, i.e. one picks a virtual ball centered in the display and drags the
picked point around the center of the display.
Press right mouse button to select major interaction mode from popup menu,
or use keyboard keys to temporarily switch between different modes (just keep
a button pressed to switch mode temporarily). Some essential modes are:
