The Open Scene Graph library, which current FlightGear uses for its 3D
graphics, provides excellent support for multiple views of a
scene. FlightGear uses the osgViewer::Viewer class, which implements a
"master" camera with "slave" cameras that are offset from the master's
position and orientation. FlightGear provides the "camera group"
abstraction which allows the configuration of slave cameras via the
property tree.

Slave cameras can be mapped to windows that are open on different
screens, or all in one window, or a combination of those two schemes,
according to the video hardware capabilities of a machine. It is not
advisable to open more than one window on a single graphics card due
to the added cost of OpenGL context switching between the
windows. Usually, multiple monitors attached to a single graphics card
are mapped to different pieces of the same desktop, so a window can be
opened that spans all the monitors. This is implemented by Nvidia's
TwinView technology and the Matrox TripleHead2Go hardware.

The camera group is configured by the /sim/rendering/camera-group node
in the property tree. It can be set up by, among other things, XML in
preferences.xml or in an XML file specified on the command line with
the --config option.

Here are the XML tags for defining camera groups.

camera-group
For the moment there can be only one camera group. It can contain
window, camera, or gui tags.

 window
 A window defines a graphics window. It can be at the camera-group
 level or defined within a camera. The window contains these tags:

  name - string
  The name of the window which might be displayed in the window's
  title bar. It is also used to refer to a previously defined
  window. A window can contain just a name node, in which case
  the whole window definition refers to a previously defined window.

  host-name - string
  The name of the host on which the window is opened. Usually this is
  empty.

  display - int
  The display number on which the window is opened.

  screen - int
  The screen number on which the window is opened.

  x, y - int
  The location on the screen at which the window is opened. This is in
  the window system coordinates, which usually puts 0,0 at the upper
  left of the screen XXX check this for Windows.

  width, height - int
  The dimensions of the window.

  decoration - bool
  Whether the window manager should decorate the window.

  fullscreen - bool
  Shorthand for a window that occupies the entire screen with no
  decoration.

 camera
 The camera node contains viewing parameters.

  window
  This specifies the window which displays the camera. Either it
  contains just a name that refers to a previous window definition, or
  it is a full window definition.

  viewport
  The viewport positions a camera within a window. It is most useful
  when several cameras share a window.

   x, y - int
   The position of the lower left corner of the viewport, in y-up
   coordinates.

   width, height - int
   The dimensions of the viewport

  view
  The view node specifies the origin and direction of the camera in
  relation to the whole camera group. The coordinate system is +y up,
  -z forward in the direction of the camera group view. This is the
  same as the OpenGL viewing coordinates.

   x,y,z - double
   Coordinates of the view origin.

   heading-deg, pitch-deg, roll-deg - double
   Orientation of the view in degrees. These are specified using the
   right-hand rule, so a positive heading turns the view to the left,
   a positive roll rolls the view to the left.

  perspective
  This node is one way of specifying the viewing volume camera
  parameters. It corresponds to the OpenGL gluPerspective function. 

   fovy-deg - double
   The vertical field-of-view

   aspect-ratio - double
   Aspect ratio of camera rectangle (not the ratio between the
   vertical and horizontal fields of view).

   near, far - double
   The near and far planes, in meters from the camera eye point. Note
   that FlightGear assumes that the far plane is far away, currently
   120km. The far plane specified here will be respected, but the sky
   and other background elements may not be drawn if the view plane is
   closer than 120km.

   offset-x, offset-y - double
   Offsets of the viewing volume specified by the other parameters in
   the near plane, in meters.

  frustum
  This specifies the perspective viewing volume using values for the near
  and far planes and coordinates of the viewing rectangle in the near
  plane.

   left, bottom - double
   right, top - double
   The coordinates of the viewing rectangle.

   near, far - double
   The near and far planes, in meters from the camera eye point.

  ortho
  This specifies an orthographic view. The parameters are the sames as
  the frustum node's.

 gui
 This is a special camera node that displays the 2D GUI.

  viewport
  This specifies the position and dimensions of the GUI within a
  window, *however* at the moment the origin must be at 0,0.

Here's an example that uses a single window mapped across 3
displays. The displays are in a video wall configuration in a
horizontal row.

<PropertyList>
  <sim>
    <rendering>
      <camera-group>
        <window>
          <name>wide</name>
          <host-name type="string"></host-name>
          <display>0</display>
          <screen>0</screen>
          <width>3840</width>
          <height>1024</height>
          <decoration type = "bool">false</decoration>
        </window>
        <camera>
          <window>
            <name>wide</name>
          </window>
          <viewport>
            <x>0</x>
            <y>0</y>
            <width>1280</width>
            <height>1024</height>
          </viewport>
          <view>
            <heading-deg type = "double">0</heading-deg>
          </view>
          <frustum>
            <top>0.133</top>
            <bottom>-0.133</bottom>
            <left>-.5004</left>
            <right>-.1668</right>
            <near>0.4</near>
            <far>120000.0</far>
          </frustum>
        </camera>
        <camera>
          <window>
            <name type="string">wide</name>
          </window>
          <viewport>
            <x>1280</x>
            <y>0</y>
            <width>1280</width>
            <height>1024</height>
          </viewport>
          <view>
            <heading-deg type = "double">0</heading-deg>
          </view>
          <frustum>
            <top>0.133</top>
            <bottom>-0.133</bottom>
            <left>-.1668</left>
            <right>.1668</right>
            <near>0.4</near>
            <far>120000.0</far>
          </frustum>
        </camera>
        <camera>
          <window>
            <name>wide</name>
          </window>
          <viewport>
            <x>2560</x>
            <y>0</y>
            <width>1280</width>
            <height>1024</height>
          </viewport>
          <view>
            <heading-deg type = "double">0</heading-deg>
          </view>
          <frustum>
            <top>0.133</top>
            <bottom>-0.133</bottom>
            <left>.1668</left>
            <right>.5004</right>
            <near>0.4</near>
            <far>120000.0</far>
          </frustum>
        </camera>
        <gui>
          <window>
            <name type="string">wide</name>
          </window>
        </gui>
      </camera-group>
    </rendering>
  </sim>
</PropertyList>

Here's a complete example that uses a seperate window on each
display. The displays are arranged in a shallow arc with the left and
right displays at a 45.3 degree angle to the center display because,
at the assumed screen dimensions, the horizontal field of view of one
display is 45.3 degrees. Each camera has its own window definition;
the center window is given the name "main" so that the GUI definition
can refer to it.  Note that the borders of the displays are not
accounted for.

<PropertyList>
  <sim>
    <rendering>
      <camera-group>
        <camera>
          <window>
            <host-name type="string"></host-name>
            <display>0</display>
            <screen>0</screen>
            <fullscreen type = "bool">true</fullscreen>
          </window>
          <view>
            <heading-deg type = "double">45.3</heading-deg>
          </view>
          <frustum>
            <top>0.133</top>
            <bottom>-0.133</bottom>
            <left>-.1668</left>
            <right>.1668</right>
            <near>0.4</near>
            <far>120000.0</far>
          </frustum>
        </camera>
        <camera>
          <window>
            <name type="string">main</name>
            <host-name type="string"></host-name>
            <display>0</display>
            <screen>1</screen>
            <fullscreen type = "bool">true</fullscreen>
          </window>
          <view>
            <heading-deg type = "double">0</heading-deg>
          </view>
          <frustum>
            <top>0.133</top>
            <bottom>-0.133</bottom>
            <left>-.1668</left>
            <right>.1668</right>
            <near>0.4</near>
            <far>120000.0</far>
          </frustum>
        </camera>
        <camera>
          <window>
            <host-name type="string"></host-name>
            <display>0</display>
            <screen>2</screen>
            <fullscreen type = "bool">true</fullscreen>
          </window>
          <view>
            <heading-deg type = "double">-45.3</heading-deg>
          </view>
          <frustum>
            <top>0.133</top>
            <bottom>-0.133</bottom>
            <left>-.1668</left>
            <right>.1668</right>
            <near>0.4</near>
            <far>120000.0</far>
          </frustum>
        </camera>
        <gui>
          <window>
            <name type="string">main</name>
          </window>
        </gui>
      </camera-group>
    </rendering>
  </sim>
</PropertyList>
