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
physical-dimensions
The physical dimension of the projection surface.
Use this together with the master-perspective, right-of-perspective
left-of-perspective, above-perspective, below-perspective or
reference-points-perspective
width, height - double
The dimensions of the projection plane, if unset the veiwport values
are taken as default.
bezel
Gives informantion about the bezel of monitors for a seamless view.
right
right bezel with in the same units than with and height above
left
left bezel with in the same units than with and height above
top
top bezel with in the same units than with and height above
bottom
bottom bezel with in the same units than with and height above
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.
master-perspective
Defines a persective projection matrix for use as the leading display
in a seamless multiscreen configuration. This kind of perspective
projection is zoomable.
eye-distance - double
The distance of the eyepoint from the projection surface in units of
the physical-dimensions values above.
x-offset, y-offset - double
Offset of the eyelpint from the center of the screen in units of
the physical-dimensions values above.
left-of-perspective, right-of-perspective, above-perspective,
below-perspective
Defines a perspective projection matrix for use as derived display
in a seamless multiscreen configuration. The projection matrix
is computed so that the respective edge of this display matches the
assiciated other edge of the other display. For example the right edge
of a left-of-perspective display matches the left edge of the parent
display. This also works with different zoom levels, leading to distorted
but still seamless multiview configurations.
The bezel with configured in the physical dimensions of this screen and
the parent screen are taken into account for this type of projection.
parent-camera - string
Name of the parent camera.
reference-points-perspective
Defines a perspective projection matrix for use as derived display
in a seamless multiscreen configuration. This type is very similar to
left-of-perspective and friends. It is just a more flexible but less
convenient way to get the same effect. A child display is configured
by 2 sets of reference points one in this current camera and one in
the parrent camera which should match in the final view.
parent-camera - string
Name of the parent camera.
this
reference points for this projection.
point - array of two points
x, y - double
x and y coodinates of the reference points in units of this
physical-dimensions.
parent
reference points for the parent projection.
point - array of two points
x, y - double
x and y coodinates of the reference points in units of the
parents physical-dimensions.
texture
This tag indicates that the camera renders to a texture instead of the
framebuffer. For now the following tags are supported, but obviously
different texture formats should be specified too.
name - string
The name of the texture. This can be referred to by other cameras.
width, height - double
The dimensions of the texture
panoramic-distortion
This tag cause the camera to create distortion geometry that
corrects for projection onto a spherical screen. It is equivalent to
the --panoramic-sd option to osgviewer.
texture - string
The name of a texture, created by another camera, that will be
rendered on the distortion correction geometry.
radius - double
Radius of string
collar - double
size of screen collar.
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.
wide
0
0
3840
1024
false
wide
0
0
1280
1024
0
0.133
-0.133
-.5004
-.1668
0.4
120000.0
wide
1280
0
1280
1024
0
0.133
-0.133
-.1668
.1668
0.4
120000.0
wide
2560
0
1280
1024
0
0.133
-0.133
.1668
.5004
0.4
120000.0
wide
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.
0
0
true
45.3
0.133
-0.133
-.1668
.1668
0.4
120000.0
main
0
1
true
0
0.133
-0.133
-.1668
.1668
0.4
120000.0
0
2
true
-45.3
0.133
-0.133
-.1668
.1668
0.4
120000.0
main
This example renders the scene for projection onto a spherical screen.
main
0
0
1024
768
0
0.133
-0.133
-.1668
.1668
0.4
120000.0
mainview
1024
768
main
768
0
0
1024
-1.0
1.0
mainview
main
Here is an example for a 3 screen seamless zoomable multiscreen
configuration using 3 533mmx300mm displays each with a 23mm bezel.
The side views are angled with 45 deg.
The commented out reference-points-perspective shows the
aequivalent configuration than the active right-of-perspective.
This is done by just using two reference points at the outer
edge of the bezel of the respective display.
0.0
0.0
0
0
true
0.1
0
1
true
CenterCamera
0.0
0
0
1920
1080
0.0
0.0
0.0
533
300
23
23
23
23
450
0
130
RightCamera
0.0
1920
0
1920
1080
-45
0
0
533
300
23
23
23
23
CenterCamera
LeftCamera
0.1
0
0
1920
1080
45
0
0
533
300
23
23
23
23
CenterCamera
0.0