Mini-HOWTO: Configuring Views in FlightGear
24 April 2002
Jim Wilson
Introduction
This mini-HOWTO explains how to configure views in FlightGear.
Views define the position of the eye in the rendered scene and
what if any object the eye is focused on or tracking. The
eye is the imaginary or constructed eye that is simulated
by the 3D rendering. It makes the picture on your screen represent
what things would look like if you were standing or sitting at a
given location.
Formerly there were two views hard coded into FlightGear, but as
this document describes, it is possible to configure any number of
views with XML configuration files. Views may be configured to switch
between pilot and co-pilot, tower view, or view of an observer on the
ground.
1. Types of Views
There are two basic types of views:
LookFrom - This type requires a single coordinate position. The
direction of the view is independant of any particular object. Adjusting
the heading and pitch offsets moves the direction of observation around.
In the default Flight Gear configuration panning with the mouse will
move the view around as if the observer was turning their head.
LookAt - This type requires a two coordinate positions. The
direction of the view is always oriented toward a target and tracks the
target if it moves.
(see configuration examples below for usage
of the <type> paramter)
2. Defining Positions
There are two methods to define a postion and orientation for use in
configuring a view. One method is tied to a model which means the view
references the position and orientation of a model (e.g. Aircraft 3D Model)
and the other is an idependant postion which simply means that the
view references location data directly (via the Flight Gear property tree):
Tied to a Model - This method allows definition of the "eye",
that is where you are looking from and/or the "target" or objected being
looked at based on a model position and orientation. The following
illustrates defining the positon of a "Cockpit View" that is positioned
and oriented according to the position and orientation of a model:
<sim>
<view>
<name>Cockpit View</name>
<type>lookfrom</type>
<internal type="bool">true</internal>
<config>
<from-model type="bool">true</from-model>
<from-model-idx type="int">0</from-model-idx>
</config>
</view>
</sim>
Syntax for the eye position is:
<from-model type="bool">true or false</from-model>
<from-model-idx type="int">modelnumber</from-model-idx>
Syntax for the target or at position used in views of type "lookat" is:
<at-model type="bool">true or false</at-model>
<at-model-idx type="int">modelnumber</at-model-idx>
Note that from-model or at-model must be true in order for the
position and orientation data from the model to be used.
Independant Position - This method allows definition of the eye,
that is where you are looking from or the target (objected being
looked at) based on arbitrary position and orientation data obtained from the
property tree (i.e. any source). The following illustrates a Tower defined as
a "lookat", a view that can be used to track or follow a moving model:
<sim>
<view>
<name>Tower View</name>
<type>lookat</type>
<config>
<eye-lat-deg-path>/sim/tower/latitude-deg</eye-lat-deg-path>
<eye-lon-deg-path>/sim/tower/longitude-deg</eye-lon-deg-path>
<eye-alt-ft-path>/sim/tower/altitude-ft</eye-alt-ft-path>
<eye-roll-deg-path>/sim/tower/roll-deg</eye-roll-deg-path>
<eye-pitch-deg-path>/sim/tower/pitch-deg</eye-pitch-deg-path>
<eye-heading-deg-path>/sim/tower/heading-deg</eye-heading-deg-path>
<at-model type="bool">true</at-model>
<at-model-idx type="int">0</at-model-idx>
</config>
</view>
</sim>
Note that the definition is constructed as a path to a numeric data item in
the global property tree. If you are unfamiliar with these properties, take
a look at the "Property Picker" dialog while Flight Gear is running and you
can see the kinds of values in use. You may define your own static values, say
for a fixed location of an observer, by adding the following xml to your
setup (prefrences.xml or another file):
<somepath>
<ground-observer-lon-deg>48.6124</ground-observer-lon-deg>
<ground-observer-lat-deg>63.1243</ground-observer-lat-deg>
<ground-observer-alt-ft>123.5</ground-observer-alt-ft>
</somepath>
These values can then then be referenced in your view by including their
path in the definition. For example to reference the above longitude for
the ground observer:
<sim>
<view>
<config>
<eye-lat-deg-path>/somepath/ground-observer-lat-deg></eye-lon-deg-path>
..
All three longitude, latitude, and altitude must be defined to have a valid
position (otherwise the missing items default to 0). You may also define
orientation such as heading (the direction to look toward) and pitch (the
angle up or down to be looking). In the case of a cockpit view the orientation
will be that of the aircraft itself, but in the case of a "lookfrom" view like a
ground observer or tower it can be an arbitrary view direction that is either
static, or changing.
Syntax for the eye position and orientation is:
<eye-lat-deg-path>path-to-property-value</eye-lat-deg-path>
<eye-lon-deg-path>path-to-property-value</eye-lon-deg-path>
<eye-alt-ft-path>path-to-property-value</eye-alt-ft-path>
<eye-roll-deg-path>path-to-property-value</eye-roll-deg-path>
<eye-pitch-deg-path>path-to-property-value</eye-pitch-deg-path>
<eye-heading-deg-path>path-to-property-value</eye-heading-deg-path>
Syntax for the target or at position used in views of type "lookat" is:
<target-lat-deg-path>path-to-property-value</target-lat-deg-path>
<target-lon-deg-path>path-to-property-value</target-lon-deg-path>
<target-alt-ft-path>path-to-property-value</target-alt-ft-path>
3. View Offsets
There are two types of offsets, but only one is used in building viewer configurations.
This is an offset from the eye position applied to the position defined for the eye as
described in the section on Defining Positon.
These offsets are defined in meters. Here is an example of a Cockpit View definition
that includes offsets for the pilot's eye:
<sim>
<view>
<name>Cockpit View</name>
<type>lookfrom</type>
<internal type="bool">false</internal>
<config>
<from-model type="bool">true</from-model>
<from-model-idx type="int">0</from-model-idx>
<x-offset-m type="double">-0.22</x-offset-m>
<y-offset-m type="double">0.30</y-offset-m>
<z-offset-m type="double">-0.45</z-offset-m>
</config>
</view>
</sim>
Note that these offsets are actually best applied differently to specific
aircraft models. The size of the aircraft and the actual position of the
pilot's seat make a big difference. So typically these offset entries
are included per aircraft in the aircraft-set.xml files.
In other circumstances, such as defining a Chase View, you can possibly get away with
defining the x-offsets in a more global location so that they apply to all
aircraft. Here is an example of a Chase View configuration that
includes offsets to define the eye as being 25 meters behind the model
origin:
<sim>
<view>
<name>Chase View</name>
<type>lookat</type>
<config>
<from-model type="bool">true</from-model>
<from-model-idx type="int">0</from-model-idx>
<at-model type="bool">true</at-model>
<at-model-idx type="int">0</at-model-idx>
<x-offset-m type="double">0</x-offset-m>
<y-offset-m type="double">0</y-offset-m>
<z-offset-m type="double">-25</z-offset-m>
</config>
</view>
</sim>
At the time of this writing (and this is subject to change), offsets
define the change in eye position as follows:
x-offset-m
Positive values to the right of origin (view position).
y-offset-m
Positive values move up from origin.
z-offset-m
Positive values move back (or in) from origin.
4. Ground Level Near Plane
Generally, a higher Near Plane value will result in a better quality
rendering of distant scenery and models. Currently when the aircraft is
in flight the Near Plane is set at 10 meters.
Any object (including terrain) that is closer to the eye or
camera than 10 meters will not be visible. For this reason, when
the eye is close to the ground, the Near Plane must be set lower. Here is
an example of a Cockpit view that is configured to have only a 0.5 meter Near
Plane when sitting on the ground (or below a given flight level which currently
defaults to 30ft AGL altitude).
<view>
<name>Cockpit View</name>
<type>lookfrom</type>
<internal type="bool">false</internal>
<config>
<from-model type="bool">true</from-model>
<from-model-idx type="int">0</from-model-idx>
<ground-level-nearplane-m type="double">0.5f</ground-level-nearplane-m>
</config>
</view>
The example Tower View however, is able to set the value to 10 meters since
the tower position is 50ft AGL altitude to start with:
<view>
<name>Tower View</name>
<type>lookat</type>
<config>
<eye-lat-deg-path>/sim/tower/latitude-deg</eye-lat-deg-path>
<eye-lon-deg-path>/sim/tower/longitude-deg</eye-lon-deg-path>
<eye-alt-ft-path>/sim/tower/altitude-ft</eye-alt-ft-path>
<eye-roll-deg-path>/sim/tower/roll-deg</eye-roll-deg-path>
<eye-pitch-deg-path>/sim/tower/pitch-deg</eye-pitch-deg-path>
<eye-heading-deg-path>/sim/tower/heading-deg</eye-heading-deg-path>
<at-model type="bool">true</at-model>
<at-model-idx type="int">0</at-model-idx>
<ground-level-nearplane-m type="double">10.0f</ground-level-nearplane-m>
</config>
</view>
5. Other examples
Example 1 - The configuration of a Chase View without a rotating frutstum
(the view does not tilt with the aircraft's roll and pitch) is easy. Since the aircraft model's
orientation is contained in it's location definition, you must set from-model
property to false so you may define your own source for position and orientation. Then
define eye position properties for the appropriate values. If you leave
eye-roll-deg and eye-pitch-deg out then they will be fixed at 0 degrees.
Note that technically this example does rotate, but only on the heading :)
<view>
<name>Chase View</name>
<type>lookat</type>
<config>
<from-model type="bool">false</from-model>
<from-model-idx type="int">0</from-model-idx>
<eye-lat-deg-path>/position/latitude-deg</eye-lat-deg-path>
<eye-lon-deg-path>/position/longitude-deg</eye-lon-deg-path>
<eye-alt-ft-path>/position/altitude-ft</eye-alt-ft-path>
<eye-heading-deg-path>/orientation/heading-deg</eye-heading-deg-path>
<at-model type="bool">true</at-model>
<at-model-idx type="int">0</at-model-idx>
<ground-level-nearplane-m type="double">0.5f</ground-level-nearplane-m>
<x-offset-m type="double">0</x-offset-m>
<y-offset-m type="double">0</y-offset-m>
<z-offset-m type="double">-25</z-offset-m>
</config>
</view>
more to come?
Jim Wilson, 24 April 2002 - last update 11 June 2002
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