Users Guide to FlightGear panel configuration
Version 0.7.7, May 16 2001
Author: John Check <j4strngs@rockfish.net>

This document is an attempt to describe the configuration of
FlightGear flight simulator's aircraft panel display via XML.  The
information was culled from the fgfs-devel@flightgear.org mailing list
and my experiences making alternate panels.  Corrections and additions
are encouraged.

Some History:
------------
Older versions of FGFS had a hard coded display of instruments.  This
was a less than ideal state of affairs due to FGFS ability to use
different aircraft models. Being primarily developed on UNIX type
systems, a modular approach is taken towards the simulation. To date,
most alternatives to the default Cessna 172 aircraft are the product
of research institutions interested in the flight characteristics and
not cosmetics.  The result of this was that one could fly the X-15 or
a Boeing 747 but be limited to C172 instrumentation.

A rewrite of the panel display code was done around v0.7.5 by
developer David Megginson allowing for configuration of the panel via
XML to address this limitation. Some major changes and additions were
made during the course of version 0.7.7 necessitating a rewrite and
expansion of this document.


About The Property Manager:
--------------------------
While not absolutely necessary in order to create aircraft panels,
some familiarity with the property manager is beneficial....
FlightGear provides a hierarchical representation of all aspects of
the state of the running simulation that is known as the property
tree.  Some properties, such as velocities are read only. Others such
as the frequencies to which the navcom radios are tuned or the
position of control surfaces can be set by various means.  FlightGear
can optionally provide an interface to these properties for external
applications such as Atlas, the moving map program, or even lowly
telnet, via a network socket. Data can even be placed on a serial port
and connected to, say a GPS receiver.  Aside from its usefulness in a
flight training context, being able to manipulate the property tree on
a running copy of FG allows for switching components on the fly, a
positive boon for panel authors.  To see the property tree start FG
with the following command line:

fgfs --props=socket,bi,5,localhost,5500,tcp

Then use telnet to connect to localhost on port 5500. You can browse
the tree as you would a filesystem.

XML and the Property Manager:
----------------------------
Panel instruments interface with the property tree to get/set values
as appropriate. Properties for which FG doesn't yet provide a value
can be created by simply making them up. Values can be adjusted using
the telnet interface allowing for creation and testing of instruments
while code to drive them is being developed.

If fact, the XML configuration system allows a user to combine
components such as flight data model, aircraft exterior model, heads
up display, and of course control panel. Furthermore, such a
preconfigured aircraft.xml can be included into a scenario with
specific flight conditions. These can be manually specified or a FG
session can be saved and/or edited and reloaded later. Options
specified in these files can be overridden on the command line. For
example:

--prop:/sim/panel/path=Aircraft/c172/Panels/c172-panel.xml

passed as an option, would override a panel specified elsewhere.
Property tree options all have the same format, specify the node and
supply it a value.

The order of precedence for options is thus:

Source          Location                Format
------          --------                ------
command line
.fgfsrc         Users home directory.   command line options
system.fgfsrc   $FG_ROOT                ""      ""
preferences.xml $FG_ROOT                XML property list


Loading Panels on the fly:
-------------------------
When editing a panel configuration, pressing Shift +F3 will reload the
panel. If your changes don't seem to be taking effect, check the
console output.  It will report the success or failure of the panel
reload*. Editing textures requires restarting FGFS so the new textures
can be loaded. Panels can be switched on the fly by setting the
/sim/panel/path property value and reloading.

Regarding Window Geometry:
-------------------------
For the sake of simplicity the FGFS window is always considered to be
1024x768 so all x/y values for instrument placement should relative to
these dimensions.  Since FG uses OpenGL 0,0 represents the lower left
hand corner of the screen. Panels may have a virtual size larger than
1024x768. Vertical scrolling is accomplished with
Shift+F5/F6. Horizontal scrolling is via Shift+F7/F8. An offset should
be supplied to set the default visible area. It is possible to place
items to overlap the 3D viewport.

Panel Architecture:
-------------------
All of the panel configuration files are XML-encoded* property lists.
The root element of each file is always named <PropertyList>. Tags are
almost always found in pairs, with the closing tag having a slash
prefixing the tag name, i.e </PropertyList>. The exception is the tag
representing an aliased property. In this case a slash is prepended to
the closing angle bracket.  (see section Aliasing)

The top level panel configuration file is composed of a <name>, a
<background> texture and zero or more <instruments>.Earlier versions
required instruments to have a unique name and a path specification
pointing to the instruments configuration file.

[ Paths are relative to $FG_ROOT (the installed location of FGFS data files.) ]
[ Absolute paths may be used.Comments are bracketed with <!-- -->.            ]

Old style instrument call in top level panel.xml:
------------------------------------------------
  <clock>         <!-- required "unique_name" -->
   <path>Aircraft/c172/Instruments/clock.xml</path>
   <x>110</x>     <!-- required horizontal placement -->
   <y>320</y>     <!-- required vertical placement -->
   <w>72</w>      <!-- optional width specification -->
   <h>72</h>      <!-- optional height specification -->
  </clock>

The difference between the old and new styles, while subtle, is rather
drastic.  The old and new methods are indeed incompatible. I cover the
old style only to acknowledge the incompatibility. This section will
be removed after the next official FGFS release.

New Style Example Top Level Panel Config:
----------------------------------------
<PropertyList>
 <name>Example Panel</name>
 <background>Aircraft/c172/Panels/Textures/panel-bg.rgb</background>
 <w>1024</w>                      <!-- virtual width -->
 <h>768</h>                       <!-- virtual height -->
 <y-offset>-305</y-offset>        <!-- hides the bottom part -->
 <view-height>172</view-height>   <!-- amount of overlap between 2D panel and 3D viewport -->

 <instruments>                    <!-- from here down is where old and new styles break compatibility -->

  <instrument include="../Instruments/clock.xml">
   <name>Chronometer</name>   <!-- currently optional but strongly recommended -->
   <x>150</x>                 <!-- required horizontal placement -->
   <y>645</y>                 <!-- required vertical placement -->
   <w>72</w>                  <!-- optional width specification -->
   <h>72</h>                  <!-- optional height specification -->
  </instrument>

 </instruments>

</PropertyList>


Indexed Properties
------------------
This is a lot to do with the compatibility break so lets get it out of
the way.  The property manager now assigns incremental indices to
repeated properties with the same parent node, so that

 <PropertyList>
 <x>1</x>
 <x>2</x>
 <x>3</x>
 </PropertyList>

shows up as

 /x[0] = 1
 /x[1] = 2
 /x[2] = 3

This means that property files no longer need to make up a separate
name for each item in a list of instruments, layers, actions,
transformations, or text chunks. In fact, the new panel I/O code now
insists that every instrument have the XML element name "instrument",
every layer have the name "layer", every text chunk have the name
"chunk", every action have the name "action", and every transformation
have the name "transformation" -- this makes the XML more regular (so
that it can be created in a DTD-driven tool) and also allows us to
include other kinds of information (such as doc strings) in the lists
without causing confusion.

Inclusion:
----------
The property manager now supports file inclusion and aliasing.
Inclusion means that a node can include another property file as if it
were a part of the current file.  To clarify how inclusion works,
consider the following examples:

If bar.xml contains

 <PropertyList>
 <a>1</a>
 <b>
 <c>2</c>
 </b>
 </PropertyList>

then the declaration

 <foo include="../bar.xml">
 </foo>

is exactly equivalent to

 <foo>
 <a>1</a>
 <b>
 <c>2</c>
 </b>
 </foo>

However, it is also possible to selectively override properties in the
included file. For example, if the declaration were

 <foo include="../bar.xml">
 <a>3</a>
 </foo>

then the property manager would see

 <foo>
 <a>3</a>
 <b>
 <c>2</c>
 </b>
 </foo>

with the original 'a' property's value replaced with 3.

This new inclusion feature allows property files to be broken up and
reused arbitrarily -- for example, there might be separate cropping
property lists for commonly-used textures or layers, to avoid
repeating the information in each instrument file.


Aliasing
--------
Properties can now alias other properties, similar to a symbolic link
in Unix. When the target property changes value, the new value will
show up in the aliased property as well. For example,

 <PropertyList>
 <foo>3</foo>
 <bar alias="/foo"/>
 </PropertyList>

will look the same to the application as

 <PropertyList>
 <foo>3</foo>
 <bar>3</bar>
 </PropertyList>

except that when foo changes value, bar will change too.


The combination of inclusions and aliases is very powerful, because it
allows for parameterized property files. For example, the XML file for
the NAVCOM radio can include a parameter subtree at the start, like
this:

 <PropertyList>
 <params>
 <comm-freq-prop>/radios/comm1/frequencies/selected</comm-freq-prop>
 <nav-freq-prop>/radios/nav1/frequencies/selected</comm-freq-prop>
 </params>

 ...

 <chunk>
 <type>number-value</type>
 <property alias="/params/nav-freq-prop"/>
 </chunk>

 ...
 </PropertyList>

Now, the same instrument file can be used for navcomm1 and navcomm2,
for example, simply by overriding the parameters at inclusion:

 <instrument include="../Instruments/navcomm.xml">
 <params>
 <comm-freq-prop>/radios/comm1/frequencies/selected</comm-freq-prop>
 <nav-freq-prop>/radios/nav1/frequencies/selected</comm-freq-prop>
 </params>
 </instrument>

 <instrument include="../Instruments/navcomm.xml">
 <params>
 <comm-freq-prop>/radios/comm2/frequencies/selected</comm-freq-prop>
 <nav-freq-prop>/radios/nav2/frequencies/selected</comm-freq-prop>
 </params>
 </instrument>

Instrument Architecture:
-----------------------
Instruments are defined in separate configuration files. An instrument
consists of a base width and height, one or more stacked layers, and
zero or more actions. Base dimensions are specified as follows:

<PropertyList>                   <!-- remember, all xml files start like this -->
 <name>Airspeed Indicator</name> <!-- names are good -->
 <w-base>128</w-base>            <!-- required width spec-->
 <h-base>128</h-base>            <!-- required height spec-->
  <layers>                       <!-- begins layers section -->

Height and width can be overriden in the top level panel.xml by
specifying <w> and <h>. Transformations are caculated against the base
size regardless of the display size. This ensures that instruments
remain calibrated

Textures:
--------
FG uses red/green/blue/alpha .rgba files for textures. Dimensions for
texture files should be power of 2 with a maximum 8:1 aspect ratio.
The lowest common denominator for maximum texture size is 256 pixels.
This is due to the limitations of certain video accelerators, most
notably those with 3Dfx chipset such as the Voodoo2.

Instrument Layers**:
-------------------
The simplest layer is a <texture>. These can be combined in <switch> layers

<texture>
A texture layer looks like this:

  <layer>                      <!-- creates a layer -->
   <name>face</name>
   <texture>                   <!-- defines it as a texture layer -->
    <path>Aircraft/c172/Instruments/Textures/faces-2.rgb</path>
    <x1>0</x1>                 <!-- lower boundary for texture cropping-->
    <y1>0.51</y1>              <!-- left boundary  for texture cropping-->
    <x2>0.49</x2>              <!-- upper boundary  for texture cropping-->
    <y2>1.0</y2>               <!-- right boundary  for texture cropping-->
   </texture>                  <!-- closing texure tag -->
  </layer>                     <!-- closing layer tag -->

The texture cropping specification is represented as a decimal. There
is a table at the end of this document for converting from pixel
coordinates to percentages.

This particular layer, being a gauge face has no transformations
applied to it.  Layers with that aren't static *must* include <w> and
<h> parameters to be visible.

<type> May be either text or switch..

<type>switch</type>
A switch layer is composed of two or more nested layers and will
display one of the nested layers based on a boolean property. For a
simple example of a switch see
$FG_ROOT/Aircraft/c172/Instruments/brake.xml.

  <layer>
   <name>Brake light</name>
   <type>switch</type>                      <!-- define layer as a switch -->
   <property>/controls/brakes</property>    <!-- tie it to a property -->
    <layer1>                                <!-- layer for true state -->
     <name>on</name>                        <!-- label to make life easy -->
     <texture>                              <!-- layer1 of switch is a texture layer -->
     <path>Aircraft/c172/Instruments/Textures/brake.rgb</path>
     <x1>0.25</x1>
     <y1>0.0</y1>
     <x2>0.5</x2>
     <y2>0.095</y2>
     </texture>
     <w>64</w>                              <!-- required width - layer isn't static -->
     <h>24</h>                              <!-- required height - layer isn't static -->
    </layer1>                               <!-- close layer1 of switch -->
    <layer2>                                <!-- layer for false state -->
     <name>off</name>
     <texture>
     <path>Aircraft/c172/Instruments/Textures/brake.rgb</path>
     <x1>0.0</x1>
     <y1>0.0</y1>
     <x2>0.25</x2>
     <y2>0.095</y2>
     </texture>
     <w>64</w>
     <h>24</h>
   </layer2>
  </layer>

Switches can have more than 2 states. This requires nesting one switch
inside another.  One could make, for example, a 3 color LED by nesting
switch layers.

<type>text</type>
A text layer may be static, as in a label, generated from a property
or a combination of both.  This example is a switch that contains both
static and dynamic text:

   <layer1>                               <!-- switch layer -->
    <name>display</name>
    <type>text</type>                     <!-- type == text -->
    <point-size>12</point-size>           <!-- font size -->
    <color>                               <!-- specify rgb values to color text -->
     <red>1.0</red>
     <green>0.5</green>
     <blue>0.0</blue>
    </color>                              <!-- close color section -->
    <chunks>                              <!-- sections of text are referred to as chunks -->
     <chunk>                              <!-- first chunk of text -->
      <type>number-value</type>           <!-- value defines it as dynamic -->
      <property>/radios/nav1/dme/distance</property>      <!-- ties it to a property -->
      <scale>0.00053995680</scale>        <!-- convert between  statute and nautical miles? -->
      <format>%5.1f</format>              <!-- define format -->
     </chunk>
    </chunks>
   </layer1>
   <layer2>                                <!-- switch layer -->
    <name>display</name>
    <type>text</type>                      <!-- type == text -->
    <point-size>10</point-size>            <!-- font size -->
    <color>                                <!-- specify rgb values to color text -->
     <red>1.0</red>
     <green>0.5</green>
     <blue>0.0</blue>
    </color>                               <!-- close color section -->
    <chunks>                               <!-- sections of text are referred to as chunks -->
     <chunk>                               <!-- first chunk of text -->
      <type>literal</type>                 <!-- static text -->
      <text>---.--</text>                  <!-- fixed value -->
     </chunk>
    </chunks>
   </layer2>


Transformations:
---------------
A transformation is a rotation, an x-shift, or a
y-shift. Transformations can be static or they can be based on
properties. Static rotations are useful for flipping textures
horizontally or vertically. Transformations based on properties are
useful for driving instrument needles. I.E. rotate the number of
degrees equal to the airspeed. X and y shifts are relative to the
center of the instrument. Each specified transformation type takes an
<offset>.  Offsets are relative to the center of the instrument. A
shift without an offset has no effect. For example, let's say we have
a texure that is a circle. If we use this texture in two layers, one
defined as having a size of 128x128 and the second layer is defined as
64x64 and neither is supplied a shift and offset the net result
appears as 2 concentric circles.


About Transformations and Needle Placement:
------------------------------------------

When describing placement of instrument needles, a transformation
offset must be applied to shift the needles fulcrum or else the needle
will rotate around it's middle. The offset will be of <type> x-shift
or y-shift depending on the orientation of the needle section in the
cropped texture.

This example comes from the altimeter.xml

  <layer>
   <name>long needle (hundreds)</name>        <!-- the altimeter has more than one needle -->
   <texture>
    <path>Aircraft/c172/Instruments/Textures/misc-1.rgb</path>
    <x1>0.8</x1>
    <y1>0.78125</y1>
    <x2>0.8375</x2>
    <y2>1.0</y2>
   </texture>
   <w>8</w>
   <h>56</h>
   <transformations>                          <!-- begin defining transformations -->
    <transformation>                          <!-- start definition of transformation that drives the needle -->
     <type>rotation</type>
     <property>/steam/altitude</property>     <!-- bind it to a property -->
     <max>100000.0</max>                      <!-- upper limit of instrument -->
     <scale>0.36</scale>                      <!-- once around == 1000 ft -->
    </transformation>                         <!-- close this transformation -->
    <transformation>                          <!-- this one shifts the fulcrum of the needle -->
     <type>y-shift</type>                     <!-- y-shift relative to needle -->
     <offset>24.0</offset>                    <!-- amount of shift -->
    </transformation>
   </transformations>
  </layer>

This needles has its origin in the center of the instrument. If the
needles fulcrum was towards the edge of the instrument, the
transformations to place the pivot point must precede those which
drive the needle,

Interpolation
-------------
Non linear transformations are now possible via the use of
interpolation tables.

 <transformation>
 ...
 <interpolation>
 <entry>
 <ind>0.0</ind>            <!-- raw value -->
 <dep>0.0</dep>            <!-- displayed value -->
 </entry>
 <entry>
 <ind>10.0</ind>
 <dep>100.0</dep>
 </entry>
 <entry>
 <ind>20.0</ind>
 <dep>-5.0</dep>
 </entry>
 <entry>
 <ind>30.0</ind>
 <dep>1000.0</dep>
 </entry>
 </interpolation>
 </transformation>

Of course, interpolation tables are useful for non-linear stuff, as in
the above example, but I kind-of like the idea of using them for
pretty much everything, including non-trivial linear movement -- many
instrument markings aren't evenly spaced, and the interpolation tables
are much nicer than the older min/max/scale/offset stuff and should
allow for a more realistic panel without adding a full equation parser
to the property manager.

If you want to try this out, look at the airspeed.xml file in the base
package, and uncomment the interpolation table in it for a very funky,
non-linear and totally unreliable airspeed indicator.


Actions:
-------
An action is a hotspot on an instrument where something will happen
when the user clicks the left or center mouse button.  Actions are
always tied to properties: they can toggle a boolean property, adjust
the value of a numeric property, or swap the values of two properties.
The x/y placement for actions specifies the origin of the lower left
corner.  In the following example the first action sets up a hotspot
32 pixels wide and 16 pixels high. It lower left corner is placed 96
pixels (relative to the defined base size of the instrument) to the
right of the center of the instrument. It is also 32 pixels below the
centerline of the instrument.  The actual knob texture over which the
action is superimposed is 32x32.  Omitted here is a second action,
bound to the same property, with a positive increment value. This
second action is placed to cover the other half of the knob. The
result is that clicking on the left half of the knob texture decreases
the value and clicking the right half increases the value. Also
omitted here is a second pair of actions with the same coordinates but
a larger increment value. This second pair is bound to a different
mouse button. The net result is that we have both fine and coarse
adjustments in the same hotspot, each bound to a different mouse
button.

These examples come from the radio stack:
<actions>                              <!-- open the actions section -->
  <action>                             <!- first action -->
   <name>small nav frequency decrease</name>
   <type>adjust</type>
   <button>0</button>                  <!-- bind it to a mouse button -->
   <x>96</x>                           <!-- placement relative to instrument center -->
   <y>-32</y>
   <w>16</w>                           <!-- size of hotspot -->
   <h>32</h>
   <property>/radios/nav1/frequencies/standby</property>    <!-- bind to a property -->
   <increment>-0.05</increment>        <!-- amount of adjustment per mouse click -->
   <min>108.0</min>                    <!-- lower range -->
   <max>117.95</max>                   <!-- upper range -->
   <wrap>1</wrap>                      <!-- boolean value -- value wraps around when it hits bounds -->
  </action>
  <action>
   <name>swap nav frequencies</name>
   <type>swap</type>                   <!-- define type of action -->
   <button>0</button>
   <x>48</x>
   <y>-32</y>
   <w>32</w>
   <h>32</h>
   <property1>/radios/nav1/frequencies/selected</property1>   <!-- properties to toggle between -->
   <property2>/radios/nav1/frequencies/standby</property2>
  </action>
  <action>
   <name>ident volume on/off</name>
   <type>adjust</type>
   <button>1</button>
   <x>40</x>
   <y>-24</y>
   <w>16</w>
   <h>16</h>
   <property>/radios/nav1/ident</property>  <!-- this property is for Morse code identification of nav beacons -->
   <increment>1.0</increment>          <!-- the increment equals the max value so this toggles on/off -->
   <min>0</min>
   <max>1</max>
   <wrap>1</wrap>                      <!-- a shortcut to avoid having separate actions for on/off -->
  </action>
</actions>

More About Textures:
-------------------
As previously stated, the usual size instrument texture files in FGFS
are 256x256 pixels, red/green/blue/alpha format. However the mechanism
for specifying texture cropping coordinates is decimal in nature. When
calling a section of a texture file the 0,0 lower left convention is
used.  There is a pair of x/y coordinates defining which section of
the texture to use.

The following table can be used to calculate texture cropping
specifications.

# of divisions | width in pixels | decimal specification
per axis
        1   =   256 pixels              1
        2   =   128 pixels,             0.5
        4   =   64 pixels,              0.25
        8   =   32 pixels,              0.125
        16  =   16 pixels,              0.0625
        32  =   8 pixels,               0.03125
        64  =   4 pixels,               0.015625
        128 =   2 pixels,               0.0078125

A common procedure for generating gauge faces is to use a vector
graphics package such as xfig, exporting the result as a postscript
file. 3D modeling tools may also be used and I prefer them for pretty
items such as levers, switches, bezels and so forth.  Ideally, the
size of the item in the final render should be of proportions that fit
into the recommended pixel widths.  The resulting files can be
imported into a graphics manipulation package such as GIMP, et al for
final processing.

How do I get my panels/instruments into the base package?
-------------------------------------------------------
Cash bribes always help ;) Seriously though, there are two main
considerations.  Firstly, original artwork is a major plus since you
as the creator can dictate the terms of distribution.All Artwork must
have a license compatible with the GPL.  Artwork of unverifiable
origin is not acceptable.  Secondly, texture sizes must meet the
lowest common denominator of 256e2 pixels.  Artwork from third parties
may be acceptable if it meets these criteria.

*  If there are *any* XML parsing errors, the panel will fail to load,
   so it's worth downloading a parser like Expat (http://www.jclark.com/xml/)
   for checking your XML. FlightGear will print the location of errors, but
   the messages are a little cryptic right now.

** NOTE: There is one built-in layer -- for the mag compass ribbon --
   and all other layers are defined in the XML files.  In the future,
   there may also be built-in layers for special things like a
   weather-radar display or a GPS (though the GPS could be handled with
   text properties).