Users Guide to FGInput - Joystick And Keyboard Bindings For FlightGear
Or
"The document formerly know as The Users Guide to Joystick Usage Under
FlightGear Flight Simulator"

version 0.7.7.3 07/02/2001
Author John Check <j4strngs@rockfish.net>

This document is written with versions of FlightGear 0.7.7 and greater
in mind. It assumes a working joystick present on your system. It
is written from the perspective a Linux user, but the information presented
is valid on other platforms. The most current version can be found at:

http://rockfish.net/shell/aboutFGInput.txt

Thanks to David Megginson, who aside from actually implementing FGFS XML
features, lets me rip off his descriptions of how stuff works so I can look
smart.

Some History:
------------
Earlier versions of FGFS had assignments of joystick axis/buttons
and key bindings hard coded. If you had a joystick that did not use
the default channel assignments, or wanted different key bindings
you had to edit the source code and recompile.

Fortunately, around about v0.7.5 a "property manager" was implemented,
which facilitated being able to set the parameters for the joystick at runtime.
Version 0.7.7 saw an expanded role for the property manager and the
addition of a "command manager" that allows for binding of events to commands.
The code that does this is known as FGInput and is used to configure
keyboard command bindings as well as joysticks.

Storing alternate keyboard or joystick bindings can be done in a variety of ways.
The order of precedence for options is thus:

Source          Location                Format                 Scope
------          --------                ------                 -----
command line    STDIN                   see examples           session
.fgfsrc         Users home directory.   command line options   single user
system.fgfsrc   $FG_ROOT                command line options   system wide
joystick.xml    $FG_ROOT                XML property list      system wide
keyboard.xml    $FG_ROOT                XML property list      system wide

First Things First
------------------
I will cover joysticks first and save the keyboard stuff for later.
FGInput treats things in a generic enough way that the line between
joystick buttons and keyboard events starts to blur.

About XML
---------
In case you were wondering, XML stands for eXtensible Markup Language.
It looks a lot like HTML, except you get to define your own tags. Well,
in the case of FGFS we defined the tags you need to configure things.
It is well suited for describing hierarchically organized structures, such as
the property tree FGFS uses to expose the it's innards to external applications.
Your XML configuration files for FGFS must start and end with the following
pair of tags.
     <PropertyList>
       <!-- this is a comment, See I told you it was like HTML -->
     </PropertyList>

Determining your joystick output:
--------------------------------
FlightGear ships with a utility called js_demo. It will report
the number of joysticks attached to a system and their capabilites.
By observing the output of js_demo while working your joystick you can
determine what controls are where.
It should be noted that, at least on UNIX, numbering generally starts with
zero. In the following example the system has 1 joystick (js0) connected.
The output shown is from an analog Gravis BlackHawk with four buttons and
a throttle.

Typical output of js_demo:

Joystick test program.
~~~~~~~~~~~~~~~~~~~~~~
Joystick 1 not detected              <!-- remember we start at 0 -->
Joystick 2 not detected
+---------------JS.0-----------------+
| Btns Ax:0 Ax:1 Ax:2                |
+------------------------------------+
| 0000 +0.0 +0.0 -1.0   .    .    .  |

The buttons are handled internally as a binary number in which bit 0 (the
least significant bit) represents button 0, bit 1 represents button 1, etc.,
but this number is displayed on the screen in hexadecimal notation, so:
  0001 => button 0 pressed
  0002 => button 1 pressed
  0004 => button 2 pressed
  0008 => button 3 pressed
  0010 => button 4 pressed
  0020 => button 5 pressed
  0040 => button 6 pressed
  ... etc. up to ...
  8000 => button 15 pressed
  ... and ...
  0014 => buttons 2 and 4 pressed simultaneously
  ... etc.


Default Joystick properties:
----------------------------

Axis 0   = Aileron
Axis 1   = Elevator
Axis 2   = Rudder
Axis 3   = Throttle
Button 0 = All brakes
Button 1 = Elevator trim (up)
Button 2 = Elevator trim (down)


Okay, Now what?
---------------
Now that you know what the output of the devices connected to the joystick
port (or USB port joystick devices) is, you probably want to dive straight
in and to start connecting to FGInput. If you are familiar with configuring
the joystick on versions of FGFS prior to 0.7.7 you can skip down to the
section "The Command Manager".
If you are a new FGFS user, you should at least skim the next bit since it
explains some concepts you may or may not know. It also covers some legacy
joystick options which have not been implemented yet in the context of the
command manager.


Modifiers For Raw Joystick Values
---------------------------------
These concepts are expressed by supplying arguments to the joystick bindings.
The raw values coming from the joystick axes may not be suitable to use directly.
For that matter not all joysticks are created equal so understanding the basic
concepts should save you some time when experimenting to get the best performance.

The full order of precedence for axis properties is

1. The raw axis value ...
2. is adjusted to dead-band, ... deadband is implemented outside the command manager
3. then adjusted to offset, ...
4. then multiplied by factor, which ...
5. is assigned to the FlightGear control property.

Put another way....

	cooked_value = (( raw_value > dead-band ) + offset) * factor


Axis properties:

	dead-band

-1          0          1
 .......................
-1         | |         1
            ^
        dead-band


This is an area where signals are ignored. It is used to compensate
for noise or potentiometers of dubious quality by creating a threshold
below which any signal is ignored.

The default of 0.1 for elevators and ailerons is very forgiving. A lower
number results in a tighter feel. In some cases such as throttle you may
wish to not set a deadband. Use a value of 0.0 in this case.



	offset

-1  0                  1
 .......................
-1  ^                  1
 offset

Used to maximize a controls use of it's axis, as in the case of a
throttle where zero would be a minimum and not a center point like in the case
of a rudder. Typical value -1.0.


	factor

Controls sensitivity of an axis. If the factor is too low it results in
control not reaching its maximum possible limit. Negating the number will result in
the control moving counter to the default. The default value is 1.0, think unity gain.

In my case, throttle behavior was inverted from what I preferred.
I set this value to -1.0 and everything was groovy.


The Command Manager
-------------------
Previous versions of FGFS allowed joystick output to be bound directly
to the property manager. This has changed for FGFS v0.7.7 and now events
are bound to commands. Commands *must* be specified for a binding to have
an effect. The current list of commands is broken down here into two
categories, mainly for my convenience.

Visual And File Related:

command            options          used for
-------            -------          --------
null               none             useful for clearing a previous binding
exit               none             Exiting FGFS
load               file name*       Load a saved flight
save               file name*       Save current flight
load-panel         path **          Change/reload panel
load-preferences   path **          Load preferences ***
screen-capture     none             Save a screenshot to ./fgfs-screen.ppm
view-cycle         none             Change the direction of the pilots view

*Saved/loaded relative to current working directory.
**The path includes the filename you wish to load and is relative to $FG_ROOT,
  which is the location of the installed base package. The default for load-panel
  is the value of /sim/panel/path (from preferences.xml) or if that is unset
  Panel/Default/default.xml. The default for load preferences is preferences.xml.
***This might make a good first binding to experiment with, since it's not currently
   bound to anything. Reloading preferences will allow you to test settings
   without having to sit through a restart of FGFS for every edit. You can
   always (re)move it later.

Flight Control:
command         options                effect
-------         -------                ------
property-toggle property               toggle the property full on
property-assign "" "", value           targets a property for action
property-adjust "" "", step            Increment size for changes
property-swap   "" ""[0], "" ""[1]     Set values in a switch
property-scale  "" "", offset, factor  Processes the raw joystick value


Bindings
--------
A command may have more than one binding.  By default, the examples below
use just /binding or <binding>, but /binding[0] or <binding n="0"> is implied.
When bindings are specified in XML the indices are created automagically. If
you wish to avoid XML you must supply the index number for multiple bindings
in your command line formatted options.


Joystick Axes
-------------

Here's a sample Joystick axis declaration in XML:

  <axis n="0">                        <!-- target an axis -->
   <desc>Aileron</desc>               <!-- descriptive name (optional) -->
   <binding>                          <!-- open a container for the binding -->
    <command>property-scale</command>           <!-- pick a command -->
    <property>/controls/aileron</property>      <!-- target a property -->
   </binding>                                   <!-- closing tag for binding -->
  </axis>                                       <!-- closing tag for axis -->

Remember how I said the property tree was a hierarchy? Thoughtful readers
will notice how the nested tags keep things organized.
This binding appears in the context /input/joysticks/js/, so the
command-line option equivalent of this declaration (leaving out the
'desc', which isn't strictly necessary), is

  --prop:/input/joysticks/js[0]/axis[0]/binding/command=property-scale
  --prop:/input/joysticks/js[0]/axis[0]/binding/property=/controls/aileron

Do you see how the command line versions uses a path to represent the hierarchy?
Cool! You should read README.xmlpanel next, working with FGFS XML configuration
system is easy and it's fun for the whole family! ( not sold in stores. excludes
tax and title. void where prohibited by law.)

Ok, back to business. The 'property-scale' command also recognizes 'offset' and
'factor' arguments. The "type" arguments shown in the following example are
specifying a double precision floating point number. A double has more discrete
steps and gives a smoother action than a plain float.

  <axis n="2">
   <desc>Throttle</desc>
   <!-- See important note about dead-band below -->
   <binding>
    <command>property-scale</command>
    <property>/controls/throttle</property>
    <offset type="double">-1.0</offset>
    <factor type="double">-0.5</factor>
   </binding>
  </axis>

or

  --prop:/input/joysticks/js[0]/axis[2]/binding/command=property-scale
  --prop:/input/joysticks/js[0]/axis[2]/binding/property=/controls/throttle
  --prop:/input/joysticks/js[0]/axis[2]/binding/offset=-1.0
  --prop:/input/joysticks/js[0]/axis[2]/binding/factor=-0.5

*Important Note About dead-band*
--------------------------------
You may recall from the section about raw axis value modifiers that dead-band
is implemented outside the command manager. This means that if you want to apply
a dead-band, the tag *must* precede the binding tag. If you are using the command
line format you must omit the 'binding' part like so:

  --prop:/input/joysticks/js[0]/axis[2]/dead-band=0.005

Joystick Button Properties
--------------------------
Buttons, being boolean by nature, can use a little help. By this I mean that there
are times when you need momentary action, times where you need a repeating action
and sometimes you just want a plain old toggle. In order to facilitate this need
we have some tags that modify the actions of buttons.

<repeatable>
 <!-- Will be either true or false. If it is true, holding down a button (or key) will
      cause repeated events, say, for moving the trim; if false (the default), pressing
      a key will cause only a single event. -->

<step>
 <!-- The property-adjust command takes a 'step' argument specifying the
      amount of the adjustment for each event. In the following example, the elevator
      trim moves 0.1% for each event (without automatic repetition, you'd
      have a pretty sore finger). -->

<value>
 <!-- Use value on non-repeatables to supply the value for each consecutive press-->

<mod-up>
 <!-- This stands for "modifier up", my favorite I think. This is used to set up a binding
      for when you *release* a key. As you'll see, it comes in handy -->

Here's a sample joystick button declaration in XML:

  <button n="1">                         <!-- target a button -->
   <desc>Elevator trim up</desc>         <!-- optional description -->
   <repeatable>true</repeatable>    <!-- Ok, repeatable is outside the command manager too -->
   <binding>                             <!-- Open the "binding" node of the tree-->
    <command>property-adjust</command>           <!-- pick a command type to bind -->
    <property>/controls/elevator-trim</property> <!-- target a property -->
    <step type="double">0.001</step>
   </binding>
  </button>

In command-line option syntax, this would appear as

  --prop:/input/joysticks/js[0]/button[1]/repeatable=true     <!-- See? no 'binding' -->
  --prop:/input/joysticks/js[0]/button[1]/binding/command=property-adjust
  --prop:/input/joysticks/js[0]/button[1]/binding/property=/controls/elevator-trim
  --prop:/input/joysticks/js[0]/button[1]/binding//step=0.001


Here's a slightly fancier declaration, that applies the left (differential) brakes
when button 4 is pressed, and releases them automatically when the user releases the
button:

  <button n="4">
   <desc>Left brake</desc>
   <binding>
    <command>property-assign</command>
    <property>/controls/brakes[0]</property>
    <value type="double">1.0</value>    <!-- brakes are a toggle so 1.0 represents on -->
   </binding>
   <mod-up>                    <!-- it's not a parking brake so we need to release it -->
    <binding>
     <command>property-assign</command>
     <property>/controls/brakes[0]</property>
     <value type="double">0.0</value>       <!-- 1.0 is on so 0.0 is off, right? -->
    </binding>
   </mod-up>
  </button>

The first binding is straight-forward: when the button is pressed, the
'property-assign' command assigns the value 1.0 (i.e. full) to the left brake
property.  The second binding, however, is nested inside a 'mod-up' element,
it will be fired when the user *releases* the button, and will use the
'property-assign' command to assign the value 0.0 (i.e. none) to the left brake
property.  Repetition is left at the default value of false, so that the same
value will not be assigned over and over again.

Here's the command-line equivalent:

  --prop:/input/joysticks/js[0]/button[4]/binding/command=property-assign
  --prop:/input/joysticks/js[0]/button[4]/binding/property=/controls/brakes[0]
  --prop:/input/joysticks/js[0]/button[4]/binding/value=1.0
  --prop:/input/joysticks/js[0]/button[4]/mod-up/binding/command=property-assign
  --prop:/input/joysticks/js[0]/button[4]/mod-up/binding/property=/controls/brakes[0]
  --prop:/input/joysticks/js[0]/button[4]/mod-up/binding/value=0.0

Remember that more than one binding can be included in each context. Here's a
very hairy example from the default bindings that fires all three brakes when
button 0 is pressed, and releases all three when button 0 is released:

  <button n="0">
   <desc>Brakes</desc>
   <binding>
    <command>property-assign</command>
    <property>/controls/brakes[0]</property>
    <value type="double">1.0</value>
   </binding>
   <binding>
    <command>property-assign</command>
    <property>/controls/brakes[1]</property>
    <value type="double">1.0</value>
   </binding>
   <binding>
    <command>property-assign</command>
    <property>/controls/brakes[2]</property>
    <value type="double">1.0</value>
   </binding>
   <mod-up>
    <binding>
     <command>property-assign</command>
     <property>/controls/brakes[0]</property>
     <value type="double">0.0</value>
    </binding>
    <binding>
     <command>property-assign</command>
     <property>/controls/brakes[1]</property>
     <value type="double">0.0</value>
    </binding>
    <binding>
     <command>property-assign</command>
     <property>/controls/brakes[2]</property>
     <value type="double">0.0</value>
    </binding>
   </mod-up>
  </button>

For people who take pleasure in avoiding XML, here's the command-line
equivalent (note the subscripts to distinguish multiple bindings; the
XML will handle this automatically):

  --prop:/input/joysticks/button[0]/binding[0]/command=property-assign
  --prop:/input/joysticks/button[0]/binding[0]/property=/controls/brakes[0]
  --prop:/input/joysticks/button[0]/binding[0]/value=1.0
  --prop:/input/joysticks/button[0]/binding[1]/command=property-assign
  --prop:/input/joysticks/button[0]/binding[1]/property=/controls/brakes[1]
  --prop:/input/joysticks/button[0]/binding[1]/value=1.0
  --prop:/input/joysticks/button[0]/binding[2]/command=property-assign
  --prop:/input/joysticks/button[0]/binding[2]/property=/controls/brakes[2]
  --prop:/input/joysticks/button[0]/binding[2]/value=1.0
  --prop:/input/joysticks/button[0]/mod-up/binding[0]/command=property-assign
  --prop:/input/joysticks/button[0]/mod-up/binding[0]/property=brakes[0]
  --prop:/input/joysticks/button[0]/mod-up/binding[0]/value=0.0
  --prop:/input/joysticks/button[0]/mod-up/binding[1]/command=property-assign
  --prop:/input/joysticks/button[0]/mod-up/binding[1]/property=brakes[1]
  --prop:/input/joysticks/button[0]/mod-up/binding[1]/value=0.0
  --prop:/input/joysticks/button[0]/mod-up/binding[2]/command=property-assign
  --prop:/input/joysticks/button[0]/mod-up/binding[2]/property=brakes[2]
  --prop:/input/joysticks/button[0]/mod-up/binding[2]/value=0.0


This is a good time to remind you that command line formatted options are used
in your .fgfsrc file. You can put them in $FG_ROOT/system.fgfsrc to make them
global.

Digital Coolie Hats
-------------------
Many common joysticks come with digital coolie hats. These are detected as 
axes rather than as buttons, although they are in fact just four (or eight) 
simple switches. FGFS provides 2 virtual buttons to every axis which are 
triggered whenever the axis reaches one of the end positions. These virtual 
buttons can be addressed via two sub-properties »low« and »high« and accept any
of the common button properties.  

For example, if you just want to control view direction, you can map two of the 
axes to /sim/view/axes/long and /sim/view/axes/lat:

  --prop:/input/joysticks/js/axis[5]/binding/command=property-scale
  --prop:/input/joysticks/js/axis[5]/binding/property=/sim/view/axes/lat

  --prop:/input/joysticks/js/axis[6]/binding/command=property-scale
  --prop:/input/joysticks/js/axis[6]/binding/property=/sim/view/axes/long

If you want to use them as buttons (say, to scroll the panel
vertically or horizontally), you can use the high/low bindings.
Here's an example of how to use an axis to adjust the elevator trim:

  --prop:/input/joysticks/js/axis[1]/low/repeatable=true
  --prop:/input/joysticks/js/axis[1]/low/binding/command=property-adjust
  --prop:/input/joysticks/js/axis[1]/low/binding/property=/controls/elevator-trim
  --prop:/input/joysticks/js/axis[1]/low/binding/step=0.001

  --prop:/input/joysticks/js/axis[1]/high/repeatable=true
  --prop:/input/joysticks/js/axis[1]/high/binding/command=property-adjust
  --prop:/input/joysticks/js/axis[1]/high/binding/property=/controls/elevator-trim
  --prop:/input/joysticks/js/axis[1]/high/binding/step=-0.001

You could also bind some axes to brakes, so that you can use positions
between between 0.0 (no brakes) and 1.0 (full brakes).


Keyboard Bindings
-----------------

Keyboard bindings are almost exactly identical to joystick-button
bindings, as in the following example (the context is
/input/keyboard):

  <key n="1">
   <name>Ctrl-A</name>
   <desc>Toggle autopilot altitude lock.</desc>
   <binding>
    <command>property-toggle</command>
    <property>/autopilot/locks/altitude</property>
   </binding>
  </key>

There are a few gotcha's though.  First, the index of the key
specifies the key code that you're binding; in the above example, '1'
corresponds to Ctrl-A (as mentioned in the friendly comment).  Regular
key codes go up to 255; codes from 256 up represent special keys like
function and arrow keys (you can get the value from include/GL/glut.h,
but adding 256 to the special key code).  Here is the command-line
equivalent of the above, leaving out the documentation properties:

  --prop:/input/keyboard/key[1]/binding/command=property-toggle
  --prop:/input/keyboard/key[1]/binding/property=/autopilot/locks/altitude

(The 'property-toggle' command switches a boolean value between true and
false, so it needs no arguments except for the property name.)  DON'T
LEAVE OUT THE INDEX!!!

The second gotcha is that keys can take more modifiers than joystick
buttons.  In addition to mod-up (representing the key release), a key
may use any combination of nested 'mod-alt', 'mod-ctrl', and
'mod-shift' modifiers, as in the following example:

  <key n="49">
   <name>1</name>
   <mod-shift>
     <desc>Look back left</desc>
     <binding>
      <command>property-assign</command>
      <property>/sim/view/goal-offset</property>
      <value type="double">135</value>
     </binding>
   </mod-shift>
  </key>

In this example, the '1' key combined with shift causes the view to
switch to back left.  Note that this will work only with the keypad 1,
since pressing shift+1 on the regular keyboard will give a '!'
character instead.

The input module tries to be smart about supplying control and shift
modifiers automatically where they make sense -- note that it wasn't
necessary to use a nested 'mod-ctrl' element for the ctrl-A binding
earlier, since a keycode less that 32 implies a control character
anyway.

With the new input module, the key-up events can also be detected for
the keyboard, so it's possible to have touch-sensitive brakes (etc.)
just as with the joystick:

  <key n="44">
   <name>,</name>
   <desc>Left brake</desc>
   <binding>
    <command>property-assign</command>
    <property>/controls/brakes[0]</property>
    <value type="double">1.0</value>
   </binding>
   <mod-up>
    <binding>
     <command>property-assign</command>
     <property>/controls/brakes[0]</property>
     <value type="double">0.0</value>
    </binding>
   </mod-up>
  </key>

Now here is a different way to bind a brake. In this example, there is
no <mod-up> tag, so it *does* work like a parking brake.

<key n="66">
  <name>B</name>
  <desc>Toggle parking brake on or off</desc>
  <binding>
   <command>property-toggle</command>
   <property>/controls/brakes[0]</property>
  </binding>
  <binding>
   <command>property-toggle</command>
   <property>/controls/brakes[1]</property>
  </binding>
  <binding>
   <command>property-toggle</command>
   <property>/controls/brakes[2]</property>
  </binding>
 </key>