Document started 27/01/2008 by Tiago Gusmão
Updated 02/02/2008 to reflect syntax changes
Updated 03/02/2008 to add trails (connected particles)

This is a short specification/tutorial to define particle systems in
FlightGear using XML

Meaningless example (what i had accumulated due to tests):

  <particlesystem>
    <name>fuel</name>
<!--     <texture>particle.rgb</texture> -->
    <emissive>false</emissive>
    <lighting>true</lighting>

    <offsets>
      <x-m>35</x-m>
      <y-m>-0.3</y-m>
      <z-m>0</z-m>
      <!--<pitch-deg>90</pitch-deg>-->
    </offsets>

    <!--<condition>
      <and>
        <equals>
          <property>engines/engine/smoking</property>
          <value>true</value>
        </equals>
        <less-than>
          <property>position/altitude-agl-ft</property>
          <value>12000</value>
        </less-than>
      </and>
    </condition>-->

    <attach>world</attach>

    <placer>
      <type>point</type>
    </placer>

    <shooter>
      <theta-min-deg>84</theta-min-deg>
      <theta-max-deg>86</theta-max-deg>
      <phi-min-deg>-1.5</phi-min-deg>
      <phi-max-deg>1.5</phi-max-deg>
      <speed>
        <value>10</value>
        <spread>2.5</spread>
      </speed>
      <rotation-speed>
        <x-min-deg-sec>0</x-min-deg-sec>
        <y-min-deg-sec>0</y-min-deg-sec>
        <z-min-deg-sec>0</z-min-deg-sec>
        <x-max-deg-sec>0</x-max-deg-sec>
        <y-max-deg-sec>0</y-max-deg-sec>
        <z-max-deg-sec>0</z-max-deg-sec>
      </rotation-speed>
    </shooter>

    <counter>
      <particles-per-sec>
        <value>1</value>
        <spread>0</spread>
      </particles-per-sec>
    </counter>

    <align>billboard</align>

    <particle>
      <start>
        <color>
          <red>
            <value>0.9</value>
          </red>
          <green>
            <value>0.09</value>
          </green>
          <blue>
            <value>0.09</value>
          </blue>
          <alpha>
            <value>1.0</value>
          </alpha>
        </color>
        <size>
          <value>0.25</value>
        </size>
      </start>

      <end>
        <color>
          <red>
            <value>1</value>
          </red>
          <green>
            <value>0.1</value>
          </green>
          <blue>
            <value>0.1</value>
          </blue>
          <alpha>
            <value>0.0</value>
          </alpha>
        </color>
        <size>
          <value>4</value>
        </size>
      </end>

      <life-sec>
        <value>10</value>
      </life-sec>

      <mass-kg>0.25</mass-kg>
      <radius-m>0.1</radius-m>
    </particle>

    <program>
      <fluid>air</fluid>
      <gravity type="bool">true</gravity>
      <wind type="bool">true</wind>
    </program>
</particlesystem>

Stick this inside any model XML like it was an animation and it should
work (notice the condition requires wheel on the ground)

Specification:

Note:
<VALUEORPROP/> means you can either specify a property with factor and
offset (result = (prop*factor)+offset ) in the usual way


<particlesystem>  = the base tag
  <type>string</type> = can be "normal" or "trail", normal is the usual quad
                        particles, trail is a string of connected line shapes
                        by default.
  <offsets>  = this places the source of the particles (the emitter) in relation
               to the perhaps already offset model (see model-howto.html for details)
    <x-m>float</x-m>
    <y-m>float</y-m>
    <z-m>float</z-m>
    <pitch-deg>float</pitch-deg>
    <roll-deg>float</roll-deg>
    <heading-deg>float</heading-deg>
  </offsets>
  <condition> = a typical condition that if not true stops particles from being
    ....        emitted (PPS=0)
  </condition>
  <name>string</name> = the name of the particle system (so it can be referenced
                        by normal animations)
  <attach>string</attach> = can be "world" or "local". "world means the particles
                            aren't "physically linked" to the model (necessary for
                            use outside moving models), "local" means the opposite
                            (can be used for static objects or inside moving objects)
  <texture>string</texture> = the texture path relative to the XML file location
  <emissive>bool</emissive> = self-explanatory
  <lighting>bool</lighting> = yet to be tested, but seems obvious
  <align>string</align> = can be "billboard" or "fixed"
  <placer> = where particles are born
    <type>string</type> = can be "sector" (inside a circle), "segments"(user-defined
                          segments) and "point" (default)
    *<radius-min-m>float</radius-min-m> = only for sector, inner radius at which
                                          particles appear
    *<radius-max-m>float</radius-max-m> = only for sector, outer radius at which
                                          particles appear
    *<phi-min-deg>float</phi-min-deg> = only for sector, starting angle of the
                                        slide at which particles appear
    *<phi-max-deg>float</phi-max-deg> = only for sector, ending angle of the slide
                                        at which particles appear
    <segments> = only for segments, encloses sequential points that form segments
      <vertex> = specifies one point, put as many as you want
        <x-m>float</x-m>
        <y-m>float</y-m>
        <z-m>float</z-m>
      </vertex>
      ....
      <vertex>
      ...
      </vertex>
    </segments>
  </placer>
  <shooter> = the shooter defines the initial velocity vector for your particles
    *<theta-min-deg>float</theta-min-deg> = horizontal angle limits of the particle cone
    *<theta-max-deg>float</theta-max-deg>
    *<phi-min-deg>float</phi-min-deg> = vertical angle limits of the particle cone
    *<phi-max-deg>float</phi-max-deg>   for an illustration of theta/phi see
    http://www.cs.clemson.edu/~malloy/courses/3dgames-2007/tutor/web/particles/particles.html
    <speed-mps> = the scalar velocity (meter per second)
     <VALUEORPROP/> = see note
     *<spread> = the "tolerance" in each direction so values are in the range
                 [value-spread, value+spread]
    </speed-mps>
    <rotation-speed> = the range of initial rotational speed of the particles
      *<x-min-deg-sec>float</x-min-deg-sec>
      *<y-min-deg-sec>float</y-min-deg-sec>
      *<z-min-deg-sec>float</z-min-deg-sec>
      *<x-max-deg-sec>float</x-max-deg-sec>
      *<y-max-deg-sec>float</y-max-deg-sec>
      *<z-max-deg-sec>float</z-max-deg-sec>
    </rotation-speed>
  </shooter>
  <counter>
    <particles-per-sec>
      <VALUEORPROP/> = see note
      *<spread> = the "tolerance" in each direction so values are in the range
                  [value-spread, value+spread]
    </particles-per-sec>
  </counter>
  <particle> = defines the particle properties
    <start>
      <color> = initial color (at time of emission)
        <red><VALUEORPROP/></red> = color component in normalized value [0,1]
        <green><VALUEORPROP/></green>
        <blue><VALUEORPROP/></blue>
        <alpha><VALUEORPROP/></alpha>
      </color>
      <size> = as above, but for size
        <VALUEORPROP/>
      </size>
    </start>
    <end>
      <color> = final color (at the end of the particle life)
        <red><VALUEORPROP/></red>
        <green><VALUEORPROP/></green>
        <blue><VALUEORPROP/></blue>
        <alpha><VALUEORPROP/></alpha>
      </color>
      <size>
        <VALUEORPROP/>
      </size>
    </end>
    *<life-sec> = the time the particles will be alive, in seconds
      <VALUEORPROP/>
    *</life-sec>
    *<radius-m>float</radius-m> = each particles is physically treated as a sphere
                                  with this radius
    *<mass-kg>float</mass-kg> = mass in KG
  </particle>
  <program> = defines external forces acting upon a particle
    <fluid>string<fluid> = can be "air" or "water"
    <gravity>bool</gravity> = can be "true" or "false". uses standard gravity
    <wind>bool</wind> = can be "true" or "false". uses user position wind (not the
                        model position, but shouldn't be noticeable, you want to
                        disabled it when using local attach)
  </program>
</particles>

Remarks:
* Don't forget you can use existing animations with particles, so if you want to
  direct or translate the emitter, just use translate, rotate, spin and so on
  (other animations might have interesting effects too I guess)

* Particle XML should be compatible with plib, as the tags will be ignored (you
  might get some warning if you attach them to animations though)

* Try not to use a lot of particles in a way that fills the screen, that will demand
  lots of fill rate and hurt FPS

* If you don't use any properties nor conditions, your particle system doesn't need
  to use a callback a so it's slightly better on the CPU (mostly useful for static models)

* If your particle lifetime is too big you might run out of particles temporarily
  (still being investigated)

* Use mass and size(radius) to adjust the reaction to gravity and wind
  (mass/size = density)

* Although at the moment severe graphical bugs can be seen in the trails,
  they are usable.

* Consider your options correctly! You should consider giving them no initial
  velocity and most important, no spread, otherwise particles will race and the
  trail will fold. Start simple (no velocities and forces) and work your way up.