Particle effects : snow and rain
  by Nicolas VIVIEN
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							Patch v4 (only test)


FEATURES :
----------
This first try permits to add a basic snow and rain effects in using
particle from OSG.

For the moment, the patch uses the METAR informations to enable / disable
rain or snow effects. (intensity of effects is : low, meddium, high)


CHANGELOG v4 :
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- Follow the Tim'advice, Precipitation object has to link to each tile... but to the scene.
- First test with wind...


CHANGELOG v3 :
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- The origin of Precipitation object is calculate with worldcoordinate Matrix.
- Precipitations fall vertically anywhere on the earth...


CHANGELOG v2 :
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- Link the precipitation object to tile element :
Now, there is one precipitation object per tile. It permits to use the world coordinate.
- Add links between precipitation and cloud layers :
When you are above the cloud layer, the precipitations are stopped.


CHANGELOG v1 :
--------------
Firt step : test the precipitation effect.


TO DO :
-------
For the next release of patch :
1) FIXME wind effect
2) FIXME origin object
3) it's raining cats and dogs in my plane !!! (fixed this issue)
4) Precipitation density depends on altitude. (rain becomes snow...)


INSTALL :
---------

- You have to remove the others patch about precipitation.

- You have to add the two files :
precipitation.hxx and precipitation.cxx 
in the directory simgear/simgear/environment

- You have to apply simgear patch

- You have to add the two files :
precipitation_mgr.hxx and precipitation_mgr.cxx 
in the directory flightgear/src/Environment

- You have to apply flightgear patch

- Then, build simgear and flightgear.

