Property Root

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Warning

This documentation is currently under development! It should not be relied upon at this time.

Property Root#

Each model has a property root. When the model refers to a property, e.g.

<animation>
  ...
  <property>path/to/property</property>
  ...
</animation>

the path/to/property is relative to the model property root.

The model property root is usually part of FlightGear’s global property tree. For example:

  • For the main aircraft model, the property root is the root / of the property tree itself.

  • For an AI or multiplayer model, the property root is /ai/models/<model_name>/.

Be careful not to confuse the property root with the model XML file. Both are property trees, but with very different roles:

  • The model property root usually lives in the global property tree. Properties from the model XML file never do.

  • Any <property> tag or similar in the model file refers to a property path relative to the property root.

  • Any alias="path" attribute refers to a property path in the model XML file.

Be also careful of relative vs absolute property paths. This is a relative path (relative to the model property root):

<property>path/to/property</property>

This is an absolute path (due to the leading /), it refers to the global property tree:

<property>/path/to/property</property>

If you have any doubt, use only relative paths in XML model files. Absolute paths are rarely correct: the model property root exists for a reason, and you should use properties in it.

Example#

This is a typical animation for the elevator in an aircraft model:

<animation>
  <type>rotate</type>
  <object-name>elevator</object-name>
  <property>surface-positions/elevator-pos-norm</property>
  <factor>30</factor>
</animation>

When flying this aircraft, the model is loaded with property root /, so the elevator is animated by the property /surface-positions/elevator-pos-norm. In this case, there is no difference between relative and absolute paths.

When this aircraft is loaded as multiplayer, the model is loaded with property root e.g. /ai/models/multiplayer[0]/, so the elevator is animated by the property /ai/models/multiplayer[0]/surface-positions/elevator-pos-norm. This property is transmitted over multiplayer, and the animation will work correctly.

If an absolute path had been used instead, then it is still /surface-positions/elevator-pos-norm which would have been used for the multiplayer aircraft animation. This is the elevator position for the main aircraft, not the multiplayer one!

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