Howto:Animate models

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The real world is full of motion. To simulate this in FlightGear, models must be animated.

FlightGear allows you to animate models in response to property changes: for example, the propellers can spin when the engine is on and the elevators can move up and down with your controller. There is no fixed limit on what parts can be animated: the only requirements are that the part is named in the 3D model file, and that there is a property in the main tree that you can use to get the positioning information.

This document provides basic information for all kind of animations. When animating your model, it is very helpful to find an aircraft with parts similar to yours and use it as an example. Cut and paste the code into your wrapper file and then edit to suit.

Notes

File name of main model and animation XML file

1rightarrow.png See Aircraft-set.xml#Not used for loading multiplayer aircraft for the main article about this subject.

The file name of the main model and animation XML file, or the .ac file if there is no XML file, (in essence the property /sim/model/path) will be the name of the aircraft that is transmitted when using multiplayer and will also be used for loading multiplayer aircraft.

There is also a mechanism to substitute a full aircraft model with a simpler AI aircraft model if one is available at the same file path (including for example Models/Boeing-797-800.xml), but in $FG_ROOT/AI/Aircraft/ instead of $FG_ROOT/Aircraft/.

.ac files

1rightarrow.png See AC files: Understanding and changing .ac code#Identifying an object for the main article about this subject.

When referring to an .ac file in your xml animation, it is important that the <object name> exactly matches the object named in the .ac file (this includes cases!).

Note for SketchUp users: The spatial reference X/Y/Z used in animation to locate an object or a point are different from the ones in AC3D ie X values are the same in both but Y in animation must be matched to AC3D's -Z (Z value but opposite sign) and Z value in animation must be matched to AC3D's Y value.

Note for SketchUp users: when exporting to AC3D in Sketchup, the .ac file will name the objects in your model to "blah" by default. You need to amend the relevant object names in your .ac file using text edit, so that the xml will work.

Animation order

Animations are executed by FlightGear in the order that they are read in the model's .xml file. Therefore, it is very important to pay attention to the order, especially when multiple animations are applied to the same object(s).

Tags used in most animations

Name

With a name animation, you can group multiple objects.

 <animation>
  <name>Collection1</name>
  <object-name>Object1</object-name>
  <object-name>Object2</object-name>
  <object-name>Object3</object-name>
 </animation>

The example above creates a "virtual object" with the name Collection1. In animation, we can animate this group of objects, by using:

 <object-name>Collection1</object-name>

Object-name

These names are set in the 3D model. Each single object has a unique name; for easy identification it is advised to use descriptive names (LeftElevator, Rudder etc.). Animations are only applied to those objects that are mentioned in an object-name line (one object per line!). Animations lacking those, will be applied to the entire model.

Property

Each animation must be associated with exactly one property from the main FlightGear property tree (remember that the properties in the wrapper file are not part of the main tree), using <property> to provide the property path:

 <animation>
  <type>rotate</type>
  <object-name>Rudder</object-name>
  <property>controls/rudder</property>
 </animation>

Note the omission of the leading slash '/' when reffering to the property. This assures that when the model is used for AI or multiplayer traffic the animations will follow that of the AI controller instead of that of the user.

Axis

An axis part is required in every animation that involves a rotating or moving thing.

  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>

The axis are similar to the ones of the 3D model. There is a difference between rotation and translation:

  • In rotation animations, the axis part defines around what axis the object rotates. Negative/positive values make the difference between counterclockwise and clockwise rotations.
  • In translate animations, the part defines along what axis the object moves. If the x-axis is poiting backwards, an x-value of -1 will result in forward motion.

You could also define two points, between which FlightGear will calculate the correct axis. This makes the use of a <center> tag redundant! Such coordinates are extremely useful for animating control surfaces (rudder, elevators etc.).

  <axis>   
   <x1-m> 4.9</x1-m>
   <y1-m> 7.1</y1-m>
   <z1-m>-1.0</z1-m>
   <x2-m> 5.9</x2-m>
   <y2-m>11.2</y2-m>
   <z2-m>-0.5</z2-m>
  </axis>

Center

Various animations (rotate, spin, scale) move around a center point.

  <center>
   <x-m>-1.50</x-m>
   <y-m> 1   </y-m>
   <z-m> 0.25</z-m>
  </center>

The axis are similar to the ones of the 3D model, so finding coordinates is easily done in 3D modeling software.

Using a geometry object for axis and centre (2017.2)

Added in V2017.2 is support to allow a geometry object (a line segment with two vertices) that is used to define both the centre and the axis for an animation. This will work with rotate, translate and the knob animations.

The syntax for this is

<axis>
    <object-name>some-object</object-name>
</axis>

Once the object-name used for the axis has been processed the geometry object will be hidden. This also allows a visual check for any axis objects that are not yet assigned.

It is possible to reuse the same object definition multiple times within a single XML file.

Illustration of where an axis object (2017.2) can be placed for a canopy

Illustration of where an axis object (2017.2) can be placed for cockpit elements

Additional tags that can be used in most animations

Conditions

Multiple animations can make use of a conditional. Check $FGDATA/Docs/README.conditions for some more details.

  • equals: property value (or second property) is equal to value/(first)property.
  • greater-than: property value (or second property) is larger than value/(first)property.
  • greater-than-equals: property value (or second property) is greater than or equal to value/(first)property.
  • less-than: property value (or second property) is smaller than value/(first)property.
  • less-than-equals: property value (or second property) is smaller than or equal to value/(first)property.

The example below is true when n1 has a value greater than 25.

  <condition>
   <greater-than>
    <property>engines/engine[1]/n1</property>
    <value>25</value>
   </greater-than>
  </condition>

Then there are some special tags:

  • and:
  • not:
  • or:

In the example below, the condition is true when either n1 is greater than 25% or equal to 0%.

  <condition>
   <or>
    <greater-than>
     <property>engines/engine[1]/n1</property>
     <value>25</value>
    </greater-than>
    <equals>
     <property>engines/engine[1]/n1</property>
     <value>0</value>
    </equals>
   </or>
  </condition>

An example of implementation into an animation looks as follows:

 <animation>
  <object-name>Object</object-name>
  <type>rotate</type>
  <property>suface-positions/left-aileron-pos-norm</property>
  <factor>25</factor>
  <condition>
   <greater-than>
    <property>suface-positions/left-aileron-pos-norm</property>
    <value>10</value>
   </greater-than>
  </condition>
  <center>
   <x-m>-1.50</x-m>
   <y-m> 1   </y-m>
   <z-m> 0.25</z-m>
  </center>
  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>
 </animation>

Interpolation

For non-fixed factors, an interpolation "table" can be created.

  <interpolation>
   <entry>
    <ind> 0.0</ind>
    <dep> 0.0</dep>
   </entry>
   <entry>
    <ind> 0.667</ind>
    <dep> 0.0</dep>
   </entry>
   <entry>
    <ind> 1.0</ind>
    <dep> 0.5</dep>
   </entry>
  </interpolation>

The lines above represent the following table:

Input Output
0.0 0.0
0.667 0.0
1.0 0.5

You can add as many entries as you need. Interpolation tables are often used for gear animations (eg. to open doors during gear-movements and close them again once the gear is either retracted or fully extended).

Expressions

For some animations it is possible to define complex animations by using Expressions. This even allows to drive the animation from multiple properties without the need for additional Nasal scripts. Here is an example for a translate animation depending on two properties and the cosine function:

 <animation>
     <type>translate</type>
     <expression>
       <product>
         <property>/my/factor-property</property>
         <cos>
           <deg2rad>
             <property>/my/angular-property</property>
           </deg2rad>
         </cos>
       </product>
     </expression>
     [..]more elements[..]
 </animation>

Animations which can utilize Expressions are:

See more detailed info at Expressions

Object animations

Alpha-test

 <animation>
  <type>alpha-test</type>
  <object-name>Object</object-name>
  <alpha-factor>0.01</alpha-factor>
 </animation>

This "animation" is a way to set an alpha test on a model branch. The effect is to avoid depth buffer writing of pixel that are not seen because they are transparent. This is particulary useful when modeling a metallic structure or a tree with a billboard. The threshold of transparency is set with the <alpha-factor> element. See also Pixel testing in effects.

Blend

Blends an object with the surrounding. Comparable to a translucency animation. A value of 0 corresponds to no transparency, i.e. and ordinary solid object, and a value of 1 makes the object fully transparent, i.e., not visible at all.

 <animation>
  <type>blend</type>
  <property>/velocities/airspeed-kt</property>
  <factor>0.00025</factor>
  <min>0.2</min>
  <max>0.7</max>
 </animation>

Note that when using the Project Rembrandt renderer, all transparent and translucent objects must be registered to display properly. More information here.

Billboard

This faces an object towards the viewer. Often used on 2D objects, like clouds, trees and lights.

 <animation>
  <type>billboard</type>
  <object-name>Object</object-name>
  <spherical type="bool">true</spherical>
 </animation>
  • spherical:

Dist-scale

Used to scale an object, based on the distance to the viewer.

 <animation>
  <type>dist-scale</type>
  <object-name>Object</object-name>
  <interpolation>
   <entry>
    <ind>0</ind>
    <dep>1</dep>
   </entry>
   <entry>
    <ind>300</ind>
    <dep>4</dep>
   </entry>
   <entry>
    <ind>1500</ind>
    <dep>8</dep>
   </entry>
  </interpolation>
 </animation>

You can optionally add <center> coordinates, to scale the object around that point.

Flash

Used to scale an object based on the cosine of the angle between the axis provided in the animation and the view vector.

 <animation>
  <type>flash</type>
  <object-name>Object</object-name>
  <offset>0.0</offset>
  <factor>1.0</factor>
  <power>2</power>
  <two-sides type="bool">false</two-sides>
  <min>0.0</min>
  <max>1.0</max>
  <center>
   <x-m>0.0</x-m>
   <y-m>0.0</y-m>
   <z-m>0.0</z-m>
  </center>
  <axis>
   <x>0.0</x>
   <y>-1</y>
   <z>0.1</z>
  </axis>
 </animation>
  • offset:
  • factor:
  • power:
  • two-sides: if false, nothing is drawn if the cosine is negative.
  • min:
  • max:

scale = factor * pow( cosine, power ) + offset

scale is then clamped between min and max.

and this scale factor is applied to the object, from the center specified. It works best if scale is less than 1. Otherwise, there will be clipping issues.

Noshadow

This animation is used to make sure an object will cast no shadow.

 <animation>
  <type>noshadow</type>
  <object-name>Object</object-name>
 </animation>

Range

See also Modeling - Getting Started#Level of Detail (LOD).

To prevent objects -like instruments- being drawn when the aircraft is actually too far away for them to be seen anyway, a range animation is used.

 <animation>
  <type>range</type>
  <min-m>0</min-m>
  <max-m>30</max-m>
 </animation>
  • min-m: the shortest distance (in meters) from the object center at which it is visible.
  • max-m: the largest distance (in meters) from the object center at which it is visible.

You could also use the generic level of detail (LOD) properties, which can be set by the user through View > Adjust LOD rangers:

Property Description Default value
/sim/rendering/static-lod/bare only a rough exterior model 30,000 m
/sim/rendering/static-lod/rough most should be visible 9,000 m
/sim/rendering/static-lod/detailed all details should be visible 1,500 m

The animation code will look like this:

 <animation>
  <type>range</type>
  <min-m>0</min-m>
  <max-property>sim/rendering/static-lod/bare</max-property>
 </animation>

You can have both ranges (max and min) bound to a property, or just one of them.

Rotate

One of the most important and frequently used animations of all. It rotates an object to an absolute position in degrees, as provided by the property-value.

 <animation>
  <object-name>Object</object-name>
  <type>rotate</type>
  <property>surface-positions/left-aileron-pos-norm</property>
  <factor>25</factor>
  <offset-deg>25</offset-deg>
  <center>
   <x-m>-1.50</x-m>
   <y-m> 1   </y-m>
   <z-m> 0.25</z-m>
  </center>
  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>
 </animation>
  • factor: is optional.
  • offset-deg: is optional. Offset in degrees.
  • expression: is optional. For more details see Expressions

Scale

A scale animation scales (resizes) an object. This can be either property-value dependant (first example) or a fixed scale (second example).

 <animation>
  <type>scale</type>
  <object-name>Object</object-name>
  <property>sim/time/sun-angle-rad</property>
  <x-min>1.0</x-min>
  <y-min>1.0</y-min>
  <z-min>1.0</z-min>
  <x-factor>1.4</x-factor>
  <y-factor>1.4</y-factor>
  <z-factor>2.0</z-factor>
 </animation>
  •  ?-min: the mimimum scale factor for each axis. If the property value would result in a smaller factor than this setting, the scale animation will hold.
  •  ?-factor: the scale factor for each axis (factor*property=scale factor).
 <animation>
  <type>scale</type>
  <x-offset>0.5</x-offset>
  <y-offset>0.5</y-offset>
  <z-offset>0.5</z-offset>
 </animation>
  • x.offset: the scale factor.
  • Add <center> coordinates, to scale the object around that point.
  • You can optionally use an expression in the <factor> or <offset> inputs. For more details see Expressions

Select

This animation selects (or unselects) objects when certain conditions are true (or false). The example below shows the object when the n1 of engine[1] is higher than 25%.

 <animation>
  <object-name>Object</object-name>
  <type>select</type>
  <condition>
   <greater-than>
    <property>engines/engine[0]/n1</property>
    <value>25</value>
   </greater-than>
  </condition>
 </animation>

Shader

 <animation>
  <type>shader</type>
  <shader>chrome</shader>
  <texture>chrome2.png</texture>
  <object-name>Object</object-name>
 </animation>
  • shader:
  • texture: path to the texture used by the shader.

Spin

Very similar to rotate, but the property provides a value in revolutions per minute (RPM) rather than an absolute position in degrees, and offset cannot be used.

 <animation>
  <object-name>Object</object-name>
  <type>spin</type>
  <property>engines/engine[0]/n1</property>
  <factor>25</factor>
  <center>
   <x-m>-1.50</x-m>
   <y-m> 1   </y-m>
   <z-m> 0.25</z-m>
  </center>
  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>
 </animation>
  • factor: is optional.

Timed

Swtiches between objects at specified intervals. This example switches between a lights-on model and a lights-off model. Lights on are shown 0.2 seconds, while lights off are displayed for 0.8 seconds.

 <animation>
  <type>timed</type>
  <object-name>BacklightOn</object-name>
  <object-name>BacklightOff</object-name>
  <use-personality type="bool">true</use-personality>
  <branch-duration-sec>0.8</branch-duration-sec>
  <branch-duration-sec>0.2</branch-duration-sec>
 </animation>

Tracking

The new (in 2.11) locked-track animation can do exactly the same thing as the Locked Track constraint available in Blender. However it can also be used to simulate simple inverse kinematic systems consisting of two bones connected with a revolute joint (aka hinge). See detailed explanantion

Translate

The same as textranslate, but this animation moves a whole object (so including fixed textures). The example below will move an object in the y-direction:

Property value Output
-1 -2.5
0 2.5
1 7.5
 <animation>
  <type>translate</type>
  <object-name>Object</object-name>
  <property>controls/seat/pilot/position-norm</property>
  <factor>5</factor>
  <offset-m>2.5</offset-m>
  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>
 </animation>

When using interpolation tables such as:

<interpolation>
	<entry><ind>0.0</ind><dep> 0</dep></entry>
	<entry><ind>0.666</ind><dep>0.18</dep></entry>
	<entry><ind>1.0</ind><dep>0.27</dep></entry>
</interpolation>

The first figure (<ind> refers to the value of the property associated with the object and the second figure (in <dep>) refers to the amount that the object moves in metres. For example, in this case, the object moves 18cm when the value reads 0.66 and 27cm when the value is 1.

IF used WTIHOUT property : The object is placed at some (Value of Offset) meters away from its original position, along the virtual axis formed by said original position and the point with coordinates x/y/z defined in the <axis> tag. Mathematically, assuming the Object to translate is (in the model space) placed at point A (x1, y1, z1) and you want to relocate it to point B (x2, y2, z2) then x2,y2,z2 are the values in the <axis> tag of the Translate animation and <offset> can be computed as SQRT((x2-x1)^2+(y2-y1)^2+(z2-z1)^2)


  • factor: is optional.
  • offset-m: is optional. Offset in meters.
  • expression: is optional. For more details see Expressions

Material animation

An animation type that can be used in various ways. Of course you can combine the below mentiond systems into one (big) animation.

 <animation> 
  <type>material</type> 
  <object-name>Object</object-name>
  <property-base>sim/model/c172p/material</property-base>
  <global type="bool">true</global> <!-- This tag is no longer supported -->
  ...
  lines as mentioned below
  ...
 </animation>

Optional:

  • property-base: when using prop(erties), you might want to set a property-base. All props will be relative to this path.
  • global (depreciated): by setting this to true, all objects using the same material as the defined object(s) (via <object-name>) will be affected by the animation. This is preferred to listing several objects in <object-name> tags. It's not only faster, but also doesn't break animations by forcing objects together. This tag is no longer supported

Notes:

  • Numbers are clamped to 0.0-1.0, except "shininess", which is clamped to 0-128.
  • By appending -prop each of the material properties can read its value from another property.

Ambient

  <ambient>
   <red>1.0</red>
   <green>0.2</green>
   <blue>0.0</blue>
  </ambient>

Diffuse

  <diffuse>
   <red>1.0</red>
   <green>0.2</green>
   <blue>0.0</blue>
  </diffuse>

Emission

1rightarrow.png See Howto: Illuminate faces for the main article about this subject.

  <emission>
   <red>1.0</red>
   <green>0.2</green>
   <blue>0.0</blue>
   <factor-prop>controls/lighting/panel-norm</factor-prop>
  </emission>

Emission colors are multiplied by the factor-prop value. 1 is maximum color intensity, while 0 is the minimum. Colors are calculated according to the RGB color model.

Shininess

Shininess is clamped to 0-128.

  <shininess>105</shininess>

Specular

  <specular>
   <red>1.0</red>
   <green>0.2</green>
   <blue>0.0</blue>
  </specular>

Texture

Used for the Livery over MP system.

  <property-base>sim/model/livery</property-base> 
  <texture-prop>engine</texture-prop> 
  <texture>KLM.png</texture>

Transparency

  <transparency>
   <alpha-prop>rotors/tail/rpm</alpha-prop>
   <factor>-0.0015</factor>
   <offset>1</offset>
  </transparency>

Threshold

  <threshold>0.001</threshold>

Texture Animations

Applying different matrix transformations to the textures of an object.

Textranslate

A very important animation for cockpits! This animation moves textures over a surface.

 <animation>
  <type>textranslate</type>
  <object-name>Object</object-name>
  <property>autopilot/settings/target-speed-kt</property>
  <bias>0.0001</bias>
  <factor>0.001</factor>
  <step>100</step>
  <axis>
   <x>0</x>
   <y>1</y>
  </axis>
 </animation>
  • bias: Adds an offset to the property before factor/step. A small value is needed to compensate for floating point accuracy.
  • factor: property * factor * texture width/height = the amount of pixels that the texture should be translated. If your texture is 256 pixels, an textranslate of 0.1 will result in the texture moving with 26 pixels, into the direction specified by the axis settings.
  • step: the step size at which the texture is translated. If this is set to 0.1, the texture will only be translated at 0.1, 0.2, 0.3 etc.
  • axis: the direction in which the texture is translated. Y is up/down, while X is left/right.

Texrotate

<animation>
  <object-name>Object</object-name>
  <type>texrotate</type>
  <property>some/property/path</property>
  <factor>25</factor>
  <offset-deg>25</offset-deg>
  <center>
    <x>0.5</x>
    <y>0.5</y>
    <z>0</z>
  </center>
  <axis>
    <x>0</x>
    <y>0</y>
    <z>1</z>
  </axis>
</animation>

Textrapezoid

<animation>
  <type>textrapezoid</type>
  <object-name>HUD.l.canvas</object-name>
  <property>/hud/trapezoid-correction</property>
  <side>bottom</side>
</animation>
  • side: side of quad which should be scaled (top (default)/right/bottom/left)

Texmultiple

Only one texture matrix can be applied to each object. With textmultiple multiple texture animations can be combined into a single matrix, applied to the specified object.

<animation>
  <type>texmultiple</type>
  <object-name>HUD.l.canvas</object-name>
  <transform>
    <subtype>textranslate</subtype>
    <property>/hud/offset-x</property>
    <axis>
     <x>1</x>
     <y>0</y>
     <z>0</z>
   </axis>
  </transform>
  <transform>
    <subtype>textranslate</subtype>
    <property>/hud/offset-y</property>
    <axis>
     <x>0</x>
     <y>1</y>
     <z>0</z>
   </axis>
  </transform>
  <transform>
    <subtype>textrapezoid</subtype>
    <property>/hud/trapezoid-correction</property>
  </transform>
</animation>

Object interaction animations

Enable-hot

Scenery objects are automatically defined as solid by FlightGear, meaning that an aircraft can taxi on them and/or crash when touching. For certain objects (groundmarkings, beacon light-beams etc.) this might be an unwanted feature. The solidness can be disabled with the following animation:

 <animation>
  <object-name>Object</object-name>
  <enable-hot type="bool">false</enable-hot>
 </animation>
  • enable-hot: can be either true or false. Remember that objects are automatically solid, so it should not be necessary to set this at all when wanting solidness.

Interactions

 <animation> 
  <type>interaction</type> 
  <object-name>Object</object-name> 
  <interaction-type>carrier-wire</interaction-type> 
 </animation>
  • interaction-type: can have the following values:
    • carrier-catapult:
    • carrier-wire: makes the object act as an arresting wire, as used on aircraft carriers.

Direct manipulation animations

Knob / slider (v. 2.11-)

1rightarrow.png See Knob / slider animation for the main article about this subject.

Pick

1rightarrow.png See Howto: Make a clickable panel#Pick for the main article about this subject.

Shadow Handling

There exist several possibilites for handling of shadows.
See ALS Technical Notes and more specific Fuselage Shadow Effect with ALS for a relatively simple shadow handling.
See Project Rembrandt which - amongst other functionality - implements a very realistic shadow mapping.

References

References