FlightGear Flight Simulator

3. Animating the Model

Now for the interesting part. FlightGear allows you to animate models by having parts rotate or spin 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.

Currently, there are several types of animation recognized:

none
billboard
rotate
scale
blend
select
spin
timed
translate
texrotate
textranslate
textmultiple
material
range
alpha-test
noshadow

Typically you will use combinations of rotate, spin, and translate to animate the main control surfaces for most standard-configuration aircraft. none is a no-op; spin rotates the object around an axis with a known rotational velocity (not worrying about the exact position), and rotate rotates the object around an axis to an exact position.

Every animation appears inside an animation element, and contains a type property and at least one object-name property:

<animation>
 <type>rotate</type>
 <object-name>Rudder</object-name>
</animation>

The object name must match exactly the object name used in the 3D file (including case). You may include more than one object name to apply the same transformation to more than one object, assuming that they rotate around exactly the same line:

<animation>
 <type>rotate</type>
 <object-name>LeftElevator</object-name>
 <object-name>RightElevator</object-name>
</animation>

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 name:

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

This example rotates the rudder according to the value of the /controls/rudder property (or at least, it will when we specify the center and axis of rotation below); however, /controls/rudder is normalized from -1.0 to 1.0, and we probably want to rotate the rudder more than that; as a result, we need to use the factor property to do scaling. For example, if the rudder on the actual aircraft rotates 18 degrees in each direction, we would use a factor of 18 to scale the rudder position from -18 degrees to 18 degrees:

<animation>
 <type>rotate</type>
 <object-name>Rudder</object-name>
 <property>/controls/rudder</property>
 <factor>18</factor>
</animation>

If you wanted to reverse the direction of rudder movement, you would use a factor of -18.

There is also an offset property that can be useful for starting the rotation from a point other than center. For example, let's say that you want the rudder to start 1% to the left rather than dead center; you could specify that like this:

<animation>
 <type>rotate</type>
 <object-name>Rudder</object-name>
 <property>/controls/rudder</property>
 <offset>-0.01</offset>
 <factor>18</factor>
</animation>

The offset is applied after the factor.

Finally, there are min and max properties that can constrain the amount of rotation in degrees, as in this (very complicated) example for the Cessna 310 landing gear:

<animation>
 <type>rotate</type>
 <object-name>NoseWheel</object-name>
 <property>/gear/gear[0]/position-norm</property>
 <factor>120</factor>
 <offset>-1</offset>
 <min>-90</min>
 <max>0</max>
 <center>
  <x-m>-2.28</x-m>
  <y-m>0.0</y-m>
  <z-m>-0.65</z-m>
 </center>
 <axis>
  <x>0</x>
  <y>1</y>
  <z>0</z>
 </axis>
</animation>

In this example, the gear position (from 0.0 for fully retracted to 1.0 for fully extended) is multiplied by a factor of 120 and an offset of -1, then clamped to between -90 and 0. In the 3D model, the gear is extended by default, so we end up with the following rotations through the gear's range of movement:

position-norm	+ offset -1	* factor 120	clamped to min/max -90:0	comments
0.0	-1.0	-120.0	-90.0	fully retracted
0.25	-0.75	-90.0	-90.0	still retracted
0.5	-0.5	-60.0	-50.0	1/3 extended
0.75	-0.25	-30.0	-30.0	2/3 extended
1.0	0.0	0.0	0.0	fully extended (default position)

The gear does not move at all during the first 1/4 of the position-norm value, giving the doors a chance to open and close in a separate animation. Obviously, this would be easier to manage with an interpolation table, and future versions of the model animation will likely support interpolation.

Update: Interpolation tables our now supported. Each table entry plots a dependency point and output values are interpolated in a linear manner between two points. In the following example an input property value of 25 will result in an application of a value 65 to the animation (65 degrees rotation maybe):

 <interpolation>
   <entry>
    <ind>0</ind>
    <dep>0</dep>
   </entry>
   <entry>
    <ind>16.67</ind>
    <dep>60</dep>
   </entry>
   <entry>
    <ind>25</ind>
    <dep>65</dep>
   </entry>
   <entry>
    <ind>33.3333</ind>
    <dep>70</dep>
   </entry>
   <entry>
    <ind>66.6667</ind>
    <dep>71</dep>
   </entry>
   <entry>
    <ind>100</ind>
    <dep>75</dep>
   </entry>
  </interpolation>

For a spin animation, the property provides a value in revolutions per minute (RPM) rather than an absolute position in degrees, and offset is not used. You can still use factor to scale the property value if it is not in RPM.

Now, it is necessary to specify the axis of rotation for the object, its virtual hinge. This is often the hardest part, requiring a lot of trial-and-error when the axis of rotation is not lined up with the x-, y-, or z- axis (think of ailerons on a swept wing with a non-zero dihedral angle). You need to provide two groups of information: a point through which the axis of rotation passes, and the direction in which the axis is moving.

Update: there is now an alternate method for specifying axes that avoids the trial-and-error approach. See below after the Cessna 172 animation examples.

For the point through which the axis passes, you use the /center/x-m, /center/y-m, and /center/z-m properties to specify a position in meters, using the aircraft's coordinate system. Note that this is the system before repositioning: if the original model was pointing sideways, then your fuselage will run along the y-axis rather than the x-axis. Here is an example for a rudder:

<animation>
 <type>rotate</type>
 <object-name>Rudder</object-name>
 <property>/controls/rudder</property>
 <factor>18</factor>
 <center>
  <x-m>5.45</x-m>
  <y-m>0.0</y-m>
  <z-m>0.0</z-m>
 </center>
</animation>

In this example, the point is right on the y and z axes, but 5.45 meters along the x axis (i.e. towards the back of the plane).

Finally, in addition to the center point, it's necessary to specify the direction of the axis that passes through it, using the properties /axis/x, /axis/y, and axis/z. These are unitless values showing the rate of change in each direction; for example, a straight up-and-down rotational axis could be specified like this:

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

or like this:

<axis>
 <x>0</x>
 <y>0</y>
 <z>1000000</z>
</axis>

Since there's 0 movement along the other two axes, it doesn't matter. On the other hand, let's say that the rudder hinge sloped back 5% because of a swept tail. In that case, the x-axis would have to show some movement as well:

<axis>
 <x>0.05</x>
 <y>0</y>
 <z>1</z>
</axis>

That could just as easily be written as

<axis>
 <x>1</x>
 <y>0</y>
 <z>20</z>
</axis>

as long as the ratios are the same. For a complicated rotation axis, like that for an aileron on a swept-back wing with a significant dihedral angle, you'll have to specify movement along all three axes:

<axis>
 <x>0.15</x>
 <y>1.00</y>
 <z>0.01</z>
</axis>

Here's a complete example, showing the animation for the rudder on the Cessna 172:

<animation>
 <type>rotate</type>
 <object-name>Rudder</object-name>
 <property>/controls/rudder</property>
 <factor>18</factor>
 <center>
  <x-m>5.45</x-m>
  <y-m>0.0</y-m>
  <z-m>0.0</z-m>
 </center>
 <axis>
  <x>0.72</x>
  <y>0.0</y>
  <z>1.0</z>
 </axis>
</animation>

And here's an example of a spin, using the Cessna 172 propeller:

<animation>
 <type>spin</type>
 <object-name>Propeller</object-name>
 <property>/engines/engine[0]/rpm</property>
 <center>
  <x-m>0</x-m>
  <y-m>0</y-m>
  <z-m>-.25</z-m>
 </center>
 <axis>
  <x>1.0</x>
  <y>0.0</y>
  <z>0.0</z>
 </axis>
</animation>

There is an alternate method of specifying the axis that can often be find much easier. This method is particularly helpful for rotating on axes that are at odd angles, such as ailerons on dihedral swept wings.

It is not necessary to specify a <center> tag for this method as it will be calculated automatically. The <axis> tag requres two sets of coordinates (x1-m,y1-m,z1-m and x2-m,y2-m,z2-m), one for each end of the axis. With an aileron, for example, you would need to specify coordinates from both the right and left corner of the aileron along the hinged edge. If your object rotates in the direction opposite of what you intended you may correct it by simply swapping the two sets of coordinate values (swap x1 and x2, y1 and y2, z1 and z2).

Here is an example from the 747-400:

<animation>
  <type>rotate</type>
  <object-name>AileronLeftOuter</object-name>
  <property>/surface-positions/left-aileron-pos-norm</property>
  <factor>-30</factor>
  <axis>
   <x1-m>18.28</x1-m>
   <y1-m>-21.55</y1-m>
   <z1-m>-0.37</z1-m>
   <x2-m>22.51</x2-m>
   <y2-m>-28.37</y2-m>
   <z2-m>0.08</z2-m>
  </axis>
 </animation>

You can get a lot of your initial measurements by viewing the model in a 3D editor like PPE, and you can also use PPE to name or rename objects so that you can animate them in FlightGear. In the end, though, you'll almost have to do a little tweaking by trial and error until everything looks right.

This document will likely be out of date by the time you read it. Look at the actual XML wrapper files (currently in $FG_ROOT/Aircraft/aircraft-type/Models/) to look at how FlightGear is doing things now. In the future, we'll be adding other animation types, including selecting among different versions of the same object (such as a translucent propeller disk for high RPM), non-rotational transformations, scaling, and conditionals (i.e. draw engine exhaust only above a certain velocity).

"scale" animation type

Here is a contrived example of one way to use the "scale" animation type. In this case a shadow map is placed below the aircraft and we can control the size based on some property value (i.e. alitutude above the ground.) In real life, the shadow size should stay pretty much fixed, so this technique might ultimately be more useful for a landing light.

 <animation>
  <type>scale</type>
  <object-name>ShadowMap</object-name>
  <property>/position/altitude-agl-ft</property>
  <x-factor>0.05</x-factor>
  <x-offset>1.0</x-offset>
  <y-factor>0.05</y-factor>
  <y-offset>1.0</y-offset>
  <z-factor>0.0</z-factor>
  <z-offset>1.0</z-offset>
 </animation>

As with the other animations, you need to specify an object-name and the property that will drive this animation. And, as with other types of transform animations, the order they are applied is important. For a given object, you will probably want to put the scaling animation after any other translate or rotate animations.

You have some individual control over the X, Y, and Z axis scaling. For each dimension you can provide factors (x-factor, y-factor, and z-factor) and also offsets (x-offset, y-offset, and z-offset.) You can also specify minimum values (x-min, y-min, z-min) and maximum values (x-max, y-max, and z-max) for scaling results.

The amount of scaling for each dimension is computed by the following formula (and then clamped if a min or max value is specified.)

  scale = offset + property * factor

Specifying an offset of 1.0 and a factor of 0.0 is equivalent to no scaling at all.

As you can see in the above example, at ground level (the property value = 0.0) the scaling formula will evaluate to 1.0 (no scaling.) As altitude above the ground increases, the scaling factor will increase by 1.0 + agl * 0.05. Note that because a shadow has no depth we use a z-factor of 0.0 and a z-offset of 1.0 so the formula always evalutes to a scaling factor of 1.0 in the Z dimension.

There is still a plib issue to be aware of when using scale animation. When rendering an object, the plib bounding sphere culling system isn't aware of the scale transformation so it will do the cull based on the object's original size. If you have made the object bigger, it will disappear if the space occupied by the original size of the object goes out of the view frustum. If you make the object smaller then this won't be a problem because the larger original object will always contain it's currently smaller version. So one work around would be to build the object at it's maximum size and then use the scaling transform to always make it smaller.

"blend" animation type

The blend animation type can be used to adjust the transparency of an object. To controll it's behavior it is possible to define:

property use the value of this property in the calculation

offset offset, ranges from 0.0 to 1.0

factor multiplication factor for the property

The formula used is:

alpha = property * factor + offset

The result should be between 0.0 and 1.0 where 0.0 equals to a fully visible model and 1.0 equals to a fully transparent model.

To gain greater controll over the result it is possible to limit the result by defining a <min> and/or a <max> parameter.

Note: This effect will only work if there is already a transparent component in the object, either in the vertex color or in the texture.

"texrotate" texture rotation animation type

This animation will adjust the texture mapping on an object so that the texture image appears to rotate about the specified center on the object. Note that when talking about texture mapping, meaningful values are from 0.0 ~ 1.0. Texture coordinates are commonly specified as (u,v). A square object's lower left corner would generally be specified as (u=0,v=0) and the upper right corner as (u=1,v=1). The center will be (u=0.5, v=0.5). These values will be vary (within the 0.0 ~ 1.0 range) if only a portion of a texture is mapped (see compass rose example below).

For the purpose of specifying the <center> tag you need only use the x and y coordinates. The <axis> tag usually only requires a value for z. It is possible to obtain some unusual texture mapping effects, like as giving the illusion of perspective on a flat stationary surface by using the other axes. Here is an example of a rotating compass rose on the 747-400 PDF (primary flight display):

 <animation>
  <type>texrotate</type>
  <object-name>rose</object-name>
  <property>/orientation/heading-magnetic-deg</property>
  <center>
   <x>0.75</x>
   <y>0.75</y>
   <z>0.0</z>
  </center>
  <axis>
   <x>0</x>
   <y>0</y>
   <z>-1</z>
  </axis>
 </animation>

The center in this example is (0.75, 0.75) because the compass rose only occupies the upper right corner of the total texture image that it is contained in. The other three quardrants have other textures for the same model. As mentioned before, the coordinates are (0,0) lower left to (1,1) upper right, the center of the upper right quadrant is thus (0.75, 0.75).

"textranslate" texture translation animation type

This animation will adjust the texture mapping on an object so that the texture image appears to slides around on the object. An <axis> tag is required to specify the axis along which the texture is moved (the direction of movement). The following example displays the right most digit in the altitude indicator on the primary flight display:

 <animation>
  <type>textranslate</type>
  <object-name>alt1</object-name>
  <property>/position/altitude-ft</property>
  <factor>0.01</factor>
  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>
 </animation>

There are also some special properties available for use in the texture animations. These are:

step
scroll

The <step> property is useful if you want animations to happen in steps rather than smoothly. It causes values that are fractional (between) steps size to be ingnored. It can be used for things like changing digits on nurmeric displays or a ticking second hand on a clock. The <scroll> property works with the <step> property. The scroll value specifies the distance the input property value should be from a step when scrolling to the next position begins. This type of scrolling is common on EFIS cockpit displays. The following example gives an "odometer effect" where the hundreds position digit doesn't move until the input value is within is within 1 (e.g. > 99):

<animation>
  <type>textranslate</type>
  <object-name>asi3</object-name>
  <property>/velocities/airspeed-kt</property>
  <factor>0.001</factor>
  <step>100</step>
  <scroll>1</scroll>
  <axis>
   <x>0</x>
   <y>1</y>
   <z>0</z>
  </axis>
 </animation>

"textmultiple" type, combining texture animation types

The textmultiple animation type isn't a real animation type, but instead it is a method of combining multiple texture tansform operations on the same object. Unlike object vertex operations, texture operations can not be stacked (a plib limitation), which means that if you configure two or more texture animation tags on the same object, only the first in the list will be used. By listing operations using texmultiple they are combined together in Simgear and then handled as a single operation.

If you need to combine together a translation and rotation this is the only way to do it currently. Note that transform operations are applied in the same order that they are listed in the configuration file. The following example for the horizon texture from the 747-400 primary flight display explains the format clearly:

<animation>
  <type>texmultiple</type>
  <object-name>horizon</object-name>
  <transform>
    <property>/orientation/pitch-deg</property>
    <subtype>textranslate</subtype>
    <factor>0.0045</factor>
    <axis>
     <x>0</x>
     <y>1</y>
     <z>0</z>
   </axis>
  </transform>
  <transform>
    <property>/orientation/roll-deg</property>
    <subtype>texrotate</subtype>
    <center>
     <x>0.50</x>
     <y>0.50</y>
   </center>
    <axis>
     <x>0</x>
     <y>0</y>
     <z>-1</z>
   </axis>
  </transform>
 </animation>

"range" animation type

The range type is used to select part of a model using its distance from the viewer as the criterion. To be viewed, the model must be between a minimum distance and a maximum one. Minimum distance can be specified either with the <min-m> element or the <min-property> element that are exclusive. The first one allow to specify a fixed distance in meters, the latter allow to lookup the value from a property. Both can be multiplied by a fixed amount, specified in the <min-factor> element. The same apply for the maximum value with <max-m>, <max-property> and <max-factor> elements. The example below shows how it can be used :

<animation>
  <type>range</type>
  <object-name>Detailed</object-name>
  <min-m>0</min-m>
  <max-property>/sim/rendering/static-lod/detailed</max-property>
</animation>

"alpha-test" animation type

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 like this :

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

"noshadow" animation type

This "animation" prevents the shadow casting code to include this object, which might be handy when it doesn't cast a shadow (exhaust flame) or when it looks ugly.

<animation>
 <type>noshadow</type>
 <object-name>Wires.1</object-name>
 <object-name>Wires.2</object-name>
</animation>

"material" animation type

This "animation" can set any of the material properties on a model branch, including the texture file path. The following minimalistic example animation allows to change the "panel" object's emissive color from (0,0,0) to (1,.2,0) by setting the factor property to values between 0.0 and 1.0.

<animation>
  <type>material</type>
  <object-name>panel</object-name>
  <emission>
    <red>1.0</red>
    <green>0.2</green>
    <blue>0.0</blue>
    <factor-prop>/controls/lighting/instruments-norm</factor-prop>
  </emission>
</animation>

Changes made with this animation type are by default only effective for the objects listed in <object-name> tags, whereby you can use more than one of those. The next example, however, sets the optional <global> property, so that changes affect all objects that share the same material. This is the preferred method and should be used whereever possible. It isn't only faster, but also doesn't break other animations by forcing objects into the same branch. Note that all material properties can be set to fixed values, for instance <red> or, by appending "-prop", to the contents of another property node: <red-prop>. Because a "material" animation can contain a lot of such property paths, which is a nuisance to write, hard to read, and a potential source for typos, there's a <property-base> component. Its string value is prepended to all material property names (but not to property paths in a <condition> statement!). The following example shows all available elements:

<animation>
  <type>material</type>
  <object-name>fuselage</object-name>
  <condition>
    <property>/sim/model/foo/animate-fuselage-material</property>
  </condition>
  <global type="bool">true</global>
  <property-base>/sim/model/foo/material/fuselage</property-base>
  <diffuse>
    <red-prop>diffuse/red</red-prop>
    <green-prop>diffuse/green</green-prop>
    <blue-prop>diffuse/blue</blue-prop>
    <factor-prop>diffuse/factor</factor-prop>
    <offset-prop>diffuse/offset</offset-prop>
  </diffuse>
  <ambient>
    <red-prop>ambient/red</red-prop>
    <green-prop>ambient/green</green-prop>
    <blue-prop>ambient/blue</blue-prop>
    <factor-prop>ambient/factor</factor-prop>
    <offset-prop>ambient/offset</offset-prop>
  </ambient>
  <emission>
    <red-prop>emission/red</red-prop>
    <green-prop>emission/green</green-prop>
    <blue-prop>emission/blue</blue-prop>
    <factor-prop>emission/factor</factor-prop>
    <offset-prop>emission/offset</offset-prop>
  </emission>
  <specular>
    <red-prop>specular/red</red-prop>
    <green-prop>specular/green</green-prop>
    <blue-prop>specular/blue</blue-prop>
    <factor-prop>specular/factor</factor-prop>
    <offset-prop>specular/offset</offset-prop>
  </specular>
  <shininess>
  <transparency>
    <alpha-prop>transparency/alpha</alpha-prop>
    <factor-prop>transparency/factor</factor-prop>
    <offset-prop>transparency/offset</offset-prop>
    <min>0.0<min>  <!-- no min-prop! -->
    <max>1.0<max>  <!-- no max-prop! -->
  </transparency>
  <texture>texture</texture>
  <threshold>threshold</threshold>
</animation>

A change to one of the components in each color group does always update all three color components. Unset values default to zero. The <texture> path is relative to the model's <texture-path> directory, or, if unset, to the model directory. All numerical values are clamped to 0.0-1.0, except <shininess>, which is clamped to 0.0-128.0. The <alpha> property (<transparency> group) makes an object fully transparent (and thus invisible) with 0.0, and fully opaque with 1.0. ("alpha" could also be called "opaqueness".) The <threshold> property sets the alpha/opaqueness threshold. It is only relevant for semitransparent textures. Only parts of the texture that are more opaque than this are diplayed at all. If it is set to 0.0, all parts of the texture will be shown. If it is 0.5, only parts with opaqueness greater than 0.5 (or transparency less than 0.5) are shown.

Note that defining two or more of these animations for the same object, or a mixture of "material" and "blend" animations, will probably not yield the result that you expect. Try to put all state manipulations for one object or material in one "material" animation, and use its "transparency" property instead of an extra "blend" animation.

To make a texture replaceable at runtime, use a "material" animation like this:

<animation>
  <type>material</type>
  <object-name>tail-vert</object-name>
  <texture-prop>/sim/model/foo/texture</texture-prop>
</animation>

and put something like this into your aircraft's *-set.xml file:

<sim>
  <model>
    <foo>
      <texture>logos/tail-logo.rgb</texture>
    </foo>
  </model>
</sim>

You can explore the influence of material changes at runtime, if you use a material animation like the above complete example, and call the material dialog on it, like in the following Nasal example. Note, that the path is the same that you would set in <property-base>.

material.showDialog("/sim/model/foo/material/fuselage");

David Megginson, 11 March 2002

About

Mini-HOWTO: 3D Aircraft Models in FlightGear

11 March 2002

David Megginson

Introduction

1. Loading the model

2. Repositioning the Model