107 lines
4.7 KiB
Text
107 lines
4.7 KiB
Text
Expressions (or SGExpressions) are a feature of the SimGear library
|
|
and provide a nice way of implementing complex math formulas using XML
|
|
syntax. They are supported in many systems within the FlightGear code.
|
|
|
|
Caution: Expressions do not check if your math creates floating point
|
|
exceptions (like division by zero conditions, taking the square root
|
|
of a negative number, etc.). This can cause undefined behavior and may
|
|
result in NaNs or even Cascading NaNs. Usage
|
|
|
|
Expressions are supported in
|
|
|
|
* Autopilot configuration files
|
|
* Particle system configuration files
|
|
* Animations (translate, rotate, scale, range, blend)
|
|
* The shader technique
|
|
|
|
Sample Expressions
|
|
|
|
This is a sample expression for the euation c = sqrt(a*a + b^2).
|
|
|
|
Children/arguments are parsed in the order they appear in in the file
|
|
(or the order in which they are set via property methods).
|
|
|
|
<expression>
|
|
<sqrt>
|
|
<sum>
|
|
<product>
|
|
<property>/value/a</property>
|
|
<property>/value/a</property>
|
|
</product>
|
|
<pow>
|
|
<property>/value/b</property>
|
|
<value>2</value>
|
|
</pow>
|
|
</sum>
|
|
</sqrt>
|
|
</expression>
|
|
|
|
Supported elements
|
|
|
|
NOTE: #c in the table below is the number of child nodes required.
|
|
|
|
+-------------------+----+--------------------------------------------------+
|
|
| Function | #c | Notes |
|
|
+-------------------+----+--------------------------------------------------+
|
|
| <abs> <fabs> | 1 | |
|
|
| <acos> | 1 | |
|
|
| <asin> | 1 | |
|
|
| <atan2> | 2 | |
|
|
| <atan> | 1 | |
|
|
| <ceil> | 1 | |
|
|
| <clip> | 3 | clipMin, clipMax, expression |
|
|
| <cos> | 1 | |
|
|
| <cosh> | 1 | |
|
|
| <deg2rad> | 1 | |
|
|
| <difference> <dif>| 1+ | |
|
|
| <div> | 2 | |
|
|
| <exp> | 1 | |
|
|
| <floor> | 1 | |
|
|
| <log10> | 1 | |
|
|
| <log> | 1 | |
|
|
| <max> | 1+ | |
|
|
| <min> | 1+ | |
|
|
| <mod> | 2 | |
|
|
| <pow> | 2 | |
|
|
| <product> <prod> | 1+ | |
|
|
| <property> | 0 | Property name e.g.<property>node/value</property>|
|
|
| <rad2deg> | 1 | |
|
|
| <sin> | 1 | |
|
|
| <sinh> | 1 | |
|
|
| <sqr> | 1 | |
|
|
| <sqrt> | 1 | |
|
|
| <sum> | 1+ | |
|
|
| <table> | 2+ | <entry><ind> 0.0 </ind><dep> 10 </dep></entry> |
|
|
| <tan> | 1 | |
|
|
| <tanh> | 1 | |
|
|
| <value> | 0 | Constant Value e.g. <value>0</value> |
|
|
+-------------------+----+--------------------------------------------------+
|
|
|
|
Hints and tips
|
|
--------------
|
|
|
|
1. There is no function for rounding, hwoever "Round half up" can be
|
|
achieved as follows
|
|
|
|
<expression>
|
|
<floor>
|
|
<sum>
|
|
<property>your/property/here</property>
|
|
<value>0.5</value>
|
|
</sum>
|
|
</floor>
|
|
</expression>
|
|
|
|
2. Interpolation tables can be useful when a formula cannot be found,
|
|
In the example below <ind> is the indepdendant variable and
|
|
<dep> is the dependant variable. What this table does is as follows
|
|
- values below 0.2 will be 10
|
|
- values above 0.2 and between 1.0 will be interpolated between 10
|
|
and 0
|
|
- values above 1.0 will be 0
|
|
|
|
<table>
|
|
<entry><ind> 0.0 </ind><dep> 10 </dep></entry>
|
|
<entry><ind> 0.2 </ind><dep> 10 </dep></entry>
|
|
<entry><ind> 1.0 </ind><dep> 0 </dep></entry>
|
|
</table>
|