Expressions: Difference between revisions
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Expressions are a feature of the SimGear library and provide a nice way of implementing complex math formulas using XML syntax. | '''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. | 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 == | == Usage == | ||
Expressions are supported in | Expressions are supported in | ||
* [[ | * [[Autopilot Configuration Reference#Expressions|Autopilot configuration files]] | ||
* | * Particle system configuration files | ||
* Animations (translate, rotate, scale, range, blend) | * Animations (translate, rotate, scale, range, blend) | ||
* The shader technique | * The shader technique | ||
== Sample Expressions == | == Sample Expressions == | ||
This is a sample expression for c = sqrt(a*a + b^2) | This is a sample expression for <code>c = sqrt(a*a + b^2)</code>. Children/arguments are parsed in the order they appear in in the file (or the order in which they are set via property methods). | ||
<syntaxhighlight lang="xml"> | |||
<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> | |||
</syntaxhighlight> | |||
== Supported elements == | == Supported elements == | ||
<syntaxhighlight lang="xml"> | |||
<abs> <!-- also: fabs --> | |||
<acos> | |||
<asin> | |||
<atan> | |||
<atan2> | |||
<ceil> | |||
<clip> | |||
<cos> | |||
<cosh> | |||
<difference> <!-- also: dif --> | |||
<div> | |||
<exp> | |||
<floor> | |||
<log> | |||
<log10> | |||
<max> | |||
<min> | |||
<mod> | |||
<pow> | |||
<product> <!-- also: prod --> | |||
<property> <!-- Unlike elsewhere, 'prop' does not work in expressions. --> | |||
<rad2deg> | |||
<deg2rad> | |||
<sin> | |||
<sinh> | |||
<sqr> | |||
<sqrt> | |||
<sum> | |||
<table> | |||
<tan> | |||
<tanh> | |||
<value> | |||
</syntaxhighlight> | |||
== Related content == | |||
=== Wiki articles === | |||
* [[Conditions]] | |||
* [[PropertyList XML files]] | |||
=== Source code === | |||
* {{simgear file|simgear/structure/SGExpression.hxx}} | |||
* {{simgear file|simgear/structure/SGExpression.cxx}} | |||
[[Category:XML]] | |||
Revision as of 17:17, 3 May 2019
| This article is a stub. You can help the wiki by expanding it. |
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 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
<abs> <!-- also: fabs -->
<acos>
<asin>
<atan>
<atan2>
<ceil>
<clip>
<cos>
<cosh>
<difference> <!-- also: dif -->
<div>
<exp>
<floor>
<log>
<log10>
<max>
<min>
<mod>
<pow>
<product> <!-- also: prod -->
<property> <!-- Unlike elsewhere, 'prop' does not work in expressions. -->
<rad2deg>
<deg2rad>
<sin>
<sinh>
<sqr>
<sqrt>
<sum>
<table>
<tan>
<tanh>
<value>