Expressions

Work in progress This article or section will be worked on in the upcoming hours or days. See history for the latest developments.

General

Expressions 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.

Usage

Expressions are supported in

• Autopilot configuration files
• The 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)

<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

<acos>
<asin>
<atan>
<ceil>
<cos>
<cosh>
<exp>
<floor>
<log>
<log10>
<rad2deg>
<sin>
<sinh>
<sqrt>
<tan>
<tanh>
<atan2>
<div>
<mod>
<pow>
<value>
<property>
<abs> (same as fabs)
<sqr>
<clip>
<div>
<mod>
<sum>
<difference> (same as dif)
<product> (same as prod)
<min>
<max>
<table>