Expressions

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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.

Beware: 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>
<rad2deg>
<sin>
<sinh>
<sqr>
<sqrt>
<sum>
<table>
<tan>
<tanh>
<value>