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