Added section on conditions

Modified Files:
 	README.xmlpanel.html

Docs/README.xmlpanel.html

@@ -17,11 +17,11 @@ Author: John Check <j4strngs@rockfish.net>
  <li><A HREF="#example_panel">Example Top Level Panel Config</A>
  <li><A HREF="#indexed_properties">Indexed Properties</A>
  <li><A HREF="#inclusion">Inclusion</A>
- <li><A HREF="#aliasing">Aliasing</A>
  <li><A HREF="#instrument_architecture">Instrument Architecture</A>
  <li><A HREF="#textures">Textures</A>
  <li><A HREF="#instrument_layers">Instrument Layers</A>
  <li><A HREF="#transformations">Transformations</a>
+ <li><A HREF="#conditions">Conditions</A>
  <li><A HREF="#needle_placement">About Transformations and Needle Placement</A>
  <li><A HREF="#interpolation">Interpolation Tables</A>
  <li><A HREF="#actions">Actions</A>
@@ -580,6 +580,145 @@ This example is a switch that contains both static and dynamic text:
 </pre>
 <br>
 
+<HR WIDTH="20%">
+<H2><A NAME="conditions">Conditions:</A></H2>
+<HR WIDTH="20%">
+
+<p>
+A condition may appear in a binding (for keyboard, joystick, or panel
+action) or in most parts of a panel configuration file (specifically,
+for an instrument, layer, transformation, text chunk, or action).  If
+a condition is present, it must succeed for the component to be used
+(i.e. if the condition fails, the binding will not be fired, the layer
+will not be drawn, etc.).  Conditions are compiled to an efficient
+internal form and load time.
+</p>
+<p>
+For example, to ensure that a layer is drawn only if the /foo/bar
+property has the value "active", you would do the following:
+</p>
+<pre>
+  &lt;layer&gt;
+   &lt;name&gt;FooBar Layer&lt;/name&gt;
+   &lt;condition&gt;
+    &lt;equals&gt;
+     &lt;property&gt;/foo/bar&lt;/property&gt;
+     &lt;value&gt;active&lt;/value&gt;
+    &lt;/equals&gt;
+   &lt;/condition&gt;
+   ...
+  &lt;/layer&gt;
+</pre>
+<p>
+To draw a different layer when the /foo/bar property does not have the
+value "active", you could follow with this:
+</p>
+<pre>
+  &lt;layer&gt;
+   &lt;name&gt;FooBar Layer&lt;/name&gt;
+   &lt;condition&gt;
+    &lt;not-equals&gt;
+     &lt;property&gt;/foo/bar&lt;/property&gt;
+     &lt;value&gt;active&lt;/value&gt;
+    &lt;/not-equals&gt;
+   &lt;/condition&gt;
+   ...
+  &lt;/layer&gt;
+</pre>
+<p>
+While conditions were originally created for panel designers, they
+have proven to  be very useful for binding input devices (such
+as the keyboard or joystick), since they effectively allow any key,
+axis, button, or panel action to act as a modifier for any other key,
+axis, button, or panel action, and in fact, allow mind-numbingly
+complicated combinations for people who like them (i.e. Emacs users).
+</p>
+<pre>
+  &lt;!-- make 'u' a modifier key --&gt;
+  &lt;!-- could also use property-toggle to make it a toggle modifier --&gt;
+  &lt;key n="117"&gt;
+   &lt;name&gt;u&lt;/name&gt;
+   &lt;desc&gt;Special 'u' modifier&lt;/desc&gt;
+
+   &lt;binding&gt;
+    &lt;command&gt;property-assign&lt;/command&gt;
+    &lt;property&gt;/modifiers/u&lt;/property&gt;
+    &lt;value&gt;1&lt;/value&gt;
+   &lt;/binding&gt;
+
+   &lt;mod-up&gt;
+    &lt;binding&gt;
+     &lt;command&gt;property-assign&lt;/command&gt;
+     &lt;property&gt;/modifiers/u&lt;/property&gt;
+     &lt;value&gt;0&lt;/value&gt;
+    &lt;/binding&gt;
+   &lt;/mod-up&gt;
+
+  &lt;/key&gt;
+
+  &lt;!-- take different actions based on the u modifier --&gt;
+  &lt;key n="118"&gt;
+   &lt;name&gt;v&lt;/name&gt;
+
+   &lt;binding&gt;
+    &lt;condition&gt;
+     &lt;property&gt;/modifiers/u&lt;/property&gt;
+    &lt;/condition&gt;
+    &lt;command&gt;property-toggle&lt;/command&gt;
+    &lt;property&gt;/foo/bar&lt;/property&gt;
+   &lt;/binding&gt;
+
+   &lt;binding&gt;
+    &lt;condition&gt;
+     &lt;not&gt;
+      &lt;property&gt;/modifiers/u&lt;/property&gt;
+     &lt;/not&gt;
+    &lt;/condition&gt;
+    &lt;command&gt;property-toggle&lt;/command&gt;
+    &lt;property&gt;/bar/foo&lt;/property&gt;
+   &lt;/binding&gt;
+
+  &lt;/key&gt;
+</pre>
+<p>
+The &lt;condition&gt; element acts as an implicit 'and' group for everything
+contained in it.  Here are the elements that can appear inside:
+</p>
+<pre>
+  &lt;property&gt;[name]&lt;/property&gt;
+
+  - true if the named property returns a true boolean value (i.e. a
+    non-0 numeric value)
+
+  &lt;and&gt;[condition...]&lt;/and&gt;
+
+  - true if all of the conditions contained in it are true
+
+  &lt;or&gt;[condition...]&lt;/or&gt;
+
+  - true if any of the conditions contained in it is true
+
+  &lt;not&gt;[condition]&lt;/not&gt;
+
+  - true if the condition contained in it is false
+
+  &lt;less-than&gt;...&lt;/less-than&gt;
+  &lt;less-than-equals&gt;...&lt;/less-than-equals&gt;
+  &lt;equals&gt;...&lt;/equals&gt;
+  &lt;not-equals&gt;...&lt;/not-equals&gt;
+  &lt;greater-than&gt;...&lt;/greater-than&gt;
+  &lt;greater-than-equals&gt;...&lt;/greater-than-equals&gt;
+</pre>
+<p>
+  - all of these take two child elements; the first, "property", is
+    the property to test; the second argument may be "property" or
+    "value".  If it is also "property" (i.e. property[1]), the named
+    property's value will be used; otherwise, the literal value
+    provided will be used
+  - be warned that any string comparison other than equals/not-equals
+    is guaranteed to be flaky
+</p>
+
 <HR WIDTH="20%">
 <H2><A NAME="transformations">Transformations:</A></H2>
 <HR WIDTH="20%">
@@ -591,7 +730,7 @@ based on properties are useful for driving instrument needles. I.E. rotate the
 number of degrees equal to the airspeed. X and y shifts are relative to the
 center of the instrument. Each specified transformation type takes an &lt;offset&gt;.</p><p>
 Offsets are relative to the center of the instrument. A shift without an offset
-has no effect. For example, let's say we have a texure that is a circle. If we
+has no effect. For example, let's say we have a texture that is a circle. If we
 use this texture in two layers, one defined as having a size of 128x128 and
 the second layer is defined as 64x64 and neither is supplied  a shift and offset
 the net result appears as 2 concentric circles.
@@ -887,7 +1026,7 @@ Here is a list of property names including appropriate units:
 /engines/engine0/rpm =&gt; /engines/engine[0]/rpm
 /environment/clouds/altitude += "-ft"
 /environment/magnetic-dip += "-deg"
-/environment/magnetic-varation += "-deg"
+/environment/magnetic-variation += "-deg"
 /environment/visibility += "-m"
 /environment/wind-down += "-fps"
 /environment/wind-east += "-fps"