Add offset polygons example

tests/feature_offset_polygons.earth

@@ -0,0 +1,110 @@
+<!--
+osgEarth Sample
+
+This one demonstrates how to read polygon data from a shapefile and "drape" it
+on the map using the overlay technique.
+
+It also shows the "selector" technique of using SQL queries to selectively style
+the feature data.
+-->
+
+<map name="Feature Overlay Demo" type="geocentric" version="2">
+
+    <options>
+        <lighting>false</lighting>
+        <overlay_blending>false</overlay_blending>
+		<terrain min_lod="8"/>
+    </options>
+
+	<extensions>
+		<terrainshader>
+			<description>
+				A GLSL snippet that pushes the terrain back to prevent high-altitude z fighting.
+			</description>
+			<vertex>
+				<![CDATA[
+					#version 110
+					#pragma vp_entryPoint "dp_vert"
+					#pragma vp_location   "vertex_view"
+					void dp_vert(inout vec4 v) {
+						const float R    = 6371000.0;	  // Approximate radius of the earth
+						const float dmin = 100000.0;      // Minimum vertex distance at which to apply effect
+						const float dmax = R;             // Vertex distance at which to apply maximum effect
+						const float maxOffset = 250000.0; // Maximum offset (applied at dmax)
+
+						float d = length(v.xyz);
+						float t = clamp( (d-dmin)/(dmax-dmin), 0.0, 1.0 );
+						float offset = t*maxOffset;
+						vec3 n = normalize(v.xyz);
+						v.xyz = v.xyz + n*offset;
+					}
+				]]>
+			</vertex>
+		</terrainshader>
+	</extensions>
+
+    <image name="world" driver="gdal">
+        <url>../data/world.tif</url>
+    </image>
+
+    <model name="countries" driver="feature_geom">
+
+        <features name="states" driver="ogr">
+            <url>../data/world.shp</url>
+            <buffer distance="-0.05"/>
+			<resample max_length="5.0"/>
+        </features>
+
+        <styles>        
+            <style type="text/css">
+                p1 {
+                   fill:               #ffff8066;
+                }       
+                p2 {
+                   fill:               #80ffff66;
+                }   
+                p3 {
+                   fill:               #ff80ff66;
+                }       
+                p4 {
+                   fill:               #ff808066;
+                }     
+                p5 {
+                   fill:               #80ff8066;
+                }                                      
+            </style>
+
+            <selector class="p1">
+                <query>
+                    <expr><![CDATA[ POP_CNTRY <= 14045470 ]]></expr>
+                </query>
+            </selector>
+
+            <selector class="p2">
+                <query>
+                    <expr><![CDATA[ POP_CNTRY > 14045470 and POP_CNTRY <= 43410900 ]]></expr>
+                </query>
+            </selector>
+
+            <selector class="p3">
+                <query>
+                    <expr><![CDATA[ POP_CNTRY > 43410900 and POP_CNTRY <= 97228750 ]]></expr>
+                </query>
+            </selector>
+
+            <selector class="p4">
+                <query>
+                    <expr><![CDATA[ POP_CNTRY > 97228750 and POP_CNTRY <= 258833000 ]]></expr>
+                </query>
+            </selector>
+
+            <selector class="p5">
+                <query>
+                    <expr><![CDATA[ POP_CNTRY > 258833000 ]]></expr>
+                </query>
+            </selector>
+
+        </styles>
+
+    </model>
+</map>