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This map shows how to use the preemptive loading mode of osgearth. Preemptive
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mode prioritizes the highest visible imagery LOD, skipping intermediate
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levels. It also demonstrates per-layer resource allocation. Preemptive mode
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uses a pool of loading threads to fetch tiles. You can use "loading_weight"
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to give some layers more threads than others.
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Please note that use of ESRI's free maps is subject to certain restrictions:
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http://resources.esri.com/arcgisonlineservices/index.cfm?fa=content
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<map name="ESRI Imagery and Roads" type="geocentric" version="2">
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<!-- Set a "preemptive" loading policy, and set the number of
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loading threads to use. -->
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<loading_policy mode="preemptive" loading_threads_per_core="3"/>
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<!-- loading_weight specifies the relative number of threads
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to use for this particular layer. For heightfield layers,
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the highest specified loading_weight for all heightfield layers
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will be used. For imagery layers, a separate group of threads
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will be allocated to handle each layer. -->
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<image name="arcgisonline esri imagery" driver="arcgis" loading_weight="1">
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<url>http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_Imagery_World_2D/MapServer</url>
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<nodata_image>http://server.arcgisonline.com/ArcGIS/rest/services/ESRI_Imagery_World_2D/MapServer/tile/100/0/0.jpeg</nodata_image>
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<compress_textures>true</compress_textures>
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<!-- We give the transportation layer 5x the resources as the image layer: -->
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<image name="arcgisonline transportation" driver="arcgis" loading_weight="5">
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<url>http://server.arcgisonline.com/ArcGIS/rest/services/Reference/ESRI_Transportation_World_2D/MapServer</url>
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<compress_textures>true</compress_textures>