OE之earth文件序列化(上)

在理解Config的概念之后，就可以进入OE对Earth文件的解析过程，从earth的读写插件osgdb_earth插件开始

打开ReaderWriterOsgEarth.cpp文件

熟悉的自定义osg插件读写框架，直接进入解析正题

virtual ReadResult readNode(std::istream& in, const osgDB::Options* options ) const

 在这里看到了亲切的 XmlDocument类，通过它我们将XML文档存储到了Config中

接下来就是根节点辨析了，可以看到支持<<map>>和<<earth>>两种根节点字符串

然后是earh文件的版本控制，新版本增加了一些新的节点序列化，具体可以深入

序列化的最终结果就是将earth文件解析为mapNode 节点，可以将其置入osg的节点体系，进入到osg的渲染过程

继续来到版本2的序列化 EarthFileSerializer2

关键函数 

MapNode
EarthFileSerializer2::deserialize( const Config& conf, const std::string& referenceURI ) const
{
    MapOptions mapOptions( conf.child( "options" ) );

    // legacy: check for name/type in top-level attrs:
    if ( conf.hasValue( "name" ) || conf.hasValue( "type" ) )
    {
        Config legacy;
        if ( conf.hasValue("name") ) legacy.add( "name", conf.value("name") );
        if ( conf.hasValue("type") ) legacy.add( "type", conf.value("type") );
        mapOptions.mergeConfig( legacy );
    }

    Map* map = new Map( mapOptions );

    // Yes, MapOptions and MapNodeOptions share the same Config node. Weird but true.
    MapNodeOptions mapNodeOptions( conf.child( "options" ) );

    // Read the layers in LAST (otherwise they will not benefit from the cache/profile configuration)

    // Image layers:
    ConfigSet images = conf.children( "image" );
    for( ConfigSet::const_iterator i = images.begin(); i != images.end(); i++ )
    {
        Config layerDriverConf = *i;
        layerDriverConf.add( "default_tile_size", "256" );

        ImageLayerOptions layerOpt( layerDriverConf );
        layerOpt.name() = layerDriverConf.value("name");
        //layerOpt.driver() = TileSourceOptions( layerDriverConf );

        map->addImageLayer( new ImageLayer(layerOpt) );
    }

    // Elevation layers:
    for( int k=0; k<2; ++k )
    {
        std::string tagName = k == 0 ? "elevation" : "heightfield"; // support both :)

        ConfigSet heightfields = conf.children( tagName );
        for( ConfigSet::const_iterator i = heightfields.begin(); i != heightfields.end(); i++ )
        {
            Config layerDriverConf = *i;
            layerDriverConf.add( "default_tile_size", "15" );

            ElevationLayerOptions layerOpt( layerDriverConf );
            layerOpt.name() = layerDriverConf.value( "name" );

            map->addElevationLayer( new ElevationLayer(layerOpt) );
        }
    }

    // Model layers:
    ConfigSet models = conf.children( "model" );
    for( ConfigSet::const_iterator i = models.begin(); i != models.end(); i++ )
    {
        const Config& layerDriverConf = *i;

        ModelLayerOptions layerOpt( layerDriverConf );
        layerOpt.name() = layerDriverConf.value( "name" );
        layerOpt.driver() = ModelSourceOptions( layerDriverConf );

        map->addModelLayer( new ModelLayer(layerOpt) );
        //map->addModelLayer( new ModelLayer( layerDriverConf.value("name"), ModelSourceOptions(*i) ) );
    }

    // Overlay layers (just an alias for Model Layer with overlay=true)
    ConfigSet overlays = conf.children( "overlay" );
    for( ConfigSet::const_iterator i = overlays.begin(); i != overlays.end(); i++ )
    {
        Config layerDriverConf = *i;
        if ( !layerDriverConf.hasValue("driver") )
            layerDriverConf.set("driver", "feature_geom");

        ModelLayerOptions layerOpt( layerDriverConf );
        layerOpt.name() = layerDriverConf.value( "name" );
        layerOpt.driver() = ModelSourceOptions( layerDriverConf );
        layerOpt.overlay() = true; // forced on when "overlay" specified

        map->addModelLayer( new ModelLayer(layerOpt) );
    }

    // Mask layer:
    ConfigSet masks = conf.children( "mask" );
    for( ConfigSet::const_iterator i = masks.begin(); i != masks.end(); i++ )
    {
        Config maskLayerConf = *i;

        MaskLayerOptions options(maskLayerConf);
        options.name() = maskLayerConf.value( "name" );
        options.driver() = MaskSourceOptions(options);

        map->addTerrainMaskLayer( new MaskLayer(options) );
    }

    
    //Add any addition paths specified in the options/osg_file_paths element to the file path.  Useful for pointing osgEarth at resource folders.
    Config osg_file_paths = conf.child( "options" ).child("osg_file_paths");
    ConfigSet urls = osg_file_paths.children("url");
    for (ConfigSet::const_iterator i = urls.begin(); i != urls.end(); i++) 
    {
        std::string path = osgEarth::getFullPath( referenceURI, (*i).value());
        OE_DEBUG << "Adding OSG file path " << path << std::endl;
        osgDB::Registry::instance()->getDataFilePathList().push_back( path );
    }

    MapNode* mapNode = new MapNode( map, mapNodeOptions );

    // External configs:
    Config ext = conf.child( "external" );
    if ( !ext.empty() )
    {
        mapNode->externalConfig() = ext;
    }

    return mapNode;
}

从上面可以看到

1 earth插件 map 统筹OE中所有"层"的概念，支持多层影像，高程，模型，镂空层（mask）等

2各个层相应配置的属性由指定的Option类从Config中获取，如果对应Config中有Option类指定元素相应值的配置Option将其写入

现将OE支持的层节点和部分属性分列如下(仅供参考)