[原][osgearth]API加载earth文件的解析

参考：http://blog.csdn.net/cccstudyer/article/details/17691893

通过srcosgEarthDriversearthReaderWriterOsgEarth.cpp文件

ReaderWriterEarth类，用来解析“earth”文件

在文件225行的readNode函数

  virtual ReadResult readNode(std::istream& in, const osgDB::Options* readOptions) const
        {
            // pull the URI context from the options structure (since we're reading
            // from an "anonymous" stream here)
            URIContext uriContext( readOptions ); 

            osg::ref_ptr<XmlDocument> doc = XmlDocument::load( in, uriContext );
            if ( !doc.valid() )
                return ReadResult::ERROR_IN_READING_FILE;

            Config docConf = doc->getConfig();

            // support both "map" and "earth" tag names at the top level
            Config conf;
            if ( docConf.hasChild( "map" ) )
                conf = docConf.child( "map" );
            else if ( docConf.hasChild( "earth" ) )
                conf = docConf.child( "earth" );

            osg::ref_ptr<osg::Node> node;

            if ( !conf.empty() )
            {
                // see if we were given a reference URI to use:
                std::string refURI = uriContext.referrer();

                if ( conf.value("version") == "1" )
                {
                    OE_INFO << LC << "Detected a version 1.x earth file" << std::endl;
                    EarthFileSerializer1 ser;
                    node = ser.deserialize( conf, refURI );
                }

                else
                {
                    if ( conf.value("version") != "2" )
                        OE_DEBUG << LC << "No valid earth file version; assuming version='2'" << std::endl;

                    // attempt to parse a "default options" JSON string:
                    std::string defaultConfStr;
                    if ( readOptions )
                    {
                        defaultConfStr = readOptions->getPluginStringData("osgEarth.defaultOptions");
                        if ( !defaultConfStr.empty() )
                        {
                            Config optionsConf("options");
                            if (optionsConf.fromJSON(defaultConfStr))
                            {
                                //OE_NOTICE << "

Original = 
" << conf.toJSON(true) << "
";
                                Config* original = conf.mutable_child("options");
                                if ( original )
                                {
                                    recursiveUniqueKeyMerge(optionsConf, *original);
                                }
                                if ( !optionsConf.empty() )
                                {
                                    conf.set("options", optionsConf);
                                }
                                //OE_NOTICE << "

Merged = 
" << conf.toJSON(true) << "
";
                            }
                        }
                    }

                    EarthFileSerializer2 ser;
                    node = ser.deserialize( conf, refURI );
                }
            }

            MapNode* mapNode = MapNode::get(node.get());
            if (mapNode)
            {
                // If the user passed in a cache object, apply it to the map now
                CacheSettings* cacheSettings = CacheSettings::get(readOptions);
                if (cacheSettings && cacheSettings->getCache())
                {
                    mapNode->getMap()->setCache( cacheSettings->getCache() );
                    OE_INFO << LC << "Applied user-supplied cache to the Map
";
                }
            }

            return ReadResult(node.get());
        }
};

看到65行的读取调用

进入srcosgEarthDriversearthEarthFileSerializer2.cpp文件

EarthFileSerializer2类的deserialize函数

 void addImageLayer(const Config& conf, Map* map)
    {
        ImageLayerOptions options( conf );
        options.name() = conf.value("name");
        ImageLayer* layer = new ImageLayer(options);
        map->addLayer(layer);
        if (layer->getStatus().isError())
            OE_WARN << LC << "Layer "" << layer->getName() << "" : " << layer->getStatus().toString() << std::endl;
    }

    void addElevationLayer(const Config& conf, Map* map)
    {
        ElevationLayerOptions options( conf );
        options.name() = conf.value( "name" );
        ElevationLayer* layer = new ElevationLayer(options);
        map->addLayer(layer);
        if (layer->getStatus().isError())
            OE_WARN << LC << "Layer "" << layer->getName() << "" : " << layer->getStatus().toString() << std::endl;
    }

    void addModelLayer(const Config& conf, Map* map)
    {
        ModelLayerOptions options( conf );
        options.name() = conf.value( "name" );
        options.driver() = ModelSourceOptions( conf );
        ModelLayer* layer = new ModelLayer(options);
        map->addLayer(layer);
        if (layer->getStatus().isError())
            OE_WARN << LC << "Layer "" << layer->getName() << "" : " << layer->getStatus().toString() << std::endl;
    }

    void addMaskLayer(const Config& conf, Map* map)
    {
        MaskLayerOptions options(conf);
        options.name() = conf.value( "name" );
        options.driver() = MaskSourceOptions(options);
        MaskLayer* layer = new MaskLayer(options);
        map->addLayer(layer);
        if (layer->getStatus().isError())
            OE_WARN << LC << "Layer "" << layer->getName() << "" : " << layer->getStatus().toString() << std::endl;
    }

    // support for "special" extension names (convenience and backwards compat)
    Extension* createSpecialExtension(const Config& conf)
    {
        // special support for the default sky extension:
        if (conf.key() == "sky" && !conf.hasValue("driver"))
            return Extension::create("sky_simple", conf);

        if (conf.key() == "ocean" && !conf.hasValue("driver"))
            return Extension::create("ocean_simple", conf);

        return 0L;
    }

    bool addLayer(const Config& conf, Map* map)
    {
        std::string name = conf.key();
        Layer* layer = Layer::create(name, conf);
        if (layer)
        {
            map->addLayer(layer);
            if (layer->getStatus().isError())
                OE_WARN << LC << "Layer "" << layer->getName() << "" : " << layer->getStatus().toString() << std::endl;
        }
        return layer != 0L;
    }

osg::Node*
EarthFileSerializer2::deserialize( const Config& conf, const std::string& referrer ) const
{
    // First, pre-load any extension DLLs.
    preloadExtensionLibs(conf);
    preloadExtensionLibs(conf.child("extensions"));
    preloadExtensionLibs(conf.child("external"));

    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 );

    // Start a batch update of the map:
    map->beginUpdate();

    // Read all the elevation layers in FIRST so other layers can access them for things like clamping.
    for(ConfigSet::const_iterator i = conf.children().begin(); i != conf.children().end(); ++i)
    {
        if ( i->key() == "elevation" || i->key() == "heightfield" )
        {
            addElevationLayer( *i, map );
        }
    }

    Config externalConfig;
    std::vector<osg::ref_ptr<Extension> > extensions;

    // Read the layers in LAST (otherwise they will not benefit from the cache/profile configuration)
    for(ConfigSet::const_iterator i = conf.children().begin(); i != conf.children().end(); ++i)
    {
        if (i->key() == "options" || i->key() == "name" || i->key() == "type" || i->key() == "version")
        {
            // nop - handled earlier
        }

        else if ( i->key() == "image" )
        {
            addImageLayer( *i, map );
        }

        else if ( i->key() == "model" )
        {
            addModelLayer( *i, map );
        }

        else if ( i->key() == "mask" )
        {
            addMaskLayer( *i, map );
        }

        else if ( i->key() == "external" || i->key() == "extensions" )
        {
            externalConfig = *i;
            
            for(ConfigSet::const_iterator e = i->children().begin(); e != i->children().end(); ++e)
            {
                Extension* extension = loadExtension(*e);
                if (extension)
                    extensions.push_back(extension);
                //addExtension( *e, mapNode.get() );
            }
        }

        else if ( !isReservedWord(i->key()) ) // plugins/extensions.
        {
            bool addedLayer = addLayer(*i, map); //mapNode.get());

            if ( !addedLayer )
            {
                Extension* extension = loadExtension(*i);
                if (extension)
                    extensions.push_back(extension);
                //OE_INFO << LC << "Tried to load "" << i->key() << "" as a layer; now trying extension
";
                //addExtension( *i, mapNode.get() );
            }
        }
    }

    // Complete the batch update of the map
    map->endUpdate();

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

    // Create a map node.
    osg::ref_ptr<MapNode> mapNode = new MapNode( map, mapNodeOptions );

    // Apply the external conf if there is one.
    if (!externalConfig.empty())
    {
        mapNode->externalConfig() = externalConfig;
    }

    // Install the extensions
    for (unsigned i = 0; i < extensions.size(); ++i)
    {
        mapNode->addExtension(extensions.at(i).get());
    }

    // return the topmost parent of the mapnode. It's possible that
    // an extension added parents!
    osg::Node* top = mapNode.release();

    while( top->getNumParents() > 0 )
        top = top->getParent(0);

    return top;
}

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

为了统筹OE各个层的属性配置，OE提供了ConfigOptions类，ConfigOptions类就是对某个Config类的封装，提供了到Config层次上的调用，例如Config赋值，拼接等

为不同层的属性配置提供了存储载体,结合上文map包含的图层有以下关系

通过这些类，可以很方便的将特定的XML节点下属性配置生成Config归类到某一个图层中，以下列出部分图层的属性(仅供参考)

MapOptions：XML<<options>>节点下配置的信息，主要是配置地形渲染时的属性；包括如下

<elevation_interploation> 插值，nearset 最近点插值  average 最近点平均值 bilinear 线性插值，  triangulate 三角面片插值
<elevation_tile_size> 瓦片大小
<elevation_texture_size> 纹理大小
<overlay_wapring> 纹理装载
<overlay_blending>混合
<overlay_minmapping>  纹理映射
<overlay_texture_size>
<overlay_attach_stencil>模板

ImageLayerOptions： XML<<image>>节点下配置的信息，很明显，影像属性配置，部分属性值

<nodata_image> 无数据显示的url
<opacity>    透明度
<min_range> 最小可视范围
<max_range> 最大可视范围
<min_level>  最小显示细节层级
<max_level>  最大显示细节层级
<min_resolution> 数据的最小分辨率，单位的像素
<max_resolution>数据的最大分辨率，单位的像素
<enable> 是否包含进map层
<visible> 可视

ElevationLayerOptions： XML<<elevation>>or<<heightfield>>节点下配置的信息，高程层

<min_level>  最小显示细节层级
<max_level>  最大显示细节层级
<min_resolution> 数据的最小分辨率，单位的像素
<max_resolution>数据的最大分辨率，单位的像素
<enable> 是否包含进map层

ModelLayerOptions： XML<<Model>>节点下配置的信息 模型层装载 矢量数据，模型，几何体等

<name>
<driver>
<enable><span style="font-family:Arial, Helvetica, sans-serif;">是否包含进map层</span>
<visible>是否可见
<overLay>

MaskLayerOptions： XML<<mask>>节点配置下的信息，镂空层

一个比较特殊的节点 ext(Config); 暂且叫他功能层，XML <<external>>,它由mapNode直接读取配置信息，实现一些经常用到的功能点，例如 视点定位 <<<viewpoint>> 星空时刻设置<<sky>>等