osgEarth的Rex引擎原理分析（十九）request请求加载瓦片优先级的含义

目标：（十七）中的问题40

这要先从TileNode的load说起，它会计算出一个priority，但这个优先级还不是request的优先级

osgEarthDrivers/engine_rex/TileNode.cpp
void
TileNode::load(TerrainCuller* culler)
{    
    const SelectionInfo& si = _context->getSelectionInfo();
    int lod     = getKey().getLOD();
    int numLods = si.getNumLODs();
    
    // LOD priority is in the range [0..numLods]
    float lodPriority = (float)lod;
    if ( _context->getOptions().highResolutionFirst() == false )
        lodPriority = (float)(numLods - lod);

    float distance = culler->getDistanceToViewPoint(getBound().center(), true);

    // dist priority uis in the range [0..1]
    float distPriority = 1.0 - distance/si.visParameters(0)._visibilityRange;

    // add them together, and you get tiles sorted first by lodPriority
    // (because of the biggest range), and second by distance.
    float priority = lodPriority + distPriority;

    // normalize the composite priority to [0..1].
    //priority /= (float)(numLods+1); // GW: moved this to the PagerLoader.

    // Submit to the loader.
    _context->getLoader()->load( _loadRequest.get(), priority, *culler );
}

紧接着在PagerLoader的load函数中会根据上面生成的优先级设置request的优先级

osgEarthDrivers/engine_rex/Loader.cpp
bool
PagerLoader::load(Loader::Request* request, float priority, osg::NodeVisitor& nv)
{ 

    // update the priority, scale and bias it, and then normalize it to [0..1] range.
    unsigned lod = request->getTileKey().getLOD();
    float p = priority * _priorityScales[lod] + _priorityOffsets[lod];            
    request->_priority = p / (float)(_numLODs+1);
}