1、問題描述

我們?yōu)槭裁词褂胓ooglEarth或者osgEarth的時候，原則上可以加載無限大的數(shù)據(jù)呢，其原因就是因為其使用的是樹狀的組織結(jié)構(gòu)，如下：

我們?nèi)匀粊砟玫厍騺硐胂?#xff0c;Level0是最粗的球，我們離的很遠的時候是這個球。Level1是我們拉近一點，地球會一分為八，然后我們再對著其中一個角拉近就分裂成Level2的模樣，這個角又一分為八。

粗的級別就顯示粗的內(nèi)容，細的級別就加載細的內(nèi)容，就像高清影像的瓦片一樣，第0級可以是整個地球，分辨率是256X256，然后一分為八，每張圖片的范圍變成了第0級的八分之一，但是分辨率仍然是256x256，就越往下拉越清晰。

本節(jié)我們就來構(gòu)建這樣一個八叉樹的結(jié)構(gòu)，本節(jié)如果你搞明白了，就入門了八叉樹的結(jié)構(gòu)，因為在構(gòu)建樹狀結(jié)構(gòu)的時候往往會用到遞歸。理解起來還是有點費勁的。LOD和PagedLOD都大量的用作構(gòu)建數(shù)字地球等，其實原理都和本節(jié)差不多。

2、本節(jié)功能

1、隨機生成5000個球，坐標范圍在[-500, 500]。
2、對這5000個球生成八叉樹，其結(jié)束條件是這樣的：當結(jié)點的孩子小于16個時認為是葉結(jié)點，就不再往下分了。或者樹的深度大于32也不往下分了。3、每一層我們用一個LOD來保存，當離的很遠時顯示父親（把包圍盒繪制出來），當拉近時父親就一分為八。直到顯示葉結(jié)點。

3、具體實現(xiàn)

osgPro227.cpp

// osgPro227.cpp : 此文件包含 "main" 函數(shù)。程序執(zhí)行將在此處開始并結(jié)束。#include <osg/Group> #include <osgDB/ReadFile> #include <osgUtil/PrintVisitor> #include <osgViewer/ViewerEventHandlers> #include <osgViewer/Viewer> #include <osgViewer/ViewerEventHandlers> //事件監(jiān)聽 #include <osgGA/StateSetManipulator> //事件響應(yīng)類，對渲染狀態(tài)進行控制#include "OctreeBuilder.h"#pragma comment(lib, "OpenThreadsd.lib") #pragma comment(lib, "osgd.lib") #pragma comment(lib, "osgDBd.lib") #pragma comment(lib, "osgUtild.lib") #pragma comment(lib, "osgGAd.lib") #pragma comment(lib, "osgViewerd.lib") #pragma comment(lib, "osgTextd.lib")float randomValue(float min, float max) {return (min + (float)rand() / (RAND_MAX + 1.0f) * (max - min)); }osg::Vec3 randomVector(float min, float max) {return osg::Vec3(randomValue(min, max),randomValue(min, max),randomValue(min, max)); } class PrintNameVisitor : public osgUtil::PrintVisitor { public:PrintNameVisitor(std::ostream& out) : osgUtil::PrintVisitor(out) {}void apply(osg::Node& node){if (!node.getName().empty()){output() << node.getName() << std::endl;enter();traverse(node);leave();}else osgUtil::PrintVisitor::apply(node);} };int main(int argc, char** argv) {osg::BoundingBox globalBound;std::vector<OctreeBuilder::ElementInfo> globalElements;for (unsigned int i = 0; i < 5000; ++i){osg::Vec3 pos = randomVector(-500.0f, 500.0f);float radius = randomValue(0.5f, 20.0f);std::stringstream ss; ss << "Ball-" << i + 1;osg::Vec3 min = pos - osg::Vec3(radius, radius, radius);osg::Vec3 max = pos + osg::Vec3(radius, radius, radius);osg::BoundingBox region(min, max);globalBound.expandBy(region);globalElements.push_back(OctreeBuilder::ElementInfo(ss.str(), region));}OctreeBuilder octree;osg::ref_ptr<osg::Group> root = octree.build(0, globalBound, globalElements);std::ofstream out("octree_output.txt");PrintNameVisitor printer(out);root->accept(printer);osg::ref_ptr <osgViewer::Viewer> viewer = new osgViewer::Viewer;viewer->setSceneData(root.get());viewer->addEventHandler(new osgGA::StateSetManipulator(viewer->getCamera()->getOrCreateStateSet()));viewer->addEventHandler(new osgViewer::StatsHandler());//實現(xiàn)狀態(tài)信息統(tǒng)計viewer->addEventHandler(new osgViewer::WindowSizeHandler());return viewer->run(); }

OctreeBuilder.h

#ifndef H_COOKBOOK_CH8_OCTREEBUILDER #define H_COOKBOOK_CH8_OCTREEBUILDER#include <osg/Geode> #include <osg/LOD>class OctreeBuilder { public:OctreeBuilder() : _maxChildNumber(16), _maxTreeDepth(8), _maxLevel(0) {}int getMaxLevel() const { return _maxLevel; }void setMaxChildNumber( int max ) { _maxChildNumber = max; }int getMaxChildNumber() const { return _maxChildNumber; }void setMaxTreeDepth( int max ) { _maxTreeDepth = max; }int getMaxTreeDepth() const { return _maxTreeDepth; }typedef std::pair<std::string, osg::BoundingBox> ElementInfo;osg::Group* build( int depth, const osg::BoundingBox& total,std::vector<ElementInfo>& elements );protected:osg::LOD* createNewLevel( int level, const osg::Vec3& center, float radius );osg::Node* createElement( const std::string& id, const osg::Vec3& center, float radius );osg::Geode* createBoxForDebug( const osg::Vec3& max, const osg::Vec3& min );int _maxChildNumber;int _maxTreeDepth;int _maxLevel; };#endif

OctreeBuilder.cpp

#include <windows.h> #include <iostream> #include <fstream> #include <sstream> using namespace std;#include <osg/ShapeDrawable> #include <osg/Geometry> #include <osg/PolygonMode> #include "OctreeBuilder.h"osg::Group* OctreeBuilder::build( int depth, const osg::BoundingBox& total,std::vector<ElementInfo>& elements ) {int s[3]; // axis sides (0 or 1)//存放當前包圍盒的最大、中間、最小點，以為劃分八叉樹做準備osg::Vec3 extentSet[3] = {total._min,(total._max + total._min) * 0.5f,total._max};std::vector<ElementInfo> childData;//遍歷父結(jié)點的所有孩子，讓包含在當前盒子里的，不完全包含但是中點在盒子里的，都壓入到當前盒子的子結(jié)點for ( unsigned int i=0; i<elements.size(); ++i ){const ElementInfo& obj = elements[i];if ( total.contains(obj.second._min) && total.contains(obj.second._max) )childData.push_back( obj );else if ( total.intersects(obj.second) ){osg::Vec3 center = (obj.second._max + obj.second._min) * 0.5f;if (total.contains(center)){childData.push_back(obj);}}}//如果當前結(jié)點的孩子數(shù)量已經(jīng)達標，或者層數(shù)已經(jīng)達標，則認為是葉結(jié)點bool isLeafNode = false;if ((int)childData.size() <= _maxChildNumber || depth > _maxTreeDepth){isLeafNode = true;}//當前八叉樹根osg::ref_ptr<osg::Group> group = new osg::Group;//如果不是葉結(jié)點，繼續(xù)分，把空間一分為八if ( !isLeafNode ){osg::ref_ptr<osg::Group> childNodes[8];//空間一分為八2*2*2for ( s[0]=0; s[0]<2; ++s[0] ) //x{for ( s[1]=0; s[1]<2; ++s[1] ) //y{for ( s[2]=0; s[2]<2; ++s[2] ) //z{// Calculate the child extent//extentSet 0是最小，1是中間，2是最大//下面這個小算法有點磨人，分別求出min和max的x, y, z自己好好推幾個試試osg::Vec3 min, max;for ( int a=0; a<3; ++a ){min[a] = (extentSet[s[a] + 0])[a];max[a] = (extentSet[s[a] + 1])[a];}//這么求id是為了確保唯一性int id = s[0] + (2 * s[1]) + (4 * s[2]);childNodes[id] = build( depth+1, osg::BoundingBox(min, max), childData );}}}//八個子結(jié)點構(gòu)建完畢后，加入到根結(jié)點當中for ( unsigned int i=0; i<8; ++i ){if ( childNodes[i] && childNodes[i]->getNumChildren() )group->addChild( childNodes[i] );}}else //找到葉結(jié)點，遞歸就結(jié)束了{for ( unsigned int i=0; i<childData.size(); ++i ){const ElementInfo& obj = childData[i];osg::Vec3 center = (obj.second._max + obj.second._min) * 0.5;float radius = (obj.second._max - obj.second._min).length() * 0.5f;//創(chuàng)建一個球group->addChild( createElement(obj.first, center, radius) );}}osg::Vec3 center = (total._max + total._min) * 0.5;float radius = (total._max - total._min).length() * 0.5f;//最后創(chuàng)建一個LOD，離的遠了顯示調(diào)試盒子，離的近了顯示分的組osg::LOD* level = createNewLevel( depth, center, radius );level->insertChild( 0, createBoxForDebug(total._max, total._min) ); // For debug uselevel->insertChild( 1, group.get() );return level; }osg::LOD* OctreeBuilder::createNewLevel( int level, const osg::Vec3& center, float radius ) {osg::ref_ptr<osg::LOD> lod = new osg::LOD;lod->setCenterMode( osg::LOD::USER_DEFINED_CENTER );lod->setCenter( center );lod->setRadius( radius );lod->setRange( 0, radius * 5.0f, FLT_MAX );lod->setRange( 1, 0.0f, radius * 5.0f );if ( _maxLevel<level ) _maxLevel = level;return lod.release(); }osg::Node* OctreeBuilder::createElement( const std::string& id, const osg::Vec3& center, float radius ) {osg::ref_ptr<osg::Geode> geode = new osg::Geode;geode->addDrawable( new osg::ShapeDrawable(new osg::Sphere(center, radius)) );geode->setName( id );return geode.release(); }osg::Geode* OctreeBuilder::createBoxForDebug( const osg::Vec3& max, const osg::Vec3& min ) {osg::Vec3 dir = max - min;osg::ref_ptr<osg::Vec3Array> va = new osg::Vec3Array(10);(*va)[0] = min + osg::Vec3(0.0f, 0.0f, 0.0f);(*va)[1] = min + osg::Vec3(0.0f, 0.0f, dir[2]);(*va)[2] = min + osg::Vec3(dir[0], 0.0f, 0.0f);(*va)[3] = min + osg::Vec3(dir[0], 0.0f, dir[2]);(*va)[4] = min + osg::Vec3(dir[0], dir[1], 0.0f);(*va)[5] = min + osg::Vec3(dir[0], dir[1], dir[2]);(*va)[6] = min + osg::Vec3(0.0f, dir[1], 0.0f);(*va)[7] = min + osg::Vec3(0.0f, dir[1], dir[2]);(*va)[8] = min + osg::Vec3(0.0f, 0.0f, 0.0f);(*va)[9] = min + osg::Vec3(0.0f, 0.0f, dir[2]);osg::ref_ptr<osg::Geometry> geom = new osg::Geometry;geom->setVertexArray( va.get() );geom->addPrimitiveSet( new osg::DrawArrays(GL_QUAD_STRIP, 0, 10) );osg::ref_ptr<osg::Geode> geode = new osg::Geode;geode->addDrawable( geom.get() );geode->getOrCreateStateSet()->setAttribute(new osg::PolygonMode(osg::PolygonMode::FRONT_AND_BACK, osg::PolygonMode::LINE) );geode->getOrCreateStateSet()->setMode( GL_LIGHTING, osg::StateAttribute::OFF );return geode.release(); }

總結(jié)

以上是生活随笔為你收集整理的使用八叉树结构来管理场景的全部內(nèi)容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網(wǎng)站內(nèi)容還不錯，歡迎將生活随笔推薦給好友。