OSG 学习第一天：场景的组织及渲染

OSG 场景的组织及渲染

前言：翻开导师给的OSG学习资料，打开各个参考书。如《OpenSceneGraph三维渲染引擎编程指南 》、《OpenSceneGraph程序开发指南 》。发现osgChina是十年前从我们实验室走出去的，看着大师兄们留下的资料感觉无比的荣幸；看着他们最初的奋斗历程异常艰辛，他们是多么的努力多么对学习充满激情，深感自愧不如。

有意愿了解osg在中国的朋友，可以登录osgChina 查看。

本文参照：《OpenSceneGraph三维渲染引擎编程指南 》

1.Geode

osg::Geode是OSG中的叶节点，用于保存几何信息以便渲染。同时，作为叶节点，它就不会再包含子节点。在应用程序中，所有相关的几何体的渲染都必须与Geode节点相关联。

类说明：Geode 结点，是个几何结点，可以说是一个几何 Group 结点，一般的可绘制几何体都是通过它来 传向 root 进行渲染。是 OSG 几何绘制的最高管理结点。 

类继承图：

 

 

1.1布告板示例

Billboard节点继承自Geode节点，因此它是一个叶节点，不可再包含其他的子节点，只能像叶节点那样通过添加Drawable 来绘制信息。Billboard 有下面3种模式：

enum   Moda{      POINT_ROT_EYE; //绕视点      POINT_ROT_WORLD;  //绕世界坐标      AXIAL_ROT;     //绕轴}

说白了，布告板就是相当于纹理贴图。下面附上一段书中代码：

#include <osgViewer/Viewer>#include <osg/Node>#include <osg/Geode>#include <osg/Geometry>#include <osg/Group>#include <osg/Billboard>#include <osg/Texture2D>#include <osg/Image>#include <osg/PositionAttitudeTransform>#include <osg/MatrixTransform>#include <osgDB/ReadFile>#include <osgDB/WriteFile>#include <osgUtil/Optimizer>osg::ref_ptr<osg::Node> createBillboardTree(osg::ref_ptr<osg::Image> image){//创建四边形osg::ref_ptr<osg::Geometry> geometry = new osg::Geometry();//设置顶点osg::ref_ptr<osg::Vec3Array> v = new osg::Vec3Array();v->push_back(osg::Vec3(-0.5f,0.0f,-0.5f));v->push_back(osg::Vec3(0.5f,0.0f,-0.5f));v->push_back(osg::Vec3(0.5f,0.0f,0.5f));v->push_back(osg::Vec3(-0.5f,0.0f,0.5f));geometry->setVertexArray(v.get());//设置法线osg::ref_ptr<osg::Vec3Array> normal = new osg::Vec3Array();normal->push_back(osg::Vec3(1.0f,0.0f,0.0f)^osg::Vec3(0.0f,0.0f,1.0f));geometry->setNormalArray(normal.get());geometry->setNormalBinding(osg::Geometry::BIND_OVERALL);//设置纹理坐标osg::ref_ptr<osg::Vec2Array> vt = new osg::Vec2Array();vt->push_back(osg::Vec2(0.0f,0.0f));vt->push_back(osg::Vec2(1.0f,0.0f));vt->push_back(osg::Vec2(1.0f,1.0f));vt->push_back(osg::Vec2(0.0f,1.0f));geometry->setTexCoordArray(0,vt.get());//绘制四边形geometry->addPrimitiveSet(new osg::DrawArrays(osg::PrimitiveSet::QUADS,0,4));if (image.get()){//状态属性对象osg::ref_ptr<osg::StateSet> stateset = new osg::StateSet();//创建一个Texture2D属性对象osg::ref_ptr<osg::Texture2D> texture = new osg::Texture2D();//关联imagetexture->setImage(image.get());//关联Texture2D纹理对象，第三个参数默认为ONstateset->setAssociatedTextureModes(0,texture,osg::StateAttribute::ON);//启用混合stateset->setMode(GL_BLEND,osg::StateAttribute::ON);//关闭光照stateset->setMode(GL_LIGHTING,osg::StateAttribute::OFF);geometry->setStateSet(stateset.get());}//创建Billbroad 对象一osg::ref_ptr<osg::Billboard> billbroad1 = new osg::Billboard();//设置旋转模式为绕视点billbroad1->setMode(osg::Billboard::POINT_ROT_EYE);//添加Drawable,并设置其位置，默认位置为osg::Vec3(0.0f,0.0f,0.0f)billbroad1->addDrawable(geometry.get(),osg::Vec3(5.0f,0.0f,0.0f));osg::ref_ptr<osg::Billboard> billbroad2 = new osg::Billboard();//设置旋转模式为绕轴转，因此还需要设置旋转轴billbroad2->setMode(osg::Billboard::AXIAL_ROT);//设置旋转轴billbroad2->setAxis(osg::Vec3(0.0f,0.0f,1.0f));billbroad2->addDrawable(geometry.get(),osg::Vec3(10.0f,0.0f,0.0f));osg::ref_ptr<osg::Group> billbroad = new osg::Group();billbroad->addChild(billbroad1.get());billbroad->addChild(billbroad2.get());return billbroad.get();}int main(){osg::ref_ptr<osgViewer::Viewer> viewer = new osgViewer::Viewer();osg::ref_ptr<osg::Group> root = new osg::Group();//读取图像osg::ref_ptr<osg::Image> image = osgDB::readImageFile("Images/tree0.rgba");//缩放，达到合适的大小osg::ref_ptr<osg::PositionAttitudeTransform> pat = new osg::PositionAttitudeTransform();pat->setScale(osg::Vec3(5.0f,5.0f,5.0f));pat->addChild(createBillboardTree(image.get()));root->addChild(pat.get());//读取cow的模型，进行对比root->addChild(osgDB::readNodeFile("cow.osg"));//优化场景数据osgUtil::Optimizer optimizer;optimizer.optimize(root.get());viewer->setSceneData(root.get());viewer->realize();viewer->run();return 0;}

1.  建立新的Geometry实例，用于输入顶点，颜色等数据的osg::Vec3Array，osg::Vec4Array变量数组，以及用于建立点索引的osg::UByteArray，osg:: IntArray等。示例代码如下：

osg::ref_ptr<osg::Geometry> geo = new osg::Geometry;osg::Vec3Array* vecarray = new osg::Vec3Array;     //顶点坐标数组osg::UByteArray* vecindex = new osg::UByteArray;   //顶点索引数组osg::Vec4Array* colarray = new osg::Vec4Array;     //颜色RGB值数组osg::UByteArray* colindex = new osg::UByteArray;   //颜色索引数组osg::Vec3Array* norarray = new osg::Vec3Array;     //法线坐标数组osg::UByteArray* norindex = new osg::UByteArray;   //法线索引数组 

 2.向顶点坐标变量数组中输入顶点数据，osg::Vec3Array是一个模板类，继承自STL的vector，因此可以使用push_back方法送入osg::Vec3的坐标数据。颜色数据，法线坐标数据的保存与此类似，不过一般来说颜色数据使用osg::Vec4Array加以保存，除了RGB值之外，还包括一个Alpha分量。示例代码如下：

vecarray->push_back(osg::Vec3(1.0, 0.0, 1.0));vecarray->push_back(osg::Vec3(-1.0, 0.0, 1.0));vecarray->push_back(osg::Vec3(-1.0, 0.0, -1.0));vecarray->push_back(osg::Vec3(1.0, 0.0, -1.0)); colarray->push_back(osg::Vec4(1.0, 0.0, 0.0, 1.0));    //Redcolarray->push_back(osg::Vec4(0.0, 1.0, 0.0, 1.0));    //Greencolarray->push_back(osg::Vec4(0.0, 0.0, 1.0, 1.0));    //Bluecolarray->push_back(osg::Vec4(1.0, 1.0, 1.0, 1.0));    //White norarray->push_back(osg::Vec3(0.0, -1.0, 0.0));

2.Group

类描述：对结点起到组织作用，一般做为父结点或者根结点出现。它允许有一系列的孩子，相同的孩子或 者孩子之间有引用重得加入的会共享内存，是通过父类 osg::Referenced 实现自动管理内存句柄的。

类继承图： 

 

附上Group 的子类程序，位置变换节点示例：

#include <osgViewer/Viewer>#include <osg/Node>#include <osg/Geode>#include <osg/Group>#include <osg/PositionAttitudeTransform>#include <osgDB/ReadFile>#include <osgDB/WriteFile>#include <osgUtil/Optimizer>int main(){//创建Viewer对象，场景浏览器osg::ref_ptr<osgViewer::Viewer> viewer = new osgViewer::Viewer();//创建场景组节点osg::ref_ptr<osg::Group> root = new osg::Group();//创建一个节点，读取牛的模型osg::ref_ptr<osg::Node> node = osgDB::readNodeFile("cow.osg");//创建位置变化节点 pat1osg::ref_ptr<osg::PositionAttitudeTransform> pat1 = new osg::PositionAttitudeTransform();//设置位置为osg::Vec3(-10.0f, 0.0f, 0.0f)pat1->setPosition(osg::Vec3(-10.0f, 0.0f, 0.0f));//设置缩放，在X.Y.Z方向都缩小一倍pat1->setScale(osg::Vec3(0.5f, 0.5f, 0.5f));//添加子节点pat1->addChild(node.get());//创建位置变换节点pat2osg::ref_ptr<osg::PositionAttitudeTransform> pat2 = new osg::PositionAttitudeTransform();pat2->setPosition(osg::Vec3(10.0f, 0.0f, 0.0f));pat2->addChild(node.get());//添加到场景root->addChild(pat1.get());root->addChild(pat2.get());//优化场景数据osgUtil::Optimizer optimizer;optimizer.optimize(root.get());//设置场景数据viewer->setSceneData(root.get());//初始化并创建窗口viewer->realize();//开始渲染viewer->run();return 0;}