Learning OSG programing---osgAnimation（2）

 osg::Node* createBase(const osg::Vec3& center,float radius)
 {

     int numTilesX = ;
     int numTilesY = ;

     float width = *radius;
     float height = *radius;

     osg::Vec3 v000(center - osg::Vec3(width*0.5f,height*0.5f,0.0f));
     osg::Vec3 dx(osg::Vec3(width/((float)numTilesX),0.0,0.0f));
     osg::Vec3 dy(osg::Vec3(0.0f,height/((float)numTilesY),0.0f));

     // fill in vertices for grid, note numTilesX+1 * numTilesY+1...
     osg::Vec3Array* coords = new osg::Vec3Array;
     int iy;
     for(iy=;iy<=numTilesY;++iy)
     {
         for(int ix=;ix<=numTilesX;++ix)
         {
             coords->push_back(v000+dx*(float)ix+dy*(float)iy);
         }
     }

     //Just two colours - black and white.
     osg::Vec4Array* colors = new osg::Vec4Array;
     colors->push_back(osg::Vec4(1.0f,1.0f,1.0f,1.0f)); // white
     colors->push_back(osg::Vec4(0.0f,0.0f,0.0f,1.0f)); // black

     osg::ref_ptr<osg::DrawElementsUShort> whitePrimitives = new osg::DrawElementsUShort(GL_QUADS);
     osg::ref_ptr<osg::DrawElementsUShort> blackPrimitives = new osg::DrawElementsUShort(GL_QUADS);

     int numIndicesPerRow=numTilesX+;
     for(iy=;iy<numTilesY;++iy)
     {
         for(int ix=;ix<numTilesX;++ix)
         {
             osg::DrawElementsUShort* primitives = ((iy+ix)%==) ? whitePrimitives.get() : blackPrimitives.get();
             primitives->push_back(ix    +(iy+)*numIndicesPerRow);
             primitives->push_back(ix    +iy*numIndicesPerRow);
             primitives->push_back((ix+)+iy*numIndicesPerRow);
             primitives->push_back((ix+)+(iy+)*numIndicesPerRow);
         }
     }

     // set up a single normal
     osg::Vec3Array* normals = new osg::Vec3Array;
     normals->push_back(osg::Vec3(0.0f,0.0f,1.0f));

     osg::Geometry* geom = new osg::Geometry;
     geom->setVertexArray(coords);

     geom->setColorArray(colors, osg::Array::BIND_PER_PRIMITIVE_SET);

     geom->setNormalArray(normals, osg::Array::BIND_OVERALL);

     geom->addPrimitiveSet(whitePrimitives.get());
     geom->addPrimitiveSet(blackPrimitives.get());

     osg::Geode* geode = new osg::Geode;
     geode->addDrawable(geom);

     return geode;
 }

createBase函数用于建立模型模拟时的地板，作为飞机飞行的参照。

23行中，循环向变量coords中添加坐标点，作为地板的控制点。

36-46行中，交替取出白色面元和黑色面元，并向其中每次添加四个顶点（索引值），指示面元的角点坐标（按照索引值从coords数组中取出）。

随后设置所有面元的法向量，并将黑白面元分别添加进绘制几何节点中，并将绘制几何节点添加到集合节点中，由函数返回。

接下来分析第三个函数：

 osg::Node* createMovingModel(const osg::Vec3& center, float radius)
 {
     float animationLength = 10.0f;

     osg::AnimationPath* animationPath = createAnimationPath(center,radius,animationLength);

     osg::ref_ptr<osg::Group> model = new osg::Group;

     osg::ref_ptr<osg::Node> glider = osgDB::readRefNodeFile("glider.osgt");
     if (glider)
     {
         const osg::BoundingSphere& bs = glider->getBound();

         float size = radius/bs.radius()*0.3f;
         osg::MatrixTransform* positioned = new osg::MatrixTransform;
         positioned->setDataVariance(osg::Object::STATIC);
         positioned  ->setMatrix(osg::Matrix::translate(-bs.center())*
                                      osg::Matrix::scale(size,size,size)*
                                      osg::Matrix::rotate(osg::inDegrees(-90.0f),0.0f,0.0f,1.0f));

         positioned->addChild(glider);

         osg::PositionAttitudeTransform* xform = new osg::PositionAttitudeTransform;
         xform->setUpdateCallback(new osg::AnimationPathCallback(animationPath,0.0,1.0));
         xform->addChild(positioned);

         model->addChild(xform);
     }

     osg::ref_ptr<osg::Node> cessna = osgDB::readRefNodeFile("cessna.osgt");
     if (cessna)
     {
         const osg::BoundingSphere& bs = cessna->getBound();

         float size = radius/bs.radius()*0.3f;
         osg::MatrixTransform* positioned = new osg::MatrixTransform;
         positioned->setDataVariance(osg::Object::STATIC);
         positioned->setMatrix(osg::Matrix::translate(-bs.center())*
                                      osg::Matrix::scale(size,size,size)*
                                      osg::Matrix::rotate(osg::inDegrees(180.0f),0.0f,0.0f,1.0f));

         positioned->addChild(cessna);

         osg::ref_ptr<osg::MatrixTransform> xform = new osg::MatrixTransform;
         xform->setUpdateCallback(new osg::AnimationPathCallback(animationPath,0.0f,2.0));
         xform->addChild(positioned);

         model->addChild(xform);
     }

     #ifndef OSG_GLES2_AVAILABLE
     model->getOrCreateStateSet()->setMode(GL_NORMALIZE, osg::StateAttribute::ON);
     #endif

     return model.release();
 }

这个函数的作用是创建运动模型。

　　首先利用上一节介绍的createAnimationPath 函数创建模拟路径，并以此为基础加载模型，并控制模型的运动。

　　动态更新的核心就是设置数据变度属性DataVariance，它决定了OSG在多线程渲染的过程中的执行策略：只有所有DYNAMIC属性的对象被渲染 完毕之后，OSG才会开始执行下一帧的用户更新操作；这样有效地可以避免数据的过快更新造成当前的渲染动作出错，以致系统崩溃。

　　将模型进行平移，缩放，并绕z轴旋转180度。创建转换函数，并设置其更新回调函数。AnimationPathCallback的构造函数为：

osg::AnimationPathCallback::AnimationPathCallback     (     AnimationPath *      ap,
        double      timeOffset = 0.0,
        double      timeMultiplier = 1.0
    )     

该函数的第三个参数决定了节点旋转的速度。因此cessna模型的旋转速度是glider的二倍。这两个模型的加载和回调更新函数的设定过程相同，只是参数有所不同。