osg:多方位观察牛~

#include <osg/Camera>
#include <osgDB/ReadFile>
#include <osgViewer/Viewer>

osg::Camera* createBirdsEye( const osg::BoundingSphere& bs )
{
    osg::ref_ptr<osg::Camera> camera = new osg::Camera;
    camera->setClearMask( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
    camera->setReferenceFrame( osg::Transform::ABSOLUTE_RF );
    
    double viewDistance = 2.0 * bs.radius();
    double znear = viewDistance - bs.radius();
    double zfar = viewDistance + bs.radius();
    float top = bs.radius();
    float right = bs.radius();
    float bottom = top;
    float left = right;
    //osg左手系坐标(z+向上，x+向右，y+向里)

    camera->setProjectionMatrixAsOrtho( -left, right, -bottom, top, znear, zfar );//设置正投影

    //牛的身长变短了（在视口大小没有变化的情况下，加大left和right(原本是6.3),牛的左右相对于裁剪区域来说，就缩小了,我是这么理解。）
    //camera->setProjectionMatrixAsOrtho( -10, 10, -bottom, top, znear, zfar );

    //牛的身长变窄了（同上理）
    //camera->setProjectionMatrixAsOrtho( -left, right, -20, 20, znear, zfar );

    //改变了znear和zfar，好像没什么变化，为什么呢，待找答案。****
     //camera->setProjectionMatrixAsOrtho( -left, right, -bottom, top, 1, 1 );

    /********* openGL解释***************************************************************************
        setProjectionMatrixAsOrtho(left,right,bottom,top,zNear,zFar);

        left和right表示最小和最大x值，top和bottom是最小和最大的y值，near和far是最小和最大的z值。

        left：裁剪区域最左边的坐标。
        right:裁剪区域最右边的坐标。

        bottom：最底部的坐标。
        top:最顶部的坐标。

        near：从原点距离观察者的最小位置。
        far：从原点距离观察者的最大位置。
    **************************************************************************************************/
    
    osg::Vec3d upDirection( 0.0,1.0,0.0 );
    osg::Vec3d viewDirection( 0.0,0.0,1.0 );
    osg::Vec3d center = bs.center();
    osg::Vec3d eyePoint = center + viewDirection * viewDistance;
    //从前面看牛
    //eyePoint.set(0.77,-10,0.5);

    //从牛头看牛
    //upDirection.set(0.0,0.0,1.0);
    //eyePoint.set(10,-0.4,0.5);

    //从牛下巴看牛
    //upDirection.set(1.0,0.0,-1.0);
    //eyePoint.set(100,-0.4,-100);

    camera->setViewMatrixAsLookAt( eyePoint, center, upDirection );//相机位置（眼睛位置，眼睛看到的场景中心位置，视点向上的量）
    //eyePoint（0.77，-0.4，12.7） center(0.77,-0.4,0.0) upDirection(0,1,0) zfar = 19,redius = 6.3
    
    return camera.release();
}

int main( int argc, char** argv )
{
    osg::ArgumentParser arguments( &argc, argv );
    osg::Node* model = osgDB::readNodeFiles( arguments );
    if ( !model ) model = osgDB::readNodeFile( "cow.osg" );
    
    osg::Camera* camera = createBirdsEye( model->getBound() );
    camera->addChild( model );
    
    osgViewer::Viewer viewer;
    viewer.setSceneData( camera );
    viewer.setUpViewInWindow(40,40,800,600);
    return viewer.run();
}