为什么三角形在这个网格中被裁剪?

我有以下代码:

#include <irrlicht.h>

using namespace irr;

using namespace core;
using namespace scene;
using namespace video;
using namespace io;
using namespace gui;

int main(int argc, char** argv)
{
    IrrlichtDevice *device =
        createDevice(EDT_SOFTWARE, dimension2d<unsigned int>(640, 480), 16,
            false, false, false, 0);

    device->setWindowCaption(L"Train Simulator Demo");

    IVideoDriver* driver = device->getVideoDriver();
    ISceneManager* smgr = device->getSceneManager();
    IGUIEnvironment* guienv = device->getGUIEnvironment();

    IAnimatedMesh* mesh1 = smgr->getMesh("media/simple_engine.b3d");
    IAnimatedMesh* mesh2 = smgr->getMesh("media/simple_track.b3d");

    IAnimatedMeshSceneNode* node1 = smgr->addAnimatedMeshSceneNode( mesh1 );
    IAnimatedMeshSceneNode* node2 = smgr->addAnimatedMeshSceneNode( mesh2 );

    if (node1)
        node1->setMaterialFlag(EMF_LIGHTING, false);

    if (node2)
    {
        node2->setMaterialFlag(EMF_LIGHTING, false);
        node2->setMaterialTexture( 0, driver->getTexture("media/grass.jpg") );
    }

    ICameraSceneNode * pCamera = smgr->addCameraSceneNode(0, vector3df(5,0,5), vector3df(0,0,0));

    pCamera->setNearValue(0.5f);
    pCamera->setFarValue(40.0f);

    while(device->run())
    {
        driver->beginScene(true, true, SColor(0,0,0,0));

        u32 cur_time = device->getTimer()->getTime();
        float f_time = (float)cur_time / 1000.0f;

        // Change the camera
        pCamera->setPosition(vector3df(sinf(f_time) * 8.0f ,3.0f ,cosf(f_time) * 8.0f));

        smgr->drawAll();
        guienv->drawAll();

        driver->endScene();
    }

    device->drop();

    return 0;
}

…运行时会产生以下结果:

……这很好,只是草应该是16×16的正方形网格,看起来有些三角形是因为某种原因被修剪的。这似乎还与三角形仅部分显示在屏幕上有关,因为仅显示完全位于窗口视区内的三角形。我该怎么解决?

附加:

操作系统: Ubuntu 10.04 64位
鬼火 版本:1.7 beta