如何在OpenGL中始终将对象保留在其他所有内容之前？

例如,我在地面上绘制了一个平坦的地形,在XZ平面上Y = 0.摄像机位于Y = 1.75(模拟一般人的身高).如果我抬头,它工作正常,如果我向下看,它会被地平面夹住.

向上看：http://i.stack.imgur.com/Q0i6g.png
往下看：http://i.stack.imgur.com/D5LIx.png

我调用的函数在角落绘制轴系统是这样的：

void Axis3D::DrawCameraAxisSystem(float radius,float height,const Vector3D rotation) {
    if(vpHeight == 0) vpHeight = 1;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glViewport(0,vpWidth,vpHeight);
    gluPerspective(45.0f,1.0 * vpWidth / vpHeight,1.0f,5.0f);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    glTranslatef(0.0f,0.0f,-3.0f);

    glrotatef(-rotation.x,0.0f);
    glrotatef(-rotation.y,0.0f);

    DrawAxisSystem(radius,height);
}

我认为现在有几个主要功能与问题相关：

glutdisplayFunc(renderScene);
glutReshapeFunc(changeSize);

void changeSize(int width,int height) {
    if(height == 0) height = 1;

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();

    glViewport(0,width,height);

    gluPerspective(60.0f,1.0 * width / height,1000.0f);

    glMatrixMode(GL_MODELVIEW);
}

void renderScene(void) {
    glClearColor(0.7f,0.7f,0.0f);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    changeSize(glutGet(gluT_WINDOW_WIDTH),glutGet(gluT_WINDOW_HEIGHT));

    glLoadIdentity();

    SceneCamera.Move(CameraDirection,elapsedtime);
    SceneCamera.LookAt();

    SceneAxis.DrawCameraAxisSystem(0.03f,0.8f,SceneCamera.GetRotationAngles());

    glutSwapBuffers();
}

建议？