the camera renderes only a small part of the virtual world. only the area between the near clipping plane and the far clipping plane. anything outside will be clipped away.
your models are partly outside of this area.
modify, like gri suggested, the near clipping plane of the camera for example:
int main()
{
...
camera.clip_near = 1;
...
}
you should find the best values for the clipping plane that your game looks good. the near clipping plane as high as possible and the far clipping plane as small as possible (but clip_near < clip_far). then the engine can clip as many as possible models away and this may increase the rendering speed and the FPS.
look in the manual for further information (for example how particels behave in the clipping volume - it is really good explained in the manual)
