> With the camera in the right place, all that remained was to avoid the problem of objects behind the output portal occluding the view through the portal, which I accomplished simply by setting the distance to the near plane in the projection matrix to be equal to the viewer's distance from the portal.
вот этот пункт у меня ваще не сделан. Мне кажется near plane не поможет, потому что отсекать надо поверхностью портала, а она не перпендикулярна оси взгляда, в отличие от near plane