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Gamebryo Technic:
NiPortal objects are grouping nodes that support aggressive visibility culling. They represent flat polygonal regions through which a part of a scene graph can be viewed. In other words, portals represent doorways and windows in a scene, as well as the objects seen through such openings.
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Example:
- Door = Look / transit from outside world into a inside world space.
- Door = Look / transit from inside world into a outside world space.
- Window = Look from the outside world into a inside world space.
- Window = Look from the inside world into a outside world space.
You can open a door and then look / move inside that world.