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(Here I eat my words on Meta about posting Unity questions on Unity Answers... since that site is less responsive than this one.)

Unity provides cell-based Occlusion Culling (via Umbra, I believe). However, a newer feature that it supports is Occlusion Portals.

The question is, if BSP-based occlusion culling is already a feature of Unity, what do portals add, and how?

PS. This question is not "What are portals?" -- I'm aware of the original Quake BSP-style portals -- which is partly why I find the explicit portal concept in Unity odd, since it uses BSP anyway.

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If you can't get any satisfactory answers here, you might try and ask Aras Pranckevičius (!/aras_p). Something tells me he might know... ;) – Koarl Apr 20 '12 at 14:01
According to this, the point of occlusion portals is that they can be enabled/disabled (closed/open). I'm curious if they are built into the BSP used by Umbra or function more like Unreal Engine 2's anti-portals..‌​. – Torious Jun 6 '12 at 22:38
up vote 3 down vote accepted

As far as I understand, portal culling is one of many culling techniques.

That being said, we can infer that Unity could pre-compute potentially visible sets (PVSs) in many complementary ways: computing what's inside the frustum (visibility culling), hierarchically subdividing space and querying it (traditional occlusion culling), dividing the space in rooms and discarding objects that are in a room disconnected from the viewer's room (part of portal culling), etc.

So, what would portal culling add to BSP-based occlusion culling?

PVSs could be determined faster.

Also, saying that you can open/close portals in runtime means you'll recompute your PVS dynamically so that maybe you can determine visibility more accurately.

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I believe this is explained in detail in the Umbra 3 article here.

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Could you quote the relevant parts of the article for us, in case it goes down? – BlueRaja - Danny Pflughoeft Jan 31 '14 at 23:17

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