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#2 How Occlusion Culling Works: Portal-based Occlusion Culling

This video introduces portal based occlusion culling, a relatively fast and accurate technique that's stood the test of time, despite it's limitations. Algorithm overview below. Preprocessing: 1) Divide scene into "nodes." Division boundaries become "portals" 2) In each node, store which portals are connected to it and which nodes are behind those portals. Runtime: 1) Find node the camera is in, and mark it as visible. 2) If the node has portals in it, check each portal against the camera's view frustum. Otherwise, return. 3) If the portal is entirely outside the view frustum, ignore it. 4) If the portal goes entirely through the view frustum (i.e. the portal start/end points are outside the frustum, but the portal plane goes through it), then trivially accept the node behind the portal, and recurse to step 2 with the new node. 5) If the portal is only partially intersecting the view frustum, create a new frustum that is clipped against the portal so the frustum does not go outside the portal boundaries. Then accept the node behind the portal, and recurse to step 2 with the new node and frustum.