Occlusion Culling System Setup for Hidden Graphics Optimization

Occlusion Culling System Setup for Hidden Graphics

Occlusion culling—one of few optimization tools giving noticeable VR performance improvement almost free. Essence: don't render objects behind other objects, physically invisible. In complex VR scenes with multiple rooms or enclosed spaces, this reduces draw calls 2–5x.

Where occlusion culling yields results and where it's useless

Occlusion culling effective in enclosed spaces with opaque geometry: corridors, rooms, buildings, caves. Wall between two rooms—excellent occluder. Camera looks into one room, everything in neighboring room doesn't render.

Open spaces, natural scenes, landscapes—occlusion culling barely helps. With horizon open, most objects hit frustum and nothing occludes them. Here frustum culling works (already built into Unity), LOD groups and portals matter.

VR specificity: player has two eyes and two frustums. Headset FOV (100–110° on Meta Quest 3) larger than typical monitor. Meaning culling efficiency slightly lower than desktop games with narrow FOV—more objects enter frame. So correct bake is important.

Umbra configuration in Unity

Unity uses Umbra as occlusion culling engine. Configure via Window → Rendering → Occlusion Culling. Key bake parameters:

Smallest Occluder—minimum object size considered as occluder. Default value 5 units—for VR games with detailed interiors often need lowering to 1–2, else thin walls and columns don't become occluders. If 0.3m wall doesn't block render of room behind—check this parameter.

Smallest Hole—size of opening through which occluder "sees" geometry beyond. Too small—doorways and windows leak, objects behind still render. Too large—occluder ignores real openings.

Backface Threshold—how aggressively PVS graph simplifies. Value 100 = maximum aggression (less data, pop-in artifact risk). For VR better 25–50: pop-in in VR more noticeable than flat screen.

Critical: all static occluder objects must be tagged Occluder Static and Occludee Static in Inspector. If one large wall missed—culling hole.

Dynamic objects and occlusion culling

Occlusion culling in Unity works only with static geometry on occluder side. Dynamic objects (enemies, pickups, particles) can't be occluders—don't participate in bake. But they can be culled by static occluders if tagged Occludee.

For VR games with many dynamic objects in enclosed spaces—also consider Portal-based culling via custom scripts or third-party solutions like SECTR or Portals & Rooms in Unity. Portal culling works with explicitly defined portals (doorways) and gives more predictable result than Umbra in complex cases.

Verifying configuration correctness

After bake—mandatory visualization via Occlusion Culling → Visualization in Scene View. See PVS (Potentially Visible Set) for any camera position: which objects considered visible from that point.

Typical artifact—occluder cuts geometry too aggressively at edges: player sees object appear from air when turning head. In VR this especially unpleasant. Fixed by increasing Smallest Hole or adding additional occluders in transition zones.

Performance check tool: Unity Frame Debugger after bake—look at object count in Render.OpaqueGeometry for each camera position before and after culling.

Cost is determined after analyzing project scenes and target platforms.