Fix: Unity Occlusion Portal Not Culling at Runtime

Here is how to fix Unity OcclusionPortal components that do not actually cull anything when closed. You add a portal at a doorway, code that sets portal.open = false when the door shuts, but the room beyond keeps rendering at full cost. Frame Debugger shows every object inside the closed room still being drawn. The fix is almost always missing baked occlusion data — OcclusionPortal does not work standalone.

The Symptom

An OcclusionPortal component is on a door GameObject. portal.open toggles correctly in the inspector. But objects on the other side still render even when closed. stat batches shows no reduction. The Occlusion Culling window shows the scene as “Not Baked” or with stale data.

What Causes This

No baked occlusion data. OcclusionPortal works only as part of Unity’s baked occlusion culling system. Without baking, the portal has nothing to cull through.

Static flags missing. Surrounding walls must be Occluder Static; objects to be hidden must be Occludee Static. Dynamic objects can be culled by portals only if a static occluder defines the volume.

Portal positioned incorrectly. The portal’s box must completely span the gap (doorway). Gaps around the edges leak visibility.

Stale bake after scene changes. Adding or moving static geometry invalidates the bake. Without re-bake, runtime culling uses old data that may not match the current scene.

The Fix

Step 1: Mark walls Occluder Static. Select all walls around the door. In the inspector, click Static dropdown and enable Occluder Static. Walls block visibility for the bake.

Step 2: Mark hidden room contents Occludee Static. Select the renderers inside the room you want to hide. Set their Static flag to Occludee Static (and not Occluder unless they should also block). Dynamic objects (NPCs, the player) can still be culled at runtime if the static topology is sound.

Step 3: Add the OcclusionPortal at the door. On the door GameObject, add the OcclusionPortal component. Resize its Center/Size box to span the doorway exactly. Sketch it slightly larger than the visual frame; gaps cause leaks.

Step 4: Bake occlusion. Open Window → Rendering → Occlusion Culling → Bake. Click Bake. Wait for completion. Confirm the scene’s baked data updates by viewing Visualization while moving the camera; objects on the wrong side of the closed portal should disappear.

Step 5: Toggle from script.

using UnityEngine;

[RequireComponent(typeof(OcclusionPortal))]
public class DoorOcclusion : MonoBehaviour
{
    private OcclusionPortal portal;

    void Awake() { portal = GetComponent<OcclusionPortal>(); }

    public void SetDoor(bool isOpen)
    {
        portal.open = isOpen;
        // Also toggle the visual mesh, animations, audio, etc.
    }
}

open = true means visibility passes through (door open). open = false means the portal blocks (door closed).

Common Mistakes

Adding OcclusionPortal but forgetting Static flags — the most common cause. Without static markers, the bake has nothing to occlude. Result: portal toggle does nothing.

Marking the door itself as Occluder Static. The door is a moving object; it cannot participate as a static occluder. Use OcclusionPortal instead.

Forgetting to rebake after moving walls. The bake snapshots geometry; subsequent moves are not reflected.

When To Use Other Approaches

OcclusionPortal works well for static buildings with discrete doors. For procedurally generated levels or open worlds, baked PVS is impractical. Consider:

• Dynamic occlusion (CullingGroup API) for sphere-based culling tied to gameplay regions.
• Streaming + LOD via World Streamer or Addressables to avoid loading hidden content at all.
• Manual SetActive(false) on whole rooms when entering/leaving via triggers, when you can be sure the player cannot peek around.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Unity Engine, the underlying behavior is shaped by how the engine layers its abstractions - the public API you call, the runtime systems that respond, and the platform-specific implementations underneath. A bug at any layer can produce symptoms that look like they originate at a different layer. Triaging effectively means recognizing which layer the symptom belongs to, even when the gameplay code is what's visible.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

After applying the fix, the verification step has three parts: confirm the original repro is resolved, confirm no obvious regressions in adjacent functionality, and (for shipping titles) deploy to a small player cohort first and watch the crash and report rates. Each step catches something the others miss.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

In shipping builds, this issue may interact with other production-only behavior. Stripping, encryption, asset bundling, and platform-specific code paths can each modify the symptoms. When players report a related issue, capture build SHA, platform, and any feature flags - those three fields cover most of the production-only variations.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

If this issue manifests under high load (many actors, many particles, many network connections), profile the post-fix code path with realistic counts. The original cost was a bug; the new cost is real work, and real work has a budget.

Diagnostic approach

Before applying any fix, gather enough context to be confident you're addressing the actual cause and not a similar-looking symptom. The cheapest diagnostic step is reproducing the bug deterministically - if you can't get the same failure twice in a row, your fix attempts will be hard to evaluate. Lock down the reproduction first.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Edge cases for this class of issue often involve specific timing: the first frame after a state change, the last frame before a transition, frames where multiple subsystems update simultaneously. Reproducing these reliably is part of what makes the bug class hard to test.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“OcclusionPortal is a runtime toggle on top of baked PVS. Without the bake, there is nothing to toggle.”

Related Issues

For occlusion not working at all, see Occlusion Culling Not Working. For LOD issues, see LOD Group Not Switching.

Static flags. Bake. Then portal.open works.