Fix: Unity Occlusion Portal Not Occluding Static Geometry

A castle level uses Occlusion Portals on doors to cull entire rooms when doors are closed. The portals are placed, the doors animate, but the rooms beyond render unchanged regardless of door state. Memory bandwidth doesn’t drop; frame rate doesn’t improve.

Three Required Pieces

Working Occlusion Portals need:

1. An OcclusionPortal component on a child GameObject with a fitted bounding box (set the OcclusionPortal’s Center and Size to match the door’s opening).
2. The portal’s frame (door frame, walls around the opening) marked Occluder Static.
3. The scene rebaked via Window → Rendering → Occlusion Culling → Bake with the portal in the scene.

At runtime, toggle portal.open = false to close (occlude beyond) or true to open (render normally).

Step 1: Add the Portal

On your door GameObject (or a child), add an OcclusionPortal component. Set Center and Size to match the door opening exactly. Too small: leaks light through edges. Too large: door area includes adjacent geometry that shouldn’t be portal-controlled.

Step 2: Mark Surrounding Geometry Static

The door frame, walls flanking the opening, and the floor/ceiling at the opening must be Occluder Static. The bake uses these as anchors for portal-aware visibility. Without static neighbors, the portal can’t meaningfully cull.

Step 3: Rebake

Open Window → Rendering → Occlusion Culling. Click Bake. The bake takes 1–30 minutes depending on scene size. Watch the bake progress; the result is an Occlusion.asset stored next to the scene.

Step 4: Runtime Toggle

using UnityEngine;

public class Door : MonoBehaviour
{
    [SerializeField] OcclusionPortal portal;

    public void Open() {
        portal.open = true;
        // play animation, etc.
    }

    public void Close() {
        portal.open = false;
    }
}

Close the door, watch the room beyond stop rendering. Frame rate should improve in large multi-room levels.

Diagnose with the Stats Window

Game view → Stats. Note “Saved by occlusion culling”. Should be non-zero when closed portals hide a room. If it stays at 0, the portal isn’t affecting culling — check the three-step setup again.

Common Mistakes

• Portal Size too small to fully cover the opening.
• Door geometry itself marked Occluder Static — baked occlusion treats it as a fixed wall and ignores the portal.
• Forgot to rebake after adding the portal.

Verifying

Walk into the doorway. Toggle portal.open via a debug command. With it false, scene complexity behind should drop dramatically; with true, restored. Stats panel confirms the saved-by-occlusion count changing.

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

Related bug classes often share the same root cause. If you find yourself fixing this issue, look for cousins: similar symptoms in adjacent systems, the same data flow but a different value, or the same fix pattern in another module. The catalog of 'we've seen this before' becomes valuable institutional knowledge.

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

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

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

The tooling around this bug class matters as much as the fix itself. Good logging, accessible profilers, and clear error messages turn 30-minute investigations into 5-minute ones. If your project doesn't have visibility into this code path, the first fix should add the visibility - the second fix uses it.

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.

“Portals + Static + Bake. All three for working occlusion. Miss one and the door is decoration.”

For procedural levels where rebaking is impractical, use Occlusion Areas + manual visibility scripting instead — portals don’t fit dynamic geometry.