Quick answer: Set Smallest Hole lower than the smallest doorway, ensure dynamic objects are not marked Occludee Static, and rebake. Inspect the visibility cells visualization to find pops.

The player walks toward a doorway. Two steps in, the chandelier in the next room vanishes. Three more steps, it re-appears. The bake completed cleanly with no warnings. Something in the visibility data is wrong.

How Occlusion Culling Decides

Unity’s occlusion culling divides the world into a grid of view cells. For each cell, the bake precomputes which renderers are visible from any camera position within the cell, considering all Occluder Static geometry as blockers. At runtime, the camera’s current cell is looked up and only the precomputed-visible set is drawn.

The system has three knobs:

Pop bugs almost always trace back to mis-tuned Smallest Hole or to dynamic objects accidentally marked static.

Fix 1: Set Smallest Hole Below Your Doorway Width

If your narrowest passage is 1.5 m wide, set Smallest Hole to 0.5 (one-third of the passage). Larger values cause the baker to treat the doorway as solid and hide everything beyond it; smaller values bloat the bake.

Open Window → Rendering → Occlusion Culling → Bake and adjust Smallest Hole. Rebake. Check the Visualization tab to confirm the previously invisible room is now in the visible set when the camera is inside the doorway frustum.

Fix 2: Audit Static Flags

Select every dynamic object in the scene (players, enemies, projectiles, doors that move) and ensure Static is unchecked. Specifically, both Occluder Static and Occludee Static must be off.

For a door that opens, you need it to not be Occluder Static — the bake captured it closed. When it opens at runtime, the visibility data still thinks the doorway is blocked. Either keep doors out of the static set, or use Occlusion Areas with separate baked variants for open and closed states.

Fix 3: Use Occlusion Areas for Cell Density

In large levels, view cells get coarse and pops increase at cell boundaries. Add an Occlusion Area volume around regions where the camera actually goes (corridors, rooms) and leave large open spaces without one. The bake produces denser cells inside the area, more accurate visibility within them, and far fewer pops at cell transitions.

Fix 4: Mesh Bounding Box Sanity

If a mesh’s bounding box extends far beyond its visible geometry — common with characters whose rig includes off-mesh helper bones — the renderer is correctly culled when its bounds enter occluded space, even when the visible part is in plain sight. Inspect the MeshRenderer Bounds in the Scene view (toggle “Show bounds” in the inspector). Recalculate bounds with SkinnedMeshRenderer.updateWhenOffscreen = true for animated meshes:

GetComponent<SkinnedMeshRenderer>().updateWhenOffscreen = true;

This costs per-frame bounds recomputation but eliminates “hidden when on screen” bugs for animated characters.

Step-by-Step Diagnosis

  1. Open Occlusion → Visualization and move the camera to where the pop happens.
  2. Note the highlighted cell.
  3. Click “Lock” and rotate the camera. The visible renderer list updates with each rotation. If the popping object never appears in the list, it’s missing from the cell’s visibility data.
  4. Check the object’s static flags and renderer bounds. Likely culprit.
  5. If everything looks correct, lower Smallest Hole and rebake.

Verifying

Build a navigation script that pans the camera through the affected area and logs Renderer.isVisible on the popping object. Before the fix, you’ll see false at the pop location with true a step later. After the fix, the value stays true across the transition.

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

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

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

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

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

Live games surface this bug class at scale. What's a rare edge case in development becomes a daily occurrence once you have a few thousand concurrent players. The class isn't 'this player has a unique setup'; it's 'one in N thousand sessions will trigger this exact combination'.

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.

“Occlusion culling is a precomputed lie. If the lie doesn’t match runtime reality, your scene pops. Tune the bake to match reality.”

When in doubt, lower Smallest Hole and add an Occlusion Area — both errors are towards over-conservative visibility.