Quick answer: Tetrahedralization needs probes distributed in 3D. All-coplanar probe placements produce degenerate triangulation. Add probes at multiple heights (floor, head, ceiling) and at least one near the sky for outdoor scenes.

Here is how to fix Unity Light Probe systems that warn about tetrahedralization failure, leave dynamic objects unlit in some areas, or produce sudden dark patches when characters move between rooms. Light probes need 3D distribution; flat rings of probes form degenerate volumes that the engine cannot interpolate over.

The Symptom

Console warning: Light probe placement is degenerate; tetrahedralization may fail. Dynamic objects (NPCs, player) appear flat-lit or unlit in certain areas. Crossing certain spots produces sudden lighting changes.

What Causes This

Coplanar probes. Light Probe Group with all probes at Y=0 produces a flat plane; the engine cannot tetrahedralize a flat point set.

Holes in coverage. Probes around a doorway but none inside the next room leaves a gap; objects in the gap fall outside the probe convex hull.

No sky probe. Outdoor objects above ground level fall outside the volume; sample wrong.

Probe Group inactive. An inactive probe group’s probes do not contribute. Verify groups are enabled.

The Fix

Step 1: Add probes at multiple heights. For each room, place probes at:

This forms tetrahedra throughout the volume.

Step 2: Cover doorways with probes. Place probes at the threshold of each room transition, both inside and outside, so the volume connects continuously.

Step 3: Add a high sky probe for outdoor scenes. Place at least one probe well above the playable area (e.g., 50 m up). Captures sky color; dynamic objects above ground sample it.

Step 4: Use multiple Light Probe Groups for clarity. Per-room groups make placement maintainable. Unity merges them into a single tetrahedralization.

Step 5: Visualize and verify. Open Window → Rendering → Lighting. Enable Light Probe Visualization. Walk dynamic objects through your scene; the probe blend cells should color them correctly without abrupt changes.

Use Adaptive Probe Volumes For Easier Setup

Unity 2022+ added Adaptive Probe Volumes (APV) which auto-place probes based on geometry. Switch from Light Probe Groups to APV for a far smoother experience — no manual probe placement needed.

Understanding the issue

AI bugs are emergent. The code is correct in isolation; the behavior emerges from interaction with other systems. Reproducing means controlling the interaction; fixing means deciding which interaction was wrong.

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

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

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

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

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

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

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

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

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.

“Probes need 3D distribution. Multi-level placement avoids tetrahedralization failure. Sky probe for outdoors. APV for the easy life.”

Related Issues

For lightmap leaks, see Baked Light Leaks. For bake stalls, see Lightmap Bake Stuck.

Floor, head, ceiling. Cover doorways. Sky probe. The dynamic objects light correctly.