Fix: Unity Light Baking Stuck at Percent

Here is how to fix Unity Progressive lightmap bakes that freeze at 30%, 70%, or some other oddly stable percentage and never finish. The progress bar is steady. CPU/GPU usage hovers near zero. The editor is technically responsive but the bake is dead. Recovering means understanding which sub-step stalled and either changing the lightmapper or reducing scene complexity.

The Symptom

Hit Generate Lighting. Progress climbs to some value (commonly 30%, 70%, or 92%) then stops. CPU/GPU activity drops. No error in the console. Cancel only sometimes works; otherwise you have to kill the editor.

What Causes This

GPU memory exhaustion. Progressive GPU loads the entire bake context into VRAM. A scene larger than your GPU’s available memory stalls when paging fails.

OptiX denoiser unavailable. If you enabled OptiX but lack a CUDA-capable NVIDIA GPU, the denoiser request silently fails and the bake stalls.

Single oversized chart. One mesh with a 4096x4096 lightmap chart can dominate the bake. Other charts are starved while it processes.

Editor.log silent crash. The bake worker process can crash without surfacing to the editor UI. Check the log file directly.

The Fix

Step 1: Try Progressive CPU. Open Window → Rendering → Lighting → Scene. Set Lightmapper to Progressive CPU. Slower but memory-tolerant. Often finishes when GPU stalls.

Step 2: Lower bake size limits.

Lighting Settings -> Lightmapping Settings:
Max Lightmap Size:    512     (was 1024)
Lightmap Resolution:  10       (was 40)
Direct Samples:       32       (was 64)
Indirect Samples:     128      (was 512)
Environment Samples:  128      (was 256)

Lower these and re-bake. If it succeeds, your earlier bake was hitting memory or timeout limits.

Step 3: Disable OptiX if your GPU does not support it. In Lighting → Filtering, set Denoiser to None or Optix only if you have a recent NVIDIA card. Open Image Denoise (OIDN) works on AMD/Intel and does not stall on missing CUDA.

Step 4: Find oversized charts.

// Editor script: find renderers with huge lightmap area
[UnityEditor.MenuItem("Tools/Find Big Lightmap Charts")]
static void Find()
{
    foreach (var r in Object.FindObjectsByType<Renderer>(FindObjectsSortMode.None))
    {
        Vector4 st = r.lightmapScaleOffset;
        float coverage = st.x * st.y;
        if (coverage > 0.1f)
            Debug.Log($"{r.name}: {coverage:P0} of lightmap", r);
    }
}

Anything over 10% of a single lightmap is usually a chart you should split (mark the renderer’s Scale In Lightmap lower).

Step 5: Bake in chunks. Split the scene into smaller scenes (BG terrain, props, hero areas). Bake each separately. Combine via additive scene loading or lightmap groups.

Reading Editor.log

If the bake is stuck, open Editor.log:

# macOS
~/Library/Logs/Unity/Editor.log

# Windows
%LOCALAPPDATA%\Unity\Editor\Editor.log

# Linux
~/.config/unity3d/Editor.log

Look for repeated “Out of memory”, “OptiX initialization failed”, or worker process exit codes. The error usually appears within a few lines of where the stall began.

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

This bug class disproportionately affects late-stage development. The work to surface it is interactive testing in realistic conditions, which only really happens after the gameplay is in place and assets are populated. Catching it early requires deliberate testing of conditions that look unimportant.

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

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

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

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

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

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

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.

“If GPU bake stalls, switch to CPU. If CPU stalls, the scene is too big — split it.”

Related Issues

For baked GI seams, see Baked GI Seams. For occlusion baking, see Occlusion Portal.

CPU first when GPU stalls. Smaller charts. Editor.log tells the truth.