Quick answer: Snap shared vertices between adjacent LightOccluder2D nodes to identical coordinates and close each polygon. Increase Light2D.shadow_filter_smooth to mask sub-pixel residue.

A 2D dungeon scene lit by a single Light2D looks great except in one corner: a thin slice of light is making it into a room that should be in total darkness. The occluder walls are sitting right next to each other but a hairline leak betrays the seam.

The Geometry Problem

Light2D builds shadows by extruding each LightOccluder2D polygon away from the light source. If two occluders meet at a corner but their meeting vertices are off by even 0.001 pixels, the extrusion produces two non-overlapping shadow volumes with a gap between them. Light passes through that gap.

Causes of misaligned vertices:

Fix 1: Snap Vertices

Enable Use Snap in the 2D editor toolbar and set step size to 1 px (or your tile pixel size). Open each LightOccluder2D and re-edit its polygon — vertices will snap to integer coordinates. For TileSet occlusion shapes, ensure the “Snap Options” in the TileSet editor are enabled and the polygon corners sit on the tile boundaries.

For programmatically built occluders, quantize coordinates:

func snap_poly(poly: PackedVector2Array, grid: float) -> PackedVector2Array:
    var result = PackedVector2Array()
    for v in poly:
        result.append(Vector2(round(v.x / grid) * grid, round(v.y / grid) * grid))
    return result

Fix 2: Close the Polygon

OccluderPolygon2D has a closed property. When true (default), the polygon is treated as a solid loop. When false, it’s a polyline — only the line segments cast shadows, not the interior. A polyline occluder will cast correct shadows only from the line itself, leaving rays parallel to or behind the line unhandled. For room walls, always use closed = true.

Fix 3: Increase shadow_filter_smooth

Once the geometry is correct, residue from precision limits can still show as a thin bright line. Increase the Light2D’s shadow filter smoothness:

@onready var light: Light2D = $TorchLight

func _ready():
    light.shadow_filter = Light2D.SHADOW_FILTER_PCF13
    light.shadow_filter_smooth = 2.0   # pixels

PCF13 samples 13 points around each shadow edge pixel; the smooth value scales the sampling kernel. Higher values blur the shadow edge more — great for masking sub-pixel leaks, less great for sharp tactical shadows. Tune for your art style.

Fix 4: Bias and Extension

Each OccluderPolygon2D can be inset or outset via vertex editing. If shadows still leak after snapping, expand the polygon outward by 1 pixel along each edge so neighboring occluders overlap rather than merely touching:

# Inflate a polygon outward by ‘offset’ pixels
func inflate(poly: PackedVector2Array, offset: float) -> PackedVector2Array:
    var n = poly.size()
    var result = PackedVector2Array()
    for i in n:
        var prev = poly[(i - 1 + n) % n]
        var curr = poly[i]
        var nxt = poly[(i + 1) % n]
        var normal = ((curr - prev).orthogonal().normalized() + (nxt - curr).orthogonal().normalized()).normalized()
        result.append(curr + normal * offset)
    return result

Overlapping occluders combine cleanly into a single shadow volume with no gap.

Verifying

Use the Editor’s 2D Debug → Visible Collision Shapes to inspect occluder geometry. Run the game and move the light source around the suspect corner — if leaks reappear at specific angles, the geometry mismatch is still there. After the fixes, the shadow stays solid at all light angles.

Understanding the issue

This bug class falls into a pattern that's worth understanding beyond the specific case. In Godot 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 Godot. 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

For shipping games, the safest verification is a staged rollout. Apply the fix to 1% of players for 24 hours; watch the affected metric; expand if green. Skipping the staged rollout means the verification is the entire player base, which is too high a stakes for most fixes.

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

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Godot-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 Godot, 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.

“Light leaks are a geometry problem dressed up as a shader problem. Fix the polygons first; the filter is icing.”

When building dungeon walls procedurally, snap to integer pixels — floating point accumulates and you’ll see it three rooms down.