Fix: Unity 2D Tilemap Composite Collider Jagged Edges

Here is how to fix Unity Tilemap CompositeCollider2D producing jagged outlines that snag characters or look broken in debug. The composite stitches per-tile shapes; small numerical drift at boundaries produces unwanted micro-edges. The settings to clean it up are subtle.

The Symptom

Tilemap with TilemapCollider2D + CompositeCollider2D shows jagged or zig-zag outlines along straight tile rows. Character snags or stops on these.

What Causes This

Polygon geometry artifacts. Polygon mode internally triangulates; numerical jitter near boundaries produces visible micro-edges.

Per-tile collision misalignment. If tile collision shapes are slightly off-pixel-grid, composite output reflects that.

No edge radius. Default 0 edge radius leaves sharp corners that snag capsules.

The Fix

Step 1: Switch to Outlines geometry.

// CompositeCollider2D inspector
Geometry Type:    Outlines
Generation Type:  Manual // or Synchronous for runtime updates
Edge Radius:      0.05

Outlines walks the perimeter; smoother result for tile-based worlds.

Step 2: Verify per-tile shapes. Open the TileSet. For each solid tile, ensure the collision shape exactly fills the tile (corners at tile bounds). Slight offsets compound at boundaries.

Step 3: Use Capsule for character collider. Round bottom rolls over micro-edges instead of catching.

Step 4: Snap character physics positions.

void FixedUpdate()
{
    Vector3 p = transform.position;
    p.x = Mathf.Round(p.x * 100f) / 100f;   // 1cm grid
    p.y = Mathf.Round(p.y * 100f) / 100f;
    transform.position = p;
}

Snapping character to a fine grid avoids float-position-dependent snagging.

Step 5: For runtime tile changes, regenerate composite.

composite.GenerateGeometry();

Call after editing the Tilemap programmatically; otherwise the composite uses stale geometry.

Understanding the issue

Tilemaps are dense data structures. A single tile change touches several other systems: rendering, collision, possibly navigation. Bugs at the intersection often look like 'I changed one tile, why did three other things break'.

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

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

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

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 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

Boundary conditions deserve specific testing attention. What happens when the input is zero, maximum, negative, or NaN? What happens at the start of a session vs hours in? What happens at the boundary between two systems handling the same data? These are where bugs hide and where regression tests are most valuable.

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.

“Outlines geometry. Edge radius. Capsule character. Tilemap collisions stop snagging.”

Related Issues

For 2D ghost collisions in Godot, see CharacterBody2D Ghost. For Physics2D effector, see Effector Not Applying.

Outlines mode. Edge radius up. Capsule on character. Edges run smooth.