Why does my convex MeshCollider lose detail?

PhysX caps convex hulls at 255 vertices. A high-poly mesh gets simplified to that limit before becoming a collider. For non-convex shapes, decompose with V-HACD and add multiple convex MeshColliders, or use primitive Box/Sphere/Capsule composites.

Volumetric Hierarchical Approximate Convex Decomposition. An algorithm that breaks a concave mesh into overlapping convex pieces. Each piece becomes a convex collider; together they approximate the concave shape with reasonable count.

Should I just use non-convex MeshCollider?

Non-convex MeshColliders work for static geometry only — they cannot collide with other non-convex MeshColliders. For dynamic Rigidbodies, you need convex. Decompose.

Fix: Unity MeshCollider Convex Hull Too Coarse

Quick answer: PhysX convex hulls cap at 255 vertices. For complex shapes, decompose into multiple convex pieces (V-HACD or Mesh Baker plugin) and add a MeshCollider per piece. For dynamic Rigidbodies, this is the only correct approach — non-convex Mesh-vs-Mesh isn’t supported.

Detailed kart mesh used as a Rigidbody collider. Convex MeshCollider produces a wrap that loses the wheel arches and rolls oddly. The simplification is the 255-vertex cap; multiple convex pieces are the answer.

The Symptom

Convex MeshCollider on a complex mesh produces a coarse hull visibly different from the source. Physics interactions feel wrong — objects can’t fit into negative spaces, balance is off.

The Fix

Decompose with V-HACD. Use the Asset Store package “Convex Decomposition” or run V-HACD offline (Blender plugin, command-line tool) and import the pieces.

Result: the original mesh becomes a hierarchy of child meshes, each approximating a convex region. Add a MeshCollider with Convex = true to each child.

Vehicle (root)
  — VisualMesh (MeshFilter + MeshRenderer)
  — Collision (Empty)
       — Hull_001 (MeshCollider Convex)
       — Hull_002 (MeshCollider Convex)
       — Hull_003 (MeshCollider Convex)
       ...

Multiple convex hulls collide with everything, including dynamic-vs-dynamic.

Primitive Composite

For simpler shapes, hand-build with Box/Sphere/Capsule colliders. A car can be one big Box plus four Capsules for wheel wells. Cheaper than mesh colliders and very controllable.

Performance

Each convex hull is a separate collision object. 10 hulls cost roughly 10x of one. Aim for 4–12 hulls per object for vehicles or large props; 1–3 for smaller items.

Verifying

Show physics colliders in Scene view. The combined collider hull should match the visible mesh outline. Drop the object on a complex surface; behavior should match expectations.

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

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

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

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

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.

“Decompose. Compose. Multiple convex pieces are the rigid-body-friendly path.”

Related Issues

For Rigidbody sleep, see Rigidbody sleep. For tilemap collider edges, see tilemap collider.

Decompose. Multi-hull. Physics behaves.