Fix: Unreal Procedural Mesh Missing Collision

Here is how to fix Unreal procedural mesh missing collision. You generate terrain or a destructible wall at runtime using ProceduralMeshComponent. The mesh renders perfectly — you can see it, lighting works, materials apply. But the player walks straight through it. Line traces pass through it. Physics objects fall through it. The mesh has zero collision presence despite being visually solid.

The Symptom

A ProceduralMeshComponent renders correctly but has no collision. Pawns walk through it, projectile traces ignore it, and physics simulations treat it as non-existent. The component’s collision settings in the Details panel show collision enabled, but at runtime nothing collides with the generated geometry.

What Causes This

bCreateCollision not set in CreateMeshSection. The CreateMeshSection function has a boolean parameter bCreateCollision that defaults to false. If you omit it or pass false, no collision geometry is generated for that section.

bUseComplexAsSimpleCollision not enabled. Even with bCreateCollision = true, the physics engine needs to know what shape to use. Without bUseComplexAsSimpleCollision, it looks for simple collision (convex hulls). If none exist, there is no collision.

Collision cooking failed silently. If the mesh has degenerate triangles (zero-area faces, NaN vertices), the physics cooking step fails without visible errors. The mesh renders fine but the collision body is empty.

Component collision settings overridden. The component’s collision enabled setting or collision profile may be set to NoCollision or QueryOnly without physics. Check both the component defaults and any runtime modifications.

The Fix

Step 1: Enable collision in CreateMeshSection.

// Create mesh section WITH collision
ProceduralMesh->CreateMeshSection_Linear(
    0,              // Section index
    Vertices,       // TArray<FVector>
    Triangles,      // TArray<int32>
    Normals,        // TArray<FVector>
    UVs,            // TArray<FVector2D>
    VertexColors,   // TArray<FLinearColor>
    Tangents,       // TArray<FProcMeshTangent>
    true            // bCreateCollision = TRUE
);

The last parameter is the key. Without it, you get a visual-only mesh.

Step 2: Set bUseComplexAsSimpleCollision.

// In constructor or BeginPlay:
ProceduralMesh->bUseComplexAsSimpleCollision = true;

// Also ensure collision is enabled on the component:
ProceduralMesh->SetCollisionEnabled(ECollisionEnabled::QueryAndPhysics);
ProceduralMesh->SetCollisionProfileName("BlockAll");

This tells the physics engine to use the mesh triangles directly as collision geometry. Without this flag, it expects separate convex hulls which procedural meshes do not have by default.

Step 3: Validate mesh data before creation. Degenerate geometry causes silent cooking failures:

// Validate triangles before creating the mesh
bool ValidateMeshData(const TArray<FVector>& Verts,
                      const TArray<int32>& Tris)
{
    for (int32 i = 0; i < Tris.Num(); i += 3)
    {
        FVector A = Verts[Tris[i]];
        FVector B = Verts[Tris[i+1]];
        FVector C = Verts[Tris[i+2]];
        FVector Cross = FVector::CrossProduct(B - A, C - A);
        if (Cross.SizeSquared() < KINDA_SMALL_NUMBER)
        {
            UE_LOG(LogTemp, Warning, TEXT("Degenerate tri at %d"), i/3);
            return false;
        }
    }
    return true;
}

Step 4: Use convex collision for dynamic objects. Complex collision (triangle mesh) only works for static objects. For dynamic procedural meshes, add convex hulls:

// Add convex collision for a dynamic procedural mesh
TArray<FVector> ConvexVerts;
// ... compute convex hull vertices ...
ProceduralMesh->AddCollisionConvexMesh(ConvexVerts);

// For multiple convex sections (decomposition):
for (const auto& Hull : ConvexHulls)
{
    ProceduralMesh->AddCollisionConvexMesh(Hull.Vertices);
}

Understanding the issue

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

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

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

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 Unreal-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

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Unreal, 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

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.

“Rendering and collision are separate systems. A visible mesh is not automatically physical. Explicitly create collision or the physics world does not know it exists.”

Related Issues

For collision channel configuration on procedural meshes, see Collision Channel Not Blocking. For level streaming and dynamic content, see Level Instance Not Loading.

bCreateCollision true, bUseComplexAsSimple true, valid geometry. Procedural mesh collides.