Why does Unreal ISM collision not match the visual after I move an instance?

UpdateInstanceTransform updates the visual but not the physics body unless you pass bMarkRenderStateDirty=true and bTeleport=true. Pass both, then call MarkRenderStateDirty to refresh; or rebuild the physics state via ReleasePhysicsState/RecreatePhysicsState.

How do I efficiently move many ISM instances?

Call BatchUpdateInstancesTransforms with an array of new transforms, true for bMarkRenderStateDirty and bTeleport. Single GPU upload, single physics rebuild. Faster than per-instance UpdateInstanceTransform calls.

Should I use ISM or HISM?

HISM (Hierarchical) supports per-instance LODs and culling; ISM is simpler and slightly faster when you do not need LODs. For static foliage clusters, HISM is preferred. For player-modifiable structures (movable boxes), ISM with explicit updates.

Fix: Unreal InstancedStaticMesh Collision Stale After Update

Quick answer: Pass both bMarkRenderStateDirty = true and bTeleport = true to UpdateInstanceTransform. Without them, the render state may update but the physics body keeps the old transform. For batches of moves, prefer BatchUpdateInstancesTransforms.

Here is how to fix Unreal InstancedStaticMesh collisions that diverge from the visible mesh after instance transforms change. The visual is in the new place; physics queries still hit the old place. The fix is the right flags on the update call.

The Symptom

Move an instance via UpdateInstanceTransform. Visually it moved. Click on it: the line trace hits empty space where it used to be. Collision lags the visual.

What Causes This

Default flags do not refresh physics. bMarkRenderStateDirty defaults to false; physics body is not rebuilt unless explicitly requested.

Teleport flag missing. Without bTeleport, physics treats the move as a continuous transition; can produce wrong intermediate states.

Per-instance updates without batch. Calling Update once per instance for thousands of instances is expensive; physics rebuild for each is wasteful.

The Fix

Step 1: Call with correct flags.

FTransform NewTransform(/* ... */);
ISM->UpdateInstanceTransform(InstanceIndex, NewTransform,
    /*bWorldSpace*/ true,
    /*bMarkRenderStateDirty*/ true,
    /*bTeleport*/ true);

Step 2: For batches, use BatchUpdateInstancesTransforms.

TArray<FTransform> NewTransforms;
// fill array
ISM->BatchUpdateInstancesTransforms(0, NewTransforms,
    /*bWorldSpace*/ true,
    /*bMarkRenderStateDirty*/ true,
    /*bTeleport*/ true);

Single GPU buffer upload, single physics rebuild for the affected range.

Step 3: Force MarkRenderStateDirty for visibility.

ISM->MarkRenderStateDirty();

Useful after manipulating instance properties like CustomData per-instance.

Step 4: Recreate physics if needed.

ISM->ReleasePhysicsState();
ISM->RecreatePhysicsState();

Heavy-handed but reliably refreshes all instance bodies.

Step 5: For HISM, the same flags apply. HierarchicalInstancedStaticMeshComponent inherits ISM’s API. Use the same calls.

“Visual update needs MarkRenderStateDirty. Physics needs Teleport. Set both, every move.”

Related Issues

For static batching equivalent, see Static Batching Runtime. For physics constraint after load, see Physics Constraint After Load.

bMarkRenderStateDirty + bTeleport. Batch where possible. Collisions track visuals.