"Hand-rolled visual smoother fights engine FixedUpdateInterpolation — 50 Hz model flicker"

Third-person character model flickers visibly on stepped or voxel terrain (coarse-collision block risers), but appears smooth on continuous/lowpoly terrain. The flicker is a 50 Hz sawtooth — the model judders up and down each physics tick. Toggling the hand-rolled visual smoother off immediately resolves it.

FixedUpdateInterpolation is true by default in the engine (GameTransform.cs). A GameObject whose WorldPosition is written inside OnFixedUpdate is already interpolated per render frame — ModelRenderer renders Transform.InterpolatedWorld, and a child composes parent.InterpolatedWorld * childLocal.

A hand-rolled per-frame visual smoother that eases a child's LocalPosition.z toward the body each render frame — subtracting the raw, un-interpolated fixed-tick z while the parent renders at the interpolated z — creates a disagreement. The composed child world-z becomes parentInterp_z + (visZ - feetZ[t]): the parent term sweeps between tick positions across each 20 ms interval while the offset pins to the discrete tick value, producing a per-tick 50 Hz sawtooth whose amplitude equals the z-delta per step.

On stepped terrain (voxel, coarse collision) that delta is large (~0.9 m per block riser), so the flicker is obvious. On continuous terrain the per-tick z-delta approaches zero, so the sawtooth vanishes. That terrain discriminator is the diagnostic signature.

Delete the manual smoother when engine interpolation is active. The visual child sits at its natural LocalPosition and engine interpolation carries the step motion.

Before hand-smoothing any fixed-tick transform, check whether engine interpolation is already active:

• No NoInterpolation flag set
• Not IsStatic
• Position written in a fixed-update context

If all three hold, a second per-frame smoother will double-smooth and fight it.

If manual smoothing is genuinely needed on an interpolated root, either:

• Compute the offset against the interpolated parent state, not raw fixed-tick state
• Feed the smoothed value back through the fixed-update write so interpolation smooths the smoothed value

Engine interpolation already blends between discrete physics-tick positions to produce smooth render-frame positions. Adding a second smoothing layer that reads the raw (un-blended) tick state creates two competing interpolations that disagree by the per-tick delta — visible as a sawtooth at the physics rate. Removing the redundant layer leaves only the engine's interpolation, which handles the step motion correctly.