fix: Lerp smoothing frame rate dependence and freeze at maximum interpolation time

[NoelStephensUnity]

Purpose of this PR

Fixes two pre-existing defects in the lerp smoothing pass used by the Lerp and SmoothDampening interpolation types.

1. Smoothing was frame rate dependent. The factor was 1.0f - MaximumInterpolationTime applied once per frame with no delta time, so the wall clock smoothing rate scaled with the frame rate. It is now raised to the number of 60fps reference frames elapsed, making the rate a function of elapsed time.

2. A maximum interpolation time of 1.0 froze the transform. That is the upper bound of the inspector [Range], and it produced a factor of exactly 0, so the value never advanced. Affects all three axes and both interpolation types. The retained portion is now clamped just below 1.0.

Three things worth knowing that aren't obvious from the diff:

• Results at 60fps are byte-identical for every legal setting except 1.0. Verified across 0.1 / 0.5 / 0.9 / 0.99. Only 1.0 changes, and it was a hard freeze, so nothing could have been relying on it. Other frame rates now match the 60fps curve rather than diverging from it.
• The blast radius is smaller than it looks. InterpolationTypes.LegacyLerp is enum value 0 with no field initializer, no Reset(), and no editor default, so a freshly added NetworkTransform uses the legacy path — which was already frame rate correct and is untouched here. Only projects that explicitly opted into Lerp or SmoothDampening were exposed.
• The two defects are independent, which is why there are two constants rather than one fix. pow(1.0, x) == 1, so the delta time fix alone still yields a factor of 0 at 1.0.

For scale: settle time at MaxInterpolationTime = 0.9 was 2.13s @30fps vs 0.275s @240FPS; it is now a flat ~1.02s at 30/60/120/240. At the 0.1 default the effect was invisible, which is why this went unnoticed.

The doc comments on the lerp smoothing fields described the LegacyLerp formula for all interpolation types and have been corrected.

Jira ticket

TODO: add ticket

Changelog

com.unity.netcode.gameobjects

• Fixed: Issue where lerp smoothing was applied per frame instead of over time, which caused the Lerp and SmoothDampening interpolation types to smooth by different amounts at different frame rates. Results at 60fps are unchanged.
• Fixed: Issue where setting a maximum interpolation time of 1.0 would stop a NetworkTransform from interpolating at all when using the Lerp or SmoothDampening interpolation types.

Documentation

• Includes edits to existing public API documentation.

The remarks on PositionLerpSmoothing, RotationLerpSmoothing, ScaleLerpSmoothing, MaximumInterpolationTime, and the InterpolationTypes.Lerp / SmoothDampening enum entries all described the LegacyLerp formula regardless of the selected type. They now state that smoothing is frame rate independent and note that the same value does not produce the same result under LegacyLerp as under the other types.

Testing & QA (How your changes can be verified during release Playtest)

Both regression tests were written first and confirmed red against unfixed code, failing on the intended assertions rather than on timeouts:

• freeze: Interpolated value only advanced 0 from 3.17 over 2.11s (once for Lerp, once for SmoothDampening)
• frame rate: produced 6.626402 at 30fps but 6.688997 at 240fps

After the fix, all 13 tests in InterpolatorTests pass, including the 10 pre-existing ones.

For release playtest, the highest value check is an object using Lerp or SmoothDampening with a maximum interpolation time above ~0.5, observed at a non-60fps frame rate — that is where the old and new behaviour differ. At the 0.1 default there should be no perceptible change at any frame rate.

Functional Testing

Manual testing :

• Manual testing done

Automated tests:

• Covered by existing automated tests
• Covered by new automated tests

Does the change require QA team to:

• Review automated tests?
• Execute manual tests?
• Provide feedback about the PR?

Up-port

#4132 is the up-port.
Required. Will be up-ported to develop-3.x.x once this PR lands.

Backports

Not needed.

[codecov-github-com]

Codecov Report

All modified and coverable lines are covered by tests ✅

@@                Coverage Diff                @@
##           develop-2.0.0    #4130      +/-   ##
=================================================
+ Coverage          73.86%   73.96%   +0.09%     
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  Files                172      172              
  Lines              28099    28103       +4     
=================================================
+ Hits               20756    20786      +30     
+ Misses              7343     7317      -26     

Flag	Coverage Δ
NGOv2_project_testproject_ubuntu_pinnedTrunk	73.68% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...ponents/Interpolator/BufferedLinearInterpolator.cs	86.46% <100.00%> (+0.15%)	⬆️
...gameobjects/Runtime/Components/NetworkTransform.cs	88.42% <ø> (ø)

... and 2 files with indirect coverage changes

Components	Coverage Δ
com.unity.netcode.gameobjects	73.96% <100.00%> (+0.09%)	⬆️

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