Snapshot interpolation: buffering a stable remote timeline

Smoothness requires a timeline

Packets rarely arrive with the same spacing with which they were sent. Rendering the newest arrival directly makes packet timing visible as motion. Snapshot interpolation instead reconstructs remote presentation on a deliberately delayed timeline. [1] The design question is how much missing-data risk to accept for a given amount of visual delay.

Buffer and bracket

Keep snapshots ordered by simulation timestamp and sample between the two that bracket the chosen render time. Mirror documents an independently testable snapshot buffer and sampling implementation. [2] Interpolation stays inside known samples. Extrapolation estimates beyond the newest sample and therefore introduces a different failure mode when motion changes unexpectedly.

Worked example

Suppose an illustrative client estimates current server time as 2.16 s and chooses a presentation delay of 0.12 s. It renders remote objects at 2.04 s. The buffer contains position 4.0 m at 2.00 s and 5.0 m at 2.10 s.

The interpolation fraction is (2.04 - 2.00) / (2.10 - 2.00) = 0.4, producing position 4.4 m. Packet arrival times do not appear in that position formula. They determine whether the required samples are available.

If the 2.10 s sample has not arrived, the same result cannot be called interpolation. Choose an explicit policy: briefly hold the latest known pose, extrapolate within a defined horizon, or adjust the presentation timeline. The example's delay is a design choice, not a recommended value for every network.

Implementation workflow

Establish how client time maps to server time and keep clock corrections from abruptly reversing presentation. Separate snapshot identity, simulation timestamp, and local receive timestamp. Reject duplicates and stale data according to the protocol while retaining useful out-of-order samples when supported.

Treat spawn, despawn, teleport, and attachment changes as lifecycle events with defined interpolation boundaries. For rotations, use a rotation-appropriate blend; for discrete state, specify the switch time rather than averaging identifiers. Keep locally predicted control distinct from delayed remote presentation.

Limits and measurement

Longer buffering can absorb more arrival variation but adds visible delay. It cannot recover arbitrary outages or make an extrapolated collision reliable. Higher-order motion curves also need suitable input data and explicit treatment of discontinuities.

Measure buffer occupancy, sample age, underruns, correction distance, and end-to-end presentation delay. Test burst loss, packet reordering, clock drift, and a client returning from suspension. Tune against recorded arrival patterns and the game's motion, not average ping alone. A stable algorithm should recover a useful timeline after disruption without accumulating unlimited delay.

Sources

[1] Snapshot interpolation — Glenn Fiedler.

[2] Mirror snapshot interpolation — Mirror Networking contributors.