feature: Lag Compensation V1 (#3534)

* Lag Compensation Example based on Snapshot Interpolation example

* rename scene

* increase latency

* update scripts

* instructions

* better

* rename to rollback

* rename scene

* rebase

* rename to LagComp

* collider, onclick

* CmdClick

* flash color

* cleanup

* LagCompensationSettings

* caputre in interval

* comment

* syntax

* catpure, comp

* ns

* stuff

* source

* syntax

* naming

* history and draw history

* adjust

* cleanups

* tests

* sample and tests

* simplify tests

* out interpolation factor t

* tostring

* show result duration

* sample and interpolate

* fix

* logs

* better gizmos

* store size

* EstimateTime

* demo: estimate time

* estimation tests

* perf: Queue instead of List

* comment

* syntax

* cleaner

* cleanup

* syntax

* comment

* DrawGizmo(s)

* syntax

* extrapolation tests

* fix insert and test

* extrapolation

* fix extrapolation out values

* comments

* TODO

* TODO

Assets/Mirror/Core/LagCompensation.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: d2656015ded44e83a24f4c4776bafd40
+timeCreated: 1687920405

Assets/Mirror/Core/LagCompensation/Capture.cs

@@ -0,0 +1,13 @@
+namespace Mirror
+{
+    public interface Capture
+    {
+        // server timestamp at time of capture.
+        double timestamp { get; set; }
+
+        // optional gizmo drawing for visual debugging.
+        // history is only known on the server, which usually doesn't render.
+        // showing Gizmos in the Editor is enough.
+        void DrawGizmo();
+    }
+}

Assets/Mirror/Core/LagCompensation/Capture.cs.meta

@@ -0,0 +1,3 @@
+fileFormatVersion: 2
+guid: 347e831952e943a49095cadd39a5aeb2
+timeCreated: 1687921461

Assets/Mirror/Core/LagCompensation/LagCompensation.cs

@@ -0,0 +1,144 @@
+// standalone lag compensation algorithm
+// based on the Valve Networking Model:
+// https://developer.valvesoftware.com/wiki/Source_Multiplayer_Networking
+using System.Collections.Generic;
+
+namespace Mirror
+{
+    public static class LagCompensation
+    {
+        // history is of <timestamp, capture>.
+        // Queue allows for fast 'remove first' and 'append last'.
+        //
+        // make sure to always insert in order.
+        // inserting out of order like [1,2,4,3] would cause issues.
+        // can't safeguard this because Queue doesn't have .Last access.
+        public static void Insert<T>(
+            Queue<KeyValuePair<double, T>> history,
+            int historyLimit,
+            double timestamp,
+            T capture)
+            where T : Capture
+        {
+            // make space according to history limit.
+            // do this before inserting, to avoid resizing past capacity.
+            if (history.Count >= historyLimit)
+                history.Dequeue();
+
+            // insert
+            history.Enqueue(new KeyValuePair<double, T>(timestamp, capture));
+        }
+
+        // get the two snapshots closest to a given timestamp.
+        // those can be used to interpolate the exact snapshot at that time.
+        // if timestamp is newer than the newest history entry, then we extrapolate.
+        //   't' will be between 1 and 2, before is second last, after is last.
+        //   callers should Lerp(before, after, t=1.5) to extrapolate the hit.
+        //   see comments below for extrapolation.
+        public static bool Sample<T>(
+            Queue<KeyValuePair<double, T>> history,
+            double timestamp, // current server time
+            double interval,  // capture interval
+            out T before,
+            out T after,
+            out double t)     // interpolation factor
+            where T : Capture
+        {
+            before = default;
+            after  = default;
+            t = 0;
+
+            // can't sample an empty history
+            // interpolation needs at least one entry.
+            // extrapolation needs at least two entries.
+            //   can't Lerp(A, A, 1.5). dist(A, A) * 1.5 is always 0.
+            if(history.Count < 2) {
+                return false;
+            }
+
+            // older than oldest
+            if (timestamp < history.Peek().Key) {
+                return false;
+            }
+
+            // iterate through the history
+            // TODO faster version: guess start index by how many 'intervals' we are behind.
+            //      search around that area.
+            //      should be O(1) most of the time, unless sampling was off.
+            KeyValuePair<double, T> prev = new KeyValuePair<double, T>();
+            KeyValuePair<double, T> prevPrev = new KeyValuePair<double, T>();
+            foreach(KeyValuePair<double, T> entry in history) {
+                // exact match?
+                if (timestamp == entry.Key) {
+                    before = entry.Value;
+                    after = entry.Value;
+                    t = Mathd.InverseLerp(before.timestamp, after.timestamp, timestamp);
+                    return true;
+                }
+
+                // did we check beyond timestamp? then return the previous two.
+                if (entry.Key > timestamp) {
+                    before = prev.Value;
+                    after = entry.Value;
+                    t = Mathd.InverseLerp(before.timestamp, after.timestamp, timestamp);
+                    return true;
+                }
+
+                // remember the last two for extrapolation.
+                // Queue doesn't have access to .Last.
+                prevPrev = prev;
+                prev = entry;
+            }
+
+            // newer than newest: extrapolate up to one interval.
+            // let's say we capture every 100 ms:
+            // 100, 200, 300, 400
+            // and the server is at 499
+            // if a client sends CmdFire at time 480, then there's no history entry.
+            // => adding the current entry every time would be too expensive.
+            //    worst case we would capture at 401, 402, 403, 404, ... 100 times
+            // => not extrapolating isn't great. low latency clients would be
+            //    punished by missing their targets since no entry at 'time' was found.
+            // => extrapolation is the best solution. make sure this works as
+            //    expected and within limits.
+            if (prev.Key < timestamp && timestamp <= prev.Key + interval) {
+                // return the last two valid snapshots.
+                // can't just return (after, after) because we can't extrapolate
+                // if their distance is 0.
+                before = prevPrev.Value;
+                after = prev.Value;
+
+                // InverseLerp will give [after, after+interval].
+                // but we return [before, after, t].
+                // so add +1 for the distance from before->after
+                t = 1 + Mathd.InverseLerp(after.timestamp, after.timestamp + interval, timestamp);
+                return true;
+            }
+
+            return false;
+        }
+
+        // never trust the client.
+        // we estimate when a message was sent.
+        // don't trust the client to tell us the time.
+        //   https://developer.valvesoftware.com/wiki/Source_Multiplayer_Networking
+        //   Command Execution Time = Current Server Time - Packet Latency - Client View Interpolation
+        // => lag compensation demo estimation is off by only ~6ms
+        public static double EstimateTime(double serverTime, double rtt, double bufferTime)
+        {
+            // packet latency is one trip from client to server, so rtt / 2
+            // client view interpolation is the snapshot interpolation buffer time
+            double latency = rtt / 2;
+            return serverTime - latency - bufferTime;
+        }
+
+        // convenience function to draw all history gizmos.
+        // this should be called from OnDrawGizmos.
+        public static void DrawGizmos<T>(Queue<KeyValuePair<double, T>> history)
+            where T : Capture
+        {
+            foreach (KeyValuePair<double, T> entry in history)
+                entry.Value.DrawGizmo();
+        }
+    }
+}

Assets/Mirror/Core/LagCompensation/LagCompensation.cs.meta

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+fileFormatVersion: 2
+guid: ad53cc7d12144d0ba3a8b0a4515e5d17
+timeCreated: 1687921483

Assets/Mirror/Core/LagCompensation/LagCompensationSettings.cs

@@ -0,0 +1,19 @@
+// snapshot interpolation settings struct.
+// can easily be exposed in Unity inspectors.
+using System;
+using UnityEngine;
+
+namespace Mirror
+{
+    // class so we can define defaults easily
+    [Serializable]
+    public class LagCompensationSettings
+    {
+        [Header("Buffering")]
+        [Tooltip("Keep this many past snapshots in the buffer. The larger this is, the further we can rewind into the past.\nMaximum rewind time := historyAmount * captureInterval")]
+        public int historyLimit = 6;
+
+        [Tooltip("Capture state every 'captureInterval' seconds. Larger values will space out the captures more, which gives a longer history but with possible gaps inbetween.\nSmaller values will have fewer gaps, with shorter history.")]
+        public float captureInterval = 0.100f; // 100 ms
+    }
+}

Assets/Mirror/Core/LagCompensation/LagCompensationSettings.cs.meta

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+guid: fa80bec245f94bf8a28ec78777992a1c
+timeCreated: 1687920412

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Assets/Mirror/Examples/LagCompensation/Capture2D.cs

@@ -0,0 +1,32 @@
+using UnityEngine;
+
+namespace Mirror.Examples.LagCompensationDemo
+{
+    public struct Capture2D : Capture
+    {
+        public double timestamp { get; set; }
+        public Vector2 position;
+        public Vector2 size;
+
+        public Capture2D(double timestamp, Vector2 position, Vector2 size)
+        {
+            this.timestamp = timestamp;
+            this.position = position;
+            this.size = size;
+        }
+
+        public void DrawGizmo()
+        {
+            Gizmos.DrawWireCube(position, size);
+        }
+
+        public static Capture2D Interpolate(Capture2D from, Capture2D to, double t) =>
+            new Capture2D(
+                0, // interpolated snapshot is applied directly. don't need timestamps.
+                Vector2.LerpUnclamped(from.position, to.position, (float)t),
+                Vector2.LerpUnclamped(from.size, to.size, (float)t)
+            );
+
+        public override string ToString() => $"(time={timestamp} pos={position} size={size})";
+    }
+}

Assets/Mirror/Examples/LagCompensation/Capture2D.cs.meta

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+fileFormatVersion: 2
+guid: a2ccdcd0db384bf08f4150ffb08fd09b
+timeCreated: 1687921611

Assets/Mirror/Examples/LagCompensation/ClientCube.cs

@@ -0,0 +1,240 @@
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+namespace Mirror.Examples.LagCompensationDemo
+{
+    public class ClientCube : MonoBehaviour
+    {
+        [Header("Components")]
+        public ServerCube server;
+        public Renderer render;
+
+        [Header("Toggle")]
+        public bool interpolate = true;
+
+        // snapshot interpolation settings
+        [Header("Snapshot Interpolation")]
+        public SnapshotInterpolationSettings snapshotSettings =
+            new SnapshotInterpolationSettings();
+
+        // runtime settings
+        public double bufferTime => server.sendInterval * snapshotSettings.bufferTimeMultiplier;
+
+        // <servertime, snaps>
+        public SortedList<double, Snapshot3D> snapshots = new SortedList<double, Snapshot3D>();
+
+        // for smooth interpolation, we need to interpolate along server time.
+        // any other time (arrival on client, client local time, etc.) is not
+        // going to give smooth results.
+        internal double localTimeline;
+
+        // catchup / slowdown adjustments are applied to timescale,
+        // to be adjusted in every update instead of when receiving messages.
+        double localTimescale = 1;
+
+        // we use EMA to average the last second worth of snapshot time diffs.
+        // manually averaging the last second worth of values with a for loop
+        // would be the same, but a moving average is faster because we only
+        // ever add one value.
+        ExponentialMovingAverage driftEma;
+        ExponentialMovingAverage deliveryTimeEma; // average delivery time (standard deviation gives average jitter)
+
+        // debugging ///////////////////////////////////////////////////////////
+        [Header("Debug")]
+        public Color hitColor      = Color.blue;
+        public Color missedColor   = Color.magenta;
+        public Color originalColor = Color.black;
+
+        [Header("Simulation")]
+        bool lowFpsMode;
+        double accumulatedDeltaTime;
+
+        void Awake()
+        {
+            // defaultColor = render.sharedMaterial.color;
+
+            // initialize EMA with 'emaDuration' seconds worth of history.
+            // 1 second holds 'sendRate' worth of values.
+            // multiplied by emaDuration gives n-seconds.
+            driftEma = new ExponentialMovingAverage(server.sendRate * snapshotSettings.driftEmaDuration);
+            deliveryTimeEma = new ExponentialMovingAverage(server.sendRate * snapshotSettings.deliveryTimeEmaDuration);
+        }
+
+        // add snapshot & initialize client interpolation time if needed
+        public void OnMessage(Snapshot3D snap)
+        {
+            // set local timestamp (= when it was received on our end)
+            // Unity 2019 doesn't have Time.timeAsDouble yet
+            snap.localTime = NetworkTime.localTime;
+
+            // (optional) dynamic adjustment
+            if (snapshotSettings.dynamicAdjustment)
+            {
+                // set bufferTime on the fly.
+                // shows in inspector for easier debugging :)
+                snapshotSettings.bufferTimeMultiplier = SnapshotInterpolation.DynamicAdjustment(
+                    server.sendInterval,
+                    deliveryTimeEma.StandardDeviation,
+                    snapshotSettings.dynamicAdjustmentTolerance
+                );
+            }
+
+            // insert into the buffer & initialize / adjust / catchup
+            SnapshotInterpolation.InsertAndAdjust(
+                snapshots,
+                snapshotSettings.bufferLimit,
+                snap,
+                ref localTimeline,
+                ref localTimescale,
+                server.sendInterval,
+                bufferTime,
+                snapshotSettings.catchupSpeed,
+                snapshotSettings.slowdownSpeed,
+                ref driftEma,
+                snapshotSettings.catchupNegativeThreshold,
+                snapshotSettings.catchupPositiveThreshold,
+                ref deliveryTimeEma);
+        }
+
+        void Update()
+        {
+            // accumulated delta allows us to simulate correct low fps + deltaTime
+            // if necessary in client low fps mode.
+            accumulatedDeltaTime += Time.unscaledDeltaTime;
+
+            // simulate low fps mode. only update once per second.
+            // to simulate webgl background tabs, etc.
+            // after a while, disable low fps mode and see how it behaves.
+            if (lowFpsMode && accumulatedDeltaTime < 1) return;
+
+            // only while we have snapshots.
+            // timeline starts when the first snapshot arrives.
+            if (snapshots.Count > 0)
+            {
+                // snapshot interpolation
+                if (interpolate)
+                {
+                    // step
+                    SnapshotInterpolation.Step(
+                        snapshots,
+                        // accumulate delta is Time.unscaledDeltaTime normally.
+                        // and sum of past 10 delta's in low fps mode.
+                        accumulatedDeltaTime,
+                        ref localTimeline,
+                        localTimescale,
+                        out Snapshot3D fromSnapshot,
+                        out Snapshot3D toSnapshot,
+                        out double t);
+
+                    // interpolate & apply
+                    Snapshot3D computed = Snapshot3D.Interpolate(fromSnapshot, toSnapshot, t);
+                    transform.position = computed.position;
+                }
+                // apply raw
+                else
+                {
+                    Snapshot3D snap = snapshots.Values[0];
+                    transform.position = snap.position;
+                    snapshots.RemoveAt(0);
+                }
+            }
+
+            // reset simulation helpers
+            accumulatedDeltaTime = 0;
+        }
+
+        void OnMouseDown()
+        {
+            // send the command.
+            // only x coordinate matters for this simple example.
+            if (server.CmdClicked(transform.position))
+            {
+                Debug.Log($"Click hit!");
+                FlashColor(hitColor);
+            }
+            else
+            {
+                Debug.Log($"Click missed!");
+                FlashColor(missedColor);
+            }
+        }
+
+        // simple visual indicator for better feedback.
+        // changes the cube's color for a short time.
+        void FlashColor(Color color) =>
+            StartCoroutine(TemporarilyChangeColorToGreen(color));
+
+        IEnumerator TemporarilyChangeColorToGreen(Color color)
+        {
+            Renderer r = GetComponentInChildren<Renderer>();
+            r.material.color = color;
+            yield return new WaitForSeconds(0.2f);
+            r.material.color = originalColor;
+        }
+
+        void OnGUI()
+        {
+            // display buffer size as number for easier debugging.
+            // catchup is displayed as color state in Update() already.
+            const int width = 30; // fit 3 digits
+            const int height = 20;
+            Vector2 screen = Camera.main.WorldToScreenPoint(transform.position);
+            string str = $"{snapshots.Count}";
+            GUI.Label(new Rect(screen.x - width / 2, screen.y - height / 2, width, height), str);
+
+            // client simulation buttons on the bottom of the screen
+            float areaHeight = 150;
+            GUILayout.BeginArea(new Rect(0, Screen.height - areaHeight, Screen.width, areaHeight));
+            GUILayout.Label("Click the black cube. Lag compensation will correct the latency.");
+            GUILayout.BeginHorizontal();
+            GUILayout.Label("Client Simulation:");
+            if (GUILayout.Button((lowFpsMode ? "Disable" : "Enable") + " 1 FPS"))
+            {
+                lowFpsMode = !lowFpsMode;
+            }
+
+            GUILayout.Label("|");
+
+            if (GUILayout.Button("Timeline 10s behind"))
+            {
+                localTimeline -= 10.0;
+            }
+            if (GUILayout.Button("Timeline 1s behind"))
+            {
+                localTimeline -= 1.0;
+            }
+            if (GUILayout.Button("Timeline 0.1s behind"))
+            {
+                localTimeline -= 0.1;
+            }
+
+            GUILayout.Label("|");
+
+            if (GUILayout.Button("Timeline 0.1s ahead"))
+            {
+                localTimeline += 0.1;
+            }
+            if (GUILayout.Button("Timeline 1s ahead"))
+            {
+                localTimeline += 1.0;
+            }
+            if (GUILayout.Button("Timeline 10s ahead"))
+            {
+                localTimeline += 10.0;
+            }
+
+            GUILayout.FlexibleSpace();
+            GUILayout.EndHorizontal();
+            GUILayout.EndArea();
+        }
+
+        void OnValidate()
+        {
+            // thresholds need to be <0 and >0
+            snapshotSettings.catchupNegativeThreshold = Math.Min(snapshotSettings.catchupNegativeThreshold, 0);
+            snapshotSettings.catchupPositiveThreshold = Math.Max(snapshotSettings.catchupPositiveThreshold, 0);
+        }
+    }
+}

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Assets/Mirror/Examples/LagCompensation/LagCompensation.unity.meta

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Assets/Mirror/Examples/LagCompensation/ServerCube.cs

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+using System;
+using System.Collections.Generic;
+using UnityEngine;
+using Random = UnityEngine.Random;
+
+namespace Mirror.Examples.LagCompensationDemo
+{
+    public class ServerCube : MonoBehaviour
+    {
+        [Header("Components")]
+        public ClientCube client;
+        public new BoxCollider collider;
+
+        [Header("Movement")]
+        public float distance = 10;
+        public float speed = 3;
+        Vector3 start;
+
+        [Header("Snapshot Interpolation")]
+        [Tooltip("Send N snapshots per second. Multiples of frame rate make sense.")]
+        public int sendRate = 30; // in Hz. easier to work with as int for EMA. easier to display '30' than '0.333333333'
+        public float sendInterval => 1f / sendRate;
+        float lastSendTime;
+
+        [Header("Lag Compensation")]
+        public LagCompensationSettings lagCompensationSettings = new LagCompensationSettings();
+        double lastCaptureTime;
+
+        // lag compensation history of <timestamp, capture>
+        Queue<KeyValuePair<double, Capture2D>> history = new Queue<KeyValuePair<double, Capture2D>>();
+
+        public Color historyColor = Color.white;
+
+        // store latest lag compensation result to show a visual indicator
+        [Header("Debug")]
+        public double resultDuration = 0.5;
+        double resultTime;
+        Capture2D resultBefore;
+        Capture2D resultAfter;
+        Capture2D resultInterpolated;
+
+        [Header("Latency Simulation")]
+        [Tooltip("Latency in seconds")]
+        public float latency = 0.05f; // 50 ms
+        [Tooltip("Latency jitter, randomly added to latency.")]
+        [Range(0, 1)] public float jitter = 0.05f;
+        [Tooltip("Packet loss in %")]
+        [Range(0, 1)] public float loss = 0.1f;
+        [Tooltip("Scramble % of unreliable messages, just like over the real network. Mirror unreliable is unordered.")]
+        [Range(0, 1)] public float scramble = 0.1f;
+
+        // random
+        // UnityEngine.Random.value is [0, 1] with both upper and lower bounds inclusive
+        // but we need the upper bound to be exclusive, so using System.Random instead.
+        // => NextDouble() is NEVER < 0 so loss=0 never drops!
+        // => NextDouble() is ALWAYS < 1 so loss=1 always drops!
+        System.Random random = new System.Random();
+
+        // hold on to snapshots for a little while before delivering
+        // <deliveryTime, snapshot>
+        List<(double, Snapshot3D)> queue = new List<(double, Snapshot3D)>();
+
+        // latency simulation:
+        // always a fixed value + some jitter.
+        float SimulateLatency() => latency + Random.value * jitter;
+
+        // this is the average without randomness. for lag compensation math.
+        // in a real game, use rtt instead.
+        float AverageLatency() => latency + 0.5f * jitter;
+
+        void Start()
+        {
+            start = transform.position;
+        }
+
+        void Update()
+        {
+            // move on XY plane
+            float x = Mathf.PingPong(Time.time * speed, distance);
+            transform.position = new Vector3(start.x + x, start.y, start.z);
+
+            // broadcast snapshots every interval
+            if (Time.time >= lastSendTime + sendInterval)
+            {
+                Send(transform.position);
+                lastSendTime = Time.time;
+            }
+
+            Flush();
+
+            // capture lag compensation snapshots every interval.
+            // NetworkTime.localTime because Unity 2019 doesn't have 'double' time yet.
+            if (NetworkTime.localTime >= lastCaptureTime + lagCompensationSettings.captureInterval)
+            {
+                lastCaptureTime = NetworkTime.localTime;
+                Capture();
+            }
+        }
+
+        void Send(Vector3 position)
+        {
+            // create snapshot
+            // Unity 2019 doesn't have Time.timeAsDouble yet
+            Snapshot3D snap = new Snapshot3D(NetworkTime.localTime, 0, position);
+
+            // simulate packet loss
+            bool drop = random.NextDouble() < loss;
+            if (!drop)
+            {
+                // simulate scramble (Random.Next is < max, so +1)
+                bool doScramble = random.NextDouble() < scramble;
+                int last = queue.Count;
+                int index = doScramble ? random.Next(0, last + 1) : last;
+
+                // simulate latency
+                float simulatedLatency = SimulateLatency();
+                // Unity 2019 doesn't have Time.timeAsDouble yet
+                double deliveryTime = NetworkTime.localTime + simulatedLatency;
+                queue.Insert(index, (deliveryTime, snap));
+            }
+        }
+
+        void Flush()
+        {
+            // flush ready snapshots to client
+            for (int i = 0; i < queue.Count; ++i)
+            {
+                (double deliveryTime, Snapshot3D snap) = queue[i];
+
+                // Unity 2019 doesn't have Time.timeAsDouble yet
+                if (NetworkTime.localTime >= deliveryTime)
+                {
+                    client.OnMessage(snap);
+                    queue.RemoveAt(i);
+                    --i;
+                }
+            }
+        }
+
+        void Capture()
+        {
+            // capture current state
+            Capture2D capture = new Capture2D(NetworkTime.localTime, transform.position, collider.size);
+
+            // insert into history
+            LagCompensation.Insert(history, lagCompensationSettings.historyLimit, NetworkTime.localTime, capture);
+        }
+
+        // client says: "I was clicked here, at this time."
+        // server needs to rollback to validate.
+        // timestamp is the client's snapshot interpolated timeline!
+        public bool CmdClicked(Vector2 position)
+        {
+            // never trust the client: estimate client time instead.
+            // https://developer.valvesoftware.com/wiki/Source_Multiplayer_Networking
+            // the estimation is very good. the error is as low as ~6ms for the demo.
+            double rtt = AverageLatency() * 2; // the function needs rtt, which is latency * 2
+            double estimatedTime = LagCompensation.EstimateTime(NetworkTime.localTime, rtt, client.bufferTime);
+
+            // compare estimated time with actual client time for debugging
+            double error = Math.Abs(estimatedTime - client.localTimeline);
+            Debug.Log($"CmdClicked: serverTime={NetworkTime.localTime:F3} clientTime={client.localTimeline:F3} estimatedTime={estimatedTime:F3} estimationError={error:F3} position={position}");
+
+            // sample the history to get the nearest snapshots around 'timestamp'
+            if (LagCompensation.Sample(history, estimatedTime, lagCompensationSettings.captureInterval, out resultBefore, out resultAfter, out double t))
+            {
+                // interpolate to get a decent estimation at exactly 'timestamp'
+                resultInterpolated = Capture2D.Interpolate(resultBefore, resultAfter, t);
+                resultTime = NetworkTime.localTime;
+
+                // check if there really was a cube at that time and position
+                Bounds bounds = new Bounds(resultInterpolated.position, resultInterpolated.size);
+                if (bounds.Contains(position))
+                {
+                    return true;
+                }
+                else Debug.Log($"CmdClicked: interpolated={resultInterpolated} doesn't contain {position}");
+            }
+            else Debug.Log($"CmdClicked: history doesn't contain {estimatedTime:F3}");
+
+            return false;
+        }
+
+        void OnDrawGizmos()
+        {
+            // should we apply special colors to an active result?
+            bool showResult = NetworkTime.localTime <= resultTime + resultDuration;
+
+            // draw interpoalted result first.
+            // history meshcubes should write over it for better visibility.
+            if (showResult)
+            {
+                Gizmos.color = Color.black;
+                Gizmos.DrawCube(resultInterpolated.position, resultInterpolated.size);
+            }
+
+            // draw history
+            Gizmos.color = historyColor;
+            LagCompensation.DrawGizmos(history);
+
+            // draw result samples after. useful to see the selection process.
+            if (showResult)
+            {
+                Gizmos.color = Color.cyan;
+                Gizmos.DrawWireCube(resultBefore.position, resultBefore.size);
+                Gizmos.DrawWireCube(resultAfter.position, resultAfter.size);
+            }
+        }
+    }
+}

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Assets/Mirror/Examples/LagCompensation/Snapshot3D.cs

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+// a simple snapshot with timestamp & interpolation
+using UnityEngine;
+
+namespace Mirror.Examples.LagCompensationDemo
+{
+    public struct Snapshot3D : Snapshot
+    {
+        public double remoteTime { get; set; }
+        public double localTime { get; set; }
+        public Vector3 position;
+
+        public Snapshot3D(double remoteTime, double localTime, Vector3 position)
+        {
+            this.remoteTime = remoteTime;
+            this.localTime = localTime;
+            this.position = position;
+        }
+
+        public static Snapshot3D Interpolate(Snapshot3D from, Snapshot3D to, double t) =>
+            new Snapshot3D(
+                // interpolated snapshot is applied directly. don't need timestamps.
+                0, 0,
+                // lerp unclamped in case we ever need to extrapolate.
+                Vector3.LerpUnclamped(from.position, to.position, (float)t));
+    }
+}

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Assets/Mirror/Examples/LagCompensation/_DISABLE VSYNC_

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+otherwise it may not look entirely smooth.
+even on 120 hz.

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Assets/Mirror/Examples/LagCompensation/_README.txt

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+Rollback / Lag Compensation is a standalone, Unity / netcode independent algorithm.
+This is a simple demo to test it, without Mirror.
+We want this to be usable in all game engines.
+
+The demo intentionally introduces latency so that server / client cubes are
+at different positions when clicking.

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Assets/Mirror/Tests/Editor/LagCompensation/LagCompensationTests.cs

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+using NUnit.Framework;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace Mirror.Tests
+{
+    // a simple snapshot with timestamp & interpolation
+    struct SimpleCapture : Capture
+    {
+        public double timestamp { get; set; }
+        public int value;
+
+        public SimpleCapture(double timestamp, int value)
+        {
+            this.timestamp = timestamp;
+            this.value = value;
+        }
+
+        public void DrawGizmo() {}
+    }
+
+    public class LagCompensationTests
+    {
+        // buffer for convenience so we don't have to create it manually each time
+        Queue<KeyValuePair<double, SimpleCapture>> history;
+
+        // some defaults
+        const int HistoryLimit = 4;
+        const double Interval = 1;
+
+        [SetUp]
+        public void SetUp()
+        {
+            history = new Queue<KeyValuePair<double, SimpleCapture>>();
+        }
+
+        [Test]
+        public void Insert()
+        {
+            // insert a few
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            LagCompensation.Insert(history, HistoryLimit, 2, new SimpleCapture(2, 20));
+            LagCompensation.Insert(history, HistoryLimit, 3, new SimpleCapture(3, 30));
+
+            Assert.That(history.Count, Is.EqualTo(3));
+            Assert.That(history.ElementAt(0).Key, Is.EqualTo(1));
+            Assert.That(history.ElementAt(1).Key, Is.EqualTo(2));
+            Assert.That(history.ElementAt(2).Key, Is.EqualTo(3));
+            Assert.That(history.ElementAt(0).Value.value, Is.EqualTo(10));
+            Assert.That(history.ElementAt(1).Value.value, Is.EqualTo(20));
+            Assert.That(history.ElementAt(2).Value.value, Is.EqualTo(30));
+        }
+
+        [Test]
+        public void InsertAboveLimit()
+        {
+            // inserting more than limit, should evict the oldest one
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            LagCompensation.Insert(history, HistoryLimit, 2, new SimpleCapture(2, 20));
+            LagCompensation.Insert(history, HistoryLimit, 3, new SimpleCapture(3, 30));
+            LagCompensation.Insert(history, HistoryLimit, 4, new SimpleCapture(4, 40));
+            LagCompensation.Insert(history, HistoryLimit, 5, new SimpleCapture(5, 50));
+
+            Assert.That(history.Count, Is.EqualTo(4));
+            Assert.That(history.ElementAt(0).Key, Is.EqualTo(2));
+            Assert.That(history.ElementAt(1).Key, Is.EqualTo(3));
+            Assert.That(history.ElementAt(2).Key, Is.EqualTo(4));
+            Assert.That(history.ElementAt(3).Key, Is.EqualTo(5));
+            Assert.That(history.ElementAt(0).Value.value, Is.EqualTo(20));
+            Assert.That(history.ElementAt(1).Value.value, Is.EqualTo(30));
+            Assert.That(history.ElementAt(2).Value.value, Is.EqualTo(40));
+            Assert.That(history.ElementAt(3).Value.value, Is.EqualTo(50));
+        }
+
+        [Test]
+        public void Sample_Empty()
+        {
+            Assert.That(LagCompensation.Sample(history, 0, Interval, out _, out _, out _), Is.False);
+        }
+
+        [Test]
+        public void Sample_Single_Interpolate()
+        {
+            // always need at least two values
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            Assert.That(LagCompensation.Sample(history, 0.5, Interval, out _, out _, out _), Is.False);
+        }
+
+        [Test]
+        public void Sample_Single_Extrapolate()
+        {
+            // insert a few
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            Assert.That(LagCompensation.Sample(history, 1.5, Interval, out _, out _, out _), Is.False);
+        }
+
+        [Test]
+        public void Sample_Double_Interpolate()
+        {
+            // always need at least two values
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            LagCompensation.Insert(history, HistoryLimit, 2, new SimpleCapture(2, 20));
+
+            // sample older than first
+            Assert.That(LagCompensation.Sample(history, 0, Interval, out SimpleCapture before, out SimpleCapture after, out double t), Is.False);
+
+            // sample exactly first
+            Assert.That(LagCompensation.Sample(history, 1, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(10));
+            Assert.That(after.value, Is.EqualTo(10));
+            Assert.That(t, Is.EqualTo(0.0));
+
+            // sample between first and second
+            Assert.That(LagCompensation.Sample(history, 1.5, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(10));
+            Assert.That(after.value, Is.EqualTo(20));
+            Assert.That(t, Is.EqualTo(0.5));
+        }
+
+        [Test]
+        public void Sample_Double_Extrapolate()
+        {
+            // insert a few
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            LagCompensation.Insert(history, HistoryLimit, 2, new SimpleCapture(2, 20));
+
+            // sample newer than newest: should extrapolate even if we only have one entry
+            Assert.That(LagCompensation.Sample(history, 2.5, Interval, out SimpleCapture before, out SimpleCapture after, out double t), Is.True);
+            Assert.That(before.value, Is.EqualTo(10));
+            Assert.That(after.value, Is.EqualTo(20));
+            Assert.That(t, Is.EqualTo(1.5));
+        }
+
+        [Test]
+        public void Sample_Triple_Interpolate()
+        {
+            // insert a few
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            LagCompensation.Insert(history, HistoryLimit, 2, new SimpleCapture(2, 20));
+            LagCompensation.Insert(history, HistoryLimit, 3, new SimpleCapture(3, 30));
+
+            // sample older than first
+            Assert.That(LagCompensation.Sample(history, 0, Interval, out SimpleCapture before, out SimpleCapture after, out double t), Is.False);
+
+            // sample exactly first
+            Assert.That(LagCompensation.Sample(history, 1, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(10));
+            Assert.That(after.value, Is.EqualTo(10));
+            Assert.That(t, Is.EqualTo(0.0));
+
+            // sample between first and second
+            Assert.That(LagCompensation.Sample(history, 1.5, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(10));
+            Assert.That(after.value, Is.EqualTo(20));
+            Assert.That(t, Is.EqualTo(0.5));
+
+            // sample exactly second
+            Assert.That(LagCompensation.Sample(history, 2, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(20));
+            Assert.That(after.value, Is.EqualTo(20));
+            Assert.That(t, Is.EqualTo(0.0));
+
+            // sample between second and third
+            Assert.That(LagCompensation.Sample(history, 2.5, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(20));
+            Assert.That(after.value, Is.EqualTo(30));
+            Assert.That(t, Is.EqualTo(0.5));
+
+            // sample exactly third
+            Assert.That(LagCompensation.Sample(history, 3, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(30));
+            Assert.That(after.value, Is.EqualTo(30));
+            Assert.That(t, Is.EqualTo(0.0));
+        }
+
+        [Test]
+        public void Sample_Triple_Extrapolate()
+        {
+            // let's say we capture every 100 ms:
+            // 100, 200, 300, 400
+            // and the server is at 499
+            // if a client sends CmdFire at time 480, then there's no history entry.
+            // => adding the current entry every time would be too expensive.
+            //    worst case we would capture at 401, 402, 403, 404, ... 100 times
+            // => not extrapolating isn't great. low latency clients would be
+            //    punished by missing their targets since no entry at 'time' was found.
+            // => extrapolation is the best solution. make sure this works as
+            //    expected and within limits.
+
+            // insert a few
+            LagCompensation.Insert(history, HistoryLimit, 1, new SimpleCapture(1, 10));
+            LagCompensation.Insert(history, HistoryLimit, 2, new SimpleCapture(2, 20));
+            LagCompensation.Insert(history, HistoryLimit, 3, new SimpleCapture(3, 30));
+
+            // sample at 3.9, just before we capture the next one.
+            // this should return before=after=3, with t=1.9.
+            // the user can then extrapolate manually.
+            Assert.That(LagCompensation.Sample(history, 3.9, Interval, out SimpleCapture before, out SimpleCapture after, out double t), Is.True);
+            Assert.That(before.value, Is.EqualTo(20));
+            Assert.That(after.value, Is.EqualTo(30));
+            Assert.That(t, Is.EqualTo(1.9));
+
+            // exactly interval is still fine
+            Assert.That(LagCompensation.Sample(history, 4, Interval, out before, out after, out t), Is.True);
+            Assert.That(before.value, Is.EqualTo(20));
+            Assert.That(after.value, Is.EqualTo(30));
+            Assert.That(t, Is.EqualTo(2.0));
+
+            // it should never extrapolate further than one interval.
+            Assert.That(LagCompensation.Sample(history, 4.01, Interval, out before, out after, out t), Is.False);
+        }
+
+        [Test]
+        public void EstimateTime()
+        {
+            // server is at  t=100
+            // client has an rtt of 80ms
+            // snapshot interpolation buffer time is 30ms
+            // 100 - 0.080/2 - 0.030 = 99.93
+            Assert.That(LagCompensation.EstimateTime(100, 0.080, 0.030), Is.EqualTo(99.93).Within(0.001));
+        }
+    }
+}

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