I have a rewind system in my game that i am working on. The purpose of this system is to allow the server to be able to look back in time for the purpose of projectile hit validation. While it mostly seemed to be working fine, I find that there is a desync that occurs between 1-2 minutes into operation.
In the screenshot above, I have a system setup where I have a dummy moving back and forth across the baseplate. Pressing the P key makes a visual representation of a snapshot of the dummy’s movement. Blue represents what the client sees the moment the P key is pressed, and red represents what the server found when it looks back in time to the timestamp sent by the client. As you can see, some of them line up while others don’t. The ones that line up were taken before approximately one minute of operation, and the ones that do not line up were taken afterward.
I’ve noticed that the moment this desync occurs, the rig visibly rubberbands, moving backward a few studs. The distance of the desync remains consistent afterwards. Is this a studio issue or is there something wrong with my script?
--[[
Setup:
- Place this script in ServerScriptService
- Ensure workspace.Entities contains models with hitbox parts
--]]
local RunService = game:GetService("RunService")
local EntitiesFolder = workspace:WaitForChild("Entities")
-- Configuration
local MAX_SNAPSHOTS = 3000
local SnapshotInterval = 1 / 30 -- 30 Hz sampling
-- Data storage
local entitySnapshots = {} -- [entity] = { [index] = {timestamp = t, parts = { [part] = CFrame }} }
local snapshotIndex = 0
-- Utility: Get all BaseParts in a model
local function getHitboxParts(model)
local parts = {}
for _, part in ipairs(model:GetDescendants()) do
if part:IsA("BasePart") then
table.insert(parts, part)
end
end
return parts
end
-- Utility: Record snapshot for all entities
local function recordSnapshot()
snapshotIndex = (snapshotIndex + 1) % MAX_SNAPSHOTS
local timestamp = workspace:GetServerTimeNow()
for _, entity in ipairs(EntitiesFolder:GetChildren()) do
if not entity:IsA("Model") then continue end
local parts = getHitboxParts(entity)
entitySnapshots[entity] = entitySnapshots[entity] or {}
local snapshot = { timestamp = timestamp, parts = {} }
for _, part in ipairs(parts) do
snapshot.parts[part] = part.CFrame
end
entitySnapshots[entity][snapshotIndex] = snapshot
end
end
-- Utility: Find closest snapshots before and after timestamp
local function findSnapshots(entity, timestamp)
local snapshots = entitySnapshots[entity]
if not snapshots then return nil end
local before, after
local minBeforeDist, minAfterDist = math.huge, math.huge
for _, snap in pairs(snapshots) do
local t = snap.timestamp
if t <= timestamp and (timestamp - t) < minBeforeDist then
before = snap
minBeforeDist = timestamp - t
elseif t > timestamp and (t - timestamp) < minAfterDist then
after = snap
minAfterDist = t - timestamp
end
end
return before or after, after or before
end
-- Main Function: GetHitboxAtTimestamp
function GetHitboxAtTimestamp(entity, timestamp)
local before, after = findSnapshots(entity, timestamp)
if not before or not after then return nil end
local alpha = (timestamp - before.timestamp) / (after.timestamp - before.timestamp)
alpha = math.clamp(alpha, 0, 1)
local hitboxModel = Instance.new("Model")
hitboxModel.Name = "ReconstructedHitbox"
hitboxModel.Parent = workspace
for part, cfBefore in pairs(before.parts) do
local cfAfter = after.parts[part]
if not cfAfter then continue end
local interpCF = cfBefore:Lerp(cfAfter, alpha)
local clone = Instance.new("Part")
clone.Size = part.Size
clone.CFrame = interpCF
clone.Anchored = true
clone.CanCollide = false
clone.Transparency = 0.5
clone.Material = Enum.Material.Neon
clone.BrickColor = BrickColor.new("Persimmon")
clone.Parent = hitboxModel
end
return hitboxModel
end
-- Start recording snapshots
local timeAccumulator = 0
RunService.Heartbeat:Connect(function(dt)
timeAccumulator += dt
if timeAccumulator >= SnapshotInterval then
recordSnapshot()
timeAccumulator = 0
end
--accurate server time definition
game.ReplicatedStorage.ServerTime.Value = game.Workspace:GetServerTimeNow()
end)
--test function to spawn clones
game.ReplicatedStorage.ServerRemotes.CLONE_communicate.OnServerEvent:Connect(function(player, humanoid, timestamp, brickcolor)
timestamp -= .15
local copy = GetHitboxAtTimestamp(humanoid.Parent, timestamp)
local copy2 = GetHitboxAtTimestamp(humanoid.Parent, timestamp - .01)
copy2 = GetHitboxAtTimestamp(humanoid.Parent, timestamp + .01)
local time_Min = math.huge
local time_Max = 0
local totalEntry = 0
for _, i in pairs(entitySnapshots[humanoid.Parent]) do
if i.timestamp < time_Min then time_Min = i.timestamp end
if i.timestamp > time_Max then time_Max = i.timestamp end
totalEntry += 1
end
print("Timestamp range: " .. tostring(time_Max - time_Min))
print("Timestamp Count: " .. tostring(totalEntry))
end)