dead reckoning
DESCRIPTION
Dead Reckoning. References: Gamasutra ( 1 ), Gamedev ( 1 ) Forum articles ( 1 ). Introduction. Fast-paced interaction in twitch games Players expect the level of performance of distributed games to approximate that of single computer/player game - PowerPoint PPT PresentationTRANSCRIPT
Dead Reckoning
References: Gamasutra (1), Gamedev (1)Forum articles (1)
Fall 2012 2
Introduction
Fast-paced interaction in twitch gamesPlayers expect the level of performance of distributed games to approximate that of single computer/player gameSolution: dedicated network (end-to-end latency 150-200 ms)
Fall 2012 3
Dead Reckoning
From DIS (distributed interactive simulation) protocol of SIMNET project, DoD, USA Was for networking tank simulators Objectives: latency hiding, bandwidth reduction
Basic Ideas: Agree in advance on a set of algorithms that can
be used by all players to extrapolate the behavior of entities
Update threshold: how far reality should be allowed to get from these extrapolations before a correction is issued
Fall 2012 4
Dead Reckoning
[point-to-point]01 tt PositionPosition
0101ttvPositionPosition tt
20101 2
101
tta
ttvPositionPosition tt [quadratic]
[linear]
If the motion is still within theDR threshold (on Owner), no updates required.
Fall 2012 5
DR Algorithm (point-to-point)
No path
Noticeably jerky unless one packet is received per frame
New packet [p]
New packet [p]
Fall 2012 6
DR Algorithm (linear)moves along this v, until …
New packet [p,v]
Path does not consider change of velocity
Fall 2012 7
DR Algorithm (quadratic)moves along this v and a, until …
NewPosition = OldPosition + Velocity*time + 0.5*Acceleration*(time)2
New packet [p,v,a]
Path does not consider change of acceleration – jerk
Fall 2012 8
PDU (protocol data unit)
Data packet representing each entity Kinematic state: position, velocity,
acceleration, orientation Other info: damage level, turret … Which dead reckoning algorithm to use
Threshold: jerkiness vs. more PDUs to be sent
Fall 2012 9
Smoothing
Jerkiness: due to sudden update of positionUse smoothing algorithm to lessen the apparent jerkiness [more later]
Fall 2012 10
Extension
Predictive Contract State-based, rather than kinematic-
based State: “drive along road to waypoint”
If the definition of roads, the specific waypoint, the way of driving (right-side), … are known to all players, the vehicle could be computed w/o any network traffic
Others: “turn on/off the sensors”, “send out radio report”
Fall 2012 11
Summary
Dead reckoning is not free: every computer runs an algorithm to extrapolate each entityTrade processor cycles to reduced network use and apparent latencyIf all entities behave unpredictably all the time, DR offers little gain
Fall 2012 12
DR Smoothing with Cubic Splines (ref)
Jerkiness is due to the sudden update of positionThe following method ensures smooth positional transition while ensuring accuracy by packet (server) updates
Only cubic (parametric) curve can represent R3 curve [quadratic curves are planar]Hermite curves are most common. Two points and their corresponding tangents need to be specified (by quadratic kinematics laws)
A Draftsman’s Spline
Fall 2012 13
Fall 2012 14
Math of Hermite Curves
h1(s) = 2s33s2 + 1h2(s) = 2s3 + 3s2
h3(s) = s3 2s2 + s h4(s) = s3 s2
P(s) = P1h1(s) + P2h2(s) + T1h3(s) + T2h4(s)P’(s)=P1h1’(s) + P2h2’(s) + T1h3’(s) + T2h4’(s)
h1’ = 6s2 6s h2’ = 6s2+6s h3’= 3s2 4s+1 h4’= 3s2 – 2sNote: end tangent vectors are dP/ds
Fall 2012 15
s=0 t=0
s=1 t=T
)0()0(dt
dpT
ds
dP
)()1( Tdt
dpT
ds
dP
Reparameterization
dt
ds
ds
sdP
dt
tdp
tsPtp
t
)()(
))(()(
:(time) isparameter theIf
Tds
sdP
dt
ds
ds
sdP
dt
tdpT
dtds
T
ts
1)()()(velocity
Fall 2012 16
Algorithm
When a packet [p,v,a] arrives, begin creating a cubic spline for next position Approximate (extrapolate) the [p,v] after time
T
A piece of Hermite curve is defined by two end points and two end tangents P1: current position T1: (scaled) current velocity P2: extrapolated position after T T2: (scaled) velocity after T T: estimated
interval between packets
T: estimated interval between packets
Fall 2012 17
Details
New packet [p,v,a]
Start a new spline by1. Current [p,v]2. New [p,v] computed by [p,v,a] after time T
Move with current spline
P1
T1
P2T2 The position gradually
changes from P1 (s=0) to P2 (s=1, t=T)