"Subclassing and Composition – A Pythonic Tour of Trade-Offs", Hynek Schlawack
CS4344 09/10 Lecture 3: Dead Reckoning and Local Perception Filter
1. Lecture 3
Dead Reckoning
and
Local Perception Filter
25 January 2010
Saturday, January 23, 2010 1
2. states
Simulate Collect Events
events
Game
Game
or States
Game Render
States
Wait
Saturday, January 23, 2010 2
3. What if a state changes continuously (i.e., is a
function of time) ?
Player A
Server
Player B
Saturday, January 23, 2010 3
4. Consider position updates of players. Players
send move command. Server replies with
positions periodically.
Player A
Server
Player B
Saturday, January 23, 2010 4
5. Two issues:
1. Message overhead
2. Delay jitter
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6. Delay jitter causes player’s movement to appear
erratic.
Player A
Server
time
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7. Recall: Short circuiting is used to predict own
states. We can similarly predict opponent’s
state.
Player A
Server
Player B
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8. Suppose the velocity remains constant, then we
can predict every position at all time.
predicted position
Player A
Server
time
Saturday, January 23, 2010 8
9. x[t] position of entity at time t
v velocity of the entity
x[ti ] = x[ti−1 ] + v × (ti − ti−1 )
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10. We send over the initial position x[t], t, and
velocity. (Why do we need to send t?)
predicted position
Player A
Server
time
Saturday, January 23, 2010 10
11. But velocity may change (e.g. a car accelerating).
To counter this, we send position, velocity, and
acceleration as update.
Player A
Server
time
Saturday, January 23, 2010 11
12. x[t] position of entity at time t
v velocity of the entity
a acceleration of the entity
1
x[ti ] = x[ti−1 ] + v(ti − ti−1 ) + a(ti − ti−1 )2
2
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13. We will still need substantial number of updates
if the direction changes frequently (e.g. in a FPS
game).
B
Player A
Server
time
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14. idea: trade-off message
overhead and accuracy.
No need to update if error
is small.
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15. Player A
error threshold
Server
time
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16. Player A
Server
time
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17. Player A
Server
time
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18. predicted position at A
A’s predicted position at the server
Player A
Server
time
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19. A’s version of the entity’s position is now too far
away from the correct position. Server updates A
with the new velocity and position.
Player A
Server
time
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20. A converges the entity to the correct
position smoothly.
Player A
Server
time
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21. how to set error
threshold?
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22. adapt based on game
requirement
(e.g. distance to other players)
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23. Space inconsistency: due to error threshold and
convergence
Time inconsistency: due to message delay and
clock asynchrony
Player A
Server
time
Saturday, January 23, 2010 23
24. What is the difference between
the actual and predicted
position ?
How long does the difference
last?
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25. error
time
inconsistency =
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26. what are the different components of distance
error?
Player A
Server
time
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31. “return to base”
“drive along this road”
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32. Server and clients must have a
common notion of “time”
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33. Two choices:
Wall Clock
Game Clock
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34. Wall Clock: clients and
server have to synchronize
their physical clocks using
NTP or SNTP.
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35. now is
10:18:32 now is
10:18:29.5
now is
10:18:30
now is
10:18:33.5
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36. now is
10:18:32 now is
10:18:29.5
now is
10:18:30
now is
10:18:33.5
Time now should be
10:18:30.5
RTT = 1s
OWD = 0.5s
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37. Players try to sync the game states with the
server at the same wallclock time, and predict
ahead with an amount of time equal to one-
way delay.
Player A
Server
Player B
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38. or: synchronize states to game clock, which
runs behind the server by an amount of time
equals to the one-way delay.
Player A
Server
Player B
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39. what A sees
what B sees
A S B
3 4 2 game clock
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46. A decides to move. Send event to S.
A S B
0 0 0
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47. S receives event, moves A, and
tells A and B that A is moving at t = 1
A S B A S B
0 0 0 1 1 1
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48. A moves itself to where it should be at t = 2.
A S B A S B A S B
0 0 0 1 1 1 2 2 2
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49. B finds out A moves at t = 1 and
moves A to where A should be at t = 3
A S B A S B A S B A S B
0 0 0 1 1 1 2 2 2 3 3 3
Saturday, January 23, 2010 49
