57
« on: 03:23 PM - 02/02/12 »
Lag Compensation
Understanding interpolation is important in designing for lag compensation because interpolation is another type of latency in a user's experience. To the extent that a player is looking at other objects that have been interpolated, then the amount of interpolation must be taken into consideration in computing, on the server, whether the player's aim was true.
Lag compensation is a method of normalizing server-side the state of the world for each player as that player's user commands are executed. You can think of lag compensation as taking a step back in time, on the server, and looking at the state of the world at the exact instant that the user performed some action. The algorithm works as follows:
1. Before executing a player's current user command, the server:
1. Computes a fairly accurate latency for the player 2. Searches the server history (for the current player) for the world update that was sent to the player and received by the player just before the player would have issued the movement command 3. From that update (and the one following it based on the exact target time being used), for each player in the update, move the other players backwards in time to exactly where they were when the current player's user command was created. This moving backwards must account for both
connection latency and the interpolation amount 8 the client was using that frame. 2. Allow the user command to execute (including any weapon firing commands, etc., that will run ray casts against all of the other players in their "old" positions). 3. Move all of the moved/time-warped players back to their correct/current positions
Note that in the step where we move the player backwards in time, this might actually require forcing additional state info backwards, too (for instance, whether the player was alive or dead or whether the player was ducking). The end result of lag compensation is that each local client is able to directly aim at other players without having to worry about leading his or her target in order to score a hit. Of course, this behavior is a game design tradeoff.
Game Design Implications of Lag Compensation
The introduction of lag compensation allows for each player to run on his or her own clock with no apparent latency. In this respect, it is important to understand that certain paradoxes or inconsistencies can occur. Of course, the old system with the authoritative server and "dumb" or simple clients had it's own paradoxes. In the end, making this tradeoff is a game design decision. For Half-Life, we believe deciding in favor of lag compensation was a justified game design decision.
The first problem of the old system was that you had to lead your target by some amount that was related to your latency to the server. Aiming directly at another player and pressing the fire button was almost assured to miss that player. The inconsistency here is that aiming is just not realistic and that the player controls have non-predictable responsiveness.
With lag compensation, the inconsistencies are different. For most players, all they have to do is acquire some aiming skill and they can become proficient (you still have to be able to aim). Lag compensation allows the player
to aim directly at his or her target and press the fire button (for instant hit weapons 9 ). The inconsistencies that sometimes occur, however, are from the points of view of the players being fired upon.
For instance, if a highly lagged player shoots at a less lagged player and scores a hit, it can appear to the less
lagged player that the lagged player has somehow "shot around a corner" 10 . In this case, the lower lag player may have darted around a corner. But the lagged player is seeing everything in the past. To the lagged player, s/he has a direct line of sight to the other player. The player lines up the crosshairs and presses the fire button. In the meantime, the low lag player has run around a corner and maybe even crouched behind a crate. If the high lag player is sufficiently lagged, say 500 milliseconds or so, this scenario is quite possible. Then, when the lagged player's user command arrives at the server, the hiding player is transported backward in time and is hit. This is the extreme case, and in this case, the low ping player says that s/he was shot from around the corner. However, from the lagged player's point of view, they lined up their crosshairs on the other player and fired a direct hit. From a game design point of view, the decision for us was easy: let each individual player have completely responsive interaction with the world and his or her weapons.
In addition, the inconsistency described above is much less pronounced in normal combat situations. For first-person shooters, there are two more typical cases. First, consider two players running straight at each other pressing the fire button. In this case, it's quite likely that lag compensation will just move the other player backwards along the same line as his or her movement. The person being shot will be looking straight at his attacker and no "bullets bending around corners" feeling will be present.
The next example is two players, one aiming at the other while the other dashes in front perpendicular to the first player. In this case, the paradox is minimized for a wholly different reason. The player who is dashing across the line of sight of the shooter probably has (in first-person shooters at least) a field of view of 90 degrees or less. In essence, the runner can't see where the other player is aiming. Therefore, getting shot isn't going to be surprising or feel wrong (you get what you deserve for running around in the open like a maniac). Of course, if you have a tank game, or a game where the player can run one direction, and look another, then this scenario is less clear-cut, since you might see the other player aiming in a slightly incorrect direction.
Conclusion
Lag compensation is a tool to ameliorate the effects of latency on today's action games. The decision of whether to implement such a system rests with the game designer since the decision directly changes the feel of the game. For Half-Life, Team Fortress and Counter Strike, the benefits of lag compensation easily outweighed the inconsistencies noted above.
Show Posts
