I'm writing a little 2D overhead multiplayer space shooter. I use
client-side prediction to make movement smooth for the local player,
and it looks great. For the local player. My problem is that I want
to show the player the current location of the other spaceships in
regards to the local player, and to do that I need to predict them,
too. Now, fortunately, the movements of the enemy spaceships are
pretty smooth, since they smoothly accelerate rather than zip left and
right instantly.
My problem, though, is turning. When a player presses left or right,
his ship immediatley begins to rotate at a set rotational velocity, and
when the player releases the key, the ship immediately stops turning.
This, again, works great for the local player. However, when the local
player receives notice that another player began rotating 5 game states
ago, the local player predicts the motion ahead a bit and continues to
extrapolate how the other ship is rotating. With a bit of smoothing,
that looks fine, too.
The problem is that when the other ship stops turning, the local
machine has already predicted it turning a bit further, which it then
needs to take back, causing it to snap back after finishing a turn like
it's on a spring. It happens every time, and if there's 500ms of lag
or so, it can be more than a 180 degree turn.
In order to solve this problem, I've tried a few techniques. The
first, obviously, was to stop the prediction of turning. I still use
the predicted turn angle to compute motion (if you're accelerating), so
if you have quite a bit of lag, you might notice the ship not quite
flying in the right direction briefly on occasion, but it's hardly
noticeable. I haven't quite worked out a good way to smooth between
these messages yet, especially in a situation with high variance in the
network connection's lag, but I'm sure I can work out something that
more or less works.
So, I'm asking for general input as to how to handle players controlled
elsewhere in a system that uses client prediction. In half-life 2, for
instance, I do not notice the other players take a step back every time
they stop walking forwards. Does half-life 2 not predict the locations
of other players? Or does it use some other, neater trick?
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