The code is now available on GitHub, too. github.com/jwatte/EPIC

When writing a networked game, you quickly run into the problem that, on client B, when you get data from client A, that data will be old, because of the transmission latency inherent in any networking. On a LAN, you may get away with using the old data as-is, but when playing on the general Internet, you want to display a guess of where the player might be right now.

There are various ways of doing this. The simplest way is to just read the position and velocity of the entity update packet, and forward the position of the entity by the velocity in the packet to the current time. The main draw-back of this approach is that the entity on screen will jump around ("snap" or "lag") for each update packet you get, because there are discontinuities in the interpolated position.

A better method involves interpolating between the last position you actually rendered the entity at, and some position derived from the last update. Because we are drawing the entity forward extrapolated to our current time frame, the actual position we get out of the update will need to be extrapolated by the time until the next update, to give us a point to aim for. This method is implemented in the template class Extrapolator which is included in the EPIC download.