Setting The Standard in Online Multiplayer Doom
Odamex is a free, cross-platform (Mac, Windows, Linux) modification of the Doom engine that allows players to easily join servers dedicated to playing Doom online. The goal of Odamex is to add enhancements to Doom while still retaining that "classic Doom feel" we all know and love about the original game.
Odamex is free software, however a copy of Doom or Doom II is recommended to play it. If you do not own Doom, it can be purchased on Steam. Other alternatives include using Freedoom, or the Shareware version of Doom.
Latest version: 0.7.0
News & Updates
Check out the details for our first session of UptightDM RIGHT HERE.
Odamex has several advanced features, including on-the-fly wad changing, the ability to trigger between vanilla Doom and ZDoom physics and nuances, map voting, an optional 32bpp renderer, network packet-based multiplayer demos, and backwards reconciliation netcode optimizations for high latency players.
However the last release of Odamex was on March 29, 2014. There have been many code updates following the release but no new versions due to some plans not being completed. There are currently no “horrible” bugs and repeat offender crashes but the project definitely needs optimizations and maintenance.
If you are interested in helping Odamex, please respond here and/or join #odamex on irc.oftc.net
Note that these apply to Odamex 0.7 and may change in future versions!
rate ("Bandwidth" in the Network Options menu)
Sets the maximum bandwidth the client can receive, measured in kilobytes per second. The default value is 200, though setting it higher to 750 will allow for faster in-game downloading if the server allows it.
cl_unlag ("Adjust weapons for lag" in the Network Options menu)
Allows the client to enable or disable backwards reconciliation ("unlagged") for hitscan weapons. Enabled by default, this setting allows a player to aim directly at enemies when firing hitscan weapons such as the shotgun and chaingun.
It works as follows:
Every gametic (there are 35 per second), the server records the position of every player.When the server sees that a player with round-trip latency of X milliseconds pressed the fire button, it temporarily moves all other players to the position they were in X milliseconds ago.The server determines if the shot hit anyone.Players are moved back to their current position.
cl_updaterate ("Position update freq" in the Network Options menu)
Indicates the frequency of position updates a client wishes to receive. The default value of 1 indicates they wish to receive an update every gametic while a value of 2 indicates an update every 2 gametics. More frequent updates lead to greater accuracy at the expense of more bandwidth usage.
cl_interp ("Interpolation time" in the Network Options menu)
Allows a client to smooth the movement of enemy players due to network jitter in the connection between that particular client and the server. This smoothness comes at the expense of increased visual latency.
If the most recently received position update is N, the default value of 1 will display players at position update N - 1. Similarly, a value of 2 will display players at position update N - 2.
Network jitter causes the client to not always receive position updates in a timely manner. If a client displayed position update N - 1 last gametic and has yet to receive a new position update from the server, it can display position N, maintaining accurate, smooth movement even when there is jitter or small latency spikes.
Allows the client to enable or disable client-side prediction of a client's player position. This prediction, enabled by default, allows the client to immediately move its player position based on controller input instead of waiting for confirmation of a new position from the server. This is much more responsive compared to the alternative, which does not change the player position (and in turn the rendered view of the world) until the the client's round-trip latency time to the server has passed.
Although prediction allows for immediate visual feedback, it can also be incorrect. This is often the result of collision with other players or being thrust by weapon damage. When the prediction is incorrect, the client considers the server to be authoritative about the real position of the client's player.
Although most players would find it difficult to play with this CVAR disabled, it is immensely useful in diagnosing recordings of games for debugging purposes.
cl_prednudge ("Smooth collisions" in the Network Options menu)
Sets the amount of interpolation between a client's mispredicted player position and the corrected player position from the server. Valid values range from 0.05 to 1.0. The client can choose how they want to trade off between smoothness and accuracy when their player position is mispredicted (see cl_predictplayerposition for further background information). A value of 0.05 will nudge the player towards the correct position gradually and as smoothly as possible where as a value of 1.0 will instantly jerk the player to the correct position to be as accurate as possible.
For the examples in this explanation, we will consider two players: Player A with a 150ms ping and Player B with a 20ms ping.
Odamex has a fixed frame-rate of 35fps, which means each frame lasts roughly 29ms. That also conveniently corresponds to the network update rate and the game physics update rate.
Odamex also uses a term "world index" to refer to the particular set of actor positions that the client has received from the server and uses to draw the sprites for actors in the game (enemy players, etc).
How it works:
When Player A aims directly at Player B and presses the fire button, his Odamex client will send all of his input (eg, fire-button, mouse angle, etc) to the server as well as the current world index. The server will receive this input from Player A 75ms later. During this time, Player B may have moved significantly and without latency compensation, Player A would miss his shot.
With latency compensation, the server will temporarily move all players except the shooter to their position in the world index that was received from the shooter. In the case of Player A shooting, it means that Player B would be moved to his position 150ms ago. The server then performs hitscan collision detection and then moves all of the players back to their current real position. This compensation allows Player A to aim directly at Player B and successfully hit him, even with adverse network conditions.
One side effect of this compensation can be seen from the point-of-view of Player B. It is possible for Player B to have gotten behind cover of a wall and still be shot by Player A. This is due to the fact that Player A has high latency and thus his input takes a noticeable amount of time to reach the server and during this time, Player B can move behind a wall. The latency compensation will temporarily move Player B to his position at the time Player A pressed fire, which was directly in the line-of-sight of Player A.
Hopefully this description is not too technical and also did not omit any details that are necessary for understanding the way latency compensation works in modern FPS games. Let me know and I can further clarify any points which are not clear.