First-person shooters (FPS) are probably the most controversial games that exist. For some they represent the essence of video gaming, others despise the ubiquitous violence or simply find them extremely boring. Nevertheless, the genre has played an important role in computer gaming history. Therefore it is logical that quite a few open source FPS games have been created.
In some cases one has to differentiate between the game engine and the actual game content (graphics, maps, sounds). Several engines of the Doom and Quake series have been put under the GPL by id Software, and they continue to be the building basis on top which many of today's open source shooters are built. However, there are games which offer their content under a non-free (whether this is a GPL violation or not is another question) license. For example, this is the case with Tremulous (since CC licenses are highly problematic) and Sauerbraten.
Entering the arena are three different offsprings of id Software's various liberated engines. First, there is PrBoom, which is based on the Doom engine. Moving one step further in FPS history comes Nexuiz, being built on top of the DarkPlaces engine, which in turn is derived from Quake I. The Quake III Arena engine is the one most recently released under the GPL and serves as the basis for OpenArena.
PrBoom is an "old school" first-person shooter, meaning that rather than hunting computer-controlled bots there are dozens of levels filled with monsters. In order to complete a level you only have to reach the exit, but that sounds easier than it usually is. There are also two multiplayer modes, one being the usual deathmatch, where the players try to kill each other. The other option is to play the single-player levels cooperatively and fight together against the swarm of monsters. PrBoom is usually being distributed together with FreeDoom, which contains all the graphics and levels necessary for actually playing the game.
Some things that have been said about Quake III Arena apply to OpenArena as well. Unlike to PrBoom, the player faces only a few humans or bots in relatively small-sized levels. When a character dies, it immediately returns to the game. The game offers four playing modes:
"Bringing deathmatch back to the basics" is the slogan of Nexuiz. Indeed, the game is deathmatch-centered, even in singleplayer mode. There, all opponents are bots. Besides the playing modes OpenArena offers, there are some other variants as well:
In addition, there are lots of possible variants like vampire deathmatch (you get healed by doing damage) or low-gravity matches.
Gentlemen, start your engines
Since the three games have their roots in different ages of FPS history, the quality of the game engines varies significantly. When you play PrBoom for the first time, you will immediately realize that the foundations of the rendering engine date back to the early nineties. Although the rendering is done via OpenGL and the resolution has been pushed up from the original 320x200, the graphical effects look rather poor from today's point of view. Another problem is that the engine does not deliver a full 3D experience, possibly bringing back the long forgotten game sickness. An obvious boon of the antique engine, however, is its ability to run fast on almost any machine.
In contrast, the graphics of OpenArena are much better, especially the dynamical light effects made by weapon fire are quite impressive. Some textures look a bit boring. However, since OpenArena is a high-speed arcade action shooter you hardly have time to notice it. Another advantage is the game's relatively modest hardware hunger — any not too out-dated machine with a 3D graphics accelerator will do the job.
Of all three games, Nexuiz's rendering engine produces the best result. The DarkPlaces engine, upon which the game is based, offers realtime lighting and shadowing effects, bump mapping and other eye-catchers. The downside of this is that Nexuiz has pretty hefty hardware requirements. Even with all advanced visual effects switched off, a decent 3D graphics accelerator is a must.
Choose your weapon
The weapons arsenal is pretty similar in all games, ranging from shotguns to rocket launchers. In general, each weapon is characterized by three characteristics: the damage done, the rate of fire, and the speed of the projectile or beam. Usually you start with two weapons, however the second should often be used only as a last resort.
OpenArena has the most balanced system, with no "super-weapon" ruling all others. Nexuiz offers a total of nine weapons. Some are very straight-forward to use, but especially the more powerful ones require a fair amount of training. It might be debatable whether the rocket launcher is too powerful since missing rocket may be detonated remotely, inflicting splash damage. Players in the explosion radius will also be catapulted away, which can be used as a tactical move. This is even more important on open maps without boundaries, as blasting someone off the map counts as a kill as well.
Single player modes
PrBoom is the game most focussed on single player experience. It offers five difficulty levels, ranging from "I'm too young to die" to "Nightmare". Higher difficulty means more monsters and less ammo, medkits and armor lying around. As the labels indicate, there should be a difficulty suited to everyone's ability. In the FreeDoom package there are 30 normal levels + 2 secret ones, like in Doom II.
OpenArena features pretty intelligent bots with adjustable strengths. They show tactical behavior like switching weapons when appropriate or retreating before running out of health. Long-term fun is some problem, however. Unless you install additional ones there are only seven maps to play and hunting bots gets a bit dull after some time.
Nexuiz's singleplayer campaign mode is quite entertaining. Besides playing alternately in the about twenty maps there are often modifications to the game rules that add another twist. For example, one level includes reduced gravity, only sniping weapons and you die when you run out of ammo. The only downside of the campaign mode is that you cannot adjust the difficulty. So, while it is very challenging for beginners, an FPS expert will find it far too easy. The bots, however, are certainly not too bright. Especially when equipped with explosive weapons like the rocket launcher, you often only have to wait until they blow themselves up.
In summary, the clear overall winner is Nexuiz. It offers the best graphics, the most playing variants and the best single player mode. OpenArena comes next, but may be the better choice if you don't have a machine fast enough for Nexuiz. PrBoom is only an option for true Doom enthusiasts or people with really slow computers.