How to make and watch Doom2 Deathmatch recordings
 

When you play Doom2, its possible to tell Doom2.exe to record everything you do and save this information as a file, and then later when your game is over, you can tell doom2 to replay your game like a recording. For all practical purposes, doom2 recorded your game, but if we want to get technical about it, its not a recording at all, its a re-enactment.
 

If you wanted to tell doom2.exe to record your game, you would run Doom2 with an added parameter. The parameter would look like this, if you wanted to record a game named "mygame" :
 

doom2 -record mygame
 

After the game was done, you would notice a new file in your doom2 directory named mygame.lmp . This is the file that Doom2 created, and it contains all of the information about your movement and firing while you played. All doom2 recordings create files with the ending prefix .lmp .

If you wanted to tell doom2 to play back your game, you would type
 

doom2 -playdemo mygame
 

Note that you are not saying "doom2 -playdemo mygame.lmp". The program does not want to see the .lmp extention in the command line parameter, just the demo name itself.
 

If you want to watch a demo recorded by someone else, you must keep these facts in mind:

(1) There are several versions of Doom2.exe. The most common ones are 1.9, 1.7, 1.7a, and 1.666. The recordings produced by these different versions are NOT compatible. You cannot have doom2.exe version 1.666 and watch a recording made by someone who was using doom2.exe version 1.9. The standard for recording distribution is 1.9, and all of the demos on my site are for doom2 v1.9. If you have a different version of doom2.exe, you will need to run IDsoftware's patch programs to upgrade your doom2 to the compatible version. I have these files on my site at  my utilities page if you need to patch.

(2) Its possible to play Doom2 on levels that did not come with Doom2 itself. If the people who recorded the game you want to watch were playing on one of these custom levels, you must download the file that contains the new data for the level, and put it into your doom2 directory. Then to watch it, you would use the -file parameter to tell doom2 to use the new level. For example, if you wanted to watch a recording that was done on a level called "Dwango5", you would download the Dwango5 level, and unzip dwango5.wad into your doom2 directory. Then when you play back the recording, your line would look something like this:
 

doom2 -playdemo newdemo -file dwango5.wad
 

Note that the level file (the .wad file) does require you to use the .wad extension in the command line parameter, which is opposite of how Doom2 handles the .lmp files at the command line.
 

Other trivia about deathmatch recordings:
 

Single-player cheat code use is not picked up in recordings. If you use a cheat code while you were recording, and then play it back, the demo does not activate the cheat code. The demo continues playing back your movement and firing, and typically becomes nonsensical when you die and Doom2 does not really acknowledge it.

Once in a long while, you will find a recording (especially in a recording that has a long pause in it) that becomes messed up sometime in the MIDDLE of playback and performs like nonsense. This is called a "desynched" demo. Sucks when it happens.

If you forget to load the correct .wad file for a recording, or load the wrong one, the demo will attempt to play back on the wrong map, not knowing there is a problem. The resulting playback typically looks like nonsensical running into walls and firing at nothing, similar to a desynchd demo as described above. If you see this AT THE BEGINNING, you know you're not loading the correct map.