Viewing demos is really easy. Playing back demos can often times work different depending on what game it is, for half-life, playing a demo requires the console to be open and a little knowledge on how to operate its built in demo player. For others (like Half-Life 2, csgo), they have a GUI based player and also functions as a very limited video editor (for trimming, camera operation. ect)
Half-life uses the command:
playdemo <demoname>
as its standard way of playing back newly created demos in the form of ".dem". Half-life uses this filetype, since mpeg video recording was impossible on late 90's pcs at the time and because it was a feature left over from the quake engine, which also uses the same ".dem" filetype to record demos.
The problem with .dems: they only work locally, and i mean that by the literal definition. You cannot stream these files, you can't record a demo on one map and share the demo with someone who's never downloaded that map. Demos are strange, they're not a video clip in the traditional sense, its like theater mode on call of duty, it will load the entire map in a private server with bots.
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