How to create animated background like valves?

like in half life 2, the video background. how do i create one like that?

I remember reading a question like this a little while ago. the animated backrounds are special maps specially designed for that purpose. i dont know anymore details aside from that but you can always search up more on these “special maps”

ah, thanks.

One way around having to let a map load is to make a background map, load it in-game, use Source video recorder to export it as an .avi file and use Bink & Smacker video tools to convert it to a .bik file (like what valve games use). Not sure how this could be implemented in Gmod though, but it’s great for making custom L4D backgrounds.