Left 4 Dead (engine branch)

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The Left 4 Dead engine branch was released in 2008 with Left 4 Dead, and continued in 2009 with Left 4 Dead 2. It was maintained side-by-side with the Orange Box branch, but was ultimately discontinued with the release of Portal 2.

Unlike other branches, L4D does not require earlier versions of Source to be installed. It stands entirely alone.

Features

New since the Orange Box is:

Subtitles for intro movies and other Bink-based videos
Allows subtitles (including custom ones) to be put on timelines for when they should show
"Counter-Strike bot" AI
Source's original AI routines appear to have been completely replaced by the successor to Mike Booth's Counter-Strike bot. As well as unknown changes to code structure, the old nodegraph system has been superseded by polygonal navigation meshes that are automatically generated by the engine.
AI director
A "dungeon master" for the entire game which controls NPC spawning, music, weather effects, map layout, and more.
Ambient occlusion
Used on the survivors' eyes.
VPK add-ons
An archive format for easy mod distribution.
Lag compensated props
prop_physics entities can be selectively lag compensated.
Split-screen
Up to four players can play on the same screen at once.
Water flow (New with Left 4 Dead 2)
Water can now flow through a map.

Availability

Source code for this branch is not available. However, server plugins are possible.