Left 4 Dead (engine branch)
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.
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 As well as unknown changes to code structure, the old nodegraph system has been superseded by polygonal navigation meshes that are automatically generated by the engine.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.
- VPK add-ons
- An archive format for easy mod distribution.
- Lag compensated props
- prop_physics entities can be selectively lag compensated.
- Up to four players can play on the same screen at once.
- Shader and effect detail options
- New options that allow for better performance on low-end machines.
- Swaying trees
- A material parameter that gives the effect of trees swaying in the wind.
- A powerful new scripting system using Squirrel.
- Water can now flow through a map using flowmaps.
Source code for this branch is not available. However, server plugins are possible, and authoring tools exist for custom content.