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 is currently being maintained side-by-side with the Orange Box branch; the two branches share technology, but it is unclear if they will ever be merged.
Unlike other branches, L4D does not require earlier versions of Source to be installed. It stands entirely alone.
New since the Orange Box is:
- "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 As well as unknown changes to code structure, the old nodegraph system has been superseded by polygonal navigation meshes that are automatically generated by the engine.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 for the survivors' eyes. See $ambientocclusion.
- VPK add-ons
- An archive format for easy mod distribution. See VPK.
- Lag compensated props
- prop_physics entities can be selectively lag compensated.