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 branches but was ultimately discontinued with the release of the 
Alien Swarm engine branch.
Unlike other branches, L4D (and later branches) does not require earlier versions of Source to be installed or bundled with Half-Life 2 content. It stands entirely alone.
Features
New since the Orange Box is:
- VPK files and add-ons
- A new archive format for easy content distribution, replacing the old GCF format.
- NextBot AI
- Source's original AI routines appear to have been completely replaced by the NextBot, 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 As well as unknown changes to code structure, the old nodegraph system has been superseded by polygonal navigation meshes that are automatically generated by the engine.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.
- Rewritten speech response system
- The speech response system has been rewritten, with several new features and improved performance. Details here and here.
- Split-screen
- Up to four players can play on the same screen at once.
- Subtitles for intro movies and other Bink-based videos
- Allows subtitles (including custom ones) to be put on timelines for when they should show.
- Lag compensated props
- prop_physics entities can be selectively lag compensated.
- Shader and effect detail options
- New options that allow for better performance on low-end machines, entirely replacing the old DirectX level system.
- New post-processing effects
- More post-processing effects have been added, such as film grain and vignette.
- Radial fog and fog volumes
- Radial fog replaces the previous planar-based fog, and multiple fog controllers can now be used within defined volumes.
- Local light shadows
- Render-to-texture shadows can now be calculated per-entity, and their direction determined by the closest light source.
- Swaying trees
- A material parameter that gives the effect of trees swaying in the wind.
- Per-texel color tint masking for models
- Models can now use the alpha channel of their base textures to define areas to tint with a specific color.
- VScript (in all games since )
- A powerful new scripting system primarily using Squirrel.
- Water flow maps (in all games since )
- Water can now flow using flow maps, and water fog can now receive lightmaps.
Availability
Source code for this branch is not available. However, server plugins are possible, and authoring tools exist for custom content.
