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 succeeded with the release of the 
Alien Swarm engine branch, being the successor of this branch. However, as of 2024, this branch, aswell as Left 4 Dead and Left 4 Dead 2, is still being actively updated in order to fix security issues and other bugs, as well as adding new contents (such as Left 4 Dead 2's Last Stand update back in 2021), and features such as Steam Input API support.
Unlike other branches (with the exception of some third-party games), L4D (and later branches) does not require earlier versions of Source to be installed or bundled with Half-Life 2 content in order to run. It stands entirely alone.
Aside from branching off to the Alien Swarm engine branch, L4D's branch also branched off to create the 
CS:GO engine branch.
Features
Added
New since the Orange Box engine branch is:
- VPK files and add-ons
- A new archive format for easy content distribution, replacing the old GCF format used in Source 2009 and earlier. VPK was later eventually backported to previous branch during the transition to the new SteamPipe content distribution system.
- 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 As well as unknown changes to code structure, the old nodegraph system has been superseded by polygonal navigation meshes that are automatically generated by the engine.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 "gamemaster" 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.
- HDR is now required.
- LDR support has mostly been dropped with the option to disable compiling HDR being absent, meaning the only way to have LDR is with the VRAD -ldr syntax.
Since Left 4 Dead 2
- 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.
- BSP version 21
- New BSP version, with modified lumps.
Removed
Features that have been removed in this branch are:
- Half-Life 2 assets
- Starting with the Left 4 Dead engine branch and later engine branches, HL2 content is no longer required in order to run the game, through the game may still come with some
Half-Life 2 assets (such as skyboxes). - DirectX compatibility levels support,
mat_dxlevel-related commands. - DirectX compatibility levels (and support for GPUs made for DirectX 8.x), including all
dxlevelconsole command and command line, has been removed. Left 4 Dead engine branch also requires GPUs with DirectX 9 with Shader Model 2.0.
Availability
2008 Left 4 Dead2009 Left 4 Dead 22011
Nuclear Dawn2012
Revelations 20122013
Contagion
Source code for this branch is not publicly available. However, server plugins are possible, and authoring tools exist for custom content.
