Left 4 Dead engine branch
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Left 4 Dead engine branch
In-game screenshot of Left 4 Dead 2, running on Left 4 Dead engine branch, with one of it's new features, flowing water ↓, shown on this screenshot.
Developer(s)
Release date(s)
November 17, 2008
Status
Actively updated (as of 2026)
SDK
Left 4 Dead Authoring Tools
Tools only, SDK Base & source code not available.
Tools only, SDK Base & source code not available.
Predecessor
Successor
The 
Left 4 Dead engine branch was released in 2008 with the launch of Left 4 Dead. It was maintained side-by-side with the 
& 
Orange Box branches, while subsequently updated in 2009 with 
Left 4 Dead 2, adding additional features. It was ultimately succeeded with the release of the 
Alien Swarm engine branch, being the successor of this branch.
This branch, aswell as Left 4 Dead and Left 4 Dead 2, is still being actively updated (as of 2026) and maintained 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), Left 4 Dead (and subsequent branches released after it) no longer requires Half-Life 2 content to be present in order to run. It stands entirely alone.
Aside from branching off to the Alien Swarm branch, L4D's branch also branched off to create the 
Portal 2 engine branch (forked between L4D & L4D2), 
CS:GO engine branch and the 
Dota 2 engine branch[2] (all of which implemented features from newer branches since Alien Swarm onward).
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 Orange Box branches during the transition to the new SteamPipe content distribution system and the release of Source 2013 (which have native implementation of VPK). Additionally Source 2007, Source 2006 and earlier branches also received VPK mounting support via Steam2 / Steam.dll wrapper.
- Left 4 Dead branch and subsequent branches after that until CS:GO & Source 2013 (which switches to VPK version 2), uses VPK version 1.
- Improved multithreading support (also in )
- Multi-thread support was expanded, allowing for features like split screen multiplayer, additional post-processing effects, event scripting with Squirrel, and the highly-dynamic AI Director.
- The improved multi-threaded support in Left 4 Dead branch was later backported to titles running on Source 2009 engine (part of Orange Box branch).
- 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 two players can play on the same screen at once. While this system is present on PC, it is difficult to access outside of Xbox 360 (check out this guide for help using split-screen).
- 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 (also in
, 
20th, 
, 
, 
)[Note] 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.
- 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 rendering is now required.
- LDR rendering support has mostly been dropped and deprecated with the option to disable compiling HDR lighting being absent, meaning the only way to have LDR is to compile the map with VRAD
-ldrsyntax. Games on this branch will also default to HDR by default and will fallback to LDR if the map lighting is only compiled in LDR. The command that was associated with switching LDR/HDR rendering still exists but has been hidden (marked as launcher-only), and cannot be changed normally via Developer Console without having SourceMod installed, using VScript or add+mat_hdr_level 0/1/2on launch options. - DirectX 10 hardware feature level on Direct3D 9 (
dxlevel 100) (also in) (not in ) - All games since L4D branch (including Alien Swarm, SFM) runs on
dxlevel 100, except Portal 2 and CS:GO. This is likely used to force sRGB behavior on DirectX 10 graphics card like Nvidia GeForce 8 series to match the behavior seen in previous DirectX 9 graphics card (Nvidia GeForce 7, and AMD equivalent or earlier), aswell tell the engine to load the-dx10version of the assets (often used by tool textures). Textures that are marked as-dx10would also have the sRGB flag. The game will still render in Direct3D 9, though. 
Note: In L4D2 and Alien Swarm, mat_dxlevel 100can be only seen using SourceMod (sm_cvar mat_dxlevel).- Raw mouse input (also in all games since
and 
) - Directly taking unchanged data from mouse drivers before it goes through operating system enhancements, meaning that mouse input will be exactly 1 to 1.
Mac OS X support- This engine branch can now run on Mac after October 5, 2010 (for ), and October 27, 2010 (for ). However, Mac support outside of official games may be spotty.
However with the release of macOS since 10.15 (Catalina), it drops support for all 32-bit apps and games in general (which also includes every single Source engine games that are complied for 32-bit only).
Since Left 4 Dead 2
- Swaying trees
- A material parameter that gives the effect of trees swaying in the wind.
- VScript (in all games since )
- A powerful new scripting system primarily using
Squirrel. - Water flow maps (in all games since ) (also in
) - Water can now flow using flow maps, and water fog can now receive lightmaps.
- BSP version 21
- New BSP version, with modified lumps.
Linux support- The engine can now run on Linux. However, support for these OS outside of official games may be spotty.
Changed
- Protocol versions
- Since Left 4 Dead March 11, 2009 update, protocol version are now tied to
PatchVersiononsteam.inf. Additionally later in Left 4 Dead 2 (but not later games like Portal 2), it is tied toNetworkVersion. This meant that old demos are no longer viewable unless downgrading the game to older version, or editsteam.inf(and change PatchVersion to "37" for example, to view demos from protocol version 37), doing this will prevent you from connecting to any servers not matching your game version. - VPhysics optimization
- In Left 4 Dead engine branch and subsequent branches (later backported to Orange Box-based branch under Team Fortress 2 branch), it features optimization to VPhysics (compiled using floats instead of doubles), meant to improve performance on console and lower end hardware, and possibly avoid network prediction errors & CPU load on servers. However this leads to unreliable collision detection due to reduced physics precision.[1] This means in some cases props and other physics objects will briefly glitch through map geometry, other props or other physics objects.[1]
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 and usage
The following titles run on the Left 4 Dead engine branch:
Valve
See also: Category:Left 4 Dead engine branch games
- 2008 - Left 4 Dead
- 2009 - Left 4 Dead 2
Third-Party
- 2010 -
Zeno Clash (
Xbox 360 only) - 2011 -
Nuclear Dawn - 2012 -
Hybrid ( Xbox 360 exclusive) - 2012 -
Revelations 2012 - 2013 -
Contagion
Source code and SDK Base for this branch is not publicly available. However, server plugins are possible, and authoring tools exist for custom content:
History and changes
November 18, 2008: Left 4 Dead was released, running on it's own branch. Unlike other Source engine branches and games, Left 4 Dead does not require Half-Life 2 assets to operate.
Early 2009: At some point, the Left 4 Dead branch was forked to create the Portal 2 engine branch, with the early Portal 2 build (as far back as earliest found July 2009 build) features protocol version "37", matching the release build of Left 4 Dead. The Portal 2 branch around July 2009 (or much earlier) would eventually feature VScript, VTF 7.5 along with new features as the Left 4 Dead 2 code (Portal 2 and L4D2 are being developed side-by-side until the Nov 2009 release of L4D2), followed by some Alien Swarm code were later upstreamed to the then in-development Portal 2 branch. At the same time, the Portal 2 July 2009 build also contains FGD files from Dota 2 (
dota.fgd), Alien Swarm (infested.fgd, infested_base.fgd) and cancelled Half-Life 2: Episode Three (ep3.fgd), which contains the weapon entity name for the Weaponizer (the model of the Weaponizer would later be re-used for the cut Paint Gun during Portal 2 development). As Dota 2 FGD files were included in Portal 2 2009 build along with a build of Dota 2 from 2010, in the same Steam2 Teraleak, the Dota 2 branch was likely also forked from the Left 4 Dead branch, or the very early version of the Portal 2 branch during it's development.
November 17, 2009: Left 4 Dead 2 was released, which runs on updated version of the Left 4 Dead branch (this is also referred as the L4D2 branch). The Left 4 Dead 2 version of the branch appears to contain few leftover source code from the scrapped F-Stop & 2 Bots 1 Wrench (codenamed Zion), based on the leak from LunchHouse Software (who later develops "PUNT" and "Exposure" in 2020s, the latter used some of the code from F-Stop).
References
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