View source for Left 4 Dead engine branch

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{{source topicon}}{{l4d topicon}}{{l4d2 topicon}}
[[File:L4d2 flowmap user.jpg|thumb|320px|In-game screenshot of {{l4d2|1}}, running on {{l4dbranch|1}}, with one of it's new features, [[#Water flow maps|flowing water]], shown on this screenshot.]]
The {{l4dbranch|4}} was released in 2008 with {{l4d|4}} and continued in 2009 with {{l4d2|4}}. It was maintained side-by-side with the {{src07}} & {{src09}} [[Orange Box]] branches but was ultimately succeeded with the release of the {{asbranch|4}}, being the successor of this branch. However, as of 2024, this branch, aswell as {{l4d|1}} and {{l4d2|1}}, is still being actively updated in order to fix security issues and other bugs, as well as adding new contents (such as {{l4d2|1}}'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 [[Alien Swarm engine branch|later]] branches) does not require earlier versions of Source to be installed or bundled with {{hl2|1}} content in order to run. It stands entirely alone.

Aside from branching off to the {{asbranch|3.1}}, L4D's branch also branched off to create the {{csgobranch|4}}.

==Features==
===Added===
New since the {{src07|4|nt=0}} is:

; [[VPK]] files and add-ons
: A new archive format for easy content distribution, replacing the old [[GCF]] format used in {{src09|4}} 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 As well as unknown changes to code structure, the old [[nodegraph]] system has been superseded by polygonal [[Nav Mesh|navigation meshes]] that are automatically generated by the engine.
; [[info_director|AI director]]
: A "[[Wikipedia:Gamemaster|gamemaster]]" for the entire game which controls NPC spawning, music, weather effects, map layout, and more.
; Rewritten speech [[Response System|response system]]
: The speech response system has been rewritten, with several new features and improved performance. Details [https://cdn.cloudflare.steamstatic.com/apps/valve/2012/GDC2012_Ruskin_Elan_DynamicDialog.pdf here] and [https://github.com/mapbase-source/source-sdk-2013/pull/108 here].
; Split-screen
: Up to four players can play on the same screen at once.
; [[Closed_Captions#Captions_for_BINK_videos|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 compensation|Lag compensated]] props
: {{ent|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 Versions|DirectX level]] system.
; New [[postprocess_controller|post-processing]] effects
: More post-processing effects have been added, such as film grain and vignette.
; [[env_fog_controller|Radial fog]] and [[fog_volume|fog volumes]]
: Radial fog replaces the previous planar-based fog, and multiple fog controllers can now be used within defined volumes.
; Local light [[shadow_control|shadows]]
: [[Lighting#Dynamic_Shadows|Render-to-texture shadows]] can now be calculated per-entity, and their direction determined by the closest light source.
; [[$treeSway|Swaying trees]]
: A material parameter that gives the effect of trees swaying in the wind.
; [[$color#Additional Model Parameters|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]] {{since|{{l4d2}}}}
: A powerful new scripting system primarily using {{sq|4}}.
; Water flow maps {{since|{{l4d2}}}}<span id="Water flow maps"></span>
: Water can now flow using [[Water (shader)#Flow|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:
; {{hl2|3.1}} assets
: Starting with the {{l4dbranch|4}} and later engine branches, HL2 content is no longer required in order to run the game, through the game may still come with some {{hl2|2}} assets (such as [[Skybox|skyboxes]]).
; [[DirectX]] compatibility levels support, {{code|mat_dxlevel}}-related commands.
: DirectX compatibility levels (and support for GPUs made for DirectX [[DirectX Versions#DirectX 8.0|8.x]]), including all {{code|dxlevel}} console command and command line, has been removed. {{L4dbranch|1}} also requires GPUs with DirectX 9 with Shader Model 2.0.

==Availability==
{{see also|[[:Category:Left 4 Dead engine branch games]]}}
* {{Code|2008}}{{l4d|4}}
* {{Code|2009}}{{l4d2|4}}
* {{Code|2011}}{{nd|4}}
* {{Code|2012}}{{rl|4}}
* {{Code|2013}}{{con|4}}

Source code for this branch is '''not''' publicly available. However, [[Left 4 Dead Plugins|server plugins are possible]], and authoring tools exist for custom content.
* {{Game link|Left 4 Dead Authoring Tools}}
* {{l4d2}} [[Left 4 Dead 2 Authoring Tools]]
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