View source for Left 4 Dead engine branch

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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 discontinued with the release of the {{asbranch|4}}, being the successor of this branch. However, as of 2024, this branch is still being actively updated in order to fix security issues and other bugs, aswell as adding new contents (such as {{l4d2|4}}'s Last Stand update back in 2021).

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.

== 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 As well as unknown changes to code structure, the old [[nodegraph]] system has been superseded by polygonal [[Nav Mesh|navigation meshes]] that are automatically generated by the engine.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.
; [[VScript]] {{since|{{l4d2}}}}
: A powerful new scripting system primarily using {{sq|4}}.
; Water flow maps {{since|{{l4d2}}}}
: Water can now flow using [[Water (shader)#Flow|flow maps]], and water fog can now receive lightmaps.

=== Removed ===
Features that have been removed in {{l4dbranch|4}} are:
; {{hl2|4}} content
: Starting with {{l4dbranch|4}} and later engine branch, HL2 content are no longer required in order to run the game, through the game may still come with some {{hl2|4}} assets (such as [[Skybox|skyboxes]]).
; DirectX 8.x and earlier support, {{code|mat_dxlevel}}-related commands.
: All previous version of DirectX mode (used for compatibility with older GPUs that does not support all D3D9 features) has been removed.

== Availability ==

* {{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]]