Alien Swarm engine branch

In-game screenshot of Alien Swarm, running on Alien Swarm engine branch. The screenshot demonstrates the new "depth of field" effect.

Developer(s)

Valve Corporation

Release date(s)

July 19, 2010

Latest release(s)

September 24, 2020^[Note]

Status

Discontinued / occasionally maintained^[Note]

Platform(s)

Windows

SDK

Alien Swarm - SDK

Predecessor

Left 4 Dead engine branch

Successor

Portal 2 engine branch

The TileGen mapping tool in Alien Swarm.

The Alien Swarm engine branch was released in 2010 with Alien Swarm. It succeeds the Left 4 Dead engine branch and precedes the Portal 2 engine branch.[1] This branch is the only Source engine one (post-Orange Box) that has officially available source code, with all the new features introduced in Left 4 Dead engine branch (BSP 21, swaying trees, VScript, water flow maps), as well as VTF 7.5 and other mentioned below which are later used in Portal 2 and Counter-Strike: Global Offensive. Some of these features however, have later been backported to the Orange Box-derived branch, under the Team Fortress 2 branch.

As of 2026, this branch has not received any updates since September 2020.

Features

TileGen: A new quick-build mapping tool. Level designers can create connectible rooms and corridors which can be arranged either by hand or programmatically. TileGen maps can be played in-editor, and/or can be exported to Hammer for further refinement.illuminate an entire scene.
VTF version 7.5 and PCF version Binary 5 PCF 2: The latest versions of Source's texture and particle formats.
Depth of field: A real-time depth of field shader blurs backdrop scenery.
Vertex animation: The ability to animate vertexes directly is now supported.
Various entity improvements: Numerous entities have been improved and have new features, such as env_fire and env_projectedtexture.
Water debris flow: Water now supports using a base texture layer that is flowed using a flow map. This implemention is less advanced than Portal 2's, however.
Lightmap influence on cubemapped materials: Lightmaps can now tint cubemap reflections in the LightmappedGeneric shader.

See also: Category:Alien Swarm engine branch games

Gamecode for this branch is included in the Alien Swarm - SDK.

Last engine update was in August 19, 2020 (engine last compiled in July 29, 2020). The September update simply removes bin/swarm_rel_latest_good.txt file, but everything else is unchanged.
With the branch itself hasn't been updated since 2020 (5+ years), and unlike other Source branches which received feature updates (DXVK/Vulkan API, Steam Deck compatibility, etc.) plus having a native Linux port, the branch is considered in maintenance mode with rarely released security updates (or possibly discontinued).