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.

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] Unlike previous and later branches, this is the only Source engine branch (post-Orange Box) that has it's source code publicly available.

Features

TileGen: A new quick-build mapping tool. Level designers 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 the ability to use 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.
Steamworks integration: Unclear how useful this will be, as currently Steamworks distribution is required to implement persistence, Steam Cloud, achievements, etc.