Alien Swarm engine branch
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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]
Features
New since Left 4 Dead engine branch is:
- 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.
Availability
Gamecode for this branch is included in the Alien Swarm SDK .
