From Valve Developer Community
The Orange Box engine branch was released in 2007 with the Orange Box (Episode Two, Portal, Team Fortress 2). It existed side-by-side with the Left 4 Dead branch until 2011, when it was obsoleted by the release of Portal 2.
The Orange Box builds on the original Half-Life 2 branch, requiring its content installed to work.
There are actually four Orange Box branches, referred to in the SDK Launcher as Source '2006', '2007', '2009' (2013 Singleplayer) and 'MP' (2013 Multiplayer). The 2009 and MP branches do not have a different feature set, but are incompatible with 2007 due to various under-the-bonnet changes (which include a different SteamID scheme and Mac OS X compatibility).
New since the Episode One branch is:
- Logic, rendering and audio have been threaded for performance gains in multi-core environments.
- Soft particles
- An all-new, artist-driven particle system.
- Shadow mapping
- A modern dynamic lighting and shadowing technology.
- Alpha to coverage
- Vector-like edges to raster images, fast.
- Self-shadowing bump maps
- For a more realistic illusion of a 3D surface.
- Automatic displacement texturing
- Avoids ugly stretching.
- Maplist Thumbnails
- A console-oriented server browser UI.
- Xbox 360 support
- Of little use to modders, needless to say!
- Mac OS X support
- GNU/Linux support
- (since 14 February 2013)
- SteamPipe support
- Source SDK Base 2007 (formerly Source SDK Base - Orange Box)
Gamecode for the 2007 branch is included in the Source SDK.
Source 2013 Singleplayer
Formerly Source 2009
Gamecode for the 2013 singleplayer branch is included in the Source SDK 2013.
Source 2013 Multiplayer
Formerly Source MP
Gamecode for the 2013 multiplayer branch is included in the Source SDK 2013.