The Orange Box engine branch was released in 2007 with the Orange Box (Ep2, Portal, TF2). It is currently being maintained side-by-side with the Left 4 Dead branch; the two branches share technology, but it is unclear if they will ever be merged or if one will come to be favoured.
The OB builds on the original HL2 branch, requiring its content installed to work.
There are actually two Orange Box branches, referred to in the SDK Launcher as Source '2007' and '2009'. The 2009 branch does not have a different feature set, but is 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
- Source code is coming, with support for Mac builds as well as Windows.
- Source SDK Base 2007 (formerly Source SDK Base - Orange Box; Gamecode for the 2007 branch is included in the Source SDK.)