The Episode One engine branch was released in 2006 with Half-Life 2: Episode One (also known as Source 2006 engine). It was obsoleted by the Orange Box branch. It requires the original HL2 'branch' of Source to operate.
New since Half-Life 2:
- High Dynamic Range rendering
- A simulation of brightness values outside computer monitors' actual range, and of aperture adjustment.
- Phong shading
- Diffuse reflections for skin and other roughened surfaces.
- Facial animation upgrades
- More detail and intenser shapes for close-up performances.
- DirectX 9 Shader Model 3
- Unlocks greatly improved shader performance.
- Detail prop shapes
- Improved detail sprites that sway in the breeze and bend away from nearby players.
Gamecode for this branch is included in the Source SDK.