Source 2006

The Source 2006 engine branch, also known as the Episode One engine branch, was released in 2006 with Half-Life 2: Episode One. It was obsoleted by the Source 2007 branch. It requires Half-Life 2 content to operate.

The SDK and SDK Base game for this branch are free to all Steam users.

Features

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.
Dynamic interactions: Dynamic interactions between NPCs using special animations.
Lightwarp textures: 1D textures that tint a material per-texel depending on their brightness.
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 the 2006 branch is included in the now-deprecated Source SDK app.