Source 2006

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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. Most Valve games introduced with Source 2006 and previous titles that are upgraded from Source 2004 to Source 2006 are now currently using Source 2013 Singleplayer or Source 2013 Multiplayer.