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Goldsource (also known as GoldSrc), is a 3-D video game engine created by Valve in 1996, which was the driving force behind many famous games of the late 90s such as Half-Life, Team Fortress Classic, and Counter-Strike. GoldSource is a heavily modified version of the Quake engine, and runs on C++ programming code. GoldSource and its level editor, the Hammer World Editor, were released by Valve for public use, making it the source of countless community-made modifications. GoldSource was replaced by its sequel engine, Source, in 2004, which currently holds 1st place as the choice for modders. Though GoldSource is past its prime, many gamers still seek to use it for mods and level design.
For information about converting GoldSrc content to the Source engine, see Porting Goldsource content to Source.
AI flocking - NPCs can group together, seen with Houndeyes
Skeletal animation - allowing for easier animating of models
Colored lighting - Quake simply had monotone lighting
Scripted sequences - Used extensively to tell Half-Life's story
Transparent textures - Glass can now be used in maps
Higher poly counts - GoldSrc allows for far more detailed models than Quake
The GoldSrc SDK is known as the Half-Life SDK. As of August 2013, it is available on Steam under the "Tools" section. With it comes Hammer 3.4, the 3ds Max sources for many cut and retail NPCs as well as player and weapon models, tools for packing textures into WADs, and detailed documentation on implementing baseline features such as voice chat into a mod.
The full source code for the SDK can be found on Valve's Github Page.