Half-Life High Definition Content
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The Half-Life High Definition Content was the content used to increasing texture quality and more polygons for models. It is included with the Half-Life: Blue Shift installation of Half-Life (which allowed for various custom content packs, such as ones made by users). With the release of Blue Shift on Steam, the content was added as an on-demand update (although it was never mentioned).
The High Definition Content originated in the Dreamcast version, and later, the PS2 version. The poly-count of many of the models was increased, though not to the same extent as the current High Definition Content.
The High Definition Pack on Steam contains only the animations required for Half-Life, Blue Shift and Opposing Force, making many mods that use skinned Half-Life models unusable. Some models such as Holographic Assistant does not have their HD versions, unlike Half-Life (PS2).
In 
Half-Life: Source, the High Definition Pack used in this game, which was added in 2013 was later discovered to be originated from GameBanana, instead of reusing the one created by Gearbox. It is currently unknown whether Valve licensed it or stolen this pack and using it without licensing it (through Valve did credit the original author), and it's still remain existed in game files to this day.
Gallery
PC
Comparison between original and HD models of HECU soldiers
PlayStation 2 / Dreamcast
The HD Health Charger from the PS2 version of
Half-Life. Also this is the only version of the game that has the Holographic Assistant (Gina Cross) model in High Definition.
