HDR
Half-Life 2: Lost CoastHigh Dynamic Range (HDR) rendering simulates brightness above that which a computer monitor is capable of displaying, before downconverting to 
Standard Dynamic Range (SDR, internally referred to in Source and Source 2 as LDR, for Low Dynamic Range) video. This mainly involves "blooming" colors above 100% brightness into neighboring areas and adjusting a virtual camera aperture to compensate for any over-exposure that results.
Besides the obvious effects of this (see right), HDR gives richer colors and finer gradients: since bright and dark areas are pushed into white and black, correctly exposed areas are drawn with a far wider range of values.
HDR rendering is available with Source 2006 and later, and requires a GPU that supports DirectX 9 or newer for Windows (or OpenGL 1.4 and later for macOS/Linux). However, some maps in certain games such as 
Counter-Strike: Source, 
Half-Life 2 (both at the time were running Source 2006) weren't fully compiled with HDR support until the engine update in 2010, upgrading it to 
Source 2009 along with changes to the maps and skybox textures to support HDR.)
Since 
Left 4 Dead engine branch and later games, HDR rendering is now required and LDR/SDR rendering has been deprecated, with the option to disable HDR removed outside of console commands.
Note:Third-party games like 
Team Fortress 2 Classic also require HDRHDR in Source
Source does not have a physically accurate HDR simulation. Its camera has a far wider range than the human eye (let alone real cameras), and it also adjusts to changes in brightness far faster. Both of these are expedient to gameplay of course, especially in multiplayer.
In fact, by default, Source does not even create "proper" HDR images. Instead, it collapses the image down to LDR early, like many other games in the 2000s/early 2010s, this is hardware limitations at the time and proper HDR display output are only introduced later in mid-2010s with supported games, graphics card, displays, operating systems (Windows 10 and later), and newer HDMI/DisplayPort versions. The average user would be hard-pressed to tell the difference, however, and the benefits include support for all DX9 GPUs, MSAA compatibility, and excellent performance.
Todo: How does 
improve HDR?
