Detail textures (GoldSrc)
Detail textures in 
GoldSrc are textures that are overlaid on a WAD texture to increase the perceived detail of the surface, without increasing texel size (which would result in using up more AllocBlock). They are extensively used by 
Day of Defeat (since version 1.2) and 
Counter-Strike: Condition Zero.
Unlike Source, detail textures in GoldSrc always use DecalModulate, and are always 24-bit.
Bug:Detail textures are broken in the latest HL25 builds and may not display correctly when scaled. [todo tested in?]
Important:Don't rely upon detail textures for a map to look "correct"!Users whose GPUs have blocked the GL_ARB_multitexture extension (such as for 
enabling overbright lighting on pre-25th anniversary versions of the engine) will not see detail textures.
Additionally, users are able to disable detail textures via the r_detailtextures cvar or the video settings menu.
Note:Detail textures are also unavailable in software mode, but that comes with a bunch of other limitations and is often disregarded.Bug:Detail textures may not work using the native 
Linux binaries. (relevant issue on GitHub)
[todo tested in?]
Bug:Non-power-of-two detail textures do not display correctly when Mipmapping is enabled (broken since late 2019 NPoT update; relevant issue on GitHub). Power-of-two detail textures work fine on non-power-of-two WAD textures.
[todo tested in?]Note:Detail textures can only be used on BSP and WAD textures. To allow more detail to be added to an MDL texture, use a higher resolution canvas, as MDL textures aren't subject to AllocBlock (since they aren't lightmapped). UVs can also be realigned to use more textures, if a max-resolution 512x512 (1024x1024 in 
) canvas is insufficient.
Tip:Use 
Half-Life Asset Manager to enable mipmaps for such high-resolution textures, in order to greatly reduce shimmering on the textures when at a distance.Note:Detail texture scale cannot go lower than 1, unlike Source. To reduce repetition on large surfaces, use megatextures.