Surf
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Surfing is a exploit of a quirk in 
QuakeWorld's hull collision detection, inherited by 
GoldSrc and 
Source, where the player can slide on sloped surfaces, allowing them to gain speed and fling themselves given the right circumstances.
How it works
To surf, the player gets onto a slope and strafes into it, which will make them start sliding forward. During this, the player can move their camera up or down to control how fast or slow they gain speed, and depending on the size of the slope, may be able to fling themselves off by moving the mouse correctly.
Only certain slopes are viable to surf off of, as it needs to not be too steep or shallow to really work. Most typical maps not don't include many places that work, if at all, and if there is it usually isn't intended.
Rotated instances will not be correctly surfable, due to not creating axis-aligned brushside "bevels" for angled QPhysics geometry.
Gamemode
Some servers, particularly in 
Counter-Strike: Source and 
Garry's Mod, will run a gamemode called, Surf (Garry's Mod even having a section in it's server browser for it.)
Note: While Surfing exists in 
Source 2 (with servers in 
Counter-Strike 2), it is currently broken and the player will frequently get stuck on seemingly random parts of a slope. It's not recommended to play on these for nowThese servers host maps designed as courses to play through, most consisting of a series of floating triangular sections with some sort of theme. The course will usually get progressively harder as the player goes through it, with placement, distance and other obstacles placed to make a section harder or easier.
Pixelsurf
A pixelsurf is a unintended bug mostly used in 
Counter-Strike: Global Offensive. It's defined mostly by the fact that the area to do it is very small, can be done in any map where two separate sections of a wall meet, along with some other areas (one of the easiest ones to do being against the fence in CT Spawn of de_nuke, although it doesn't really accomplish much.)
These can be very hard to find and perform, as there's very little room to work with and can be easy to miss or fall off of.
See also
External links
- 3kliksphilip video about pixelsurfs
- The Absolutely Bizarre World of Slopes in Quake by Quake Speedruns Explained on Youtube
