0ms pushable boosting under water to skip past entities
[OUTDATED. See    • Skipping walls swimming in Half-Life   ]

I just recently found out that the game has optimization mechanics that caps the collision tracing of entities to 256 units in each direction.
Meaning that just like skipping triggers, with high enough speed to get past the trigger area in one frame so the trigger never was touched, you could also skip entities (all non-worldspawn brush- and point-entities like func_door, func_wall, enemies...)
Apparently this was known years ago and brought up by speedrunner Proto. However no successful tricks was made.

With some math the problem is apparent. The player width is 32 units and the lowest physics rate is 20 fps or 50 ms.
To skip a 1 unit trigger in 50 ms, only 660 ups is required.
To skip a 256+1 unit entity in 50 ms, 5780 ups is required.

Since the game instantly caps to 2000 ups with sv_maxvelocity 2000, even the thinnest entities seems impossible(?) to skip.
In water however, the maxvelocity cap isn't applied for some reason. And pushables used with 0ms frames can get you up to millions of ups.
So it's actually possible to skip entities provided the rare circumstances of both water and pushables exist near the entity you want to skip.

At Pit Worm's Nest, the water level is below the desired door. So during the 1 ms frame of 700000 ups I collide with a slanted box to redirect the velocity up towards and lastly behind the door. All this in 1 ms frame while the game still thinks I'm crouched in water.
Travelling at 700 000–1 500 000 ups in 1 ms is visually a teleport making it hard to follow. It took 2 hours of fine tuning to get the position and angles right.


Because of the extreme precision and amount of FPS required, it's pretty much a TAS only strategy possible only on the Steam versions.



hltas scripts: https://pastebin.com/qWtuTV4B

Half-Life TASing Introduction v2:    • Half-Life TASing Introduction v2  
HL TAS tools: https://github.com/YaLTeR/BunnymodXT/...
Half-Life Physics Reference: https://www.jwchong.com/hl/