There are 3 "main" versions of HL2:
- Steampipe, the one of steam, with clipbrushes so it's slow
- New Engine, with ABH, without clipbrushes
- Old Engine, with BHOP
There are a far larger set of differences than that.
First and foremost: Old engine -The primary method of movement in Old Engine is bunny hopping, holding w and jumping repeatedly. I believe speed gain per hop is linear, and can be quite slow to reach the movement cap. -Propclimbing/propflying is another feature in Old Engine, Since Valve didn't really program what happens when a prop is underneath you, you can hold a prop underneath you and repeatedly jump on it to gain speed. You don't let go of it. -In Old Engine the command 'wait' works differently to how it does in New Engine. You can perform a trick called a Change Level Delay, where you can move near freely through a level change and appear there on the next map. -In Scripted Old Engine you can use a script called savewarping which is loading a save during a trigger delay. This allows you to load the next map in a position of your choice, Savewarping also allows for wrongwarping which is a bit more complicated. -When you enter a vehicle regardless of engine, you enter noclip, and when you exit a vehicle in a standard manner, the game removes noclip. If you save warp out of a vehicle you get given noclip glitch, which allows you to fly freely in noclip through the level. -When exiting noclip via barnacle you get a new glitch which I believe is called 'QuickClip' Which allows you to take a prop and travel through walls, pretty cool. -Jclip is where a user save and loads while crouched (in old engine). When the game loads for a short timespan you can pull an object into your top half (uncrouched). When the game realises that you are stuck in the object, you can save and load in order to clip. -CLIP DIRECTION IN OLD ENGINE IS UPWARDS. -Simply saving and loading while stuck allows you to travel through brushes (areas that are solid). -Falling for a short period of time before save loading allows you to travel through the void (areas with free movement.) -Pausing and save loading both reset your velocity to 0 while in the void. -Save deletion (Creating a Save, loading it, deleting it and reloading it) works in Old Engine.
New Engine -Has ABH/ASH/AFH, (Accelerated Backwards/Sideways/Forward Hopping) which is exponential movement, reaching the speedcap (3500 on each axis) is far easier to get. -Propclimbing is limited, You must jump and regrab the prop each time you want to climb higher. -Propflying is possible though is far harder. -Propclipping by pushing the player against an object which is against a wall or brush will eventually cause the prop to clip into the player, allowing them to clip. -CLIP DIRECTION IN NEW ENGINE IS DOWNWARDS -Maintaining velocity through a brush is easy through repeated save loading. -Maintaining velocity in the void requires saving and loading with precise timing, otherwise you lose all your speed. -Save Deletion also works in New Engine.
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