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How the movement works

This page explains why SUCC behaves the way it does. You don't need it to use the addon, but it helps when deciding which values to change.

Why holding jump makes you faster

In most games, your speed is clamped: the game works out how fast you should be going and won't let you exceed it. Quake didn't do that.

Quake caps how much speed it can add each frame, not how fast you can be. On the ground that distinction doesn't matter, because friction removes speed as fast as acceleration adds it. The moment you leave the ground, friction stops, and the cap behaves differently.

Air acceleration works like this:

  1. Take the direction you're asking for.
  2. Measure how much of your current speed already points that way.
  3. If that's less than the cap, add the difference. Otherwise add nothing.

The catch is step 2. It measures speed along the direction you asked for. Turn the mouse slightly and hold a strafe key, and the direction you're asking for is nearly sideways to where you're travelling, so your speed along it is almost zero, and the game gives you the full allowance again. Repeat every frame and you accumulate speed indefinitely.

That is bunnyhopping and strafe-jumping. It falls out of these two config values:

  • air_acceleration, how hard the game pushes
  • max_air_speed, the per-frame allowance, 0.762 m (30 units) in every Valve and id engine

The allowance being small is the point. It's small enough that you can't accelerate forward in mid-air, but non-zero, so the sideways trick works. Raising it breaks bunnyhopping by making forward air acceleration too easy.

Why surfing works

A slope steeper than the character can stand on leaves them permanently airborne. They slide, gravity keeps pulling, and because they're airborne, air acceleration is running.

So a surf ramp is bunnyhopping against a wall. You hold a strafe key into the slope and the same accumulation happens, except the ramp keeps you from falling. All SUCC needs for this is a surface steeper than max_floor_angle (50° by default). The demo's ramps are 55°.

enable_surf projects air acceleration along steep ramp faces. Surf contact remains airborne, reports FALLING, and ignores jump input until the player reaches a walkable floor.

Why stairs need special handling

Godot's move_and_slide() treats a 20 cm step like a wall, so you stop against it. Every Quake-lineage engine solves this the same way, and SUCC copies it.

Rather than detecting steps and teleporting the body upward, the engines do the move twice:

  1. Move normally. Remember where that got you.
  2. Go back to the start, lift up by one step height, move again, then drop back down.
  3. Keep whichever attempt covered more ground.

This is Source's StepMove and Quake's SV_WalkMove. The second attempt must be a real move rather than a position fix, because the move itself produces the velocity. Teleporting the body and re-applying speed by hand turns every ramp into a catapult, since the synthesised speed never decays.

The step attempt only runs when the normal move was blocked. Running it every frame lifts and drops the character constantly, which stalls ordinary walking.

step_height decides the tallest ledge you can walk up. All four engines use 18 units (0.457 m).

Why the camera lags behind on stairs

When you climb a step, the body jumps upward instantly. If the camera followed exactly, every step would be a visible jolt.

So the camera doesn't follow. SUCC records the height the body gained and applies it as a downward offset to the camera, cancelling the jump, so your view stays level while your feet climb. The offset then closes at a constant 3.81 m/s (Source's 150 units/s).

The rate is constant because a percentage-based approach is framerate-dependent and jitters when you climb several steps in quick succession.

Why crouching moves the body differently in the air

On the ground, crouching keeps your feet planted and lowers your head.

In the air, Source does the opposite: it raises the origin by the full height difference, so your head stays where it was and your feet tuck up underneath you. That's why crouch-jumping reaches ledges an ordinary jump can't: the jump gains you height, and the duck gains you the hull difference on top.

SUCC copies this, including the origin raise and the camera drop cancelling out, so the view doesn't move during an air duck. Landing absorbs the raise, and standing up afterwards checks for room below rather than overhead, because the legs are extending downward.

Surf ramps count as air for this purpose, so boarding a ramp while ducking tucks the same way.

Why friction uses only horizontal speed

Friction slows you along the ground. If it measured your full 3D speed, falling fast would make ground friction stronger, so landing at speed would brake harder than landing gently, for no reason a player could see.

Every engine, and SUCC, measures horizontal speed only. Measuring 3D speed makes deceleration inconsistent after a fall.

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