What is tunneling in physics?

Tunneling happens when a fast-moving body travels more than its own width during a single physics step. The body is on one side of a wall at tick N and the other side at tick N+1; the broadphase never sees an overlap, so no collision is reported. The default Godot solver uses discrete collision detection which is vulnerable to this; continuous CD sweeps the body’s shape along its motion vector to detect intermediate collisions.

How do I enable continuous collision detection in Godot 4?

On RigidBody2D or RigidBody3D, set the continuous_cd property: CCD_MODE_CAST_RAY (cheap, casts a ray from the body’s center) or CCD_MODE_CAST_SHAPE (more expensive, sweeps the full shape). For CharacterBody nodes, use move_and_slide—it already performs swept motion. For Area2D/Area3D, no equivalent exists; thicken your shape or split the area into multiple overlapping pieces.

Does raising physics_ticks_per_second help?

Yes. Doubling the tick rate halves the per-step distance, so a body must move twice as fast before it can tunnel. The cost is CPU: physics work doubles. 120 Hz is a common compromise for action games; 240 Hz is rare but valid for bullet-hells where projectiles routinely exceed 2000 px/s.

Fix: Godot Physics Body Passes Through Thin Walls

Quick answer: Set continuous_cd to CCD_MODE_CAST_SHAPE on fast rigid bodies, or thicken thin walls to at least speed_per_tick wide. For projectile-style objects, prefer a swept raycast over a physics body.

A bullet leaves the muzzle at 3000 pixels per second. At 60 Hz physics, it moves 50 pixels per tick. Your wall colliders are 4 pixels wide. On most ticks the bullet is in empty space; on the tick where it should hit the wall, it has already crossed to the other side. The collision is never detected and the bullet sails through.

Why Discrete Collision Misses

Godot’s default solver checks for overlap between shapes at the end of each physics step. If shape A and shape B are non-overlapping at tick N and non-overlapping at tick N+1 — even though A’s motion would have intersected B somewhere in between — the solver reports no collision. This is fine for bodies moving slower than their own thickness per step. It fails when speed exceeds the smallest collider dimension along the motion axis.

The math: tunneling occurs when velocity * delta > min(body_thickness, wall_thickness) along the motion vector. Faster bodies, thinner walls, or larger delta — any of those crossing the threshold — produces tunneling.

Fix 1: Continuous Collision Detection

On the moving body, set continuous_cd:

# bullet.gd
extends RigidBody2D

func _ready():
    continuous_cd = CCD_MODE_CAST_SHAPE

Two modes exist:

CCD_MODE_CAST_RAY — casts a single ray from the body’s center along its motion vector. Cheap. Works for bullets where the visual shape is small relative to the cast distance.
CCD_MODE_CAST_SHAPE — sweeps the body’s actual collision shape along the motion vector. More accurate; catches glancing impacts the ray would miss. Pay the cost only for bodies that genuinely tunnel.

CCD has no effect on Area2D/Area3D or StaticBody. If your projectile is an Area for damage events, switch it to a RigidBody with a sensor-like collision setup, or replace the area entirely with a raycast (see Fix 3).

Fix 2: Thicken Walls or Bodies

If you can’t enable CCD — for example, on a CharacterBody using move_and_slide with a thin platform — thicken the geometry. Make wall colliders at least as thick as the maximum distance a body can travel in one tick:

# Compute minimum safe wall thickness
var max_body_speed = 2000.0   # pixels per second
var tick_rate = Engine.get_physics_ticks_per_second()
var min_wall_thickness = max_body_speed / tick_rate
print("Walls must be at least %s px thick" % min_wall_thickness)

At 60 Hz and 2000 px/s, walls need to be 34 pixels thick for guaranteed catches. If your art has 4-pixel walls, add an invisible thicker CollisionShape2D for physics, separate from the visual sprite.

Fix 3: Replace Projectiles with Raycasts

For bullets and other linear projectiles, the most reliable solution is to skip physics entirely. Each frame, cast a ray from the previous position to the new position and resolve the first hit:

# bullet.gd (non-physics version)
extends Node2D

var velocity: Vector2

func _physics_process(delta):
    var prev = global_position
    var next = prev + velocity * delta
    var space = get_world_2d().direct_space_state
    var query = PhysicsRayQueryParameters2D.create(prev, next)
    var hit = space.intersect_ray(query)
    if hit:
        _on_hit(hit.collider, hit.position)
        queue_free()
    else:
        global_position = next

This is mathematically equivalent to perfect CCD for point-shaped projectiles. It also runs faster than RigidBody simulation for the same workload because the broadphase, contact manifold, and integration steps are all skipped.

Fix 4: Raise the Physics Tick Rate

In Project Settings → Physics → Common, raise physics_ticks_per_second from 60 to 120 or 240. Halves or quarters the per-tick travel distance respectively. Use this only after profiling — physics work scales linearly with tick rate.

Verifying

Add a temporary trace in _physics_process that draws a line from previous to current position. Tunneling shows up as a line passing cleanly through a wall. After applying CCD or raycasting, the line stops at the wall surface.

“Fast plus thin equals tunneling. Pick one: slow it down, thicken it up, sweep it, or raycast it. Hoping isn’t on the list.”

CCD costs CPU per body — enable it per-projectile, not project-wide.