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.

Understanding the issue

The challenge with physics-related bugs is reproducibility. A symptom you see in a 30 fps build may vanish at 60 fps because the integrator's step size changed. Reproducing reliably means controlling both your inputs and the engine's tick rate.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

The triage path for this kind of bug is long. The symptom appears in gameplay, but the cause is in a different system. The reporter describes the gameplay effect; the engineer has to translate that into a hypothesis about the underlying cause. Misdirection is common.

At the engine level, the behavior comes from a deliberate design decision in Godot. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

Diagnosing this class of bug benefits from a structured approach: confirm the symptom, isolate the variables, hypothesize the cause, and verify the hypothesis before writing fix code. Skipping the isolation step is the most common mistake; without it, fixes often address symptoms while the underlying cause continues to produce other variations.

For Godot-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Third-party plugins often provide better diagnostics for their own behavior than the engine does. If the affected code is in a plugin, check the plugin's documentation for debug modes, verbose logging, or inspector tools - these can save hours of investigation when they exist.

Within Godot, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Platform-specific edge cases are worth enumerating explicitly. iOS handles backgrounding differently than Android; Windows handles focus changes differently than macOS. A fix that works on the development platform may not work on every target. Test on each shipping platform deliberately.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

Document the fix and its rationale in the commit message or attached engineering doc. Future engineers will encounter related issues; the rationale tells them whether your fix is reusable or specific to the case at hand. Without rationale, the fix gets reverted or copied incorrectly.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“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.