Demystifying Collision Jitters: Step-by-Step Fixes
The Frustration of Jitters
I remember my first real encounter with collision jitters. My character, a nimble rogue, would occasionally vibrate uncontrollably when pressing against a wall. It felt like a subtle earthquake, undermining all the smooth movement I’d painstakingly crafted. I tweaked, I cursed, I even considered a whole new physics system. The frustration was real.
The Subtle Culprits
Sometimes, the simplest design choices introduce the most baffling jitters. For instance, scale inconsistencies within your game world can throw physics engines into a tizzy. If your character is 1 unit tall but your world is built for objects 100 units tall, floating-point inaccuracies become glaring. Similarly, rapid, successive input changes, like spamming a movement key, can create micro-adjustments that accumulate into visible judder.
Common Code-Related Mistakes
Beyond design, many jitters stem from common coding errors.
FixedUpdate vs. Update
A frequent culprit is performing physics calculations outside of FixedUpdate. The Update loop runs at varying frame rates, while FixedUpdate operates at a consistent, fixed timestep. Physics operations, like applying forces or moving rigidbodies, must occur in FixedUpdate for stable and predictable behavior.
Interpolation and Extrapolation
For visual smoothness, understanding interpolation and extrapolation is key. These settings in engines like Unity tell the renderer how to visually position objects between physics updates. Interpolation smooths movement by displaying objects where they were in the previous frame, while extrapolation tries to predict future positions. Incorrect settings here can lead to visible stuttering, even if physics calculations are perfectly stable.
Tiny Colliders and Precision Issues
Working with very small colliders or objects can introduce precision problems. Floating-point numbers, used for nearly all game calculations, have limitations. At extremely small scales, these inaccuracies can lead to objects interpenetrating or behaving erratically. Consider scaling up your game world or using different units if you encounter this.
High-Speed Collisions
Fast-moving objects can “tunnel” through colliders, especially thin ones. This happens because in a single physics step, the object might move entirely past the collider without registering a collision. Continuous Collision Detection (CCD) is a setting in many engines (like Unity’s Rigidbody component) designed to prevent this by performing additional checks for high-speed objects.
The Debugging Toolkit
To pinpoint jitter sources, you need a robust debugging toolkit. Visualize your colliders in the editor. Slow down time in your game to observe physics interactions frame by frame. Log relevant variables like velocity, position, and collision states. Many engines also offer physics debug views that show collision points and forces.
Step-by-Step Fixes (Narrative Integration)
Let’s revisit my rogue’s wall-vibration issue. My initial journal entry looked something like this:
- Observation: Rogue vibrates against walls. Only happens when holding movement key.
- Hypothesis 1: Input is sending too many tiny movements.
- Hypothesis 2: Physics update order is wrong.
My first fix attempt was to clamp input, but the jitter persisted. I then focused on the physics update order.
Fix 1: Incorrect Update Order
- Problem: Physics calculations being performed in
Update. - Solution: Move all rigidbody manipulations (e.g.,
Rigidbody.MovePosition,Rigidbody.AddForce) fromUpdatetoFixedUpdate.
In Unity, this means ensuring your movement script’s physics logic resides within FixedUpdate(). For Godot, _physics_process(delta) is the equivalent.
- Journal Entry (after Fix 1): Moved all movement code to
FixedUpdate. Jitter reduced, but still present. Less severe. Focus on other physics settings.
My next journal entry, after more observation:
- Observation: Jitter still there, especially when camera is moving.
- Hypothesis 1: Visual interpolation issue.
- Hypothesis 2: Collider shape is too complex.
Fix 2: Interpolation Issues
Problem: Visual desynchronization between rendering and physics updates.
Solution: In Unity, set your
Rigidbody's Interpolate mode toInterpolate. For Godot, ensure yourCamera2DorCamera3Dfollows a_physics_processupdated object, or manually interpolate positions in_process.Journal Entry (after Fix 2): Set Rigidbody interpolation to Interpolate. Jitter nearly gone! Visuals are much smoother. Minor vibration still occurs if I really mash against a corner.
My final breakthrough came with the next entry:
- Observation: Tiny jitter only in corners, when multiple forces are active.
- Hypothesis: Collider precision issue with multiple contacts.
- Solution: Slightly increase the player collider’s size and roundness. Also, check physics material for excessive friction or bounce.
Fix 3: Tiny Colliders and Precision Issues
Problem: Floating-point inaccuracies at small scales, especially with complex shapes or multiple simultaneous contacts.
Solution: Consider slightly increasing the size of problematic colliders. Ensure the collider shape is as simple as possible (e.g., Capsule instead of Mesh Collider for characters). Adjust physics material properties like friction and bounciness.
Journal Entry (after Fix 3): Slightly increased capsule collider radius. Jitter is completely gone. Lesson learned: even subtle design choices, like a perfectly flush collider, can cause issues. Documenting each step really helped me connect the dots.
As you tackle these challenging bugs, documenting your process can be your greatest asset. Keeping a detailed log of your observations, hypotheses, and solutions, much like the developer in our story, dramatically accelerates your understanding and problem-solving. This kind of systematic approach, often overlooked by indie and beginner devs, is crucial for tracking game development progress effectively. Start improving your debugging workflow and keep a detailed game development log today with our dedicated journaling tool: Organize Your Game Dev Journey.
Conclusion: Beyond Jitters
Demystifying collision jitters requires patience and a methodical approach. By understanding the common culprits, from subtle design choices to core coding errors, and by embracing a systematic debugging process, you can achieve smooth, stable physics in your games. Remember, consistent journaling is your secret weapon for turning frustration into breakthroughs and truly mastering your game development journey.