home / skills / dylantarre / animation-principles / arc-mastery
This skill helps you design natural motion paths by applying arc-based principles to character movement, UI motion, and motion graphics.
npx playbooks add skill dylantarre/animation-principles --skill arc-masteryReview the files below or copy the command above to add this skill to your agents.
---
name: arc-mastery
description: Use when designing motion paths, character movement trajectories, gesture animations, or any motion that should feel natural rather than robotic.
---
# Arc Mastery
## The Natural Path Principle
Almost all natural movement follows curved paths. Arms swing in arcs. Heads turn on arcs. Thrown objects travel parabolic arcs. When animation moves in straight lines, it immediately feels mechanical and artificial. Arcs are the signature of organic motion.
## Core Theory
**Anatomical basis**: Bodies are systems of hinges and pivots. When a joint rotates, everything attached to it moves in an arc centered on that joint. Straight lines require mechanical rails—biology doesn't have those.
**Physics basis**: Gravity creates parabolic curves on projectiles. Momentum creates curved paths when direction changes. Straight-line direction changes require infinite force.
## Arc Types
**Circular arcs**: Pure rotation around a single pivot (pendulum, door swing)
**Parabolic arcs**: Projectile motion under gravity (jumps, throws)
**S-curves**: Complex motion through multiple pivots (whip crack, spine movement)
**Figure-8 patterns**: Continuous flowing motion (hips during walk, flowing cloth)
## The Pivot Chain
Understanding arcs requires understanding pivot hierarchies:
- Shoulder rotates → elbow travels an arc
- Elbow rotates → wrist travels a compound arc
- Wrist rotates → fingertips travel triple-compound arcs
Each joint adds complexity to the motion path of everything downstream.
## Interaction with Other Principles
**Follow through follows arcs**: Appendages settle along curved paths, not straight returns.
**Anticipation creates arc initiation**: The wind-up often establishes the arc's direction.
**Timing affects arc perception**: Fast motion through arcs needs exaggerated curves to read clearly.
**Overlapping action creates offset arcs**: Secondary elements trace their own arc patterns behind the primary.
## Domain Applications
### UI/Motion Design
- **Page transitions**: Elements should enter/exit on curved paths, not straight slides
- **Menu expansions**: Items fan out on arcs from origin point
- **Gesture responses**: Swipe follow-through curves naturally
- **Notification entries**: Slight arc on slide-in prevents robotic feel
- **Dragging**: Release trajectories should curve based on velocity vector
### Character Animation
- **Walk cycles**: Every limb segment traces arcs; hips follow figure-8
- **Head turns**: Chin traces arc, doesn't translate linearly
- **Arm gestures**: Hand paths curve even during "straight" pointing
- **Jumping**: Parabolic body path with internal rotation arcs
### Motion Graphics
- **Logo reveals**: Elements following arc paths feel choreographed, not random
- **Text animation**: Letter paths curving adds elegance
- **Infographic builds**: Data points connecting along arcs creates flow
### Game Design
- **Projectile physics**: True parabolic arcs for believable ballistics
- **Character controllers**: Turn animations following arcs prevent pivot-in-place
- **Camera movement**: Arc-based camera paths feel cinematic
## Common Mistakes
1. **Straight-line interpolation**: Default animation software behavior—must be corrected manually
2. **Inconsistent arc centers**: Motion arcs should have logical pivot points
3. **Symmetric arcs**: Natural arcs are often asymmetric (fast one way, slow return)
4. **Flat arcs on fast motion**: Quick movements need exaggerated arcs to register
## The 3D Arc Challenge
In 3D, arcs must read from camera view. A perfect arc in world space may look straight or even reversed from certain angles. Always verify arc readability from final camera.
## Arc Drawing Technique
Traditional method: Draw through the motion path with a single stroke. If your pencil naturally curves, the arc is working. If you must consciously redirect, the motion is likely too linear.
## Implementation Heuristic
For any point-to-point motion: add a control point to create a curve. The control point should be offset perpendicular to the direct line, placed closer to the start for ease-out feeling, closer to end for ease-in. For complex motions, trace the intended path first—if it's not curved, it needs work.
Default arc height: 10-20% of travel distance for subtle organic feel, 30-50% for dramatic sweeping motion.
This skill helps designers and animators create natural, organic motion by applying arc-based movement principles. It codifies how joints, gravity, and momentum produce curved paths and gives actionable heuristics for drawing and implementing arcs across UI, character, and game motion. Use it to replace robotic straight-line movement with readable, expressive curves.
The skill inspects motion intents and suggests arc types (circular, parabolic, S-curve, figure‑8) based on anatomy and physics. It recommends pivot centers, control point offsets, default arc heights, and timing adjustments. It also checks readability from camera/view and flags straight-line interpolations or inconsistent pivot hierarchies.
How tall should an arc be relative to travel distance?
Use 10–20% for subtlety and 30–50% for bold, sweeping motion; adjust by eye and timing context.
What if an arc reads straight from the camera?
Reproject the control points toward the camera plane or exaggerate curvature until the arc is visible from the final view.
How do I balance physics and stylization?
Start with anatomically and physically motivated arcs, then push curvature and timing to match the style while keeping readable pivot logic.