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fitts-law-pointer-acceleration

Use this skill when the question involves cursor speed, pointer acceleration curves, or interaction with input devices that vary in precision (mouse, trackpad, stylus, touchscreen, eye-tracking). Trigger when designing for varied input devices, building drag interactions, designing scrubbers / sliders / handles, or analyzing why a UI feels "fiddly." Sub-aspect of `fitts-law`; read that first if you haven't already.

plugins/HDeibler/universal-design-principles/plugins/interaction-and-control-principles/skills/fitts-law-pointer-acceleration/SKILL.md

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# Fitts's Law and pointer acceleration

The mouse and trackpad are not "raw" inputs — operating systems apply *acceleration curves* that translate physical movement into cursor displacement. Fast physical motion produces disproportionately fast cursor motion; slow physical motion produces precise, slow cursor motion. This curve interacts with Fitts's Law in ways that affect how target size and distance are perceived.

## What pointer acceleration does

A non-accelerated cursor maps physical motion to screen motion 1:1. To cross 1000 pixels, the user must move the mouse 1000 mouse-equivalent units. Fast or slow physical motion makes no difference to the resulting distance.

An accelerated cursor varies the multiplier based on physical speed:

- **Slow physical motion → low multiplier** (e.g., 0.5×). Used for precise positioning over small targets.
- **Fast physical motion → high multiplier** (e.g., 3–8×). Used for crossing large screen distances quickly.

The acceleration curve is what lets a single mouse on a 4K display traverse the full screen without lifting and re-anchoring (a problem 1990s mice without acceleration had).