What this line jitter test measures
The browser reports a sequence of pen coordinates through Pointer Events. GearCheck Lab calculates an orthogonal best-fit line through the captured stroke and measures each sample's perpendicular distance from that fitted trajectory.
RMS deviation summarizes the typical sideways departure in CSS pixels. Maximum deviation shows the largest observed departure. Normalized RMS divides RMS deviation by the fitted stroke span so the result is less dependent on how long the line was.
Why the guide is not a pass/fail ruler
The diagonal guide gives you a repeatable movement target, but your hand will never land on it perfectly. Scoring absolute distance from the guide would mix aiming error, hand position and digitizer behavior.
The fitted-line method deliberately removes small angle and offset mistakes. It asks: how much did this captured stroke wobble around its own overall straight direction?
Why slow diagonal strokes are useful
Slow diagonal lines are commonly used when reviewing pen behavior because periodic wobble or coordinate quantization can be easier to see when movement is deliberate and software smoothing has less visual help from speed. Run a slow stroke first, then a normal-speed stroke for context.
Do not force the nib or grip the pen unusually tightly just to chase a smaller number; that changes the human part of the measurement.
Path excess and sample cadence
Path excess compares the total captured path length with the straight span of the fitted trajectory. A perfectly straight sampled path would be close to zero excess; wobble, hand tremor and sampling detail can increase it.
The browser sample-rate figure uses the timestamps of Pointer Event samples, including coalesced samples where the browser exposes them. It is not the tablet's certified hardware report rate.
How to compare results correctly
- Use the same browser, zoom level, display orientation and pen.
- Run at least three slow strokes rather than judging one attempt.
- Compare the median RMS and the shape of the visible traces.
- Repeat at normal speed to see whether the symptom is mainly a slow-stroke issue.
- If one area of the display seems worse, repeat the same motion in another screen region or a native drawing app.
Important limitations
- Human hand movement is part of every freehand stroke.
- CSS-pixel measurements depend on browser zoom, device pixel ratio and page geometry; compare like with like.
- Drivers, operating-system pen stacks and browsers can filter, quantize or smooth coordinates before this page receives them.
- The browser cannot identify the exact digitizer component responsible for a repeated wobble pattern.
- No application-level line smoothing is applied by GearCheck Lab, but upstream software processing may still exist.
- A single high deviation does not prove hardware failure.
Stylus line jitter FAQ
What result is a good stylus jitter score?
GearCheck Lab intentionally does not publish a universal pass/fail threshold. Device size, browser scaling, hand motion, driver filtering and pen technology differ too much for one CSS-pixel cutoff to be honest. Repeated comparisons under the same setup are more useful.
Why does a ruler-straight physical movement still show deviation?
The input path can include digitizer noise, coordinate quantization, driver processing, browser sampling and small mechanical movement between the nib and surface. A repeated pattern is more informative than one isolated spike.
Can I use a mouse or finger?
The page identifies those pointer types but does not save them as valid stylus runs. This tool owns active-pen line-straightness intent; mouse or finger traces would answer a different question.
Does this replace the pressure test?
No. Line jitter measures spatial trajectory consistency. The Stylus Pressure & Tilt Test measures pressure, tilt, twist, buttons and browser sample telemetry. Use both when diagnosing a pen comprehensively.
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