02 · Grip Size Measurement

Feature 1 of 4. Two entry points lead to the same three-system size output (Yonex L0–L5 / Wilson in fractions / Head–Prince–Babolat equivalent):

• 🤖 AI measurement — snap a palm photo, on-device MediaPipe Hand computes palm length + width + finger count
• 📏 Manual measurement — measure with a ruler, type the number in, goes through the exact same size formula

Both paths share the same sizing formula, so use whichever is more accurate; cross-validate with both if you like.

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Two paths at a glance

	🤖 AI measurement (photo)	📏 Manual measurement (ruler)
Entry	Home → "AI Grip Size" → Start	Home → same card → "Manual Tier Picker"
Requires	Rear camera + on-device ML model	Any ruler
Time	30 sec (shoot + adjust + result)	10 sec (measure + type)
Accuracy	RMSE ±4.2 mm ≈ ±⅓ tier	Depends on ruler precision
When to use	No ruler / quick first read / want palm + wrist texture	No camera / camera denied / AI keeps failing / want precise control over start point

Covered in detail below. AI is the default for most users; manual is the fallback + a great ground-truth check for AI.

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🤖 AI measurement (photo)

What it does

Hold the phone up to your palm. The App estimates the following on-device by default (no upload):

• Palm length (mm)
• Palm width (mm)
• Finger count + ratios (calibration across hand shapes)
• → Recommended grip size in three systems simultaneously

How to shoot accurately

Key 1: place on a flat desk, lay an A4 sheet, palm-up flat with fingers spread

<img src="/images/grip_measurement_1.jpg" alt="Before the shot: palm flat on A4 paper, green frame in place — "Place an A4 paper flat on the desk"" style="max-width: 320px; border-radius: 12px; margin: 12px 0;" />

This is the live preview before shooting: the screen when the phone is directly overhead — preview + pre-computed green frame at 30 cm working distance + bottom hint "Place an A4 paper flat on the desk". Get the A4 fully inside the green frame, palm up with fingers spread, then tap the shutter.

• Desk must be flat — a hard flat surface with a white A4 sheet. Don't hold the palm in mid-air, don't rest on a soft surface.
• Palm flat on the paper, fingers spread — skin-on-paper gives the AI a clear edge to anchor on; the A4 dimensions also back out the camera distance (close to 30 cm).
• Palm facing the camera — the back-of-hand has too little texture, AI will struggle to localize; the palm-side that's on the paper has the richest texture.
• Fingers straight — natural straight + spread, even if you have to push a bit; never curled.

Key 2: even lighting

   ✅ Good              ❌ Bad
┌────────────┐     ┌────────────┐
│  by wall   │     │ cluttered  │
│   🖐       │     │   🖐 + 📦   │
│            │     │            │
└────────────┘     └────────────┘

• Light should be even; angled window light is better than overhead (less harsh shadows).
• Turn off flash if there's enough light.
• White A4 paper also "diffuses" overhead hard light — yet another reason to lay it down.

Key 3: shoot straight down

• Not at an angle — straight overhead.
• Camera ≈ 30–50 cm above the paper.
• Use the rear camera (front has heavy distortion).

Capture flow

1. Home → "AI Grip Size"
2. "Start" → camera permission prompt (first time)
3. Shoot straight down (≈ 30–50 cm), frame on the flat palm, hold still for 1 sec while pose pre-check runs
4. Shutter (or auto-trigger)
5. Wait 1–2 sec: on-device MediaPipe Hand runs → result page

Result page

The result page shows the recognition result overlaid on your photo:

Panel	Content
Photo + overlay	Your palm + AI-drawn transverse line (the boundary between palm and wrist)
Measurements	Palm length + palm width (mm)
Recommendations	Yonex / Wilson / Head–Prince–Babolat simultaneously

Three-system grid example:

             Yonex   Wilson    Head / Prince / Babolat
             L3      4½"       4¾

Two correction paths for the AI result

Before we dive into the result page details, here are the two correction modes the App provides — pick whichever fits the situation.

Mode 1 — drag the transverse line on the result page (preferred)

This mode lets you drag the transverse line on the photo. The line is the AI's "palm start position" — if it's 5 mm too high (toward the fingertips), the recommended size jumps one tier.

• After the shot, you'll see an AI-drawn wrist transverse line on the photo
• Long-press, then drag up/down to move it
• The "Measurements" + "Recommendations" panels update in real time
• Drag range is clamped to ±15 mm from the AI's initial position (to avoid absurd values)
• Tap "Save to history" when satisfied

Use this mode when "AI number is roughly right, just a bit off (typically 3–10 mm)".

Mode 2 — Adjust Green Frame page: fit the A4 sheet to the green frame

If the overall scale is wrong — e.g., you wore multiple overgrips and the image looks elongated/compressed — switch to the Adjust page to give the App a known physical reference.

The page title is "Adjust Green Frame", showing:

• The full shot (palm + A4)
• A cyan-green rectangle the App computed for a 30 cm standard working distance
• A bottom hint: "Drag with one finger; pinch to zoom (rotation disabled). Fit the A4 inside the green frame, then Next."
• A green Next (Analyze) button

How to operate:

• One finger drag — pan the photo so the A4 sheet is centered in the green frame
• Two-finger pinch / spread — zoom the photo so all four edges of the A4 sheet hug the green frame
• Stop when the four paper edges overlap the green frame — the tighter, the more accurate
• Rotation is disabled (so you don't accidentally rotate the photo)
• When satisfied, tap Next (Analyze) — the App re-runs palm-length detection at the corrected scale

An A4 sheet is 210 × 297 mm; telling the App "this green frame = an A4 at this working distance" nails down absolute scale — something Mode 1 alone can't do.

Combine the two modes: Mode 2 fixes scale, Mode 1 fixes palm start.

Error budget and mitigations

Source	Impact	Mitigation
Palm tilted	Length ±5–8 mm	App says "place flat"
Focal distortion (front cam)	Length ±3 mm	Default forces rear cam
Skin tone / lighting	Keypoint shift ±2 mm	MediaPipe Hand 2024 model is robust across tones
Partial occlusion (ring, long nails)	Failure / oversized	Remove rings, trim nails first
Hand-shape training bias	±3 mm	Use Mode 1 (drag line) or Mode 2 (adjust page)

Measured RMSE: 4.2 mm on-device. In tier terms: about ±⅓ tier in the Yonex system.

Limits and edges

• ❌ Gloves not supported — gloves drop keypoints
• ❌ Children's hands — training set is mostly adults; kids won't be accurate
• ❌ Extreme lighting (very dark, strong backlight) — shot fails → use Adjust page, or 📏 manual measurement
• ❌ Multiple hands in frame — App auto-picks the largest (often wrong)

No original photo saved

V1.1 does not save the original photo. After measurement, only the keypoint coordinate array (~80 bytes) is cached for history replay; the photo is discarded immediately. If you need to keep a copy, go to Settings → Data management → Enable original photo archive (off by default).

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📏 Manual measurement (ruler)

This path uses the exact same size formula as the AI path — only the entry differs. So if your ruler reading is accurate, the result will match what AI computes; if AI was off, manual measurement is a great ground-truth check.

Entry

Home → "AI Grip Size" card → "Manual Tier Picker" link below.

Steps

1. Pick your dominant hand (left / right) — the diagram on the page flips accordingly
2. Measure with a ruler: from directly below the ring fingertip down to the transverse line (the second crease counting up from the wrist crease — the palm / finger boundary)
3. That distance is your rulerMm — the "palm length" used in the sizing formula
4. Type it into the Ruler Method Measurement input (mm); you can leave it blank and pick a tier directly
5. Optional: fill in Palm Length (mm, optional) if you measured from the wrist crease to the ring fingertip instead of the transverse line to the ring fingertip
6. The App applies the exact same formula as AI camera mode and outputs Yonex / Wilson / Head–Prince–Babolat

When to use

• Camera permission denied / no camera
• Repeated AI failure (extreme lighting, gloves, very small hands)
• You already have a ruler reading and just want to type it in
• You want a sanity check against the AI result

How to measure well

• A soft tape measure or rigid ruler + caliper both work; mm precision is enough (output is ⅛" tiers)
• Ring finger is the standard sizing finger (index and middle are longer, which cross-system comparisons won't agree on)
• Transverse line: the boundary between palm and fingers. Curl your hand, then release — you'll see 3 creases: wrist, palm-mid, finger-base. Take the middle one
• Measure with the hand naturally relaxed — don't force it open (that adds 1–3 mm)

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FAQ

iPhone / iPad?

Not supported in this version. See 00 · Supported platforms.

How powerful does the device need to be for on-device detection?

Snapdragon 7xx / Dimensity 800 or higher recommended. Low-end devices may feel "spinning" for a few seconds.

Does the data get uploaded?

No. All computation runs on-device; the photo never leaves your phone.

Will AI and manual produce the same size?

In theory, yes — both paths share the same formula; in practice, expect ~⅓ tier difference (sources: AI measurement error ±4.2 mm + ruler reading error). If the two paths differ by ≥ 1 tier, run each twice and take the median, or treat 📏 manual's rulerMm as ground truth.

I don't agree with the AI result — what are my options?

From highest precision to lowest:

1. Drag the transverse line on the result page — long-press its handle and drag; the measurement + recommendation panels update in real time. Use this when AI is roughly right but a bit off (typically 3–10 mm).
2. Adjust page — on the result page tap "Adjust scale only" → a ±10 mm slider. Use this when you've already measured with a ruler and just want to type the number in, or when on-device detection fails entirely and you can't see the photo.
3. Skip AI entirely → 📏 Manual measurement — the fallback path; completely independent of AI.