How Scanning Works

Every scan mode shares one pipeline; only the parsing layer differs. This page walks the full flow, then each mode in order: passport (MRZ) → credit card → business card.

The shared pipeline

Camera (vision-camera)
  → onFrame worklet            … your code, throttled (e.g. every 5th frame)
  → scan(frame)                … native OCR: Apple Vision / ML Kit
      · crops to the central band (roi: 'centralBand' default)
      · returns text lines, top-to-bottom, left-to-right within a row
  → scheduleOnRN(lines)        … hand off to the JS thread
  → parseXxx(lines)            … mode parser (TS) → structured fields
  → session.push(parsed)       … multi-frame confirmation
  → final result               … render / autofill

Key properties:

• Native does OCR only. scan() runs real OCR on every call (hundreds of ms) with no hidden throttling — you decide the cadence in the worklet. All structuring happens in TypeScript.
• One frame is never trusted. Every mode ships a scan session that only confirms a result once frames agree, so a single misread ("HONG" → "HONO") never surfaces.
• frame.dispose() in finally. A leaked frame stalls the camera pipeline.

const scanner = useMemo(() => getOcrScanner(), []);
const session = useMemo(() => createMrzScanSession(), []); // per mode

const frameOutput = useFrameOutput({
  pixelFormat: 'yuv',
  onFrame: (frame) => {
    'worklet';
    try {
      const g = globalThis as unknown as { __ocrFrameCount?: number };
      g.__ocrFrameCount = (g.__ocrFrameCount ?? 0) + 1;
      if (g.__ocrFrameCount % 5 !== 0) return;
      const ocr = scanner.scan(frame);
      if (ocr.lines.length > 0) scheduleOnRN(onLines, ocr.lines, ocr.lineItems);
    } finally {
      frame.dispose();
    }
  },
});

1. Passport (MRZ)

lines → parseMrz() → MrzResult → createMrzScanSession()

• parseMrz finds the two/three <<<-filled MRZ lines, parses them (ICAO 9303), and verifies the check digits — document number, birth date, and expiry each carry one, so a read can prove itself correct.
• MrzResult.valid = composite checksum passed; fieldsValid = the reported fields' own digits passed (OCR noise in unused filler often breaks only the composite).
• The session accepts a read only when its field checksums pass, confirms after minReads (default 3) agreeing reads, and takes the name by majority vote — names have no check digit.

MRZ is first for a reason: the checksum makes it the one mode where free on-device OCR can be provably right. See MRZ Scanning.

2. Credit card

lines → parseCard() → CardResult → createCardScanSession()

• parseCard anchors on the card number: digit runs are windowed along group boundaries and must pass a brand check (BIN ranges) or the Luhn checksum. Expiry prefers a labeled date ("VALID THRU"); holder name is best-effort.
• CardResult.valid = Luhn passed. The session requires the same Luhn-valid number to repeat, then votes on expiry/name.
• No native changes were needed for this mode — it reuses the same scan() output. See Credit Card Scanning.

3. Business card

lines → parseBusinessCard() → BusinessCardResult → createBusinessCardScanSession()

• A business card has nothing to checksum, so the anchor flips to contact info: parseBusinessCard returns null unless an email, phone, or website is found. There is no valid flag.
• Strong-pattern fields (email, phones with conservative type labels, website, address with continuation-line merging) are extracted first; name/company/title are heuristics: name shape + surname boost, legal suffix / domain-label / logo-text company, keyword titles plus an elimination fallback (the short line next to the name that nothing else explains) with departments split out.
• The session anchors on the contact identity (email, else phone digits) repeating — default minReads: 2 — and votes the unchecked fields.

See Business Card Scanning.

Auto-detect

detectDocument(lines) runs the parsers in confidence order:

1. Valid MRZ (checksum) → { type: 'mrz', valid: true }
2. Luhn-valid card → { type: 'card', valid: true }
3. Structurally-parsed MRZ, then card, with valid: false
4. Business card, only if an email is present → { type: 'bizcard' } — a phone number alone would false-positive on receipts and posters.

When your screen already knows what it's scanning, call the specific parser instead — fewer false positives and a mode-specific guide box.