# lightcraft-meta (L1)

Photo metadata: EXIF, XMP (read + write), IPTC-IIM, and the containers that carry them.

## API

| Function | Purpose |
|---|---|
| `read_exif(&[u8]) -> Metadata` | TIFF-structured Exif block (whole TIFF/DNG/raw file, or JPEG APP1 payload with/without `Exif\0\0`). `try_read_exif` reports header errors. |
| `from_tiff(&Tiff) -> Metadata` | Same, from an already parsed stream (used by `lightcraft-raw`). |
| `embedded(&[u8]) -> Embedded` | Sniffs JPEG / PNG / WebP / TIFF / PSD / PSB and returns the Exif, XMP (+ extended XMP), ICC and IPTC blocks. PSD/PSB EXIF resource 1058 and XMP resource 1060 are read from the bounded image-resource section without decoding pixels. `jpeg_segments`, `png_chunks`, `webp_chunks` are the per-container walkers. |
| `parse_xmp(&str) -> Result<XmpData>` | Metadata + the opaque `lc:settings` JSON + every property (`prefix:name`, structs flattened as `a:b/c:d`). |
| `write_xmp(&Metadata, Option<&str>) -> String` | Complete `<?xpacket?>` packet; the second argument is LightCraft's full develop settings JSON. |
| `parse_iptc(&[u8]) -> Metadata` | IIM record 2: title (2:5), keywords (2:25), by-line (2:80), copyright (2:116), caption (2:120), date/time created (2:55/2:60). |
| `extract(&[u8]) -> Metadata` | Everything for a whole file. Precedence: EXIF → IPTC fills gaps → XMP overrides user fields (rating, label, title, caption, artist, copyright, keywords, GPS) and fills the rest. |
| `try_extract(&[u8]) -> Result<Metadata, ExtractError>` | Like `extract`, but reports malformed PSD/PSB headers, image-resource sections, or embedded EXIF/XMP structure (including duplicate EXIF/XMP resources) so callers can fail closed when location absence cannot be established. Other formats remain best-effort. |

`Metadata` carries camera (make, model, serial, software), lens (make, model, serial, specification), capture
(date-time with UTC offset and milliseconds, exposure time, f-number, ISO incl. extended ISO, focal length and
35 mm equivalent, flash, exposure bias, program, metering, white balance), orientation, GPS, dimensions and the
user fields (artist, copyright, title, caption, keywords, hierarchical keywords, rating −1…5, colour label).

## XMP namespaces

Written: `dc`, `xmp`, `photoshop`, `exif`, `exifEX`, `tiff`, `lr` (`hierarchicalSubject`) and ours,
`lc` = `http://ns.lightcraft.app/lc/1.0/` (`lc:settings`, an opaque JSON string). Reading canonicalises
prefixes by namespace URI, handles attribute and element forms, `rdf:Seq`/`Bag`/`Alt` (x-default first),
`rdf:parseType="Resource"` and nested-description structs, predefined and numeric entities. Nesting depth
and node count are limited.

An explicitly foreign namespace using a recognized prefix (for example `lc` or `xmp`) cannot supply
recognized properties. Its keys use `Q{hex-encoded namespace URI}local` in `properties` and `values`;
hex encoding keeps `/` reserved for struct paths. Other unknown prefixes keep their existing QName keys.
Merging preserves the original foreign XML rather than writing these opaque keys into the packet.
The parser and merge scanner each cap generated opaque names at 8 MiB per packet, returning `TooComplex`
if repeated use of a long namespace URI would exceed that budget.

## Robustness

No panics on malformed input: proptests feed random bytes to every container walker and random text to the
XMP parser; truncation tests cut a sample JPEG at every byte and exercise truncated PSD/PSB headers and
image-resource records with hostile declared lengths.
PSD/PSB XMP resources require valid UTF-8; trailing NUL padding is trimmed after validation.

## Sources

CIPA DC-008 (Exif 2.32), TIFF 6.0, Adobe DNG 1.7 (tag numbers), ISO 16684-1 / Adobe XMP Specification
Parts 1–3 (packet, RDF serialisation, standard schemas, JPEG extended XMP), IPTC-IIM 4.2 and the IPTC Photo
Metadata Standard, ITU-T T.81 (JPEG markers), ICC.1 (APP2 embedding), PNG Specification 3rd ed. (`eXIf`,
`iTXt`), the WebP RIFF container specification, Adobe Photoshop File Format Specification (image resource
blocks). XML parsing: `quick-xml` (MIT). No third-party metadata code was consulted.