photocraft/crates/codecs/README.md

# photocraft-codecs

Codecs for flat raster image formats, written in pure Rust. The crate is standalone: it depends on no other
workspace crate and defines its own `Image` type. It also builds for `wasm32-unknown-unknown` and
does no I/O: decoders take `&[u8]` and encoders return `Vec<u8>`.

It follows the "avoid GIMP's hole" rules in `plan/architecture.md` §1.1:

* **Symmetric.** Every format we write, we can also read. The only exceptions are listed in
  `ASYMMETRIC_EXCEPTIONS`, each with a reason. The test suite checks this.
* **Depth-preserving.** An image can hold U8, U16, F16 or F32 samples, and each format keeps
  what it can hold.
* **Metadata-preserving.** ICC, EXIF, XMP, DPI and text are kept wherever the format supports
  them.
* **Honest.** `caps(format)` says what each format holds, and
  `fidelity_warnings(&image, format)` lists what an export will lose before you write it. Both
  functions use the same encode plan as `encode`, so a warning appears exactly when the file
  changes.

```rust
use photocraft_codecs::*;

let img = decode(&bytes)?;                               // detect + decode (default Limits)
for w in fidelity_warnings(&img, Format::Jpeg) {         // e.g. "16-bit will be reduced to 8-bit"
    println!("{w}");
}
let out = encode(&img, Format::Tiff, &EncodeOptions::default())?;
```

## Capability matrix (default build)

| Format | Read | Write | Depths (native) | Layouts (native) | Alpha | ICC | EXIF | XMP | DPI | Text | Lossy write | Backend |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PNG | yes | yes | U8, U16 | Gray, GrayA, RGB, RGBA | yes | yes (iCCP) | yes (eXIf) | yes (iTXt `XML:com.adobe.xmp`) | yes (pHYs) | yes (tEXt/zTXt/iTXt) | no | `png` |
| JPEG | yes | yes | U8 | Gray, RGB, CMYK | no | yes (multi-segment APP2) | yes (APP1) | yes (APP1) | yes (JFIF) | no | yes | `zune-jpeg` / `jpeg-encoder` |
| TIFF | yes | yes | U8, U16, F32 | all six (CMYK, CMYK+A included) | yes | yes (tag 34675) | no | yes (tag 700) | yes | yes (Description, Make, Model, Software, DateTime, Artist, Copyright) | no | `tiff` |
| WebP | yes | yes (lossless only) | U8 | RGB, RGBA | yes | yes | yes | yes | no | no | no | `image-webp` |
| GIF | yes | yes | U8 | RGBA | 1-bit | no | no | no | no | no | yes (256-colour palette) | `image` |
| BMP | yes | yes | U8 | RGB, RGBA | yes | no | no | no | no | no | no | `image` |
| TGA | yes | yes | U8 | Gray, GrayA, RGB, RGBA | yes | no | no | no | no | no | no | `image` |
| ICO | yes | yes | U8 | RGBA (at most 256×256) | yes | no | no | no | no | no | no | `image` |
| Netpbm (PBM/PGM/PPM/PAM/PFM) | yes | yes | U8, U16, F32 (float: PFM gray/RGB only) | all six (PAM `CMYK`/`CMYK_ALPHA`) | yes (PAM) | no | no | no | no | no | no | built in |
| QOI | yes | yes | U8 | RGB, RGBA | yes | no | no | no | no | no | no | `image` |
| OpenEXR | yes | yes | F16, F32 | Y, YA, RGB, RGBA | yes | no | no | no | no | no | no | `exr` |
| Radiance HDR | yes | yes | F32 | RGB | no | no | no | no | no | no | yes (RGBE) | `image` |
| AVIF | **no** | only with feature `avif` | U8 | RGB, RGBA | yes | no | no | no | no | no | yes | `image` + `ravif` |

"Native" means the data is stored and read back without conversion. Anything else is converted by
the encode plan, and `fidelity_warnings` reports the conversion when it loses information:

* A float image written to an integer format goes to U16 where the format supports it. Values
  outside 0..1 raise `RangeClipped`.
* Integer data written to a float-only format (EXR, HDR) becomes F32 without loss.
* CMYK is converted to RGB with a naive formula that is not colour-managed. `CmykConverted` is
  raised and the ICC profile is dropped, because a CMYK profile does not describe RGB data.
* Gray written to an RGB-only format is expanded to RGB without loss, so no warning is raised.

`Image::convert` applies the same ICC rule: it keeps the profile only when the colour model stays
the same.

## Documented asymmetries and limitations

* **Camera raw files** (DNG, CR2, NEF, ARW… which are TIFF-structured) are recognised and refused
  with `CodecError::Unsupported`: they are sensor data, not flat images. `photocraft-raw` decodes
  and develops them, and `photocraft-io` routes them there.

* **AVIF (the only asymmetric format).** Encoding uses `ravif`, which is pure Rust. Decoding
  needs `dav1d`, which is C. AVIF is therefore read-unsupported, and write support is gated
  behind the non-default `avif` feature. In a default build it is neither readable nor writable,
  so the symmetric guarantee holds. It is listed in `ASYMMETRIC_EXCEPTIONS`.
* **Lossy WebP.** There is no pure-Rust lossy WebP encoder. We always write lossless WebP, and
  `webp_lossless: false` returns `CodecError::Unsupported`. We can read both lossy and lossless
  files.
* **Animation and multi-page files** (APNG, animated GIF/WebP, multi-page TIFF): only the first
  frame or page is decoded, and a single frame is written. `FormatCaps::animation` marks
  containers that can hold more frames.
* **EXIF in TIFF** is stored as a sub-IFD rather than a blob, so it is not preserved yet.
  `caps.exif = false` for TIFF, and a warning is raised.
* **EXIF orientation** is preserved as data but not applied to the pixels.
* **JPEG**
  * 8-bit only. Neither decoder backend supports 12-bit.
  * CMYK is always written 4:4:4 (subsampled CMYK is not portable) as Adobe-inverted CMYK with an
    APP14 marker.
  * Text is not written because there is no COM-segment support.
  * EXIF and XMP must each fit in one 64 KiB APP1 segment, otherwise the encoder returns an error.
    Extended XMP is not written.
* **TIFF decode**
  * Supports 1, 2 and 4-bit gray (best effort, not yet covered by tests), 8/16/32-bit integers (32-bit is reduced to U16), F16, F32 and
    F64 (converted to F32), and WhiteIsZero.
  * Palette and YCbCr TIFFs, and compressions that the `tiff` crate lacks (CCITT, JPEG-in-TIFF),
    return `Unsupported`.
* **EXR**
  * Reads the first valid layer at full resolution, from its data window.
  * Channel names are matched by suffix, so `layer.R` counts as `R`.
  * Subsampled channels and deep data are unsupported.
  * Only lossless compressions are offered for writing: None, RLE, ZIP1, ZIP16 and PIZ.
* **Netpbm writing** picks the subtype from the image: float gives PFM, gray gives P5, RGB gives
  P6, and anything with alpha or CMYK gives P7 (PAM). Writing a `.pbm` file therefore produces
  whichever of these fits the data.
* **Colour conversions** (CMYK↔RGB, RGB→gray) are naive. Colour-managed conversion belongs in
  `photocraft-color`.

## Limits (decompression bombs)

`DecodeOptions { limits: Limits { max_width, max_height, max_pixels, max_alloc } }` guards
decoding. Header dimensions are checked before the pixel buffer is allocated, and the budget is
also passed to the underlying decoders. A violation returns `CodecError::LimitExceeded`. The
defaults are 262144 px per side, 2^30 pixels and 8 GiB (2 GiB on 32-bit targets such as wasm).

## Tests

`cargo test -p photocraft-codecs` covers:

* the full round-trip matrix (every writable format × 4 sample types × 6 layouts), exact for
  lossless formats and above a PSNR threshold for lossy ones;
* metadata preservation per format, checked against `caps`;
* magic-number detection, including negatives;
* fidelity warnings, and a cross-check that the warnings match the actual round-trip result;
* the symmetric-capability guarantee;
* malformed input: truncation, bit flips and proptest random bytes;
* Adam7 PNG against progressive PNG and against the `image` crate as an oracle;
* limit enforcement.

If `corpus/pngsuite/*.png` exists at the repo root, every file in it is compared against the
`image` crate's decoder. Without it, that test is skipped.