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. On the way in, Image::warnings lists what the decoded image doesn't show: frames or pages left out, or data that ended early.

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
HEIF/HEIC	with feature heif	no	U8, U16 (10/12-bit decodes to U16)	RGB, RGBA	yes (auxiliary alpha)	yes (colr prof)	yes	yes	no	no	n/a	heic-rs

"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.

• HEIF/HEIC (read-only, feature heif). Decoding lives in the optional photocraft-heif crate behind the non-default heif feature (official PhotoCraft builds enable it; distributors choose). Without it, HEIF is still detected and decoding returns CodecError::Unsupported ("HEIC/HEIF support isn't included in this build"). iPhone and Mac photos decode with heic-rs (pure Rust, no unsafe): single pictures and grid-tiled photos, 8 to 12 bits, auxiliary alpha, ICC, EXIF and XMP. The container's irot/imir/clap are applied and EXIF Orientation is rewritten to 1 (HEIF's EXIF tag only repeats the container's, and applying both would turn the photo twice). Grayscale decodes to RGB. Image sequences, overlays and identity derivations, multilayer HEVC and some 4:2:2/4:4:4 streams return CodecError::Unsupported or Malformed, never wrong pixels. Writing needs an HEVC encoder and every mature one is C, so HEIF is listed in ASYMMETRIC_EXCEPTIONS.

• AVIF. 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. The decoded image then carries a DecodeWarning::MoreFrames / MorePages in Image::warnings, with the total when the file states it (frames and TIFF directories are counted without decoding them; reduced-resolution TIFF directories and transparency masks are not pages).

• 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.

• Orientation (EXIF tag 274 in JPEG, PNG eXIf and WebP; the IFD0 tag in TIFF) is applied on decode, like Photoshop: the pixels come back upright and the EXIF/XMP orientation is rewritten to 1 (DecodeOptions::keep_orientation opts out). Encoders always write Orientation = 1 (upright_exif, upright_xmp, which change only the tag's value, kept in its own type), so upright pixels are never rotated twice. Parsing is strict: an IFD0 inside the 8-byte header or cut off before its next-IFD pointer, an Orientation that is not exactly one SHORT or LONG, or an out-of-range value reads as 1; with duplicate entries the first wins. Limits are checked again on the upright size (5–8 swap width and height), and the rotated buffer is allocated fallibly. Image::oriented turns any layout and depth in parallel bands (about 10 ms for 24 MP RGB8 in release).

• JPEG

  • 8-bit only. Neither decoder backend supports 12-bit.
  • A file cut off inside its image data decodes leniently (a baseline JPEG's missing rows come out grey) but never silently: the image carries DecodeWarning::Truncated. A file that ends before any scan data is an error.
  • 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.