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feat(export): expose JPEG XL Save As and Export As, document limitations and test output

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Nigel Ihza Firdausy
2026-10-11 17:06:29 +07:00
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# Raster formats
`photocraft-codecs` provides format detection, capability declarations, decode limits, metadata handling, and encoding for flat images. The default build declares read/write support for PNG, JPEG, TIFF, WebP, GIF, BMP, TGA, ICO, Netpbm/PFM, QOI, OpenEXR, and Radiance HDR. HEIF/HEIC (iPhone and Mac photos) is read-only, decoded in pure Rust by `heic-rs` through the optional `photocraft-heif` crate. It is a build-time choice behind the `heif` cargo feature (`cargo build -p photocraft --features heif`): official builds include it; without it HEIC files are still recognised and opening one reports that HEIC support isn't included in that build. AVIF encoding is optional; AVIF decoding is not implemented in the current crate.
`photocraft-codecs` provides format detection, capability declarations, decode limits, metadata handling, and encoding for flat images. The default build declares read/write support for PNG, JPEG, TIFF, WebP, GIF, BMP, TGA, ICO, Netpbm/PFM, QOI, OpenEXR, and Radiance HDR. HEIF/HEIC (iPhone and Mac photos) is read-only, decoded in pure Rust by `heic-rs` through the optional `photocraft-heif` crate. It is a build-time choice behind the `heif` cargo feature (`cargo build -p photocraft --features heif`): official builds include it; without it HEIC files are still recognised and opening one reports that HEIC support isn't included in that build. AVIF encoding is optional; AVIF decoding is not implemented in the current crate. Lossless JPEG XL (.jxl) export supports 8/16-bit integer gray and RGB images with optional alpha through the pure-Rust `zune-jpegxl` encoder. Flat JPEG XL import is not yet available (see PR #2868).
## Layered TIFF
A TIFF saved by Photoshop with its layers carries them in two private tags: 37724 (`ImageSourceData`, the same layer section a PSD has, preceded by the string `Adobe Photoshop Document Data Block`) and 34377 (the PSD image resources). `photocraft-codecs` passes both through as opaque bytes on `Metadata`; `photocraft-io` rebuilds a PSD model from them plus the TIFF's pixels and hands it to the PSD importer, so groups, masks, layer styles, type, shapes, smart objects and adjustment layers open from a TIFF exactly as from a PSD, and are written back the same way. In Intel-order (`II`) files Photoshop stores the whole block structure byte-swapped (reversed keys such as `MIB8` and `ryaL`, little-endian integers and doubles) while pixel samples stay big-endian; `photocraft-psd`'s `tiff` module converts between the two orders field by field, and drops, with a warning, any block whose layout it does not know rather than pass byte-swapped data to other readers. Limits: a TIFF holds one transparency sample, so extra alpha channels are not written; Lab documents are saved flat. Save As asks "Including layers will increase file size" (with "Discard Layers and Save a Copy") while *Ask Before Saving Layered TIFF Files* is on in Preferences › File Handling. Scripted and agent saves write a flat TIFF unless they ask for layers: `--tiff-layers` for `photocraft-cli convert`, `run` and `batch`, and `"tiffLayers": true` in the save, batch and export commands and MCP `doc_save`.
## JPEG XL
Save As, Export As and the CLI can write a lossless, single-image `.jxl`
codestream with 8- or 16-bit grayscale, grayscale+alpha, RGB or RGBA samples.
The pure-Rust encoder writes the bare codestream (without ICC, EXIF, XMP or DPI
metadata boxes), so export converts non-sRGB tagged images to sRGB and surfaces
metadata losses as fidelity warnings. Opening ordinary JPEG XL files remains a
separate feature under review in #2868.
## WebP
Lossless WebP (VP8L) is encoded and decoded by `image-webp`. Lossy WebP is written by PhotoCraft's own VP8 key-frame encoder in `crates/codecs/src/codecs/vp8`, a clean-room implementation of RFC 6386: BT.601 4:2:0 conversion, 16×16 and 4×4 intra prediction chosen per macroblock by reconstruction error, the forward DCT and Walsh-Hadamard transforms paired with the bit-exact inverse transforms, quantisation on the RFC's `dc_qlookup`/`ac_qlookup` tables, token coding with per-frame probability updates, and skipped macroblocks for flat areas. `EncodeOptions::webp_lossless` picks the encoder and `webp_quality` (0–100, the libwebp scale) sets the quantiser; alpha travels losslessly in an `ALPH` chunk and ICC/EXIF/XMP in their own chunks of the extended (`VP8X`) container. The decoder applies the frame's loop filter; no C library is involved on either side.