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LR-HDR-EXPORT: ISO 21496-1 gain map JPEG codec

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`lightcraft_codecs::gainmap`:
- `compute` measures a gain map (luminance or RGB, log2, block-averaged)
  from an SDR and an HDR rendition; `apply` rebuilds the HDR rendition.
- ISO 21496-1 metadata: writer (fixed denominator) and reader (also the
  common-denominator form; refuses unknown versions, backward maps and zero
  denominators). Adobe `hdrgm` XMP reader and writer, per-channel values as
  rdf:Seq; Apple's HDRGainMap/HDRToneMap description of the same map.
- `encode_jpeg`: base with CIPA DC-007 MPF index (gain map typed 0x050000),
  the ISO marker and Container XMP merged into the user's packet, followed by
  the gain map JPEG. `read_jpeg` / `is_gain_map_jpeg` find and decode the map.

Also: gain map images are no longer picked as embedded thumbnails (an Ultra
HDR JPEG could show its greyscale gain map in the grid). The JPEG APP-segment
building is shared with `encode_jpeg`. Fuzzed with the other decoders.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
akvaithi
2026-10-09 22:01:35 -05:00
co-authored by Claude Opus 5.5
parent 791526aa8c
commit e11db8fea0
6 changed files with 957 additions and 36 deletions
Download Patch File Download Diff File Expand all files Collapse all files
crates/codecs/src/encode.rs
+41 -29
View File
@@ -79,6 +79,46 @@ pub enum ChromaSubsampling {
const XMP_NS: &[u8] = b"http://ns.adobe.com/xap/1.0/\0";
/// The APP segments [`encode_jpeg`] writes for `meta` (marker byte, payload): JFIF density, EXIF,
/// XMP and the ICC profile in chunks, in that order.
pub(crate) fn jpeg_app_segments(meta: &EncodeMeta) -> Result<Vec<(u8, Vec<u8>)>> {
let mut segs: Vec<(u8, Vec<u8>)> = Vec::new();
let ppi = meta.ppi.filter(|p| *p > 0).map_or([0u8, 0, 1, 0, 1], |p| {
let [hi, lo] = p.to_be_bytes();
[1, hi, lo, hi, lo]
});
segs.push((0xE0, [&b"JFIF\0\x01\x02"[..], &ppi, &[0, 0]].concat()));
if let Some(exif) = meta.exif {
if exif.len() > 65_527 {
return Err(Error::Encode("EXIF larger than one APP1 segment (64 KiB)".into()));
}
segs.push((0xE1, [&b"Exif\0\0"[..], exif].concat()));
}
if let Some(xmp) = meta.xmp {
segs.push((0xE1, xmp_segment(xmp)?));
}
if let Some(icc) = meta.icc {
// ICC.1 Annex B.4: "ICC_PROFILE\0", 1-based chunk number, chunk count
let chunks: Vec<&[u8]> = icc.chunks(65_519).collect();
if chunks.len() > 255 {
return Err(Error::Encode("ICC profile too large for JPEG".into()));
}
for (i, c) in chunks.iter().enumerate() {
segs.push((0xE2, [&b"ICC_PROFILE\0"[..], &[i as u8 + 1, chunks.len() as u8], c].concat()));
}
}
Ok(segs)
}
/// An APP1 XMP payload (namespace + packet), refusing packets larger than one segment.
pub(crate) fn xmp_segment(xmp: &str) -> Result<Vec<u8>> {
let seg = [XMP_NS, xmp.as_bytes()].concat();
if seg.len() > 65_533 {
return Err(Error::Encode("XMP larger than one APP1 segment (extended XMP not supported)".into()));
}
Ok(seg)
}
/// Encode baseline JPEG (8-bit; alpha is dropped). `quality` 1..=100.
pub fn encode_jpeg(img: &EncodeImage, quality: u8, subsampling: ChromaSubsampling, meta: &EncodeMeta) -> Result<Vec<u8>> {
img.validate()?;
@@ -97,35 +137,7 @@ pub fn encode_jpeg(img: &EncodeImage, quality: u8, subsampling: ChromaSubsamplin
};
if subsampling != ChromaSubsampling::S422 {
// Parallel encoder (restart-interval bands on all cores).
let mut segs: Vec<(u8, Vec<u8>)> = Vec::new();
let ppi = meta.ppi.filter(|p| *p > 0).map_or([0u8, 0, 1, 0, 1], |p| {
let [hi, lo] = p.to_be_bytes();
[1, hi, lo, hi, lo]
});
segs.push((0xE0, [&b"JFIF\0\x01\x02"[..], &ppi, &[0, 0]].concat()));
if let Some(exif) = meta.exif {
if exif.len() > 65_527 {
return Err(Error::Encode("EXIF larger than one APP1 segment (64 KiB)".into()));
}
segs.push((0xE1, [&b"Exif\0\0"[..], exif].concat()));
}
if let Some(xmp) = meta.xmp {
let seg = [XMP_NS, xmp.as_bytes()].concat();
if seg.len() > 65_533 {
return Err(Error::Encode("XMP larger than one APP1 segment (extended XMP not supported)".into()));
}
segs.push((0xE1, seg));
}
if let Some(icc) = meta.icc {
// ICC.1 Annex B.4: "ICC_PROFILE\0", 1-based chunk number, chunk count
let chunks: Vec<&[u8]> = icc.chunks(65_519).collect();
if chunks.len() > 255 {
return Err(Error::Encode("ICC profile too large for JPEG".into()));
}
for (i, c) in chunks.iter().enumerate() {
segs.push((0xE2, [&b"ICC_PROFILE\0"[..], &[i as u8 + 1, chunks.len() as u8], c].concat()));
}
}
let segs = jpeg_app_segments(meta)?;
let ch = if img.channels == 2 { 1 } else { img.channels as usize };
// `data` is already gray for 2-channel input (see above)
return Ok(crate::jpeg_par::encode(&data, img.width as usize, img.height as usize, ch, quality, subsampling, &segs));