A CMYK JPEG came out with JFIF's APP0 segment at its head and four components
in the frame:
APP0/JFIF APP14/Adobe APP2[ICC_PROFILE 1/4 … 4/4] SOF0: 4 components
JFIF describes one- and three-component images, greyscale and YCbCr, so an APP0
segment in front of a four-component frame says two different things about the
file's colours: a reader that trusts JFIF decodes nonsense, and one that checks
refuses the file. Adobe's own CMYK JPEGs carry the APP14 marker the encoder
already writes, and no JFIF. `jpeg-encoder` writes JFIF unconditionally, so the
segment is replaced here.
The resolution JFIF carried moves to an EXIF APP1 segment -- `XResolution`,
`YResolution` and `ResolutionUnit` -- which is where `cmd::fileio::ppi` already
looks when a file has no JFIF, so nothing is lost; with no resolution asked for
the segment is simply dropped. RGB and greyscale files keep JFIF, which is what
it is for.
Found while looking at #3084, whose default RGB exports are valid here; this is
the file that is not.
document.export {format: "jpg"} takes colorModel (rgb|cmyk|gray), quality 0–100, method
(baseline|optimized|progressive), scans (3–5), embedIcc and imageMap (none|client|server).
JPEGs are written with jpeg-encoder. CMYK files hold ink amounts drawn as the renderer's ink
planes, so CMYK colours keep their inks (black type stays black ink alone). Profiles are made
in code from the CMS (cms::icc_bytes): the RGB spaces from their primaries, the CMYK spaces as
Lab lookup tables sampled from their model, a grey with sRGB's tone curve, and loaded
profiles as loaded. Image maps write <stem>.html or <stem>.map beside the image for objects
with a URL and an Image Map shape. The JPEG Options dialog gains Color Model, Quality 0–10,
Method and Scans, Image Map and Embed ICC Profile; CMYK documents default to CMYK.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Hamza