Photo Merge decoded each raw itself and wrote the reference frame's own colour
tags into the merged DNG. A CR2 (and NEF, ARW and the other raws whose colour is
fitted from the camera JPEG) has none, so an HDR merge of CR2s rendered with no
camera colour: green and about 16 L* dark.
`lightcraft_merge::load_frame_with` takes the raw's colour tags from the caller,
and the engine passes `starting_colour_tags`: the colour the raw opens with
(`starting_colour`) as DNG tags. That is one forward matrix with a colour matrix
and AsShotNeutral that agree with it at the as-shot white, the look's hue/sat
table as ProfileHueSatMap and its tone curve as ProfileToneCurve. Raws with
their own matrices (DNG) keep their tags. The look's chroma curve has no DNG
tag and is left out.
On two CC0 CR2s (EOS 550D, 40D) an HDR merge of three copies against the photo
alone: mean CIE76 dE 26.15 -> 1.07 and 22.89 -> 0.47.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
