Module apple 

Apple iOS MakerNote HDR gain-map metadata extraction.

Apple HEIC/JPEG captures encode their HDR gain-map headroom inside the EXIF MakerNote (TIFF tag 0x927C), not in XMP — the embedded gain-map image’s XMP carries only HDRGainMapVersion. The headroom is derived from two Apple MakerNote tags, per exiftool’s Image::ExifTool::Apple:

Tag	Name	Type	Role
0x0021	HDRHeadroom	rational64s	“maker33”
0x0030	HDRGain	rational64s	“maker48”
0x000A	HDRImageType	int32s	3 = HDR, 4 = SDR

The headroom (in stops, i.e. log2 luminance ratio) is computed from maker33 and maker48 with the community/Apple-derived piecewise formula (see AppleHdrInfo::headroom_stops). The exact constants are not in a published Apple specification; final fidelity is validated by the gain-map round-trip test, not asserted to be bit-exact here.

References:

• exiftool lib/Image/ExifTool/Apple.pm
• https://juniperphoton.substack.com/p/decoding-some-hidden-magic-of-makerapple
• https://photoinvestigator.co/blog/the-mystery-of-maker-apple-metadata/

This module is pure byte parsing: no_std + alloc, no transcendental math, zero new dependencies. The IFD-walk mechanics mirror the proven reader in zenraw::apple, with the corrected exiftool tag IDs.

Modules

compatibility

Compatibility with ultrahdr-core 0.6.0’s published Apple helpers.

tags

Apple MakerNote tag IDs (per exiftool Apple.pm).

Structs

AppleHdrInfo

HDR information recovered from an Apple MakerNote.

AppleMakerNote

Apple MakerNote inspection view. Other vendors remain opaque. No allocation or value copies; unknown entries are visible for audit/diff.

MakerNoteEntry

One borrowed vendor entry, including unknown/private tags.

MakerNoteEntryError

An entry that could not be read. Inspection does not silently hide it.

Functions

from_apple_headroom

Map recovered Apple headroom to the canonical GainMapMetadata (= zencodec::GainMapParams).

parse_apple_makernote

Parse an Apple iOS MakerNote blob ("Apple iOS\0" + version + byte-order marker + IFD) and pull out the HDR tags.

parse_exif_for_apple_hdr

Extract Apple HDR info directly from EXIF TIFF bytes (e.g. the payload of a HEIF Exif item or a JPEG APP1 segment, with any leading TIFF-offset prefix tolerated).