What byte layout does a second independent ISO 21496-1 implementation (libavif) use for gain map metadata, and does it agree with libultrahdr's min_version 0 layout?

libavif (commit b1e4d339) and libultrahdr v2.0.2 (commit e5f5a022) write the same first four bytes of the ISO 21496-1 gain-map metadata and the same top two flag bits (bit 7 multi-channel, bit 6 use-base-colour-space, confirmed in libultrahdr's gainmapmetadata.h). libavif writes and reads the remaining six bits as reserved without checking them, while libultrahdr assigns flag values 4 (backward direction) and 8 (common denominator, which switches the body layout). Bytes match the producer pins, re-read 2026-10-22. That files written by one library could be misread by the other is an unexecuted inference. Curator qualifications: Consequence for cross-reading (mis-parse or 'remaining bytes' failure) is the producer's unexecuted inference; needs an authorised L10 fixture. Which layout the final ISO 21496-1 specifies is unknown (standard unread; libavif changelog says it followed a draft). 'Two independent code bases' is used loosely; shared organisational lineage not assessed. Static reading only; no image encoded/decoded, no library built. Amendments applied: CA-AM-0037. Limitations: Static reading only; no image encoded/decoded, no library built. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/AOMediaCodec/libavif/b1e4d339c1ff5b547664c85f1713279f4997d694/CHANGELOG.md",
      "note": "write.c lines 1067-1082; read.c lines 2262-2280; gainmapmetadata.cpp lines 122-162, 221, 231"
    },
    {
      "url": "https://github.com/AOMediaCodec/libavif/blob/b1e4d339c1ff5b547664c85f1713279f4997d694/src/write.c",
      "note": "write.c lines 1067-1082; read.c lines 2262-2280; gainmapmetadata.cpp lines 122-162, 221, 231"
    },
    {
      "url": "https://github.com/AOMediaCodec/libavif/blob/b1e4d339c1ff5b547664c85f1713279f4997d694/src/read.c",
      "note": "write.c lines 1067-1082; read.c lines 2262-2280; gainmapmetadata.cpp lines 122-162, 221, 231"
    },
    {
      "url": "https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/lib/src/gainmapmetadata.cpp",
      "note": "write.c lines 1067-1082; read.c lines 2262-2280; gainmapmetadata.cpp lines 122-162, 221, 231"
    },
    {
      "url": "https://developer.android.com/media/platform/hdr-image-format",
      "note": "write.c lines 1067-1082; read.c lines 2262-2280; gainmapmetadata.cpp lines 122-162, 221, 231"
    }
  ]
}