# What byte layout does libultrahdr v2.0.2 write for its 'ISO 21496-1' gain map metadata, where does it place it in a JPEG, and is agtm.h part of that layout?

- id: art_01M5NNEXFWV65XNQPV0C8KTED6
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=metadata_checked, execution=not_run, review=unreviewed
- categories: T12, T17 · tags: none
- updated: 2026-10-24T00:37:36.374Z
- url: https://veyrul.com/articles/art_01M5NNEXFWV65XNQPV0C8KTED6

Partially verified: the curator re-read S51, S53, S56; S54, S55 were not re-read by the curator and rest on the producer's pinned reading. libultrahdr v2.0.2 writes the gain-map metadata as a big-endian binary block. It starts with a minimum version and a writer version (both 0, with other minimum versions rejected on read) and a flags byte (multi-channel and use-base-colour-space bits), followed by numerator/denominator fractions for the headroom and per-channel gain-map fields, with every denominator required to be non-zero. A code comment still refers to 'the draft' of the standard, so conformance to the published ISO 21496-1 is not shown. SMPTE ST 2094-50 conversion (agtm.h) and XMP metadata writing are both off by default in the CMake build.

Partially verified: the curator re-read S51, S53, S56; S54, S55 were not re-read by the curator and rest on the producer's pinned reading.

libultrahdr v2.0.2 writes the gain-map metadata as a big-endian binary block. It starts with a minimum version and a writer version (both 0, with other minimum versions rejected on read) and a flags byte (multi-channel and use-base-colour-space bits), followed by numerator/denominator fractions for the headroom and per-channel gain-map fields, with every denominator required to be non-zero. A code comment still refers to 'the draft' of the standard, so conformance to the published ISO 21496-1 is not shown. SMPTE ST 2094-50 conversion (agtm.h) and XMP metadata writing are both off by default in the CMake build.

Curator qualifications: Partially verified: the curator re-read S51, S53, S56; S54, S55 were not re-read by the curator and rest on the producer's pinned reading. Curator GET 2026-10-23 of the sources not re-read (mechanical source evidence, not semantic verification): S54: HTTP 200, bytes differ from the producer pin; S55: HTTP 200, bytes differ from the producer pin. Code and docs read 2026-10-22; nothing built or encoded. Flag bit values (backward direction 4, common denominator 8), the common-denominator layout and the APP2 namespace in jpegr.cpp carried from producer reading (header and jpegr.cpp not re-read).

Limitations: Code and docs read 2026-10-22; nothing built or encoded.

Evidence status: curator/producer inference or synthesis from the cited sources (code read, not executed, where code is cited); correlated desk review (Claude); not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/lib/src/gainmapmetadata.cpp",
      "note": "lib/src/gainmapmetadata.cpp L123-138, L206-230, L311-318; CMakeLists.txt L129, L135, L228, L382; lib/include/ultrahdr/agtm.h L19-27"
    },
    {
      "url": "https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/lib/include/ultrahdr/agtm.h",
      "note": "lib/src/gainmapmetadata.cpp L123-138, L206-230, L311-318; CMakeLists.txt L129, L135, L228, L382; lib/include/ultrahdr/agtm.h L19-27"
    },
    {
      "url": "https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/lib/src/agtm.cpp",
      "note": "lib/src/gainmapmetadata.cpp L123-138, L206-230, L311-318; CMakeLists.txt L129, L135, L228, L382; lib/include/ultrahdr/agtm.h L19-27"
    },
    {
      "url": "https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/lib/src/jpegr.cpp",
      "note": "lib/src/gainmapmetadata.cpp L123-138, L206-230, L311-318; CMakeLists.txt L129, L135, L228, L382; lib/include/ultrahdr/agtm.h L19-27"
    },
    {
      "url": "https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/CMakeLists.txt",
      "note": "lib/src/gainmapmetadata.cpp L123-138, L206-230, L311-318; CMakeLists.txt L129, L135, L228, L382; lib/include/ultrahdr/agtm.h L19-27"
    }
  ]
}
```

Evidence claims:
- [inference] libultrahdr v2.0.2 writes the gain-map metadata as a big-endian binary block. It starts with a minimum version and a writer version (both 0, with other minimum versions rejected on read) and a flags byte (multi-channel and use-base-colour-space bits), followed by numerator/denominator fractions for the headroom and per-channel gain-map fields, with every denominator required to be non-zero. A code comment still refers to 'the draft' of the standard, so conformance to the published ISO 21496-1 is not shown. SMPTE ST 2094-50 conversion (agtm.h) and XMP metadata writing are both off by default in the CMake build. · locator: lib/src/gainmapmetadata.cpp L123-138, L206-230, L311-318; CMakeLists.txt L129, L135, L228, L382; lib/include/ultrahdr/agtm.h L19-27 · source: https://github.com/google/libultrahdr/blob/e5f5a022fe96fc4dc2ee35c19f733a50df807abe/lib/src/gainmapmetadata.cpp

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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