# How is an HDR still stored as an SDR JPEG plus gain map (Ultra HDR / Adobe gain map / ISO 21496-1), and what colour-profile and metadata rules decide what a reader reconstructs?

- id: art_01M5MDK5X90BD1A3JASESJ1G4V
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T12, T17 · tags: none
- updated: 2026-10-23T13:00:52.825Z
- url: https://veyrul.com/articles/art_01M5MDK5X90BD1A3JASESJ1G4V

Android's Ultra HDR specification says the SDR primary image's colour profile defines the HDR image's colour space and that Ultra HDR and ISO 21496-1 gain-map metadata can share one gain-map image in a JPEG; AVIF 1.2 allows a tmap derived image item grouped with the base item; libjxl defines a jhgm box whose metadata is an ISO 21496-1 blob - three container families for gain maps.

Android's Ultra HDR specification says the SDR primary image's colour profile defines the HDR image's colour space and that Ultra HDR and ISO 21496-1 gain-map metadata can share one gain-map image in a JPEG; AVIF 1.2 allows a tmap derived image item grouped with the base item; libjxl defines a jhgm box whose metadata is an ISO 21496-1 blob - three container families for gain maps.

Curator qualifications: Adobe Gain Map page (S14) not reinspected (Adobe help pages returned 403 earlier in this window); its draft-version and patent-licence statements are unverified. Rights conflict preserved: Android documentation licence vs Adobe patent-licence reference not reconciled. libultrahdr README API table and libavif changelog (E6, E9) not reinspected; tool preservation of gain maps on re-save unknown. Specification and header text only; no image encoded.

Limitations: Live Android page.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://developer.android.com/media/platform/hdr-image-format",
      "note": "developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b"
    },
    {
      "url": "https://helpx.adobe.com/camera-raw/using/gain-map.html",
      "note": "developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b"
    },
    {
      "url": "https://github.com/google/libultrahdr/blob/47d9f00726adba87e3f04059a50a8b34492e057a/README.md",
      "note": "developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b"
    },
    {
      "url": "https://github.com/AOMediaCodec/av1-avif/blob/bf4c18d1f3971069b75e87d6ee469790589f4f09/index.bs",
      "note": "developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b"
    },
    {
      "url": "https://github.com/libjxl/libjxl/blob/f23e813be793fa5cd15c0eeca4ae32c2ed7c6add/lib/include/jxl/gain_map.h",
      "note": "developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b"
    },
    {
      "url": "https://github.com/AOMediaCodec/libavif/blob/b1e4d339c1ff5b547664c85f1713279f4997d694/CHANGELOG.md",
      "note": "developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Android's Ultra HDR specification says the SDR primary image's colour profile defines the HDR image's colour space and that Ultra HDR and ISO 21496-1 gain-map metadata can share one gain-map image in a JPEG; AVIF 1.2 allows a tmap derived image item grouped with the base item; libjxl defines a jhgm box whose metadata is an ISO 21496-1 blob - three container families for gain maps. · locator: developer.android.com/media/platform/hdr-image-format (fetched 2026-10-22); av1-avif index.bs line 399 at bf4c18d1; libjxl lib/include/jxl/gain_map.h lines 10, 40-44 at f23e813b · source: https://developer.android.com/media/platform/hdr-image-format

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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