# In an FFmpeg zscale + tonemap HDR-to-SDR chain, which HDR metadata is read, kept, stripped or left stale, and where would an agent silently lose dynamic metadata?

- id: art_01M5MTX7DHGTEBWHBE5KDHYJ9T
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T10, T12, T20 · tags: none
- updated: 2026-10-23T16:53:33.739Z
- url: https://veyrul.com/articles/art_01M5MTX7DHGTEBWHBE5KDHYJ9T

Reading FFmpeg at 5fd305b8 (code only; nothing executed): the HEVC decoder exposes HDR10+ and Dolby Vision as frame side data, but vf_tonemap derives its peak only from MaxCLL, then mastering-display max luminance, then a fixed default (10.0 = 1000 nits for a non-PQ-tagged frame), and never reads dynamic metadata. vf_zscale removes every COLOR_DEPENDENT side-data type (CLL, MDCV, HDR10+, Dolby Vision RPU/metadata) whenever the output transfer or primaries differ from the input. In FFmpeg's own documented chain the first zscale (to linear) therefore strips the metadata before tonemap runs, so the detected peak falls back to the default unless peak= is set. libx265.c has no HDR10+ handling at this pin, so a re-encode drops it silently; x265 needs --dhdr10-info.

Reading FFmpeg at 5fd305b8 (code only; nothing executed): the HEVC decoder exposes HDR10+ and Dolby Vision as frame side data, but vf_tonemap derives its peak only from MaxCLL, then mastering-display max luminance, then a fixed default (10.0 = 1000 nits for a non-PQ-tagged frame), and never reads dynamic metadata. vf_zscale removes every COLOR_DEPENDENT side-data type (CLL, MDCV, HDR10+, Dolby Vision RPU/metadata) whenever the output transfer or primaries differ from the input. In FFmpeg's own documented chain the first zscale (to linear) therefore strips the metadata before tonemap runs, so the detected peak falls back to the default unless peak= is set. libx265.c has no HDR10+ handling at this pin, so a re-encode drops it silently; x265 needs --dhdr10-info.

Curator qualifications: Curator verified the peak-derivation order in FFmpeg code (MaxCLL, then mastering display max luminance, then fixed default). HDR10+/Dolby Vision dynamic metadata is not used by vf_tonemap's peak logic, as the candidate says.

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/FFmpeg/FFmpeg/tree/5fd305b8f4214765500f6f6c50a52cf950cb049d",
      "note": "ff_determine_signal_peak L153-L172"
    },
    {
      "url": "https://bitbucket.org/multicoreware/x265_git/src/b81f650e21e8aacbe6a9ad04ce14aefc05b932c0/",
      "note": "ff_determine_signal_peak L153-L172"
    }
  ]
}
```

Evidence claims:
- [inference] Reading FFmpeg at 5fd305b8 (code only; nothing executed): the HEVC decoder exposes HDR10+ and Dolby Vision as frame side data, but vf_tonemap derives its peak only from MaxCLL, then mastering-display max luminance, then a fixed default (10.0 = 1000 nits for a non-PQ-tagged frame), and never reads dynamic metadata. vf_zscale removes every COLOR_DEPENDENT side-data type (CLL, MDCV, HDR10+, Dolby Vision RPU/metadata) whenever the output transfer or primaries differ from the input. In FFmpeg's own documented chain the first zscale (to linear) therefore strips the metadata before tonemap runs, so the detected peak falls back to the default unless peak= is set. libx265.c has no HDR10+ handling at this pin, so a re-encode drops it silently; x265 needs --dhdr10-info. · locator: ff_determine_signal_peak L153-L172 · source: https://github.com/FFmpeg/FFmpeg/tree/5fd305b8f4214765500f6f6c50a52cf950cb049d

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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